Report on Methods for Estimating In-Place Concrete Strength · 2020. 5. 12. · ACI228.1R-19 Report...

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Report on Methods for Estimating In-Place Concrete Strength Reported by ACI Committee 228

� American Concrete Institute � Always advancmg

American Concrete Institute Always advancing

irst Printing January 2019

ISBN: 978-1-64195-050-3

Report on Methods for Estimating In-Place Concrete Strength Copyright by the American Concrete Institute, Farmington Hills, MI. All rights reserved. This material may not be reproduced or copied, in whole or part, in any printed, mechanical, electronic, film, or other distribution and storage media, without the written consent of ACI.

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ACI228.1R-19

Report on Methods for Estimating In-Place Concrete

Strength

Reported by ACI Committee 228

Andrew J. Boyd. Chair

Todd Allen Muhanuned P. A. Ba heer

Michael D. Brown icholas J. Carino

Wil liam Ciggelakis Aldo De La Haza

Ethan C. Dodge

Boris Dragunsky Christopher C. Ferraro

Michael C. Forde Mosta(a Mohamed Gad Alia

Eric R. Giannini Kerry . Hall

Julie Ann HaneU

B mard H. Hertlein. Secretary

Frederick D. Heidbrink Michael W. Hoag Robert S. Jenkins

Liying Jiang Keith E. Kesner

Hai . Lew Malcolm K. Lim

Larry D. Olson Stephen Pes iki JohnS. Popovics

athaniel Steven Rende Patrice Rivard Patll L. iwek

Patrick J. E. Sulli an

Consulting Members

John H. Bungey Honggang ao

1-lermenegildo aratin • estor E. Chonillo

Gerardo G. Clemena eilA. umming

AI Gh rhanpoor Alexander M. Le hchinsky

Kenneth M. Lozen V. M. Malhotra

Clau� Germann Petersen George V. Teodoru

Herbert Wiggenhauser

ubcommittee Member

hris M. McDenn u

This report provides methods for esh"maling the in-place strength of

concrete in new and existing com/ruction. These methods include:

rebound numbet; penetration resistance. pullout. pull-off, ultra

sonic pulse velocity, maturity, and cast-in-place cylinders. The

principle. inherent limitations, and repeatability of each method

are reviewed. Pmcedures are presented for developing the relation

ship needed to estimate compressive strength fi"Om in-place results.

Factors to consider in planning in-place tests are discussed, and

statistical techniques to inte1pret lest results are presented. The

use of in-place tests for acceptance of concrete is inft"Oduced.

The Appendix A pmvides information on the number of strength

levels that should be used to develop the strength relationship and

explains a regression analysis procedure that accoums for ermr in

both dependent and independent variables.

ACJ Committee Reports, Guides, and ommentaries are intended for guidance in planning, designing, executing, and inspecting construction. This document i intended for the use of individual who arc competent to evaluate the significance and limitations of its content and recommendations and who will accept responsibility for the application of the material it contains. TI1e American oncrete In titute disclaims any and all respon ibility for the stated principles. The Institute shall not be liable for any lo s or damage arising therefrom.

Reference to this docmnent hall not be made in contract documents. If item found in this document are desired by the Architect/Engineer to be a part of the contract documents, they shall be restated in mandatory language for incorporation by the Architect/Engineer.

Keywords: coefficient of variation; compressive strength; construction safety; in-place tc ts: nondcstructi c test : sampling; statistical analysis.

CONTENTS

CHAPTER 1 -INTRODUCTION, p. 2 I . I -Scope,p. 2 1 .2- eed for in-place te ts during con tmction, p. 2 1 .3-fnftuence of A T 3 1 8 , p. 3 1 .4-Rccommendations in other ACT documents, p. 3 1 .5-Exi ting constm rion, p. 4

1 .6-Report obj ctivc, p. 4

CHAPTER 2-NOTATION AND DEFINITIONS, p. 4 2. 1 - 1otation, p. 4 2.2-Definitions, p. 5

CHAPTER 3-REVIEW OF METHODS, p. 5 . 1-Tntroduction, p. 5

3 .2-Rebound number {A TM 805/ 805M), p. 6 3 .3-Pcnctration resistance (A TM 803/C803M ). p. 7 3 . Pullout te r (A TM C900), p. 8

ACI 228.1 R-19 supscrscdes ACI 228.1 R-03 and was adopted and published January

2019. opynght 2019. American Concrete Institute.

All rights reserved including rights of reproduction and use in any form or by

""Y means, including the making of copies by any photo process, or by eleclronic

or mechanical dcvu�:c, prinled. wnttrn. or oral, or recording for sound or visual

reproduction or for use in any knowledge or retrlc al system or dev1ce, unlc s pennis ion in writing is obtained from the copyright proprietors.

2 REPORT ON METHODS FOR ESTIMATING IN-PLACE CONCRETE STRENGTH (AC1 228.1 R-19)

3.5-Pull-off test (A TM C 1 583/C1583M ), p. I I 3 .6 U ltrasonic pulse velocity ( TM C597), p. 12 3 .7 Maturity method ( TM C 1 074), p. 1 4 3 . ast-in-place cylinders ( TM 873/ 73M), p. 1 6 3. trength limitation , p. 16 3 .10 ombined method . p. 16 3 .11 ummary, p. 17

CHAPTER 4-STATISTICAL CHARACTERISTICS OF TEST RESULTS, p. 17

4.1- eed for tatistical analysis, p. 17 4.2-Repeatabil ity of test result , p. 1 8

CHAPTER 5-DEVELOPMENT OF STRENGTH RELATIONSHIP, p. 23

5 .1 General, p. 23 5.2 ew construction, p. 24 5 .3 Existing con truction, p. 27

CHAPTER 6-IM PLEMENTATION OF IN-PLACE TESTING, p. 28

6. 1- ew construction. p. 28 6.2-Existing con truction, p. 31

CHAPTER ?-INTERPRETING AND REPORTING RESULTS, p. 32

7.1 General, p. 2 7.2 tatistical method , p. 33 7.3 Reporting re ults, p. 36

CHAPTER S-IN- PLACE TESTS FOR ACCEPTANCE OF CONCRETE IN NEW CONSTRUCTION, p. 38

8.1- eneral, p. 38 8.2-Acceptance criteria, p. 38 8.3 Early-age testing p. 3

CHAPTER 9-REFERENCES, p. 39 Authored documents, p. 40

APPENDIX A , p. 44 A. l Minimum nw11ber of strength level , p. 44 A.2 Regression analysis with X-error (Mandel 1984), p. 44 A.3- tandard deviation of estimated Y-value ( tone and

Reeve 19 6), p. 46 A. Example, p. 46

CHAPTER 1-INTRODUCTION

1.1 -Scope In-place te ts are performed typically on concrete within a

tructure, in contrast to te t performed on molded specimen made from the concrete to be u ed in the tructure. Hi Iorically, they ha e been cal led nonde tructive tests becau e some of the early tests, such as rebound number and uln·asonic pul e elocity, were noninvasive and did not damage the concrete. Over the years, howe er, new method have developed that re ult in superficial local damage. There-

fore, the tem1inology "in-place tests" is used as a general name for these test methods, which includes those that do not damage the concrete and those that result in some near-urface damage. In thi report, the principal application of

in-place tests is to estimate the compressive trength of the concrete. The pull-off test can be used to e tim ate the tensile trength of concrete or evaluate bond trength between

layers. The ignjficant characteristic of most of the e tests i that they do not drrectly measure the compre ive trength of the concrete in a structure. In tead, they mea ure ome other property that can be correlated to com pres ive trength (Popovics 1998). The strength is then estimated from a previously establi bed relationship between the measured property and concrete strength. The uncertainty of the estimated compre sive trength depends on the ariability of in-place test results and the uncertainty of the relationship between these two parameters. The e ource of uncertainty are di cussed in this report. An alternative approach for correlation between te ts re ults and concrete trength i pre en ted in E I 3 79 I (2007) and B 60 9 (20 I 0).

In-place test can be used to e timate concrete trengtb during construction so that operations requiring a specific strength can be performed safely or curing procedures termjnated. They can also be used to e timate concrete trength during the evaluation of existing structures. These two applications require l ightly different approaches, so parts of this report are eparated into section dealing with new and exi ring con truction.

A variety of technique are a ai lable for estimating the

in-place trength of concrete (Malhotra 1976; Bungey et al . 2006; Malhotra and Carino 2004). o attempt i made to review all method in thi report· only tho e method that have been tandardized by A TM International are discussed. Examples of methods not co ered include internal fracture test (Chabowski and B1yden- mith 19 0; Domone and Castro 1987) and torque te t ( toll 1985 ).

1.2-Need for in-place tests during construction In North American practice, the mo t widely used test for

concrete is the compressive strength te t of standard cylinder ( STM C39/39M). This te t procedure i relatively ea y to p rform in t rm of ampl ing, pecimen preparation

( STM C31/C31 M), and trength measurement. hen properly performed this test has low single-operator variation and low interlaboratory variation and therefore, the method lend itself to use as a standardized te ting procedure. The compressive strength so obtained is u ed to verify that the specified strength (f;') used to calculate the nominal strengths of tructural member has been achieved. Therefore the compres ive strength of tandard cylinders is an e sential parameter in design code and project specifications.

hen carried out according to tandard procedure , howe er, the re ult of the cylinder compres ion te t represent the poten6al strength of the concrete as delivered to a site. The test is u ed mainly as a basis for quality assurance of the concrete to ensure that contract requirement are met. It is not intended for determining the in-place strength of the concrete because it makes no al lowance for the effects

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REPORT ON METHODS FOR ESTIMATING IN-PLACE CONCRETE STRENGTH (ACI 228.1 R-19) 3

of placing, consolidation or curing. It is unusual for the concrete in a structure to have the same properties as a standard-cured cylinder at the arne test age. lso standardcured cylinders are usually tested for acceptance purpo es at an age of28 days· therefore, the result of these te ts cannot be used to determine whether adequate trength exists at earl ier ages for safe removal of formwork or the application of po !-tensioning. The concrete in some part of a tructure, uch a colwnn may develop trength equal to the tandard 28-day cylinder trength by the time it i ubjected

to design loads. Concrete in flexural member , especially pretensioned members, can be required to support a large percentage of the design load before an age of 28 days. For these reason , in-place tests are used to estimate the concrete trength at critical location in a structure and at times when

crucial construction operations are scheduled. Traditional ly, a mea ure of the trength of the concrete in

the structure ha been obtained by using field-cured cyl inders prepared and cured in accordance with A TM C3 1 /C3 1 M. The e cyl inder are cured on or in the tru ture under, a nearly a possible, the arne condition as the concrete in the tructure. Measured strengths of field-cured cylinders may

be significantly different from in-place strengths because it i difficult, and often impossible, to ha e identical bleeding con olidation, and curing condition for concrete in cylinder and concrete in structures ( outsos et al. 2000). Fieldcured pecimens should be handled with care and tored properly to avoid misleading test result .

Con truction schedules often require that operation uch a form removal, post-tensioning, termination of curing, and removal of re hore b carried out a early a po ible to keep the project on chedule. To enable the e operation to proceed afely at the earliest pos ible time requires the use of reliable in-place tests to estimate the in-place strength. The need for uch strength information is emphasized by everal con truction fai lure that po ibly could have been

prevented had in-place testing been u ed ( Lew 1 9 0; 'arino et al . 1 9 a) . In-place te ting not only increases safety but can re ult in substantial co t saving by permitting accelerated construction schedules (Bickley 1 982a).

1.3-lnfluence of ACI 318 Before 1 9 3 , ACI 3 1 required testing o f field-cured cylin

ders to demonstrate the adequacy of concrete strength before removal of formwork or re horing. ln 1 983, ACI 3 1 8 first al lowed the use of other procedures instead of tests for fieldcured cylinders, if approved by the building official ( I

ommittee 3 1 8 1 983). The design professional, when requested by the building official however, wa required to approve the altemative procedure before its u e. Since 1 9 3, A I 3 1 has permitted the u e of in-place testing as an alternative to te ting field-cw-ed cylinders if approved by the licen ed de ign profe -

ional and, if reque ted, approved by the building official. The commentaiy to ACI 3 1 - 1 4 ( ection R26. 1 1 .2. 1 (e)) l i ts fowprocedures, which are covered in this report, that may be used pro ided there are sufficient correlation data.

Most design provision in I 3 1 are based on the compressive strength of standard cylinders. Thu to evaluate

tructural capacity under construction loading 1t IS nece -ary to have an estimate of the equivalent cylinder trength

of the concrete as it exists in the structure. If in-place tests are u ed a valid relation hip between the re ults of in-place tests and the compressive strength of cylinder is required.

I present, there are no standard practices for developing the required relation hip.

1.4-Recommendations in other ACI documents After the 1 995 ver ion of thi report wa published, other

I documents incorporated in-place test as alternative procedures for estimating in-place strength. One of these documents is I 30 I a specification for new concrete con truction. In the 20 1 6 ve1 ion of I 30 I , ection 1 .6.4.2 on in-place testing of hardened concrete include the following:

U e of the rebound hammer in accordance with A TM C805/C805M or the pulse-velocity method in accordance with ASTM CS97 may be specified by rchitect!Engineer to evaluate uniformity of in-place concrete or to elect areas to be cored. These method shall not be used to evaluate in-place strength.

Regarding the validity of in-place strength test , A I 30 1 -16 states in ection 1 .6.S.3(a):

Result of in-place strength tests wil l be evaluated by Architect/Engineer 311d are valid only if tests are condu ted u ing properly cal ibrated equipment in a cordance with recognized standard procedure and an acceptable correlation between te t re ult and concrete compres ive trength i established and submitted.

ection 1 .6.6.4 of I 30 1 - 1 6, however, restricts the u e of the e te ts in acceptance of concrete by stating that ''In-place tests shall not be used as the sole basis for accepting or rejecting concrete," but they may be used if specified to "evaluate" concrete if standard-cured cyl inder trength fail to meet the pecified strength criteria.

A I 30 1 - 1 6 al o mention in-place te t in ection 2.3.4 dealing with required strength for removal of formwork.

pecifically, it is stated that if specified in Contract Documents the following methods may be used to estimate in-place trength:

(a) TM 73/ 873M (ca t-in-place cylinder )

(b) A TM 803/C803M (penetration resi tance) (c) TM 900 (pul lout) (d) TM C I 074 (maturity method) The e same methods may be u ed if pecified, as alterna

tive to testing field-cmed cylinder for e timating in-place tr ngth for the purpo e of terminating curing procedure .

A J 562- 1 6, the repair code for exi ting concrete building , al lowed the use of in-place test methods for assessment of concrete trength. ection 6.4.3.2 state :



ondestructive strength testing to evaluate in-place strength of concrete shall be permitted if a alid correlation is established with core sample com pre -

i e trength te I re ult and nondestructive te t re ult . Quantifications of concrete compre sive strength by DT alone shall not be permitted as a substitute for core amp l ing and te ting.

ACJ 308. 1 also mention in-pia e te t a acceptable method for e timating in-place trength for the purpo e of terminating curing procedures. Project specifications can, therefore, reference standard specifications that al low in-place testing as an alternative to testing field-cured cylinders. A I 325. 1 1 R di cu ses the use of in-place te t for e timating early-age concrete strength in fast-track concrete paving projects.

In all cases, ufficient correlation data are required, and permis ion may have to be granted before using in-place test method . Thi report explains how the required correlation data can be acquired and provide guidance on how to implement an in-place te ting program.

1 .5-Existing construction Reliable estimates of the in-place concrete trength are

required for structural evaluation of exi ting structure ( CI 437R and A I -62). H istorically, in-place strength has been e timated by te ting core drilled from the tructure. In-place tests can supplement coring and penni! more economical c aluation of concrete in the structure. The critical step in such applications is to establi h the relationship between in-place te t re ult and concrete str ngth. More recently the approach i to correlate re ult of in-place te t performed at selected locations with the strength of corresponding cores. Although in-place testing does not eliminate the need for coring, it can reduce the total amount of coring needed to evaluate a large volume of concrete. ound sampling plan is required to acquire the correlation data, and appropriate stati tical methods should be u ed for reliable interpretation ofte t re ult .

1 .6-Report objective This report re iew TM te t method for e timating

the in-place trength of concrete in new construction and existing structures. The overall objective is to provide the potential user with a guide to as ist in planning, conducting and interpreting the result of in-place test .

hapter 2 inc ludes the notation and definition of terms used in this report. hapter 3 discusses the underlying principles and inherent l imitation of in-place te ts. hapter 4 review the statistical characteri tic of in-place test .

hapter 5 outl ines procedures to develop the relationhip needed to e timate in-place compre i e trength.

Chapter 6 di cu e factors to be con idered in planning the in-place testing program. Chapter 7 pre ents statistical techniques to interpret in-place test results. Chapter 8 discusses in-place testing for acceptance of concrete. Appendix A pro ides detail and an i l lustrative example on the statistical principles di cus ed.

CHAPTER 2-NOTATION AND DEFINITIONS

2.1 -Notation a b

c

d,_. I i In

lnl lnPO = K k

M(t)

111

11; n,

n,

PO Q

if

sx

intercept of l ine slope of l ine, u ed m development of trength relationship average of in-place compressive strengths individual compres ive strength test results de iation of each te t point from the best-fit l ine in-place test result indjvidual in-place trength te t re ults average of natural logarithms of compre 1 e trength

average of natural logarithms of in-place te t re ult average of natural logarithms of pullout load one-sided tolerance factor (Table 6. 1 .2a) b/'J..., where A. is obtained from the single-operator variabil ity during correlation te ting as given by

q. ( .2d). temperature-time factor at age t. deg-days or deg-hours number of replicate in-place tests perfonned on an element of a structure number of te t point used to e tabli h the trength relation hip number of replicate in-place te ts number of replicate in-place tests at each strength level number of replicate compressive strength tests at each trength le el average of natural logarithms of pullout loads apparent activation energy di ided by the gas con tant. K (kelvin) estimated re idual tandard deviation modified sum of the quares as given by Eq. (A.2i(a)) urn of quare of the deviation about X of the X alue us:_d to develop the trength relationship, Sa

=L(X-Xf tandard deviation of the logarithms of concrete

strength in the structure tandard deviation of logarithm of compre si e trength in the laboratory

error of fit of trength relation hip a given by Eq.

( .2j) tandard deviation of logarithms of the in-place

re ult in the laboratory tandard deviation of the logarithm of the in-place

te ts perfonned on the structure standard deviation of the logarithms of the in-place tests at strength levelj standard deviation of estimated value of Y (average concrete strength) standard deviation of the logarithms of the compre -

ive strength test at strength levelj average concrete temperature during time interval

r, oc (°F} or K datum temperature, 0 (°F)

pecified curing temperature, K

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I� equivalent age at a specified temperature 'fs, days or hours

t,.1.a tudent's /-value for m- 1 degrees of freedom and confidence le el a

t.v-2.o12 = tudent's /-value for -2 degrees of freedom and ignificance level a

Vi coefficient of variation of in-place test V, coefficient of variation of standard test Vr coefficient of variation (ex pre ed a a decimal) W width of the I 00( 1 - a) percent confidence interval

for the estimated mean value of Y for the value X X average of the logarithms of the in-place tests

perfonned on the structure

� average of the logarithm of the in-place tests at strength le elj

X grand average of X values during coJTelation tests q. (A.2g(a))

Y logarithm of the e timated average in-place compre i e trength

Y grand averag of Y value during correlation te t ,

Eq. (A.2g(b)) Yo.1o lower tenth percentile of strength (I 0 percent

defective)

>j average of the logarithms of the compress• e trength tests at strength level}

Y10,. lower confidence limit of e timated average in-place trength at confidence level a

1 time interval, day or hour A. - ratio of variance of average compre sive strength

te t re ults to ariance of average in-pia e test result

2.2-Defi n itions A I pro ide a comprehensive li t of definitions through

an online resource, I oncrete Terminology. Definition pro ided herein complement that ource.

apparent activation energy-an empirical factor that indicate the temperature sensitivity of the rate of trength development of a cementitious mixture after final setting has occurred.

datum temperatur the temperature value that is used for calculating the temperature-time factor.

equivalent ag the number of days or hours of curing of a concrete mixture at a specified temperature required to produce a maturity equal to the maturity achieved by a given curing period at concrete temperatures different from the specified temperature.

lower confidence limit-the value above which the true average, in-place strength i expected to occur with a specified probability or confidence level.

lower tenth percentile strength-the trength below which I 0 percent of test results are expected to occur.

maturity-the extent of the development of a property of a cementitious mixture.

maturity function a mathematical expression that u es the measured temperature hi tory of a cementitious mixture during the curing period to calculate an index that is indicative of the maturity at the end of that period.

maturity index-an indicator of marurity, such as equi alent age or temperature-time factor, that is calculated from the temperature history of the cementitiou mixture by using a maturity function.

residual tandard deviation-a measure of the scatter of test data about a regres ion line· the tandard deviation of the test value from the value c timated from the regre sion line.

single-operator variability-the standard deviation or coefficient of variation of repl icate test detenninations obtained on identical test specimens by a single operator using the arne apparatus.

strength relationship--empirical equation obtained from regression analy is of test data that relate the compressi e strength of concrete to the re ult of an in-place test method.

temperature-time factor-the maturity index computed as the area between the concrete temperature and the datum temperature from the plot of mea w-ed concrete temperature

crsu time, ex pre sed in unit of degree-days or degree-hours.

CHAPTER 3-REVIEW OF METHODS

3.1-lntroduction ften, the objective of in-place testing is to estimate

the compre sive trength of concrete in the tructure. To make a trength e timate, it i necessary to have a known relation hip between the re ult of the in-place te t and the concrete strength. For new con truction, thi relationship i u ually e tabli hed empirically in the laboratory. For exi ting con truction, the relationship is con entionally e tablished by perf01ming in-place te t at selected locations in the structure and determining the strength of cores drilled from adjacent location . Cl 2 1 4 .4R discu es the number of core pecimens needed to obtain, at a given confidence level, an e timate of average trength that is within a certain percentage of the true average strength. Figure 3.1 is a schematic of a strength relationship in which the cylinder compressive trength is plotted a a function of an in-place te t re uJt. Thj relation hip would be u ed to e timate the

tr ngth of concrete in a tructure ba ed on the value of the in-place te t result obtained from te ting the structure. The accuracy of the strength estimate depend on the degree of correlation between the strength of concrete and the quantity measured by the in-place test. The user of in-place tests should understand what property i measured by the test and how this property i related to the strength of concrete.

The purpo e ofthi chapter i to explain the underlying principles of the widely used in-place te t methods, and to identify the factors other than concrete strength, that can influence the te t re ult . Additional background information on the e methods i available in the refi renee by Malhotra ( 1 976), Bw1gey et al. (2006), and Malhotra and Carino (2004).

The fol lowing method are discus ed: (a) Rebound number (b) Penetration resistance (c) Pullout test (d) Pull-off test (e) ltrasonic put e velocity (t) Maturity (g) a t-in-place cylinder



Cyl inder

Compressive

Strength

In-Place Test Value

Fig. 3.1 chemotic of relationship between cylinder compressive strength and in-place test value.

3.2-Rebound number (ASTM C805/C805M) Operation of the rebound hammer, also called the chmidt

Hammer or \ i Hammer, is i l lustrated in Fig. 3.2. The device consi ts of the following main component : I) outer body; 2) metal plunger and guide rod· 3) hammer; and 4) impact pring. The fol lowing equence de cribes how the tc t i pcrfonned. From the initial locked condition, the plunger i placed in contact with the concrete urface and the plunger i extended from the body of the in trument by mean of a compres ion pring. A latch lock the hammer in place and the instrument i ready for testing ( Fig. 3 .2(a)). Then, the body i pu hed toward the te t object, which extend the impact spring connected between the hammer and body (Fig. 3.2(b)). When the body i pu hed to its l imit of tra el, the hammer i released and travels toward the test object due to the energy tored in the tretched impact spring (Fig. 3.2(c)). Finally, the

hammer trike the plunger and rebounds a certain di tance along the guide rod (Fig. 3 .2(d)). The rebound number can be based on the rebound di lance expre ed as a percentage of the tretch dj tance of the impact pring, or it can be ba ed on the ratio of the hammer peed after impact to the hammer peed before impact with the plunger.

The key to understanding the inherent limitations of thi te t for estimating strength i recognizing the factors influencing the rebound number. The impact loading and re ulting wave propagation within the hammer-concrete sy tern re ult in a compl icated dynamic sy tern that is difficult to model from a fundamental point of view.

The rebound number depends on the kinetic energy in the hammer before impact with the plunger and the amount of that energy ab orbed during impact. Part of the energy i ab orbed a mechanical friction in the in trument, and part of the energy is ab orbed in the interaction of the plunger with the concrete. The latter factor is what make the rebound number an indicator of the concrete properties.

nergy ab orbed by the concrete depends on the tres - train relationship of the concrete. Therefore, ab orbed energy i related to the strength and sti ffne of the concrete. lowstrength, low- tiffnc s concrete will absorb more energy than a high-strength, high-stiffnes concrete. Thu , the low- trength concrete wil l l ikely re ult in a lower rebound number. Because it i po ible for two concrete mixture to have the arne trength but different st iffne e , there could be different rebound number e en if the trengths are equal.

(a) Instrument ready

for test

(b) Body pushed

toward test object (c)

Hammer is released

(d) Hammer rebounds

Fig. 3.2- -S hematic illustrating operation of rebound hammer.

Conversely, it i pos ible for two concrete with ditrerent trength to have the arne rebound number if the tiffne

of the low- trength concrete i greater than the stiffne of the high- trcngth concrete. Bccau c aggregate type and the

olume of coar e aggregate affect the stilfne of concrete, it i nece sary to develop the strength relation hip on concrete made with the arne aggregates and imilar proportions that will be u ed for the concrete in the structure.

ln rebound-hammer te ting, the concrete near the point where the plunger impact influence the rebound value. Therefore, the test is ensitive to the urface condition , local

ariation in concrete con olidation, and relative tiffne of the member at the location where the te t i performed. If the plunger i located over a hard-aggregate particle an unu ually high rebound number will re ult. By contra t, if the plunger i located over a large air oid or a oft aggregate particle, a lower rebound number will occur. Reinforcing bars with hallow concrete co er could al o affect rebound numbers if test are done directly o er the bars. To account for these po sibilitie TM C 051 05M require that 1 0 rebound numbers be taken for a test. I f a reading differs by more than ix units from the average, that reading i di carded and a new a erage i computed ba ed on the remaining reading . I f more than two reading differ from the a erage by i unit , the entire et of reading i di carded.

Becau e the rebound number i affected mainly by the ncar- urfacc layer of concrete. the rebound number might not repre ent the interior concrete. The pre ence of urface carbonation can re ult in higher rebound numbers that are not indicative of the interior concrete. imilarly, a dry surface will re ult in higher rebound number than for the moi t, interior concrete. Ab orptive oiled plywood can ab orb moi ture from the concrete and produce a harder urface layer than concrete ca t again t teel form . imi

larly, curing condition affect the trength and tiffne of the near- urface concrete more than the interior concrete. The urface tc turc can al o influence the rebound number. When the te t i performed on rough concrete, local crushing occur under the plunger and the indicated concrete trength



will be lower than the true value. Rough surfaces should be ground before te ting. I f the formed urfaces are smooth, grinding is unneces ary. hard, smooth surface, such as a urface produced by trowel fini hing, can result in higher

rebound number . Finally the rebound di tance is affected by the orientation of the instrument· the strength relationship has to be developed for the arne instrument orientation as will be u ed for in-place testing.

Ln wnmary, while the rebound number te t is imple to perform, there are many factor other than concrete trength that influence test re ults. As a result, estimated compre -sive strengths are tmreliable unless a correlation is developed between rebound number and compressive strength for a given concrete mixture or from core taken from a structure. Refer to hapters 5 and 6 for additional information on developing the relationship and on using the relation hip to e timate in-place strength.

3.3-Penetration resistance (ASTM C803/C803M) In the penetration-re i lance technique, depth of pene

tration of a rod (probe) or a pin forced into the hardened concrete by a dri er unit i measured.

The probe-penetration technique involve the use of a specially designed gun to drive a hardened steel probe into the concrete. One well-known commercial test system is the Wind or Probe. The penetration depth of the probe is an indicator of concrete strength. This method is imilar to the rebound number te t, except the probe impact the concrete with much higher energy than the plunger of the rebound hammer. The probe penetrates the concrete, compared with the plunger of the rebound hammer which only produce a minor urface indentation. A theoretical analysi of thi te t i even more complicated than the rebound test, but again, the e sence of the test invol es the initial kinetic energy of the probe and energy absorption by the concrete. The probe penetrates the concrete until it initial kinetic energy i absorbed. The initial kinetic energy i governed by the charge of smokeles po' der used to propel the probe the location of the probe in the gun barrel before firing, and frictional lo ses as the probe travels through the barrel. An e sential requirement of thi test is that the probe have a con i tent alue of initial kinetic energy. A TM C 03/

03M require that the probe exit-velocitie do not ha e a coefficient of variation greater than 3 percent based on I 0 tests by appro ed balli tic methods.

As the probe penetrates the concrete, some energy is absorbed by friction between the probe and the concrete, and some is ab orbed by cru bing and fracturing the concrete. There are no rigorous tudies of factor affecting the geometry of the fracture zone, but it general shape i mo t likely similar to Fig. 3.3a. There i usually a cone-shaped region in which the concrete i heavily fractured and a ignificant pot1ion of the probe energy i absorbed in thi zone.

The probe tip can travel through mot1ar and aggregate; in general, cracks in the fracture zone will be through the mortar matrix and the coarse-aggregate particles. l -Ienee, the strength properties of both the mortar and coarse aggregate influence penetration distance. This contra t with the

Exposed length

f'S·.:: •.. ··,\iiii�� Coarse aggregate

Fig. 3.3a-Approximate shape of failure zone in concrete during probe penetration test.

0 4

40

l1l 0... � .s:: 30

0, c:: Q) .... U5 20

Q) > "iii (/) Q) 1 0 .... a. E 0 u

0

1 0

Penetration Depth, in. 0 8 1 2

. . . . . . . . . -� . . . . . . . . . . . . . i . . . . . . . . . . . . . . . . . . . . . . . . . . . .

20 30

1 6

40

Penetration Depth, mm

2

5800

"iii a. .s::

4350 0, c:: Q) .... .... (f) 2900 Q) > "iii (/)

� 1 450

a. E 0 (.)

0

50

Fig. 3.3b-Effect of aggregate type on relation hip between concrete strength and depth of probe penetration.

behavior of normal- trength concrete in a compre sion te t , where mortar trength has the predominant influence on mea ured compre ive trength. Thu , an important characteri tic of the probe penetration te t i that the type of coar e aggregate affect greatly the relation hip between concrete trength and depth of probe pen tration. For example Fig.

3.3b compare empirical relationship between compre -ive trengtb and prob penetration for con rete made with

a oft aggregate uch as Lime tone, and concrete made with a hard aggregate uch a chert. For equal cornpre si e trength , the concrete with the oft aggregate al low greater

probe penetration than concrete with hard aggregate. More detailed information on the influence of aggregate type on

trength relation hip i found in Malhotr. ( 1 976), Bungey et al. (2006), and Malhotra and arino (2004).

Penetration value may be affected by urface v ith coar e texture . ASTM C803/C 03M require grinding of urfa e that are coar er than burlap-dragged fini be before te ting.

hard urface layer, a would o cur with trowel finishing, can re ult in low penetration values and exces ive scatter of data. The orientation in which the te t i performed i not critical provided the probe is driven perpendicular to the

American Concrete lnstitu1e - Copyrighted © Material - www.concrete.org


surface. The penetration, however wil l be affected by the pre ence of reinforcing bars within the zone of influence of the penetrating probe. Thus, the location of the reinforcing tee) should be determined before electing test ites. over

meter can be used for thi purpo e ( I 228.2R). In practice, it is cu tomary to measure the exposed length of

the probe . The fundamental relationshjp however, i between concrete trength and resistance to penetration. Therefore when a se sing the ariabi)jty of te t re ult (Chapter 4), it i preferable to expre the coefficient of variation in tenn of penetration depth rather than expo ed length.

Before 1 999, the hardened steel probes were l imited to � e in concrete with compre sive strength less than approxImately 40 M Pa (6000 p i ) . There wa a tendency for the probe to fracture > ithin the threaded region when testing stronger concrete. Al-Manaseer and qui no ( 1 999) reported that a newer probe made with tre s-relieved alloy tee) was successfully used to test concrete with a compres ive strength of 1 1 7 M Pa ( 1 7,000 p i ) .

A pin penetration te t device, which u e le energy than the Wind or Probe y tem, wa developed by a ser ( asser and Al-Manaseer 1 987a,b), and the procedure for its use was sub equently incorporated into A TM C 031 C803M. spring-loaded device i u ed to dri e a pointed 3.56 mm (0. 1 40 in . ) diameter hardened tee! pin into the concrete. Penetration by the pin creates a mall indentation (or hole) in the concrete urface. The pin i removed from the hole, the hole cleaned with an air jet, and the hole depth mea ured with a uitable depth gauge. Penetration depth is u ed �o e timate compre si e strength from a pre iously e tabh bed trength relation hip.

The kinetic energy deli ered by the pin penetration device i � t im a ted to be approxjmately 1 .3 percent of the energy dell ered by the Windsor Probe system (Carino and Tank

_1 989). Becau e of low energy le el, penetration of the pin IS reduced greatly if the pin encounters a coarse-aggregate particle. Thu , the test is intended a a penetration test of the mortar fraction of the concrete. Results of tests that penetrate coar e-aggregate particles are not con idered in detennining the average pin-penetration resistance (ASTM C803/C803M). A pin can b come blunted during penen·ation. Becau e the degree of blunting affe t penetration depth, TM C803/C803M require that a new pin be u ed for each penetration test.

ensiti ity of the pin penetration to changes in compressive strength decreases for concrete strength above 2 M Pa (4000 psi) (Carino and Tank 1 9 9). Therefore the pin penetration te t ystem i not recommended for testing concrete having an expected compre ive n·ength above 2 MPa (4000 p i ).

In summary concrete strength can be estimated by mea uring the penetration depth of a probe or pin driven into the concrete at con tant energy. Penetration te t are le affected by urface condition than the rebound number method. The coar e aggregate, however ha a sigruficant effect on the re ult ing penetration. For the gun-driven probe system, the type of coar e aggregate affects the strength relationship; for the spring-driven pin system tests that impact coar e aggregate particle are di regarded.

d, <: 1 .25 d, 2 0 d, < d, < 2 4 d,

,,. '

Fig. 3.4a-Dimensiona/ requirements of pullout test in accordance with A TM 900.

3.4-Pullout test (ASTM C900) The pullout test measures the maximum force required to

pull an embedded metal insert with an enlarged head from a concrete specimen or structure. A po itivc feature of the pullout te t i that it produce a well-defined failure urface in the concrete and mea ure a tatic, rather than dynamic, strength property of the concrete.

The test requires a metal insert that is either cast into fresh concrete or installed into hardened concrete. The pullout force is applied by a loading system that react against the concrete surface through a reaction ring concentric with the in ert (Fig. 3 .4a). A the insert is pulled out, a roughly c�ne-shaped fragment of the concrete is extracted. The large d1amcter of the conic fragment, d2, i detennined by the inner diameter of the reaction ring, and the mall diameter d1 i dctennined by the insert-head diameter. Requirement for the te ting configuration are given in TM 900. The embedment depth and head diameter have to be qual, but there i no requirement on the magnitude of the e dimension . The inner diameter of the reaction ring can be between 2.0 and 2.4 times the insert-head diameter. This means that the apex angle ("2a") of the conic frustum defined by the in ert-head diameter and the inside diameter of the reaction ring can vary between 54 and 70 degree . The arne test geometry ha to be used for de eloping the trength relationship and for the in-place testing.

Unlike the rebound hammer and probe-penetration te t , the pullout te t ubject the concrete to a tatic loading that lend it elf to tre analy is. The finite-element method ha been u ed to calculate the stre e induced in the concrete before cracking ( tone and arino 1 984) and where the concrete has cracked ( ttosen 1 9 I · Ferretti 2004). In the e analyses the concrete was as umed to be a homogeneous solid and the influence of discrete coarse-aggregate particles was not modeled. There is agreement in cited literature that the test subject the concrete to a nonuniform, three-dimensional state of stres . Figure 3.4b show the approximate directions (trajectorie ) of the principal tre e acting in radial plane (tho e pas ing through the center of the in ert) before cracking for apex


REPORT ON METHODS FOR ESTIMATING IN-PLACE CONCRETE STRENGTH (AC1 228.1 R-19) 9

Tensile stress trajectory

Compressive stress trajectory

-- Tensile stress trajectory

- - - - · Compressive stress trajectory

Fig. 3.4b-Prin ipal stre s trajectorie before cracking for pullout test in a homogeneous material and measured fracture swfaces in physical tests (Stone and Carino 1 984).

angles of 54 and 70 degrees. Becau e of symmetry, only onehalf of the pecimen i shown. The e trajectories would be expected to change after cracking de elop . Before cracking, there are ten iJe stre e that are approximately perpendicular to the eventual fai lure surface mea ured by Stone and Carino ( 1 984). Compressive tre es are directed from the insert head toward the ring. The principal stresses are nonuniform and are greatest near the top edge of the in ert head.

A serie of analytical and experimental studies some of which are critically reviewed by Yener and hen ( 1 984 , has been carried out to detennine the fai lure mechanism of the pullout test. While the conclu io11s have been different, it i generally agreed that circumferential cracking (producing the failure cone) begin in the highly tres ed region next to the in ert head at a pullout load that i a fraction of the ultimate value. With increasing load, the circumferential cracking propagates from the in ert head toward the reaction ring. There

Fig. 3.4c-Circumferential cracks predicted by nonlinear fracture mechanics analysis of pullout test by Hellier et a!. {1987).

is no agreement on the nature of the :final failure mechani m governing the magnitude of the ultimate pullout load.

Ottosen ( 1 9 1 ) concluded that fai lure i due to "crushing" of concrete in a narrow band between the insert head and the reaction ring. Thu , the pullout load is related directly to the compre ive trength of concrete. In another analyt

ical tudy Ycner ( 1 994) concluded that failure occurred by outward cmshing of concrete around the perimeter of the failure cone near the reaction ring. Using linear-elastic fracture mechanic and a two-dimensional mode� Ballarini et al . ( 1 9 6) concluded that ultimate pullout load is governed by the fracture toughness of the matrix. In an experimental study,

tone and Carino ( 1 983) concluded that before ultimate load, circumferential cracking extends from the insert head to the reaction ring, and that additional load is re isted by aggregate interlock aero the circumferential crack. In thi ca e, fai lure occurs when ufficient aggregate particle have been pulled out of the mortar matrix. According to the aggregate interlock the01y, maximum pullout force i not directly related to the compre i e trength. There i good correlation, however, between ultimate pul lout load and compre i e trength of concrete becau e both value are influenced by the mortar trength (Stone and Carino 1 984). In other studies, using

nonlinear fracture mechanics and discrete cracking models, Hellier at al. ( 1 987) and Ferretti (2004) showed excellent agreement between the predicted and ob erved internal cracking in the pullout te t. Figure 3.4c shows the displaced

hape of the finite-element model used by Hell ier et al. ( 1 987). The analy e by Hell ier et al. and Ferretti indicate that a plimaty circumferential crack de eloped at the corner of the in ert head and propagated outward at a shallow angle. Thi crack cea ed to grow when it penetrated a ten i le-free region. A secondary crack developed ub equently and propagated as shown in the figure. The econdary crack appears


10 REPORT ON METHODS FOR ESTIMATING IN-PLACE CONCRETE STRENGTH (ACI 228.1 R-19)

Flat Surface

Core h = d2 hole

d,

"'

(a) Drill hole, plane surface, and undercut slot

(c) Expand tnsert

Expandable insert

Expansion tool

(b) Insert expansion tool and expandable msert

(d) Install bearing ring and loading system, pullout insert assembly

Fig. 3.4d-Techniquefor post-in tailed pul/out te I (adapted from ASTM C900).

to coincide \ ith the final fra ture urface ob er ed when the

conical fragment i extracted from the concrete ma during

pul lout te ting. The e tudie al o concluded that the ult i

mate pul lout load is not governed by unia ial compre i e

fai lure in the concrete.

Pul lout strength i primarily governed by the concrete

located next to the conic fru tum defined by the in crt head

and reaction ring. ommercial in crt ha e embedment

depth of appro imately 25 to 0 mm ( 1 to 1 .2 in . ). Thu ,

only a mal l olumc of concrete i tc ted and, becau c of

the inherent heterogeneity of concrete, the average ingle

opcrator coeffi ient of variation of the e pul lout te t h

been found to be between approximately 4 and 1 5 percent.

In ne> con !ruction, the mo t de irable approa h for

pullout testing is to attach the in crt to formwork before

concrete placement. It i a! o po ible, howe er, to place

in crt into unformed urface , uch a top of slab , by

placing the in crt into fresh concrete that i ufficiently work

able. The hardware include a metal plate attached to the in crt

to pro ide a mooth bearing urface, and a pia tic cup to al lo�

embedment of the plate lightly bclo> the urfacc. The pia tic

cup al o en urc that the in crt ' i l l float in the fre h concrete

and not ettle before the concrete set . I f insett are placed

manually, care i required to maintain repre entative con rete

propertie at placement location and to reduce the amount of

air that i entrapped on the under ide of the plate . In an early

study, Vogt et at . ( 1 9 4) reported higher-than-exp cted ingle

operator variabi l i ty \ hen using manually placed in crt .

Sub equent work by Dil ly and Vogt ( 1 98 ) re ulted in an

abi l ity imi lar to that expected with insert fastened to form

work. The recommended approach is to pu h the insert into

fre h concrete and then float i t horizontal ly o er a distance of

50 to I 00 mm (2 to 4 in.) to al low aggregate to A ow into the

pullout fai lure zone. fter in ertion, the in crt hould be ti l ted

about 20 to 0 degree from the vertical to allow entrapped air

to c cape from beneath the tcel plate. Care hould be taken to

en urc that the plate i completely belo the con rete urface.

To prevent mo ement of the in crt before the on rete et ,

fre h concrete can be placed in the cup.

In e i l ing con truction, it i po ible to perform pul lout

te t u ing post-in tailed insert . The procedure for

performing po t-in tai led pul lout te ts, which \ a added to

A TM C900 in 1 999, i summarized in Fig. 3 .4d. The proce

dure involve five ba ic tep :

( I ) Dri l l ing a hole perpendicular to the urface of the

concrete u ing a core dri l l

( 2 ) Grinding the t e t area t o make i t flat

(3) Undercutting a lot to engage an e pandablc in crt

(4) Expanding an in crt into the mi l led lot

(5 ) Pull ing the in crt out of the on crete

The te t geometry i the ame as for the ca t-in-place

in crt. In a commercial te t sy tern known a the APO


REPORT ON METHODS FOR ESTIMATING IN-PLACE CONCRETE STRENGTH (ACI 228. 1 R-1 9) 11

(Cut and Pull Out) test the insert is a coiled, split ring that i expanded with specially designed hardware. The CAPO system perfonns similarly to the cast-in-place system of the arne geometry ( Petersen 1 9 4, 1 997). are i required during preparation to ensure that the hole i drilled perpendjcular to the test surface. The urface has to be flat o that the bearing ring of the loading sy tem is supp01ted unjfonnly when the in ert i extracted. onunifonn bearing of the reaction ring can re ult in an incomplete circle for the top urface of the extracted frustum. lf thi occur , the te t re ult

mu t be rejected (A TM C900). Water u ed for dri l l ing and undercutting should be removed from the hole a soon as the undercutting is completed, and the hole should be protected from ingres of water unti l the te t is completed. This is to prevent penetration of water into the fracture zone, which might affect the measured pullout load.

Other types of pullout te t configmation are a ailable for existing construction (Mai lhot ct al. 1 979· habowski and Bryden-Smith 1 980; Dom n and Castro 1 9 7) . The e typically in olve drilling a hole and in erting an expanding anchorage device that will engage in the concrete and cau e fracture in the concrete when the device is extracted. The e methods, however do not have the same fai lure mechanisms as the standard pullout test. These techniques have not been tandardized as A TM te ts methods; howe er, the internal

fracture test by habow ki and Bryden- mith ( 1 980) has been incorporated into a British standard (B 1 8 1 -Part 207).

In ummary. the pullout test can be u ed to estimate the strength of concrete by measuring the force required to extract an insert embedded in fresh concrete or installed in hardened concrete. The test re ult in a complex, tbreedimen ional state of stres in the concrete. Although there i no consensu on the exact failure mechanism of the pullout test there is a strong relationship between the compressi e strength and pullout . trength of concrete.

3.5-Pul l-off test (ASTM C1 583/C1 583M) The pull-off test as de cribed in A TM 1 5 3/C I 83M,

i u ed to detennine the near- urface ten ile trength of the substrate as an indicator of the adequacy of surface preparation before application of a repair or o erlay material; the bond trength of a repair or an overlay material to the ub trate; and the tensil strength of a repair or overlay

material, or an adhesi e used in repairs, after the material has been applied to a surface (Long and Murray 1 984). The pull-off method is i l lustrated in Fig. 3 .5 . In the A TM test procedure, the surface to be tested is cored to a depth of at lea t I 0 mm (0.5 in.), as shown in Fig. 3 .5(a). If testing the bond trength of an overlay or a concrete repair the core depth is extended to at lea t I 0 mm (0.5 in.) below the concrete overlay inte1face, as hown in Fig. 3 .5(b). metal dj c i then bonded to the te t urface u ing a uitable adhe-

ive. A temporary packing material can be placed in the kerf to prevent the adhe ive from fil ling in the annular cut. The ruse is then pulled from the stuface at a con tant rate of 35 ± 1 5 kP (5 :r: 2 psils) using a device that ensure the force i parallel to and coincident with the axis of the core. The system to provide reaction to the applied load can be a ring

Swivel JOtnt

Partial depth core at least 10 mm (0.5 1n )

into c:oocrete

P rtllll depth core through overlay or repalf matenal and least 10 mm (0 5 '" )

into base concrete

Base concrete

(a) Test of base concrete

Base concrete

Tensile load1ng deYIOe

Adhesrve

(b) Test of material applied to base concrete

(i) F aiture In (iQ Bond failure at base concrete 011ertay/ccncrete

lntelfaal

(c) Potential failure locations

(IV) Disc bond r llure

Fig. 3.5-Schematic of pull-off te t and potential failure location (adapted from ASTM 1 583/C /583M).

or tripod. ourard et al. (20 1 4) invc tigated the influence of pull-off force mi alignment and concluded that a 4-degrec mi alignment could redu e th mea ured pull-off trength by appro imately 1 5 p rcent.

An alternative procedure (not tandardized a of 20 1 6) i to first prepare a flat surface u ing a planing tool that \ orks without water, remove the du t, bond the disc, and then cut


1 2 REPORT ON METHODS F O R ESTIMATING IN-PLACE CONCRETE STRENGTH (AC1 228.1 R-1 9)

the partial-depth core. This avoids waiting for the surface to dry after cutting the core and before bonding the disc. This procedure, however, require a core barrel that is properly ized for the di c being used o that the partial-depth core is

centered with the disc. The failure location in the pull-off test will be in the mate

rial with the lowe t strength. As hown in Fig. 3.5(c), the fai lure location can be at the concrete urface (dis bond fai lure), at the interface between materials (bond fai lure), in the overlay material (overlay fai lure), or in the ba e material ( ubstrate failure). Because the failure location carmot be predicted before the test is complete, both the fai lure load and location are recorded after completion of the test. The a erage strength in the pull-off te t is based on test re ults with the same failure location.

The pull-off test was initially developed at Queen's Uni ersity Belfa t in the 1 970s for strength testing of in-place concrete. Initial research resulted in the development of L IMPET te t equjpment (Bungey and Madandoust 1 992), although everal type of commercial equipment are a ai lable. Early research by Bungey and Madandoust ( 1 992) examined the effect of the disc material disc size, effect of coring depth, reaction system, and loading rate. These factors were examined in a series of experimental studies and finite-element simulation . Thi research howed that te t re ult are ensitive to different disc material (steel or aluminum), di c diameter and thickne s, and core depth. Experimental result showed that pull-off strength \ ere not influenced by whether the loading system used a ring or tripod arrangement to provide reaction to the applied load. The re ult indicated that a 50 mm (2 in.) d iameter di c with a di c thickne of30 mm ( 1 .2 in.) will produce a reasonably uniform concrete stress distribution regardle of the ela tic properties of the concrete or disc material. TM 1 583/

1 5 3M requires a steel disc diameter of 50 mm (2 in.) with a thickne s of at least 25 mm ( I in.).

The pull-otr te t has been u ed widely in evaluating the bond strength between material in repair applications { I RI 2 1 0.3 R). In this application, the test can be used to a es the surface preparation quality a well as bond trength. between a repair material and ub trate. The I Rl reference provides a de cription of the variou failure mechani m that can occur when used in thi application. The pull-off test wa u ed extensively to asses the effect of different concrete removal or surface preparation methods on the bond of repair materials to the concrete substrate { I I indo 1 990). The research by H indo identified that "bru ising " or micro-fracturing, of the sub trate surface layer during concrete removal has a significant effect on the bond strength of repair material , with lower bond strength observed with more aggre i e removal methods. The more aggre sive remo al methods re ult in damage to the concrete ub trate, often re ulting in bond fai lure immediately below the interface between the repair material and ubstrate.

Results from three different pull-off test device were compared by ay hurd and McDonald ( 1 999). Two commercially available pull-off devices and a modified device were used in the research. The modified device was

a commercially available instrwnent designed to asses the pull-out strength of adhesive anchors and had been modified for pull-off testing. Pull-off tests were performed on a concrete pecimen that included a imulated concrete repair. The re earch program included an examination of the effects of coring depth, with l inear elastic, finite-element analy es performed to examine the stre s concentration that occur with different coring depth . Test results from the two comm rcially available pull-off te t device were found to be imilar, with lower pull-off trength obtained from the modified device. The overall re ults of the pull-off tests had high variabil ity, with a preferential fai lure location observed in the concrete substrate, rather than in the repair material or at the bond line between materials. The result obtained in this study were used for the preci ion tatement in TM

1 583/ 1 583M (4.2 .4) . In summary the pull-off test can be u ed to a es the

bond between new materials and a concrete substrate, and for qual ity of urface preparation before overlay placement. Ba ed on the nature of the te t, both the failure location and fai lure load are u d to asse s the test resul t . Te t re ult can be affected significantly by surface preparation methods and test sp cimen geometry.

3.6-Uitrasonic pulse velocity (ASTM C597) The ultra onic pulse velocity te t, as prescribed in TM

597, determine the propagation velocity of a pulse of compres ional {P-wave) energy through a concrete member (Jonc 1 949; Lc lie and Chcc man 1 949). The operational principle of modern te ting equipment is i l lustrated in Fig. 3.6a. A pul er ends a hort-duration high-voltage ignal to a tran ducer, causing the tran ducer to ibrate at it re onant frequency and generate a stres wave pul e in the concrete (refer to I 228.2R for a description of stress waves in ol ids). At the start of the electrical pulse, an electronic t imer

i witched on. The transducer vibrations are tran ferred to the concrete through a viscou coupling fluid. The pul e travels through the member and is detected by a receiving tran ducer coupled to the opposite concrete surface. hen the pulse is received, the electronic t imer is turned off and the elap d travel t ime is dj played. The direct path length between the tran ducer i divided by the travel tim to obtain the pul e velocity through the concrete.

From the principles of elastic wave propagation, the P-wave elocity is proportional to the square root of the elastic modulu (A I 228.2R). Because the elastic modulus and strength of a given concrete increase with maturity, it follows that pulse veloci ty can provide a means of e timating trength of concrete, even though there i no direct phy -

ical relation hip between these two properties. concrete mature , however, the elastic modulu and compre si e trength increa e at different rate . t early maturities, the

ela tic rnodulu increa e at a higher rate than trength, and at later maturities, the elastic modulu increa es at a lower rate. A a result, over a wide range of maturity, the relation-hip between compre sive trength and pul e velocity is

highly nonlinear. Figure 3 .6b shows a typical relationship between compressive trength and pulse velocity. ote that



· - - - - - - - - - - - - - - - - - - - - - - - - - - - - -1 I I I

I Pulser

Fig. 3.6a- chematic of apparatu. to mea ure ultrasonic pulse l'elocity.

this is only an il lustrative example and the actual relation-hip depend on the pecific concrete mixture. At early

maturities, a gi en increase in compre sive strength is accompanied by a relatively large increase in put e velocity, while at later maturitie , the velocity increase i maller for the arne trength increa e. For example, a trength increa e from 3 to MPa (400 to 1200 p i roughly) can be associated with a velocity increa e from approximately 2400 to

3040 tnls (7900 to I 0,000 ftls roughly). However, a trength increase from 25 to 30 MPa (3600 to 4400 psi roughly) could correspond to a velocity increa e of only 3800 to 3920 m/s ( 1 2,500 to 1 2,900 ftls roughly). Thu , the sensitivity of the put e elocity as an indicator of change in concrete trength decrease with increasing maturity and strength.

Factors other than concrete trength can affect put e elocity, and changes in put e velocity due to the e factor

could over hadow changes due to trength ( turmp et al . 1 984). For example, although the pulse velocity depends strongly on the type and amount of aggregate in the concrete, the strength of normal-strength concrete ( les than approximately 40 MPa [6000 psi ] ) is less sensiti e to the e factors. As the volumetric aggregate content of concrete increa es the pulse velocity may increase or decrease, depending on the aggregate type, but the compre ive strength may not be affected appreciably (Boga et al . 20 13; Jones 1 962).

Another factor i moi ture content. A the moi ture content of concrete increa e from the air-dry to saturated condition,

put e elocity could incr a up to 5 percent (Bungey et al . 2006). I fthe effects of moisture are not considered, erroneous conclu ions can be drawn about in-place strength, especially in mature concrete. The curing proce s also affects the relationship between pulse velocity and strength, especially when accelerated methods are used (Teodom 1986 ).

The amount and orientation of steel reinforcement will at o influence pulse velocities. Because the pulse velocity through steel is approximately 40 percent greater than

Pulse Velocity, ft/s 6560 8200 9840 1 1480 13120

� t-��&00 ca 30 a.. � 5 25

0> c � 20

1/)

-� 15 .., .., I!? a. 10 E 0

u 5

25 3 3 5 4 Pulse Velocity, km/s

'iii 4000 a. £ Cl c

3000 � 1/) Cll >

2000 ·� I!? a. E 0

1000 u

Fig. 3. 6b-Schematic of typical relationship between pul e velocity and com pre sive trength of a given concrete mixture.

through oncrete, th pulse velocity through a heavily rein-

forced concrete member could be greater than through one with little reinforcement. Thi i e pccially trouble orne when reinforcing bar are oriented parallel to the pul epropagation direction. The pul e may be refracted into the bar and transmitted to the recei er at the pul e elocity in

teel. The resulting apparent elocity through the member wil l be greater than the actual clocity through the con rete. Failure to account for the pre en c and orientation of reinforcement may lead to incorre t conclu ion about concrete trength. Although con·ection fa tor have been propo ed, uch a tho e di cu ed in Malhotra ( 1976) and Bungey et al .

(2006), their accura y ha not been establi hed conclu ively. The mea ured pulse velocity can al o be affected by the

pre ence of crack or oids along the propagation path from !Tan miller to recei er. The put e could be diffracted around the di continuitie , thereby increa ing the travel path and ITa el time. Without additional knowledge about the interi r condition of the concrete member, the apparent dccrca c in put e velocity could be incorrectly interpreted a low ompre i e strength.

In thi te t method, all the con rete between the tran mitling and receiving transducer affect the tra el time. Te t re ult are, therefore, relatively in ensitive to the normal heterogeneity of concrete. on equently, the te t method has been found to ha e a low ingle-operator coefficient of

ariation. Thi doe not mean, however, that trength e timate are neces arily highly rel iable.

TI1e u e of the velocity of other ultra onic wa e mode ( for example, urfacc wave [Gudra and tawi ki 20 0; Gallo and PopO\.ic 2005]) and other wave chara tcri tic ( for c ample, P-wa e wa e energy attenuation [damping] [Teodoru 1988; I mail et al. 1996· Te famariam et al. 2006]) to e timate in-pia e concrete trength have at o been reported. The application of the e methods to concrete tmcture ha not, however, been evaluated e tensively; furthermore, standard testing procedure for these method have not yet been de eloped.

Tn summary, pulse velocity can be u ed to e timate trength in new and exi ting con !ruction, pro ided a trength-rela-

American Concrete Institute - Copyrighted © Material - www.concrete.org ((]'CiJ

1 4 REPORT ON METHODS FOR ESTIMATING IN-PLACE CONCRETE STRENGTH (ACI 228.1 R-19)

tionship for the concrete mixture has been developed. or a given concrete, a change in P-wave velocity is fundamentally related to a change in ela tic modulus. Because elastic modulus and trength are not linearly related pulse velocity i inherently a less-sensitive indicator of concrete strength as strength increases. The amount and type of aggregate has a strong influence on the pul e elocity versus strength relation-hip, and the in-place pulse velocity is affected by moisture

content and the pre ence of t el reinforcement. Refer to I 228.2R for additional di cu sion of the pulse velocity method.

3.7-Maturity method (ASTM C1074) Freshly placed concrete gains strength because of the

exothennic chemical reactions between the water and cementit ious material in the mixture. Provided sufficient moisture is present, the reaction rate are influenced by the concrete temperature; an increa e in temperature causes an increa e in the reaction rates. The extent of the reactions and, therefore, strength at any age depend on the thermal hi tory of the concrete.

The maturity method ( TM I 074) i a technique to estimate in-place strength by considering the relationship of temperature and time on strength development. The thermal history of the concrete and a maturity function are used to calculate a maturity index that quantifies the combined effects of t ime and temperature. The strength of a specific concrete mixture i expressed a a function of its maturity index by means of a strength-maturity relationship. If samples of the same concrete are subjected to different temperature conditions, the tr ngth-maturity relation hip for that concrete and the temperature hi tories of the ample can be used to e timate their trengths.

The maturity function is a mathematical expression that converts the temperature history of the concrete to a maturity index. everal such functions have been proposed and are reviewed in Malhotra ( 1 97 1 ), RIL M omm1s 1on 42-CE ( 1 98 1 ) and Carino (2004). As explained by Carino (2004) the maturity function i related to the rate of strength development after fu1al setting has occurred. Therefore, a key feature of a maturity function is the expression used to represent th influence of temperature on the initial rate of trength development. Two expres ion are commonly u ed.

In one approach, it is as umed that the initial rate of strength development is a l inear function of temperature, which leads to the imple maturity function, commonly known as the

urse- au I function ur e 1 949; au I 1 95 1 ) shown in Fig. 3 .7a. In this ca e the maturity index at any age is the area between a datum temperature T0 and the temperature curve of the concrete. The tenn temperature-time factor i u ed for this area and is calculated as follows

M(t) = L.( Tu - To)!J.t (3.7a)

where M(t) is temperature-time factor at age t, deg-days or deg-homs; t::..t i a time interval, days or hours; To is average concrete temperature during time interval tJ.t, 0 (°F); and T0 is datum temperature, °C (°F).

Temperature T

To

Time t

Fig. 3. 7a-Maturity function based on assumption that the initial rate of strength gain varies linearly with temperature; shaded area is the temperature-time factor (Eq. (3. 7a)).

Traditionally, the datum temperature u ed in Eq. (3.7a) has been taken a the temperature below which tTength gain cea es which ha been a umed to be approximately -1 0°C ( I 4°F). Tt ha been ugge ted, however, that a ingle value for the datum temperature i not the mo t accurate approach and that for better a curacy, the datum temperature hould be evaluated for the pecific cementitiou material and admixture in the concrete mixtw-e (Carino 1 984). ASTM C l 074 recommends a datum temperature of 0° (32°F) for concrete made with A TM Type I ement when the concrete temperature i e peeled to be between 0 and 40°C (32 and 1 04°F). A TM

I 074 provides a procedure to determine experimentally the datum temperature for other type of cementitious material and for different range of curing temperature.

Tn the econd approach it i a umed that the initial rate of trength gain aries exponentially with concrete temperature in accordance � ith the AJThcniu function ( Frcic !ebenHan en and Pedersen 1 977). Thi second maturity function i u ed to compute an equivalent age of the concrete at a specified curing temperature a follows

(3.7b)

where 1, i equivalent age at a pecified temperature T,., day or hours; Q i apparent acti ation energy di ided by the ga con taot, K (kelvin)· To i average ab olute temperature of concrete during time interval t, K· T; i specified curing temperature, K; and t is time inter al, days or hours.

In Eq. (3.7b), the exponential function can be considered as an age conversion factor that convert a time interval t at the actual concrete temperature to an equivalent interval (in tenn of trength gain) at a pecified curing temperature. In

orth America, the peci:fied curing temperature i typically taken ro be 23°C (296 K), whcrca in Europe, 20°C (293 K) i typically u ed. To calculat the equivalent age of a concrete mixture, the alue of a parameter (known a the apparent acti ation energy) i required; thi depends primarily on the types of cementitiou material and, to a le er extent on the water-cementitiou materials ratio (w/cm) ( arino and Tank 1 992). The Q-value in Eq. (3.7b) is the activation energy divided by the ga constant (8.3 1 joules/(mole· K)). A TM



1 074 recommends a Q- alue of 5000 K for concrete made with ASTM Type I cement without any admixtures or additions and provides procedures for determining the Q-value for other cementitious y terns. A Q-value of 5000 K corresponds to an apparent activation energy of about 42 kJ/mol. Figure 3 .7b show how the age conversion factor varies with concrete temperature for different Q-values and a specified curing temperature of23°C (73°F). As the Q-value increa e , the relation hip betw en age conver ion factor and temperature become more nonlinear. Researcher have obtained apparent activation values for various cementitious systems (Carino 2004· Brooks et al. 2007). Kjellsen and Detwi ler ( 1 993) suggested a modified version of the equivalent age function to account for the observation that the apparent activation energy decrease a hydration progres es, but indicated that their model required further validation.

To use the maturity method requires establ ishing the strength-maturity relation hip in the laboratmy for the concrete that will be used in the tructure. A de cribed in A TM C I 074, this i accomplished by preparing concrete specimens (cylinders, beam , or cube ) to develop the trength gain relationship in the laboratory, placing tempera

ture sensors in two specimens, and measuring the strength (compressive or flexural) and maturity index at regular strength intervals. The temperature history of the in-place concrete is monitored continuously and the in-place maturity index (temperature-rime factor or equivalent age) is computed from the recorded temperature history. The in-place strength can be estimated from the maturity index and strength-maturity relation hip. There are in trument that automatically compute the maturity index; however, becau e the value of T0 or Q used by some in tmments may not be appropriate for the concrete in the structure, these should be used with care. A TM 1 074 gives the procedure for using the maturity method and provides examples to i l lustrate calculation of the temperature-time factor or equivalent age from the recorded temperature history of the concrete. A l 306R illu trates the use of the maturity method to estimate in-place strength during cold-weather concreting operations. CJ 325 .9R discusse the use of the maturity method for e timating when the in-place flexural strength i ufficient to open a pavement to traffic.

The maturity method is intended for e tirnating trength development of newly placed concrete. trength estimate are ba ed on four important assumptions:

( l ) The maturity function consta11ts (datum temperature or Q-value) accurately reflect the temperature dependence of strength development.

(2) There is ufficient water for continued reaction of the cementitiou materials.

(3) The concrete in the stlucture is the same a that used to de elop the trength-matUJity relationship.

(4) The trength potential of the concrete in the structure is the same as that used to develop the trength-maturity relationship.

The accuracy associated with the first asswnprion can be increased by detennining the maturity constants experimentally in accordance with the procedures in A TM C I 074

7 � 0 6 -0 ('0 L1. 5 c: .Q 4 (/) � Q) 3 > c: 0 2 () Q) C> 1 <{

0 0 10 20 30

Temperature, oc 40 50

Fig. 3. 7h-Age conversion factor for different Q-va/ues and specified curing temperature of 23°C based on Eq. (3. 7b).

for the particular concrete. Proper curing procedures ( CI 308R) will ensure that water is available to sati fy the econd condition. Careful control of eoncret proportions in

accordance with A TM C94/C94M, and proper placement and consolidation are needed to satisfy the third assumption; however, explicit verification of constituent proportions is not always practical . The trength potential of some concrete mixtures can be significantly affected by high early-age temperature ( arino 1 984 2004; Brooks et a!. 2007). Thus, there could be some deviation in trength potential if the field concrete is placed and cured initially at temperatures substantially different than used in developing the strengthmatulity relation hip. The third and fourth a umptions require additional confirmation that the concrete in the truelure ha the correct strength potential. This can b achieved by pcrfonning early-age trcngth te ts on concrete sampled from the structure in accordance with STM C9 1 8/C9 1 M or by pcrfonning other in-place te t that gi c indications of the actual strength level. Such verification i essential when

c timatc of in-place trcngth arc u ed for timing critical operation uch a form work removal or application of po ttcn ioning. It may also be prudent in omc case to de clop trcngth-maturity cur c for different initial curing tempera

ture (Brooks ct al . 2007) or verify lhe curve based on field infonnation aflcr placcm nt begins {Tex-426-A 20 I 0).

Although the maturity method is most often used to estimate eompres ive strength development, it can be used to estimate the development of other properric that arc related to the extent of reaction of ccmcntitious materials with water. The method ha been u ed to estimate the development of concrete ten i le trcngth or flexural trength (ACI "25 .9R- l - ; Bergstrom 1 953· Whiting ct al . 1 994· Hoerner and Darter 1 999; Delatte ct al. 2000) early-age modulu of elasticity (Pinto and Hover L 999a), and overlay bond

trcngth ( Delatte ct al. 2000). Pinto and I lo cr ( ! 999b) and Wade et al. (20 l 0) demon tratcd the applicability of the maturity method for the estimation of times of ctting under different curing temperature . Wade et al. (20 I 0) however indicated that the apparent activation energy for setting rime may be lower than for trcngth development.



Fig. 3.8----special mold and support hardware to obtain castin-place concrete specimen.

ln summary, the maturity method is used to e timate strength de elopment during con truction. Becau e thi method relie on the mea urement of in-place temperature with re pect to time, and the a umption that the trengthmaturity relation hip is alid, other information i required to en urc the in-place concrete ba the as umcd trcngth potential. The correct datum temperature or Q-valuc i required to impro e the accuracy of the trength e timatiou at early age .

3.8-Cast-in-place cyl inders (ASTM C873/C873M) Thi i a technique for obtaining cylindrical concrete

pecimens from oewly ca t slab without drilliog core . The method i described in A T 7 /C873M and involve u ing a mold, a il lu trated in Fig. 3.8. The outer lee e i nailed to the formwork and i u ed to upport a cylindrical mold. The slee e can be adju ted for different lab thickne se with the top of the lccvc flu h to the floor elevation. The mold i filled a the lab i ca t, and the concrete ubjectcd to imilar con olidation and curing operation a the lab concrete. The concrete in the mold i al lowed to cure ith the lab.

The objective of the technique i to obtain a te t pecimen that ha been ubjected to the arne placement and thermal hi tory a the concrete in the tructure. To determine the in-place trength, the mold i removed from the leeve at the time of te ring and tripped from the concrete cyl inder. The cylinder i te ted in compre ion in accordance with ASTM

73/ 7 M. For ca e in which the length-diameter ratio of the cylinder i I . 75 or le , the mea urcd comprc i c

trcngth are corrected by the factor in TM C42/ 42M. Strength r ults of ca t-in-place cylinders ha e b en fi und to

be, on average, 5 to 20 percent higher than tho e of companion core removed from the general icinity of the cylinders (Popovtc et al. 20 1 6· Bloem 1 96 ). The actual trength difference \ ill depend on the concrete age material damage caused by the coring proce , pre ence of embedded reinforcing bars within the cores, and the manner in which the core are treated during the time between e traction and te ting (A I 2 1 4.4R).

In ummary, because the ca t-in-place cylinder technique involve a comprc ive trcngth te t of a cylindrical pecimen, a trcngtb relation hip i not required. To obtain an accurate e timate of the in-place trength, are i required to

Table 3.9-Useful compressive strength ranges for In-place test methods -

I Range of compressive strength*

Test method MPa psi

Rebound number 1 0 to 40 1 500 to 6000 [Probe penetration

-10 to 1 20 I 00 to 1 7,000' - -

I Pin penetration 3 to 30 500 to 4000 f- -Pullout 2 to 1 301 300 to 1 9,0001

1- -ltro oni pul c

I to 70 1 00 to 1 0,000 velocity

Pull-ofT o limit (docs not mcm.urc compressive

strength)

Maturity o limit

Cast-in-place cylinder No limit

. . Htghcr strengths may be tested 1f satisfactory datu arc presented for the test method

and equipment to be used.

'For •Lrcngth abo\c 40 MPa (6000 p i). pcctul probes arc rcqutrcd.

'for >trengths above 55 MPa ( 000 psi). spe ial lugh- trength bolt£ are required to extra 1 pullout inscn

ensure that the concrete in the mold i properly consol idated in accordance with A TM 873/ 873M.

3.9-Strength l imitations Most test procedure have orne l imitation regarding the

applicable trength range. In some ca e , the te t apparatus has not been designed for testing low-strength or highstrength concrete, and in other cases there is l imited experience in u ing the methods to test high-strength concrete. The u eful strength range for the arious method are ummarized in Table 3 .9. The e range are approximate and can be extended if the user can show a reliable trength relation hip at higher trength .

3.10-Combined methods The term "combined method" refers to the u e of two or

more in-place te t method to e timate concrete trength. By combining results from more than one in-place test, a multi

ariable correlation can be established to estimate strength. ombined methods are reported to increa e the reliability

of the e timated strength. The underlying concept is that if the two method are influenced in different way by the same factor, their combined use results in a canceling effect that improves the accuracy of the estimated trength; for example, an increa e in moisture content increa es pul e

elocity but decrea es the rebound number. ombined methods were developed and have been used

in Ea tern Europe to evaluate concrete strength in existing construction or in precast elements (Facaoaru 1 970, 1 984; Teodoru 1 9 6, 1 9 8). ombination of test methods, such as pulse elocity and rebound nwnber (or pulse velocity, rebound number, and pulse attenuation) have resulted in strength relation hips with higher correlation coefficients than when these methods are u ed individually. The improvements, however, have u ually only been marginal (Tanigawa ct al. 1 984; amarin and Dhir 1 9 4· a marin and Meynink 1 9 I ; Teodoru 1 988).



Breysse (20 1 2 ) conducted a comprehensive critical review of available data and regression models for the combined method of rebound hammer and ultrasonic pulse

elocity. Brey e noted that the quality of trength e timation from any in-place test depend on three element : I ) the sensitivity of concrete strength to the property measured by the in-place test; 2) the range of the in-place te t values and corre ponding concrete strength alue used to establi h the trength relation hip; and 3) the mea urement etTor a ociated with the in-place te t which may overwhelm errors associated with the regression model. From the literature review, Breysse concluded that attempts to identify a univer al trength relation hip are simply wasted energy. For a given et of data everal regression models will lead to similar strength e timate , but the key is to adapt the coefficients of the model to provide the be t estimates of actual in-place trength as oppo ed to trength estimates of laboratory specimens. Brcysse explained why combined methods do not alway re ult in an improvement in the quality of estimated trength : to be beneficial, each of the method u ed in combination have to provide the same quality of infonnation about concrete strength.

Another approach is to use the maturity method in combination with another in-place test that measures an actual trength property of the concrete, uch a a pullout test. The maturity method i u ed to determine when the in-place concrete should have reached the required trength, then the other te t method i cariied out to verify that the strength has been achieved. This approach is especially beneficial when in-place te ts involve embedded hardware. The use of the maturity method to determin � hen the other te t hould be performed may avoid premature te ting. In addition, maturity readings can be u ed to as es the significance of lower or higher than expected in-place test results ( outsos et al. 2000).

ote that combining methods i not an end in itself; a combined method should be used in those ca es where it i the mo t economical way to obtain a reliable e timate of concrete strength (Le hchin ky 1 99 1 ) . In North America, the use of combined methods ha hown minimal intere t among researchers and practitioners. There have been no efforts to develop ASTM tandard for their use.

3.11 -Summary Methods that can be used to estimate the in-place trength

of concrete or to measure bond strength of repair materials and overlay have been reviewed. While other procedures have been proposed (Malhotra 1 976; Bungey et al. 2006; Malhotra and Carino 2004), the discus ion has been l imited to tho e techniques that ha e been tandardized a A TM test method .

Table 3 . 1 1 summarizes the relative performance of the in-place te t di cu ed in thi report in term of accuracy of e timated trength and ea e of u e. The table al o indicates which methods are applicable to new con truction and which are applicable to existing constmction. General ly tho e methods requiring embedment of hardware are l imited to use in new construction. In general those technique that involve preplanning of test locations and embed-

Table 3.11 -Relative performance of in-place tests

A TM Test method

Rebound number

1--Penetration resistance

Pullout 900

Pulse velocity 597

H

H

+

Ht

H

I Ease of

use'

H

H +

test method with a-++ results in a more accurate strength estimate or is easier to usc than

a method with a +. I indicates that the method 1 not appltcable to e'istmg constnuctton. 'Rcqutrc. vcrincation by other tests.

ment of hardware require more effort to u c. Tho e method , hO\ e er al o tend to gi e more reliable trength e tirnate . The u er hould consider the relative importance of accuracy and ea e of u e hen electing the mo 1 appropriate in-place te ting y tem for a particular application.

In-place test provide alternative to core test for e timating the trength of concrete in a structure or can upplement the data obtained from a limited number of core . The e methods are ba ed on measuring a concrete property that has

orne relation hip to trength. The accuracy of the e method i , in part, determined by the degree of COITelation between trength and the pby ical quantity mea ured by the in-place

te t. For proper evaluation of te t re ult , the u er bould be aware of tho e factor other than oncrete trengtb that

n affect the te t re ult . dditional fundamental re earch i needed to improve the under tanding of bow the e method are related to concrete strength and bow the test results are affected by factors other than trength.

An e entiat tep for using the e method to e timate the in-place trength i the development of a relationship between trength and the quantity mea ured by the in-place te t. The data acquired for de eloping the trength relation-

hip pro ide aluable infom1ation on the reliability of the e timate . Sub cquent chapter of thi report di cu the tali tical cbaracteri tic of the te ts, method for de eloping trength relation hip , planning of in-place te t , and inter

pretation of the re ult . The final chapter deal with the u e of in-place te ts for acceptance of concrete.

CHAPTER 4-STATISTICAL CHARACTERISTICS OF TEST RESULTS

4.1 -Need for statistical analysis In de igning a tructure to afely re i t the e pected

load , the engineer usc the pecified compre i e trengtb .fc' of the concrete. The trength of the concrete in a tructure i ariable and, a indicated in ACI 2 1 4R, the pecified compre ive trength i e peeted to be e ceeded with


1 8 REPORT ON METHODS FOR ESTIMATING IN-PLACE CONCRETE STRENGTH (AC1 228.1 R-19)

about 90 percent probability· that is, no more than about I 0 percent of strength test re ults are expected to be le than the pecified strength. Thus the specified strength can be considered a the tenth-percentile trength. To ensure that no more than I 0 percent of the te t results are below f/, the concrete supplied for the structure hould have an average standard-cured cyl inder strength that i greater than fc' as di cus ed in A I 2 1 4R. The difference in required concrete cylinder trength and de ign compre ive trengtb depend on the variability of the producer's previou trength-te t results for similar concrete.

In assessing the abi l ity of a partially completed structure to re i t con truction load , the committee bel ieves that the tenth-percentile in-place compre sive trength (strength exceeded with 90 percent probabil ity) should be used for comparison against the required compressive strength at the time of application of the construction load . The required strength mean the compre sive strength used in computing the nominal load re i tance of structural element at the applicable load tage.

The use of the tenth-percentile value to interpret re ult from in-place tests in partially completed structures is con idered reasonable by u ers of in-place tests. The critical nature of construction operations in partially completed structures the ensitivity of early-age strength on the previous thermal history of the concrete, and the general lack of careful con ideration of construction loading during the design of a tructure dictate the use of a con ervative procedure for e aluating in-place test results. For ituations where the con equence of a failure may not be serious, the e timated mean trength may be an a ceptable mea ure to as e the adequacy of the in-place strength for proceeding with con !ruction operations. Examples of such situations would include slabs-on-ground, pavements, and some repairs. Inadequate trength at the time of a proposed construction operation can u ually be remedied by simply providing for additional curing before proceeding with the operation.

In-place test may al o be u ed to evaluate the concrete strength in a structure when que tion ari e because of low strengths of standard-cured cyl inders during construction. If the compre i e trength of standard-cured cylinder fails to meet ACI 3 1 8 acceptance criteria, te ting of core may be required. Ba ed on C I 3 1 if the average compres ive strength of three cores taken from the questionable portion of the structure exceeds 85 percent of the specified compressive strength and no single core strength is less than 75 percent of the specified trength, the concrete strength is deemed to be acceptable. There are, however no analogous acceptance criteria for the e timated in-place compre i e

trength ba ed on in-place test . Chapter 8 di cu se how in-place testing could be used for acceptance of concrete.

l rre pecti e of the objective of in-place te ting, a reliable e timate of the in-place compre ive trength require the use of tati tical methods that account for the following primary ources of unce11ainty:

(a) The a erage value of the in-place test re ult (b) The relationship between compressive strength and the

in-place test re ults

(c) The inherent variability of the in-place compressi e strength

The first source of uncertainty is associated with the inherent ariability (repeatabil ity) of the te t method which i discus ed in this chapter. haptcrs 5 and 7 addres the other main source of uncertainty.

4.2-Repeatabil ity of test results The un ertainty of the average value of the in-place te t

re ult i a function of the tandard de iation of the re ult and the number of test . The standard deviation is, in tum, a function of the repeatabil ity of the test and the ariability of the concrete in the structure.

In this report, repeatability means the standard deviation or coefficient of variation of repeated te ts by the same operator on the ame material. This is often called the ingleoperator variation and how the inherent catter a ociated with a particular test method.

Data on the repeatabi l ity of some in-place te ts are provided in the preci ion tatement of the A T tandard go erning the test . orne information on the repeatability of other tests may be found in published reports. nfortw1ately, mo t published data deal \ ith correlations with standard strength test , rather than with repeatability. s will be seen, conclu ions about repeatabil ity are often in conflict becau e of differences in experiment de igllS or in data analysis.

4.2. 1 Rebound number-The precision statement of A TM 805/ 805M tate that the ingle-operator tandard deviation of the rebound hammer test is 2.5 rebound numbers which limit the expected range of 1 0 readings to 1 2. The re ult of three studie that evaluated the performance of variou in-place te t provide additional in ight into the repeatabi l ity of the rebound number te t. Keiller ( 1 9 2) used eight different mixtures and took 1 2 replicate rebound reading at age of 7 and 2 day . Carette and Malhotra ( 1 984) used four mixture and took 20 replicate readings at ages of I , 2, and 3 days. Yun et al. ( 1 9 ) u ed five mixtures of concrete and took 1 5 replicate readings at age ranging from I to 9 1 days.

F igure 4.2. 1 a shows the tandard deviations oft he rebound number as a function of the average rebound numb r. The data from the three tudie appear to follow the arne pattern. In the study by Carette and Malhotra ( 1 984), the average maximum rebound number ranged from 1 5 to 22 and the average standard deviation was 2.4. In the study by Keil ler ( 1 9 2) the average rebound number ranged from I to 35, and the average standard deviation was 3 .4. In the work by Yun et al . ( 1 98 ) the range in average rebound number was 1 2 to 32 and the average tandard deviation wa 2.5.

Examination of Figure 4.2. 1 a how that there may be a trend of increasing standard de iation with increasing average rebmmd number, in \ hich ca e the coefficient of

ariation i a better mea ure of repeatabi lity. Figure 4.2. 1 b shows the coefficients of variation plotted a functions of average rebound number. There doe not appear to be any trend with increasing rebound number. In contra t, Le hchin ky et al . ( 1 990) found that the coefficient of variation and its variability tended to decrea e with increasing



concrete strength. The average coefficients of variation from the tudies by Carette and Malhotra ( 1 9 4) and by Keiller ( 19 2) have equal values of 1 1 .9 percent while the average

alue from the tudy by Yun et al . ( 1 9 ) wa I 0.4 percent.

6

5

c: 0 � 4 ·:;;: Q) 0 "0 3 ..... (II "0 c: (II U5 2

1 0

• Ketller (1982)

. :

• •

Carette & Malhotra (1lj84) · · · ·:· · · ··· · · · · ·-:- · · · • • • • • • • Y un et a l ( 1 988) . . · =

. . . . . . . · - · · · · · · · ·� ·· · · ·� · · · · · ·; · · ·r · � ·� ' · · · · · · · · ·

: : .. . :

:

· · · · · · · · · ·� · · · · · · · · · · ·� · · · · · · · · · ··· · · · · · · · · · ·:- · · · · · · · · · � · · · · · · · · · · . . . . . , . .. ' . : . .. : . :.

• . ,. «

.

. : .

: . . : : · · · · · ·�· · ·r· · · · �r�· · · · ·�- � - �- - -�· ·T· · · · · · · ·T · · · · · · · · · ·

1 5 20 25 30 35 40

Average Rebound Number

Fig. 4.2. /a-Sing/e-operator standard deviation as a function of average rebound numbe1:

20 ,-----o----�-=----�----�-----------, � .

.

.

: : . : :: 1 5 + · · · · · · ·+ · · · · · · · · ·+ · · ·=·· · · ·+ · · · · · · · · · ·+ · · ··· · · · · · ·1· · · · · · · · · · ·

·� ! • • . ·,· t I : • · � ·;:: . : . �

.

.

:

� 1 0 + · · · · · · · · · · � · · · · · ··� ·�· .... · · · · · ·� · · · · - - - - - - -� · · · · · · · · -•L . . . . . . . . : ... . : :. ... �- : -0 -c: Q) ·u = Q) 0 (.)

. : .. � . .. . : . . :· ;·

5

- .. .----=-'---�e-�-=-:...:.�98-M-�-Iho-tra__:(1�984--,)

• • •

r . . . . . . . T .. . . . . . . . . • Yun et al (1988) o ��+=��i��+�� I 1 0 1 5 20 25 30 35

Average Rebound Number

40

Fig. 4.2. 1 b--Single-operafor coefficient of variation as a function of average rebound numbe1:

In Figure 4.2 . 1 b the coefficients of variation are not con tant. ote, however, that the values are based on sample estimate of the true averages and tandard de iation . With finite ample ize there wil l be variation in the e e timate , and a random variation in the computed coefficient of variation i e pected. although the true coefficient of ariation can be con tant. Thus, it appears that the repeatability of the rebound number technique can be de cribed by a constant coefficient of variation, which ha an a erage value of approximately I 0 percent.

4.2.2 Penetration re istance-The single-operator preci-ion statement in A TM 03/C 03M for probe and pin

testing are urnmarized in Table 4.2.2.a and Table 4.2.2b, re pecti ely.

The data reported by arette and Malhotra ( 1 9 4) and Keiller ( 1 9 2), which include concrete strength in the range of I 0 to 50 M Pa ( 1 500 to 7000 p i), give additional in ight into the underlying mea ure of repeatability for this te t. Figure 4.2.2a how the tandard deviation of the expo ed length of the probe a a fun tion of the average expo ed length. The values from arette and Malhotra ( 1 9 4) are based on the average of six probe , while Keiller ( 1 9 2 ) r e ults are based o n three probes. xcept for one outlying point, there i a trend of decreasing single-operator variabil ity with increasing exposed length. In Fig. 4.2.2b the coefficients of variation of exposed length are shown a a function of the a erage exposed length. The decrea ing trend with increa ing concrete trength is more pronounced than in Fig. 4.2.2a. Thus, the repeatability of the expo ed length i described neither by a constant standard deviation nor a con tant coefficient of variation.

The cu tomary practice i to measure the expo ed length of the probes but concrete trength has a direct effect on the depth of penetration. A more logical approach is to express the coefficient of variation in terms of depth of penetration. Figure 4.2.2c show the coefficient of variation of the penetration depth a a function of average penetration. In thi case, there is no clear trend with increasing penetration. The higher catter of the values from the Keiller ( 19 2) te t might be due

to their smaller sample size compared with the tests of arette and Malhotra ( 1 984). ote that the standard de iation ha the arne value whether expo ed length or penetration depth i u ed. The coefficient of variation, howe er, dep nd on

Table 4.2.2a-Single-operator precision for penetration-resistance testing with probes (ASTM C803/C803M)

I Mnximum range of three I

Maximum expected difference bel\ cen two tests (each Maximum size of aggregate Standard deviation individual measurements calculated as the average of three measurements)

Mortar--4.75 mm ( o. 4 ) 2.0 m m (0.08 in.) 6.6 111m (0.26 in.) 3.3 mm (0. 1 3 in.) I l oncrete-25 mm ( I in.) 2.5 111111 (0. 1 0 in.) .4 111111 (0.33 in.) 4.1 111111 (0. 1 6 in.)

I Concrete 50 mm (2 in.) 3.6 mm (0. 1 4 in.) 1 1 .7 mm (0.46 in. ) 5.6 mm (0.22 in.)

Table 4.2.2b-Single-operator precision for penetration-resistance testing with pins (ASTM C803/C803M)

Material

Con ret 3 to 28 MPa

(450 [0 4000 p i)

tandard deviation

0.4 mm (0.0 1 6 in.)

Maximum range of six individual measurement

1 .6 nun (0.064 in. )

Ma imum expected differcn e bet\\CCn two tests (ca h calculated a the average of ix measurement�)

0.5 mm (0.01 in.)


20 REPORT ON METHODS FOR ESTIMATING IN-PLACE CONCRETE STRENGTH (ACI 228.1 R-19)

E E c 0 � ·;; � 'E 01 "0 c g (/)

Average Exposed Length, in. 0 4 0 8 1 2 1 6 2 2 4 2 8

5.0 +-.,....,.....,....f ........ ,.....,..+-.,....,.......-f-.-......-+-.,....,.......-f-.-......-+ 0 20

4 0

3.0

2.0

1 0

20 30 40 50 60 Average Exposed Length, mm

0 16

0 12

0 08

0 04

0 00

10

!: c .Q iii ;; � 'E 01 "0 c g (/)

Fig. 4.2.2a-Single-operator standard deviation a a .fimction of average e.xpo ed length of probes.

Average Exposed Length, in . 0.4 0.8 1 .2 1 .6 2 2.4 2.8

14 1-�-r�rr,-��-r,-�.....-�� : ! : !

� :: . T� J� I I r ••• � : : . : . :

� : ••.. .. T •..•..•• i � ?r.········r··· ·· ····r ••· • Q) : • : : ••:

·o . •.; . • . � 4 : -� · . . . . "t "�'""�'""""' 8 2 � • Kcdler (1982) , . . . . . . . l . . � ... . . . . . ; . . . . . . . . . . . � • CareHe & Malhotra (1984) •; • :

i i : • i o �� 10 20 30 40 50 60 70

Average Exposed Length, mm

Fig. 4.2. ingle-operator oef]i ienl of varia/ion a a function of average exposed length of probe .

Average Penetration, in. 0.4 0.8 1 .2 1 .6 2 2.4 2.8

1 5 4-rT,-�,-� ........ �� ........ � ........ �-r��

c 0 � 10 ·c ro > -0 c Q) ·o :e Q) 0 (.)

5

10

i • • . . • •

Ka.ller (11182)

. . . . . . . � . . . . . . .. . . ! ... . . . -: - - - . - · · - -·-�-- . . . - . .. . . . -� - - - - - - - - - . l � : .

. ': . . • ! • : . . . . . f . . . ... .. . - f - . . . . . . . . . .f - . � . .. .. . l . . . .. ill . . � . . . . . . . . . . : . . :

:•

.

;

: . � i : i •

20 30 40 50 60 70

Average Penetration, mm

Fig. 4.2.2c ingle-operator coefficient of varia/ion as a June/ion of average pene/ration of probes.

whether the standard deviation is divided by average exposed length or average penetration depth.

Hence, it appears that a con tant coefficient of variation of the penetration depth can be u ed to de cribe the ingleoperator variability of the probe penetration te t. The work by Carette and Malhotra ( 1 9 4) is the first known study that u es tbjs method for defining the repeatabil ity of the penetration te t. Other te t data u ing the probe penetration ystem, ho ever, can be manipulated to yield the coefficient

of ariation of penetration depth, provided tv o of the e three quantities are given: a erage exposed length, standard deviation, or coefficient of variation of exposed length. sing the data given in Table 6 from the Malhotra ( 1 976) review the following alues for average coefficients of variation for depth of penetration are calculated:

Maximum izc aggregate Coefficient of variation of penetration

mm in. depth, percent -50 2 1 4

25 I .6 -1 9 314 3.5, 4.7, and 5.6

ln the rudy by Carcttc and Malhotra ( 1 9 4), the maximum aggregate size wa 1 9 mm (3/4 in.) and the a cragc coefficient of ariation wa 5 .4 percent. \ hcrcas in the study by Keillcr ( 1 9 2 ), it was 7.8 percent for the arne maximumsize aggregate. Other work (Swamy and Al-l lamad 1 984) used I 0 rnm (3/ in. ) maximum- izc aggregate, and the coefficient of variation ranged bct\vecn 2.7 and 7 percent. For commonly used 1 9 mm (3/4 in.) aggregate, it is concluded that a coefficient of ariation of 5 percent is rca onablc.

There arc l imited data on the repeatability of the pin penetration te t. a er and !-Mana ecr ( 1 9 7b) reported an average coefficient of variation of approximately 5 percent for replicate tc t on 1 00 mm (4 in.) thick lab pecimen and on the bottom urfaces of 1 50 x 300 mm (6 x 1 2 in.) cylinders. The variability wa ba ed on the b t five of seven readings with the lowe t and rughc t d leted; the concrete strength aried from approximately 3 .5 to 25 M Pa (500 to 3500 p i) . I n another study (Carino and Tank 1 9 9) eight replicate pin tc ts were petfomlcd at the midhcight of I 00 x 200 mm (4 x 8 in. ) cylinder . The comprc sivc strength ranged from approximately 7 to 40 M Pa ( I 000 to 5800 psi).

ach et of replicate pin tc t wa analyzed for outlier due to penetration into large air void or coar c aggregate particle . On average, two of the eight readings were discarded. Figure 4.2.2d shows the standard deviation ofvaHd penetration value plotted a a function of a crage penetration. ote that a rugh pen tration eorrc pond to low concrete trcngtb. There is no clear trend between the standard deviation and average penetration. The average tandard deviation is 0.4 1 mm (0.0 1 6 in.) which i the value adopted in the precision statement of A TM 803/C803M. To compare with the

atiability reported by a cr and Al-Mana ecr ( 1 987b), the result in Fig. 4.2.2d arc pre entcd in term of coefficient of

ariation in Fig. 4.2.2c. The average coefficient of variation i 7.4 percent. Ba ed on available infom1ation, a coefficient



E E c .Q a; ·:;; Q) 0 -e m " t: !!! en

Average Penetration, in.

0. 14 0.16 0. 1 8 0.2 0.22 0.24 0.26 0.28

1 00 Q�

0.75

0 50

0.25

: . e WIC • 0 50 • WIC • 0 65

· · ·t· · · · · · ·r-;;· · · ·r· · ··

� -·-::��65

� • • .: • ... •• + ,_ . . . . . . . T .

�

... . �-r . . . J. ... ..... : . . . . . . . ., .; .. . . . -r · · . . . . _ . . . . . . .. � • !\ ·� • : i • • j . . . . � . . v� . . � . . · -� j . . � . . . . ·t .. . . /-:-- �- .. � .. . . . .

: . • : . : : • : � . ; ; l· ; • � � 3.5 4.0 4.5 5.0 5.5 6.0 6.5 7.0


0.03

0.02

0.01

.s c .2 a; ·:;; Cll 0 -e m " t: m en

Fig. 4.2.2d-Standard deviation of pin penetration test on 100 x 200 mm (4 x 8 in.) cylinders (Carillo and Tank 1 989).

�

0 c .2 � (ij > -0 c Cll '(j � Cll 0

(.)

Average Penetration, in. 0.14 0. 16 0.1 8 0.2 0.22 0.24

20 �rr�!�rr+!.�.�-�!:

rT�!-rrT�-rr+�� 0.26 0.28

1 5

1 0

5

. . . . . . . . ; . . . . . . . . . 1 . . .• . . . . �- - - · · · · · · ·�- -. . .. : . : : .

•

• • WIC = O SO

WIC = 0 55

WIC = 0 60

-- Average

e • • I we • , : • : .I' : • : : : : J : , J : • : : • . •: . . . . . . . . .. . .. . ..;. . .. .. .. .. . . . . ... . . , . . .. . . . ... .. . . . . . -. . . . . . . . . . : . • : . 1 : : .: l !_ : : • -� -.�· : • : . . o ��i��i��i=��

3.5 4.0 4.5 5.0 5.5 6.0 6.5 7 .0


Fig. 4.2.2e-Coefficient of variation of pin penetration tests on 100 x 200 mm (4 x 8 in.) cylinders (Carillo and Tank 1 989).

of variation of 8 percent is recommended for planning pin penetration tests.

4.2.3 Pullout test-1\.STM C900 tates that the average single-operator coefficient of variation is 8 percent for castin-place pullout tests with emb dments of about 25 mm ( J in.) in concrete with nominal maximum aggregate izc of 1 9 mm (3/4 in.) . Thi value i based on the data summarized a follows. A similar single-operator variabi lity is suggested for post-in tailed test of the same geometry (Peter en 1 997).

tone ct al. ( 1 986) examined whether tandard deviation or coefficient of variation i the best mea urc of repeatabil ity. Four tc t series were pcrfonncd. Three of them used a 70-dcgrcc apex angle (2a) with different aggregate type : sil iceous river gravel, crushed limestone, and expanded lm: -den ity ( l ightweight) shale. The fourth cries was for a 54-degree angle with river-gravel aggregate. These test series arc identified as G70, LS, LW, and G54 in Fig. 4.2.3a and 4.2.3b. The embedment depth was about 25 mm ( I in.)

� c 0 7ii ·:;; Cl> 0 " m

"0 t: <0 en

Pullout Load, lb

0 2000 4000 6000 8000 10000

7 �� 6

5

4

- : � r · r : : r · -• LW ! • 4

· · · i· · · · · · · · · · · · ·t· · · · · · · · · · · ;

3

2

· · · · · · · · · · · -�· - · · · · · · · · · · ·+· · · · · · · · · - -�· - · · �- - · · · : · · · · · · . . .

� :.! : � � � � ...... . . . . -r·�� .. -�

�

-

� -

�:��::.

·� �-

-.r.-

.

·::::::::::.1::::::.

·: · · -

i • i • : o ��,_��,_��,_��-r��-r 0 10 20 30

Pullout Load, kN 40 50

1 575

1 350

:9. 1 1 25 c 0 900 �

·:;; Cll 675

0 -e Ill

450 " t: <0

225 en

0

Fig. 4.2.3a-Single-operator standard deviation as a function of pullout load (Stone et a/. 1 986).

c .2 a; ·;;:: (tJ > 0 c Q) '(j

!E Q) 0 (.)

0

0

Pullout Load, lb

2000 4000 6000 . . . . .

8000 1 0000

. . . . . . . . . . . . � . . . . . . . . �- - - - · 1_

. . . . . . . . . . . . . � . . . . . . ... . . . . � . . . . . . . . . . . . . : . : : : : . . . . . . . . . . . . , . . . . . . . . . . . . . . . . . . . . . . . . . . . - . . . . . . . . . . . . . � . . . . . . . . . . . . . . . . .

. . . : : • . . . . . . . . . . . ; . . . . . . . . . . . . . � . . . . . . A. . . . . . � . . . . . . . . . . . . . � - - ·� · · · · · · · ·

: � ; � : .

. . . . . . . . . . . . • . . . . . . . . . . . . i . . . . . . · · · · · · · -� - - . . . . " " ' 'to' ' � • • • • • • • • • • • • • : . : .... : ! • • : • . • : � :

· · · · · · · · · · · · : · · · · · · · · · · A· ; · · · · · · · · · · · · ·� · · · · · · · · · · · · · ; . . . . ... . . . . . . . .

. . . . • G70

• G54 L$

. . . . . . . . . . . . ] . . � . . . . . . . . . . t . . . . . . . . . . . . . i . . . . t . .

. . . . . . . . . . . . j . . . . . . . . . . . . . .:. . . . . . . . . . . . . . : !

. • !

10 20 30

Pullout Load, kN

:.I • LW

40 50

Fig. 4.2.3b---Single-operator coe. fficienl of variation as a function ofpullout load (Stone et a/. 1 986).

and compressive strength of concrete ranged from about I 0 to 40 MPa ( 1 500 to 6000 psi). Figure 4.2.3a show the tandard deviation, using 1 1 replication , as a function of the average pullout load. ote that there is a tendency for the standard deviation to increase with incrca ing pul lout load. Figure 4.2.3b hows the coefficient of variation as a function of the a erage pullout load. In thi ca e, there i no trend between the two quantities. Thus, i t can be concluded that the coefficient of variation should be used a a mea urc of the repeatabi l i ty of the pullout test.

Table 4.2.3a gives the reported coefficients of variation from different laboratory studies of the pullout te t . Be ides these data, the work of Krenchel and Petersen ( 1 984)

ummarizcs the repeatability obtained in 24 correlation testing programs involving an insert with a 25 mm ( I in.) embedment and a 62-dcgrec apex angle. The reported co fficients of ariation ranged from 4. 1 to 1 5 .2 percent, with an average of 8 percent. The tests reported in Table 4.2.3a and

American Concrete Institute - Copyrighted © Material - www.concrete.org <aCi'J


Table 4.2.3a-Summary of single-operator coefficient of variation of pul lout test -- -

pex angle, Embedment depth Maximum aggregate size

Aggregate o. of replicate Coefficient of variation, percent

Reference dcg mm in. mm in. type specimens Range Average

Malhotra and 67 0 2 25 I ra el I 2 0.9 to 1 4.3 5.3

Carcnc ( 1 980) I Malhotra ( 1 975) 67 50 2 6 1/4 Limestone 3 2.3 to 6.3 3.9

- - - - - -

Bickley ( 1 982b) 62 25 I 1 0 /8 Unknown 3.2 to 5.3 4. 1 - - -- -

Khoo ( 1 9 4 ) 70 25 I 1 9 3/4 Granite 6 1 .9 to 1 2.3 6.9 - ·- - - -

arcrtc and r-- 67 50 2 1 9 3/4 Lime tone 4 1 .9 to I I . 7. 1

Malhotra ( 19 4) 62 - - - -

25 I 1 9 314 Limestone 1 0 5.2 t o 1 4.9 8.5

Keiller ( 1 982) 62 25 I 1 9 3/4 Limestone 6 7.4 to 3 1 1 4.8

70 25 I 1 9 3/4 Gravel I I 4.6 to 1 4.4 1 0.2 r- - - -- - -

70 25 I 1 9 3 4 Limestone I I 6.3 to 1 4.6 9.2 tone et al. ( 1 9 6)

70 25 I I 1 9 314 Low density I I 1 .4 to 8.2 6.0

54 25 I I 1 9 3/4 Gravel I I 4.3 to 1 5.9 1 0.0 - -- -- -Lsocca ( 1 9 4} 67 30 1 .2 L 1 3 l /2 nknown 24 2. to 6. 1 4.3

- -- -- - -

Table 4.2.3b-Summary of results from i nvestigation of pul lout test (Stone and Giza 1 985)

Apex angle. Embedment depth Maximum aggregate size

No. of replicate 1- oeffictent of vanation, percent

Test series deg mm m. mm m.

30 25 0.98 1 9 314

46 25 0.98 1 9 314

54 25 0.9 1 9 314 - - 1- -

pex allgle 2 0.98 1 9 314 - r- -

62 25 0.9 1 9 3/4 r- - r-70 25 0.9 1 9 314

f-6 25 0.98 1 9 314

- 1-58 1 2 0.47 1 9 3'4

58 20 0.78 1 9 3/4

58 23 0.9 1 1 9 3/4 Embedment 1-- --

58 25 0.98 1 9 3/4

58 27 1 .06 1 9 314

58 43 1 .69 1 9 314

70 25 0.98 6 1 14

70 25 0.98 1 0 31 ggregate -1-- --size 70 25 0.9 1 3

I 1- - 1-70 25 0.9 1 9

- 1-I 70 25 0.98 1 9

ggrcgatc 70 I 25 0.98 1 9

type 70 25 0.98 1 9

70 25 0.98 1 9

•The tcnn. '"2 ' I I"" indicates two groups of I I replicate per group.

by Krenchel and Peter en in ol ed different test geometrie and different type and ize of coar e aggregate. In addition the geometry of the pecimen containing the embedded in ert wa different, with cylinders, cube beam , and lab being ommon hape . Becau e of the e te ting difference , it i difficult to draw firm conclu ion about the repeatability of the pullout te t .

1 2

3/4

3 4

314

3/4

3/4

Aggregate type specimens• Range Average

Gravel 2 X I I 9. 1 to 1 1 .4 1 0.3

Gravel 4 X 1 1 5.6 to 1 8.7 1 1 . 1

Gravel 2 X I I 6.3 to 6.7 6.5 -r- -- -

Gravel 2 X 1 1 8.6 to 1 0.0 9. - -- - - -

Gravel 2 X 1 1 7.5 to 9.6 8.6 i- - - - -

ravel 4 X 1 1 .0 to 1 0. 1

Gravel 2 x I I 9.0 to 1 0.8 9.9 - - -

Gravel 1 X I I - 1 2.9

Gravel 2 X 1 1 7.7 to 1 4.0 1 0.9

Gravel 2 X 1 1 6.5 to 6.7 6.6 - -

Gravel 2 x I I 8.8 to 1 0.7 9.8

Gravel 2 X 1 1 9. 1 to 1 1 . 1 1 0. 1

Gravel 2 X 1 1 1 1 .5 to 1 1 .9 1 1 .7

Gravel - X 1 1 6.5 to 7.0 6.8

ravel 5 x 1 1 4.9 to 6.5 6.0 - - -

Gravel 5 X I I 3 .3 to 1 0.6 6.7 1- -- - - -

Gravel 4 X 1 1 8.0 to 10 . 1 - - -

Low dcnsiry 2 x I I 5.6 to 5.7 5.7

Gravel 4 x I I 8.0 to 10 . 1 8.8

Crushed gnei>" 2 X 1 1 7.2 to 1 6.8 1 2.0

Porous limestone 2 x I I 7.7 to 1 0.9 9.3 -

Table 4.2.3b ummarize the coefficient of variation obtained in a tudy by tone and Giza ( 1 985) de igned to e amine the effect of different variable on te t repeatabi l ity. The column labeled ample ize how the number of group of te t , \! ith each group containing I I repl ication . For the condition tudied, il wa found that embedment depth and apex angle did not greatly affect repeatabi l ity. The maximum nominal aggregate ize, however,



appeared to have some affect, with the 1 9 mm (3/4 in.) aggregate resulting in slightly greater variability than the smaller aggregates. The aggregate type also appears to be important. For test with low-den ity aggregate, the variabil i ty was lower than for tests " ith normal-density aggregate . In thi study, companion mortar pecimens were also te ted, and the coefficients of variation varied between 2.8 and I 0.6 percent, with an average value of 6.2 percent. Thu , the repeatability with low-den ity aggregate i imilar to that obtained with mortar.

xperimental evidence suggests that the ariability of the pullout test should be affected by the ratio of the mortar strength to coarse-aggregate strength and by the maximum aggregate size. A aggregate strength and mortar trength become similar, repeatability is improved. This explains why the tests re ult by tone and iza ( 1 9 5) with low-density aggregate were imilar to te t re ult with plain mortar. Re ults from Bocca ( 1 984), summarized in Table 4.2.3b, al o lend upport to this pattern of behavior. Jn thi ca e, high- trength

concrete wa u ed, and the mortar trength approached that of the coarse aggregate. This condition, and the use of mall maximum aggregate size, could explain why the coefficients of ariation were lower than typically obtained with similar pullout test configurations on lower-strength concrete.

In summary a ariety of test data has been accumulated on the repeatabil ity ofthe pullout test . Differences in result are often due to difference in material and te ting condition . In general, it appear that an average ingle-operator coefficient of variation of8 percent is typical for pullout tests confotming with the requirement of ASTM C900 and embedment depths of about 25 mm ( I in.). The actual value expected in any particular ituation will be affected primarily by the nature of the coarse aggregate, a discussed in previous paragraphs.

4.2.4 Pull-off resr-A TM 1 5831 l 58 3 M states that an inter-laboratory testing program to determine the precision and bias of the test method has not been completed. ay.burd and McDonald ( 1 999) published data that provide an estimate of the single-operator variability. In the study, three repl icate test were perfonned for each condition. The mean

alue of the pull-off bond strength ranged from 0.4 1 to 3.44 M Pa (60 to 500 psi). Replicate te t " ith the arne failure mode were u ed to d !ermine the ingle-op rator tandard deviation for each test condition. ignificant ariation in pull-off bond strengths were observed between replicate specimens in the study. The o erall pooled tandard deviation, for all data sets was found to be 0.29 M Pa (42 psi) .

4.2.5 Ultrasonic pulse veloci In contrast to the previous test technique that examine a relatively thin layer of the concrete in a tructure the pulse-velocity method (using through transmis ion) examine the entire thicknes of concrete between the transducers. Localized differences in the compo ition of the concrete becau e of inherent

ariability are expected to ha e a negligible effect on the mea ured travel time of the ultra onic pul e . Thus the repeatability of this method i expected to be better than the pre ious techniques.

Table 4.2.5 reports the single-operator variability of pulsevelocity measurement obtained by different investigator .

Table 4.2.5-Single-operator coefficient of variation of pulse-velocity tests

Reference

Kcillcr ( 1 9 2) ;-

arellc and Malholra ( 1 9 4)

Bocca ( 1 9 4)

Yun ct al. ( 198 )

Lc hchin ky ct nl. ( 1 990)

L Phoon c1 al. ( 1 999)

oefficicnt of variation. percen1 Range

0.5 10 1 .5

O. l lo 0.8

0.4 10 1 .2

0.4 IO 1 . 1

0.2 10 4.0

1 . 1 10 1 .2

Average

1 . 1

0.4

0.7

0.6

1 .9

1 .2

TM C597 state that the repeatabil ity of test results is within 2 percent for path lengths from 0.3 to 6 m ( I to 20 ft) through Olllld concrete and for differ nt operator using the arne in trumcnt or on operator using differ nt in trumcnt .

4.2.6 Maturity method-ll1 the maturity method, the temperature history of the concrete is recorded and used to compute a maturity index. Therefore, the repeatabil ity of the maturity indcxc depends on the in tmmcntation used. One would expect the repeatabil ity to be better when u ing an electronic matutity meter than when the maturity index is computed from temperature reading on a strip-chart recorder. There arc howe cr, no publi hcd data on repeatabil ity of maturity measurements u ing different in trumentation. The pre ision of temperature mea uremcnt by the instrument i not an important issue, provided that step arc taken to cnsw·c that the instrument i operating properly b fore it is used. Temperature probes can be embedded in temperaturecontrolled water baths to verify that they arc operating properly. The maturity index, after a given time in the bath, can be calculated readi ly and compared with the in trumcnt reading. Of greater importance than accurate temperature measurement is u ing the datum temperate or Q-valuc that rcprc cnts the temperature sen itivity of the particular concrete.

4.2.7 Ca t-in-place cylinder- This tc t method involve the dctcm1ination of the com pres i c strength of cylindrical

p cimen cured in the pc ial mold located in the nucturc. The repeatability would be expected to be imilar to other comprc ion tests on cylinders. Little data have been published. Bloem ( 1 968) reported a ingle-opcrator coefficient of variation ranging from 2. 7 to 5.2 percent with an average of 3. percent for three replicate tc ts at ages from I to 9 1 days. Data from Carino ct al. ( 1 983b ), in which three

replicate cylinders were tested at age ranging from I to 32 day , how an average coefficient of variation of3.8 percent.

A TM 73/C873M tatcs that the single-operator coef-ficient of variation is 3 .5 percent for a range of compre si e

trengtb between I 0 and 40 M Pa ( 1 500 and 6000 p i ) .

CHAPTER 5-DEVELOPMENT OF STRENGTH RELATIONSHIP

5.1 -General Manufacturers of in-place tc ting equipment typically

provide generalized relation hip in the form of graph or equation that relate the property measured by the partic-

American Concrete Institute - Copyrighted © Material - www.concrete.org (clci)


ular te t device to the compressive strength of standard concrete pecimens. These relationship , however, often do not accurately represent the specific concrete being tested. The e relationships hould not be u ed unles their validity has been established through correlation testing on concrete similar to that being inve tigated and with the pecific test in trument that will be used in the investigation. The general approach in correlation te ling is to perform replicate in-place te t and tandard trength test at ariou trengtb level , and then to u e tati tical procedures to e tabli h the strength relationship. The details, however, will depend on whether the in-place tests are to be used in new construction or in existing structure .

tandard pecimens can be cylinders, cubes, or beam . The in-place te ts are often correlated with the compre sive strength of cores becau e core strength is the most establi hed and accepted mea ure of in-place trength. ast-inplace cyl inders are also useful in determining the in-place strength of ne� concrete, and their use doe not require a pree tabli hed correlation. The tati tical technique for e tabli bing the strength relationship are independent of the type of standard specimen. The specimen type, however, is important when interpreting the re ults of in-place tests.

5.2-New construction 5.2 . 1 General-For new construction, the preferred

approach is to e tablish the trength relationship by a laboratory-te ting program that is performed before u ing the in-place test method in the field. The testing program typically in ol e preparing tc t specimens using the same con rete mixture proportions and material to b u ed in con truction. At regular interval , mea urement are made u ing the in-place test technique, and the compressive strengths of standard specimens are also measured. The paired data are ubjected to regres ion analysis to determine the best-fit e timate of the strength relation hip.

For orne techniques it might be po ible to perfonn the in-place test on standard specimen without damaging them and the specimens can be sub cquently tested for com pres ive strength. Usually, in-place tests are carried out on separate specimen , and it i extremely important that the in-place te t and tandard te ts are performed on p cimen having imilar con olidation and at the same maturity. Thi can be achieved by u ing curing conditions that ensure similar internal temperature histories. Alternatively internal temperature can be recorded and te t ages adjusted so that the in-place and standard te ts are perforn1ed at the same maturity index.

In developing the test plan to obtain a rel iable strength relation hip, the user hould con ider the fol lowing question :

(a) l low many trength levels (test points) are needed? (b) Hm many replicate tests should be perforn1ed at each

trength level? (c) l lo\ hould the data be analyzed? 5.2.2 Number of strength level -The nwnber of trength

levels required to develop the strength relation hip depends on the desired level of preci ion and the cost of additional tests. ection A. I in ppendix discus es how the number ofte t point used to develop the strength relationship affects

the uncertainty of the estimated strength. From that discu -sion in . I , it was concluded that in planning the correlationtesting program, ix to nine strength levels hould be considered. sing fewer than ix strength levels may re ult in high uncertaintie in the estimated strength and using more than nine level may not be ju tifiable economically.

The range of strengths u ed to e tabli h the correlation hould cover the range of strengths that are to be e timated

in the tructure. Thi will en ure that the trength relation-hip wil l not be u ed for extrapolation beyond the range

of the correlation data. Therefore if low in-place strengths are to be estimated, such as during l ipforming, the testing program mu t include the e low strength levels. The cho en trength level hould be evenly distributed within the trength range.

5.2.3 umber of replications-The number of replicate te t at each trength le el affects the uncertainty of the average value . The tandard deviation of the computed average aries with the in er e of the square root of the number of replicate te t u ed to obtain the average. The effect of the nwnber of te t on the preci ion of the average is simi lar to that shown in Fig. . I ( ppendix A).

Statistics show (A TM E 1 21) that the required number of replicate tests depends on: I ) the single-operator variabil ity of the method· 2) the al lowable error between the sample average and the true average· and 3) the confidence level that the al lowable error is not exceeded. The number of replicate tests i , howe er, often ba cd on customary practice. For example, in acceptance te ring, ACI 3 1 8 considers a test result as the average compressive trength of two molded cylinder . Therefore, in correlation te ting, two replicate tandard compre sion te t can be a umed to be adequate for measuring the average compre!> ive strength at each level.

The nwnber of companion in-place tests at each strength level should be chosen so that the averages of the in-place te ts and compre sive trengths have imilar uncertainty. To achieve this condition, the ratio ofthe number ofte ts should equal the square of the ratio of the corresponding ingleopcrator coefficient of variation. If the number of replicate compression test at each strength level is t� o the required number of replicate in-place te ts is

(5.2.3)

where n ; is number of replicate in-place tests; V, i coefficient of variation of in-place test; and Vs is coefficient of variation of standard test.

For planning purposes, the coefficients of variation given in hapter 4 can be u ed for the in-place tests. For molded cylinder prepared cured, and te ted according to A TM standards the single-operator coefficient of ariation can be a sumed to be 3 percent (A TM C39/39M). For core , a value of 5 percent may be assumed (A TM C42 C42M).

5.2.4 Regres ion analysis-After the data are obtained, the trength relation hip should be determined. The u ual prac

tice i to treat the average alues of the replicate compre -



sive strength and in-place test results at each strength level as one data pair. The data pairs are plotted using the in-place test value a the independent value (or X variable) and the compre sive strength a the dependent value (or Y variable). Regre ion analy is i performed on the data pair to obtain the be t-fit estimate of the strength relation hip.

I L istorically, mo t strength relation hip have been a swned to be traight l ine , and ordinary lea t- quares (OL ) analy i ba been u ed to e timate the corre ponding lope and intercept . The u e of OL i acceptable if an

estimate of the uncertainty of the strength relationship is not required to analyze in-place test results such as if the procedure in 7.2. 1 and 7.2.2 are u ed. If more rigorous methods, such a those in 7.2.3 and 7.2 .4 are used to analyze in-place te t re ult a procedure that i more rigorou than OL should be u ed to e tablish the strength relationship and its associated uncertainty.

The limitations of OLS analy is arise from two of its underlying a umption :

(a) There i no error in the X value and (b) The error ( tandard de iation) in the Y alue i con tant

xcept for mea ured maturity indexes the first of the e asswnptions is violated because in-place test (X value generally have greater single-operator variabil ity than compression test ( Y value). In addition, i t is generally accepted that the ingle-operator variabil ity of standard cylinder compre ion test i described by a con tant coefficient of ariation (ACI 2 1 4R) . Therefore the standard deviation increa es with increasing compre sive trength, and the econd of the aforementioned assumptions is al o

iolated. a re ult, OL aualy i wil l undere t imate the uncertainty of the trength relation hip (Carino 1 993). There are, however, approaches for dealing \ ith these problems.

First, the problem of increasing tandard deviation with increa ing average trength i discussed. If test results from group that have the same coefficient of variation are transformed by taking their natural logarithms the tandard deviation of the logarithm values in each group will have the same value (Ku 1 969). For example, if the coefficient of variation of a group of numbers, expressed a a decimal, equal 0.05, the tandard de iation of the tran formed values will be approxi

mately 0.05. Thus, th econd a un1ption of OLS can be ati fied by p rforrning regre sion analy i using the average

of the natural logarithms of the test re ults at each trength level. If a linear relationship is used, it fom1 is as fol lows

In = a -r B lnl (5 .2.4a)

where In i the average of natural logarithms of com pre -sive strengths; a i intercept of l ine; B is slope of l ine; and lnl is average of natural logarithms of in-place test re ults.

By obtaining the antilogarithm of ln , Eq. (5 .2.4a) can be tran formed into a power function

(5 .2.4b)

The exponent B determines the degree of nonlinearity of the power function. If B = I , the trength relation hip

is a straight l ine pa sing through the origin with a slope = A . If B =F I , the relationship has positive or negative curvature, depending on whether B is greater than or less than I . Regres ion analy is u ing the natural logarithms of the test result pro ides two benefit :

( I ) atisfics an underlying as umption of OL analysis (constant error in Y value), and

(2) llo� s for a nonlinear strength relation hip if such a

relationship is needed Use of the transformed data implies that concrete strength

i distributed a a lognormal raU1er than a normal distribution. It has been argued that, for the usual variabil ity of concrete strength, the possible errors from this assumption are not significant ( tone and Reeve 1 986).

ext, a method for dealing with the problem of error in the X alues is discussed. Fortunately, regression analysis that account for X error can be performed with linlc additional computational effort compared with OL analysis. One uch procedure wa proposed by Mandel ( 1 984) and wa u ed by tone and Reeve ( 1 9 6) to de elop a rigorou procedure to analyze in-place te t re ult (7.2.3). Mandel' approach invol es the use of a parameter A defined as the ariance (square of the standard de iation) of the Y ariable di ided by the variance of the X variable. For the correlation-te ting

program the alue of A i obtained from the standard deviat ions of the a erage compressive strength and in-place test result . If the numbers of replicate for compre sive te ts and in-place test arc cho en so that average alue arc mea urcd with comparable preci ion, the value of A should be c lo c to I .

The parameter A and the correlation te ting re ult -that is, the a erages of the logarithms of the in-place results (X

alue ) and the average of the logariU1ms of compre si e strengths ( Y alues}-are used to determine the strength relationship using the calculation outlined in A.2 (Appendix

). The calculations involve the usual sum of squares and cross-product used in OL analysi (Mandel 1 984). The procedure is well suited for application on a personal computer ith a pread hcet program.

Figure 5.2.4 i a graphical rcpre entation of the difference between OL analysi and Mandel ' pro edure. In OLS analy i , the be t-fit traight line i the one that minimizes th sum of quare of ilie vertical deviation of ilie data points from the l ine, as hown in Fig. 5 .2 .4(a). Mandel's analysis minimize the sum of squares of the de iations along a direction inclined to the straight line, as shown in Fig. 5 .2 .4(b). The direction of minimization depend on the

alue of A, which in turn depends on the ratio of the errors in the Y and X values. As the error in the X value increa e , the value of A decrea e and the angle () in Fig. 5 .2.4(b) incrca e . An important feature of the Mandel analy is is that the e timated tandard de iation of the predicted alue of Y for a ne� alue of X account for error in the new X

alue and the error in the trength relationship {A.3). I n ummary, regre ion analysis hould be performed

u ing the natural logarithm of the te t result to establi h the strength relationship. This approach wil l accommodate the increase in single-operator variabil ity with increasing



Q.) 15 ro ·.:::: ro > >-

Q.) .0 ro ·.:::: ro > >-

(a)

X - Variable

(b)

X - Variable Fig. .-A-Direction of e!Tor minimi:ation in: (a) ordin(lly lea t- quare (OLS) analy i : and (b) Mandel ' procedure (Carino 1993).

trcngth. U ing a traight line to reprc ent the relationhip between logarithm alue i equivalent to as uming

a power function trength relation hip. The power function can accommodate a nonlinear relation hip, if nece ary. To be rigorou , the regre ion analysi procedure hould account for the uncertainty in the in-place te I re ult (X error). Failure to account for the X error will undere timate the uncertainty of future e timate of in-place compre ive tTength. Thi rigorou pro edure, howe er, i ju tified only when an equally rigorous method will b u cd to interpret in-place te t re uh ( hapter 7); otherwi e, OLS analy i i a ceptable.

5.2.5 Procedures for correlation te ling- Ideally, it i de irable to determine the compre i e trength and the in-place te t re uh on the ame pecimen o that companion te t re uhs are obtained at the ame maturity. Unfortunately, thi i only po sible with those method that are truly nondestructive, such a pulse velocity and rebound number. For method that cau e local damage to concrete, eparate pecimen are needed for obtaining compre sive trength

and the in-place te t re ult. Tn uch ca e , it i important that companion pccimen arc tc ted at the arne maturity. Thi i e pccially critical for early-age te t when trength at a given age depend highly on the thermal hi tory. The problem ari e becau e of difference in early-age temperature in pecimens of different geometric . An approach for

moderating temperature differences is to cure all specimens under laboratory conditions in the same water bath.

Altemati ely, intemal temperatures can be monitored and test ages adjusted so that compression tests and in-place test arc performed at equal values of the maturity index. Failure to perform companion tc ts on specimen that arc at equal maturity will rc ult in an inaccurate strength relation hip that i l l cause y tematic errors (or bia ) \ hen it i used to e timate the in-place trength in a structure. The

following recommendations should be used in correlation testing programs.

5.2.5. 1 Rebound number-At least 1 2 standard cylinders should be cast. At each te t age, a set of I 0 rebound numbers

( TM 805/C 05M) should be obtained from each pair of cylinders held fitmly in a compres ion testing machine or other suitable de ice at a pressure of approximately 3 MPa (500 p i). Rebound tc t for the de elopmcnt of a strength relationship should be pcrfonned in the same orientation a the tc t to be made on the stntcture. However, for hammers ba ed on the ratio of hammer peed before and after impact, the ori ntation of the instrument i not a factor. The cylinders should then be tested in compression. I f it is not feasible to test the cylinders with the hammer in the same orientation that will be used to test the structure, the correction factor supplied

by the equipment manufacturer can be used to account for differences in otientation. A mentioned in 3.2, the urface produced by the material of the cylinder molds can differ from the urfacc produced by the fom1 material for the structure. This factor should at o be considered in the correlation testing. I f con id rable difference i exp cted between the surface of the tructure and the cylinders additional prismatic pecimen should be prepared for rebound tests. These specimens should be formed with the amc type of forming material that wiU be used in con truction, and they should be similar in izc to the cylinders so that they will experience similar thermal histories. hen the rebound number is dctctmincd on these specimens, en urc that the specimen arc rigidly supported o that they do not move during tc ting. l f thc pccimcn move, lower rebound nwnbcr will be recorded and the trcngth relationship � ill be bia cd.

For accurate c timatcs of in-place trcngth, the moisture content and texture of the urfa e of the cylinder at the time of the correlation te t hould be similar to tho e anticipated for the concrete in the structure at the time of in-place testing. Practically, the only easily reproducible moisture condition for concrete surfaces is the saturated condition.

5.2.5.2 Penetration resistance-For the probe penetration te t, at least 1 2 standard cylinders and a test slab large enough for at lea t I probe penetration te ts should be cast. For in-place testing of vertical element , the recommended procedure i to cast a wal l specimen and take cores next to the probe te t . All te t pecimen hould b cured under identical conctitions of moi ture and temperature. t each te t age, two compre ion tests and three probe penetration tests should be made. The recommended minimwn thicknes for the te t

lab is 1 50 mm (6 in.). The minimwn pacing between probe penetrations is 1 75 mm (7 in.) and the minimum distance from a probe to a slab edge i 1 00 mm (4 in.).



For the pin penetration test, i t may be possible to perform penetration tests on the side of cyl inders and ub equently test the cylinders for compressive strength. arino and Tank ( 1 9 9) howed that the urface damage produced by pin penetrations into I 00 x 200 mm (4 x in.) cylinders did not re ult in strength reductions. omparative test , however, were not perfonned on pecimen with concrete strength les than 25.5 M Pa (3700 psi) . Until further tudies are conducted to confinn that pin penetrations do not affect the compre ive trength of cylinder for a wide range of concrete strength it i recommended that lab specimens be used for pin penetration tests. A minimum of six penetration reading hould be performed at each test age. Di card a result when it is obvious that an aggregate particle or a large air void wa penetrated. In addition, according to TM

03/ 803M, if the range of penetration values exceeds 1 .6 mm (0.064 in.), the result with the maximum deviation from the average hould be discarded and a new test performed. Individual penetration hould b spaced between 50 and 1 50 mm (2 and 6 in.), and the minimum di tance from an edge should be 50 mm (2 in .).

5.2.5.3 Pullout test- everal techniques ha e been u ed. Pullout inserts have been cast in the bottom of standard cylinder , and a pullout test \ as pe1fonned before testing the standard cyl inder in compression (Bickley 1 982b). In this ca e the pullout test i topped when the rna ·imum load ( indicated by a drop in the load with further di placement) ha been attained. The insert i not extracted and the cylinder can be capped and tested in compression. Alternat ively, companion cylinders ha e been ca t with and without in ert and the pullout te t has been performed on one tandard cylinder and the other cylinder te ted in compre -

sion. I nvestigator have had problems \ ith both procedures, particularly at high strengths, because radial cracking occurs at the end of the cyl inder containing the pullout in ert. This cracking is believed to re ult in lower ultimate pullout loads.

A third alternative ha been to ca t tandard specimen for compression testing and to place pullout insert in cubes (or slab or beams) so that the pullout test can be made in the companion pecimen when the standard specimens are tested ( out os ct al. 2005). The latter approach i the preferred method, providing con olidation i con i tent between the tandard pecimen and the cube or other specimens containing the pullout in erts, and the maturity of all specimens at the t ime of te t ing is the same. The recommended minimum size for cubes i 200 mm (8 in.) when 25 mm ( I in.) diameter inserts are used. Four inserts can be placed in each cube one in the middle of each vertical side. For each te t age, two standard cylinder hould be te ted and eight pullout te ts perfonned. The same procedure applies to po !installed pullout test . Install the in erts on the same day that pullout te t wil l be done.

5.2.5.4 ltrasonic pul e veto ity- l t i preferable to develop the trength relationship from concrete in the structure. Tests should be performed on cores obtained from the concrete being e aluated. Tests with tandard cylinder can lead to unreliable relationships becau e of different moi -ture condition between the cylinders and the in-place

concrete. The relation hip data should be obtained from a

testing configuration that is similar to the one used in the field becau e the geometry of the test specimen may affect the determination of the pulse velocity. The recommended procedure is to select certain areas in the tructure that repre-

ent d ifferent levels of pulse velocity. At the e locations it is recommended that five elocity determination be made to obtain a repre entative average alue of the pul e elocity. For each mea urement, the tran ducer houJd be un oupled from the urfa e and then recoupled to avoid y tematic eiTor due to poor coupling {A TM C597). Then obtain at least two cores from each of the same locations for compressive trength testing. Pulse velocity measurement on these cores,

once they have been removed from the structure, will usually not be the same a the velocitie mea ured in the structure and are not representative of the pulse velocity of the structure.

5.2.5.5 Maturity method-The following procedure is given m TM I 074.

Prepare cylindrical concrete specimen according to TM C 1 92/C 1 92M u ing the mixture proportion for

the concrete intend d for the tructure. Embed temperature sensors at the centers of at least two specimens. onnect the ensors to maturity instrument or to a ui table temperature

recording device{s). Moi t cure the specimen in a water bath or in a moist

room meeting the requirements of A TM 5 1 1 . Perform compression test in accordance with A TM 39 39M at I , 3, 7, 1 4, and 2 day . Test at lea t two pecimens at each age.

At each test age, record the average maturity index for the in trumented pecimen . Plot the average compre si e tr ngth as a function of the average maturity index. Draw a

be t-fit curve through the data. A uitable empirical equation

may be fitted to the data u ing least-square curve fitting. Refer to Carino (2004) for possible equations. The resulting curve is the trength-maturity relationship to be used for estimating in-place trength.

5.2.5.6 ast-in-p/ace cylinder-If nece sary, test re ults hould be corrected for the height-diameter ratio using the alue given in TM 42/C42M. o other correlation is

needed because the specimens represent the concrete in the placement and the test is a uniaxial compression te t.

5.3--Existing construction 5.3. 1 General-There is often a need to evaluate the

in-place strength of concrete in existing structures. For example planned renovation or change in the use of a structure may require detennination of the concrete strength for an accurate a sessment of structural capacity. There also may be a need to evaluate concrete strength after a structural failure, fire damage, or environmental degradation has occurred. ometimes, errors or unforeseen conditions occur during new construction and an e aluation i needed to re ol e question about concrete trength. These ituat ions are similar because the need to detennine the in-place strength of the concrete was not preplarmed. In-place testing methods can be helpful in these evaluations.

In-place tests can be u ed in two ways to evaluate existing construction. First, they can be used qualitatively



to locate those portions of the structure where the concrete appear to be different from other portion . In this case the in-place tests can be u ed without a trength relationship for the concrete in the tn1cture. The main purpo e of the in-place testing is to establish where core should be taken for strength determinations and other pertinent tests ( Cl 437R). The rebound number and the pulse velocity method are widely u ed for thi purpo e. e ond in-place methods can be u ed for a quantitative a e rnent of the trength. I n this c a e , a trength relationship m u t be e tabli hed for the concrete in the tructure. The relationship can be developed only by pe1forming in-place tests at selected locations and taking companion cores for strength testing. Thus, the use of in-place testing does not eliminate the need for coring but i t can reduce the amount of coring required to gain an understanding of the variations of trength in a structure, and it can give a higher degree of confidence that the core taken truly represent the condition being investigated.

5.3.2 Developing trength relation hip-Typically in-place te ting for evaluation of exi ting construction i not preplanned, o the techniques that have traditionally been used are ultra onic pulse velocity, rebound number, and probe penetration. In the nited Kingdom, the pul l-off test is al o used ( Long and Murray 1 984; Murray and Long 1 9 7) . The pul l-off test involves bonding a teel disc to the concrete surface and measuring the resultant force required to pull off the di c . In candinavia and other part of Europe, a postin tailed pullout test is widely used ( Peter en 1 9 4, 1 997) . In current orth merican practice, the pull-off method is not u ed routinely to e t imate in-place compressive trength; thu , this application i not con idered in the remainder of this report.

For some te t methods, cet1ain factors hould be considered when testing exi ling structures. For example, for surface tests (rebound number, penetration resistance, and pull-off), pay special attention to those factors that could affect the near-surface strength, such as carbonation moi -ture content or surface degradation from chemical or phy -ical proce e . ttrface grinding may be nece ary to expo e concrete that represent the concrete within the tructure.

To de elop the tr ngth relationship, it i generally nece my to correlate the in-place te t param ter with the compre sive strength of cores obtained from the tructure. In selecting the core locations, it is desirable to include the widest range of concrete strengths in the structure that is possible. Often, rebound numbers or pul e velocity values are determined at points spread over a grid pattern establi hed on the area being e aluated. When the data are plotted on a map contour l ine can be ketched in to outline the variations in the concrete quality (Murphy 1 984). Ba ed on this initial survey six to nine different locations hould be elected for coring and mea urement of the in-place te t parameter. At each location a minimum oft\: o core hould be obtained to establish the in-place compre sive trength. The number of replicate in-place tests at each location depends on the test method and economic considerations, a di cussed in Chapter 6. Because at least 1 2 cores are recommended to develop an adequate strength relation hip, the use

of in-place testing may only be economical if a large olume of concrete is to be evaluated.

ores hould be tested in a moi ture condition that is repre entative of the in-place concrete. The recommended procedure i to wipe off excess dri l l ing water, allow the core to urface dry and place the cores in sealed plastic bags. Refer to TM 42/ 42M for additional guidance on the handling and testing of cores.

After the average and tandard deviation of the in-place te t parameter and core strength are detennined at each te t location, the strength relationship is developed using the same approach as for new construction (5.2.4).

In evaluating the a erage and tandard deviation of the replicate in-place re ults, the recorded values hould be checked for outlier {A TM 1 7 ). In general te t re ults that are more than two standard deviations from the average hould be scrutinized. Outl iers may occur due to an improp

erly performed test or a local ized. abnormal condition. I f an obviou cau e of the outlier i identified that result hould be ignored and the average and tandard deviation re alculated.

CHAPTER 6-IMPLEMENTATION OF IN-PLACE TESTING

6.1 -New construction 6. 1 . 1 Preconstmction con. ensus-Before starting

con truction of the components of the tructure that are to be tested in-place, a meeting hould be held among the parties who are involved. The participants typically include the owner, on tmction manager, structural engineer, testing company general contractor ubcontractor (such a formwork ontractor or po Hen ioning contractor) and concrete supplier. The objective of the preconstruction meeting i to clarify the test procedures to be used, the access requirements, the criteria for interpretation of test data, and the interaction among the partie . mutual understanding among the involved parties wil l reduce the potential for dispute during construction.

The meeting hould achie e a con en u on the following critical is ue :

(a) Agreement on type of fonnwork material that will be u ed becau e it may affect the correlation te t ing

(b) The test procedure{ ) to be u ed, number and locations of tests, the acces requirements for testing, and the assistance to be provided by the contractors in preparing and protect ing te t location and te ting equipment

(c) The criteria for acceptable test results for perfonning critical operation , uch as fonn removal, post-tensioning removal of re bores, or termination of accelerated or initial cunng

(d) Procedures for providing acce s and any modifications to formwork required to faci litate te ting

(e) Procedure and re ponsibil itie for placement of te ting hardware, where required, and protection ofte t ites

(f) Procedures for the timing and execution of te ling

(g) Reporting procedures to provide timely information to site personnel



(h) pproval procedures to allow construction operations to proceed if adequate strength is shown to have been achieved

( i ) Procedures to be followed if adequate strength i not shown to have been achieved

6. 1 .2 umber of test locations The te t should provide a reliable mea ure of the trengtb of the te ted component at the time the test are made. Therefore, sufficient test locations should be provided so that there are ufficient te I re ult to adequately characterize the concrete trength within the pmtion of the tructme being e aluated. The tenn "test location"' means a region on the structure where an in-place test procedure i to be executed. At a te t location, one or more single or replicate in-place test may be performed.

The number of test location hould account for the fol lowing considerations:

(a) Becau e te ts wil l be performed at early age � hen strength gain of concrete depend highly on temperature the initial test may how that adequate strength ha not yet been achieved. It will then be nece ary to top testing after the initial te t have been made and to rete t at a later age. ufficient test locations have to be provided to allow for repeat tests and to satisfy the criterion for number of te t required to al low critical operation to proceed.

(b) I f tests are made at ages under 1 2 hours after the concrete is cast it i expected that the in-place strength will ha e high variabil ity due to variations in temperature at the test location . In thi case, increa e the number of provided test location by I 0 to 25 percent.

Tables 6. 1 .2a to 6. 1 .2d provide recommendations for testing variou tructural component . For each te t method, the table how:

(a) The number of te t locations or acces points that should be provided per stated volume of concrete

(b) The minimum number of test locations that should be available for stati tical analysis to determine concrete strength.

The number in these tables are based on experience con idering the critical ity of the tructural component and practical ity.

6. 1 .3 Number of te. ts per location-The number of in-place te t to be petformed at a te t location could, in theory be determined ba ed on the repeatabil ity of the te t

method as discussed in 5 .2 .3 . onsider, however, the practical ity of the number of replicate tests; othenvise, in-place testing programs wil l be avoided because of the financial burden. Table 6. 1 .3 l i ts the minimum number of individual determinations per test location. A lo> er number is recommended for those in-place te t methods that require installation of hardware compared with tho e methods that do not.

6. 1 .4 Providing acce s to test location -To perform in-place te t during con truction, it i nece ary to provide acce to the hardening concrete. The pecific detail wil l depend on the test method, the type of tructural component, and the type of formwork. Test locations should be selected to avoid reinforcing steel. Finally, note that water absorption characteristics of the fonn surface at the location of the in-place testing might affect the re ults of surface tests, uch as the rebound number and pin-penetration method . Fonn material for the in-place te t specimen in the correlation testing must be similar to those u ed in construction.

For te ts on the soffit of labs formed with plywood, an acce configuration a hown in Fig. 6. 1 .4a can be u ed.

circular hole i cut in the form and the plug that i cut is attached to a backup plate that is temporarily fastened to the fonnwork with crews. Test hardware, such as a pullout in ert. is attached to the removable a sembly. When a test i to be performed test hardware, if it exists, is loosened and the backup plate and plug are removed to expose the te t urface. To provide a mooth te t utface, a sheet metal plate can be attached to the plug. A sealant should be used to seal the gap betv een the plug and backup plate to prevent leakage of fresh cement pa te. The diameter of the plug will depend on the pecific pacing requirement for the te t method a di cu ed in 6. 1 .6, and it should provide at lea t 25 mm ( I in.) of clear space around the perimeter of the plug to avoid testing concrete near the edge of the plug. For access through metal forms, a similar backup plate assembly can be fabricated of metal plate. A typical access configuration for u e on the ertical surface of a metal form is shown in Fig. 6. 1 .4b.

The acce s types hown in Fig. 6. 1 .4a and 6. 1 .4b are applicable to all the in-place testing methods except for the maturity method and ca t-in-place cylinders. Figure 6. 1 .4c i llustrate typi al technique for instal l ing maturity meter . The dispo able mini-maturity meters can be in erted directly into

Table 6.1.2a-Recommendations for suspended slabs, shear walls, and core walls*

umber of test locations provided • umber of locations to test

Te>t method First 75 m 1 ( I 00 yd1) Each additional 1 5 m1 (20 yd1) First 75 m1 ( 1 00 yd1) Each additional 1 5 m1 (20 yd1)

Rebound number 20 2 1 0 I

Probe penetration I 6 I

Pin penetration 1 5 2 1 0 I -

Pullout 1 5 2 1 0 I

Ultrasonic pul e velocity 1 5 2 1 0 I

Mantrity 5 2 5 I --Ca t-in-place cylinder'

-I 5 I

•corr walls that typ•cully surround elc.,ator halls arc u ually located at the enter of a bu•lding and fom1 the >tructural backbone of the build10g.

'l'or >lab only.



Table 6.1.2b-Recommendation for other wal ls per 1 50 m2 (200 yd2) -- -Number of test location provided umber of loeation to test

Wall thinner than 300 rum WaiL 300 mm ( I fl) thick or Wall thinner than 300 mm Wall 300 mm ( I fl) tbi k or Test method ( I ft) thicker

Rcbow1d number 20 to 25 15 to 20 -+---

Probe penetration to 1 0 6 to -+--- ---

Ptn penetration 1 0 LO 1 5 to 1 2 1- -- --Pullout ! O to 1 5 to 1 2 -- - --

Ultrasonic pulse veloeity 1 0 to 1 5 8 to 1 2

Maturity 5

Table 6.1.2c-Recommendations for individual columns*

umber of test Minimum number of Test method

Rebound number 5 LO 8 5

Probe penetration 5 to 8 5

Pin penetration 5 to 8 5

Pullout 5 to 6

'ltra onic pu1 e \ eloeity 5 to 6

Maturity 5 5

•Recommendations are based on the assumpt ions that there are 6 to 1 0 columns m each test area and that ea h olumn contams approximately I m 1 ( t .5 yd1) of concrete. Grenter numbt oftc t; 'hould bt provided and te ted for larger columns or "'here the test nrca contains more than I 0 columns.

Table 6.1.2d-Recommendations for columns with spandrel beams per 40 m3 (50 yd3)

umber of test Minimum number of Test method locations provided loeations to test

Rebound number 6 to 9 5 -Probe penetration 6 to 9 5

Pin penetration 6 to 9 5

Pullout 6 to 9 6 I-- -(.,1trasonic pulse veloeity 6 to 9 6

Maturity 5 5

•Recommendations apply to the number of t�t locauon> provided/tested before

rcmo> I of form and agam before application of construction loading from ne.•t level of con truction It is assumed that corbel . if present. arc cast integrally with columns or spandrel beams.

the top mfaccs of labs, or they can be embedded deeper into the slab using a cup-lid assembly to avoid interference with finishing operations. The cup may al o be placed within openings on the ide of vertical form . For electronic maturity meters temperature probe arc in crtcd into the structural clements. For meter with reusable probe , the u ual practice i to embed an expendable plastic tube into the frc h concrete and to place the probe within the tube (Fig. 6. 1 .4c(b )). A thermal couplant, which i a type of grease, hould be applied to the probe before in crtion into the tube to ensure accurate measurement of the concrete temperature. For meters that u c thcimocouplc wire as en ors, the wire arc fastened to reinforcing bars before concreting. After testing is completed, the

5

( I ft) thicker

1 0

6

1 0

1 0 8 -- ---1 0 8

5 5

Table 6.1.3-Number of replicate tests at each location

Tc t method Minimum number of locations to test

Rebound number I 1 0 r-

Probe penetration 3

Pin pcoctrati o I 6 1--Pullout I

ltrasonic pul vcloeiry 2

r Maturity l

Ca t-in-place cylinder l 2

thennocouple wires are cut flu h with the concrete u1face, and the exec s wire can be reu. ed.

ast-in-place cylinders do not requjrc pecial accc provi-ions. The upporting le ve for the cylinder mold is nailed

directly to the formwork. It i only nece ary to en ure that the top urface of the specimen \ ill coincide \ ith the top urface of the slab. I f the top of the pecimen i too lo , it will be difficult to locate and extract the cylinder. If the top of the specimen i too high, fini hing operations will di rupt the molds.

6. 1 .5 Distrihution of te. Is-Te t location should be di tributed throughout the component being tested o that the results provide an accurate indication of the trcngth di tribution within the component. In electing the te ting location con idcration should be given to the most critical location in the structure in term of trcngth requirement ( uch a po !-ten ioning tre ing location ) and expo ure condition ( uch as lab edge ), e pecially during cold weather. When a large number of tests are required for stmctural components such as slabs distribute the test locations in a regular pattem. For test method that require few tests such as ca t-in-place cylinders, choose location that are critical in each concrete placement.

For te t on vertical members such a column , walls and deep beam the vertical location within the placement is important. For vertical members, there is a tendency for the concrete trength to be higher at the bottom of the placement than at the top of the placement. The magnitude ofthi variation is influenced by many factor , such a mixture compo-

ition, type and degree of consolidation, aggregate shape, and environmental condition (Murphy 1 984; Munday and Dhir 1 984; Bartlett and MacGregor 1 999). It is impossible to predict accurately the magnitude of trength variation



M1n1mum diameter depends on

method

Hole (1f needed) Minimum limits to

avo1d obstruction of

Fig. 6. 1 .4a-Access for use on vertical swfaces and soffits with woodenforms.

·.·... ·: Column, beam, or .,

wall Butterfly bolt

Hole (if needed) Steel back-up plate

Fig. 6. 1 . 4b-Acces for u e on vertical swface and soffits with steel forms.

expected in a given component. AI o, code-writing committee have not addre ed the e trength ariation . A a re ult, engineering judgment i needed in planning and interpreting the re ult of in-place te t on vertical member , particularly when testing member with depth greater than 300 mm ( 1 2 in.). imilar engineering judgment will al o need to be made when te ting deep slab ection .

6. 1 .6 Critical dimensions-Te t uch a rebound number, penetration re i tance, and pullout produce orne urface damage to the concrete, and te t re ult are affected by the condition within the zone of influence of the pecific te t. A a re ult, the A TM tandard pre cribe minimum dimen-ion to a sure that test re ults are not influenced by neigh

boring te t , specimen boundarie , or reinforcing teel . Te t location hould be po itioned to conform with the dimenional requirement in Table 6. 1 .6.

(a)

(b)

Inserted 1nto top surface

Plastic

----tl� To meter

Installed In formwork using cup

Fig. 6. 1.4c-lnstallation of maturity meters illlo fresh concrete: {a) disposable mini-meter: and (b) sen or of elecn·onic meter.

6.2-Existing construction 6.2. I Prete ling meeting- di cu ed in 5. , there

are many reason for detem1ining the in-place trength f concrete in e i ling tructure . In-place testing i orten one facet of an overall inve ligation to e tabli h tructural adequacy. The guideline in ACT 437R should be followed to de clop the complete plan of the inve ligation and ident ify other a peel of the field tudy to complement concrete

trength determination. The plan for the in-place te ling program will depend

on the purpose of the inve tigation. A prete ting meeting hould be held among the member of the team who share a

common intere t in the te t re ult . t the conclu ion of the meeting, there hould be a clear under tanding of the objective of the in c tigation and agreement on the re pon ibilit ie of the team member in acquiring the te t data and the procedure for obtaining and analyzing te t re ult . When acce to the concrete for te ting i re tricted by architectural co ering , detailed plan hould be developed to accompli b thi acce .

6.2.2 Sampling p/an-111 de eloping the te t ing program, con ideration should be given to the mo t appropriate ampling plan for the pecific ituation. A TM 823/ 823M provide guideline for developing the ampling

plan . Although the tandard deal primarily with the dri l l ing



Table 6.1.6-Dimensional requirements for in-place tests according to ASTM standards*

Tes1 mclhod

Rebound number

Pin penetration

Pullout

Requiremenl

Minimum dimen ion

Th1 knes of member:

1 00 mm (4 in.)

Diameter of test area:

00 rnm ( 1 2 in.)

Minimum distance

Between lesl pomt : 25 mm ( I in.)

Minimum di. ranee

Between probes: 1 75 1111n (7 in.)

To edge ofconcre1e: 1 00 mm (4 in. )

Minimum disrnnce

Between pin : 50 nun (2 in.)

To edge of c n rete: 50 mm (2 in.)

Marimum disrnn e Between pin : I 0 mm (6 in.)

Miuimum clear spacing

Be1ween in en : 1 0 time in en head diame1er

To edge of member: Four l imes head diamcler

From edge of failure urfacc 10 reinforcing bar: ne

in en head diameler or ma imum aggregate ize.

' hichcver i larger

•The cu.,nl ' cr>�on of lhc A TM lc 1 methods hould be consullcd before plannong m-ploce 1es1s 10 ensu"' 1ha1 proper >pacing and clearance requ�rcmenJS ""' sausfied.

of cores or awn amples, there i a section addres ing in-place testing.

In general two sampling ituations can be encountered. In one ituarion all the concrete is bel ieved to be of imilar compo ition and quality. For thj case, random ampling

hould be pread out o er the entire tructure and the result treated together. onsult A TM E l 05 to w1derstand the principles of random sampling. The structure should be partitioned into different regions and a random number table used to determine objectively which area to te t. Objecri e random ampling is neces ary to apply probability theory and make

valid inferences about the propertie of the population (all the concrete in the tructure) ba ed on the ample te t re uJt .

The second sampling ituation ari es hen available infonnation uggest that the con rele in different ection of the tmcture may be of different compo it ion or quality, or when the purpo e of the in estigation i to examine fai lure or damage in a specific section of a tructure. In this case, random sampling should be conducted within each ection of the structure where the concrete is uspected of being nominally identical . Te t results from different ection of the structure should not be combined unless it i shown that there are no tatistically significant differences between the a erage test re ult in the different section .

6.2.3 umber of test As discu ed in 5.3 , the in-place te ting program for an existing tructure involve two pha e . Fir t, the trength relation hip must be e tabli hed by testing drilled core and mea uring the corresponding in-place test parameter near the core location . The locations for correlation testing hould be cho en to provide a wide range in concrete strength. s mentioned in 5.3 .2, a minimum of six to nine te t location hould be selected for

obtaining the correlation data. In general cores hould be drilled after the in-place tests are performed. At each location, two cores should be dril led, and the following number of replicate in-place tests should be performed to provide the average value of the companion in-place test parameter:

Tes1 mclhod I Replicates at each location

Rebound nwnber 1 0

Probe or pin penelrnlion 3 to 6

Ultrasomc pulse veloc1ty 5

Pullout 3

The number of replicate in-place tc t i ba cd on con idcration of the single-operator ariability of the method and the co t of additional te ting. For example the ingleoperator repeatabil ity of the ultra onic puJ c velocity te t i low, and the cost of replicate reading at one location i low. Therefore, five replicate reading arc recommended to ensure that a rcprcsentati c value will be obtained bccau c of the variability in the efficiency of the coupling of the tran -duccr to the tructurc. Ln makjng the replicate pulse velocity dctcnnination . the transducer should be moved to nearby location to evaluate the area where cores wil l be taken. The dimcn ional requirement pre cntcd in Table 6. 1 .6 hould be observed for all test methods.

The c ond phase of the in-place tc ting program involve performing the in-plac tests at other locations and c 6-mating the compre sive strength ba cd on the trcngth relationship. The number of test loca6on for thi pha c wil l depend on several factors. First, there arc the statistical factors. According to the principle set forth in A TM

1 22, the number of tc t depend on the variability of the concrete trcngth the acceptable error between the true and ample average, and the acceptable ri k that the error will

be exceeded. mong the e factors, the variabil ity of the concrete is a predominant factor in determining the number of required tc t . For a given ac cptablc error and I vel of ri k, the nwnber of te t increases with the quare of the

ariability ( TM 1 22) . Economic considerations also influence the tc t ing plan.

For orne ca c , the co t of an cxtcn ivc investigation might outweigh the economic benefit. Because the co t of an inve -tigation i related to the amount ofte ting performed, a high degree of confidence, due to a large ample ize is obtained at a higher cost. The election of a te ling plan involve tradcotr between economic and degree of confidence.

CHAPTER 7-INTERPRETING AND REPORTING RESULTS

7.1 -General Standard tatistical procedures hould be used to interpret

in-place test . I t i not ufficient to simply average the values of the in-place tc t results and then compute the equivalent compressive strength by mean of the pre iou ly e tabli hed trength relation hip. I t is nccc ary to account for the uncer

taintie that exist. While no procedure has been tandard-



ized for determining the tenth-percentile in-place strength based on the results of in-place te ts, proponents of in-place testing have developed and are using statistically based interpretations.

Four stati tical methods for evaluating in-place test re ults are revie\ ed in the following section . The fir t two methods are irnilar and are ba ed on the idea of tati tical tolerance factors. The e two method are simple to u e, requiring only tabulated tati tical factor and a calculator. Becau e of their underlying a umptions, howe er, the tati tical rigor of these method bas been questioned. As a result, more rigorous methods have been proposed. The rigorous method are more complex and require an electronic pread-heet or computer program for practical implementation.

7.2-Statistical methods 7 .2. 1 Danish method (Bickley 1 982h) Thi method ha

been de eloped for analysis of pullout test results. The pullout strengths obtained from the field test are convert d to equivalent compre sive trength by mean of the trength relationship (correlation equation) determined by regre -sion analysis of pre iously generated data for the particular concrete being used. The standard deviation of the converted data i then calculated. The tenth-percentile compre -sive strength of the concrete is obtained by subtracting the product of the standard deviation and a statistical factor K

(which varie with the number ofte ts made and the de ired level of confidence, p) from the mean of the converted data. Although Bickley ( 1 982b) did not state it explicitly, the stati tical factor is a one-sided tolerance factor (Natrclla 1 963 ), a dis u ed further in 7.2.2. The K factor for different number of te t and a p 75 percent are given in

olwnn 2 ofT able 7.2. l a. The example in Table 7.2 . 1 b i l lu -trates how the Danish method is applied. The first column shows the equivalent compressive strengths corresponding to the I 0 individual pullout te t re ults. The econd column hows the values and calculation used to obtain the tenth

percentile strength at p - 75 percent. The example use 1 0 te t rc ults, but another appropriate number may be used in larger placements.

7.2.2 General tolerance factor method (l lindo and Bergstrom 1 9 5)-The acceptance criteria for trength of concrete cylinder in ACl 2 1 4R are ba ed on the a umption that the probability of obtaining a test with strength less than .fc' i le s than approximately I 0 percent. suggested method for evaluating in-place te ts of concrete is to determine the lower tenth percentile of trength, with a pre cribed confidence level.

I t has been e tabli hed that the variation of cylinder compressive strength can be modeled by the normal or the lognormal distribution function, depending on the degree of quality control. In ca e of excellent quality control, the di tribution of compre i e trength re ult i better modeled by the normal di tribution; in ca es of poor control, it is better modeled by a lognonnal distribution ( H indo and Bergstrom 1 985).

In the general tolerance factor method, the lower tenthpercentile compre sive strength Y0. 10, is estimated from

Table 7.2.1 a-One-sided tolerance factor for 10 percent defective level (Natrella 1963)

umber of Confidence level

tests 11 75% 90% 95%

Column I olumn 2 olumn 3 Column 4 1--- -

2. 0 1 4.2 6. 1 5 1-- -1- f -I- -4 2. 1 34 3. 1 7 4. 1 63 1-- -1- -f-5 1 .96 1 2.742 3.407

� --

1 .860 -- -

6 2.494 3.006

7 1 .791 t 2.333 • . 155 - -- -

1 .740 2.2 1 9 2.582

9 1 .702 I 2. 1 33 2.454

1 0 1 .67 1 I 2.065 2.355

I I 1 .646 I 2.0 1 2 2.275

1 2 1 .624 I 1 .966 2.2 1 0 - -- -

1 3 ··�t 1 .92 2. 1 55 - -- -- -

1 4 1 .591 I . 95 2. 1 08 f- -f-- -t- -l

1 5 1 .577 1 .866 2.068

20 1 .528 1 .765 1 .926

25 1 .496 I 1 .702 1 .838

30 1 .475 + 1 .657 1 .778 -· -

35 1 .458 1 .623 1 .732

40 1 .445 I 1 .59 1 .697

50 1 .426 I 1 . 560 1 .646

in-place test results by con idering quality control, number of tc ts n, and the de ir d confidence lc cl p for the c timatcd trength. Three quality control level arc con idcrcd: excellent, average, and poor, with the di tribution function of trength as umed a normal, mixed normal-lognormal, and lognormal, respecti ely. uggested alue of p are 75 percent for ordinary structures, 90 percent for very important buildings, and 95 percent for crucial parts of nuclear power plants ( H indo and Bergstrom 1 985). Thus by selecting a different p-value the user can adjust the le el of conservatism in estimating in-place trength that is consi tent with the criticality of the project.

The tolerance factor K, the sample a crage Y, and tandard de iation r arc u cd to e tablish a lower tolerance limitthat is, the lower tenth-percentile trcngtb . For a normal distribution function, the estimate of the tenth-percentile strength Yo.to can be determined a follows

Yo. to = Y - Ksy (7.2.2a)

where Yo. 1 o is lower tenth-percentile of strength ( I 0 percent defective); Y i ample average trcngth; K is one- idcd tolerance factor (Table 7.2. l a); and sy i ample tandard de iation.

The tolerance factor i determined from tati tical cbaracteri tics of the normal probabil ity di tribution and depend on th number of test n, the confidcn e level p, and the



Table 7.2.1 b-Example of Danish method

Individual equivalent compressive slTenglh. MPa (psi)•

27.5 (3990) 25.0 (3620) 24. (3550) 25.0 (3620) 22.5 (3260) 24.0 (34 0) 25.5 (3700) 28.5 (4 1 30) 25.0 (3620) 30.0 (4350)

alculalions

Mean Y = 25.7 MPa (3730 psi)

landard dcvtalion s1 = 2.3 MPa (330 psi)

K - 1 .67 1t Tcnlb-pcr clllilc

lrcnglh Y Kst· = 2 1 .9 MPa ( 3 1 0 psi)

"Convened from pullout force measurements using trength relarionship

'The •alues of the constant K for the 75 percent confidence level are given in Column 2 of Table 6. 1 .2a.

defect percentage. Value of K are found in reference book such a that by atrella ( 1 963). Table 7.2. 1 a pro ide onesided tolerance factors for confidence levels of 75, 90, and 95 percent and a defect le el of I 0 percent.

For the lognormal distribution, Y0. 10 can be calculated in the arne manner; use the average and tandard deviation of the logarithm of trength in q. (7.2 .2a).

By dividing both ide of Eq. (7.2.2a) by the average trcngth Y, the fol lowing i obtained

(7.2.2b)

where Vy is coefficient of ariation (cxpres cd a a decimal). In Eq. (7.2.2b), the tenth-percentile trcngth i exprc ed

a a fraction of the a erage trength. F igure 7.2.2 i a plot of Eq. (7 .2.2b) for p = 75 percent and for coefficient of variation of 5, I 0 1 5, and 20 percent. Thi figure how that a the ariability of the te t re ult increa es or as fewer test are performed, the tenth-percentile strength is a maller fraction of the a erage strength.

The tolerance factor method i imilar to the Dani h method. The re ults of the in-place tests are converted to equivalent compressive strengths using the strength relationship and the equivalent compres ive trcngth are used to compute the ample average and tandard de iation.

The example in Table 7.2.2 i l lustrate the application of the tolerance factor method for probe-penetration te t . The que tion in the example i whether the in-place strength of concrete in a lab is ufficient for the application of posttensioning, if the compressive trength requirement for postten ioning i 20 MPa (2900 p i) . The numbers in the fir t column are the mea ured expo ed length of each of eight probes, and the econd column gi es the corresponding compressive strengths ba ed on the previou Iy e tabli hed strength relationship for the concrete being evaluated. For eight te t andp = 75 percent, the tolerance factor i 1 .74. It i a umed that the nonnal di tribution de cribe the variation

0.9

0.8 y __.£..!£.

y 0.7

0.6

0.5

0 .4 0

; cv=o os

·- - · · · · · · :· · - - · · · · · · ·t · · · · - - · · · ·1· · · · · · · · · · -�·cv�ii -;ci·

: . . . . . . . . . . t- - - - - - - - ·+ · - - - - - - - - -l- - - - - - - - ---�cv=o 1s

5 10 15 20 Number of Tests

: cv=o 20

25 30

Fig. 7.2.--Ratio of tenth-percentile trength to average strength a a fim lion of coefficient of variation and number of te f (normal distribution as wned).

of concrete strength. Thu , by ub tituting the cocffi icnt of variation and the tolerance factor into q. (7.2.2b), the ratio of Y0. 1 0 to the average trength i 0 . 3 . Therefore, Y0 10 i I .6 M Pa (2700 p i) . Becau e the tenth-percentile trength i Je than 20 MPa (2900 p i ), po t-ten ioning hould not be applied. Thu , additional curing time i needed. Refer to 4. I for a di cu ion of data interpretation for tructure under con !ruction.

7 .2.3 Rigorous method (Stone and Ree1·e 19 6)-The pre eding method con crt each in-place te t re ult to an equivalent compres ive strength alue by mean of trength relation hip. The average and tandard deviation of the equivalent compre i e trength are u ed to compute the tenth-percentile in-place slTength. Two major objection have been raised to the e method ( tone el al. 1 9 6· tone and Ree e 1 986):

(a) trenglh relation hip i presumed to have no error (b) Variability o f the compre i e trength in the tructure

i a umed to be equal to the ariability of the in-place tc 1

rc ult The fir t factor will make the e timate of in-pia e tenth

percentile trength uncon er alive wherea the econd fa tor will make the e timale over- on ervative.

tone and Reeve ( 1 9 6) de eloped a comprehen i e technique for tali tical analy i of in-place te I re ult that attempted to address the perceived deficiencie of the tolerance factor method . Only a general ummary of the method

i given herein. Thi rigorou method encompa e the following procedure :

( I ) Regrc ion analy i to c labli h the lrcngth relation hip

(2) E timating the variabil ity of the in-place com pre i e trength ba ed on the re ult of the correlation le 1 and te t

on the tructure (3) alculating the probability di tribution of the e ti

mated in-place, tenth-percentile strength (4) For the rea ons gi en in 5 .2.4, logarithm of the te I

result are used in the analy i , and lrength relation hip i a umed to be a power function. Regre ion analy is i



Table 7.2.2-Example of general tolerance factor method

trength relationship:

Y (MPa) = -I + 0.69L (mm) ( Y (psi) = -1 45 + 2540L [in.) )

Exposed length L. mm ( in.) Compressive strength Y. M Pa (psi)

30 ( 1 . 1 )

* 1 9.7 (2850)

3 - ( 1 .3 ) 23.2 (3360) -34 ( 1 .34) 22.5 (3260) -35 ( 1 .3 ) 23.2 (3360) -38 ( 1 .50) 25.2 (3660)

36 ( 1 .42) 23.9 (3460)

3 1 ( 1 .22) 20.3 (2950)

30 ( 1 . 1 8) 1 9.7 (2850)

Mean ( Y) = 22.2 M Pa (3220 psi ). tandard deviation (st) - 2. 1 MPa (300 psi).

Coefficient of variation CVt) = 9.3 percent.

For 11 = 8 and 75% confidence level: K = I. 74.

Y0 10 = ( 1 - KV1) Y = ( I - 1 .74 x 0.093) x 22.2 = 1 8.6 MPa (2700 psi).

performed using the Mandel procedure discussed in 5 .2 .4 and A.2. The errors associated with the best-fit strength relationship arc used to estimate the in-place tenth-percentile strength at any desired confidence level.

A novelty of the rigorou method is the approach u cd to c timatc the variability of the in-place compressive trcngth. In haptcr 4 it i shown that the single-operator ariabi l ity of in-place test result is generally greater than compressivetest rc ult , which i why objection ha c been raised against

a surning that the ariabil ity of the in-place compr ssi c strength equals the variabil ity of the in-place tc t result .

ln the rigorous mctl10d, it i as umed that the variabil ity of compressive strength divided by the variabil ity of the in-place test results i a constant. Thus, the ratio obtained during correlation testing is assumed to be valid for the test conducted in the field. This provides a mean for e timating the variability of the in-place comprcs ivc trengtb based on the results of the in-place te ts (7.2.4).

The in-place tenth-percentile strength computed by the rigorou procedure account for the error as ociatcd with the tr ngth relationship. The user can determine th tenthpercentile strength at any desired confidence level for a particular group of field test results. In addition, the user can choose the percentile to be a value other than the tenth percentile.

tone ct a!. ( 1 9 6) computed the tenth-percentile strengths by the rigorous method and compared them with those computed by the Oani h and tolerance factor method . The c calculations u cd imulatcd in-place test data having different mean value and standard de iations. lt was found that for an assumed confidence lc el, the strengths estimated by the Danish and tolerance factor methods were lower than the vaJucs ba cd on the rigorous method. The difference were a high as 40 percent when the in-place tests had high variability (coefficient of variation = 20 percent). Compared with the rigorou method, the Danish and tolerance factor

methods give more conservative estimates of in-place compressive strength, but they do not appear to provide a consistent confidence level. One reason for the inconsistency of the tolerance factor method is the assumption that the variability of the in-place compressive trength i the same as the ariabi l ity of the in-place test results. Experimental field tudie arc needed to compare the in-place, tenth-percentile trength e timated by the e method with the value obtained from many core te t . Only then can the reliability of these methods be e aluated.

7.2.4 Alternative method (Carino 1 993)-The rigorous method developed by tone and Reeve ( 1 986) has not received widespread acceptance among concrete technologists because of its complexity. arino ( 1 993) proposed

an altemative method that retain the main feature of the rigorous method but can be implemented easily with spread-

heel oftware. TI1e basic approach of the alternative method i i l lustrated

in Fig. 7.2.4. The Mandel procedure (a outl ined in .2) i u ed to obtain the trength relation hip from correlation data. The result of the in-place test and the strength relat ionship are u ed to compute the lower confidence limit of the estimated average in-place sn·engtb at a desired confidence level. Finally the tenth-percentile strength is determined assuming a lognonnal distribution for the in-place concrete strength. alculations are performed u ing naturallogarithm value .

In the following paragraphs the procedure for estimating the in-place strength is explained further. When the in-place

trength is to be estimated, repl icate test are performed on the structure. The average of the logarithm of the in-place

te t i u ed to compute the logarithm of the average in-place compressive strength using the strength relationship

Y = a + bX (7.2.4a)

where Y is logarithm of the e timated average in-place com pres ive strength; X is average of the logarithm of the in-place test performed on the structure· and a,b are intercept and slope of the trength relationship.

Next, the lower confidence l imit for the e t imated average trength i computed. Thi lower l imit i obtained u ing Eq. A.3) for the tandard deviation sr of an stimated alue of

Y for a new X. The lower confidence limit for the average concrete strength is as follows

(7.2.4b)

where Y1,"' i lower confidence l imit at confidence level a; 1111. La is tudent's /-value for m- 1 degree of freedom and confidence level a; and 111 is the number of replicate in-place te ts.

Table 7 .2.4 li t Student t- alue for m - 1 degrees of freedom and ri k (or confidence) levels of 5 and 1 0 percent. The choice of risk level depends on the criticality of in-place concrete strength in the overall assessment. When strength is

critical, a lower risk level such as 5 percent, should be used.



..... Q.) "0 c >.

() -0

E .I::. ...... ·;:: ca C) 0

....J

/ /

/ /

/

4./ e/

/

,(._ Lower �� Confidence / Limit (Y�ow)

Average

Logarithm of In-Place Test Result Fig. 7.2. 4-A/ternative method to estimate compressive strength ba ed on in-place te t (Carino 1 993).

The di tribution of in-place compres i e strength i described by a lognonnal di tribution, and the tenth-percentile trength i computed as fol lows

(7.2.4c)

where Y0_10 i logarithm of trength expected to be exceeded by 90 percent of the population; and s,r i tandard deviation

of the logarithms of concrete strength in the structure. The value of Scf i obtained from the a sumption ( tone

and Reeve 1 986) that the ratio of the tandard deviation of compressive strength to the standard deviation of in-place te t re ult has the same value in the field a " as obtained during the laboratory correlation te ling. Thus, the following relation hip is assumed

(7.2.4d)

where cfi c1 are standard deviation oflogarithm of compre -sive strength in the structure and laboratory re pectively; and sx, il are tandard deviation of logarithm of the in-place results in the structure and laboratory, re pectively.

The final step i to convert the re ult obtained from Eq. (7.2.4c) into real unit by taking the antilogarithm.

A c lose examination of the alternative procedure hows that the average compressive strength estimated by the strength relation hip ( q. (7.2.4a)) i reduced by two factors. The fir t factor, which is given by Eq. (7.2.4b), accounts for the uncertainty of the strength relationship and the uncertainty of the average of the in-place test result . The second factor which i given by Eq. (7.2.4c) accounts for the ariability of the in-place compres ive strength. Thus it is believed that the alternative procedure strikes a balance between stati -tical rigor and practicality of u e. mentioned, the procedure is well uited for implementation using a computerized

Table 7.2.4-Student's t-values for m-1 degrees of freedom and risk levels of 0.05 and 0.10 (Natrella 1 963)

m-1 Too� To 10

2 2.920 L886

3 2.353 1 .638 - -- -4 2. 1 32 1 .533 - -5 2.0 1 5 1 .476 - --- -6 1 .943 1 .440 - --- - -- -7 1 .895 1 .4 1 5

8 1 .860 1 .397

9 1 .833 1 .383

10 I. 1 2 1 .372 - -I I 1 .796 1 .363

1 2 1 .782 1 .356

1 3 L771 1 .350

1 4 1 .76 1 1 .345 - --- - -- -1 5 1 .753 I. 4 1 - --- -16 1 .746 1 .337 -1 7 1 .740 1 .333 - -1 8 1 .734 1 .330

1 9 L729 1 .328

preadsheet or a pecial ized computer program ( hang and mino 1 99 ). ection A of ppendix gives examples

that compare the e timated in-place trength using the tolerance factor and alternative method .

7.2.5 umma With the exception of ca t-in-place cylinder test , in-place test provide indirect mea urc of concrete strength. To arrive at a reliable e timate of the in-place strength, the uncertaintie involved in the e timat mu t be con idered. Thi ection ha di cu ed orne techniques de eloped for thi purpo e. The tolerance factor methods discus ed in 7.2. 1 and 7.2.2 ha e been used succes -ful ly in the analysis of pullout test data. Therefore, they may be adequate for test methods that ha e good correlation with compressive strength, uch as the pullout te t.

The tolerance factor methods however do not account for the main source of uncertainty in a rational way. This has led to the development of more rigorou procedures a di cus cd in 7.2.3 and 7.2.4. The c new methods arc designed to pro ide reliable c timatcs of in-place trcngth for any tc 1 procedure. The c rigorou method , how vcr, should be incorporated into ca y-to-usc computer program for practical usc.

7.3-Reporting results Report forn1 for the different tc ts and different purpo c

will vary. A variety of report form arc appropriate. U ually relevant TM tandard de cribc the information required on a r port. Where in-place tc ring i made at early age ,

omc pccific reporting data arc de irablc. ct of fonn , imilar to those developed by an engineer for u c in pullout

testing, i shown in Fig. 7.3a to 7.3c. These can serve a



A 8 c D E F G H J

@) @ @) Pour #2

® ® t : @)@)@ ® r- T :J @ @T 9 W @ ® ® : �

K

2

3

Fig. 7.3a-Example of form used to identifY locations of in-place tests in a floor slab ofmultistOiy building.

ABC TESTING COMPANY

Field Record of In-Place Testing

Test Test Estimated Number result

compressove Project Number strength Project Name

1 Location in

2 structure

3 4 Placement

5 Date

6 Time Size

7 Mix No 8

9 Curing Conditions 10

1 1 Maturity __ •c-h

12 Temperature at tesL

13 Ambient __ ·c 14 Within enclosure __ ·c 1 5 Appearance of top surface:

Calculations: Number of tests: ___ Remarks:

Esttmated strength: Mean:

Standard devtation: K-Value:

Technooan Mimmum Strength: Cl"oecl<ed by

Mean - (K • sd): lnsuument number

Fig. 7.3b-Sampleformfor on- ite recording a_( in-place te I results.

useful models for developing fom1s to report the re ults of other in-place tests.

Briefly, the three form provide for the fol lowing: (a) Record of test locations (Fig. 7.3a)-Thi form gives a

plan vic\ of a typical floor in a specific multi tory building. TI1e location of each test is noted. The location of maturity meter , if installed, can also be hown. Location data arc important in case of low or ariablc results. Where test arc

ABC TESTING COMPANY LETTERHEAD

l'rojcct 1\o. ____ _ licnt: ddress _____ _

Repon o. __ _ (in sequence)

Tc�tmg of In· PI cc tn:nglh Pr �e t , me: ____ _

Anent ion: ddrc

Dear ir: The fol lowing an: th results of in-place tests of the above site.

MPa concrete at

Locauoo in structure: ______ _ Individual tests n:o.ults

(MPo) Dme Time

Pour.

Proposed time of fonn removal

Test R�ul!li Summary Number of tests made.

Mean in-pi:JCC-strcnl!.th (MPa) Standard dcviation.!MI'a):

Minomum in-place stren!!,th (MPa):

Remar� Requorements of __ MPa mean and __ IPa mmimum strength before stripping and reshoring nrelare n 1 met b the abo\e rc ul�.

Your5 vel) trul). Copy gn·en to srte upermtendcnt Dtttc: Time:

igned ----:�-..,.--,---(}11' f4'VIJ« C'fAJ

(/r1 Conlmdor-; --

Fig. 7.3 ample form for reporting in-place test results.

made at very early ages and the time to complete a placement is long, there could be a significant age-strength ariation from start to finish of the placement.

(b) Record of field-test result. (Fig. 7.3b)-Thi i the form on which test data, the calculated re ults, and other pertinent data are recorded at the site. The form shm n in Fig. 7 .3b has been designed for evaluating the data with the Danish or tolerance-factor methods (minimum strength is the tenth-percentile strength). It includes pro isions for entering information on maturity data, protection details and concrete appearance to corroborate the te t data during cold weather. Due to the critical nature of formwork remo al, a recommended procedure is for the field technician to phone the data to a control office and obtain confirmation of the calculation before gi ing th re ult to the contractor.

(c) Report of test result. (Fig. 7.3c)-This fom1 i u ed to report the in-place test results. The example shown in Fig. 7.3c is a multicolor, self-carbon form designed to be completed at the site by the technician, with copies given to the contractor's and structural engineer's representatives when the results have been checked. lt provides for identification of the placement involved, the individual results, and the calculated mean and minimum strengths. It record the engineer's requirements for form removal and states if the c requirements have been met. It requires the contractor's representative signature on the testing company's copy.



CHAPTER 8-IN-PLACE TESTS FOR ACCEPTANCE OF CONCRETE IN NEW CONSTRUCTION

8.1 -General Traditionally, acceptance te ting for new construction has

been limited to judging acceptabi lity of the concrete delivered to the project on the ba i of slump air content, and compressive strength. cceptable con rete that i placed con ol idated, and cured ac ording to standard of good practice wil l perfonn according to de ign as umption . Exceptions occur when there is clear evidence of inadequate consolidation or distress, such as cold joints and excessive cracking, or when inadequate protection was provided in cold weather.

The durabil ity of exposed structure depend trongly on the curing hi tory of the concrete. Therefore, it is desirable to have a surance that the concrete in the fini hed tructure ha the neces ary propertie to attain the de ired level of performance. in-place t sting offer the opportunity to obtain thi a urance \ hen u ed a a component in a comprehen ive quality a urance program. The Great Belt L ink project in Denmark is one of the first large-scale construction projects in which the owners relied on in-place testing (pullout tests) instead of tandard laboratory trength tests to assess the acceptabil ity of the concrete layer protecting the reinforcement (Vincent en and l lenrik. en 1 992). This major construction effort erve a a model for future project where in-place quality assurance is important.

In orth America, there i a reluctance to abandon traditional acceptance procedures that ha e erved their purpose. In-place te ting however, offers th opportunity to le n the reliance on te ting of tandard-cured cylinder a the ole method to judge acceptabil ity of concrete delivered to the site. The added benefit of in-place testing is that it provides assurance that the finished construction has the propertie specified by the designer. This chapter di cu se the potential for in-place te ting a an alternative tool for acceptance te ting.

8.2-Acceptance criteria The following reviews the current acceptance criteria in

ACI 3 1 8 and propo es how in-place testing may be u ed a an altemati e to te ting standard-cured cylinder in new

sional might require le ting of field-cured cylinders to check the adequacy of curing and protection of the concrete in the tructure. The acceptabi l ity of curing and protection, a indicated by the field-cured cyl inder strength , is defined in

ection 26.5 . . 2(e)

"Procedures for protecting and curing concrete shall be con idcrcd adequate i f ( l ) and (2) arc sati ficd:

( I ) erage strength of field-cured cylinder at test age designated for determination of .fc' i equal to or at lea t 5 percent of that of companion tandard-cured cylinder

(2) verage strength of field-cured cylinder at test age exceeds.fc' by more than 3.5 M Pa (500 psi)."

8.2.2 ores-In the event that a strength te t of tandardcured cylinders i more than 3.5 MPa (500 p i) below /. ', A I 3 1 - 1 4 requires step. be taken to ensure adequacy of the tructure. ore may have to be drilled to verify the in-place trength. Three core are required for each strength test fai l ing to meet the specified criteria. In judging the acceptabil ity of the core trength , ection 26. 1 2.4. 1 (d) of ACI 3 1 8- 1 4 tate the fol lowing:

" oncrete in an area represented by core te t shall be considered structurally adequate if ( I ) and (2) are satisfied:

( I ) The average of three core i equal to at lea t 5 percent of .fc'

(2) o single core i les than 75 percent off..'." 8.2.3 In-place tests-Based on the aforementioned require

ment for judging the acceptabil ity of in-place concrete in ne\ con truction based on core strengths, the following a ceptance criteria based on in-place testing are propo ed:

The concrete in a tmcture is acceptable if: I ) the estimated average, in-pia e, compre i e trength ba ed on an ASTM tandard in-place te t procedure equal at lea t 5

percent ofj..'; and 2) no te t result estimates the compre i e strength to be less than 75 percent of.fc'.

In this ca e, an estimate of in-place strength at a test loca

tion is the value of Y1.,w computed by Eq. (7.2.4b), and the e timated average in-place strength i the average value of Yt011 • Before these criteria can be put into effect, however, standard practices need to be adopted for electing the number of in-place tests to be done at one location and for stati tical analy i of in-place te t data.

con truction. 8.3-Early-age testing 8.2. 1 Molded cylinders-According to Cl 3 1 M- 1 4 The primary rea on for using in-place te ts in new

I 3 1 - 1 4), the evaluation and acceptance of concrete con truction is to determine \ hether it i afe to perform are based on tests of cylinder molded at the job site and critical operations, such as form removal or post-tensioning. subjected to standard laboratory curing in accordance with The in-place tests provide e timates of compressi e strength A TM 3 1 /C3 1 M. ection 26. 1 2.3(b) of I 3 1 M- 1 4 at ages that are usually much earlier than the age for attaining (A I 3 1 - 1 4) tates a follows: the pecified strength. The criterion frequently u ed to judge

"Strength level of a concrete mixture hall be acceptable if the acceptabil ity of early-age strengths to permit critical ( I ) and (2) are satisfied: con truction operation uch as form work remo al, is that

( 1 ) Every arithmetic average of any three con ecuti e the e t imated in-place compre ive trength hould be at trength te t equal or exceed.fc' lea t 75 percent of.fc'. In thi ca e th e timated lower tenth-

(2) o trength te t fal l below .fc' by more than 3.5 M Pa percenti le in-place strength, Y0. 10• hould be compared to the (500 psi) i f.fc' is 35 M Pa 5000 psi ) or les ; or by more than required strength. When such a requirement is specified, 0. 1 O.fc' if.fc' exceed 35 M Pa (5000 p i ).' early-age te ting may faci l itate final acceptance of concrete.

In addition, according to 26.5.3 .2(d) of I 3 1 M- 1 4 In high-rise con !ruction, economic factors result i n accei-(A 1 3 1 8- 1 4), the building official or l icen ed de ign profe - crated schedules in which critical operation may be planned

(aCii American Concrete Institute - Copyrighted © Material - www.concrete.org


Table 8.3-Results of standard-cured cyl inder and in-place tests at 28 days (fc' = 30 M Pa [4350 psi])

Pullout tests

4

34.4t {4990)

2.7t (390)

Project I

{ 630)

3.9 {570)

Pullout tests

1 5

35.91 {52 1 0)

2.71 (390)

Project 2

3 .2 {5540)

3.5 ( 5 1 0)

Mean trength. MPa (psi)

tandard deviation . M Pa {p t) Range. MPa (psi) 30.5 to 44.5 (4420 to 6450) 29.9 to 40.5 (4340 to 6920) 32.5 to 40.51 (471 0 to 5 70) 30.9 to 43.5 {44 0 to 63 1 0)

Expected percentage of results

below/,.'

ctual percentage of results I below/,.'

l .63s

4.9

"A resuh i the a•ernge of two-cylinder test or the average of two or more pullout tests

2.23s 2. 1 . 2 .34s

1 .2 1 .4

1 .2 one one

'Mean and standard deviat ion of estimated compres ive trength based on strength relationship.

as early as I to day after concrete placement. To meet the early-age strength requirement , the contractor may choose to u e a concrete mixture that ill exceed the specified design strength. Experience has shown that requiring a minimum trength of 75 percent of .fc' at early age ( I to 3 day ) wil l u ually en ure that the in-place trength will be at lea t/,.' at 2 day , i f proper curing is u ed and the pecification do not al low mixtures that achieve all their strength gain at the time of form removal .

For example, for a specified design strength of 2 M Pa (4000 psi), the in-place trength to permit form removal

may have to be at least 2 1 M Pa (3000 p i) . A II owing for the inherent ariation of concrete strength the a erage in-place strength may ha e to be 25.5 M Pa (3700 p i) to ensure that the early-age trength criterion is ati fied. In this example, the a erage early-age, concrete trength ha to equal 93 percent of the pecified trength. Therefore, it i rea onable to a ume that if the early-age ( 1 - to 3-day trength requirement is satisfied, then at 2 day the pecified de ign strength wi l l undoubtedly be achieved. For additional assurance, in-place tests can be made on the structure at 2 day .

Bickley ( 1 9 4) reported on two demonstration projects where in-place testing was u ed for early-age strength determination of horizontal elements, as wel l as for confirmation of the 2 -day design strength. Permi sion to waive standard cylinder testing wa obtained from the building official. Innovati e project pecification defined the frequency of in-place te t and the procedure to follow in doing the test and reporting the re ults. Acceptance of the concrete ' as based on the results of pullout tests performed on the structure at 2 days. For comparison, tandard-cured cylinders were also tested at 2 day , but these strengths were not reported. Table .3 ummarizes the result . The specified design strength for both project wa 30 M Pa (4350 psi ). Individual pullout te t result were converted to com pres i e strengths ba ed on the trength relation hips, and the e estimated trength were used to compute th tat1 t 1c hown in the e ond and fourth columns of the table. Ba ed on the standard deviation , the expected percentage of trength below .f/ were computed. In all ca es, these percentage were les than I 0 percent, which i the approximate alue impl ied

in A I 1 8 . For both project , the in-place te t results clearly howed that the concrete had acceptable trength.

Tn conclusion, current legal contract for the ale and purcha e of ready mixed concrete are usually ba ed on the 28-day trength of tandard-cur d cylinder . For the time being, therefore, the e cylinde have to be ca t . When in-place test are made at an early age, however, the acceptabi l ity of the concrete can be as e ed at that time. If the concrete is ati factory, there i no need to te t the tandard cylinder . I f the early in-place te ts indicate a problem with concrete in a particular placement, the related standard cylinder are available for testing.

CHAPTER 9-REFERENCES A T committee document and document publi hed by

ther organization are li ted fir t by d cument number full t i tle, and year of publication followed by authored document l i ted alphabetically.

American oncrete Institute (A I) A I 2 1 4R- l l-Guide to Evaluation of trength Te t

Results of oncrete A T 2 1 4.4R- l 0-Guide for Obtaining ore and Inter

preting ompres ivc trength Re ults A T 228.2R- I -Report on Nondestructi c Tc t Method

for E aluation of oncrerc in rructure A I 30 1 - 1 6- pecification for Structural on rete A I 306R- 1 6-Guide to old Weather Concreting A I 308R- 1 6- uide to Extemal uring o f oncrete A I 30 . I - l l- pecification for uring oncrete A I 3 1 8- 1 4-Building ode Requirement for tructural

oncrete and ommentary A T 3 1 8M - 1 4-Building ode Requirements for true

rural oncrete and ommentary ( Metric) A T 325 .9R- 1 5-Guidc for onstruction of oncrete

Pa cments A T 325. 1 1 R-O l - ccelerated Techniques for oncrcte

Pa ing A T 437R-03- trength Evaluation of Exi ting oncrete

Building



ACI 562- 1 Code Requirements for Assessment, Repair, and Rehabil i tation of Existing Concrete tructures and Commentary

A TM lmemational A TM 3 1 /C3 I M- 1 8- tandard Practice for Making

and uring Concrete Test pecimens in the Field A TM C39/39M- 1 8-Standard Te t Method for ompre -

sive trength of Cylindrical Concrete pe imen A TM C42/ 42M- I tandard Te t Method of Obtaining

and Te ting Drilled ores and awed Beams of oncrete A TM 94/ 94M- 1 7- tandard Specification for

tandard Practice for Making and uring oncrete Test pecimen in the Laboratory

A TM CS I I - 1 3- tandard pecification for Mixing Room, Moi t abinets, Moi t Rooms, and Water torage Tanks Used in the Testing of Hydraulic Cements and

oncrete A TM 597- 1 tandard Te t Method for Pul e

elocity Through Concrete A TM 803/ 03M- 1 7- tandard Test Method for

Penetration Re istance of Hardened oncrete A TM C 05/C805M- 1 3- tandard Test Method for

Rebound umber of Hardened oncrete A TM 23/C823M- 1 2(20 1 7)- tandard Practice

for Examination and ampling of Hardened oncrele in on truction A TM 873/ 873M- 1 5 tandard Test Method for

Compres ive Strength of Concrete Cylinder ast in Place in Cylindrical Mold

A TM 900- 1 5- tandard Test Method for Pullout trength of Hardened on crete A TM 9 1 8/ 9 1 8M- 1 3- tandard Test Method for

Mea uring arly- ge ompre sive trength and Projecting Later-Age trength

A TM I 074- 1 7- tandard Practice for E t imating oncrete trength by the Maturity Method A TM C 1 5 3/ 1 583M- 1 3 tandard Tc t Method for

Tensile trength of oncrete urfaces and the Bond trengtb or Ten i le Strength of Concrete Repair and Overlay Materials by Direct Tension ( Pull-off Method)

A TM E I 05- 1 tandard Practice for Probabi l ity amp l ing of Materials A TM 1 22- 1 7- tandard Practice for Calculating

ample ize to Estimate, With pecified Precision, the Average for a haracteristic of a Lot or Proce s

A TM E 1 7 - I tandard Practice for Deal ing with Outlying Ob ervations

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APPENDIX A

A.1-Minimum number of strength levels The minimum number of trength levels required to

develop the strength relation hip depends on tatistical con ideration and cost. To gain ome insight it is useful to examine how the confidence interval for an e t imate obtained from a strength relationship is affected by the nwnb r of point u ed to e tablish that relation hip (Carino 1 993 ). Because the trength relation hip i u ed to e t imate compressive strength from in-place te t results, compressive strength i treated as the dependent variable ( Y value) and the in-place result as the independent variable (X value).

The residual standard deviation, also identified as standard error of e timate, i the basic parameter used to quantify the uncertainty of a best-fit strength relationship for a given set of data. For a linear relation hip, an estimate of the re idual standard de iation is as fol lows

s = �:r.(d,.Y • - 2 (A. l a)

where S, is estimated residual standard deviation; d,x is deviation of each test point from the best-fit l ine; and is number of te t points u ed to establish the trength relationship.

When the strength relationship i used to e t imate the mean value of Y at a new value of X, the width of the confidence interval for the mean is related to the residual standard deviation by the fol lowing expression ( atrella 1 963 : Snedecor and Co hran 1 967)

{A. I b)

where W is width of the I 00( 1 - a) percent confidence interval for the estimated mean value of Y for the value X t,v.2.a.1 is tudent's /-value for -2 degree of freedom and significance le el a; X i the average of X values u ed to develop strength relation hip; and S,,_, is um of quare of deviation about X of the X values used to develop the strength relationship Sxx - L(X X )2•

trietly peaking, Eq. (A. I b) i applicable 011ly if the a umption of ordinary lea 1- quare analy is are atisfied. It is used here to demon trate, in a implified way, the effect of the nwnber of test points on the width of the confidence interval. In practice, Eq. (A.3) hould be used to determine the lower confidence limit of the estimated mean value of Y for a new value of X.

The econd term under the square root sign in Eq. (A. I a) show that the width of the confidence interval increase a the distance between X and X increases. This means that the uncertainty of the e t imated trength is greater at the extreme l imits of the trength relationship than at its center.

To examine how the width of the confidence interval i affected by the number of test point , con ider the case where X = X o that the second term under the square root sign in

ro 2: -2 W(X) E S � c Q) "'0 1;:: c 0 ()

e

10 -.---,:--�--�------,

Recommended • . Range . 5 - . . . . . . . . . . . . . . . l"Lt · · 1· · · · · · · · . . · · · · · : . . . . . . . . . . . . . . .

� : : ' 0 0 0 ' 0 ' 0 : . . : . . : : • • • ! • • • • 0 0 0 0

0 4-��-�.��+.-��+,·�� 0 5 1 0 1 5

Nu mber of Test Points, n 20

Fig. A. I-Effect of number of point u ed to establish strength relation hip on the confidence interval width (in terms of residual tandard deviation).

q. (A. l b) equal zero. The width of the confidence interval relative to the rc idual tandard deviation i as follows

W( X) = 2t (I S,

J\ -2.al2 'JN (A. l c )

Equati n ( . I c) i plotted in Fig. A . I to how how the idth of the 95 percent confidence interval (relati e to S,) i affected by the number of tc 1 poin u cd to e tabli h the trength relation hip. It i ccn that for fc\ tc t point ( ay lc than 5) by in luding an additional te t p int there i a ignificant reduction in the relati e width of the confidence interval. For many point , however the reduction obtained by u ing an additional te I point i smal l . Therefore, the appropriate number of strength levels is determined by con ideration of preci ion and co t. It has to be decided if it i worth the additional expen e to obtain additional preci ion by u ing another te t point. From Fig. . I it i rea onable to conclude that the minimum number ofte t point i about i , v hile more than nine tc 1 would probably not be ju tificd economically.

A.2-Regression analysis with X-error (Mandel 1 984)

I f th procedure in 7.2.3 or 7.2 .4 are to be u ed to e timate the in-place characteri tic trength, the lea t- quare regre -

ion analy i procedure to determine the trength relationship should account for error in the X- ariable. The method propo ed by Mandel ( 1 9 4) can be u ed for thi purpo e. Thi section pro ide a tep-by-step procedure for carrying out the Mandel method.

At each trcngth level for the correlation te t there arc n, replicate in-place tc t re uh and n_. replicate omprc i c te t rc uh . The number of trength le el i . The objective is to find the be t-fit alue of a and 8 (and their uncertaintie ) for the traight line, trength relation hip

In = a + 8 In! (A.2a)



where a is the intercept of straight line; B is the slope of straight line· InC i the natural logarithm of com pre sive strength; and In/ is the natural logarithm of the in-place test result.

After the correlation te t data have been obtained, the fol lowing sequence of calculations is u ed to establish the trength relation hip and its uncertainty:

I . Tran form the data by taking the natural logarithm of each te t re ult

x - lni (A.2b(a))

y = Inc (A.2b(b))

where i and c are the individual in-place and compre i e trength test results, respectively.

2. For each strength level j, compute the average and standard deviation of the logarithms of the in-place and compres ive te t result :

0 = the average of the logarithm of the in-place te ts at strength level j

>j = the average of the logarithms of the compre si e strength tests at trength levelj

s_., the standard deviation of the logarithms of the in-place test at trength levelj

s,i - the tandard de iation of the logarithms of the compressive strength test at strength levelj

3. a leu late (sx)1 and (s>)2, which are the average variances (squares of the standard deviations) of the logarithm of the in-place tests and of the compressive te ts re pectively.

2 E(s f )l ( ) ) = --· - (A.2c(a))

(A.2c{b))

quations (A.2c(a)) and ( .2c(b)) a ume that the same number of replicate tests are used at each trength levei. I f orne te t re ult have to be discarded because th y are outliers, the pooled variance hould be computed to accow1t for different numbers of replicate te t at each trengtb level (refer to tone and Reeve [ 1 986] or a textbook on introductory statistics).

4. ompute the value on .. a follow

(A.2d)

n,

\ here llx is the number of replicate in-place test at each strength level; and n, is the number of replicate com pres i e strength test at each strength level.

The numerator and denominator in q. (A.2d) are the variances of the average compres ive strength and in-place result , respectively. If there are different numbers of replicate test at each trength level the a erage numbers of replication hould be u ed for "·• and n_, ( tone and Reeve 1 9 6).

5. Find the values of b and k by sol ing the following simultaneous equations

(A.2e(a))

k = !!... A. (A.2e(b))

An iterative procedure can be used to solve for k and b ( Mandel 1 9 4). First, assume a value of k such as k = 0, and

ol e for b in Eq. (A.2e(a)). Using this value of b olve for a new value of k in Eq. ( .2e(b)). ubstitute the new value of k into q. (A.2e(a)) and solve for b. Repeat the procedure until the values of k and b converge, which will usually occur in les than five iteration .

In Eq. {A.2e(a)), the term S.u. s_,, , and S.ry are calculated according to the fol lowing

( .2f(a))

(A.2f(b))

( .2f(c))

The terms X and Y are the grand averages of the logarithm of the in-place and compressive treng1h te t results.

- LX X = --' (A.2g(a))

- EY Y = -' (A.2g(b))

6. The be t-fit e timate of B and a are as follow

B - b (A.2h(a))

a = Y - bX (A.2h(b))

7. Use the fol lowing steps to compute the tandard errors of the estimates of a and B.

(a) ompute these modified sums of quares

.,, = "' + 2k .n + � s,�

(b) Compute the following error of fit, s,

s = .

(c) The error in a is gi en by the following

( .2i(a))

( .2i(b))

(A.2j )



s. = s. (A.2k)

(d) The error in B is given by the fol lowing

a = {A.21)

In ummary, the fol lowing general tep are u ed to obtain the best-fit strength relation hip and account for the error in the X variable ( in-place test re ult ):

( I ) Transform the correlation data by taking their natural logarithms

(2) At each strength level, compute the average and standard deviation of the tran formed values ( logarithms)

(3) ompute the value of A. based on the average (or pooled) ariance of the mean compres ive and in-place results

(4) ompute the value of b and k ( 5 ) ompute the slope and intercept of the be t-fit

relation hip (6) ompute the error of the fit The error of the fit, s., is needed to calculate the uncer

tainty in the e t imated mean compressi e strength when the strength relationship is used with in-place tests of the structure. This is explained in the next section.

A.J-Standard deviation of estimated Y-value (Stone and Reeve 1 986)

The strength relation hip is used to estimate the in-place compre sive strength ba ed on the re ult of the in-place te t done on the tructure. Typical ly, everal in-pia e te t are done on the structw-e, the average re ult i computed, and the strength relationship is used to estimate the average compressive strength. To obtain a reliable estimate of the a erage strength-that is a value that has a high probabi l ity of being exceeded-the standard deviation of the estimate must be known.

The approach developed by Ylandel ( 1 9 4) can be u ed to e timate the standard deviation of an estimated value of Y (average compres ive strength) for a ne'V value of X(average in-place te t re ult ) when there i X-error. Mandel ' method wa modified by tone and R C\'e ( 1 9 6) o that it al o incorporate the uncertainty of the average in-place result from tests on the structure. This modification account for the fact that the uncertainty in the average of the in-place results is typically greater for te ts on the structure compared with that from the laboratory tests used to develop the trength relation hip. The tandard deviation of the e timated value of Y (average of the logarithm of compre sive trength) i obtained by the following equation

2 z� + bz _. (A.3)

111

where S! is the standard de iation of estimated alue of Y (average concrete strength); is number of point used to obtain the strength relationship; h i e timated slope of the strength relation hip; k = b!A., where A. is obtained from the single-operator variability during correlation te ting (Eq. ( .2d)) · X is the average of the logarithms of the in-place tests performed on the structure; X i grand average of X

alues during correlation te ts (Eq. (A.2g(a))); , i error of fit of treogth relation hip ( Eq. ( .2j )) : Suu i the modified sum of the quare a gi en by Eq. (A.2i(a))· sx i tandard deviation of the logarithms of the in-place test performed on the structure; and m is the number of replicate in-place test done on the tructure.

Equation (A.3) shows that there are two sources of the uncertainty in the e timated value of Y:

( I ) The uncet1ainty of the trength relationship (s,) (2) The uncertainty of the in-place test result obtained

from testing the structure (s..\) Because Eq. ( .3) i the sum of two variances, which

could have different degree of freedom, a fonnula ha been sugge ted for computing the effective degrees of freedom for sr ( tone and Reeve 1 986). For simplicity, it can be a sumed that there are (m- 1 ) degree of freedom as ociated with St, where m is the number of in-place tests done on the structure. These degree offreedom are used in choosing the t-value to calculate a lower confidence limit for the average

alue, as di cu sed in 7.2.4.

A.4-Example An example is presented to how the application of the

Mandel { 1 9 4) method and i l lu trate the e aluation of in-place te t u ing the tolerance factor method di cu ed in 7.2.2 and the alternati e method di cus ed in 7.2.4. The correlation data are taken from the study of the pullout test by tone et al. ( 1 9 6). The pullout test geometry had an apex angle of 70 degrees and the concrete was made u ing river-gravel aggregate. ight strength level were u ed to develop the strength relationship. At each strength level, I I replicate pullout te t and five replicate cyl inder compre -sive te ts were done. soft conver ion of the inch-pound

alue reported by Stone et al. ( 1 9 6) � as used to obtain the corre ponding I value .

The data from the cited reference were converted by taking the natural logarithm of the individual pul lout loads and compressive strengths. The average standard deviation, and variance (square of tandard deviation) of the transformed pul lout load at each strength level are hown in olumns I , 3 , and 4 of Table .4a. ( I and inch-pound

er ion of orne tables are pre ented in this ppendix to reduce clutter.) The a erage, standard deviation, and variance of the transformed compres ive strength at each trength level are hown in olumn 5 7 and . For infor

mation Column 2 and 6 gi e the a erage of the logarithm value tran formed into real units.

The average values in olwnns I and 5 of Table .4a were used to calculate the various parameter to establish the strength relationship according to the procedure in .2.



Table A.4a-Average, standard deviation, and variance of correlations data from Stone et al. (1 986)

Average lnPO 1 Real value of tandard ariance for PO. k (lb) PO, kN (lb) deviation lnPO In PO

I 2 3 4

2.26 9 (7 .6842) 9.67 (2 1 74) 0. 1 085 0.0 1 1

2.49 (7.9 1 3 ) 1 2. 1 6 (273 ) 0.0459 0.002 1 -2. 076 ( .22-9) 1 6.57 (3725) 0.0700 0.0049 -2.988 ( .4().10) 1 9.36 (4465) 0. 1 065 0.0 1 1 4

-3.2945 (8.7098) 26.97 (6062) 0. 1 1 62 0.0 1 35

3.3948 (8. 8 1 00) 29. 8 1 (670 I ) 0. 1 488 0.0222

3.5244 (8.9397) 33.93 (7629) 0.0953 0.009 1

3.5725 ( 8.9877) 35.60 (8004) 0. 1 598 0.0255

Average variance of lnPO 0.0 125 - -

Table A.4b-Summary of resu lts of regression calculations using values in Table A.4a and procedure in Appendix A.2

Value. I umts Value. I unitS Parameter (in.-lb units) Parameter (in.-lb units)

8 (8) k 4.287 (4.2 4 )

"· 1 1 ( 1 1 ) h = B 1 .0 0 ( 1 .030)

n,. 5 (5 ) {I 0.0268 ( 0.5747)

x 3.0438 (8.4590) I'" 1 .027 (0.563)

y . 1 6 1 9 ( . 1 3 9) 4 . 1 55 (4 . 1 04)

I. 0.240 (0.240) S,. s ... s,

s, s.

Sn. SB

A computer preadsheet wa et up to do the e calculation . Table .4b ummarize the calculated alues.

The calculated values of a and 8 are hown in the Ia t column ofT able A.4b. Therefore, the equation of the trength r lation hip is a follow

I units: InC = 0.026 + 1 .0301nPO (A.4a)

Inch-pound unit : In = -0.5747 + I .0301nPO(A.4b

where InC is the average of natural logarithms of compre -sive strengths; and lnPO i average of natural logarithms of pullout load .

F igure A.4 shows the correlation data (a erage of logarithms) and the best-fit l ine.

Finally the trength relation. hip and the procedures in 7.2 arc u ed to e timate the in-place compre ive trength based on in-place te t re ult . Table .4c how two et of in-place pullout test re ult . Both ca e have approximately the arne a erage alue, but a e 2 ha higher ariability. In each case, there are I 0 replicate test results-that is, 111 = I 0. The pullout loads are transfonned by taking their natural logarithms. The averages of the logarithms, lnPO, are substituted into Eq. ( .4) to obtain the average of the logarithm f

-1 Average InC Real value of C. tandard ariance In for , MPa (psi ) MPn (ps1) deviation InC

5 6 7 8

2.34 1 3 (7.3 1 3) I 0.39 ( 1 508) 0.0474 0.0022

2.6522 (7.6292) 1 4. 1 9 (2057) 0.043 0.00 1 9 -2.9273 (7.9().13) I .68 (2709) 0.045 1 0.0020

r- -3. 1 275 ( . 1 047) 22.82 ( 3 3 1 0) 0.0103 0.0001 - -3.3440 (8.3209) 28.33 (4 1 09) 0.0343 0.00 1 2

3.455 1 (8.432 1 ) 3 1 .66 (4592) 0.0048 0.0005 1 3.6890 c .6660) 40.00 (5802) 0.507 0.0026

I 3.7588 (8.7358) 42.90 (6222) 0.0303 0.0009

Average variance of InC 0.00 14

3.8 • (a ) • ..s:::. 3.6 -0> r- / E c 3.4 -r-

/n(CS) = 0 0268 + 1. 030 /n(PO)

..s:::. � � U5 3.2 -r- ./ tV Q) C) >

./ 0 -- 3 -i-_J in - [; tV ...... 2.8 -r- ./ ...... a. 2 E tV 0 2.6 -1-z u / '+- 2.4 -r- . 0

2.2 I I I I I I 2.2 2.4 2.6 2.8 3 3.2 3.4 3.6

Natural Logarithm of Pul lout Load

8.8 .....------------�-----,

..s:::. 8.6 0> E c 8 4 ..s:::. � • - -

·;:::: C/) tV Q) 8.2

g> ,� _J en 8 - [; � a. 7.8 2 E Z(V 0 7.6

(.) 0 7.4

(b) •

•

'"iCS) • -o 5747 • 1 030 /"(PO/ •

./ / . /

7.2 -+-'.....L...J'-+L...L..'-+..._._-'-+ .......... ...J....f....L...I.....I....j-'-L..I....f-'-J.....I...-i 7.6 7.8 8 8.2 8.4 8.6 8.8

Natural Logarithm of Pul lout Load 9

Fig. A.4-Datafor strength relationship and best-fit line: (a) SJ units; and (b) inch-pound units.

in-place compressi e strength, In . Estimates of the tenth percentile strength ( Y0_ 10) corresponding to the two cases are obtained using the tolerance factor method (7.2.2 ) and the altemati e method (7.2.4) . The values of the various parameters used in the calculation are ummarized in Table A.4d and ' here appropriate, the corresponding equation number are hown. For the altemative method the tandard dcvia-


48 REPORT ON METHODS FOR ESTIMATING IN-PLACE CONCRETE STRENGTH (AC1 228.1 R·19)

Table A.4c-Values of pul lout force obtained from tests on structures

In inch-pound rmifs: e I a c 2

Pullout force, · LnPO LnPO Pullout force. lb lnPO Pullout force, lb LnPO 1 3.39 2. 944 1 7. 7 2. 545 30 1 0 3904

2.69 5 1 2.78 2.5479 3340 2 73

2.7453 1 4.25 2.6569 3204

2.6 1 74 I I . 7 2.4742 2669

2.4000 10.37 2.3392 2332

2.59 1 1 1 3.75 3000 3091

2.6834 1 7. 1 0 3290 3844

2.6 1 42 1 3.97 3070 3 1 40

1 1 .8 2660 2552

1 1 .83 2660 3336

Average (X) A \'eragc (X) Average (.A') Average (X) tandard deviation ( x) tandard d viation (s.,) 0. 1 670

Table A.4d-Estimate of in-place compressive strength using results in Table A.4c

In Sl rmifs: Alternative approach (7.2.4) Tolerance factor approach (7.2.2)

a e I ase 2 ase I a e 2 1--- 1- -i� � i- -Y (Eq. (A- 1 7a)) 2.6930 2.7 1 47 y 2.6930 2.7 1 47 f- - - -!- -1- -c p( Y), MPa• 1 4.7 1 - . l o expO'). MPa 1 4.78 1 5 . 1 0

1- -r- - - -,_ -r- - -- -Sy (Eq. (A.3)) 0.0454 0.0607 K (p = 0.75) 1 .67 1 1 .67 1

1- -IQ,OOS I . 33 1 .833 s,, 0. 1 1 1 0. 1 67

Y1.,. (Eq. (6-4)) 2 .6098 2.6034 I Yo ro (Eq. (6- 1 )) 2.5075 2.4356

srr(Eq. (7.2.4d)) 0.037 0.055 exp( Yo w), M Pa 1 2.27 1 1 .42

exp( Yo rol ( Eq. (6-5)) 2.5628 2.5326

exp( Yo ro). MPa 1 2.97 1 2.59

In in.-lh rmi1s: Alternative approach (7.2.4) Tolerance fact r nppr ach (7.2.2.)

1- - r- -Case I ase 2 ase I a e 2

-r- - - -- -Y (Eq. (A- 1 7b)) 7.6700 7.69 1 7 y 7.6700 7.69 1 7

-f- -!-- -cxp( Y), p i* 2 143 2 1 90 cxp( Y). psi 2 1 43 2 1 90

St (Eq. (A.3)) 0.0454 0.0607 I K (p = 0.75) 1 .67 1 1 .67 1 I- -r- I -

IQ.O Ol 1 .833 1 .833 s,, 0. 1 1 1 0. 1 67

Y,.,. ( Eq. (7.2.4b)) 7.5870 7.5804 Yo 111 (Eq. (7 .2.2a)) 7.4845 7.4 1 26

Srf (Eq. (7 .2.4d)) 0.037 0.055 exp( Y0 10), psi 1 780 1 657 r cxp( Yo IO) (Eq. (7.2.4c))

- -7.5395 7.5099

I e�tp( Yo ro), p i 1 8 I 1 826

'cxp( l') - e'.

tion of the in-place compres ive strength (sd) was computed u ing q. (7.2 .4d), while for the tolerance factor method it wa taken to equal the tandard deviation of the tran formed in-place tc t re ult . For each method, the alue of Y0.10 i a smaller fraction of the average strength for asc 2 due to

the higher variability of the in-place te ts. In this example, the trength relationship ha relatively low catter, and the estimates of Y0.10 are lower for the tolerance factor method, which does not consider this.



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