CE 555 - L13 -NDT of concrete-1-hammer probe.pdf

7/27/2019 CE 555 - L13 -NDT of concrete-1-hammer probe.pdf

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Neithalath, Spring 2006, CE 455/555 Structural Damage: Assessment, Repair, and Strengthening

Non-Destructive Testing of Concrete

Rebound Hammer

Probe Penetration

Pull out TestBreak-off Test

Lecture 13


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What is NDT / NDE ?

Test methods which measure physical

properties in place or can be used to detectflaws, and cause no significant structuraldamage to concrete

Non-destructive vs. Semi-destructive tests Some test methods may cause minor damage

like drilled cores, load testing, pin penetration,

break off and pull off tests These are considered semi-destructive


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NDT / NDE

Obtain the propertiesof concrete in-situ

Primarily, all engineersneed some estimate ofstrength

Other parameters alsocan be identified


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Other advantages of NDT

Member dimensions

Location of cracking, delamination, anddebonding

Degree of consolidation, and presence of voids

and honeycomb Steel reinforcement location and size

Corrosion activity of reinforcement

Extent of damage from freezing and thawing,fire, or chemical exposure


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Why Non-Destructive Testing ?

Determine the suitability of the structure toserve its intended functions

Be able to predict the performance withoutbeing invasive

Invasive techniques are location specific andexpensive

Invasive techniques can cause further

damage to the structure

Invasive techniques do not often allow

examination of large areas


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Why Non-Destructive Testing ?

Concrete in actual structures may exhibit adifferent behavior than the lab-tested ones

how do we know if the tested material issimilar to the one in service?

Specimen preparation issues in site and theconcreting for the structure

Curing procedures between specimen and the

structure Size effects

Laboratory testing anomalies

Sample uniformity


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Application of NDT

Both new and old structures

New structures Quality control

Dispute resolution about the material /

construction quality

Old / Existing structures

Adequacy of load carrying capacity

Age related material distress

Load related overload

Construction quality related


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ypical situations when NDT is

useful - I quality control of pre-cast units or construction in

situ

removing uncertainties about the acceptability ofthe material supplied owing to apparent non-compliance with specification

confirming or negating doubt concerning theworkmanship involved in batching, mixing,placing, compacting or curing of concrete

monitoring of strength development in relation to

formwork removal, cessation of curing,prestressing, load application or similar purpose

location and determination of the extent of cracks,voids, honeycombing and similar defects within a

concrete structure


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ypical situations when NDT is

useful - II determining the concrete uniformity, possibly

preliminary to core cutting, load testing or other

more expensive or disruptive tests

determining the position, quantity or condition ofreinforcement

increasing the confidence level of a smallernumber of destructive tests

determining the extent of concrete variability inorder to help in the selection of sample locationsrepresentative of the quality to be assessed


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Typical situations when NDT is

useful - III confirming or locating suspected deterioration of

concrete resulting from such factors as

overloading, fatigue, external or internal chemicalattack or change, fire, explosion, environmentaleffects

assessing the potential durability of the concrete monitoring long term changes in concrete

properties

providing information for any proposed change ofuse of a structure for insurance or for change ofownership.


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Situations where NDT can be used

to investigate the homogeneity of concrete mixing

to determine the density and strength of concrete in astructure

to determine the location of reinforcing bars and thecover over the bars

to determine the extent of defects such as corrosion

to determine the location of in-built wiring, piping,ducting, etc.

to determine whether internal defects such as voids,

cracks, delaminations, honeycombing, lack of bondingwith reinforcing bars, etc. exist in concrete

to determine if there is a bond between epoxy bondedsteel plates and concrete members.


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Summary of NDT Methods

See attached sheets


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Schmidt Hammer / Rebound

Hammer (ASTM C 805)


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Schmidt Hammer

Based on the sameprinciple of why a

superball bounces Mainly tests the

surface hardness of

concrete, then relatedto strength

Fundamentally acomplex problem ofimpact loading andstress-wavepropagation


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Schmidt Hammer

Springs stretched

Hammer released

Hammer

ImpactsAnd

Plunger

Rebounds

Indicator

Records

rebound

Rebound number 10 to 100


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Some things to note

Can be used in horizontal, vertical or inclinedpositions

Hammer should be perpendicular to the surfaceunder testing

Should develop a correlation between the rebound

number and the concrete being tested (theaggregate type is very significant)

Higher rebound number, higher the strength

Dont rely too much on the calibration curvesupplied with the instrument


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Limitations of Rebound hammer - I

Near surface measurement will not getproperties of the core concrete

Smoothness of surface Surface has to be smooth, variations between methods

of surface finishing

Size, shape and rigidity of the surface If the concrete moves (cracking, poor surface) the

numbers are not reliable

Age of the specimen Should not do on low strength concrete at early ages

the aggregate effect dominates and will get faultyreadings


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Limitations of Rebound hammer - II

Related to stiffness than strength

Surface and internal moisture conditions

High rebound number for wet specimens than air cureddry specimens

Presaturation preferred

Aggregate dependence

Type of cement

High alumina cement shows higher strength

Supersulfated cement 50% lower than Type I

Carbonation of the surface

Carbonation increases surface density


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Rebound number-Strengthrelationship

Guidebook on NDT for Concrete


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Rebound number-strengthrelationship

Guidebook on NDT for Concrete

Different types of gravel

as fine aggregate

Different types of

Coarse aggregates


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Probe Penetration (Windsor probe)

Drives a probe into theconcrete with a known

amount of force

Another Impact test

James Instruments, Cement Assoc of Canada


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Probe penetration (ASTM C 803)

Shoots an alloy probe into concrete

6.3 mm dia, 79.5 mm length

Exposed length of the probe measured, andrelated to the compressive strength of concrete

Much higher energy than rebound hammer

Influenced by the surface smoothness of theconcrete

Influenced by the presence of aggregates Much larger damage area than the rebound

hammer


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Relating strength and penetrationdepth

HardAggregates Soft

Aggregates


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Damage zone in probe penetration

Need to identify thelocation of

reinforcing barssince the probe willbe affected

Pin penetration modified version ofthe probe requires

less energy than theprobe


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Pullout Test (ASTM C 900)

Concrete subjected tostatic loading unlike

Rebound hammerand probepenetration


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Pullout test

Measures the maximum force required to pull anembedded metal insert with an enlarged head

from a concrete specimen or structure Force is applied by a tension jack, or center-hole

ram, that reacts against the concrete surface

through a reaction ring concentric with the insert A roughly cone-shaped fragment of the concrete is

extracted as the insert is pulled out

Diameter of the conic fragment d2

is determinedby the inner diameter of the reaction ring, and thesmall diameter d1 is determined by the insert-headdiameter


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Pulled out specimens


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Advantages and Disadvantages

Produces a well-definedfracture surface in theconcrete

Measures a static strengthproperty

Empirical relationships can

be developed between thepullout strength and thecompressive strength ofthe concrete

Pullout strength governedby that portion of concretelocated next to the conicfrustum defined by theinsert head and thereaction ring

Limited to newconstruction insert hasto be embedded

The empirical relationship

is applicable to only theparticular testconfiguration and concretematerials used in thecorrelation testing


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Break-off test (ASTM C 1150)

Measures the forcerequired to break off a

cylindrical core from alarger concrete mass

The measured force anda pre-established

strength relationship areused to estimate the in-

place compressive

strength


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Break-off test

Plastic sleeve inserted into fresh concrete toform the core

Also, the test specimens can be prepared inhardened concrete by using a special core

bit to cut the core and the counter bore Can be used to evaluate concrete in both

new and existing construction


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Break-off test procedure

Sleeve is removed and a special loading jack is placed intothe counter bore

A pump supplies hydraulic fluid to the jack that applies ahorizontal force to the top of the core

The reaction to the horizontal force is provided by a ringthat bears against the counter bore

The force on the core is gradually increased by operatingthe pump until the core ruptures at its base

The hydraulic fluid pressure is monitored with a pressuregage having an indicator to register the maximum pressure

achieved during the test The maximum pressure gage reading in units of bars [1

bar = 0.1MPa (14.5 psi)] is called the break-off number ofthe concrete.

CE 555 - L13 -NDT of concrete-1-hammer probe.pdf

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