TEST METHODS FOR THE DETERMINA TION OF CREEP AND SHRINKAGE INMASONRY

Peter Sehubertl

1. ABSTRACT

A knowledge of masonry deformation due to creep and shrinkage is important for the assessment of eraek resistanee and for design (ereep). Deformations values for shrinkage and ereep are given in the eurrent version of the "Masonry" Euroeode. At present, however, there is no standardized test method for determining shrinkage and ereep. The development and applieation of sueh methods are essential if physieal properties are to be eompared.

The paper presents a number of fundamental statements eoncerning the main criteria and eonditions for test methods, based on many years of experienee with creep and shrin­kage tests, and deseribes the in Germany used test proeedures.

The fundamental remarks refer to the ehoiee of speeimen type and size, fabrieation of the speeimens - espeeially pretreatment of the masonry units - seIeetion and arrangement of the test set-up, storage of the speeimens, exeeution of the test (application of the ereep load, test variables, test times), test duration as a funetion of speeimen size and evaluation of test results.

Finally, the test methods in use in Germany are deseribed.

Keywords: Test Methods, Creep, Shrinkage

1 Dr.-Ing., Institut fur Bauforschung, RWTH Aachen, Schinkelstr. 3, 0-52056 Aachen, Germany

777

2. TERMINOLOGYIINTRODUCTION

In the following, the term shrinkage will be used to denote changes in volume or length or strains due to loss of moisture from masonry and masonry materiaIs in the hardened state or after a certain initial hardening (masonry mortar). The reverse process will be referred to as swelling. Contraction due to hydration or early shrinkage in the still non­hardened state are not taken into account, since these processes are in practice virtually without significance for stress-inducing deformations of masonry. Shrinkage relies on a physical process and is more or less reversible depending on the construction material concemed.

Apart from shrinkage, a reduction in volume due to carbonation may occur in masonry mortars, concrete and calcium-silicate-units. This process is caused by chemical reaction of atmospheric carbonic dioxide with certain calcareous chemical compounds of the construction material, and is not revérsible.

Apart from physically-induced shrinkage or swelling, an increase in volume due to mo­lecular water binding may occur in masonry bricks. This can be reversed onIy by hea­ting to approximately 650°C. This type of deformation will not be considered in the present context. An European test standard is envisaged.

Deformation caused by long-term loading (shortening in the load axis) is referred to as creep and the change in length in relation to the originallength is termed the creep strain E k. The coefficient of creep <p indicates the ratio of creep strain to elastic strain E elo

The coefficient of creep is constant in the range of stresses up to about 50 % maximum stress.

If they are sufficiently large and are impeded, deformations in masonry components due to shrinkage and creep may cause cracks. The most precise possible knowledge of the shrinkage and creep time curves and the magnitude of the expected final values are therefore important when assessing the crack resistance of masonry components or structures. Deformation values, namely the coefficient of creep (for the buckling check) are also required for masonry designo

The October 1993 edition of Eurocode 6 (Masonry) /1/ contains a table showing defor­mation parameters, although these are not yet finally established. No European test standard for deformations in masonry is yet in preparation. Some test methods for shrin­kage in masonry units and chernical swelling (moisture expansion) of bricks are avai­lable in draft form /2/.

Suitable test methods are required in order to determine and compare the deformation parameters of masonry. The following paper considers fundamental aspects of the establishment of such test methods and describes and assesses the methods commonly used in Germany.

778

3. FUNDAMENTAL CONSIDERATIONS

3.1 Requirements and Objectives

A test method should

- be sufficient1y conclusive and reliable and be readily reproducible, - provide rapid results, - be simple and inexpensive to perforrn.

Sufficient concJusiveness and reliability mean that the deterrnined properties are suf­ficient1y relevant, i.e. the real - for example the practical - conditions are deterrnined and taken into account with sufficient precision. Particular care must be taken in this respect when time-accelerated test methods are employed.

3.2 Influencing Variables and their Assessment

3.2.1 Specimen Type and Size (see figo 1)

wallettes 0N)

length

measurhg on the \Ilit surfoce

length meosumg In the unit axIs

uni! (U)

piers (P) piers (W) (P:one uni!. 'vP. 1wo units

every course)

prism

meosumg pins

prisms from units(FU) mortar prisms(PM)

EigJ.;, Test specimen for deterrnination ofthe creep and shrinkage

The most relevant component-related test results are obtained with waIl specimens. Small walls consisting of at least 5 courses and at least one head joint in each course are adequate as specimens. In order to take practical conditions into account, it is advisable

779

to provi de a water-vapour-proof seal on the 4 narrow sides of the wall specimen. Verti­cal and horizontal deformations can be determined on wall specimens.

The masorn:y pier is a further simplified masonry specimen. It can be constructed as a single unit pier (one unit in each course) or masonry bond pier (at least two units in each course). Like the wall specimen, it should contain at least five courses. In order to obtain realistic conditions, two si de surfaces and the bottom and top surfaces should again be given a water-vapour-proof se aI.

Previous comparative studies have shown good agreement between deformation para­meters determined on walls and piers. As a rule, however, tests using piers can deter­mine only the vertical deformation. Lateral sealing of the masonry specimens is advis­able to ensure drying behaviour corresponding to that under real conditions.

Instead of masonry specimens, it is possible to test single units, provided that the mortar fraction of the masonry is small (large format units) and that the units are sealed in ac­cordance with their position in the masonry component. Sealing may be dispensed with if the value being determined is the final deformation value (final shrinkage value, fmal creep value). Numerous studies show that there are no significant differences between the final values for large-format single units and for masonry. Deformation in different axes can be determined in the single unit.

For very coarse tests, specimens (prisms) of single units may also be used. Appropriate comparative tests, at least with single units, are, however, required before apply such de­formation values to masonry.

With ali types of specimen it is necessary to ensure that deformation of the support zone of the specimen is as far as possible unrestraint (foil interlayers) or that the measuring points are far enough from the support zone not to be affected by any restraining of de­formation.

3.2.2 Fabricating the Specimens

To avoid transport effects, the masonry specimens are to be fabricated in the test room. They should be made up at least 50 cm above ground levei to ensure the most identical possible storage conditions (storage climate). The distance between the different speci­mens must be large enough to ensure identical drying conditions.

The moisture state of the masonry units during fabrication of the specimens is signifi­cant for the test results. The moisture state should be chosen on the safe side, in accor­dance with practical conditions. The need to carry out the test method without unjusti­fiably high effort should also be taken into account. Adjustrnent to a specific initial moisture content irrunediately before laying of the units would involve a very high, technically almost insoluble effort. It would entail substantial technical difficulties, especially with large-format masonry units or elements, and would lead to unjustifiably high costs. In Germany, relatively simple pretreatrnent of the units, with a margin for safety, is employed. The units are stored for two days in water at +20 °C and then for one day in the envisaged test climate; they are then laid irrunediately. Water storage

780

produces somewhat higher shrinkage and creep values than with factory-moist or moi­stened units, but the differences are limited by the fact that a substantial part of the in­itial moisture is to be found in larger pores with less effect on shrinkage. The advantages of this method are that the initial state is defined with sufficient accuracy and that the test is easy to reproduce.

It is highly advisable to determine the moisture content of the specimen prior to com­mencement of the test. In the case of masonry specimens, this can be achieved by weighing additional specimens (piers) or approximated by determining the moisture content of the single unit and the mortar.

3.2.3 Measuring Equipment

Horizontal and vertical measuring sections in the region of the unit and in the region of units and joints are to be recommended. Mechanical dial gauges with an accuracy of 0,001 mm are suitable for long-term tests. Special care must be taken to ensure proper fitting and positioning. Conic-concave positioning (see figo 2), which provides defined contact conditions, is advisabIe for base point positioning of the vertical measuring sections. Depending on tbe intended test c1imate, the measuring equipment may need to be made of stainlees steel.

Fig. 2: Equipment for determination ofthe creep and shrinkage strain

781

3.2.4 Test Climate

A test climate of +20 °C and 65 % relative humidity (20/65 climate) is generally used in Germany. This climate represents inhabited rooms, is in the range of unfavourably low relative atmospheric humidities and is the usual standard climate at the test institutes. The climate tolerances should not exceed ±2K and ±5 % relative humidity. Care must be taken to ensure that air circulation in the test space is even and not excessive.

A very low relative humidity of, for example, 30 % may, however, be selected for shrinkage tests, in order to allow for extreme cases. Shrinkage for the "normal case" can then be inferred via the relationship between shrinkage and moisture content (see Section 3.2.7).

3.2.5 Test

The initial measurement for shrinkage should take place as soon as possible - test age to :$ 3 days. The initial lengths of the test sections should also be determined at this time. Repeat measurements are made 1,3,7, 14,28, 90,180,360 ... days after the initial measurement. lt is highly advisable to determine the change in mass at the same time as the respective shrinkage measurements, so that the change in moisture content over time can be quantified with the aid of the simultaneously or subsequently deterrnined dry bulk density (essentially applicable to single units and unit specimens; see Section 3.2.7).

Since shrinkage is only partially reversible and may change due to altemating shrinkage and swelling, at least the repeat shrinkage should be determined, in order to allow better assessment of shrinkage behaviour.

At an age of 7 days, the creep test specimens should be loaded with the creep stress in successive stages (test set-up see figo 3).

The admissible stress is recommended as the creep stress. The modulus of elasticity as a secant modulus can also be deterrnined from the intermediate readings during successive loading of the creep specimens. Repeat measurements are made at 1, 3, 7, 14, 28, 90, 160,360 .. . days after the initial measurement. Care must be taken to ensure that defor­mations (shrinkage) in unloaded parallel specimens are measured at the same time as those in the creep specimens, so that the creep strain E cr may be calculated from the dif­ference between the overall strain tot E (measured on the creep specimen) and the elastic and shrinkage strain E el ' E s:

(1)

782

steel beam

s1eel beom

oif-feeld to lhe pump

a)

manometer -~L..13

compresslon cel~

tie rod ----I

mosonry woll

mortor compensoting layer

b)

Fig. 3: Creep test machine with test specimen a) for small specimen b) for masonry walls

3.2.6 Test Duration

compressed oir

hydroulic oil pump

steel beam

The necessary test duration depends on the type and grade of unit, the specimen size and the test conditions (initial and final moisture content, test climate), together with the nurnber and sequence of measuring dates. The test may be continued up to approximate constancy of the shrinkage or creep strain or until a value curve which can be extrapola­ted is adequately documented (see figo 4).

E, In mm/m

I I I .

I ---- . / 1· . I

j I i I

! I E, ~ O.33 ' (l-e-o.D's "o.,, )

I I

II I

I I !

-0.2

-0.1

0.0

o 100 200 400 500 600 700 t In d

Fig. 4: shrinkage ES versus time t test values and regression curve

783

Table 1 contains rough values for test durations.

Table 1: Deterrnination offinal creep and shrinkage values; minimum oftest duration (test c1imate: 20/65)

units type of specimen (see also figo 1) wallettes W piers P,Vpl) t=240mm t=240mm units U

without render Ca1cium-Silicate 1 y 6mon 3 .. 6 mon

Light- pumice 15y ly weight- expanded _L.) _L.) 6mon concrete c1ay Concrete _Lo) 9mon 6mon AAC (1..2 y)

prisms of unit and

mortar PU, PM

1 mon

1 . .3 mon

-(1 mon)

1) non sealed specimens (see also figo 1), otherwise use the values for wallettes 2) no inforrnation possible () values very unsure

3.2.7 Assessment

Shrinkage and creep strains should first be presented graphically as a function of test time. The curve can usually be described by means of a suitable mathematical function, e.g. the Ross function

&cr s(t) = _ t_ ; if t ~ <Xl ~ .!.. = &cr<Xl S<Xl 'a+b·t b '

(2)

or an exponential function:

(3)

The final coefficient of creep:

(4)

can also be calculated from the resulting fmal creep strain.

In order to quantify the influence of moisture content or loss on shrinkage, it is advisable to present the relationship between shrinkage and moisture content in the forrn of a graph (see figo 5) and if necessary to describe it mathematically by means of a suit­able function. The final shrinkage values for specific conditions (specific initial and fi­nal moisture values ~ equilibrium moistures, factory moisture of the units and laying moisture ofthe units) can then be deterrnined by reference to this re\ationship.

784

ES (mm)

<s1

<s2

casa 1: moisture at laying ~ production moisture content : Ea == Ea1

case 2: moisture at laying ~ moislure contenl by lhe delivery : Ea = Es1 - Es2

hv1

hv1: equilibrium moisture content (e.g . climate 20/65)

hv2: moisture content by the delivery

hvO: production moisture content

hv2 hvO

hv{Vol.%)

Fig. 5: Correlation between shrinkage Es and moisture content hv

4. TEST DESCRlPTION

Table 2 shows the test methods generally used in Gennany for detennination of creep and shrinkage strain.

Table 2: Test methods for determination ofshrinkage, swelling and creep

test specimen wallette (W) pier (P) pier (VP) units (U) prisms of (one unit (two units unit and

every course) every course) mortar (PU, PM)

test direction vertical / vertical different different horizontal

h rmml > 5 courses ~ t (40) size wrmml > 4 t I unit length unit sizes ;:: ISO (160)

thickness t [mm] i.g.240mm ofshell

resp.40mm number of ~3 specimen

ifrequired recom-man- low deforrnation restriction sealing of dations for at the bottom (steel p!ate) of specimens al!areas bui!ding (e.g.: foi!, bituminous, .. . ) beeing inte- -

ria! in masonry

if required sealing of the side and head areas

conditioning 48 h ofunits 48 h in water (20°C), 24 h test c1imate water

and prisms (20°C)

785

Table 2: continuation

test specimen wallette (W) pier (P) pier (VP) units (U) prisms of (one unit (two units unit and

every course) every course) mortar (PU, PM)

measure distance: ::2: 250 mm gauge loca- measure if required additional gauge location pads at tion pads distance

unit surface ::2: 20 mm ::2: 150 from the (160) mm

edge measuring gauge location pads should be glued, also fixing with plugs possible

set up strain gauge caliper or

mechanical dial gauges mechanical mechanical dial gauges

(axis) dial gauges

precision: 0,001 mm; long measure distances (::2: 500 mm ): 0,01 mm test climate 20/65-temperature: 20°C±2K, rei ative humidity: (65±5)%;

in special cases 20/30 measuring 0,3,7, 14,28, d; 0,3, 7, 14,21,28, d;

times 1,2,3,6,9, 12 ... mon 1,2,3,6,9, 12 ... mon (minimum)

creep, shrinkage, testing creep, shrinkage, swelling swelling; changing of

key values specimen mass moisture content

5. REFERENCES

/li Eurocode 6: Design of Masonry Structures (10.93), Part 1.1: General Rules for Building, Rules for Reinforced an unreinforced Masonry, Crack and Deflection Control, DRAFT; CENITC 250/SC 6/N 49

/2/ PrEN 772-12: 1992, Part 12: Methods ofTest for Masonry Units; Determination of Length Change During Moisture Movement in Autoclaved Aerated Concrete Masonry Units

PrEN 772-14: 1992, Part 14: Methods ofTest for Masonry Units: Determination of Moisture Movement of Aggregate Concrete Masonry Units

PrEN 772-19: 1993, Part 19: Methods ofTest for Masonry Units: Determination of Dimensional Stability ofLarge Clay Masonry Units

/3/ Schubert, P. : Formanderungen von Mauersteinen, Mauermõrtel und Mauerwerk. Berlin : Emst & Sohn. - In: Mauerwerk-Kalender 17 (1992), S. 623-637

/4/ Schubert, P. : Prüfverfahren fiir Mauerwerk, Mauersteine und Mauermõrtel. Berlin : Emst & Sohn. - In: Mauerwerk-Kalender 16 (1991), S. 685-697

786