Drying Shrinkage of Concrete

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Knowledge o the shrinkage characteristics o

concrete is a necessary starting point in the

design o structures or crack control. Such

knowledge will enable the designer to estimate

the probable shrinkage movement in reinorced orprestressed concrete and the appropriate steps

can be taken in design to accommodate this

movement.

This Data Sheet reviews the actors aecting

drying shrinkage o concrete seeking to put in

perspective their varying infuences enabling

practical and eective controls to be placed on

drying shrinkage.

dyIn ShInk MchnISM

When concrete is exposed to its service

environment it tends to reach an equilibriumwith that environment. I the environment is a

dry atmosphere the exposed surace o the

concrete loses water by evaporation. The rate o

evaporation will depend on the relative humidity,

temperature, water-cement ratio and the area o

the exposed surace o the concrete. The rst

water to be lost is that held in the large capillary

pores o the hardened concrete. The loss o this

water does not cause signicant volume change.

However, as drying continues, loss o water rom

small capillary pores and later rom gel pores

takes place. With the reduction in the vapour

pressure in the capillary pores, tensile stress in

the residual water increases. Tensile stresses in

the capillary water are balanced by compressivestresses in the surrounding concrete and as a

result the concrete shrinks. Evaporation o gel

water changes the surace energy o the solid

phase and causes urther shrinkage.

Drying shrinkage makes up a portion o the

total deormation that is observed in a concrete

member. Figure 1 shows the various components

o deormation, excluding thermal movement1.

Shrinkage strain is time-dependent and non-load-

induced.

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of 6 > DI SHIKE o Cement and Concrete

I the environment is wet or moist, the fow

o moisture will be rom the environment to the

concrete, the result being a volume increase

or swelling. schematic description o volume

changes in concrete due to alternate cycles o

drying and wetting is shown in Figure 21. The

greatest shrinkage movement occurs on rst

drying. considerable part o this shrinkage isirreversible, that is it cannot be recovered on

subsequent wetting.

FctS FFctIn dyIn ShInk

Since drying shrinkage is related to moisture loss

rom the concrete, it is infuenced by external

actors that aect drying and also internal actors

related to the concrete and its constituents

Figure 3.

xeral Faors

The external actors aecting loss o moisture rom

concrete are ambient conditions, and size andshape o the concrete member.

mbie oiios ir temperature, relative

humidity and wind velocity will aect the loss o

moisture rom the concrete surace. eerence 2

discusses how any combination o these actors

aects the evaporation rate. Dierent ambient

conditions on opposite sides o a member result in

dierential drying out, hence dierential shrinkage

with the possible consequence o warping.

When all other actors are equal, the typical

eect o varying relative humidity on the drying

shrinkage o concrete is shown in Figure 43

.In summary, higher drying shrinkage is to

be expected with rise in ambient temperature,

with decrease in relative humidity, with increase

in air movement around the concrete, and with

increase in the length o time or which concrete is

subected to drying conditions.

Loaded

Immediatestrain

Immediaterecovery

Creeprecovery

Creep

Shrinkage

Unloaded

AGE OF CONCRETE

L I N E A R

C O N T R A C T I O N

t1 t2

Stored in water

Stored in air

W e t t i n

g Initial

dryingshrinkage

Reversable shrinkage(moisture movement)

Swelling

AGE OF CONCRETE

D E F O

R M A T I O N

E x t e n s i o n

C o n t r a c t i o n

t0 t

D r y i n

g

R E L A T I V E

D R Y I N G S

H R I N K A G E

F A C T O R

RELATIVE HUMIDITY (%)

0.7

0.8

0.9

1.0

1.1

1.2

1.3

0 2010 30 40 50 60 70 80 90 100

Design anddetailing

Aggregate-pastebond strength

W/CLength of

drying period

Cement Temperature

AdmixturesRelativehumidity

Airmovement

Elasticproperties

Concentration Theoreticalthickness

CONCRETE

SHRINKAGE

Constructionpractice

Dryingconditions

Aggregate

Figure 1 Various components o deormation

Figure 2 Concrete initially dried and then

subected to cycles o drying and wetting

Figure 4 Eect o relative humidity on drying

shrinkage

Figure 3 Factors aecting drying shrinkage




Member geomer arge, thick concrete

members dry out more slowly than small, thin

ones. s a result, or the same drying period,

shrinkage o large-size members is lower than the

smaller-size ones which can dry out to their cores

more quickly.

The eect o concrete member geometry on

drying shrinkage is represented in most codesand standards by its ‘theoretical thickness’ or

hypothetical thickness which is dened as twice

the area o the cross-section o the concrete

member divided by the exposed perimeter o the

cross-section. It ollows that a higher theoretical

thickness will be associated with a lower drying

shrinkage.

Ieral Faors

The internal actors aecting drying shrinkage

o concrete are those related to its constituents:

cements, aggregates, admixtures; concrete mix

design; water-cement ratio and water content;aggregate properties and volume raction; and

those related to the construction o the concrete:

placing, compaction and curing.

cemes lthough it is generally concluded

that the composition o cement can aect drying

shrinkage the eect is not completely determined.

The C3 and alkali content have been observed to

have a dominant eect4. In turn, the eect o C3

and alkali content on shrinkage is infuenced by

the gypsum content o the cement, ie shrinkage

o cements o the same C3 content diers or

dierent gypsum contents5

.s a result, or many years, some maor

specications in ustralia have been speciying

the chemical composition o cement as a means

o controlling shrinkage o concretes to be used in

structures, such as road pavements and bridges.

n example o the chemical limits included in

those specications is a maximum C3 content

o 7% and a minimum SO3

content o 1.8%. It

was recognised, however, that there are other

cements outside these specications which

have perormed well in low shrinkage concrete

applications. This has led to the development o

a cement characterised in terms o its shrinkage

perormance ‘Shrinkage imited Cement, Type S’.

Engineers/speciers should not be concerned

with the complex details o cement chemistry as it

is the nal perormance that is important.

reliable test method and associated

perormance criteria are required to characterise

cement in terms o its shrinkage perormance.

Based on an extensive investigation by the

cement industry under the auspice o the

ustralian Standards Committee BD/10, a

standard test method or measuring the drying

shrinkage o cement mortar has been establishedand is published as S 2350.13. maximum

limit or drying shrinkage at 28 days is set at

750 microstrain or Type S cement. This maximum

limit takes into account not only the perormance

o the cement but also the precision o the test

method. The precision o the test method is

such that the dierence between two test results

or 28-day drying shrinkage obtained in two

dierent but experienced laboratories, under

conditions o reproducibility, is expected to be

up to 150 microstrain. So, or a cement to complywith the maximum limit o 750 microstrain a typical

average result should be around 600 microstrain.

This maximum limit also implies that it must not be

exceeded by a single result.

It should be pointed out that the use o Type S

cement alone will not guarantee the production

o low shrinkage concrete. s outlined in this

article other actors are involved which should be

considered as they may outweigh the eect o

cement on the drying shrinkage o the concrete.

ggregaes ggregates have a restraining

eect on shrinkage. This eect is illustrated inFigure 5. However, the restraining eect varies

and reports both rom ustralia and overseas

show that some types o aggregate, i they

shrink more than the paste, increase concrete

shrinkage signicantly 6-8. This work shows that

there is a reasonably direct relationship between

the shrinkage o an aggregate and its absorption

capacity. That is, good quality, low shrinkage

aggregates are usually characterised by low

absorption. I the aggregate shrinks less than the

paste then the aggregate restrains shrinkage and

the shrinkage o the concrete will decrease with

increase o the aggregate volume raction. The

eect o aggregate volume raction on drying

shrinkage is shown in Figure 69. Both Figures 5

a 6 illustrate the substantial restraining eect o

aggregates on the drying shrinkage o the paste.

s can be seen, shrinkage o concrete may be

only 20% o that o the cement paste.

% O

F U L T I M A T E S H R I N K A G E O F N E A T

C E M E N T

AGE (days)0 1095

0

10

20

30

4050

60

70

80

90

100

110

1207 14 28 90 180 365

Concrete*

1:3 mortar†

Neat cement†

* 400 kg cement/m3

70- x 70- x 280-mm specimens† 40- x 40- x 160-mm specimens

Figure 5 Comparative drying shrinkage o

concrete, mortar and neat cement paste at 50%

relative humidity




Other aggregate properties such as grading,maximum size, shape and texture aect drying

shrinkage indirectly. In practice variations in any o

these properties can lead to a change in the water

demand and/or paste content and their eect on

drying shrinkage is only measurable in terms o

the changes they cause to the concrete mix.

ggregates can be contaminated by other

materials such as silt, clay, coal, wood or organic

matter. Most o these materials do not restrain

shrinkage and in act can increase it, especially

in the case o clay which absorbs moisture and

shrinks considerably on drying out. S 2758.1Concrete Aggregates species maximum limits

or these materials. Most o these contaminating

materials can be removed by washing the

aggregate.

In summary, hard, dense aggregates with

low absorption and high modulus o elasticity are

important or the production o concrete o low

drying shrinkage.

The eect o aggregate on restraining the

drying shrinkage o concrete is governed by:

n The volume raction o the aggregate

n The modulus o elasticity o the aggregate

n The shrinkage o the aggregate upon drying.

mixures There are many types o admixtures

available or incorporation in concrete to achieve/

enhance certain properties or to achieve economy

or both. enerally, admixtures aect shrinkage

o the concrete to a varying degree depending

on their ormulation, their interaction with the

cement, their interaction with other admixtures

in the mix, and on the variations or adustments

they bring about in the proportions o the concrete

mix. detailed discussion o the eect o various

admixtures can be ound elsewhere10,11. It is well

established that admixtures containing calcium

chloride can increase the drying shrinkage o

concrete.

Waer oe The drying shrinkage o concrete

increases with increasing its water content. The

variation in shrinkage with water content may be

explained by the dierence in types o water lost

at the various stages o drying mentioned earlier.

It is also associated with the modulus o elasticity

o the concrete. Concrete o high water content

(and high water-cement ratio) has a lower strengthand lower modulus o elasticity and hence has a

greater tendency to shrinkage. The eect o water-

cement ratio on drying shrinkage is illustrated

in Figure 712. s can be noted, at ages beyond

28 days, higher water-cement ratios lead to

signicant increase in drying shrinkage.

It has been advocated that high cement

content always leads to higher drying shrinkage.

This is not strictly correct as can be illustrated in

Figure 813 which shows that high perormance/

high strength concrete which is characterised by

high cement content, low water content (hence low

water-cement ratio) and good quality aggregate

could have low shrinkage characteristics.

S H R I N K A G E

R A T I O

( s / s p

)

AGGREGATE VOLUME CONCENTRATION

0

0.5

0.6

0.7

0.8

0.1

0.2

0.3

0.4

0.9

1.0

0 0.20.1 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0

Figure 6 Eect o aggregate volume concentration

on shrinkage

s = shrinkage o concrete

sp

= shrinkage o paste

D R Y I N G

S H R I N K A

G E

x

1 0 -

3

AGE (days)0 270

0

0.5

1.0

1.5

2.0

2.5

3.0

3.57 28 90 180 365

W/C = 0.26

0.45

0.55

0.65

S H R I N K A G E

( m i c r o s t r a i n )

300

1000

900

800

700

600

500

400

1201008050

COMPRESSIVE STRENGTH (MPa)

7060 14013011090

Figure 7 Eect o water-cement ratio on shrinkage

o cement pastes

Figure 8 Drying shrinkage o HSC




cosruio praie Concrete placing,

compaction and curing are important actors in

minimising the magnitude o drying shrinkage.

dding urther water on site during placing o

concrete to restore slump or to aid with nal

nishing will increase the drying shrinkage o

concrete.

Proper compaction and curing are required toproduce dense concrete o reduced capillaries

and/or with discontinuous capillaries, resulting in

reduced loss o moisture rom the concrete and

reduced drying shrinkage. pplying appropriate

curing measures immediately ater nishing

the concrete will prevent drying o the concrete

surace especially in hot weather conditions.

dcIn cnct dyIn ShInk

Some o the measures that can be taken to reduce

the drying shrinkage o concrete include:

n se the minimum water content (consistent

with placing and nishing requirements).

n se highest possible volume raction o good

quality aggregate and maximum possible

aggregate size.

n se Shrinkage imited Cement (Type S)

where available.

n Do not use admixtures known to increase

drying shrinkage, eg those containing calcium

chloride.

n Ensure concrete is properly placed,

compacted and cured.

dyIn ShInk cckSDrying shrinkage is not a problem i the concrete

is ree to move. I the concrete is restrained in

any way, drying shrinkage will introduce tensile

stresses which i they exceed the tensile strength

o the concrete, will cause the concrete to crack.

educing drying shrinkage will not necessarily

prevent cracking which is also infuenced by

the restraint and the design and detailing o the

concrete element.

Shrinkage cracks, as opposed to fexural

cracks, are parallel sided and in the case o slabs

usually extend right through the slab thickness.

Such cracks can cause water penetration/leakage

and ultimately impair the durability o the concrete

element.

Thereore, control o cracking due to drying

shrinkage is important and requires appropriate

design and detailing o the concrete element.

equae reiforeme Where cracking

occurs, the spacing and width o shrinkage

cracks depend upon the percentage o

reinorcement in the restrained concrete and the

bond characteristics o the reinorcement. The

provision and location o adequate reinorcementto distribute the tensile stress caused by

drying shrinkage is particularly important in

slabs-onground and similar applications where

reinorcement may not be required or structural

reasons. S 3600 gives minimum percentages o

primary and secondary reinorcement to control

cracking due to drying shrinkage and thermal

stresses in reinorced concrete slabs, depending

on the degree o restraint.

Whilst reinorcement resists tensile stresses

in restrained concrete and helps prevent theormation o large cracks, it does not completely

prevent cracking, but ensures that the cracks,

as they occur, are more closely spaced and o

a smaller width. In properly designed reinorced

concrete, they will be invisible to the naked eye.

Jois The provision and location o contraction

oints permit movement as a result o drying

shrinkage. nreinorced concrete will tend

to develop larger cracks at irregular intervals

wherever the tensile strength o the concrete is

exceeded by the stresses induced by drying

shrinkage. To prevent such cracks, contractionoints should be installed at appropriate intervals.

It may also be more economical to install

contraction oints in reinorced concrete than to

rely on reinorcement to control shrinkage stresses.

The location o contraction oints is a matter or the

designer but normally they will be situated where

the greatest concentration o stresses due to

drying shrinkage are to be expected: at openings;

at sudden changes in cross-section; in long

walls; and in large areas o concrete pavement

where they are used to divide the concrete into

approximately square bays.

cncLSIn

The importance o the many infuences on

concrete shrinkage cannot be too highly

emphasised, including concrete constituents,

construction practices, ambient conditions,

geometry and design and detailing o the

concrete element.

Whilst the infuence o cement is important,

obviously it is not the only infuence. Indeed in

most situations other actors will have an equal,

i not over-riding infuence. It is the mix design

o concrete, proper construction techniques andappropriate design and detailing that will produce

concrete o low shrinkage characteristics.

FncS

1 Fulton’s Concrete Technology, Seventh

(revised) edition edited by B j ddis, Portland

Cement Institute, South rica, 1994.

2 CI Committee 308 Standard Practice or

Curing Concrete CI Manual o Concrete

Practice, Part 2, 2001.

3 Blakey, F , Infuence o Creep and Shrinkage

on Structural Behaviour Constructional eview,

ovember 1963, pp 15–21.




4 oper, H, Shrinkage Tensile Creep and

Cracking Tendency o Concretes ustralian

oad esearch, Vol. 5, o. 6, December 1974,

pp 26–35.

5 Pickett, , Eect o Gypsum Content and

Other Factors on Shrinkage o Concrete Prisms

Proceedings, merican Concrete Institute,

Vol. 44, October 1974, pp 149–175.6 Welch, B, Creep and Drying Shrinkage

Characteristics o NSW Concrete Concrete

Institute o ustralia, Special Publication o. 1,

Shrinkage and Creep, 1978.

7 oper, H, The Properties o Concrete

Manuactured with Coarse Aggregate o the

Sydney Area niversity o Sydney, esearch

eport 204, january, 1973.

8 ’Hermite, , Volume Changes o Concrete

Proceedings 4th International Symposium

on Chemistry o Cement, Session V, Paper 3,

1960.

9 Hobbs, D W and Parrott, j, Prediction o

Drying Shrinkage Concrete Beton r28, 1982.

10 Morgan, D , Eects o Chemical Admixtures

on Creep and Shrinkage o Concrete Paper

presented at a Workshop on the se o

Chemical dmixtures in Concrete, niversity o

SW, Sydney, 1975.

11 Popovics, S, Concrete Materials – Properties,

Specications and Testing Second Edition,

pp 221–271, oyes Publications, 1992.

12 Soroka, I, Portland Cement Paste and

Concrete Macmillan Press td, 1979.

13 High Strength Concrete ational eady MixedConcrete ssociation o ustralasia and the

Cement and Concrete ssociation o ustralia,

1992.

j2002

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Drying Shrinkage of Concrete

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