Transcript
8/3/2019 Drying Shrinkage of Concrete
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IntdctIn
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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Dy Shk of Cemen
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t S th mix design of concree,
proper consrucion ecniques and
appropriae design and deailing
a will produce concree of low
srinage caracerisics
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Page 2 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
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Page 3 of 6 > DI SHIKE o Cement and Concrete
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
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Page 4 of 6 > DI SHIKE o Cement and Concrete
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
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Page 5 of 6 > DI SHIKE o Cement and Concrete
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.
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Page 6 of 6 > DI SHIKE o Cement and Concrete
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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