Civil Engineering Reference
In-Depth Information
0.1
Effect of Concrete Ingredients on
Drying Shrinkage
Control
0.08
The most important controllable factor affecting drying
shrinkage is the amount of water per unit volume of con-
crete. The results of tests illustrating the water content to
shrinkage relationship are shown in Fig. 15-14. Shrinkage
can be minimized by keeping the water content of con-
crete as low as possible. This is achieved by keeping the
total coarse aggregate content of the concrete as high as
possible (minimizing paste content). Use of low slumps
and placing methods that minimize water requirements
are thus major factors in controlling concrete shrinkage.
Any practice that increases the water requirement of the
cement paste, such as the use of high slumps (without
superplasticizers), excessively high freshly mixed concrete
temperatures, high fine-aggregate contents, or use of small-
size coarse aggregate, will increase shrinkage. A small
amount of water can be added to ready mixed concrete at
the jobsite without affecting drying shrinkage properties
as long as the additions are within mix specifications
(
Suprenant and Malisch 2000
).
The general uniformity of shrinkage of concretes
with different types of cement at different ages is illus-
trated in Fig. 15-13. However, this does not mean that all
cements or cementing materials have similar shrinkage.
Class F
Class C
0.06
0.04
0.02
ASTM C 157
0
0
7
14
21
28
35
42
49
56
63
Age, weeks
Fig. 15-15. Drying shrinkage of fly ash concretes compared
to a control mixture. The graphs represent the average of
four Class C ashes and six Class F ashes, with the range in
drying shrinkage rarely exceeding 0.01 percentage points.
Fly ash dosage was 25% of the cementing material (
Gebler
and Klieger 1986
).
Supplementary cementing materials usually have little
effect on shrinkage at normal dosages. Fig. 15-15 shows
that concretes with normal dosages of selected fly ashes
performed similar to the control concrete made with only
portland cement as the cementing material.
Aggregates in concrete, especially coarse aggregate,
physically restrain the shrinkage of hydrating cement
paste. Paste content affects the drying shrinkage of mortar
more than that of concrete. Drying shrinkage is also
dependent on the type of aggregate. Hard, rigid aggre-
gates are difficult to compress and provide more restraint
to shrinkage than softer, less rigid aggregates. As an
extreme example, if steel balls were substituted for ordi-
nary coarse aggregate, shrinkage would be reduced 30%
or more. Drying shrinkage can also be reduced by
avoiding aggregates that have high drying shrinkage
properties and aggregates containing excessive amounts
of clay. Quartz, granite, feldspar, limestone, and dolomite
aggregates generally produce concretes with low drying
shrinkages (
ACI Committee 224
). Steam curing will also
reduce drying shrinkage.
Most chemical admixtures have little effect on
shrinkage. The use of accelerators such as calcium chloride
will increase drying shrinkage of concrete. Despite reduc-
tions in water content, some water-reducing admixtures
can increase drying shrinkage, particularly those that con-
tain an accelerator to counteract the retarding effect of the
admixture. Air entrainment has little or no effect on drying
shrinkage. High-range water reducers usually have little
effect on drying shrinkage (Fig. 15-16). Drying shrinkage
can be evaluated in accordance with ASTM C 157
(AASHTO T 160).
Water, lb/yd
3
210
250
290
340
380
420
460
1400
1200
1000
800
600
400
200
0
125
150
175
200
225
250
275
Water, kg/m
3
Fig. 15-14. Relationship between total water content and
drying shrinkage. A large number of mixtures with various
proportions is represented within the shaded area of the
curves. Drying shrinkage increases with increasing water
contents.
