Civil Engineering Reference
In-Depth Information
autogenous shrinkage starts at initial set while others eval-
uate autogenous shrinkage from time of placement.
When external water is available, autogenous shrink-
age cannot occur. When external water is not available,
cement hydration consumes pore water resulting in self
desiccation of the paste and a uniform reduction of volume
(
Copeland and Bragg 1955
). Autogenous shrinkage
increases with a decrease in water to cement ratio and with
an increase in the amount of cement paste. Normal con-
crete has negligible autogenous shrinkage; however, au-
togenous shrinkage is most prominent in concrete with a
water to cement ratio under 0.42 (
Holt 2001
). High-
strength, low water to cement ratio (0.30) concrete can
experience 200 to 400 millionths of autogenous shrinkage.
Autogenous shrinkage can be half that of drying shrinkage
for concretes with a water to cement ratio of 0.30.
Recent use of high performance, low water to cement
ratio concrete in bridges and other structures has renewed
interest in autogenous shrinkage to control crack develop-
ment. Concretes susceptible to large amounts of autoge-
nous shrinkage should be cured with external water for at
least 7 days to help control crack development. Fogging
should be provided as soon as the concrete is cast. The
hydration of supplementary cementing materials also
contributes to autogenous shrinkage, although at different
levels than portland cement. In addition to adjusting paste
content and water to cement ratios, autogenous shrinkage
can be reduced by using shrinkage reducing admixtures
or internal curing techniques. Some cementitious systems
may experience autogenous expansion.
Tazawa (1999)
and
Holt (2001)
review techniques to control autogenous
shrinkage.
Test methods for autogenous shrinkage and expan-
sion of cement paste, mortar, and concrete and tests for
autogenous shrinkage stress of concrete are presented by
Tazawa (1999)
.
Plastic Shrinkage
Plastic shrinkage refers to volume change occurring while
the concrete is still fresh, before hardening. It is usually
observed in the form of plastic shrinkage cracks occurring
before or during finishing (Fig. 15-6). The cracks often
resemble tears in the surface. Plastic shrinkage results from
a combination of chemical and autogenous shrinkage and
rapid evaporation of moisture from the surface that
exceeds the bleeding rate. Plastic shrinkage cracking can be
controlled by minimizing surface evaporation through use
of fogging, wind breaks, shading, plastic sheet covers, wet
burlap, spray-on finishing aids (evaporation retarders),
and plastic fibers.
Fig. 15-6. Plastic shrinkage cracks resemble tears in fresh
concrete. (1312)
Swelling
Concrete, mortar, and cement paste swell in the presence
of external water. When water drained from capillaries by
chemical shrinkage is replaced by external water, the
volume of the concrete mass increases. As there is no self
desiccation, there is no autogenous shrinkage. External
water can come from wet curing or submersion. Swelling
occurs due to a combination of crystal growth, absorption
of water, and osmotic pressure. The swelling is not large,
only about 50 millionths at early ages (Fig. 15-7). When the
Subsidence
Subsidence refers to the vertical shrinkage of fresh cemen-
titious materials before initial set. It is caused by bleeding
(settlement of solids relative to liquids), air voids rising to
the surface, and chemical shrinkage. Subsidence is also
called settlement shrinkage. Subsidence of well-consoli-
dated concrete with minimal bleed water is insignificant.
The relationship between subsidence and other shrinkage
mechanisms is illustrated in Fig. 15-5. Excessive subsi-
dence is often caused by a lack of consolidation of fresh
concrete. Excessive subsidence over embedded items,
such as supported steel reinforcement, can result in
cracking over embedded items. Concretes made with air
entrainment, sufficient fine materials, and low water con-
tents will minimize subsidence cracking. Also, plastic
fibers have been reported to reduce subsidence cracking
(
Suprenant and Malisch 1999
).
100
75
50
w/c = 0.35
0.30
0.45
25
Demolding
0
0
24
48
Age, hours
Fig. 15-7. Early age swelling of 100 x 100 x 375-mm (4 x 4 x
15-in.) concrete specimens cured under water (
Aítcin 1999
).
