Civil Engineering Reference
In-Depth Information
CHAPTER 15
Volume Changes of Concrete
Concrete changes slightly in volume for various reasons,
and understanding the nature of these changes is useful in
planning or analyzing concrete work. If concrete were free
of any restraints to deform, normal volume changes
would be of little consequence; but since concrete in
service is usually restrained by foundations, subgrades,
reinforcement, or connecting members, significant
stresses can develop. This is particularly true of tensile
stresses.
Cracks develop because concrete is relatively weak in
tension but quite strong in compression. Controlling the
variables that affect volume changes can minimize high
stresses and cracking. Tolerable crack widths should be
considered in the structural design.
Volume change is defined merely as an increase or
decrease in volume. Most commonly, the subject of con-
crete volume changes deals with linear expansion and
contraction due to temperature and moisture cycles. But
chemical effects such as carbonation shrinkage, sulfate
attack, and the disruptive expansion of alkali-aggregate
reactions also cause volume changes. Also, creep is a
volume change or deformation caused by sustained stress
or load. Equally important is the elastic or inelastic change
in dimensions or shape that occurs instantaneously under
applied load.
For convenience, the magnitude of volume changes is
generally stated in linear rather than volumetric units.
Changes in length are often expressed as a coefficient of
length in parts per million, or simply as millionths. It is
applicable to any length unit (for example, m/m or ft/ft);
one millionth is 0.000001 m/m (0.000001 in./in.) and 600
millionths is 0.000600 m/m (0.000600 in./in.). Change of
length can also be expressed as a percentage; thus 0.06% is
the same as 0.000600, which incidentally is approximately
the same as 6 mm per 10 m (
3
⁄
4
in. per 100 ft). The volume
changes that ordinarily occur in concrete are small,
ranging in length change from perhaps 10 millionths up to
about 1000 millionths.
EARLY AGE VOLUME CHANGES
The volume of concrete begins to change shortly after it is
cast. Early volume changes, within 24 hours, can influence
the volume changes (such as drying shrinkage) and crack
formation in hardened concrete, especially for low water
to cement ratio concrete. Following are discussions on
various forms of early volume change:
Chemical Shrinkage
Chemical shrinkage refers to the reduction in absolute
volume of solids and liquids in paste resulting from
cement hydration. The absolute volume of hydrated
cement products is less than the absolute volume of
cement and water before hydration. This change in
volume of cement paste during the plastic state is illus-
trated by the first two bars in Fig. 15-1. This does not
include air bubbles from mixing. Chemical shrinkage
continues to occur at a microscopic scale as long as
cement hydrates. After initial set, the paste cannot
deform as much as when it was in a plastic state.
Autogenous
shrinkage
(pre set)
Chemical
shrinkage
Autogenous
shrinkage
(apparent
volume
reduction)
Chemical
shrinkage
(absolute
volume
reduction)
Autogenous
shrinkage
(after set)
Unhydrated
cement
and
water
Cumulated
voids
Paste
as cast
Paste at
initial set
Paste after
final set
Paste after
final set
Fig. 15-1. Chemical shrinkage and autogenous shrinkage
volume changes of fresh and hardened paste. Not to scale.
