Drying shrinkage is defined as the contracting of a hardened concrete
mixture due to the loss of capillary water. This shrinkage causes an increase
in tensile stress, which may lead to
cracking, internal warping,
and external deflection, before the concrete is subjected to any kind of
loading. All portland cement concrete undergoes drying shrinkage or hydral
volume changeas the concrete ages. The hydral
volume change in concrete is very important to the engineer in the design
of a structure. Drying shrinkage can occur in slabs, beams, columns, bearing
walls, prestressed members, tanks, and foundations.

Drying shrinkage is dependent upon several factors. These factors include
the properties of the components, proportions of the components, mixing
manner, amount of moisture while curing, dry environment, and member size.
Concrete cured under normal conditions will undergo some volumetric change.
Drying shrinkage happens mostly because of the reduction of capillary water
by evaporation and the water in the cement paste. The higher amount of
water in the fresh concrete, the greater the drying shrinkage affects.
The shrinkage potential of a particular concrete is influenced by the amount
of mixing, the elapsed time after the addition of water, temperature fluctuation,
slumping, placement, and curing.
The makeup of concrete is also very important. Each aggregate and cement
type has distinctive characteristics, each contributing to concrete shrinkage.
The amounts of water and admixtures used during mixing also have direct
and indirect effects on drying shrinkage of concrete. Concrete shrinkage
occurs mostly due to the evaporation of the mixing capillary water. The
severity of this shrinkage depends on the physical properties of the concrete
including size of the structure, location of the structure, and the surrounding
temperature.

Properties and Proportions of Components

The compositional makeup of concrete contributes directly to the drying
shrinkage of concrete. Loss of moisture in the hydrated cement paste results
in shrinkage. Different compositions and fineness of cements have variable
effects on the shrinkage of cement paste. Difference in shrinkage is reduced
significantly due to the adjustment of the amount of gypsum added to the
different cement compositions. The size of aggregate is not as important,
but has an indirect influence on the water content of concrete. Shrinkage
decreases with the volumetric increase of aggregate concentration causing
a linear relationship between free
shrinkage and crack width. High density aggregates and high modulus
of elasticity of aggregates will decrease the compressibility and increase
the shrinkage of concrete. The use of admixtures
may alter the hydration reaction, which results directly in a high increase
of drying shrinkage.

Moisture

The concrete properties influence on drying shrinkage depends on the
ratio ofwater to cementitious materials content,
aggregate content, and total water content. The total water content is
the most important of these. The relationship between the amount of water
content of fresh concrete and the drying shrinkage is linear.
Increase of the water content by one percent will approximately increase
the drying shrinkage by three percent. Constant water to cementitous materials
ratio coincides with changes in the amount of aggregate used.

Dry Environment

The amount of drying shrinkage depends on the environmental conditions;
relative humidity, temperature, and air circulation. Concrete subjected
to a dry atmosphere will, in most cases, have a greater drying shrinkage
than if subjected to an alternative wetting and drying. Lower temperatures
generally produce a decrease in drying shrinkage because of higher humidity
and slower evaporation.