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 change as 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.
The concrete properties influence on drying shrinkage depends on the ratio of water 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.
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.
Information Compiled by David Koratich