The knowledge of shrinkage characteristics of concrete is necessary in the design of structures for crack control. When concrete is exposed to its service environment, it tends to reach an equilibrium with the environment. If the environment is dry, the exposed surface of the concrete loses water by evaporation. The rate of evaporation will depend on the relative humidity, temperature, water-cement ratio and the area of the exposed surface of the concrete. The first water to be lost is that water held in the large capillary pores of the hardened concrete. The loss of this water does not cause significant volume change. However, as drying continues, loss of water from small capillary pores and later from gel pores takes place. With the reduction in the vapour pressure in the capillary pores, tensile stress in the residual water increases. Tensile stresses in the capillary water are balanced by compressive stresses in the surrounding concrete as a result, the concrete shrinks. Evaporation of gel water changes the surface energy of the solid phase and causes further shrinkage. Shrinkage makes up a portion of the total deformation that is observed in a concrete member. If the environment is wet or moist, the flow of moisture will be from the environment to the concrete the result being a volume increase.
Factors affecting shrinkage
Since shrinkage is related to moisture loss from concrete, it is influenced by external factors that affect drying and also internal factors related to concrete and its constituents.
External factors: The external factors affecting loss of moisture from concrete are ambient conditions and size and shape of concrete member.
- Ambient conditions: Air, temperature, relative humidity and wind velocity will affect the loss of moisture from concrete surface. Higher shrinkage is to be expected with rise in ambient temperature, with decrease in relative humidity, with increase in air movement around the concrete, and with increase in the length of time for which concrete is subjected to drying conditions.
- Member geometry: Large thick concrete members dry out more slowly than small thin ones. As a result, for the same drying period, shrinkage of large size members is lower than the smaller size ones which can dry out to their cores more quickly.
Internal factors
Internal factors affecting drying shrinkage are those relating to its constituents such as cements, aggregates, admixtures, concretes mix design, water-cement ratio and water content, aggregate properties and volume fraction and also to those relating to the construction of the concrete, placing, compacting and curing.
Shrinkage in concrete mix
During the process of mixing material to form concrete, a reduction in bulk takes place. This is due to the finer particles of sand and cement filling the voids or interstices of the coarse aggregate.
In order to attain the required volume in a particular mix, we have to increase the quantities of material to compensate for the reduction in bulk. A shrinkage of 20-33% can be anticipated in such calculations.
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