Prevention of cracks contributes to the durability of concrete structures, because cracks promote the ingress of water and pollutants. Current construction codes specify limits for the width of cracks depending on environmental conditions in which a structure is built and its intended service life.
Concrete shrinkage types
Chemical shrinkage: Hydration products built up during the hydration process occupy lower volume than the total volume of individual raw materials. This results in a decrease of overall concrete element dimensions as long as the concrete is still soft.
Plastic shrinkage: One of the most common and important types. It exhibits itself through a decrease in volume caused by evaporation of water, leading to concrete contraction in all directions. Influencing factors are relative humidity, temperature and ambient wind. More severe drying conditions increase the shrinkage value.
Autogenous shrinkage: A change of volume that occurs after the initial setting of concrete due to hydration, since this process requires water and therefore reduces the internal free water.
Drying shrinkage: Hardened concrete is usually caused by evaporation of water through existing capillary pores in the hydrated cement paste. The loss of water is a progressive process that tends to stabilize with time, depending on the dimensions of the structural element. Possible measures include a reduction of cement paste volume and application of shrinkage reducing admixture.