Shrinkage

The plastic or capillary shrinkage of concrete corresponds to the reduction in volume of the concrete due to water evaporation in the first hours after installation. During this period, the concrete is still in a plastic state of the material. Due to the early time point after the concreting of the process is also known as Early shrinkage.

After placing the concrete into a formwork is the surface on which water can evaporate, usually covered by a closed planar film of water. This is getting thinner as a result of mainly caused by the evaporation loss of water, until, finally, the surface disposed on the fine solid particles are no longer covered by water. Because of adhesion forces formed between the solid particles a curved water surface. This creates a pressure differential between the liquid and the surrounding air, that is, a negative pressure in the capillary water. The resulting tensile forces cause a reduction of the distances between the solids particles and thus shrinkage deformations. However, the pressure increase with further loss of water can not be prevented by the compression of the fabric. The curvature of the surface of the water continues to increase, until eventually all of the particles are no longer distances can be bridged on the surface of the curved surface. If this is the case, air from the surface begins locally forth penetrate into the structure. Which reached at the first air inlet is known as the capillary negative pressure air inlet point. Is this pressure level is reached, there is an increased risk of cracking, because the places where air inlet was, constitute weak points in the structure of the material. The formation of cracks in desiccating suspensions sets the air intake ahead in the structure. However, the air intake does not necessarily lead to cracking. It also requires sufficient mobility of the solid particles.

The same physical processes can also be found in other desiccating suspensions, such as in sludge or cohesive soils, watching.

Under unfavorable conditions, the plastic shrinkage of concrete measures be up to 4 mm / m. Occurring Kapillarschwindrisse can reach lengths of up to 2 mm. They often form a net-like pattern and occur mostly in scale concrete components such as floor boards or traffic areas with high evaporation rates.

The capillary pressure in the plastic concrete can be measured with the help of special sensors, which are similar in function tensiometers under site conditions. Post-treatment measures can the water loss due to premature drying and thus the capillary suction pressure limit. Suitable post-treatment measures are, for example covering it with film, or the use of curing agents.