Plastic shrinkage of concrete
The plastic or capillary shrinkage of concrete corresponds to the reduction in volume of the concrete as a result of water evaporation in the first hours after installation. During this period the concrete is still in its plastic material state. Because of the early point in time after concreting, the process is also known as early shrinkage .
After the concrete has been placed in a formwork , the surface on which water can evaporate is usually covered by a closed, even film of water. As a result of the water loss mainly caused by evaporation, this becomes thinner and thinner until finally the fine solid particles arranged on the surface are no longer covered by the water. Due to adhesive forces, a curved water surface forms between the solid particles. This creates a pressure difference between the liquid and the surrounding air, ie a capillary negative pressure in the water. The resulting tensile forces cause a reduction in the distances between the solid particles and thus shrinkage deformations. However, the increase in pressure with further water loss cannot be prevented by compressing the structure. The curvature of the water surface continues to increase until finally not all the particle distances on the surface can be bridged by the curved water surface. If this is the case, local air begins to penetrate the structure from the surface. The capillary negative pressure reached at the first air entry is called the air entry point. Once this pressure level has been reached, there is an increased risk of cracking because the points where air was introduced represent weak points in the structure of the material. The formation of cracks in drying suspensions requires air to enter the structure. However, the entry of air does not necessarily lead to cracking. Sufficient mobility of the solid particles is also required.
The same physical processes can also be observed in other drying suspensions , for example in sludge or cohesive soils.
Under unfavorable conditions, the plastic shrinkage rates of concrete are up to 4 mm / m. Occurring capillary shrinkage cracks can reach widths of up to 2 mm. They often form a network-like pattern and mostly occur in flat concrete components such as floor slabs or traffic areas with high evaporation rates.
The capillary pressure in plastic concrete can be measured under construction site conditions with the help of special sensors, which function similar to tensiometers . By treatment measures , the water loss can be due to premature drying out, thus limiting the suction stress. Suitable post-treatment measures are, for example, covering with foil or the use of post-treatment agents.
literature
- Reinhardt, HW: civil engineering materials . rnst & Sohn Verlag, 2010.
- Grube, H .: Definition of the different types of shrinkage, causes, size of the deformations and practical significance in construction . In: concrete . No. 12 , 2003, p. 598-603 .
Web links
Individual evidence
- ↑ Wittmann, FH: On the cause of the so-called shrinkage cracks . In: Cement and Concrete 85/86 . 1975, p. 10-16 .
- ↑ a b Grube, H .: Causes of the shrinkage of concrete and effects on concrete components . In: Series of publications of the cement industry . No. 92 . Beton Verlag GmbH, Düsseldorf 1991.
- ↑ a b c Slowik, V., Schmidt, M .: Capillary shrinkage cracking in concrete . Bauwerk Verlag, Berlin 2010.
- ^ A b Schmidt, D., Slowik, V., Schmidt, M., Fritzsch, R .: Post-treatment of concrete surfaces based on capillary pressure measurement in the plastic material state . In: Concrete and reinforced concrete . No. 102 , November 2007, p. 789-796 .