Shrinkage classification of concrete:
(1) Plastic shrinkage (condensation) is the phenomenon of volume reduction due to the intense hydration reaction before the final setting of concrete and the gradual formation of molecular chains. Plastic shrinkage occurs within about 3 to 12 hours after the concrete is mixed, because the concrete is still in a plastic state when it occurs, so this kind of condensation is called plastic shrinkage. The size of condensation is about 1% of the absolute volume of cement, and it increases with the increase of concrete water consumption and water-cement ratio.
(2) Temperature shrinkage is the shrinkage deformation of concrete due to temperature drop (above 0°C), also known as cold shrinkage. For mass concrete, cracks are mainly caused by temperature changes.
(3) Carbonation shrinkage is the result of the chemical reaction between cement hydrate in concrete and CO2 in the air (in the presence of moisture, the real medium is H2C03). The main reason for the carbonation shrinkage is that the Ca(OH)2 crystals in the cement hydrate are carbonized to form CaCO3 precipitation. The rate of carbonization shrinkage depends on the moisture content of the concrete, the relative humidity of the environment and the size of the member, and the carbonization shrinkage stops when the relative humidity in the air is 100% or as little as 25%. Carbonation shrinkage develops relatively late and is generally limited to the concrete surface.
(4) Drying shrinkage is the volume change of concrete when it is dry, which is caused by the distribution change, movement and evaporation of water in the concrete in the newly generated cement stone skeleton. The structural shrinkage calculation is mainly for drying shrinkage. The domestic and foreign literatures have analyzed the drying shrinkage mechanism of concrete, and it is believed that the drying shrinkage is caused by the diffusion and disappearance of the capillary water inside the concrete.
(5) Autogenous shrinkage refers to the volume deformation of concrete due to the hydration reaction of cement under the condition of sealing (no water exchange with the outside world). Drying shrinkage is the volumetric deformation of concrete due to the loss of void water when exposed to air. The contraction we generally refer to is the sum of the two, that is, the total contraction. According to the research of H.E. Davis et al., the maximum limit self-shrinkage strain of ordinary concrete is only 100×10~, so from a practical point of view, its influence can be ignored (only considered in mass concrete), and only the effect of drying shrinkage should be considered. However, because of the small water-cement ratio and the large amount of cement, high-strength concrete shows earlier, faster and more obvious autogenous shrinkage. Relevant literature confirms that the drying shrinkage of high-strength concrete is much smaller than the autogenous shrinkage (about 3:7), while the autogenous shrinkage of high-strength concrete increases sharply in the initial stage, and then increases slowly with time, and more than 90% of the autogenous shrinkage occurs in The first 28d, so its influence cannot be ignored. Therefore, both autogenous shrinkage and drying shrinkage must be considered for high-strength concrete under dry conditions.
Factors Affecting Concrete Shrinkage and Expansion:
1. The higher the cement elevation, the more the dosage, the greater the water-cement ratio, and the greater the shrinkage;
2. The larger the elastic modulus of the aggregate, the smaller the shrinkage;
3. The maintenance conditions are good, the humidity of the surrounding environment is high during the induration process and the use process, the smaller the shrinkage;
4. The concrete is vibrated and compacted, and the shrinkage is smaller;
5. The larger the body-to-surface ratio of the component, the smaller the shrinkage.
6. Shrinkage deformation develops rapidly in the initial stage, 25% of the total shrinkage can be completed in 2 weeks, about 50% in 1 month, and the growth is slow after 3 months.
