Since the advent of superplasticizer in the late 1930s, the superplasticizer technology has been rapidly developed and widely used, and has become an indispensable fifth component in concrete, and is widely used in concrete production. Because the evaluation index of concrete in practical application is only the strength and slump of concrete, unless there are requirements for the impermeability and other indexes in special parts, the development process of concrete is mainly due to the continuous improvement of concrete strength grade and slump. The process, while ignoring the stability of the concrete system, also makes the production and application of superplasticizer only focus on the water reduction rate. The direct consequence of poor system stability is the initiation of cracks. In particular, the early shrinkage and cracking of concrete have attracted the attention of experts and scholars all over the world. Concrete cracks have memory. If the early shrinkage cracks of concrete are not treated or treated improperly, the deterioration of concrete will be accelerated, which will not only affect the appearance and function of the building but also constitute structural safety and durability.
With the increase of concrete slump, the crack occurrence time of concrete is generally delayed. When the crack of concrete with the concrete slump of 6 cm is delayed by about 2 h, the concrete without dehydration agent is delayed by about 2 h. It is superplasticizer that can increase the dispersion of cement agglomerates, change the ratio of adsorbed water and free water, increase the amount of free water, and increase the amount of free water that can be evaporated, although the addition of superplasticizer will increase the early water evaporation of concrete. However, the increase in the negative pressure of the pore gap may be reduced, that is, the amount of free water released is much larger than the increase in the amount of water evaporation. At the same time, because the superplasticizer adsorbed on the cement particles, the reaction between cement and water was hindered, and the production of new crystals was postponed, and the early hydration stage was significantly delayed. The cement setting time is delayed, and the cracking time of the concrete is closely related to the initial setting time so that the cracking time is pushed back.
When the concrete slump is larger, for example, more than 20 cm, the concrete will have more serious segregation and bleeding, and the concrete indicates the formation of a self-supporting layer, which delays the cracking time of the concrete. The bleeding and segregation of concrete forms a self-supporting layer for concrete, which reduces the early drying shrinkage of concrete, but also reduces the relative humidity and free moisture inside the concrete. Incorporating an appropriate amount of superplasticizer to make the concrete slump within a suitable range, for example, the slump is in the range of 11-18 cm, which may make the free water content of the concrete suitable and have a suitable retarding effect. The concrete has a small shrinkage and dry shrinkage, and the joint of the overall shrinkage and strength of the concrete is optimal so that the cracking performance is best. Concrete without admixture has a very poor slump, and the dry and hard vibrating of concrete is not dense, which makes the internal structure of concrete relatively loose. The gel produced by concrete hydration reaction fills slowly between particles and the concrete strength is low. At the same time, the relative humidity of the internal voids of the concrete is low, and the relative humidity caused by evaporation of water is reduced faster. Under the joint action of the two, the crack of the concrete develops rapidly, and the cracking area and the maximum crack width are larger than those of the concrete with the appropriate amount of superplasticizer. With the increase of the amount of superplasticizer, the dispersion of cement agglomerates is improved, the pores of the concrete are refined, and the superplasticizer is added to reduce the surface tension of the solution and reduce the capillary pore tension. According to the Laplace equation, the capillary pores. The combination of refinement and degree of surface tension reduction will ultimately determine how they will be affected.
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