The setting and hardening of concrete has to go through the three processes of initial setting, final setting and initial strength, which is mainly realized by the hydration of cement. The hydration reaction of cement releases heat, which causes the concrete to heat up, and there will be initial volume changes and possible cracks. Understanding the nature of concrete at this stage is of great benefit to controlling the construction quality of concrete.

1. The initial performance and changes of concrete

The initial volume change of concrete. When the concrete hardens in dry air, the moisture in the concrete will gradually evaporate and lose, so that the condensed colloid in the cement stone will gradually dry and shrink. This is called the dry shrinkage of the concrete; when the concrete hardens in water for a long time, due to the hydration of the cement Sufficient, internal free water fills the voids and capillary channels between the concrete particles, and the concrete will expand slightly, which is called the wet swelling of the concrete. This kind of shrinkage, wet expansion and deformation of concrete is caused by the change of moisture in the concrete. Its wet swell value is very small and will not cause concrete damage. The shrinkage deformation of concrete is more harmful to the structure. It may cause greater tensile stress on the surface of the concrete, which will cause surface cracks and affect the durability of the concrete.

2. Factors affecting concrete shrinkage

⑴Water consumption

The water consumption in concrete is closely related to the drying shrinkage value. When the water consumption increases by a certain percentage, the concrete drying shrinkage value increases by two or several times this percentage. Reducing water consumption is the primary measure to reduce the shrinkage value.

⑵ Water-cement ratio

Generally speaking, the rich mix is ​​often used for low water-cement ratio, but the amount of cement is large, and the unit water consumption is also large. Under this condition, the increased shrinkage value due to the higher water consumption and cement consumption of the rich mix exceeds the shrinkage value reduced by the low water-cement ratio. The concrete with the rich mix and the low water-cement ratio is sometimes higher than the poor mix. The shrinkage value of the ash ratio is larger. Therefore, the amount of cement and water should be controlled in the preparation of clay concrete to reduce the shrinkage value of concrete.

⑶ Cement varieties

In general, high-grade cement particles are finer and shrink more. The dry shrinkage of concrete made of slag cement and pozzolanic cement is larger; the dry shrinkage of concrete made of fly ash cement is smaller; the dry shrinkage of concrete made of bauxite cement is faster. Therefore, it is necessary to strengthen the maintenance within a period of time after the concrete is poured. The temperature difference between the surface and the inside of the concrete is not too large, and the surface of the concrete is prevented from drying and cracking due to the rapid evaporation of water.

In addition, the size and gradation of aggregates also have a greater impact on the shrinkage of concrete.

In the process of concrete hardening, the shrinkage deformation can be reduced, which can effectively improve the durability of the concrete, thereby improving the quality of the entire engineering building.