1. Concrete Factors:

(1) When the concrete hardens, the moisture decreases gradually, resulting in drying shrinkage. The dry shrinkage is related to the aggregate gradation, cement variety, standard fineness, cement dosage, etc. under the standard state, the limit shrinkage deformation value is generally 3.24*10-4, and 2.5*10-4 for reinforced concrete.

(2) After the concrete is poured, it will be hydrated and condensed, and transform from plastic state to solid state, resulting in plastic shrinkage, forming 0.05 ~ 0.1mm wide irregular cracks on the concrete surface.

(3) After the cement standard is revised, the average specific surface area of cement fineness is increased from 300 ㎡ /kg to 330 ㎡ /kg The finer the cement particles are, the larger the surface area reacts with water. The hydration is faster and more complete, so the setting and hardening is faster and the early strength is higher. However, it also increases the temperature shrinkage and drying shrinkage of the concrete.

2. Concrete Construction Reasons

(1) The whole process of concrete from mix proportion design, selection of raw materials, ready mixing, transportation, pouring and curing is a systematic project, but most construction personnel only put forward requirements for aggregate size and preparation strength, ignoring measures to ensure technical performance. In addition, during the construction, the supply channel of sand and stone materials is not fixed, and the quality is unstable, which brings some uncertain factors, thus increasing the probability of concrete cracks.

(2) Mass concrete is usually pumped during pouring. Due to the influence of construction equipment, the particle size of concrete coarse aggregate decreases, the amount of cement increases, and the sand ratio increases, resulting in greater concrete volume shrinkage. The shrinkage value of pumped concrete is 6 ~ 8*10-4, which is 2 ~ 3 times larger than that of ordinary concrete.

(3) Poor concrete pouring and vibration, untimely or poor curing are also an important reason for concrete cracks. Under the common action of light and wind, the evaporation rate of water in concrete is accelerated.

(4) The strength, rigidity and stability of the formwork and its support are not enough, or the formwork is removed too early, resulting in structural cracks.