Causes of cracking in concrete

Concrete is an important material used in engineering construction, and has many advantages such as low price and energy saving. But concrete is not a material with perfect technical performance. The problem of shrinkage cracks is a problem that has not been solved for a long time. There are many factors that cause concrete cracks, and the cement and water reducing agent used are also important ones.

Causes of concrete shrinkage and deformation cracks :

(1) Plastic crack

4-5 hours after the concrete is poured, the hydration reaction of the cement is very fierce, and the corresponding water evaporation and water reaction will occur, which will cause the loss of water shrinkage. At the same time, self-weight will cause the corresponding aggregate to sink.  The concrete does not have the corresponding hardening and deformation. Then the problem of uneven settlement between the aggregate and the cement will also occur. This is plastic deformation.

If the reinforcement framework is blocked during the sinking of the aggregate, it will lead to the problem of directional cracking of the reinforcement. In the vertical variable section of the component, such as the junction of the T-beam and the base plate, if the uniform settlement cannot be obtained before hardening, it will cause cracks on the surface. The problem of cracks is mainly because the strength of concrete before setting is small, it can even be said that this strength is almost zero. When the concrete has just set, the strength is also very small, and it is easily affected by high temperature and wind, which causes the surface to lose water too quickly. However, the strength of concrete cannot take advantage of the stress caused by the contraction of the hand itself, thus causing cracks.

(2) Hardening shrinkage

The so-called hardening shrinkage mainly refers to the hydration and hardening of concrete, due to the gradual hydration of cement particles, resulting in the capillary and the free water in each void gradually hydration with cement minerals, converted into gels and crystals to form the volume of cement stone Causes shrinkage, which is hardened shrinkage.

(3) Dehydration shrinkage

There is an immediate void in the concrete, and the cement overflows from the capillary, causing dry shrinkage. That is, when the moisture in the concrete continues to increase, causing significant volume shrinkage.

In short, as the temperature continuously changes, the concrete must accompany the shrinkage and deformation, and the shrinkage and deformation are constrained, which will accompany the corresponding semi-life tensile stress and tensile stress. As a result, concrete is subject to cracks.

Effect of water reducer types on shrinkage and cracking properties of concrete :

There is an absolute relationship between the shrinkage of concrete and the moisture retained by the 5-30mm pores in the concrete. The water reducing agent will have a certain effect in the use of concrete, and it can promote the reduction of the pore diameter of the pores in the hardened concrete. In related experiments, it has been proved that the space in the cement slurry can reach 389 for concrete without any water reducing agent, and 240 in the cement slurry with water reducing agent. It shows that after the use of high-efficiency water-reducing agent, the low water-cement ratio reduces the elastic modulus between the aggregate and the cement stone, reduces the thickness of the aggregate layer, causes the capillary pores in the transition zone to become finer, and enhances the transition zone shrinkage.

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