In building construction concrete is the most commonly used material, but in use concrete often appears more serious cracks. Cracks not only have a significant impact on the structure, but also accelerate the weathering of the building, reduce the life of the building, and occur as a chain effect, producing more cracks and causing an increasing risk factor of the building. Research on the causes and control measures of concrete cracks in buildings can improve the safety factor of buildings and protect the stability of buildings.

　　1 Types of concrete cracks

　　1.1 Concrete internal temperature difference cracks

　　Temperature cracks are gaps produced by the internal stresses generated by the heat rise and cold shrinkage of concrete, and are a key cause of cracks. Concrete in the pouring will produce a certain thermal effect, this thermal effect leads to the uneven force of the concrete as a whole, resulting in the gradual shrinkage of concrete, the shrinkage rate is different, so it forms a kind of internal temperature buildup phenomenon, so the internal concrete before the volume is larger, and so the temperature cools, the internal shrinkage, it becomes a red strong stress, when this stress exceeds the tensile strength of the concrete body, it will cause cracks on the surface of the concrete. This crack appears in the concrete surface so it does not affect the structure of the building, but if not dealt with, it is easy to have the effect of serious concrete fracture. Therefore, it is necessary to fill and eliminate the cracks generated by temperature in time.

　　1.2 Concrete shrinkage cracks

　　In the construction of concrete, exposure to air will gradually exclude the water in the concrete and the phenomenon of shrinkage will occur. This phenomenon is the phenomenon of shrinkage of concrete, but for some complex structures of concrete buildings will appear the phenomenon of deformation, changing the original stress structure. This causes a change in the forces within the structure, resulting in cracks. And yet this crack will continue to increase, gradually forming a huge crack through the entire concrete, while the repair method is not effective for this crack, so this crack will have a huge impact on the structure and life of the building.

　　1.3 Settlement cracks

　　In concrete construction, the settlement effect of concrete itself is a common phenomenon. And the cracks caused by the settlement problem are usually triggered by the concrete not being adequately mixed or not mixed evenly. In the concrete construction, if one out of the mud too much, while not timely treatment. It will affect the strength of concrete, the phenomenon of concrete dry crack, multiple gaps, multiple hollow holes, such gaps will greatly reduce the degree of concrete surface, the strength of the concrete surface has a great impact. Also conducive to weathering, reducing the life of the building.

　　2 Concrete crack formation causes

　　2.1 There are many reasons for the formation of concrete, but they can be mainly divided into those caused by stress and those caused by water evaporation.

　　Both causes of cracks can have a serious impact on the building. But the cracks in the concrete caused by stress, no cracks in the other concrete is more stable structure and can withstand stress, in the concrete caused by water evaporation, the cracks are small while the overall strength of the concrete is greatly reduced, other parts can not withstand the corresponding pressure, the concrete life greatly reduced. At the same time in accordance with the way the cracks appear in the concrete we find that a reasonable construction process is an important way to be able to avoid cracks in the concrete. So what concrete construction to be in the next step of construction after it is completely subject to shrinkage, can not be because of the rush and ignore the concrete crack management work.

　　2.2 Think about the causes of cracks from the concrete itself, cracks and the quality of concrete is by the great relationship

　　The quality of water and sand in concrete and the ratio of cement are factors that affect the concrete itself. In the ratio of concrete, the water and sand particles are small, the setting process will be accelerated, but the benefit of shrinkage in the air will become slower. If this time construction will be more likely to cause cracks. In addition, the proportion of cement in concrete is too large, it will cause the shrinkage effect to exceed the limit and cause the formation of cracks.

　　3 Crack control strategies for different causes

　　3.1 Control method of shrinkage cracks

　　The center of gravity of shrinkage crack control should fall on the control of moisture, to maintain the stability of moisture in the member or structure. In order to avoid the phenomenon of rapid moisture loss, at the early stage of concrete casting, watering maintenance can be adopted to maintain moisture, and in addition, in order to minimize the loss of moisture throughout the process, plastic film, straw mats or straw bags can be considered to cover the way. At the same time, the poured concrete should be wiped on the surface of the concrete, and the degree of wiping should be well controlled.

　　3.2 The control method of settlement process cracks

　　Settlement cracks are the phenomenon of concrete cement separated from sand and gravel due to the gravity of sand and gravel particles sinking and cement floating to the surface in the process of concrete mixing, and the existence of aggregate or reinforcement and other structural blockage. This phenomenon tends to occur within a few days of formwork dismantling, and because a certain range of concrete configured with the same mixing method is used in large quantities, this crack occurs on a wide surface, usually in the direction of concrete extension along the longitudinal axis of the reinforcement. The treatment for possible settlement cracks is to control the time of concrete mixing and to mix the concrete sufficiently, not that the longer the layering interval will be better.

　　3.3 For the prevention and control of cracks caused by sudden temperature changes

　　Objects generally have the nature of thermal expansion and contraction, and so does concrete. For the concrete which is not very elastic space, the sudden change of temperature is very easy to lead to cracks. The principle of temperature control is to balance the temperature difference between inside and outside of the member, and the threshold value is controlled within the range of less than 20 degrees. Try not to choose to pour the concrete under high temperature conditions or at night when the temperature difference changes a lot. When pouring, take the way of pouring layer by layer to facilitate the rapid temperature dispersion.

　　After demoulding, to prevent stress relaxation in the concrete, fill measures should be taken in time. The concrete is configured to avoid the selection of overheated cement and the following strategies are recommended to control the temperature: First, circulating coolable water is passed through the pouring base to increase the heat loss. Alternatively, adopt the idea of material chemistry by adding water-reducing agents as well as stones for heat absorption during the concrete configuration. Before pouring, you can simulate various crack control methods as well as the whole process of pouring under actual construction conditions, establish a physical model to predict the maximum shrinkage stress that will occur when the temperature drops, and make appropriate adjustments for exceeding the concrete tension threshold by taking one or a combination of measures according to the specific situation.

　　3.4 Reasonable improvement of structural design

　　Designers should establish the correct design concept, not only to make the necessary calculations of structural strength, but also to pay attention to the structural design, strictly in accordance with the code requirements to ensure the quality. As far as possible, ensure that the strength, water absorption rate, thermal expansion and contraction of the blocks, masonry mortar and plastering mortar used are uniform and coordinated and basically consistent. When there are more line pipes in the floor slab, the thickness of the floor slab should be increased appropriately to ensure the effective section height of the concrete, and steel mesh should be set above the line pipes for protection. Strengthen the heat insulation performance of the roof, reduce the degree of expansion and contraction of the roof and avoid cracks by increasing the thickness of heat insulation layer and heat insulation layer or setting up overhead heat insulation ventilation layer, etc. Floor reinforcement try to use double bi-directional reinforcement, the increase of the amount of steel used is very effective in the prevention of cracks.

　　Conclusion

　　The main causes of cracks in concrete construction are: the influence of temperature factors, the influence of concrete shrinkage, and the influence of the phenomenon of shrinkage. Preventive measures are taken for the three causes. The effect of temperature can be solved by pouring layer by layer, the shrinkage phenomenon can be prevented by reasonable maintenance, and the shrinkage problem can be solved by adequate mixing and reasonable proportioning of concrete. Some design solutions for improving the structure can also be used. The problem of concrete cracks is directly related to the construction technology, so strictly follow the worker process to guarantee the quality of concrete.