【正文】 microcracks, precisely because of the existence of these initial defects before the concrete showing some heterogeneity characteristics . Microcracks is usually a harmless cracks on concrete loadbearing, seepage and use the function does not cause harm. But in the concrete under load, temperature effects, the microcracks will continue to expand and connectivity, and ultimately the formation of visible macro cracks, the cracks is often said that the concrete works. Many cracks in concrete cracks caused by deformation: such as temperature change, shrinkage, expansion, uneven subsidence caused cracks。 cracks caused by improper conservation of the environment and chemical action, etc. . Common cracks in the concrete are the following: (1) shrinkage cracks。 (3) settlement cracks。 ⑤ chemical reaction caused by cracks. cracks and prevention Shrinkage cracks and prevention of Shrinkage cracks appear in a week or so after the end of the concrete curing period of time or after concrete pouring. The evaporation of the water in the slurry will produce shrinkage, and the shrinkage is irreversible. Drying shrinkage cracks are mainly caused due to the different degree of evaporation of the moisture inside and outside the concrete result of deformation of the type: concrete affected by the influence of external conditions, excessive surface moisture loss, large deformation, internal humidity changes smaller deformation smaller, the larger the surface shrinkage deformation by concrete internal constraints, have a greater tensile stress cracks. The lower the relative humidity, the greater the shrinkage of cement paste shrinkage cracks are easy to produce. Shrinkage cracks parallel to the surface of the linear or reticular light fine cracks, width of more than to mm between the flat parts more mon in mass concrete, thin flatslab along the short to distribution. Shrinkage cracks usually affect the impermeability of concrete, causing corrosion of steel affect the durability of concrete, water pressure will produce hydraulic fracturing affecting concrete bearing capacity. The concrete shrinkage main and to the watercement ratio of the concrete, cement ponent, the amount of cement used, the nature and amount of aggregate, the amount of admixture. The main preventive measures: (1) Selection of a smaller amount of shrinkage cement, monly used in low heat cement and fly ash cement, reduce the amount of cement. (2) Concrete shrinkage by watercement ratio, the greater the watercement ratio, the greater the shrinkage in the concrete mix design should try to control the selection of a good watercement ratio, while mixing the waterreducing agent . (3) Strict control than with concrete mixing and construction, concrete water consumption must not be greater than the amount of water given by the mixture ratio design. (4) To strengthen the early curing of concrete and appropriate to extend the concrete curing time. Appropriate to extend the winter construction concrete insulation cover, and brushing curing agent curing. (5) Set the appropriate contraction joints in concrete structures Plastic shrinkage cracks and prevention The plastic shrinkage refers to the before condensation, the concrete surface due to rapid dehydration resulting contraction. Plastic shrinkage cracking is generally hot and dry or windy weather, the cracks mostly wide in the middle, both ends of the thin and vary in length, noncoherent state. Short cracks as long as 20 to 30cm the longer cracks of up to 2 to 3 m, width of 1 to 5 mm. Main causes: the concrete before the final setting almost no strength or intensity is very small or just the final setting concrete small strength, high temperature or a larger wind, the concrete surface excessive water loss, resulting in capillary have a greater negative pressure of the concrete volume of a sharp contraction, but this time the strength of the concrete can not resist its own contraction, resulting in cracking. The main factors affecting the plastic shrinkage cracking watercement ratio, the setting time of the concrete, the ambient temperature, wind speed, relative humidity, and so on. The main preventive measures: (1) Selection of shrinkage values smaller high early strength Portland cement or ordinary Portland cement. (2) Strict control of watercement ratio, to increase the slump and workability of the concrete, to