【正文】 reduce the amount of cement and water mixed with superplasticizer. (3) Pouring concrete before watering the grass roots and templates evenly soaked. (4) Timely covered by plastic film or wet straw, hemp sheet to maintain final setting of concrete the moist front surface or the surface of the concrete spraying curing agent for set the sun and wind facilities . (5) At high temperatures and windy weather, timely conservation. subsidence cracks and prevention Subsidence cracks caused by structural foundation soil is uneven, soft, or backfill or flooding due to uneven settlement。 to 45 176。 C or higher). Cause the internal temperature to rise sharply due to the larger volume of concrete, the accumulation of a large amount of heat of hydration within the concrete and easy to distribute, and rapid cooling of the surface of the concrete, thus forming a larger temperature difference inside and outside, the larger the temperature difference caused by the internal and external different degrees of thermal expansion and contraction of the concrete surface tensile stress (Practice has proved that when the temperature difference between inside and outside of the concrete itself is 25 176。 C, will produce roughly about 10MPa concrete tensile stress). When the tensile stress exceeds the ultimate tensile strength of the concrete, the concrete surface cracks, cracks occurred in the concrete construction in the late. Concrete construction when the temperature changes, or concrete attacked by the cold wave will lead to concrete surface temperature sharp decline, and contraction, contraction of the concrete surface bound by internal concrete, will produce large tensile stress. cracks, such cracks are generated in the the shallow concrete surface within the range of usually only. The trend of temperature cracks usually certain rules, often crisscross a large area of structural cracks。 deep and perating temperature cracks are generally parallel or nearly parallel to the direction of the short side , cracks along the longest edge segments appear more dense middle. Crack width sizes, affected by temperature changes more obvious, summer, winter wide narrow. The high temperature expansion caused by the concrete temperature is usually middle coarse fine at both ends, while the the shrink cracks thickness less obvious changes. The emergence of such cracks can cause corrosion of steel, concrete carbonation, reduce freezethaw resistance of concrete, antifatigue and impermeability. The main preventive measures: (1) try to use low heat or moderate heat cement, slag cement, fly ash cement. (2) reduce the amount of cement to try to control the amount of cement 450kg/m3. (3) reduce the watercement ratio, generally watercement ratio of concrete is controlled below . (4) improve the aggregate gradation, fly ash or superplasticizer to reduce the amount of cement, lower heat of hydration. (5) improve the concrete mixing process, secondary air traditional cold on the basis of new technology, reduce the temperature of concrete pouring. (6)a certain amount of concrete admixture having less water, plasticizers, retarding admixture role to improve the fluidity of the concrete mixture, water retention, reduce the heat of hydration, delaying the time of occurrence of the thermal peak. (7)hot season puts up visor and other auxiliary measures to control the temperature rise of the concrete pouring, to reduce pouring concrete temperature. (8)mass concrete temperature stress associated with the structure size, the larger the size of the concrete structure, the greater the temperature stress, so reasonable arrangements for the construction process, hierarchical, block pouring, to facilitate cooling, reducing constraints. chemical reaction caused cracks and prevention Alkaliaggregate reaction cracks and steel corrosion cracks caused by the most mon due to the chemical reaction caused by cracks in the reinforced concrete structure. Concrete mixing will produce some of the alkaline ions, these ions produce a chemical reaction with certain reactive aggregate and absorbs water in the surrounding environment and the volume increases, resulting in concrete crisp, expansion cracking. Such cracks usually appear in the concrete structure during use, in the event difficult to remedy, should take effective measures to prevent it in the construction