【正文】 ixture. 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 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。 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。 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