【正文】
around the expansion stress generated, leading to protective layer cracking, peeling, cracks along the longitudinal steel, and rust penetrate into the concrete caused by temperature changes of (1)the temperature in temperature due to changing seasons, but the changeis relatively slow, on bridge structure is mainly caused vertical displacement of the bridge, usually through the bridge deck expansion joints, bearing displacement, set flexible measures to regulate piers and other structures, Only limited structural displacement only when the cracks caused by temperature, such as bridge, just bridging.(2)sun decks, the main beam or pier side by sun exposure, the temperature was higher than other parts of the distribution of the temperature gradient is to its own restraints, resulting in excessive local tensile stress, following is a sudden drop in sunshine and temperatures cause cracks in the structure of the most mon cause.(3)The sudden drop in dump heavy rain, preinvasion of cold air, the outer surface of the sun and so can lead to a sudden drop in temperature, but the relatively slow temperature changes within the structure resulting from the temperature and the sudden drop in temperature when the internal force calculation can be real bridge design specifications or reference data, without considering the reduction of concrete elastic modulus.(4)heat of hydration in the construction process, mass concrete(thickness of more than 2m)after pouring the cement hydration heat, causing the internal temperature is high, both inside and outside temperature difference is too large, resulting in surface cracks.(5)Construction of steam curing or inappropriate measures winter construction, concrete cold and heat, the temperature inside and outside the uneven, prone to cracks.(6)preT beams installed between the diaphragm, bearing embedded steel plate welded with leveling, if welded properly, iron burns near the concrete easier to : Cracks in concrete of course, far more than these, not one by one go into details to the above reasons we have given below the corresponding fracture prevention measures and measures to prevent fractures Prevention of shrinkage cracks(1)use a smaller contraction of cement, monly used in lowheat cement and fly ash cement, reducing the amount of cement.(2)shrinkage of concrete influenced by the watercement ratio, the greater the watercement ratio, the greater the shrinkage, so the concrete mix design should try to control the selection of watercement ratio, while adding the appropriate reduction agent.(3)strictly control the concrete mixing and construction in the mix, the water consumption of concrete mix design must not be greater than the given amount of water.(4)to strengthen the early curing of concrete and appropriate to extend the concrete curing construction thermal insulation of concrete cover to be extended time and brushing curing agent curing.(5)in strict accordance with the design requirements set in the proper concrete contraction Prevention of plastic shrinkage cracks(1)selection of high early strength and shrinkage value is small or ordinary Portland cement.(2)strictly control the watercement ratio, adding superplasticizer to increase slump and workability of concrete, reduce the amount of cement and water.(3)Prior to pouring concrete, water will be evenly wet the grass roots and templates.(4)timely coverage of plastic film or damp straw, hemp and other films to keep wet concrete surface before the final setting, or curing agent in the sprayed concrete surface for conservation.(5)at high temperatures and windy weather to set the sun and wind facilities, and timely Subsidence Cracks(1)of the soft soil, filling the upper structure of the foundation necessary before construction paction and reinforcement.(2)ensure that the template has sufficient strength and stiffness, and the solid support, and to the foundation in good condition.(3)to prevent the process of concrete pouring foundations were immersed in water.(4)the time the template is not removed too early, and pay attention to the order form removal.(5)puts up a template in the permafrost to take some time to pay attention to preventive The preventive measures temperature cracks(1)try to use low heat or in the hot cement, such as slag cement, fly ash and cement.(2)reduce the amount of cement, will try to control the amount of cement in the 450kg/m3 below.(3)reduce the watercement ratio, the general control of the concrete watercement ratio of or less.(4)to improve the aggregate gradation, fly ash or high water reducing agent to reduce the amount of cement to reduce heat of hydration.(5)to improve the concrete mixing process, in the traditional threetechnology based on cold air cooling with the second new technology, reduce the temperature of the concrete pouring.(6)in concrete by adding a certain amount of water, plasticizer, such as the role of retarding admixture to improve the flow of fresh concrete, water retention, reduce heat of hydration, thermal spike delay the emergence of time.(7)can be used when pouring the hot season erection of sun visor and other auxiliary measures to control the concrete temperature rise, reducing the temperature of pouring concrete.(8)the temperature of mass concrete structure sizerelated stress, the larger the size of concrete structure, the greater the thermal stress, so to arrange the construction process, hierarchical, block casting, to facilitate cooling, reducingthe constraints.(9)set in mass concrete internal cooling pipes, through water or air cooling, reducing the temperature difference between inside and outside the concrete.(10)to strengthen concrete temperature monitoring, timely cooling, protective measures.(11)reserved for temperature contraction joints.(12)reduce constraints, before pouring concrete in the basement and the old concrete should be laid on the sand cushion of about 5mm or use of asphalt and other materials brush