Research On Plastic Shrinkage And Durability Of Lightweight Aggregate Concrete

The plastic shrinkage cracking gives prominence to early cracking. It would not only effect construction appearance and promote the propagation of the cracks generated from drying shrinkage or other factors, but also provide passages for aggressive agents penetrating and moving into the concrete mass, which threatened the safety and durability of concrete construction.Lightweight aggregate concrete (LWAC) has light weight and high compression, which can reduce the load of concrete itself in massive concrete construction and had notable economic and technical benefit. Thus, it is of important significance to pay attention to the study on plastic shrinkage and cracking in order to promote the progress of LWAC and direct concrete construction practice. Characteristic of plastic shrinkage cracking of concrete made of high strength ceramsite made from shale as coarse aggregate and variability law of LWAC on the condition of instantaneous compressive load were investigated systematically in this paper. It could be concluded as following:①When the cement was same in per cubic meter, and the ration of quantity of water to cement (w/c) was between 0.35 and 0.50, evaporation quantity of water increased with the increase of w/c and there was a critical w/c ratio of maximum plastic shrinkage cracking between lower and higher w/c ratio of concrete mixtures ,which was about 0.40. LWAC with lower w/c ratio were prone to plastic shrinkage cracking, but higher w/c less prone to. ②To a certainty of sand percentage, LWAC with moderate critical sand percentage, which changed in about 35～40%,was more sensitive to plastic shrinkage cracking than concretes with lower or higher sand percentage. ③Water absorption rate of ceramsite increased with the increase of its diameter. Rate of water evaporation of concretes with big diameter ceramsite was lager, but area of plastic shrinkage cracking less. On the second hand, ceramsite dealt with water can efficiently restrain plastic shrinkage cracking of concrete made of it. ④The effects of slag and fly ash on plastic shrinkage cracking was related to quantity of them. There was a critical quantity, which was 20～30%. Below which concretes containing them were more sensitive to plastic shrinkage cracking than that of concretes without fly ash or slag, and above which, shrinkage cracking of concrete decreased. ⑤Water absorptivity of ceramsite in concrete, efficiency of "weibeng" of lightweight and good interfacial zone between lightweight aggregate and cement paste made LWAC durable. The more high compression of concrete was, the more durable was LWAC. ⑥Instantaneous compressive load weakened durability of LWAC, which related with magnitude of instantaneous compressive load. When the ration of the instantaneous compressive load to compression (compression of concrete conserved in standard condition) was between 40and 60 percent, the ability of resistance to carbonation, freeze-thaw, corrosion of sulfuric acid and sulphate of LAWC (durability coefficient) decreased no more than 5percent. When the ration of the instantaneous compressive load to compression was between 60and 80 percent, durability coefficient decreased 10 to 25 percent. The more high concentration of corrosion medium was, the more the durability coefficient fell.