Study On Behavior Of Early-age Concrete Cracking In Basement Walls

Nowadays, with the construction of large quantities of high-rise buildings, the dimensions of basement are larger and larger. So the cracking of early-age concrete of basement is a widespread problem. Cracking in concrete destroy the appearance of wall, impair structural durability, increase permeability. At present, no generally theories and codes are applicable to the cracking of early-age concrete. Therefore, it is significant to study this problem.The cracking of early-age concrete is caused by volume change; shrinkage movement and thermal movement are included. When the movements are restrained, the stress is caused. Concrete will crack when the stress is equal to the tensile strength of that time. The shrinkage movement can be transformed temperate difference at equivalence relation. Finite element method can connect the thermal and thermal stress, which make it easy to analyze unsteady temperature field and stress field.Based on the theory of the unsteady temperature field and stress field, three-dimensional finite element model is set up. The wall is analyzed by program of ANSYS finite element analysis. The results show temperature rising, temperature decreasing and temperature changing with environment occur after pouring concrete. The temperature of interior is higher than that of surface of a section. The temperature decreases from the bottom to the top of the wall. Compression stresses occur after pouring the concrete, then to the peak stress levels, from then on the compression stress tend to decrease to zero and turn into tensile stresses increasing gradually. When the temperature rise, no tensile stresses situation occurs as the stress field is dominated by the compression stresses. During the cooling process, the tensile stresses of interior are higher than that of surface of a section. The tensile stresses are decreasing from the bottom to the top of the wall. To the middle of the length of the wall, the difference between tensile stresses tends to smaller and smaller.Simultaneity, Walls with difference L/H ration of 4.2 > 6.2, 8.2> 12.2 are analyzed here. The results indicate the L/H ration has no relation to temperature field. But has effect on stress field evidently. The tensile stresses increase with the increase of L/H ration, the results from finite element analysis is consistent with those from formula in ACI. The L/B ration has no effect on temperature field and stress field.Beside, temperature field and stress field is analyzed in different environments after the formwork removed, the results show the temperature in walls decrease more quickly withambient temperature abate suddenly, cracking in concrete can occur on the first day after formwork removed. But when the concrete is covered with wet straw, the temperature in walls decrease slowly and thermal stresses levels are lower. Hence, to the basemen as a kind of large area and thin structure, the curing after formwork removed can decease largely the risk of cracking.Finally, winter concreting and spring concreting is analyzed, the peak temperature is different in the two situations, but the velocity of temperature decreasing is almost same and the thermal stresses have little difference, which indicate controlling temperature not only reduce the peak temperature but reduce the velocity of temperature decreasing. The results in this paper can be used as reference to engineers.