Stress And Strain Field Of Concrete During Steam Curing And Its Effect On Cracking

In recent years,prefabricated construction becomes one of the main research focuses of construction technology in China due to its simple construction method,short construction period and environmental friendliness.To increase the production efficiency,some rapid curing methods are usually adopted.More than 70% manufacturers apply steam curing to produce prefabricated components.Microstructure study shows that the pore size is increased and the internal cracks of concrete becomes significantly due to steam curing.Existing studies considered that temperature and humidity stresses are the main causes of concrete cracking,but the evolution of temperature and humidity field within concrete during steam curing are rarely studied.As a consequence,it is difficult to clarify the formation mechanism and interaction of concrete cracking in essence.Therefore,this paper focuses on the stress/strain field of steam curing concrete in order to reveal the formation mechanism and interaction of cracks during steam curing in essence.The main research work is as follows:(1)In order to simulate the heat and mass transfer process of steam curing concrete,a heat and mass transfer model is established based on a three-dimensional Lattice model of concrete.In consideration of the influence of hydration heat and Knudsen diffusion on the heat and mass transfer process of steam curing concrete,appropriate parameters are selected to solve the variables in the governing equations.At the same time,the temperature and humidity field in steam curing concrete is tested by embedding temperature and humidity sensors,and the measured values of temperature and humidity varying with time are obtained.Then compared them with simulation results,the accuracy of the model is verified.The model is used to study the effect of curing system,including heating rate and constant temperature on temperature and humidity field of concrete.The results show that the temperature and humidity of the surface layer change faster than that of the interior layer during heating,and the temperature varies greatly between the interior layer and the surface layer of the concrete.Due to the influence of hydration heat of concrete,the temperature of the interior layer of the concrete during the constant temperature period is slightly higher than that of the surface layer.During the heating period,the faster the heating rate,the lower the overall temperature of concrete,the greater the temperature difference between interior and surface layer.The higher the constant temperature,the higher the overall temperature of concrete,the higher the moisture content,and the greater the temperature difference between interior and surface layer during the cooling process.(2)In order to simulate the cracking process of steam curing concrete,the temperature and humidity distributions simulated by the heat and mass transfer model are applied to Lattice fracture model as local load,and the strain and stress distributions of concrete are calculated.The model is used to study the effects of curing system,including heating rate and constant temperature,on the strain and stress distribution of steam curing concrete.The results show that the concrete expands during steam curing,and the deformation of concrete during the constant temperature period is the largest in comparison with other curing periods.Compared with the constant temperature period,the displacement of concrete decreases during the cooling period.The surface layer of the concrete is under tensile stress during heating and constant temperature period and it is under compress stress during the cooling period.While the situation is just the opposite in the interior layer.During heating period,the faster the heating rate,the smaller the displacement and stress of concrete,while the situation is just the opposite during constant temperature period.The higher the constant temperature,the greater the displacement and stress of concrete.The difference of displacement is not obvious,but the stress distribution is more uneven.(3)The distribution of micro-cracks in steam curing concrete with different curing systems obtained by Lattice fracture model is counted and compared with the results of CT scanning test.The results show that there are more micro-cracks in the surface layer of steam curing concrete,and the cracks width is wider compared with the interior layer.While during the constant temperature period,the number of micro-cracks in the interior layer is more than that of surface layer,and the cracks width is wider in the interior layer.During the heating period,the faster the heating rate,the less cracks,the smaller cracks width.During the constant temperature period,the number of cracks increases at higher heating rate.The higher the constant temperature,the more cracks,the wider crack width,especially in the heating period.