Integrative Studies On Autogenous And Drying Shrinkage Of Concrete And Related Issues

Shrinkage in concrete is known as the most important reason for cracking in early-age concrete. In order to understand the mechanism of shrinkage induced cracking, the shrinkage resulted stresses must be calculated and then we may know where and when the cracking may occur in the structures. To do so, the shrinkage law in concrete, including both the relationship of shrinkage-age and shrinkage-location is critically needed. The object of present study is to establish a model for shrinkage prediction whatever it is belong to autogenous and/or drying shrinkage. The study focus on the integrative studies on autogenous and drying shrinkages, shrinkage gradients in concrete as well as shrinkage induced stresses in a typical slab structure.Three kinds of concrete with compressive strength of 30MPa, 50MPa and 80MPa at 28 days (C30, C50, C80) were selected as investigation samples. Free deformation and interior temperature & humidity of concrete were measured experimentally in the same specimen. The results show that the complete deformation of very early-age concrete behaves first plastic swelling at initial several hours after casting and then shrinking with a gradually reduced rate. The end of swelling may be defined as deformation determined setting time. The shrinkage after setting is the effective deformation that may induce stresses in structures. The effective deformations increase with decrease of water to cement ratio.In order to investigate the relationship between shrinkage and interior humidity, experimental measurements on shrinkage and interior humidity of early-age concrete under both plastic sealing and drying conditions were carried out. The experimental results show that a close relationship between shrinkage and interior humidity is existed and the interior humidity may serve as the driving force parameter for shrinkage analysis whatever the shrinkage is autogenous shrinkage or drying shrinkage. Based on these findings, a capillary stress based model for shrinkage prediction of concrete was developed. A pore structure related parameter was included in the model to taking the effect of micro structure of cement paste in account. The model predictions and experimental data agree well. The model can be used for shrinkage prediction to large range of water to cement ratio (0.3~0.62) and different drying conditions, including sealed concrete.In order to study the distribution of moisture in early-age concrete, humidity and temperature at different depth of larger concrete specimens were measured. Experimental results show that the general law of the development of relative humidity inside of concrete since casting can be described by a vapor saturated stage with 100% relative humidity (stage I) followed by stage during which the relative humidity gradually decreases (stage II),and a humidity gradient exists along the concrete specimen. Cement hydration and water diffusion produce the variation of humidity in concrete. The temperature inside of concrete rises due to accumulation of cement hydration heat at initial few hours from casting, and then declines and closes to ambient temperature finally.In order to solve the moisture diffusion coefficient of concrete at early-age, interior humidity of early-age concrete under sealed and drying conditions were measured respectively at the same time and the moisture loss resulted from drying is separated. Based on above experimental results, moisture diffusion coefficients of three kinds of concrete were calculated. The results show that the moisture diffusion coefficient is at the magnitude of 10-9m2/s. The diffusion coefficient is significantly dependent on the moisture content. The higher the moisture content, the higher the diffusion coefficient.To calculate shrinkage strain distribution in concrete, a moisture distribution model of young-age concrete was presented. In the model both cement hydration and moisture diffusion resulted humidity reduction was taking into account synchronously. The model results in terms of moisture distribution agree well with experimental results. In addition, a numerical model for temperature field prediction in concrete pavement was developed. The effect of hydration heat of cement, solar radiation and atmospheric temperature changes on the temperature field were taking into account in the model.Finally, analytical solutions of thermal & humidity variation induced shrinkage stresses in concrete pavement were presented subjected to a nonlinear shrinkage strain along the slab depth. In the model, the nonlinear shrinkage strain along the slab depth was divided into three components, an average strain component, a linear strain component and a nonlinear strain component. The thermal & humidity shrinkage stresses resulted by each strain component were calculated separately. The total thermal & humidity stresses were obtained by summing the three stress components together. Using the model, shrinkage induced stresses in three kinds of C30, C50, C80 concrete pavements were calculated separately. The calculation results show that the shrinkage stresses are nonlinearly distributed along the slab depth and its influences on mechanical behavior of concrete in early-age is great.