混凝土收缩是引发结构早期开裂的最主要原因。本研究以现代混凝土为研究对象，旨在建立混凝土早期变形及其引发的收缩应力的计算方法，重点对混凝土自身与干燥收缩一体化问题、早期内部湿度分布及变化规律、典型混凝土结构非线性收缩应力的计算等问题进行了实验与理论研究。研究采用C30、C50、C80三个强度等级混凝土，对其密封和干燥条件下的自由变形和内部温湿度发展进行了一体化测量。完整的混凝土早期变形呈先膨胀后收缩的变形特征，膨胀结束点是基于变形的混凝土凝结时间，之后的收缩变形将在结构中产生应力，称为有效变形。随着水灰比减小，有效变形增大。对干湿对比条件下混凝土有效变形和内部湿度变化的测量和分析表明，混凝土自身与干燥收缩可统一到相同的发生机理——毛细张力上来。基于弹性自洽原理，建立了水泥石孔隙中毛细张力与混凝土宏观变形的关系模型；引入了与混凝土孔径分布和孔隙含水状态相关的孔隙修正系数。所建立的混凝土自身与干燥收缩一体化计算模型能够对较宽水灰比范围(w/c=0.3~0.6)和不同失水状态的混凝土收缩变形进行计算。在较大尺寸试件中测量了早龄期混凝土不同位置处的温湿度。混凝土内部湿度随时间发展呈两阶段特征，湿度饱和期和湿度下降期；湿度沿高度呈显著的梯度分布特征。水化耗水和水分扩散是引起混凝土内部湿度下降的主要原因。混凝土内部温度初期由于水化热累积而上升，之后趋于环境温度。表面失水会带走热量并导致水泥水化加速期提前结束，因此温升值减小、温峰提前。基于密封与干燥条件下的湿度对比实验求解了混凝土早期水分扩散系数，计算的扩散系数在10-9m2/s量级。混凝土早期扩散系数受内部湿度影响显著，相对湿度越大，扩散系数越大。建立了考虑水分扩散和水泥水化耗水的湿度场计算模型，并进行了数值求解。结果表明：干燥作用使混凝土形成了表面湿度低、内部湿度高的湿度分布特征。建立了综合考虑水泥水化、太阳辐射和环境温度变化影响的早龄期混凝土内部温度有限差分计算模型。通过变形分解建立了非线性温湿度变形条件下混凝土路面板温湿度应力分项计算模型。考察了季节、浇注时间、混凝土强度对应力分布和发展规律的影响。结果表明路面板温湿度应力沿高度方向呈显著的非线性特征。
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.
