Nonlinear and Probabilistic Theory for Concrete Creep

混凝土徐变随龄期变化的线性模型已经成熟，现在适合发展非线性理论。这项研究将建立这样的理论，基于对波特兰水泥固化过程的微观力学分析以及由于应变硬化和背应力的发展而产生的非线性。该模型首先考虑恒定湿度和温度下的基本蠕变，然后将其推广到考虑温度和湿度变化的影响。该模型基于这样一种假设，即粘结断裂的速率(蠕变的根源)取决于水分子在水泥凝胶中的微扩散通量。该模型将扩展到包括材料参数不确定性的概率方面，包括随机参数的相关性和环境条件的波动。根据扩散理论中的谱近似和近似解，提出了一种新的采样方案来处理环境的随机效应。还将开发一种在有限元程序中进行时间逐步积分的数值算法。蠕变试验将在具有温度控制的新型大型三轴扭转试验机上进行。空心试验圆筒将在不同温度和围压下承受三轴载荷，以防止因干燥或温度变化而破裂或同时出现应变软化。这一结果将改善对放射性废物储存、大跨度桥梁、核反应堆事故、混凝土外壳和储罐、建筑物和其他混凝土结构的徐变预测。

The linear modeling of concrete creep with aging has matured, and it is now appropriate to develop a nonlinear theory. This research will establish such a theory, based on micromechanics analysis of the solidification process of portland cement and nonlinearities due to strain-hardening and development of backstress. The model will first be developed for basic creep at constant humidity and temperature, and is then generalized to include the effects of variable humidity and temperature. This model is based on the hypothesis that the rate of bond ruptures (which is the source of creep) depends on the microdiffusion flux of water molecules in cement gel. The model will be extended to include probabilistic aspects of materials parameter uncertainty, including correlation of random parameters, and fluctuation of environmental conditions. Random effects of environment are handled by a new sampling scheme based on spectral approximations and approximate solutions from diffusion theory. A numerical algorithm for step-by-step integration in time in finite element programs will also be developed. Creep experiments will be conducted with a novel large triaxial-torsional testing machine with temperature control. Hollow test cylinders will be subjected to triaxial loading at various temperatures with confining pressure which prevent cracking or simultaneous strain- softening due to drying or temperature change. The results will improve creep predictions for radioactive waste storage, large span bridges nuclear reactor accidents, concrete shells and tanks, buildings and other concrete structures.