多龄期加卸荷大坝混凝土的泊松效应和徐变理论研究

Creep plays an important role when the stress in concrete dam is analyzed. Since the loading and unloading creep stress in concrete dam is complex, and the assumption of the elastic creep theory used in engineering is inaccurate, there are large differences among the stress in dam concrete calculated by the numerical simulation, the stress transformed from the measured strain and the true stress..According to association the loading and unloading ages and robust test mode, multi-aged and multiaxial concrete creep spectrum under loading and unloading are accurately obtained in the same experimental condition. Then the rule among the multi-aged elastic Poisson's ratio, the creep Poisson's ratio and the effective creep Poisson's ratio under loading and unloading is investigated, and the relationship between the elastic Poisson’s ratio and the creep Poisson’s ratio in volumetric and deviatoric stress is established. Moreover, the relationship between effective creep Poisson’s ratio and instantaneous stress is quantitatively determined. Finally, similar to the different damage in tensile and compressive loading, the multi-aged creep model under loading and unloading is developed. For this new model, constitutive integration algorithm for both numerical simulation and measured strain transformation stress is proposed..The investigated results have important theoretical and practical value, and benefit to fully understanding creep properties for multi-aged concrete under loading and unloading, overcoming the shortage of elastic creep model, and improving the theory of concrete creep.

在混凝土坝应力分析时，徐变是一个不可忽略的重要因素。由于混凝土大坝处于复杂的加卸荷徐变应力状态，而工程上常用的弹性徐变理论采用的假设比较粗糙，导致大坝混凝土数值仿真计算应力、实测应变转换应力与真实应力存在较大差异。.针对上述问题，本项研究基于统一试验条件、加荷龄期和卸荷龄期相关联、鲁棒的测试方式，获得高精度多龄期多轴加卸荷徐变谱；然后挖掘多龄期加卸荷弹性泊松比、徐变泊松比和有效徐变泊松比的变化规律，建立体积应力和偏应力下弹性泊松比和徐变泊松比之间的关系，提出有效徐变泊松比和瞬时应力应变的定量关系；最后，类比拉伸压缩异性损伤，构建多龄期加卸荷徐变模型，提出新模型本构积分数值仿真算法和实测应变转换应力算法。.研究成果对深入认识多龄期加卸荷混凝土徐变特性、克服弹性徐变模型的不足以及完善混凝土徐变理论具有重要的理论和实践意义。

混凝土大坝处于复杂的加卸荷徐变应力状态，并受到内部和外界环境湿度的影响，而工程上常用的弹性徐变理论采用的假设比较粗糙，导致大坝混凝土数值仿真计算应力、实测应变转换应力与真实应力存在较大差异。.本项研究首先开展了不同水胶比水工混凝土湿度试验，首次揭示了水胶比小于0.41的高强水工混凝土存在自干燥现象；通过开展养护湿度对水工混凝土徐变影响试验，首次提出了湿胀徐变的概念。接着开展了不同水胶比和首次浸水龄期等因素影响下水工混凝土湿胀试验，揭示了随着水胶比的减小，湿胀变形达到稳定所需要的时间越长，湿胀稳定值越大且线湿胀系数越大；当首次浸水龄期大于28d时，混凝土湿胀应变的稳定值基本不再随首次浸水龄期的增大而变化，湿胀应变稳定在20με左右，湿胀系数基本稳定在3.6×10-3。进而结合水胶比为0.5，粉煤灰掺量为35%的水工混凝土，开展了统一试验条件下不同加载龄期(7d/28d/60d)的水工混凝土拉、压加卸荷徐变试验。发现0.5水胶比混凝土的拉伸徐变度大于压缩徐变度，卸荷后实测的拉伸徐变恢复大于压缩徐变恢复；基于组合指数式徐变模型和叠加原理对卸荷后的拉伸、压缩徐变恢复进行分析，发现无论是卸荷后的拉伸徐变恢复还是卸荷后的压缩徐变恢复均表现为，当叠加压缩徐变度获得的徐变恢复的计算值与实测值吻合更好。与此同时，开展了统一试验条件下不同加载龄期的单轴压缩徐变泊松试验（3d/14d/28d/60d）和常规三轴压缩徐变泊松试验（14d/28d/60d）。发现在不同应力状态下，弹性泊松比、徐变泊松比和有效徐变泊松比不相等，弹性泊松比大于徐变泊松比；此外，在常规三轴应力状态下，各个方向的有效徐变(徐变恢复)泊松比不相等，即在多轴应力状态下，各个方向的徐变(徐变恢复)不是独立的。最后，依据前述拉伸、压缩徐变不一致的试验结果，通过在主应力空间下判断应力增量主量的符号确定拉、压徐变度表达式的选取，改进了混凝土实测应变转换应力和混凝土温度应力场仿真计算的数值计算公式。.研究成果对深入认识多龄期加卸荷混凝土徐变特性、克服弹性徐变模型的不足以及完善混凝土徐变理论具有重要的理论和实践意义。