Tensile Creep of Concrete at Early Ages

混凝土徐变特性的研究很多，但几乎都是针对成熟混凝土进行的。对大体积混凝土中水泥水化热开裂的防治进行了研究。对早龄期混凝土进行了1 ~ 5天的抗压、拉伸徐变试验。研究了加载龄期、应力强度比、加载期限和温度对固化和加载过程的影响。得到如下结果：(1)抗压蠕变实验表明，在抗压强度的50%以下，应力水平对比蠕变没有影响，但超过该值时，比蠕变显著增加。随着加载龄期的增加，应力水平和比蠕变的影响逐渐减小。(2)拉伸蠕变试验表明，应力强度比的影响显著，其与蠕变应变的关系可以用指数方程来描述。加载年龄仅影响第1天的加载结果，其他加载年龄对加载结果影响不大。虽然可以通过等效水化时间来评价固化温度和加载期限的影响，但加载过程中的温度效应非常显著。(3)在实验结果的基础上，研究了应力变化的建模方法，并评价了叠加定位法对应力变化的适用性。指出本文提出的修正Voigt模型的徐变模型适用于早龄期混凝土，超定位方法也适用于热应力等应力的变化。

Creep behavior of concrete has been studied by many researchers, but almost all of them are concerned with matured concrete. This study has been conducted for prevention of cracking due to heat of hydration of cement in massive concrete. Compressive and tensile creep experiments have been carried out for early aged concrete such as from 1 day to 5 days. This study was focused on the influence of age of loading, stress-strength ratio, term of loading and temperature in curing and loading. The following results are obtained.(1) It was noted from compressive creep experiments that specific creep was constant with no influence by stress level below 50% of compressive strength, however, that specific creep significantly increased at over the value. And the influence of stress level and specific creep decreased corresponding to age of loading.(2) From tensile creep experiments it was cleared that the effect of the stress-strength ratio was significant and its relationship with creep strain could be described by an exponential equation. The age of loading influenced only the results of loading at 1 day, but little effect was observed in other age of loading. Aithouth the effect of curing temperature and term of loading could be assessed by equivalent periods of hydration, temperature effect during loading was much significant.(3) The method of modeling for change of stress has been investigated based on experimental results and applicability of super-positioning method for such stress has been assessed. It could be pointed out that a presented creep model which was modified Voigt model is applicable for early aged concrete and super- positioning method also could be applied for changing stress such as thermal stress.