Tensile Creep of Concrete at Early Ages

混凝土徐变性能的研究已有很多，但几乎都是针对成熟混凝土的。本研究是为了防止大体积混凝土中水泥水化热引起的裂缝。对早龄期混凝土进行了压缩和拉伸徐变试验。研究了加载龄期、应力强度比、加载时间、温度等因素对养护加载过程的影响。获得了以下结果。(1)从压缩蠕变实验中注意到，比蠕变是恒定的，在低于压缩强度的50%的应力水平下不受影响，然而，比蠕变在超过该值时显著增加。随着加载时间的延长，应力水平和蠕变率的影响逐渐减小。(2)拉伸蠕变实验表明，应力强度比的影响是显着的，它与蠕变应变的关系可以用指数方程来描述。加载龄期仅对第1天的加载结果有影响，对其他龄期的加载结果影响不大。养护温度和加载时间的影响可以用等效水化时间来衡量，但加载过程中温度的影响更为显著。(3)根据实验结果，研究了应力变化的建模方法，并对叠加法的适用性进行了评估。指出本文提出的徐变模型是对Voigt徐变模型的修正，它适用于早龄期混凝土，叠加法也适用于温度应力等变化应力。

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.