A Study of Properties of Creep in Concrete at Early Ages

混凝土早期徐变似乎是受温度应力影响最重要的物理性质之一。但是，混凝土早期徐变应变的测量数据很少。本研究使用两种设备进行了混凝土早期徐变试验，一种是压缩徐变，另一种是拉伸徐变。这些设备采用杠杆系统，可以控制混凝土试件的温度和湿度。选取应力速率、加载周期和温度作为蠕变试验参数，利用粘弹性流变模型从试验结果推导出蠕变应变的计算公式，并将其应用于温度应力的解析法。在此基础上，对实测温度应力的钢筋混凝土箱涵进行了考虑徐变效应的温度应力分析，并将温度应力分析结果与实测值进行了对比，对徐变模型进行了检验。研究结果表明：1.温度升高越高，压徐变和拉徐变越大，压缩荷载周期越短，此时的压应力越大，拉徐变越大。可以看出，作为混凝土早期徐变的结构公式，四单元模型在受压侧与试验结果吻合最好，而五单元模型在抗拉侧与试验结果吻合较好。利用蠕变结构公式得到的有效弹性模数对箱涵进行了温度应力分析，分析结果与实测值吻合较好。由此可见，本文所建立的蠕变模型在热应力估算中具有良好的应用前景。

Creep in concrete at early ages seems to be one of the most important physical properties affected to thermal stress. But, there are few data of creep strain measured in concrete at early ages.In this study creep tests of concrete at early ages are conducted using two kinds of equipments ; one for compressive creep and the other for tensile creep. The lever system is used for these equipments, and temperature and humidity of a concrete specimen can be controled. The rate of stress, period of loading and temperature are selected for parameters of creep tests.The equation to calculate creep strain is derived from the testing results using viscoelastic rheology model, and adopted to the analytical method for calculating thermal stress. Furthermore, the thermal stress analysis considering creep effect is applied to the reinforced concrete box culvert in which thermal stresses are measured, and creep model is examined with comparing analytical results of thermal stress to measured results.The results gained in this study are summarized as the followings ;1. It is clear that the higher the temperature rises, compressive creep as well as tensile creep become larger and the shorter the period of compressive loading and the higher the compressive stress at that time are, the tensile creep strain become larger.2. It is clear that as the structural formula for creep of concrete at early ages the 4-element model conforms the best with the test results on the compression side and the 5 element model does the same on the tension side.3. As the result of the thermal stress analysis made on the box culvert using the effective modulus of elasticity obtained from the structural formula for creep, the values gained by the analysis conform with those actually measured. From these facts, good prospect is gained for estimation of the thermal stress by applying the creep model constructed in this study.