A Study of Properties of Creep in Concrete at Early Ages

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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.