|Abstract:||The investigation was concerned with the response of plain concrete when subjected to repeated or sustained high compressive loads. The objectives were: (1) to study the strength and deformation characteristics of concrete under such loading conditions, and (2) to propose an analytical procedure to predict concrete behavior under high repeated loads.
The first objective was realized through an experimental program. Plain concrete prisms (4 in. by 4 in. by 12 in.) were subjected to high repeated and sustained loads. Compressive loads were concentrically applied. Strains in the longitudinal and lateral directions were recorded throughout the life of a specimen. The test program was divided into three phases. Phase One dealt with the effect of maximum stress, stress range and concrete age at time of loading. Phase Two concerned with the effect of speed of testing on static and repeated load behavior. The relationship between the damage, caused by high repeated and sustained loads, less than those necessary to cause failure, and the remaining load carrying capacity of a specimen, was studied in Phase Three.
To achieve the second objective, two analytical models were formulated. These were based on the cycle and time-dependence of damage and of strains of concrete when subjected to high repeated or sustained stresses. For a given set of parameters, a damage model was proposed to predict the number of cycles required to cause failure, while a failure strain model was developed to predict the total longitudinal strain accumulated at failure.
The experimental program showed that concrete response to high repeated loads is very much controlled by the time concrete has to resist stresses higher than its sustained load strength. For a maximum stress higher than the sustained load strength, a decrease in the stress range and /or the stress rate (test frequency) significantly increases the “sustained load” contribution to the overall behavior. The number of cycles to failure are smaller, and the exhibited strains throughout the loading history and at failure are larger, the greater the “sustained load” effect. Even if the effect of hydration during a test is excluded, concrete age at loading appears to have a significant effect on behavior of concrete subjected to high loads. In addition, it was shown by experiment that plain concrete, subjected to high repeated and sustained loads, undergoes a “hardening” stage manifested by an increase in the static strength over the static strength prior to sustained or repeated load test. This “hardening” is dominant during the earlier portion of the life of the specimen. The last portion is characterized by progressive crack propagation and a stress decrease until failure.
In the analytical study, a damage model was developed in which the cycle and time-dependent effects are expressed separately. The damage model was revised successively until excellent agreement, between calculated and observed failure cycles, was achieved. Also, an analytical model to determine the failure strain model was derived. The total strain was assumed to consist of an initial elastic strain, a cycle-dependent and a time-dependent strain. The agreement between calculated and observed failure strains ranged from poor to satisfactory, but the failure strains, calculated from the analytical model, gave the general tendencies which were observed in the experiments.