DEVELOPMENT OF A METHOD FOR MEASURING DESTRUCTION ENERGY AND GENERATED HEAT AT FATIGUE OF CONCRETE
Paper in proceedings, 2011
In fatigue of concrete, if the loads are not uniformly repeated and the dynamic load levels change, every particular load will contribute with certain portion to the fatigue deterioration of the concrete. For estimation of this destruction of concrete the Palmgren-Miner partial damage hypothesis has been used, but has turned out to give contradictory results. As the Palmgren-Miner hypothesis is not generally applicable to fatigue of concrete attempts have been made with volumetric, ultrasonic and acoustic emission measurements to interpret the damage accumulation in concrete. However, no acceptable relation between the measured values and the fatigue deterioration of concrete caused by different loads has been found. The present paper investigates a method for another parameter, which might be important for the deterioration process to be understood, when concrete is loaded by varying dynamic loads. The parameter is the energy absorbed by the concrete and is represented by the work in a form of load times of the deformation for each cycle of load. At each un-loading certain energy is regained, but not all. The load deformation relationship shows a hysteresis and the area within the hysteresis loop represents the absorbed energy used up in the material causing micro cracks, crushing material, redistributing stress and rising temperature due to internal friction. This research investigates methods to determine the energy for the fatigue destruction of concrete and for the rise of temperature within the concrete specimen. Different types of plates and Teflon layers are tested to avoid friction between cylinder ends and loading platens. An insulating material layer with thermocouples around the concrete cylinder is used for heat loss measurements. The paper presents the tests done and discusses problems with the measuring methods. A result is obtained which does not exclude the hypothesis that the destruction energy for concrete in compression is the same whether it is a static or dynamic fatigue failure.