Causes and modelling of the warming of fatigue-loaded concrete

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Cyclic loads evoke in concrete, as well as in other materials, deformation and damage processes, which lead to failure of the structure or specimen. This failure is even caused by loading conditions far below the material strength. Gernerally, this characteristic material behavior is referred to as fatigue.The resistance of concrete to cyclic loads is quantified in force-controlled fatigue tests. In these tests, concrete specimen exhibit significant warming. Material damping and friction processes at the micro level of the concrete are meant to be the causes. Load frequency, stress range and test duration of the fatigue tests affect the warming of the specimens significantly. Higher stress ranges and load frequencies result in higher temperatures. Experimental studies show that fatigue loaded concrete specimens tested at higher load frequencies and thus higher concrete temperatures fail earlier than at lower load frequencies with lower temperatures. Therefore, the warming is supposed to affect the fatigue resistance of concrete specimens.Within the research project, the thermal behavior of fatigue-loaded concrete specimens will be investigated experimentally and numerically. The scheduled research activities are based on an energetic approach identifying the heat release in fatigue-loaded concrete specimens. The temperature behavior of the specimens will be calculated by a finite element model considering heat transport within the concrete specimen and heat transfer to the surrounding. Measured and calculated temperatures of fatigue loaded concrete specimens will be compared. The comparison will indicate the accuracy of the established energy approach. Furthermore, the effects of the warming of concrete specimens will be evaluated. These concern the temperature deformations as well as the reduced fatigue strength due to the warming of the specimen.