Abstract
In this paper thermal analysis was applied to determine the “Critical Stress” of concrete, different from its ultimate strength, able to produce the first damage in the structures under compressive loads. The Critical Stress can be thought as the stress able to produce the beginning of fatigue rupture within the material. Several specimens of high strength concrete were tested in order to define the incipient crack phenomena, also in internal part of the specimen not accessible by direct inspections, with the aid of infrared thermography. A finite element analysis completes the study and compares, for the same static loading conditions, the stress state with the experimental thermographic images. The final results show as the coupling of normal compressive test and the acquisition of the thermal images can be a useful aid to estimate a security stress value, indeed the Critical Stress, before the Ultimate Serviceability Limit (SLU) of the structure, defined as the maximum load condition before its failure.
Highlights
Several bridges and civil structures disasters, like Genova Morandi’s viaduct, alert that the cyclical creep may have been an additional cause of the catastrophic collapse of these structures
It is known that the fatigue characterization of the concrete and of the pre-compressed concrete is not easy to perform due to the cost over time and to the necessary equipment for significant fatigue tests
The intersection of the two lines defines the Critical Stress; where micro-faults begin to appear in the internal structure of the concrete sample
Summary
Several bridges and civil structures disasters, like Genova Morandi’s viaduct, alert that the cyclical creep may have been an additional cause of the catastrophic collapse of these structures. Fatigue phenomena on civil structures, subjected to dynamic loads over time (atmospheric phenomena, alternate loads and vibrations induced by vehicular traffic) lead to dangerous cracking, limiting the life of the structures. In the last thirty years, many studies have shown that the temperature variation of a mechanical component under stress is a good parameter for estimate the energetic release, the residual fatigue life of the material [1,2,3]. Different papers showed as the thermal analysis applied to steel specimens [4,5,6], to composite specimens [7,8,9], to welding joints [10] permit to estimate the fatigue limit of the materials in easy and rapid way. It is possible to define three phases of the temperature variation on the specimen surface in relation to the applied stress (Figure 1):
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