Microstructural degradation in cementitious materials induced by cyclic loading
Niklas Schäfer, Institute for Building Materials, Ruhr University Bochum, Bochum, GermanyRobin Przondziono, Institute for Building Materials/ Ruhr University Bochum, Bochum, GermanyRolf Breitenbücher, Institute for Building Materials/ Ruhr University Bochum, Bochum, Germany
Concrete structures can be subjected to a variety of substantial influences such as static, cyclic and dynamic loads. The superposition of different stresses (thermal, hygral, load-induced, etc.) can amount to a critical level regarding the concrete’s long-time behavior. Especially cyclic stresses can result in a degradation of the concrete’s microstructure which usually can be observed in the form of very small microcracks with a width of only about 5 µm. The Institute for Building Materials of the Ruhr University Bochum systematically investigates the development of such microcracking within the concrete’s matrix under different cyclic loadings for some years already. In tests with cyclically induced stresses, the results in most cases show a larger scatter than in other tests. Thus, in order to ensure meaningful results, a larger number of tests (at least 3) must be performed under identical test conditions. Besides this, tests with several million load cycles usually take a long time each (up to weeks). To optimize such test performances, test setups for cyclic loading were developed at the Institute for Building Materials both for the four-point bending test and for the compressive strength test, which allow parallel cyclic load tests on up to 6 specimens under identical conditions. The degree of degradation induced by those stresses is continuously monitored in a non-destructive way by ultrasonic measurements at different numbers of load cycles. At the end of the cyclic test, the microstructure is investigated by microscopy where the characteristic crack parameters, (formation, orientation, width, length and cross section) are determined. A further focus is currently, to investigate to what extent the addition of microfibers to the concrete matrix can influence the formation of such micro-cracks. Herein, knowledge about the bonding and deformation behavior of microfibers in a cement-bound matrix under both static and cyclic loads is indispensable. For this purpose, the bond properties of single carbon fibers and high-strength steel fibers with diameters in relation to the magnitude of microcracks are tested in cement pastes and mortars. Pull-Out tests of single-fibers were used to investigate the interfacial bond strength. Furthermore, the toughness and load transfer between cement-based matrices and carbon microfibers as well as high-strength steel microfibers are investigated.