Mechanical and microstructural properties of HFRHSCs containing metakaolin subjected to elevated temperatures and freezing-thawing cycles

Mechanical and microstructural properties of hybrid fibers reinforced high strength concretes (HFRHSCs) containing metakaolin (MK) and metakaolin + silica fume (MK + SF) subjected to elevated temperatures and freezing-thawing (F-T) cycles were investigated in this paper. In the concrete mixtures without and with hybrid fibers, MK and MK + SF were replaced with eight percentage of cement by weight. A total of sixteen concrete mixtures were produced at a water-to-binder ratio of 0.25. Ultrasound pulse velocity (Upv), compressive strength (fc), splitting tensile strength (fsts) and flexural strength (ffs) tests were performed to evaluate the properties of HFRHSCs subjected to ambient and elevated temperatures (300, 400 and 500 °C). The Upv and fc tests of HFRHSCs subjected to F-T cycles were also conducted. Moreover, the alterations in the matrix, interface zone and aggregate of concretes without hybrid fibers subjected to ambient temperature, elevated temperatures and F-T cycles were analyzed by scanning electron microscope (SEM), energy dispersive spectroscopy (EDS) spot and polarized light microscope (PLM). The results of tests have shown that the replacement levels of MK and MK + SF have important influences on the mechanical and microstructural properties of high strength concretes. Moreover, the mechanical results have shown that the concretes with hybrid fibers have higher fc, fsts and ffs than other concretes. The results have also shown that the Upv, fc, fsts and ffs values of concretes without and with hybrid fibers decrease, as the temperature increases.