Examination of engineering properties of cement-based fiber-reinforced composites exposed to high temperature and sulpfate attack through machine learning methods

In the present study, the effects of different temperature effects (25°C, 200°C and 500°C), different curing age (7, 28 and 90 days), and high temperature and sulpfate effects on the compressive strength, flexural strength and ultrasound wave propagation speed of two different fiber types were investigated. The aim of the study is to determine the effects of these parameters on the mechanical properties of the material. The data obtained from the experimental studies were analysed by using the Extreme Learning Machine (ELM) and Group Method Data Processing (GMDH) models. The findings revealed how the determined parameters affect the material performance and demonstrated the effectiveness of ELM and GMDH in modelling these effects. Considering the high temperature parameters, tests performed at different curing ages and temperature levels showed that microstructural voids and cracks were not completely filled in the early curing age, while more stable microstructures were formed and compressive strength increased at later curing ages. While microstructural development was encouraged at 200°C, significant damage and strength losses were observed at 500°C. The hybrid combination of micro steel and polypropylene fibres provided a more stable performance at high temperatures. Fly ash increased the strength at later curing ages, while nano calcite, in general, improved the microstructural performance. UPV values varied with curing age and temperature. The combination of micro steel and polypropylene fibers increased the resistance of the materials against high temperatures, while fly ash and nano calcite improved the flexural strength by improving the fiber-matrix interface properties. These findings indicate the significant effects of high temperature on cementitious materials and that different materials and mineral admixture can help to reduce these effects. Considering sulphate effect conditions; no corrosive change due to sulphate was observed as a result of low water/binder ratio and fiber usage. The penetration of sulphate solution into the microstructure promoted additional hydration reactions and increased strength. A denser matrix was formed in mixtures containing fly ash and nano calcite and this resulted in an increase in strength values. An increase in compressive strength was observed in specimens exposed to sulphate solution after high temperature, but no critical difference was observed. Polypropylene fibers melted at high temperatures, causing sulphates to penetrate more easily into the cement-based composites, while the mixtures using steel fibers showed the highest flexural strength. UPV values measured at 7, 28 and 90 days of curing age showed that the effect of sulphate did not have a significant effect on microstructural properties.