Performance of cement paste with polypropylene fibre and powder at high temperature

Abstract

This research focuses on enhancing the elevated temperature by using ultraviolet (UV) treatment for polypropylene fibres (PPF) and powder (PPP). Cement paste is considered in this research to avoid the compounding effect of aggregate. The strength of cement paste relies on the presence of calcium silicate hydrate (C-S-H). However, at elevated temperatures, C-S-H decomposes, leading to stress and potential damage. The use of PPF and PPP in concrete initially reduced compressive strength due to poor cohesion between polypropylene and cement paste. Various experiments were conducted to evaluate these effects, including residual compressive strength testing under different humidity conditions, compressive strength and stiffness analysis at hot temperatures, microstructure investigation, and transient thermal creep testing. The use of UV-treated PPF and PPP significantly improved the compressive strength at all temperatures. The compressive strength at hot temperatures increased due to transforming the hydrophobic nature of polypropylene to hydrophilic. Additionally, UV-treated PPF improves cement stiffness at high temperatures but showed little difference at room temperature. Microstructure investigations revealed that UV-treated PPF exhibited stronger bonding with cement paste, enhancing contact at room temperature, and UV- treated PPP reduced crack formation at 20°C. Both PPF and PPP melted at around 160ºC, leading to increased porosity in the cement paste. The microstructure remained unchanged until 200°C, but at 200°C, PPF and PPP melted, creating interconnected pores while PP remains in the sample. At 350°C, increased pore formation was observed. Thermal strain and transient thermal creep results indicated the impact of PPF in cement paste. Thermal expansion occurred up to 150°C, followed by contraction due to chemical reactions such as dehydration of calcium hydroxide, decomposition of calcium silicate hydrate, and decomposition of ettringite. The inclusion of UV-treated PPF reduced thermal expansion at 150°C by approximately 41%, enhancing dimensional stability. Thermal strain under constant load is reduced by approximately 50% at 100°C. Transient thermal creep increased with rising temperatures, while PPF did not affect TTS. A review of existing models and their predictions was carried out in relation to the TTS behaviour measured in this work. The review identifies limitations in current models, particularly above 200°C, and highlights the minimal effect of polypropylene fibres on TTS, advocating for improved models to enhance accuracy in predicting cement paste behaviour under high-temperature conditions.