Piezoresistive and mechanical behaviour of one-part steel fibre-reinforced geopolymer concrete after elevated temperature exposure

In recent years, the rapid urbanization process has led to the intensification of the urban heat island effect and a growing frequency of fire incidents, thereby exacerbating the degradation of concrete properties at elevated temperatures. Concrete structures are expected to not only maintain their strength after exposure to extreme temperatures but also possess self-sensing capabilities to promptly identify potential structural vulnerabilities. This study explores the piezoresistive and mechanical properties of one-part geopolymer concrete with 0.5% steel fibre (8 mm and 13 mm) reinforcement after exposure to elevated temperatures up to 600 °C. Test results indicated that the highest compressive and flexural strengths occurred at 300 °C, reaching 63.2 and 7.2 MPa, respectively. At 600 °C, the residual strengths remained at above 70% of their initial values, demonstrating superior high-temperature resistance compared to conventional cement-based concrete. The initial electrical resistivity decreased at moderate temperatures due to matrix densification but increased notably beyond 300 °C due to fibre-matrix debonding and microcracking. The gauge factor (GF) peaked at 106.9 for 13 mm fibre-reinforced geopolymer concretes at 200 °C, retaining 64% of their sensitivity at 600 °C. Under compression, the fractional variation in resistivity exhibited stable piezoresistive responses at higher stress levels, while the flexural sensing showed a more notable response to microcrack formation, with longer steel fibres improving the electrical connectivity and sensing stability. Microstructural analysis confirmed that high-temperature exposure caused matrix degradation, fibre pullout, and porosity evolution, affecting both the mechanical and electrical performance. These findings highlight the potential of low content steel fibre-reinforced geopolymer concrete as a durable and thermally stable structural health monitoring material for fire-prone structures. • The intrinsic piezoresistive performance of geopolymer concrete highlights its suitability for SHM applications. • GPL and GSF exhibited higher residual strengths after elevated temperatures, with 30 % reduction at 600 °C. • GPL and GSF showed enhanced electrical resistivity and excellent FCR sensitivity after high-temperature exposure. • GSF13 possessed the highest GF, reaching 101.25 at 20 °C and 68.40 at 600 °C. • GSF outperformed GPL in both FCR and GF performance across all conditions.