Evolution of mechanical and physical properties of a lining concrete exposed to high temperatures and different relative humidities

This paper investigates the combined impact of temperature (23 °C, 65 °C, 85 °C) and relative humidity (RH) (55 %, 65 %, 75 %, sealed) on the mechanical and physical properties of high-strength concrete used in tunnel linings for a deep geological repository for radioactive waste. It examines hydration kinetics, pore structure, water permeability, and mineralogical changes, alongside mechanical properties like compressive strength, flexural tensile strength, and elastic moduli. Time-dependent deformations (shrinkage and creep) are analyzed in sealed and unsealed specimens loaded at 7, 28, and 90 days. Results show late hydraulic and pozzolanic reactions persisting over a year, with denser pore structures at 90 days (e.g., cumulative pore volume dropping from 0.0047 cm³/g at 7 days to 0.0036 cm³/g at 85 °C). Thermogravimetric analysis indicates increased C-S-H content with temperature, though slight degradation occurs at 85 °C after 90 days. Water-accessible porosity decreases with age (e.g., 9–2.5 % reduction at 23 °C to 85 °C over 180 days), but higher temperatures slow this due to moisture loss. Water permeability rises as temperature-induced porosity increases. Shrinkage and creep are amplified at elevated temperatures; basic shrinkage drops by 50 % when increasing the load application time from 7 to 90 days at 23 °C but rises by 35.7 % at 85 °C, while drying creep compliance increases by 99 % at 85 °C for 7-day of curing. Though RH and temperature influence properties, their effects are often statistically insignificant. These insights enhance understanding of environmental impacts on concrete and support advanced computational modeling.