Effect of biochar on mechanical and permeability properties of concrete exposed to elevated temperature.

• Wood based biochar is explored as admixture in concrete under normal and elevated temperature. • Biochar-concrete exhibits improved water tightness and mechanical strength than control. • Addition of biochar minimize internal damage to concrete micro-structure than control and concrete with 10% silica fume. • Biochar-concrete shows higher strength restoration and lower water intrusion, by 22–25%, compared to control mixes. Although many inorganic fillers have been explored as supplementary cementitious admixtures, there is limited exploration into role of porous biochar, prepared from lignocellulosic biomass, in modifying mechanical and durability performance of structural grade concrete exposed to elevated temperature. This study investigates the effect of biochar particles, prepared by pyrolysis of woody biomass at 500 °C (BC 500), on mechanical strength (compressive, flexural and split-tensile strength) and permeability properties of concrete under normal condition (only wet-curing) and after exposure to high temperature (300 °C and 550 °C in this study) at 28-day age. Biochar was added at 0.50%, 1% and 2% by weight of cement in concrete. Thermal damage was inflicted to 28-day wet-cured concrete samples by placing in an electric kiln with residence time of 1 h (at steady state) and ramp rate of 5 °C/min. The strength and permeability performance of biochar-concrete composite were compared with plain concrete and concrete with 10 wt% silica fume (SF 10%) exposed to similar conditions. The findings show that addition of 0.50 wt% and 2 wt% of BC 500 increase 28-day compressive strength of concrete by 16% and 9% respectively compared to control mix, while 2 wt% addition led to reduction in permeability by 40%, measured by water sorptivity and water penetration test. After exposure to elevated temperature, addition of 1–2 wt% BC 500 to concrete is found to minimize thermal damage and retain 20% and 11% higher strength compared to control and concrete with silica fume respectively. Investigations show that biochar addition leads to lower damage to microstructure of concrete during thermal treatment, leading to 22–25% higher water tightness compared to control mix. The findings suggest that biochar from wood waste can be applied as a sustainable admixture and alternative to silica fume in order to improve compressive strength and durability of structure exposed to high temperature. Nevertheless, this can be also be an effective means to valorize lignocellulosic waste for high value construction applications. [ABSTRACT FROM AUTHOR]