Effect of natural and synthetic fibres on fire spalling resistance of high-strength concrete with and without waterproofing admixture

High-strength concrete (HSC) is prone to explosive spalling under rapid heating in fire, which can compromise structural integrity. This study investigates the effectiveness of adding natural fibres (jute and sisal) in mitigating fire-induced spalling of HSC, including in mixes containing a waterproofing admixture for durability and benchmarking the performance with the widely used polypropylene fibres. Small-scale ring-restrained specimens were exposed to a constant 1050 °C for 30 min, and full-scale panels were exposed to a RABT 60 heating curve. Internal temperatures, pore vapour pressures, and restrained thermal stresses are monitored to evaluate the spalling behaviour. In fibre-free and steel-fibre-only HSC, spalling occurred within about 2.0 min, with spall depths of about 14 mm. In contrast, mixes containing PP (0.2 % vol) or jute fibres (0.1 % vol) exhibited no spalling during the 30 min fire exposure, even at internal moisture contents of approximately 4–6 % by mass. The superior performance of PP and jute fibres is attributed to their distinct thermal response mechanisms: PP fibre melting and the jute fibre’s straw-like morphology and thermal degradation behaviour together facilitated the formation of transient vapour escape channels during heating, effectively relieving internal pressure and delaying failure. Sisal fibres (0.1 % vol) were ineffective, with rapid spalling depth of about 11 mm occurring at ∼1.5–1.8 min despite similar moisture conditions. The addition of a crystalline waterproofing (WP) admixture (0.6 % cement mass) increased spalling risk in plain and sisal-fibre mixes (due to reduced permeability) but did not negate spalling prevention in PP- or jute-fibre mixes. The small-ring test results correlated strongly with the full-scale panel tests (R² = 0.999) and required only approximately 3 % of the furnace energy, validating it as an energy-efficient screening tool for fire-spalling resistance. The findings highlight that controlling internal vapour pressure and thermally induced strain is key to spalling prevention. PP fibres remain a highly effective solution, while jute fibres emerge as a promising sustainable alternative that can provide comparable spalling protection with significantly lower environmental impact than synthetic fibres. Combining a waterproofing admixture with these venting fibres can achieve durable concrete (low permeability in service) that remains resistant to fire spalling. • Jute fibres exhibited comparable spalling resistance to conventional polypropylene fibres in high-strength concrete. • The small ring test results correlated well with the full-scale panel test. • Controlling internal vapour pressure and thermal stress are key to spalling prevention.