A study on the flexural behaviour of lightweight concrete beams reinforced with pumice, silica fume and carbon fibers

Lightweight aggregate concrete (LWAC) incorporating pumice aggregates offers a sustainable alternative to conventional concrete, enabling a reduced self-weight while maintaining adequate strength and stiffness for structural applications. This study investigated the flexural behaviour of reinforced concrete beams incorporating 40 % pumice aggregates, 10 % silica fume, and 1 % carbon fibers. The mechanical properties were evaluated through compressive, split-tensile, and flexural strength tests, followed by full-scale four-point bending tests on under-reinforced beams. The results indicate that pumice incorporation reduces the compressive strength and elastic modulus by approximately 36 %, whereas the theoretical analysis predicts an 8–12 % reduction in the overall concrete density. The inclusion of silica fume and carbon fibers significantly enhanced the post-cracking behaviour, restoring the ultimate load capacity to 88–98 % of that of conventional beams and improving ductility and energy absorption. Microstructural analysis using SEM–EDS revealed that pumice increased porosity and weakened the interfacial transition zone, whereas silica fume promoted matrix densification through secondary C–S–H formation and reduced the Ca/Si ratio. Carbon fibers effectively bridge microcracks and improve stress transfer. Analytical validation using IS 456 confirmed the classical flexural behaviour, demonstrating the feasibility of structurally efficient and sustainable pumice-based lightweight reinforced concrete beams.