An experimental study on the steel-concrete bond performance of self-compacting coconut shell lightweight concrete

This study examines the steel-concrete bond characteristics of self-compacting lightweight concrete incorporating crushed coconut shell (CS) as a complete replacement for conventional coarse aggregate. Despite growing interest in coconut shell concrete (CSC) as a sustainable construction material, its bond behavior with steel reinforcement — a critical parameter for structural applications — remains insufficiently explored. Pull-out tests were conducted on cylindrical specimens (100 mm diameter × 200 mm height) using twisted steel bars of 8-, 10-, 12-, and 16-mm diameters, across two CSC mixes (SCCSC1 and SCCSC2) and a control mix (SCCC), monitored up to 90 days. Results demonstrated that bond strength improved consistently with increasing compressive strength and concrete age, with fly ash addition likely driving continued strength gain beyond 56 days. Conversely, bond strength decreased as bar diameter increased, though CSC mixes exhibited comparatively lower percentage reductions than the control. At 28 days, bond strengths for CSC mixes ranged from 3.95 to 6.52 N/mm², representing 19.3–30.7% of compressive strength, while the control yielded 4.53–9.80 N/mm² (17.1–37%). Remarkably, all experimental bond strengths exceeded code-predicted values from IS 456:2000 and EC2 by more than 100%. These findings confirm that eco-friendly, waste-derived coconut shell concrete possesses adequate bond performance for use in reinforced lightweight concrete structural design.