Experimental Investigation on Partial Replacement of Steel Reinforcement by CFRP Bars in Reinforced Concrete Beams

Abstract

The corrosion of conventional steel reinforcement remains a major durability concern in reinforced concrete members exposed to chlorides, carbonation, moisture and industrial chemicals, because progressive section loss and bond deterioration can reduce stiffness, serviceability and structural reliability. Carbon fiber-reinforced polymer (CFRP) bars provide high tensile strength, low density and excellent resistance to electrochemical corrosion; however, their linear-elastic response up to rupture and comparatively different bond and stiffness characteristics can produce brittle failure and limited warning when they are used as the sole longitudinal reinforcement. The present study experimentally investigates partial replacement of steel tensile reinforcement by CFRP bars in reinforced concrete beams, with the objective of identifying a hybrid reinforcement ratio that improves strength and durability while retaining an acceptable level of deformation capacity. Five M30 concrete beam specimens, each measuring 700 mm x 100 mm x 100 mm, were prepared with constant geometry, concrete grade, loading arrangement and total number of bottom tensile bars. The specimens B-S100, B-C25, B-C50, B-C75 and B-C100 represented 0%, 25%, 50%, 75% and 100% CFRP replacement, respectively. The experimental programme comprised M30 mix proportioning, reinforcement cage fabrication, tensile characterization of steel and CFRP bars, 28-day curing and four-point flexural testing in accordance with the general principles of relevant concrete and composite testing standards. During testing, first-crack load, ultimate load, mid-span deflection, ductility index, energy absorption, stiffness, crack development and expected failure characteristics were evaluated. The adopted M30 mix contained 394 kg/m 3 cement, 197 L/m 3 water, 787 kg/m 3 fine aggregate and 1082 kg/m 3 coarse aggregate at a water-cement ratio of 0.50. Material testing indicated a peak load of 39.30 kN at 4.70 mm displacement for the steel sample and 45.95 kN at 6.80 mm for the CFRP sample, confirming the higher tensile resistance of CFRP while emphasizing the need to preserve the yielding contribution of steel in hybrid members. The beam results showed a systematic increase in first-crack load from 4.2 kN for B-S100 to 5.2 kN for B-C100 and an increase in ultimate load from 18.50 kN to 31.00 kN. Conversely, ultimate deflection reduced from 10.5 mm to 6.2 mm and the ductility index decreased from 3.50 to 1.20 as CFRP replacement increased. Among the hybrid configurations, B-C50 developed an ultimate load of 24.80 kN, an ultimate deflection of 9.00 mm, a ductility index of 2.70 and the maximum energy absorption of 133 N-m. It also provided the most favorable combined response in terms of strength enhancement, crack control, deformation reserve and energy dissipation. The findings demonstrate that full CFRP replacement maximizes flexural capacity but substantially reduces ductile warning, whereas moderate hybridization permits the complementary use of steel yielding and CFRP tensile resistance, as reported in previous hybrid beam research. Accordingly, 50% CFRP replacement is recommended as the optimum configuration for the present small-scale beam system. This recommendation is limited to the adopted geometry, material properties and test conditions, and should be validated through replicated tests, larger specimens, long-term durability exposure and numerical modelling before application to full-scale structural design.