Residual flexural strength of reinforced concrete beams with unbonded reinforcement

Corrosion is the primary limit state in steel reinforced concrete (RC) beams in severe environments. Corrosion of steel bars causes a decrease in steel bar section, loss of bond, and delamination of concrete cover. Very limited models have been developed for estimating the residual flexural strength of corroded RC beams. These models do not differentiate between the decrease in flexural strength due to loss of steel section, or due to loss of bond between the steel bars and the surrounding concrete. This paper presents an analytical model for estimating the residual flexural strength of RC beams with unbonded bars. In this research, the "neutral axis depth" approach, which is widely used to obtain the ultimate stress in prestressed concrete, was adopted and modified to estimate the residual ultimate capacity of concrete beams reinforced with unbonded steel bars. A finite element analysis (FEA) model was developed, and the results were verified against experimental data by others. The experimental data of a total of 109 RC beams covered many variables: span-depth ratio, reinforcement ratio, concrete compressive strength, length of unbonded zone, and type of applied load. Using the aforementioned data and FEA model, an analytical model was developed to calculate the ultimate strength of concrete beams reinforced with unbonded steel bars. The results of this investigation suggest that the reinforcement ratio and the unbonded length have a major effect on the residual flexural strength of RC beams with unbonded reinforcement, whereas the other variables have limited impact.