Critical yielding length model for predicting mid-span debonding in FRP flexural strengthened RC beams

Yielding of the internal steel reinforcement is an important mechanism that influences the mid-span debonding behavior in FRP flexural strengthened RC member since the FRP is required to carry additional forces beyond the condition of steel yielding. However, rational design practice dictates an appropriate limit state is defined when steel yielding is assured prior to FRP debonding. This paper proposes a criterion which correlates the occurrence of mid-span debonding to the formulation of a critical steel yielding length. Once this length is exceeded the average bond stress in the FRP/concrete interface exceeds its threshold value, which proves to correlate with the average bond resistance in a FRP/concrete joint under simple shear loading. This proposed mid-span debonding concept is based on traditional sections analysis which is conventionally applied in design practice. Hence complex bond stress-slip analyses are avoided. Furthermore, the proposed model incorporates not only the bond properties of FRP/concrete interface but also the beam geometry, and properties of steel and FRP reinforcement in the analysis of mid-span debonding strength. Based upon a solid database, the validity of the proposed simple mid-span debonding criterion is demonstrated.