Empirical Approach for Determining Ultimate FRP Strain in FRP-Strengthened Concrete Beams

The flexural capacity of concrete beams can be efficiently and effectively improved through bonding fiber-reinforced plastic (FRP) plates to the tensile side. Failure of the strengthened member often occurs through debonding of the FRP from the concrete substrate. If the ultimate FRP strain at debonding failure is known, the moment capacity of the member can be obtained through a simple section analysis. In the American Concrete Institute (ACI) Design Guideline, simple empirical equations are proposed to find the ultimate FRP strain in terms of the FRP stiffness alone. However, when the proposed equations are compared to experimental data, a very large scatter is observed, indicating that the effect of other parameters cannot be neglected. In the present investigation, a new empirical approach to obtain the FRP debonding strain is developed. With a comprehensive experimental database of 143 tests, a neural network relating the ultimate FRP strain to various geometric and material parameters is trained and validated. Using the validated network, an empirical design curve and several correction equations are generated to provide a simple means to find the debonding strain in practical design. Through use of the chart and equations, the calculated ultimate failure moments for the 143 tests in our database are found to be in good agreement with experimental results. The applicability of the new empirical approach to the failure prediction of strengthened members is thus demonstrated.

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