Evaluation of tensile strength degradation of GFRP rebars in harsh alkaline conditions using non-linear genetic-based models

Glass fiber reinforced polymer (GFRP) rebars reinforced in concrete are susceptible to degradation in harsh alkaline environments such as moist reinforced concrete and seawater and sea sand concrete. The residual tensile strength of GFRP rebar is essential in designing guidelines for GFRP reinforced concrete in different codes. The residual tensile strength is reflected as an environment reduction factor (C-E) to incorporate long-term environmental exposure effects. For this purpose, an extensive database comprising 715 tested specimens were collected from literature to develop GEP tree-based model. Aging tests of GFRP rebars were carried out in the laboratory to test the trained model. Initially, nine gene expression programming (GEP) tree-based models were initially developed using RMSE, MAE, and RSE as fitness functions while varying the numbers of genes. The models were developed employing a random selection of 70% of the conditioned specimens for the training purpose in accordance with the literature. The trained models were validated using the remaining 30% data. A model was chosen to create a prediction formula evaluated from the GEP-expression trees (ETs) and derived MATLAB model based on a broader range of statistical errors and correlations. The chosen model was tested using 36 experimental accelerated aging results, which yielded a comparable statistical evaluation to training and validation data. Two types of GFRP rebars, Type-I (volume fraction of 0.50) and Type-II (volume fraction of 0.60) of three different rebar sizes, i.e., 9.5 mm, 12.7 mm, and 15.9 mm were investigated for determining tensile strength retention (TSR) and C-E. The results concluded that smaller Type-I rebars are more susceptible to degradation as compared to Type-II rebars of larger size. A value of 0.76 is recommended for a uniform C-E based on the upper bound of 95% confidence interval for design life of 100 years.

Automated summary

The residual tensile strength of Glass Fiber Reinforced Polymer (GFRP) rebars is crucial for designing GFRP reinforced concrete, and smaller GFRP rebars are found to be more susceptible to degradation compared to larger ones. A value of 0.76 is recommended as a uniform environment reduction factor (C-E) for a design life of 100 years based on statistical evaluations.