Effects of Rebar Size and Volume Fraction of Glass Fibers on Tensile Strength Retention of GFRP Rebars in Alkaline Environment via RSM and SHAP Analyses

Abstract

This study evaluates the degradation of glass fiber reinforced polymer (GFRP) rebars in alkaline environment under accelerating aging in terms of tensile strength retention (TSR). In addition to environmental conditions such as the pH of the surrounding solution, temperature, and aging duration, the manufacturing parameters of GFRP rebars (i.e., diameter of rebar, db) and volume fraction (Vf) are vital in the degradation of GFRP rebars in alkaline environments as well as scarcely reported. To assess the effect of these variables on the degradation (i.e., TSR), shapely additive explanations (SHAP analysis) based on light gradient-boosting machine (Light GBM), and statistical analysis using response surface methodology (RSM) were used. The Light GBM and RSM models were developed using 715 experimental results of TSR obtained from the existing literature. The performance of both models was reliable in terms of correlation and error analysis. The interaction among the variables was further analyzed using detailed explanations of how each variable affected the prediction of TSR. The results revealed that the TSR generally increases at higher Vf and db of GFRP rebars; however, it decreases with increasing pH, temperature, and duration of exposure. Furthermore, maximum TSR was recorded for pH of 12.6 (Vf=0.62−0.70 and db=14–16 mm). Finally, severe degradation was observed for rebars having 0.55>Vf>0.70. The findings of this study suggest that the current practice of various structural codes using GFRP rebars having minimum mass fraction of 70% (≈0.48Vf) could be improved by using the range of Vf determined in this study to minimize the degradation in alkaline environments. The current research identifies the key production parameter effects of GFRP rebars on durability in harsh alkaline environments. The effect of high pH, surrounding temperature, and aging duration, among others, is well understood. However, a study on the effects of GFRP rebar size (diameter of rebar, db) and volume fraction (Vf) of fibers would help manufacturers to improve the quality of the GFRP rebars impregnated in vinyl ester resin. Also, the practitioners may use the recommended db and Vf in designing durable GFRP-reinforced structures. This study also identifies the research gaps, where a scarce amount of literature is available on GFRP rebars exceeding 16 mm. The formulated models can further be used for long-term extrapolation of service life of the GFRP rebars based on surrounding harsh alkaline environmental conditions. The basic assumptions considered in evaluating the durability of the GFRP rebars can be overcome.