Assessing Compressive Properties of GFRP Bars: Novel Test Fixture and Statistical Analysis

This study investigates the compressive behavior of thermoset glass fiber–reinforced polymer (GFRP) bars across different grades, sizes, and length-to-diameter (L/db) ratios using a novel testing fixture. A total of 61 specimens were subjected to testing, encompassing bars in three sizes, three grades, and three L/db ratios. The new testing fixture, informed by insights from previous methods and initial investigations, easily adaptable to different bar diameters, and eliminates the need for resin, grout, or permanent attachment, allowing for reuse. Using this testing method, consistent measurements of compressive strengths were obtained for L/db = 2, reaching approximately 0.86 of their average measured tensile strength. These measurements exhibited a low standard deviation (SD) of 0.047 and a low coefficient of variation (CoV) of 5.4%. Additionally, the elastic modulus in compression closely aligned with the tensile modulus, with a ratio of 0.97, and minimal variation in measurements (SD = 0.032 and CoV = 3.3%). Increasing the L/db ratio of bars from 2 to 6 escalated result variability and reduced the compressive to tensile strength ratio (ffc/fft) from 0.86 to 0.63, mainly due to increased susceptibility to buckling as observed in the experiments. However, the elastic modulus ratios (Efc/Eft) remained consistent around 1.0. The results of a comparative statistical analysis using data from studies over the last decade highlighted a decreasing trend in ffc/fft ratios with increasing L/db ratios, having an average of 0.7. However, Efc/Eft remains stable at around 1.0 across different L/db ratios.