Correlation of Tensile Strength to Flexural Rupture Modulus of GFRP Bars

Glass fibre reinforcing polymer (GFRP) bars are becoming a more comparable alternative to steel rebar used as tensile reinforcement for concrete, due to their cost effectiveness compared to other alternatives. GFRP has material characteristics of being corrosion-resistant and low-impact to electromagnetic field interference. GFRP bars are also stronger but less stiff than traditional steel rebar. However, the biggest drawback of GFRP bars acting as reinforcement for concrete members is their brittle nature, where there is little-to-no warning of failure of a structural reinforced concrete element. As a result, it is crucial to successfully identify the tensile strength of such material prior to installment in construction projects. The most direct way to measure this quality is to perform a uniaxial, direct tensile test on GFRP bars. This test involves clamping the ends of the GFRP bar in a testing machine and pulling the bar apart with tensile force until failure. From this, the tensile stress and tensile elastic modulus of a GFRP bar can be obtained from the recorded force and displacement values. The tensile test requires large capacity test frames. Typically, the bigger sized GFRP bars are very difficult to test to their ultimate tensile stress due to lack of access of a testing machine strong enough to break the GFRP bar. Another critical consideration that needs to be made is adequately preparing steel anchorages tubes at the ends of the GFRP bar such that the grips of the testing machine would not crush the GFRP material. A common problem with this setup is that the GFRP could de-bond from the steel tube mid-way through the test, if they are not properly bonded together. As a result, there needs to be ample anchorage length and threading of the insides of the anchorage tubes to promote bonding between the GFRP material and the steel anchorage tube. If an anchorage tube length is very long, this can cause the specimen to be quite heavy and difficult to maneuver when placing it inside of the testing machine. This test is direct in obtaining key parameters, but it involves significant time and effort to conduct, discouraging the completion of quality control tests for GFRP bars, which are needed to ensure the tensile strength of the reinforcement used in concrete. An alternative test that has been investigated to obtain the tensile strength of a GFRP bar is conducting a flexural test, where the specimen is subjected to compressive and tensile stresses. Since the goal is to observe the tensile strength of the GFRP bar, the only preparations for this test that are required is having access to the proper flexural apparatus, and cutting the GFRP specimen to length and longitudinally in half to ensure tensile failure occurs first. From this test, the loading at which the tensile fibres first rupture can be converted into a rupture stress. Using Weibull’s Weakest Link model to describe the failure distribution of the GFRP material based on its flaws, the rupture stress can be related to its tensile strength. To provide a more accurate result, the GFRP material is modelled as a bi-moduli material, where its compressive and tensile elastic moduli are different. Through these set of calculations, the flexural test of the GFRP material is an efficient method in obtaining the tensile strength. This research investigates 3-point and 4-point bending tests of GFRP bars of size 8 mm, 13 mm, 16 mm, 20 mm, 25 mm, and 32 mm in diameter, to be used to determine tensile strength of these bars. Testing with two different flexural tests will examine if one of the tests yield a more accurate result for calculating the tensile strength of the GFRP bars compared to the other. Using varying sizes of GFRP bars will confirm whether the set of correlation calculations work for all sizes. Tensile testing of GFRP bars of sizes 8 mm, 13 mm, and 16 mm in diameter was completed, in order to validate the results from the flexural testing. It was found that both 3-point and 4-point bending tests can be used to determine tensile strength. Both methods are comparably accurate, with 3-point bending being slightly faster to do. In comparison with results from tensile testing (for smaller specimens) and prior research, it was found that the correlated tensile capacities from the flexure tests had minor discrepancies, having an error of less than 19%. The flexural test holds great potential to be a successful standardized test that yields accurate results to determine the ultimate tensile strength of a GFRP bar. The purpose for such testing is for quality control and quality assurance of different batches of GFRP bars to be installed in concrete infrastructure