Assessment of Current ACI 440.2R-08 Design Guidelines and Evaluation of Hoop Strain in FRP Confined Concrete
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Strengthening concrete structures with fiber reinforced polymers (FRP) is becoming increasingly common in construction practice. The currently available design guideline for FRP confined concrete is the ACI 440.2R-08 model for predicting the maximum confined compressive strength. These formulas are based on a modest test dataset of carbon fiber reinforced polymers (CFRP) and a small test dataset of glass fiber reinforced polymers (GFRP) wrapped specimens. This investigation reviewed a meta- analysis of published testing data from CFRP and GFRP-wrapped plain concrete specimens to evaluate the performance of the design guidelines. The results from 694 compression tests are compared to the predicted maximum confined compressive strength following the ACI 440.2R-08 design formulas. The investigation showed that for CFRP- wrapped plain concrete specimens the design formulas work reasonably well, 82% of the values are conservative. However for the GFRP-wrapped specimens, it was found that one third of the tested specimens had capacities that were less than the design guidelines’ prediction. Therefore the current design formula is unconservative for the design of GFRP-wrapped specimens. In order to suggest a conservative design formula, a variation of the existing formula for calculating the confined compressive strength of a GFRP- wrapped specimen is presented. Also, a variation was made for CFRP-confined concrete specimens to make the formula more conservative. Additionally, an investigation of the transfer of strain between FRP layers in FRP confined concrete specimens was completed. Glass fibers were applied to plain concrete specimens, with the number of layers ranging from one to three layers. Strain gages were placed on every layer to perform an analysis of the hoop strains under load. The results showed that the different layers of a GFRP wrap on a specimen do not receive the same amount of strain for the same axial compressive load on the cylinder. This inequality could be a possible explanation for why FRP jackets do not achieve the same tensile strength as the results from tensile coupon tests.