Analytical model to predict crack width and spacing of reinforced concrete elements externally strengthened with fiber reinforced polymers

Cracking behavior in Reinforced Concrete (RC) elements externally bonded with Fiber Reinforced Polymer (FRP) materials is still not well assessed in the literature. Few experimental data are, indeed, available about crack spacing and crack width in these elements, with reference to strengthening made of both epoxy bonded plates (EBR technique) and FRP bars or strips applied in grooves realized in the concrete cover (near surface mounted, NSM, technique). In general, although narrower crack widths are expected to occur in RC elements strengthened with FRP materials, the verification of serviceability limit states is still necessary to ensure functionality and protection of internal steel reinforcement, also because increased values of serviceability loads can be attained in the strengthened elements. This paper first presents a review of existing crack width and spacing formulations provided in some international codes with reference to RC elements with and without the FRP reinforcement. The reliability of these formulations is assessed by comparison with experimental results of tests on beams and ties extracted from the literature. Then, a cracking model included in the new MC2020 and developed by the authors for RC elements and based on equilibrium of forces is presented and adjusted to the experimental database considering both FRP strengthening techniques (EBR or NSM). The comparisons with the experimental database demonstrate a higher accuracy of the proposed model in comparison with other approaches for predicting crack width and crack spacing in RC elements strengthened with FRP materials.