Serviceability flexural ductility of FRP RC beams: a discrete rotation approach

Abstract

Flexural ductility in reinforced concrete members may be defined as concentrations of rotation at discrete points. As such, flexural ductility affects the serviceability deflection of RC beams once flexural cracking, in which there is a discrete rotation between the crack faces, has occurred and which is the subject of this paper. Design rules for quantifying the deflection of steel reinforced RC beams and slabs are generally based on a full-interaction moment–curvature (M/χ) approach that requires the flexural rigidity to be calibrated empirically. Being empirically based, these design rules should only be applied within the bounds of the tests from which they were derived that is for steel reinforcement in which the modulus is fairly constant and with ribbed bars which have a very good bond with the concrete. These bounds do not apply to FRP reinforcing bars where the modulus can vary enormously depending on the type and density of fibre and where the bond between the FRP reinforcement and concrete can also vary widely depending on the manufacturing process. Hence it is both difficult and expensive to quantify empirically, using the M/χ approach, the deflection of RC beams with FRP reinforcement due to the very wide range of these variables. In this paper, an alternative mechanics based discrete rotation approach for the non-time dependent deflection is developed for FRP reinforced flexural members and which is validated by FRP RC beam tests. Being mechanics based, this discrete rotation approach can cope with any type of FRP reinforcing bar with any type of bond characteristic. As with the M/χ approach, the material properties are determined by tests but unlike the M/χ approach in which the flexural rigidity, which is a major component of the model, has to be calibrated through tests, no component of this discrete-rotation model has to be determined experimentally. As this is a generic approach and can be used for any type of reinforcement and bond, this mechanics approach should speed up the development of new FRP products and the development of accurate design rules for deflection for these new FRP products.