Deflection behaviour of FRP reinforced concrete flexural members
The design of fibre reinforced polymer (FRP) reinforced concrete (RC) can often be governed by the serviceability limit state of deflection. Currently, the evaluation of short-term deflection of FRP RC is undertaken using radically different approaches, in both research and codes of practice. This study investigates the short-term deflection behaviour of FRP RC, both experimentally and analytically, and examines the merits of those different approaches. Experimentally, 28 RC beams and slabs with glass, carbon or steel rebars are tested under four-point loading. The main variables considered are the reinforcement ratio, modulus of elasticity and bond. In addition to measuring deflections, closely-spaced strain gauges are used to measure rebar strains between one forced crack at midspan and two naturally-occurring cracks on either side. This setup enables the investigation of rebar strains, tension stiffening and bond between flexural cracks. Furthermore, in connection with concrete strains at the extreme compressive concrete fibre, the flexural load-curvature relationship is evaluated experimentally and used to decompose the total deflection into flexural and shear-induced deflections. Analytically two numerical analysis methods are used to provide further insight into the experimental results. Finite element analysis with smeared modelling of cracks is used to predict and examine the stress-displacement response in detail. Cracked section analysis is used to provide upper-bound deflections and strains. This study also deals with the ACI and Eurocode 2 approaches for prediction of short- term deflection. The deflection prediction and tension stiffening expressions of these codes are evaluated against the experimental results of this and other studies. The main conclusion is that deflection of FRP RC is essentially due to flexural curvatures, and can be reasonably evaluated by the tension stiffening model of Eurocode 2. However, with reinforcement of relatively low axial stiffness, and depending on the reinforcement bond characteristics, shear-induced deformations become significant and may need to be evaluated.