Bond tests on concrete elements externally bonded with steel plates and assessment of bond strength models

The use of steel plates as externally bonded reinforcement for existing reinforced concrete (RC) elements can be still considered a reliable typology of strengthening intervention thanks to their relatively lower prices in comparison with fiber reinforced plastic materials, FRPs, largely used in the last decades, the ductile stress-strain behavior, and the high stiffness properties. Despite steel plates represented the first attempt of external strengthening for RC elements, design indications and practical execution rules concerning the use of such a technique nowadays are still very few. There is, indeed, a lack of experimental and theoretical studies in literature for predicting accurately the behavior of existing RC members externally strengthened with steel plates. In particular, special attention should be paid to the bond behavior at the steel-concrete interface, since it can strongly influence the performance of the strengthened elements. To this aim, an experimental campaign concerning bond tests on steel plates bonded over concrete elements and realized according to the single shear test set-up was carried out by the Authors. The experimental results are analyzed in the paper in terms of failure loads and modes, load-displacement behaviour, axial strain distributions along the steel plates, and distribution of shear stresses and slips. Then, the experimental results are compared with some well-known literature bond strength models for external FRP reinforcements in terms of failure load, transfer length and bond law. The comparisons with existing models in terms of debonding loads are developed also for an extended database of bond tests on steel plates available in literature and were prodromal to calibrate new coefficients specifically assessed for externally bonded steel plates.