Sensitivities of Strength and Ductility of Plated Reinforced Concrete Sections to Preexisting Strains

Existing reinforced concrete (RC) beams are commonly enhanced in flexure by bonding steel or fiber-reinforced polymer (FRP) plates to their tension faces. At the time of plating, preexisting loads, such as dead loads, are present on the beams. Although these loads clearly do not induce strains in the plates, flexural capacity design calculations for plated beams using RC theory tacitly incorporate the corresponding fictitious preexisting strains in the plates. If the plates are elastic at section flexural failure, as might be the case for steel plates and is always the case for FRP plates, this design approach underestimates the area of plate required for a given increase in flexural capacity, thus giving a weaker plated RC section than required. For steel-plated sections in which both the internal steel and plate steel yield before concrete crushing occurs, preexisting strains do not influence the area of plate required, but they do delay yield and so reduce ductility. In such cases, neglect of preexisting strains does not affect strength, but it does lead to overestimate of available ductility. This has implications for design of statically indeterminate plated RC beams. Owing to uncertainties in parameters such as creep and differential settlement, preexisting strains do not lend themselves to accurate evaluation. Hence it seems prudent, as a first step, to establish the sensitivity of the flexural failure behavior of plated RC sections to varying preexisting strains. In this paper, mechanical models for plated RC section flexural behavior, which allow for steel and concrete material nonlinearities and for preexisting strains, are presented. The models are first verified by comparison with experimental data, and are then used to determine areas of FRP or steel plating required for stated flexural capacity increases of a given RC section, based on either ignoring or fully recognizing varying levels of preexisting strain. The moment-curvature characteristics, based on the models, of the latter plated RC sections are then presented, using the preexisting strains considered at the design stage. The results confirm that both the ductility and strength of plated sections are appreciably sensitive to preexisting strains. It is concluded that existing approaches for design of plating for RC members should take cognizance of these potentially debilitating effects of preexisting strains.