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Forty-eight initially uncracked push-off specimens were used for the investigation of the interface shear strength of natural and recycled coarse aggregate concrete. The parameters varied in the push-off tests were the recycled aggregate replacement level (0, 50, and 100%), the clamping force across the shear-transfer plane and the concrete grade (normal-and medium-strength). The inclined cracks crossing the shear plane, noted before the development of shear cracks, were indicative of the truss like action for resisting shear in push-off specimens. An analytical approach based on the truss modeling of the force system within the push-off specimens using the method initially proposed by Mattock and Hawkins has been used for prediction of interface cracking stress of both the concrete types. Calibration of the proposed analytical procedure has been done with the help of the experimental results of the tested push-off specimens. Using the experimental results as well as comparable results from the literature, a shear-friction regression equation accounting for the effect of concrete grade, normal force on the shear plane and the recycled coarse aggregate replacement level has been proposed for shear strength. K E Y W O R D S push-off specimen, recycled aggregate concrete, shear plane, shear strength, truss-based analysis 1 | INTRODUCTIONIn many cases, shear failure is constrained to occur across interfaces that can slide relative to each other. This type of behavior is usually associated with shear-critical elements as corbels, bearing shoes, pre-cast assemblies, etc. having highly distressed shear interfaces. 1,2 Over the years, several different types of test specimens have been used to investigate interface shear capacities with the underlying objective in the specimen design being the ability to induce a relative sliding action in the specimen under the action of a load parallel to the plane of sliding, the most common being the so-called push-off specimens (Figure 1). Based on the tests carried out on push-off specimens, either uncracked or precracked prior to testing, several design expressions have been proposed to predict the shear capacity of concrete-toconcrete interfaces. 3-9 A majority of these expressions assign shear strength either to a combination of "cohesion and friction" or "cohesion" of the interface is entirely ignored and it is assumed that shear transfer is completely due to friction. Since the "cohesion" of the interface is ignored in the shear-friction model, the use of unusually high values of the coefficient of friction is required so as to fit the test data. It may be noted that the shear-friction model is the basis for the current design practice for reinforced concrete under shear stresses.Discounting the shear-friction theory, Hsu et al 10 proposed a shear-transfer theory based on truss models wherein the shear strength is controlled by the crushing strength of

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The stress-strain model of concrete depends on the degree of frictional resistance across the loading surfaces of a test specimen depending on the antifriction medium used during testing. This article presents a comparative study of platen restraint on the behavior of concrete under uniaxial compression based on an experimental investigation. The effect of four commercially available antifriction media (neoprene, polyvinyl chloride, teflon, and grease) with different layer thicknesses on platen restraint have been studied for a normal strength concrete and a relatively high-strength concrete. Subsequently, the effect of platen restraint has been quantified using the analogy of toughness. The experimental results indicate that post-ultimate response of concrete is significantly affected by platen restraint. It is shown that the stress-strain curve obtained from a conventional uniaxial compression test not only describes specimen/material behavior but also represents interaction between specimen and loading platen. Among the four antifriction media used in this investigation, grease is the most effective in reducing frictional resistance. Failure patterns of the concrete specimens for different antifriction media are also subsequently analyzed.

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