Analytical study on shear behavior of composite concrete slab

Abstract

Concrete is used significantly in many structures since, the last few centuries. The need to maximize the concrete strength and utilize it's complete potential is the main focus of researchers even in today's modern age. It is thus, important to restore the existing structures apart from developing new materials and technology to build new structures. The advancement in scientific research and technology has helped us understand the different mechanisms to strengthen the existing concrete structures such as slabs, beams, bridge decks, walls etc. One such strengthening technique is application of overlays to existing concrete structural elements. An overlay of High Strength Concrete (HSC) above a Normal Strength Concrete (NSC) slab without shear reinforcement, proves to be a successful approach in some cases. One such experimental study is performed by Dr. Randl on a composite slab with a HSC overlay on top of NSC Reinforced Concrete (RC) slab. A four point bending test is executed in the experiment on eight such slab specimens. To understand the behavior of a composite slab undergoing shear failure, an analytical study is performed using three different shear models, Eurocode 2, Critical Shear Crack Theory (CSCT) and Critical Shear Displacement Theory (CSDT). Shear resistances are obtained using these models for each composite concrete slab specimen, considering two separate cases of homogeneous slabs, one with substrate properties and other with overlay concrete properties. The average shear resistances obtained from these two limiting cases are compared with each other and their accuracy with the experimental results is also tested. Furthermore, appropriate guidelines are proposed to evaluate the shear capacity of composite concrete sections with the help of these homogeneous slabs with some validation by a numerical analysis. One shear model among the three, depending on the accuracy of results and the least Coefficient of Variation (COV), is chosen to further give a foundation to the analysis of composite concrete slabs. The composite concrete slab is modeled and a four point bending test is simulated by finite element analysis using ATENA. Although CSDT is developed for homogeneous cross-sections, in this research CSDT provides good estimation of shear behavior for composite slab cross-section. The theory also explains the experimental results better than any other analytical model with an accuracy of almost 90 % with the least COV of 2.53 %. Crack propagation is observed considering two different homogeneous slab specimens and some changes in the formulae given by CSDT are proposed. A slab factor is also introduced to explain the increase in shear capacity of concrete slab specimen. Effect of bond strength on the shear distribution along the composite slab cross-section is studied with the help of analytical models and equivalent area method. Striking resemblance in the shear stress distribution is observed in case of NSC homogeneous slab and composite concrete slab thus, further validating the idea that shear capacity of a composite concrete slab can be estimated by considering the substrate homogeneous concrete slab properties. Effect of the HSC overlay is not significant in this study. Two limiting cases are considered for further check for de-bonding/delamination of the interface between the two concrete layers. First case with perfect bond and second with no bond between the two concrete layers are considered. No delamination is observed in case of monotonic loading which agrees with the experimental observations as well.