The behavior of graded concrete, an experimental study

In structural analysis, a concrete element is assumed homogeneous in terms of strength and physical properties throughout the entire section or member. In reality, concrete is a composite material composed of mortar matrix and coarse aggregates. The actual composition of a member is therefore less likely to be uniform along its height. The presence of coarse aggregates will result in a higher density, reaching the extreme lower layers of the member, leading to a disparity in strength, elastic modulus, and Poisson's ratio. Other factors influencing this phenomenon are bleeding, and initial cracking due to the hydration heat on the exposed surfaces. However, if the non-homogeneities of these concrete properties can be observed, and further controlled and adjusted properly, the potential for a design optimization could be aimed. This study was performed to produce intentionally, and further analyze, the actual behaviors of a concrete member with a gradation in concrete compressive strength. The compressive strength of the test specimens ranged from 20 MPa to 60 MPa. The cylindrical specimens sized 100 x 200 millimeters were tested under uniaxial compression, and the strains in different layers were recorded, as the load was increased. The developed method and equipment was proven excellent in artificially creating a graded concrete specimen. The resulting specimens were visually inspected and tested using a rebound hammer to study the gradation of the material in the transition zone between the two different types of concrete strength mixes. The resulting concrete is negatively influenced by the disparity between the two concrete strengths since the cracking and fracture of the lower concrete strength part will signature the strength of the overall graded material. The brittle behavior of the resulting material is due to a combination of premature micro-cracking, and the stress differentiation in the material, that leads to stress concentrations in the corresponding layer. (C) 2015 The Authors. Published by Elsevier Ltd.