Enhancing Bond Performance by Functional Grading of Concrete

Abstract

The concrete industry is seeking solutions to reduce the carbon footprint of concrete structures. The carbon impact of concrete is mainly driven by the cement clinker content. Typically, the concrete grade is selected according to the most stringent requirements throughout the structure, even though these requirements will vary spatially. Thus, regions exist within structures where concretes with a lower performance would be sufficient. This inefficiency in cement usage can be remedied by functionally grading the concrete according to local requirements to thereby reduce the carbon footprint of the structure. The presence of concrete layers, however, increases the complexity of the design. In this study, the bond performance of layered concrete structures is investigated. Pull-out tests on single mix samples with varying concrete cover were performed using low-strength or high-strength concrete. In a second step, companion functionally graded specimens were tested. In the graded samples, a 15 mm layer of high-strength concrete was placed on the outer face, while the remainder of the sample was cast with low-strength concrete. The failure of the specimens was in all cases initiated by a splitting of the concrete. The failure load, however, was governed by the splitting resistance of the concrete enclosing the reinforcement bar and, for the parameters tested here, appeared to be independent of the layer thickness. This suggests that the splitting resistance of functionally layered concrete can be controlled in a targeted manner. In this way, the overall concrete consumption of a structural element can be reduced using concrete layers with different performance requirements.