Dynamic fractures of concrete made of recycled aggregate or reinforced with fibres

Abstract

In recent decades, there has been a lot of interest in using construction waste as a source of aggregate for the production of new concrete. Another topic of intense study is the optimal characteristics of reinforcing fibres. However, in contrast to the constant value of the static strength, it is widely known that the dynamic strength of concrete is highly dependent on the strain rate. The wide variety of possible concrete additives further complicates the assessment of the dynamic strength of concrete. Therefore, it is necessary to search for new material parameters that will allow evaluating and comparing the dynamic strength of concrete with various modifications of additivities. In this work, the effects of recycled aggregate and fibres on the fracture critical stress of original, natural aggregate concrete were studied using the incubation time criterion. It was confirmed that by estimating only one additional parameter, the incubation time, which is responsible for the preparatory processes of fracture and is invariant with respect to the history of loading in dynamic and static tests, it is possible to construct the stress/strain rate dependences of the critical stress. The intersections of the theoretical rate dependences of the critical fracture stress under static and dynamic loading considering various percentages of recycled aggregate (from 0% to 100%) and carbon, steel, and synthetic fibres were analysed. The decrease in the dynamic strength of steel- and carbon-fibre-reinforced concretes was interpreted in terms of a change in the incubation time characteristic. The influence of the fibre shape on the strain rate sensitivity of steel- and synthetic-fibre-reinforced concretes was analysed. It is here shown that the incubation time parameter can be considered to be a convenient tool for assessing the influence of recycled aggregate or fibres on the dynamic strength of concrete.