Steel fiber reinforced concrete with different recycled aggregates—In‐depth experimental investigation of fresh and hardened properties

Abstract The substitution of natural aggregates in concrete with recycled aggregates from construction waste makes ecological sense, is politically desired, and is increasingly being incorporated into technical regulations. Recycled concrete (RC) aggregates are also increasingly used for steel fiber reinforced concrete (SFRC) in the future. However, it is unclear from previous studies whether and how the RC aggregates affect the performance of the SFRC. This study addresses this subject and presents 10 test series in which the steel fiber content, the fraction of recycled aggregates (RC‐fraction), and the type of RC aggregates were systematically varied. The fresh concrete properties and the main mechanical properties, compressive strength in cubes and cylinders, Young's modulus, splitting tensile strength, and residual flexural tensile strength in three‐point flexure test, were investigated. To categorize and interpret the results, the amount and orientation of steel fibers in the crack surfaces of the flexure tests were determined using a proven opto‐analytical method. It is shown that the use of high‐quality recycled aggregates from railway sleepers without significant impurities results in mechanical properties similar to the reference concrete without recycled aggregates. At a RC‐fraction of 45 vol%, the residual flexural tensile strength is even higher than in the reference concrete, but this can be explained by an increased number of steel fibers in the failure section. The use of RC material made from mineral construction waste, including crushed bricks and impurities, leads to a reduction in mechanical properties of up to 20%. One exception is the residual flexural tensile strength with an RC‐fraction of 45 vol%, which also increases here. This is also due to an increased number of steel fibers compared to the reference specimens.