Investigation of Concrete Properties with the Use of Recycled Coarse Aggregate

This paper presents experimental results of two-stage mixing approach on properties of concrete made with recycled concrete aggregates. Concretes with recycled concrete coarse aggregates have poorer workability and lower mechanical strength than concretes with natural coarse aggregate. Concrete mixtures lose workability in early time due to their increased capillary porosity and presence of interfacial transition zone (ITZ). It consists of ettringite and calcium hydroxide. Due to the existence of two weaknesses in the ITZ, the performance of recycled aggregate concrete is much lower than that of natural aggregate concrete. When formation time of cement-based materials in concrete with recycled concrete aggregate, new appear in addition to the old ones - between recycled concrete aggregate and a new mortar part. Interfacial transition zone is less durable than the interfacial transition zone in recycled concrete aggregate itself. The article describes the method of using two-stage mixing approach (TSMA) with research of the time of formation of concrete made with recycled concrete aggregates. Concrete mixture prepared by two-stage mixing approach showed difference in time formation in early stages, without a sharp loss of workability in contrast to the concrete mixtures with normal mixing. The samples made by TSMA has higher strength than the samples made by normal mixing. The compressive strength in 3, 7, and 28 days increased by 16%, 8%, and 16%, respectively, compared with the samples made by normal mixing. The obtained data showed the positive influence of two-stage mixing on loss of workability in early time and strength of the concrete.