Influence of initial saturation degree of recycled aggregates on fresh cement paste characteristics: consequences on recycled concrete properties

Abstract

The French National Program RECYBETON aims to move the scientific boundaries relative to the use of recycled aggregates into concrete. The objective of this study is to qualify and quantify the hydric transfers between the recycled coarse aggregates and cement paste at early age. Concretes for building and civil engineering structures are tested. The concrete formulation of the reference (natural aggregates) and recycled concretes has been calculated by RECYBETON and specific initial slumps and 28 days compressive strengths are targeted. An original test is proposed to separate the fully and partially saturated recycled aggregates from the fresh cement paste. In our test conditions, absorption occurs quickly independent of the initial saturation degree of the recycled aggregates. The paste volume and water content are stable at the end of mixing. To exacerbate desorption phenomena and to provoke early age cracking, a severe drying is applied on fresh concretes. Water desorption of recycled aggregates is observed. A strong relationship between the time of initiation of the cracking and the total water content is established for recycled concretes: the time is an increasing function of the total water content. Recycled concretes tend to have wider cracks opening compared to reference concretes.