Résumé
There is increased interest in the use of recycled aggregates in concrete formulation as part of strategy of waste recovery. The French National Program RECYBETON aims to move the scientific boundaries relative to the use of recycled aggregates into concrete. In that context, the objective of this study is to qualify and quantify the hydric transfers between the recycled coarse aggregates and cement paste during the mixing and the dormant period. Two families of concrete are tested: concretes for building and concretes for civil engineering structures with Water/Cement ratios equal to 0.6 and 0.45 respectively. The concrete mix design of the reference (natural aggregates) and recycled concrete have been calculated by the national program and specific initial slumps and 28 days compressive strengths are targeted. An original test is proposed in order to separate the fully and partially saturated recycled coarse aggregates from the fresh cement paste. It was observed that in our test conditions, absorption occurs quickly independent of the initial saturation degree of the recycled aggregates and the paste volume and the paste 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. In presence of a relative high speed of wind, water desorption of recycled aggregate is observed. A strong relationship between the time of initiation of the cracking and the total water content is established for recycled concrete: the time of ignition is an increasing function of the total water content. Recycled concretes tend to have wider cracks opening compare to reference concrete. The effect of transfer phenomena on other concrete properties such as slump and compressive strength is also discussed.