Slab-on-grade

This section presents the first experimental site carried out in France on recycling concrete into concrete, within the RECYBÉTON National Project. A 2,000 m² parking lot was built, divided into six sectors made up with various concrete mixes. All mixtures matched the same set of specifications, except the replacement rate, which ranged from 0% to 100% of recycled aggregates (fine and coarse fractions). A comprehensive characterization campaign was carried out on the six concrete mixes, mainly devoted to fresh concrete properties, strength, E-modulus, and shrinkage. The usual trends—increase of cement content for high replacement rate, decrease of E-modulus, and increase of shrinkage—were noted. A comprehensive finite-element 382modeling of the slab-on-grade was performed, based on measured concrete parameters complemented by a number of data taken from the literature and from experimental results of laboratories participating with the national project. The objective was to anticipate cracks originating in a combination of thermal and hygral effects. The simulations only predicted the appearance of cracks in the joints of the slabs, and in a laboratory sample of fully recycled concrete used to perform a ring cracking test. The construction site went smoothly with no difference between the mixes, as reported by the practitioners. No crack appeared out of the joints sawn every 5 m according to usual procedures, as predicted by the simulations. From this experimental site, it was concluded that the production and casting of recycled concrete did not require any significant change in the current practices. As for the risk of shrinkage-induced cracking, it only grows in case of very high replacement rates. Even in such a case (slab-on-grade cast in winter, with moderate thickness), it was possible to place a fully recycled concrete, with a higher cement dosage but without crack. To better assess the risk in other cases, more experimental and numerical creep data have to be generated. Three years after this project completion, some supplementary slabs were cast with concrete incorporating recycled cement (i.e., a cement where 15% of the raw materials were substituted by ground recycled concrete). In terms of fresh and hardened mechanical properties, as well as behavior during and after the casting phase, no difference was noted by comparison with a control concrete batched with a regular Portland cement.