Proportioning, Microstructure and Fresh Properties of Self-compacting Concrete with Recycled Sand

The use of fine recycled aggregates from recycled concrete is limited due to the high absorption of the material and the subsequent reduction in mechanical performance. At the same time, Self-Compacting Concrete (SCC) uses large amounts of fines to ensure its flowability. Therefore, this type of concrete could allow the use of fine recycled aggregates. Hence, the aim of this work is to study the proportioning and the effects in the microstructure and the fresh basic properties of the use of recycled sand to produce SCC. The concrete mixes analyzed incorporate recycled sand (in percentages of 0%, 20%, 50% and 100%) and natural coarse aggregates. The mix design used an equivalent mortar, which allowed obtaining a suitable concrete that could be at the same time comparable between different replacement ratios and usable in real-life applications. During the design of the mixes with the mortars, the workability was measured from 10 min to 90 min using mini-cone and mini-funnel tests and the suitable ones were chosen to perform self-compacting concrete. Once this was done, these mixes were produced at concrete scale, and with these, basic properties were measured. The fine recycled aggregate changes the workability and the rheology of the mortar and concrete. These differences also affect the microstructure in terms of bonding and porosity distribution. There is a severe reduction of compressive and splitting strength as a result of the use of recycled sand, and this could be linked directly to these changes of the microstructure. The recommended substitution ratio with small decrease of mechanical performance is up to 20%.