Development of high performance concrete with coarse recycled aggregate for precast industry

Recycled aggregate (RA) is obtained by sorting, crushing and sieving inorganic material previously used in construction. When properly designed, substituting RA for natural aggregate (NA) in new construction is considered a sustainable practice that could help the European Union to achieve climate neutrality by 2050. Compared with NA, RA has complex composition, high porosity and poor engineering properties. Appropriate use of RA according to its quality class and application grade is highly critical. Several studies since 2000 have shown that it is feasible to produce high strength concrete (HSC) or high performance concrete (HPC) using RA derived from HSC waste. However, the sample size is limited, the repeatability (within-laboratory) and reproducibility (between-laboratory) are unknown, the role of RA in HSC/HPC is not fully understood yet, and design guidelines are lacking in the literature. To fill those gaps, since January 2017, the Materials and Constructions Division at KU Leuven Bruges Campus (RecyCon) has been working extensively on structural precast concrete incorporating coarse RA. This is the third PhD dissertation within that framework.

The objective of this PhD research is to develop precast concrete products using commercial coarse RAs, while attempting to identify determinants, to explore mechanisms and to provide design guidelines. The methodology includes literature review, experimental study and database analysis. The research results show that recycled concrete aggregate (RCA) conforming to NBN B 15-001 Type A+ is very promising for the development of HPC with compressive strength classes from C50/60 to C80/95. Mixed recycled aggregate (MRA) complying with NBN B 15-001 Type B+ is not recommended for mass usage in self-compacting concrete (SCC) with compressive strength classes of C50/60 and C55/67. When developing recycled aggregate concrete (RAC), four key factors should always be kept in mind, namely RA quality, concrete compressive strength class, RA quantity and water compensation degree. In particular, micro-Deval coefficient (MDE) and Los Angeles coefficient (LA) are found to be effective quality indicators for coarse RA as alternatives to or simultaneously with oven-dried particle density (ρrd) and 24 h water absorption (WA24). Furthermore, the volume fraction of coarse RA (i.e. its absolute volume per unit volume of concrete, m3/m3) is considered to be a more accurate quantity indicator than the replacement percentage of coarse NA (either in volume or in weight), and therefore a new empirical model is proposed to predict the 28 d compressive strength of concrete made with coarse RCA. Moreover, the water compensation degree for coarse MRA is recommended to be between 80% and 100% of its 24 h water absorption, so as not to alter noticeably the carbonation resistance of concrete, and therefore a concept of new mortar cover for coarse MRA is proposed. Finally, the incorporation of coarse RA affects the confinement effect of concrete. Literature using only cubic specimens may overestimate the cylinder compressive strength of RAC.

It is anticipated that this PhD dissertation will support policy makers in refining the RA classifications specified in NBN B 15-001 and relaxing the restrictions on the use of high-quality RCA in structural concrete. A proposed amendment is given in the end of this dissertation. In addition, researchers are encouraged to pay more attention than before to the mechanical properties (MDE and LA) and water compensation degree of coarse RA for use in HPC, and to start using both cubic and cylindrical specimens for the compressive strength test of RAC.