Lab tests: up to 20% recycled coarse aggregate can meet structural requirements

Crushed concrete cylinders sit at the start of a process that usually ends with waste, not a new building. At Escuela Superior Politécnica del Litoral, researchers working with the University of Surrey sorted those fragments into recycled concrete aggregate and tested whether it could replace natural stone in structural concrete. Their central result: up to 20% replacement preserved the required performance in the mixes they studied.

The team compared replacement levels of 10%, 20% and 30% using recycled concrete aggregate and mixed recycled aggregates. Every mixture used the same particle gradation, a control that allowed the researchers to separate the effect of the recycled material from the effect of particle size. They measured workability, density, compressive strength, ultrasonic pulse velocity, air permeability and microstructure.

The trade-off appeared before the concrete hardened. As recycled content rose, the mixes became lighter and less fluid because recycled particles are more porous and absorb more water. Mixed recycled aggregates were nevertheless easier to work with than pure recycled concrete aggregate in the reported tests. Once hardened, conventional concrete with up to 20% replacement maintained compressive strength and sometimes improved slightly. In high-strength concrete, the margin was narrower: 30% replacement produced significant strength losses, while 10% to 20% caused only minor reductions.

The microscope offered a reason for the difference. Recycled concrete aggregate contained calcium carbonate formed when residual cement paste carbonated, helping densify the matrix and preserve strength. Mixed recycled aggregates were more heterogeneous and showed more microcracking at high replacement levels. Durability indicators, including ultrasonic pulse velocity and air permeability, remained within acceptable ranges even at the highest recycled content tested.

So what changes in practice? Construction teams may have a defensible starting point for putting demolition waste back into structural concrete: replace up to 20% of natural coarse aggregate, then validate the specific materials and mix. That could reduce the amount of construction and demolition waste left underused, particularly in regions such as Latin America where clear rules for recycled materials remain limited. The result is a laboratory-backed limit, not a universal guarantee: the study shows what worked under its controlled conditions, while high-strength applications still demand caution.