Incorporation of recycled concrete and mixed recycled aggregate as coarse aggregate replacements in structural grade concrete

A new study challenges one of the most deeply rooted assumptions in civil engineering: that using recycled materials in structural concrete inevitably means sacrificing strength. Researchers from Escuela Superior Politécnica del Litoral, in collaboration with University of Surrey, have demonstrated that recycled aggregates can be incorporated into concrete without compromising performance—provided certain technical limits are respected.

The research addresses an urgent issue in the construction industry: the enormous volume of construction and demolition waste that remains underutilized despite its potential to reduce the sector’s environmental impact. However, the variability of these materials—particularly mixed recycled aggregates (MRA)—has raised concerns about their reliability in structural applications.

To address this uncertainty, the team designed a highly controlled experimental framework, evaluating 10%, 20%, and 30% replacement levels of natural aggregates with recycled concrete aggregate (RCA) and mixed recycled aggregates. Unlike previous studies, all mixtures were prepared using the same particle gradation, allowing the researchers to isolate the true effect of recycled materials on concrete performance.

The results show that recycled aggregates do modify key fresh concrete properties such as workability and density. As replacement levels increase, the concrete becomes less fluid and lighter due to the higher porosity and water absorption of recycled materials. Nevertheless, mixtures containing mixed recycled aggregates exhibited better workability than those with pure recycled concrete aggregates, suggesting advantages during placement and handling.

The major question, however, concerns structural performance. For conventional concrete, the results are encouraging: replacement levels up to 20% not only maintained compressive strength but in some cases slightly improved it. In high-strength concrete, the behavior was more sensitive: while lower replacement levels (10–20%) produced only minor reductions, a 30% replacement caused significant strength losses, establishing a practical limit for demanding structural applications.

In terms of durability, all mixtures—even those with the highest recycled content—remained within acceptable ranges. Tests such as ultrasonic pulse velocity and air permeability confirmed good internal material quality and resistance to fluid penetration.

Microstructural analysis helped explain these behaviors. In recycled concrete aggregate mixtures, the presence of calcium carbonate—formed through carbonation of residual cement paste—helped densify the matrix and preserve strength. In contrast, mixed recycled aggregates showed greater heterogeneity and microcracking at high replacement levels, explaining their lower performance in high-strength concrete.

Beyond the technical findings, the study offers a key conclusion for the construction industry: up to 20% of natural coarse aggregates can be replaced with recycled materials without compromising the structural performance of concrete. Supported by experimental data and statistical modeling, this threshold provides a practical guideline for integrating sustainability into construction without jeopardizing safety.

In regions such as Latin America, where clear regulations for the use of recycled materials in infrastructure are still limited, these findings could accelerate the adoption of more sustainable construction practices. The research not only validates waste as a valuable resource but also proposes a concrete pathway toward a circular economy in construction.

In a sector responsible for a significant share of global emissions, innovations like this could transform the way future cities are built: with less waste, lower environmental impact, and without compromising structural safety.