Recycled Concrete: A use still too Limited

In industrial production the guidelines on the circular economy are coming into force and permanently in all companies.

This is due to several factors: a new cultural approach of the population that is increasingly attentive to the environment, a political factor that fully embraces people's expectations, new financial-insurance rules that assess the risk level of companies based on their deviation from an average carbon footprint and, finally, on a real need for greater sustainability of consumption.

In the construction field the share of materials that go to landfill remains extremely high with important environmental consequences, not only for the quantity of waste that is not put back into circulation as new raw materials, but also due to the continuous supply of new raw materials affecting the natural resources of the environment.

Starting from the design, buildings should be designed to be built with the largest share of recycled materials and, once at the end of their life, demolition should follow an activity of recovery of all those materials that can be used again for new buildings.

What are the advantages of recycling concrete?

Unlike other materials to be recycled, such as plastics, the origin of the concrete waste includes the presence of aggregates whose natural origin is known.

Therefore, the recycling of the material coming from the demolition of buildings can be easily managed and, the share that is obtained in the context of a demolition, is generally high . The reuse of recycled material leads to a number of advantages:

• Lower cost of recycled aggregate compared to the natural one

• Less material to be sent to landfill

• Lower carbon footprint for a building made with recycled concrete compared to one made with natural aggregates

• Lower transport costs and environmental impacts

In the compositions of the concrete recipes with recycled elements we can include the following materials:

• Shattered demolition, clean and uniform color

• Brick crushed, clean and unpolluted

• Shattered glass for post consumption

• Fly ash expressed in light aggregates

• Crushed in stone as ballast or retaining walls

• Foundry sands only if clean and uniform

But let's see what the behavior of a concrete made with recycled aggregates can be compared to one with natural aggregates:

• The use of recycled aggregates up to a share of 20% has no effect on the compressive strength of the concrete, while a mixture of 100% recycled aggregates leads to a strength of about 20% of the compressive strength

• Durability over time, with the same resistance, has no influence on the percentage of use of recycled aggregates compared to natural ones

• The rigidity of the product with a use within 20% of recycled aggregates does not undergo substantial changes, while for a 100% use a reduction of the stiffness around 10%

• As far as the workability of the mixture is concerned, no reductions in the mixture have been noted using recycled aggregates up to a share of 20%.

• Using shares greater than 20% of recycled aggregates, the drop in workability of the cement paste can be substantial, the main consequence of which is the greater demand for water to make the dough is workable. This is due to the irregularity of the aggregates that increase their specific surface, the greater water absorption of the crushed aggregate and the presence of non-hydrated cement particles. In this case it is important to use plasticizer additives to reduce the use of water in the mix so as not to compromise the mechanical strength.

As regards the environmental impact of natural aggregates, it must be considered that their excavation requires 20 MJ / t of combustion energy and 9 MJ / t of electrical energy , while their crushing requires, respectively, 120 MJ / t and 50 MJ / t.

While the environmental impact of aggregates recycled from demolition waste can be estimated at 40 MJ / t of combustion energy and 15 MJ / t of electricity.

Regarding the mechanical strength between a concrete made with recycled aggregates and one with natural aggregates, which we can see in the table at the bottom of the article, except for what has been said above the technical data are very similar.

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Automatic translation. We apologize for any inaccuracies. Original article in Italian.