Concrete facts about recycling

Concrete is the second most consumed material after water and it shapes our built environment. In fact, twice as much concrete is used in construction around the world than the total of all other building materials, including wood, steel, plastic and aluminium.

The Romans first invented what today we call hydraulic cement-based concrete. They built numerous concrete structures, including the Pantheon in Rome, one of the finest examples of Roman architecture that survives to this day.

While the longevity of the material is undoubted, every construction project that involves the renovation of an existing structure will inevitably mean the removal of existing concrete.

The potential to recycle concrete is overlooked by many in the UK construction industry and it ends up as unnecessary waste in landfill. This is possibly because recovering concrete has no appreciable impact on reducing greenhouse gas emissions. 

Benefits of recycling concrete 

• It reduces the amount of waste sent to landfill, and cuts the cost of waste disposal.
• It reduces the use of new resources and the associated environmental costs of mining new materials.
• Because concrete can often be recycled on demolition or construction sites or close to urban areas where it will be reused, transport costs are reduced. 
• It can create employment opportunities and boost the local economy.

How is concrete recycled?

Although concrete is not broken down into its constituent parts, it can be recovered and crushed for reuse as low-grade aggregate in new concrete, particularly the coarse portion. While this is considered to be more sustainable than mining, processing and transporting new aggregates, it should be remembered that:

• Recycled concrete as aggregate will typically have higher absorption and lower specific gravity than natural aggregate and will produce concrete with slightly higher drying shrinkage and creep. These differences become greater with increasing amounts of recycled fine aggregates.
• The chloride content of recycled aggregates is of concern if the material will be used in reinforced concrete. The alkali content and type of aggregate in the system is probably unknown, and therefore if mixed with unsuitable materials, a risk of alkali-silica reaction is possible.

Other uses for recycled concrete aggregate

1. Civil engineering. Most recovered concrete is used as a substitute for virgin aggregate in laying a base for road work. Other uses for recovered concrete in civil engineering projects include building slabs and foundations, pavements and storm drains.

2. Cement manufacturing. Concrete can also be recycled in controlled amounts through the cement manufacturing process. It can be used either as an alternative raw material to produce clinker or as an additional component when grinding clinker, gypsum and other additives to cement.  

3. Pipe bedding. Recycled concrete can serve as a stable bed or firm foundation in which to lay underground utilities. However, this use of recycled concrete aggregate is only economical if there is a savings in the yield and transportation costs.
 
4. Landscape materials. Recycled concrete can be used in various landscape settings. Sized concrete rubble can serve as landscape feature; an attractive support that offers different architectural texture and colour. In the US, recycled concrete aggregate has been used as boulder/stacked rock walls, underpass abutment structures, erosion structures, water features and retaining walls.

As concrete recycling becomes a recognised way of using aggregates left behind when buildings are renovated or demolished, it will not only have a positive environmental impact but also help keep construction costs down.

Image credit: Mike Kniec