Recycling concrete

Reduce and reuse are priorities for good circular economy practice.  When concrete does eventually reach the end of its life, it can be recycled.  

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The majority of concrete&#;s volume/mass is aggregates, and when recycled, it becomes aggregate again. Some of this makes its way back into new concrete, but most is used &#;unbound&#; as sub-base materials, fill and hardcore, replacing primary aggregates. Concrete pieces from demolished structures can also be reused to protect shorelines, for example in gabion walls or as rip rap. The recycling process has the potential to be repeated again and again in perpetuity to provide a low carbon aggregate resource for a range of applications. 

The supply chain for recycling concrete is well established in the UK and feedback from demolition contractors, supported by data from other sources, is that practically all concrete demolition waste is recycled. The aggregates in concrete may be crushed on site for reuse in a new development on site, or transported to a local recycling centre for processing and distribution. 

Recycled and secondary aggregates account for around 29 per cent of the total aggregate use in the UK.  This plays an important role in the UK&#;s current circular economy, its use reducing the demand for primary aggregates. See Profile of the UK Mineral Products Industry on the MPA website.

Recycled aggregates 

Recycled concrete is a viable source of aggregate and has been satisfactorily used in granular subbases, soil-cement, and in new concrete. Recycled aggregates are classified in one of two ways, as: 

• Recycled Aggregate (RA) - aggregate resulting from the reprocessing of inorganic material previously used in construction.   
• Crushed Concrete Aggregate (CCA) &#; aggregate obtained from crushed concrete. 

 
A specific subset of recycled aggregates is crushed concrete aggregates (CCA) where the content of: concrete, concrete products, mortar, concrete masonry units, unbound aggregate, natural stone or hydraulically bound aggregate  must not be less than 90%. This high percentage inclusion of &#;good&#; aggregate results in CCA having better performance characteristics than RA and consequently there are fewer restrictions on the use of CCA in concrete. Provision for the use of CCA in concrete is given in BS :. 

For information on the use of recycled aggregates in concrete see below.   

Crushing concrete and carbonation 

Crushing concrete at the point of demolition substantially increases its surface area, allowing carbon dioxide to be more readily absorbed &#; &#;a process called&#;carbonation. Exposure to rain has been shown to significantly increase the rate&#;of carbonation during this stage. Carbonation occurs during demolition and while the aggregate is stored on site, but then continues during the concrete&#;s secondary life if used in groundworks, hardcore and landscaping. 

Recycled concrete fines and recycled concrete paste 

New uses for concrete demolition waste are in development. Recycled concrete fines are the residue of concrete waste after recycled aggregates are removed. It consists of cements pastes and sand. It accounts for ~15% of recycled concrete and currently has only low value uses.&#; Research and development is taking place create new uses including carbon capture use and storage,  and&#;the manufacture of recycled concrete fines and paste as a cement replacement and the manufacture of new cement.   

Recycling reinforcement  

Steel reinforcement can be recycled, used to create new reinforcing steel.  

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Yes, concrete can be recycled and can be completely reused for new concrete production.

It is important to distinguish between reusing and down cycling the material, versus reusing for the same purpose as before. When reusing the crushed concrete material as a base for roads or foundations, for example, this is down cycling. As in a linear economy model, the material has a new purpose and can no longer be brought back to its original use in new concrete production.

However, within a recycling economy, old concrete may be recycled and reused within new concrete production. This is already being done in the concrete industry. Ultimately, we strive for a circular economy, where there is no waste at all in the concrete production and recycling process.

Yes, concrete can be crushed down, separated and reused. Since the demolished concrete contains aggregates and cement stone, reuse has some limitations.

With the reCO2ver process, the Sika patented process of concrete recycling, the fragments are broken down into their component parts - namely fine powder, sand and aggregates. This technology paves the way for a circular economy in concrete.

When concrete recycling is done by crushing the concrete and reusing it as aggregates again, the concrete can only used for certain applications as described in standards. For example, in some countries it cannot be used for any applications that require freeze-thaw resistance. The high porosity of the crushed concrete leads to a lower value of frost resistance.

With Sika's new reCO2ver process, the aggregates can be cleaned from the cement paste. With this step, the potential applications can most probably be extended.

There is some difference when using either recycled concrete aggregates, also called crushed concrete, or natural aggregates in new concrete. The main difference is in the water absorption and water demand in the prepared concrete using these different aggregates.

The crushed concrete demolition waste contains aggregates and cement stone. The cement stone increases the porosity of the material and leads to a higher water demand and therefore, higher cement demand in the concrete mix. For manufactured (crushed concrete) aggregates, the surface is larger than naturally-sourced gravel or natural aggregates. This also leads to a higher water demand in the concrete.

By cleaning the recycled aggregates from cement-stone, the material can be used comparable to naturally-sourced gravel and aggregates. This reduces the cement demand in the recycled concrete and also the CO2 footprint.

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