Aggregates
Aggregates are the major component of concrete by volume and are inherently a low carbon product. Most are naturally occurring materials requiring little processing and are usually locally sourced, with the added benefit of reducing carbon associated with transportation.
The key standards relating to aggregate requirements for use in concrete include BS EN 12620:2002+A1:2008 (and the associated PD 6682-1:2009+A1:2013) and BS 8500:2023 which is the complementary British Standard to BS EN 206:2013+A1:2021.
The standard for aggregates in concrete (BS EN 12620) classifies aggregates for concrete into three main categories: Natural, Manufactured and Recycled.
Natural aggregates are either land- or marine-won and, as a resource in the UK, are abundant. Their extraction is tightly regulated, and sites of mineral extraction are restored, often to an enhanced state, delivering significant biodiversity benefits. The aggregate will undergo processing such as crushing, washing and grading to be suitable for use within concrete. They may also be referred to as primary or virgin aggregate, however some secondary aggregates can also be derived from natural sources when the aggregate is the by-product of other mineral extractive processes such as sand or crushed rock from ball clay and China clay production.
Manufactured aggregates are often by-products from other industrial processes such as blast furnace and steel slags, incinerator bottom ash, furnace bottom ash, and sintered fly ash aggregate products. As a by-product these aggregates may also fall under the definition of ‘secondary aggregates’.
Under BS 8500, the complementary British Standard to BS EN 206, recycled aggregates within concrete are split into two categories:
- Crushed concrete aggregate (CCA)
- Recycled Aggregate (RA)
BS 8500 provides further guidance on their use within concrete, giving usage limits based on exposure conditions and strength requirements. The ‘How to design concrete structures to Eurocode 2: BS 8500 for building and civil structures’ published by the Concrete Centre summarises these requirements.
The UK is one of the leaders in Europe for recycling hard construction and demolition waste. Most recycled aggregates are used in unbound applications. For use of recycled aggregates within concrete, the composition needs to be assessed to ensure that its inclusion does not affect the water demand and cement content, as this is the biggest contributor to the embodied carbon of concrete.
It is also recommended that when using CCA or RA, the transport distance from the location of demolition to the concrete producer should be less than 15 miles, as transport is the biggest contributor to the embodied carbon of the aggregate.
Specifying a precise aggregate can be more carbon intensive and less economic, so it is recommended that designers contact the local concrete supplier to find out about the local aggregates to the area.
Aggregates for concrete - definitions
There are a number of terms used within the construction sector for the use of different aggregates within concrete, however some of these terms do not align with the current standards relating to concrete. For definitions of aggregates for use in concrete refer here.
Coarse aggregates
Coarse aggregates are particles greater than or equal to 4mm in size. Coarse natural aggregates are normally grouped into gravel or crushed rock. The use of coarse CCA is permitted within BS 8500 and the section on Recycled Aggregates within ‘How to design concrete structures to Eurocode 2: BS 8500 for building and civil structures’ provides further details on this. Coarse RA is also permitted for use within BS 8500 however due to its more variable nature, further testing on the composition will be needed to assess suitability.
Fine aggregates
Fine aggregates are particles less than or equal to 4mm in size and generally consist of natural sand or crushed rock fines. As with coarse aggregates these can be from Natural, Manufactured or Recycled sources, BS 8500 does not establish general suitability for recycled fines into concrete, recommending materials should be assessed on a case-by-case basis, due to the increased risk of factors which may affect the setting or strength of the concrete.
Lightweight aggregates
Lightweight aggregates are manufactured aggregates and can be either natural or manufactured materials. The required properties of the lightweight aggregate concrete will have a bearing on the best type of lightweight aggregate to use.
Enhancing biodiversity and quarrying
Over 700 sites of special scientific interest are current and former mineral extraction sites. The significant contribution to UK biodiversity from the minerals sector is being increasingly recognised. The MPA Quarries and Nature Awards celebrate and showcase the best examples of site management and restoration delivering landscape-scale benefits, and innovation and planning for nature recovery in the future. Many of these projects are included in the publication and film celebrating 50 years of this work. For further details, visit the Biodiversity page
The UK aggregates sector is a key part of the partnership programme Nature after Minerals (NAM). NAM has stated that the restoration of sites of mineral extraction represents the largest expansion of habit in the UK. The industry’s investment in restoration is uniquely placed to enable the UK to achieve biodiversity targets that will directly safeguard the future of 960 priority species. Case studies, advice and further information can be found at www.afterminerals.com