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Optimising concrete with recycled sand, aggregates

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25 January 2021

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Virgin sand and aggregates have long been touted for their strength and quality over recycled aggregates. However, as Eunan Kelly explains, the quality and structural integrity of recycled sand and aggregates is now as good – if not better – than the originals. 

When supported with the appropriate processing practices and technology, sand and stone resources recovered from construction, demolition and excavation (CD&E) activities are suitable for high value construction and infrastructure projects.

UK statistics on waste, published by the UK Department for Environment, Food & Rural Affairs, estimates the UK generated 221 million tonnes of total waste in 2016, with CD&E activities accounting for more than three-fifths (62 per cent) of that total.1

The industry is the single largest contributor to waste generation in the UK by some margin, accounting for more than five times that of household waste which is 12 per cent of total waste generated. These waste resources can be recovered to a high specification and returned to the construction sector to further the UK’s circular economy.

In the materials processing industry, we’re having to speak out in defence of recycled sand and aggregates and lobby for attitudinal change to encourage greater acceptance and adoption of recycled materials.

We’re often told it’s not possible to produce structurally sound concrete from recycled sand and aggregates or it’s unfeasible to replicate the water-to-cement ratio with recycled products to produce a durable concrete. Some people have even suggested that concrete produced from recycled aggregates has more embodied carbon than concrete produced from natural materials. These are arguments our industry is faced with regularly, and misconceptions that we move to challenge.

Today, research is ongoing to identify alternatives to sand and aggregates in concrete production. Some of the research centres around the use of wood, shredded up vehicle components and other unnatural concrete constituents. Such research seemingly disregards CD&E, our largest waste stream, and fails to recognise the fact that much of the material in this stream originates from the natural constituents of concrete and therefore lends itself perfectly to producing concrete.

Though a huge social issue, the volume of plastic waste does not represent anywhere near the same as that of CD&E waste. Unlike plastic, sand and stone recovered from CD&E waste, a product that has been heavily processed for its original use, shares the same or similar geological make-up to that of virgin materials.

LOGICAL ANSWER

To combat depleting natural sand and aggregate resources we should better utilise the abundant incoming CD&E waste stream. 

In its appraisal, concrete produced from recycled sand and aggregates is unfairly pitted against higher strength concrete produced using virgin aggregates, such as granite or basalt, and natural sand. It is fair to say that not all granite or sandstone deposits display the same strength characteristics and therefore selective end-use logic is applied. 

The same is true for sand and aggregates recovered from CD&E waste. Given the variability of rock geologies and other man-made aggregates, such as brick and bound concrete, we must also apply the same end-use logic. 

This should not, however, undermine the potential of concrete produced from recycled materials. It is a case of identifying the strength of concrete that can be produced from recycled sand and aggregates and then pinpointing suitable applications for the product. It should also be pointed out that current wet processing technologies deployed by CDE around the world can produce washed sand and aggregates that when used in the production of similar strength concrete are comparable in cement consumption.

Low strength granite or gritstone would not be used to construct a multi-storey building, but we can identify suitable concrete strength applications for their use. Similarly, with CD&E material, we may never use it to construct that same multi-storey building, but there are still many applications for which it is suitable. For example, Thompson Recycling, based in Scotland, produces a wide range of products for the construction sector using C&D waste. With the support of CDE technology the company is able to replicate the grading of local virgin sand deposits to provide the local construction market with a viable and creditable alternative to natural resources. Such is the quality of the recycled sand and aggregates it extracts from C&D waste, its 100 per cent  recycled sand is BSI-approved for structural concrete.

Current concrete strength specifications allow for recycled aggregates to be used in the appropriate proportions to produce the required strength. It’s probably fair to say that the majority of concrete produced is C40 or below. About 75 per cent of all concrete used globally is non-structural which begs the question: Why use structural grade aggregates to produce non-structural concrete? Surely we would use the appropriate material provided it gives us the appropriate outcome. In so many cases globally CDE customers are producing competitive concrete for these non-structural – but still high value – construction projects, with some applications successfully achieving beyond C45.

Determining whether a recycled concrete product is fit for purpose or not is dependent on how it is processed and its intended use. 

Indeed, the Sheehan Group, one of the UK’s leading regional construction groups, has diverted more than 750,000 tonnes of inert waste from landfill over the past seven years. It creates 20,000 building blocks a day from 100 per cent recycled aggregates which follows a CE-certified process. Improved and advanced technology now sees cement consumption reduced by 10 per cent and Sheehan Group is still achieving the same high quality specification and end product.

The group’s success shows that with the appropriate practices and the backing of the latest wet processing technology, high quality in-spec sand and aggregate products can be extracted from CD&E waste and returned to support new construction and infrastructure, both consistently and competitively.

It is important to acknowledge, however, that poor processing practices impact uptake and acceptance in the usability of recycled materials. This is due to antiquated or less efficient technologies that fail to effectively remove contaminants, such as wood and plastic, from feedstock. The presence of these contaminants impacts upon the strength and structure of concrete.

MEETING GROWING DEMAND

Soaring urbanisation presents a global challenge to meet the demands of the construction industry, and recycled materials are an effective solution when supported with the most appropriate technologies and practices.

It was anticipated that by 2050, more than two-thirds of the world’s population (68 per cent) would be living in urban settings, according to data from the United Nations, rising from around 55 per cent of the population today.2 Combined with projections of population growth, trends in urbanisation could add up to 2.5 billion to urban settings over the next 30 years. It remains to be seen how the COVID-19 pandemic will impact population growth, but it will undoubtedly weigh on future planning considerations for construction and infrastructure as a means to mitigate against the threat of disruption caused by future pandemics.

Economies have been hit hard by the crisis and as history has shown governments tend to respond by investing in infrastructure to kickstart the economy or in construction to reshore manufacturing facilities, which we’re already seeing in the UK, USA and Australia. This is where CDE Global’s technology will add value by maximising the quality, quantity and value of waste resources.

Currently an estimated 40 to 50 billion tonnes of primary aggregates – crushed rock, sand and gravel – is extracted every year, and the Global Aggregates Information Network (GAIN), in its global outlook to 2030, estimates aggregates production will rise to 60 billion tonnes per year in the next decade to support urban population growth.3

CDE Global customer Velde Pukk AS plays a significant role in meeting the material demands in Stavanger, Norway, in the face of a construction boom. Utilising high quality recycled aggregates and its on-site concrete batching plant, Velde Pukk AS supplies the construction industry with a CE-certified concrete from 100 per cent recycled sand and aggregates. Likewise, AF Gruppen, also based in Norway, produces high quality washed sands and aggregates which also have Norwegian standard certification.

CERTIFIED

It would be wrong to assume recycled sand and aggregates are only selected as alternatives to finite virgin materials in low strength and low value applications. CDE Global’s process improves the quality of recycled sand and aggregates by removing foreign contaminants and classifying the output, resulting in higher spec recycled products that can be used to produce concrete products.

Like so many of CDE’s pioneering customers leading the charge for a circular economy, and which have demonstrated the potential in CD&E waste by achieving certification for their recycled products, we need to recognise the strength and integrity of using recycled materials.

The recycling sector will continue to grow and it will gradually become more competitive. CDE is working with its customers to stay ahead of that curve and to adopt efficient, sustainable technologies that are future-ready. Those customers who have integrated wet processing technology into their plant are reaping the commercial advantages of superior end products and are facilitating sustainable construction by recovering high quality recycled materials.

REFERENCES & FURTHER READING

1  UK statistics on waste, March 2020 Update. gov.uk/government/statistics/uk-waste-data

2  United Nations Department of Economic and Social Affairs (UNDESA). youtube.com/watch?v=XN92srq5jwg 

3  (2019) UN Environment Programme. Sand and Sustainability: Finding new solutions for environmental governance of global sand resources. wedocs.unep.org/handle/20.500.11822/28163

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