Using recycled aggregate for below ground stormwater attenuation systems can lead to environmental, planning and cost benefits. Stuart Crisp, UK Manager, Advanced Drainage Systems, talks us through the points to consider.
It is always worth considering whether recycled aggregate could be used in place of virgin aggregate for the construction of a new asset. As well as conserving natural resources for future generations, it can lead to cost savings and help reduce waste produced by construction and demolition projects.
Another reason to consider using recycled aggregate is a move towards circularity principles among certain clients and planning authorities. As well as finding ways to design out waste and extend the life of built assets, circularity considers how elements of a building or infrastructure can be reused, the aim being to redeploy materials for the highest value purpose possible.
Recycled aggregate can be used with below ground stormwater attenuation systems, which can be considered a higher value use than simply deploying them for general backfill. However, when using such aggregate, there are some additional considerations for the designer, specifier or installer, which this article will address. Specifiers need to be aware of the relevant standards, together with any technical
guidance from the manufacture of the attenuation product.
Reuse and recycling of materials has come into sharper focus recently with the introduction of new planning policies and corporate strategies. The Greater London Authority has taken a lead on this: from March 2022, its planning process has required developments over a certain size to submit Circularity Economy Statements.
A Circular Economy Statement should demonstrate how materials arising from demolition will be reused or recycled, consider resource efficiency in an asset’s design and construction and look at what will happen to the asset at end of life. Some developers are also homing in on circularity. Warehouse developer Prologis is showcasing circularity principles with projects such as the Prologis Park Waalwijk DC3 in Tarkett in the Netherlands. Closer to home, commercial property developer GPE announced in November last year that it was introducing a Circularity Score, aimed to reduce the quantities of virgin materials it uses in its development, with targets that will ramp up from 40% from April 2025 to over 50% by 2040.
Infrastructure owners are also working to include circularity principles in their construction and maintenance projects. In October 2024, the UK Water Partnership published a white paper on circularity in the water sector which references the need to reduce the use of virgin materials in construction.
This year National Highways will be developing performance and baselines for circularity. And by 2030, it will have integrated circularity assessments into its design and maintenance standards to look at the reduction of virgin materials, waste management and material flows.
Compared to many of its European counterparts, the UK has a good track record on using recycled aggregate in construction. In its 2024 report, Construction Aggregates Supply in Great Britain, the Mineral Product Association (MPA) estimates that 66.1 million tonnes of construction, demolition and excavation waste (CDEW) was recycled in 2022, or 27% of the total aggregates supply.
However, the MPA does point out that more robust means of collecting data on CDEW is needed. Digital waste tracking, which is proposed to become mandatory from April 2025, would give a more accurate picture and help encourage more circularity, says the MPA.
Although recycled aggregate can be used for drainage, including for below-ground attenuation assets, they could be deployed far more often. Since sourcing the right type of recycled aggregate could require additional time and resource, it may not make sense for contractors to pursue this route.
Where recycled aggregate is to be used for below-ground drainage, standards and guidance is available. BS EN 13242 (+A1:2013) sets out the properties required for aggregates produced from natural, manufactured or recycled materials for hydraulically bound and unbound materials for civil engineering works. Recycled aggregate from a reputable supplier will be CE marked to demonstrate that it complies with the standard.
Additional guidance on the use of BS EN 13242 is contained in document PD 6682-6:2009+A1:2013, available from the British Standards Institution. This gives information on how to apply the standard in the UK.
The Quality Protocol ‘Aggregates from inert waste, ’published by the Waste Resource and Action Programme (WRAP) and the Environment Agency, provides information on demonstrating compliance of materials, together with good practice on storage, transportation and handling of recycled products. Note that this is currently under review, with a revised document being drafted.
Manufacturers of underground stormwater attenuation devices should provide technical guidance on specifying recycled aggregate for their systems. For instance, Advanced Drainage System’s StormTech arch-shaped attenuation system has a technical note on the subject which sets out specifications for gradation, angular or subangular classification, deleterious materials and freeze-thaw resistance.
A general specification for a StormTech installation with recycled aggregate would call for a 20/40 grade which is clean, crushed, angular, with less than 5% fines. The fines content is important because the aggregate around StormTech arches is performing a water storage function as well as a structural one.
If recycled concrete is to be used, tufa precipitate from unhydrated cement may be present. This could lead to the occlusion of separation fabrics or could block infiltration or exfiltration surfaces.
When specifying recycled aggregate, it may be necessary to ask for hardness and durability testing to ensure that it meets structural requirements. Again, a reputable supplier will provide certification and assurance that the aggregate meets the specification.
A further consideration is the chemical content, alkalinity and potential toxicity of the recycled aggregate, together with the composition of the ground into which it will go. For instance, if there are sulphites in the ground, they could attack recycled concrete, impacting on its performance over time.
For below-ground stormwater attenuation systems, the amount of aggregate, whether recycled or virgin, varies considerably depending on the type of system. Modular boxes, often referred to as crates, use relatively little aggregate, installed around the outer perimeter of the tank. Systems that deploy large-diameter pipes require a greater proportion of aggregate surrounding the entire external surface of the pipe. The total amount and type of aggregate required may vary between rigid pipes, such as concrete and flexible pipes, such as plastic.
With arch-shaped attenuation chambers, such as StormTech, considerably more aggregate in relation to the product is used. This is because the aggregate has a dual function: to provide additional storage volume which contributes to the efficiency of the system with respect to its water storage capacity; and to provide structural support.
The elliptical shape of the chambers forms the aggregate around it into arches and columns, transferring loads away from the chambers so that they can be installed at shallow and greater depths.
Manufacturers may provide design aids, such as StormTech’s Site ASSIST app which provides information on embedment material and construction methodology as well as detailed installation details in the form of drawings and animated videos. Apps such as this can also help ensure that the contractor installs the underground attenuation assets as intended.
Recycled aggregate can cost less than virgin aggregate, although this is not always the case. Cost tends be location dependent.
The price of virgin stone varies between quarries and will also be affected by the distance over which it must be transported. There are more quarries than aggregate recycling facilities which may mean that, if a construction site is close to a quarry, virgin aggregate is likely to be a more cost-effective option. However, for sites that are within a reasonable distance of a recycled aggregate supplier, there could be economica dvantages of swapping virgin for recycled aggregate.
When it comes to associated carbon emissions, recycled aggregate will not always come in with a smaller carbon footprint than virgin aggregate. Recycled aggregate requires more processing, including demolition, grading and transportation to a recycling facility. If that facility is further away than a local quarry, there will be more carbon emissions associated with transport too.
One factor that may offset the transport and processing carbon emissions in favour of recycled aggregates is the use of recycled concrete that has been stockpiled to enable the material to carbonate.
Simply put, this is a process that takes carbon dioxide from the atmosphere into the surface of the concrete where it reacts with the material to create calcium carbonate.
From a carbon perspective, it is worth considering that systems with a higher aggregate-to-product ratio tend to have a lower overall carbon footprint for the same storage volume than those with lower ones. This would include systems such as arch-shaped chambers, where the aggregate is used both structurally and to store water.
Clearly, there can be tensions between reducing embodied carbon emissions associated with the construction of new assets and the use of recycled products in order to conserve natural resources and reduce waste. These issues will be addressed as policies and sustainability strategies become more nuanced to include circularity considerations.
