Understanding the ins and outs of innovative materials is increasingly vital if savings and efficiencies are to be made in road works.
The widespread use of stone mastic asphalt and thin surfacing on UK motorways has driven a significant increase in demand for high polished stone value aggregates, with a particular emphasis on 14 mm and 10 mm sizes.
As a result, the highways industry is frequently faced with availability issues, resulting in increased operational, logistical and, in turn, product costs. This approach has also introduced a major challenge with regard to future sustainability of roads and delivering resource efficiency.
SMA, by design, contains around 70 to 75 per cent of a single size aggregate, whereas its predecessor hot rolled asphalt used a 30 or 35 per cent single size.
The only high PSV requirement related to the pre-coated chippings embedded on the surface at around 15 kg per sq m.
“These surfacing materials provide an important opportunity to conserve and manage primary aggregates”
This high requirement for 14 mm and 10 mm high PSV aggregates has subsequently increased quarrying costs.
These costs are derived from the quarrying activities required to obtain the premium sizes, predominately from accelerated direct blasting and crushing procedures, but also by re-crushing larger, currently under-used sizes, including 20 mm aggregate.
It’s worth also noting that each crushing operation automatically results in the production of an under-used dust fraction. Therefore, as we increase production of the premium sizes, we also increase the volume of high-quality rock that simply becomes dust on a stockpile.
Opportunity for surface materials
In the context of greater financial, environmental and time pressures on highways projects, it is important that asphalt products not only help clients and contractors to lay materials quickly and cost-effectively, but also conserve premium primary resources and use them as effectively as possible.
Single-layer surfacing materials that include 20 mm aggregate in their composition can be used as a surface course or a dual-layer substitute to provide a solution to these challenges.
Critically, these surfacing materials provide an important opportunity to conserve and manage primary aggregates.
“There is a great socio-political need for the construction industry to do more to embed an ethos of resource efficiency”
A tonne of single-layer surfacing only contains around 20 per cent of 14 mm aggregate and virtually no 10 mm aggregate at all, therefore ensuring that for every tonne of the 20 mm product produced, approximately 0.5 tonnes of precious 14 mm and/or 10 mm aggregates are conserved for future use.
Importantly, the final application of these materials can be laid between 50 and 75 mm thick, enabling contractors to complete site operations faster.
This reduces public disruption and offers significant cost savings with regard to the associated reduction in plant, labour and traffic management costs.
Another advantage is that with only one layer to bond, the cost of bond coat operations is also reduced.
This single-layer surfacing material continues to deliver the superior rut resistance expected from the 14 and 10 mm products, with the additional benefits of providing enhanced structural contribution compared with regulating and thin surfacing materials, as well as lower in-situ voids due to better heat retention during the compaction process.
Early engagement with suppliers is vital
Local authorities and network operators are under continuous pressure to build new roads and maintain the existing road asset using methods that are efficient, cost-effective and sustainable.
In parallel, there is also a great socio-political need for the construction industry to do more to embed an ethos of resource efficiency across its operations.
Early engagement of materials suppliers, innovation and careful product selection is critical to meeting these goals and delivering effective, sustainable highways solutions.
Careful selection of single-layer surface asphalt materials, where appropriate, can support these ambitions, while also providing additional opportunities to reduce project timescales, minimise disruption to the public and deliver cost benefits to clients.
Understanding the nuances of innovative materials and engaging with the supply chain to get the right specification has never been so important.
Brian Kent is technical director at Lafarge Tarmac