The many guises of asphalt - a versatile surface
If we spent more time noticing the ground at our feet, we would see that the black, bituminous substance known as asphalt is often there. Eoin Redahan found out some of the challenges and surface solutions at The Many and Varied Uses of Asphalt event in London, UK.
Asphalt was used to impound water in Africa 3,000 years ago. David Wilson of Walo UK Ltd in Stafford and his colleagues are also using asphalt to contain water, using a dense asphaltic concrete lining system for reservoirs, landfill cells and dam faces.
The lining system must fulfil several criteria to be successful. It must be watertight enough to withstand the required hydraulic pressure, free from toxic compounds if used in reservoirs and it has to perform well on slopes. To ensure all of the above, the Walo lining system contains a stabilising/drainage layer, an asphaltic binder layer, a dense asphaltic concrete layer and a mastic seal coat. The materials used include aggregates, sands, filler and bitumen.
Wilson noted that each engineering site has its own unique materials design. Thirty-four tests are conducted to get the material fit for purpose, including an analysis of particle size distribution and material density, and the water absorption of coarse and fine aggregates.
Pavements in dockland areas are subjected to heavyloading vehicles. Surfaces need to be strong and durable to perform in this aggressive environment for long periods of time. Dr Shaun Hillier of Aggregate Industries in Surrey, UK, explained, ‘The MAFI (low-loading vehicle) catches the pavement, water seeps in and exacerbates the problem. Eventually, concrete starts to crack. It’s completely shot. The capping material is gone.’
His company was tasked with creating a flexible pavement at Southampton port. They used a pavement system with 300mm gravel road base, an 80mm asphalt binder course and a 60mm stone mastic asphalt surface course to complete the job. According to Hillier, the pavement has high rut resistance, compactability and is unaffected by cold temperatures.
Perhaps one of the most recognisable applications for asphalt is on sports pitches. All-weather pitches come in various forms, including artificial grass and clay, but as Tim Freeman of Trevor May Contractors Ltd in Kent, UK, noted, all artificial pitches require a stable form of base construction. He added that porous macadam with a stone base is particularly suitable for the UK’s climate and materials availability.
However, as other delegates noted during the event, it is not simply a case of laying the same formula every time. If a highly porous structure is required, then Freeman and his colleagues will lay a pitch by hand. If durability is more important, then the surface is machine-laid. Other factors such as energy restitution, ball bounce, force reduction, surface pace, topography, dimensions, slip resistance and traction all need to be considered before the pitch is laid.
Porous asphalts are being used in the development of permeable pavements in the draining surface layer. Phillip Tomlinson of Permavoid Ltd, in Warrington, UK, spoke about the Sustainable Urban Drainage System (SUDS), which incorporates a polymer-modified bitumen with an open-graded aggregate structure to provide a strong, durable material.
Some delegates were sceptical about the performance of permeable pavements, but Tomlinson felt that a suitable pavement design could improve drainage and help offset flood risk.
The SUDS uses polymer-modified bitumen in the mix instead of standard porous mixtures. Tomlinson explained, ‘Liquid polymer additives make the binder more viscous, allowing it to adhere to the aggregate particles more effectively so that the pores remain open – ensuring that the asphalt remains porous. It allows the binder to adhere in a thicker, more stable film so it is more resistant to oxidation and less likely to become brittle.
‘The hard surface environment doesn’t have to be part of the problem,’ he added. ‘It can be part of the solution.’