Cracking the scourge of potholes

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By Rick Ashton, bitumen division market development manager at TotalEnergies Marketing UK

POTHOLES are an evocative subject. They are defined differently and assessed without a common standard. There are many surface defects that get categorised as potholes: delamination, joint ravelling, fretting and fatigue cracking failure modes all get rounded up into this catch all description.

However potholes are defined, there is an increasing imperative to delay the onset of, or seek to prevent altogether, these phenomena.

Firstly, it must be acknowledged that the pothole is a symptom of failure. This could be a failure of material to last its intended life cycle through poor installation or actual material design; a failure to invest in the maintenance of the asset such as blocked drainage infrastructure; or even a failure of integrity of the surface due to external factors such as mechanical damage.

Assessing these various failure modes it has become clear that thin surface course design, which was historically prevalent in UK, road design was not optimal.

A drive to mimic the positive surface textures achieved with traditional mixtures took the design of these surfaces far away from the continental mixtures they were based on. In particular, the selection of basic-grade bitumen alone in these mixtures on the grounds of economy was sub-optimum compared to the highly polymerised bitumens used in such mixtures in continental Europe.

This led to highly negatively textured ‘semi porous’ surfaces. These surfaces lacked resistance to oxidation, fatigue stress and the high-water pressures seen at the tyre/road interface. The Scottish climate has proven to be a particularly harsh environment for these, with roads not meeting the expected design brief.

This has been addressed by the Transport for Scotland Specification for Stone Mastic Asphalts – TS2010 – which focuses on durability, addressing the issues seen in previously used materials.

This specification removes surface texture as a compliance requirement relying more pragmatically on measured skid resistance.

This allows the material design to achieve a denser mixture with less air voids in-situ. Porosity is now not an issue reducing the effects of freeze-thaw cycles and high rainfall events. The binding agent used is now a premium, polymer-modified bitumen with carefully selected performance characteristics reflecting best practice first seen on evaluation assessments in Germany.

One such bitumen is TotalEnergiesUK Styrelf eXtreme 100. Developed for the TS2010 specification the product extends the engineering capabilities of the bitumen to resist extreme cold temperature cracking to below minus 20 degrees, while elevating the high temperature softening point to above 75 degrees – providing a broad performance envelope built to tolerate the Scottish climate.

Material design is part of the toolkit in pothole prevention, but the surrounding infrastructure plays a key role, too.

It is frequently observed that potholes form mainly in the left-hand wheel track – which is where the most vulnerable road users, cyclists, travel. The most likely cause for this is water retention which is deposited by the camber on the road.

The aim is to get water to the grids/gullies to be removed as quickly as possible from the carriageways. When these drainage features become blocked or overwhelmed by extreme climate events such as flooding, surface water builds in these areas.

Austerity and funding challenges for network operators have sometimes seen reductions in gully clearing frequencies. Standing water can then create high surface pressures as vehicles cross this which pressure washes the asphalt.

If standing water reaches the interlayer bond between the road surface layers, delamination will follow. Micro cracks when exposed to high pressure water then allow loosening of the aggregate/mortar matrix and start the avalanche of deterioration to pothole formation accelerated by freeze thaw cycles.

The adoption of the TS2010 specification and the utilisation of compliant bitumen products is proving a value tool in the prevention of surface defects.

The broader perspective, though, is that a long-term strategy is required to embed resilience in our road networks. This means funding for a multi-year horizon and selecting suitable long-life bitumen products that will ensure roads remain safe and usable, and minimise the costly, short term, patch and fix approach to road repairs which causes disruption to users and sustains sub-standard road surfaces.