Future of Roads | Adding tyres to asphalt to bring down carbon emissions

Product certification following a successful motorway trial confirms the status of Tarmac’s waste tyre rubber modified thin surfacing as a pointer to the future of asphalt layers.

Ultipave R thin surfacing from Tarmac is based on a warm mix stone mastic asphalt to which has been added rubber milled from waste vehicle tyres.

The environment friendly system was trialled on the M1 before receiving BBA Hapas Clause 942 approval in June. Hapas certification scheme was established by Highways England, the County Surveyors Society and the British Board of Agrément (BBA) to facilitate testing methods for innovative products and systems for use in the highways sector.

The BBA Hapas approval of Ultipave R is good news on two fronts.

"There are distinct low grade polymer benefits in using rubber"

Firstly, Tarmac claims the rubber of up to 750 waste tyres can be effectively bound into every kilometre of road surfaced with 40mm of Ultipave R, substantially reducing the carbon footprint of tyre carcass disposal.

The process could help Britain overcome the problem of getting rid of waste tyres that can no longer be sent to landfill or burned. The UK currently relies heavily on exportation – up to 120,000t of tyres each year – but is vulnerable to recipient countries changing waste import policies.

Kuwait, for example, is likely to be forced to review its policy. At the small town of Sulabiya a mountain of 7M tyres is clearly visible from space. Dumping here will one day have to cease.

The rubber modified thin surfacing was otherwise conventional warm mix stone mastic asphalt

Secondly, adding rubber to what is otherwise a conventional thin surfacing mixture conveys physical advantages that include flexibility and waterproofing.

“The benefits are not just environmental ones,” says Tarmac technical director Brian Kent. “There are distinct low grade polymer benefits in using rubber.”

Tarmac’s knowledge of rubber modification was boosted in 2015 when it was acquired by United States conglomerate CRH. CRH had been using rubber since 1990 and possessed considerable data, to which Tarmac gained access.

This data revealed that asphalt containing tyre rubber enjoyed enhanced flexibility plus a delay in the formation of cracks – which, logically, pointed to delays in the formation of potholes.

Air voids

“And that’s not all,” says Kent Typically, SMAs have air voids of 4% to 6% by volume after compaction.

“We know particles of rubber can enter these voids, and thereby assist in reducing water penetration to lower levels that can lead to longer term underground movement,” he says.

His enthusiasm for rubber modified asphalt is shared in principle by Highways England head of lean and continuous improvement Martin Bolt.

“More flexibility and fewer defects to put right means less maintenance working, less road closures and fewer operatives at risk repairing roads. We can enhance safety while providing a better service to our customers.”

Highways England support has been crucial to the development of Ultipave R. In particular it has funded a comparatively onerous trial on the M1. There was exceptional interest in this trial because the asphalt used had a much higher proportion of rubber than normally employed.

“We’ve produced asphalt for some years now with a rubber content of 0.67%. On the M1 we upped the content