Lower temperature asphalts benefit the environment and can lead to rapid construction techniques as proven on a new Scottish bridge project. NCE reports.
Lower temperature asphalts are being promoted for the energy and emission savings possible in their production, and for having smaller overall carbon footprints than conventional asphalt.
But such asphalts can claim one other substantial virtue: as they are laid cooler and thereby harden faster, they can withstand loading more quickly. This means a swifter turnaround during the laying process itself and allows roads to be reopened more quickly.
The need to speed up the laying process is why lower temperature asphalt was chosen for Inchbonny bridge in the Scottish Borders. Using the material there allowed surfacing to be done to very tight deadline.
Inchbonny bridge carries the A68 Edinburgh to Newcastle road over Jed Water. The existing single span reinforced concrete arch replaced the original hump-backed crossing in 1926 and has recently been showing its age. Earlier this year it was decided that the bridge would be repaired and refurbished, including a new concrete saddle with asphalt surfacing.
“The amount of material needed and total thickness of asphalt believed to be required at the start of works was substantial and we had comparatively little time in which to lay it.”
Tony Pringle, SBC
“It was the depth of asphalt − the amount of material needed to cap the new saddle and achieve final deck level − that caused some head scratching,” says SBC Contracts’ specialist services manager Tony Pringle. “The thickness of asphalt required at the start of works was substantial and we had comparatively little time in which to lay it.”
The tight asphalt programme was due to the limited possessions allowed by road operator Bear Scotland, which acts on behalf of Transport Scotland.
Main contractor Freyssinet Makers was permitted to close Inchbonny bridge to traffic over three weekends only: one of them for erecting a temporar crossing, one for shifting the temporary structure during the works, and one for removing it and then carrying out the surfacing.
“We had a Saturday to lay the base course and the following Sunday for the other two courses,” Pringle says. The base course was planned to be made up of three layers; an initial one of between 40mm and 100mm; a second of 100mm; and a third of 100mm.
The time taken by a layer of conventional asphalt at these kind of thicknesses to cool sufficiently for a subsequent layer to be added meant that creating the bridge’s base course could not be carried out in a single working day.
Running a shift during the Saturday night was an option but against this was cost allied to the fact that no local quarries would be open.
“I knew about a lower temperature asphalt developed by Cemex that had been used on a time-critical motorway job, so telephoned Derek Simm, its technical manager in Scotland.”
The challenge with temperature-reduced asphalts is to retain workability of the mix, in particular its ability to be compacted; and he emphasises the importance of the bitumen binder being used. Cemex’s lower temperature asphalt contains Nypave PX50, a binder from bitumen specialist Nynas.
“PX50 is specially designed to enhanced performance where temperature reduction and early opening to traffic are required,” says Nynas’ asphalt engineering support manager Jukka Laitinen.
The day after the base course went in, a 60mm thick binder course of 20mm heavy-duty material was laid, followed by a 40mm surfacing of hot rolled asphalt. Soon after, white lining was applied and Inchbonny bridge fully reopened to traffic.
Client Transport Scotland
Operator BEAR Scotland
Main contractor Freyssinet Makers
Surfacing contractor SBC Contracts
Asphalt supplier Cemex
Contract period November 2009-2010