Cold mix asphalt with recycled planings is helping ensure a new Scottish car park ticks all the right sustainability boxes.
Park and ride schemes are generally deemed to be environmentally friendly through their beneficial effect on town centre congestion and in reducing vehicle emissions. Stirling Council in Scotland however, wanted more green benefits from its new Western Park & Ride car park.
This has been turned into a low-energy test bed for lighting, passenger waiting facilities and some of the asphalt that is going into access roads and hard standing. A newly developed cold mix containing recycled material has such impeccable green credentials that it is causing a great deal of interest, not least in its performance under really hard use.
"It is greener, cleaner and makes less impact in terms of energy consumption and emissions than conventional hot mix asphalt," says Tayside Contracts quarry manager Grant Milne. Tayside Contracts played a large part in developing the mix (see box below) and also has the contract for supplying materials to the Stirling car park. "For a local authority, our cold mix, called Tayset, really does tick all the right sustainability boxes."
This message got through to Stirling Council. "What may be described as 'green considerations' are an increasing part of the equation for us, in the way we work," says Stirling's roads divisional (urban) engineer Kenny Snedden. "Tayset's green credentials are claimed to be good and we were keen to trial it in a particularly tough application to see how it would perform over time."
Last year, the council was planning to expand the city's existing park and ride scheme. The car park for this, which is on a narrow green field site to the west of Stirling, seemed a natural place to put the new cold mix to the test alongside hot mix material.
"We wanted the car park to be as green and of as little environmental impact as possible. The first phase involves alterations to an existing roundabout to provide access from the A84. It also includes; an access road across site, a turning circle for buses and parking for 200 cars.
We decided to specify Tayset in part of the access road and the turning circle. This is where the road will be heavily trafficked and the turning circle is particularly tight," says Snedden. The unspoken implication is that if Tayset withstands the weight and lateral thrust imposed by the park and ride buses, it will survive anywhere.
Stirling Council's own works organisation began the £1.1M car park towards the end of 2007, with asphalting taking place in March and April of this year. The Tayset was some of the first material to be laid after being brought in by lorries from Collace quarry in Perthshire, Sector Scheme 14, where it had been produced under tight quality control. Processed recycled asphalt planings (RAP), which has been screened and crushed to the right grading, was then carefully mixed with a bitumen emulsion specially blended for Tayside by binder specialist Nynas Bitumen.
"Tayset includes a variant of our Nyset product, a bespoke penetration grade bitumen emulsion specifically designed for cold mix applications. It is a binder with a good proven history," says Nynas' cold paving technology product support manager Dennis Day.
He also makes the point that although Tayset is new, and that cold mix asphalts for primary applications are beginning to be considered in Britain as a sustainable way forward, the bitumen technology has existed here since the early 1990s. "We have confidence in our emulsion and in the way Tayside is using it in its cold mix."
The mixing of RAP and binder takes place at ambient temperature, says Milne. "The only heating required for the process is keeping the emulsion warm while it is stored. We have calculated that producing 1t of Tayset emits 10.4kg of CO2 as opposed to the 34.4kg of CO2 emitted while producing 1t of hot mix asphalt. That is a healthy reduction." Low energy consumption and consequent lower levels of emissions were perhaps the prime factors taken into account by Stirling Council. Cold mix's other advantages are, using RAP in place of virgin material and also using less bitumen.There are also health and safety benefits in handling cold material.
It was during laying and rolling that other advantages became clear. The Tayset base course required no time to cool after rolling before being topped by its Tayset binder course. This in turn, could receive its hot mix surface course quicker than is normal. The whole surfacing process was speeded up. Delivery of the material to laying machine was also quick and clean. Tayset cold mix asphalt is relatively dry and unsticky making it easier to off load from trucks.
The car park's ground is low lying and wet, according to Stirling Council's site agent Iain Baird. "We have created an attenuation pond near the entrance to the site and the car park in general has a capping of burnt colliery shale topped by a heavy geotextile, with Type 1 above that."
Then comes a combined thickness of 160mm of base and binder courses for the access road and turning circle – 125mm combined in the hard standing, with a 45mm thick wearing course – 40mm thick for the hard standing. "The Tayset, which has gone into base and binder course, amounts to about 500t in all, about 10% of the total material laid, and was placed by our normal operatives using conventional laying and rolling plant."
Once placed, the Tayset was subjected to a regime of testing. Wayne Fortune, a civil engineer from the University of Dundee on secondment to Tayside Contracts, used a Prima 100. It is a device based on a falling weight deflectometer, to determine the surface dynamic modulus of the cold mix material. "I used it to get an indication of the compaction and strength of the Tayset courses," says Fortune. "And I got some pretty good results."
Stirling's Western Park & Ride scheme should open in August this year.
The car park ground works and paving will be finished in April. The lighting, which can be dimmed when not needed, and passenger waiting and lavatory facilities, which are ecologically friendly of constructed timber with ground source heat pumps/photo-voltaic cells and sun pipes, will following on.
Tayset has been designed for the Scottish climate but is appropriate for use elsewhere. Advantages include:
Greatly reduced energy consumption and CO2 emissions
Conserves quarried materials and landfill space
Can be recycled
Extended storage life, no waste generated
A recycled asphalt that performs as well as traditional hot mix
Tayside Contracts says that it strives to promote an environmentally aware approach providing its services. It has recycled construction waste for more than a decade, initially producing low-value fill. Then in 2005, funded by the Department of Trade & Industry, the company entered into a Knowledge Transfer Partnership with the University of Dundee and Nynas Bitumen. The intention was to develop a cold recycled material that could replace traditional hot mix base and binder materials in an environmentally friendly manner. Tayset is the result.