Measuring up the options for sustainability

Climate change and sustainability have risen to the top of the agenda for professionals responsible for building and maintaining transport infrastructure; and as Colas UK senior manager Carl Fergusson writes in the latest issue of EU Infrastructure magazine, highway and airfields engineers need to know how to meet emerging sustainability policies. Solutions are available though; and if full analysis of the options is carried out for individual infrastructure projects, the most ‘sustainable’ solution can be found.

There are many different pavement construction and maintenance techniques on offer; and no single solution is the best one every time. Engineers are right to evaluate every project in turn and as a paving contractor and materials specialist, Colas has responded – historically by developing techniques to meet the many demands and now with a means for evaluating the sustainability of different methods.

“Evaluation of sustainability has added more parameters and a further level of complexity in deciding on the most appropriate solution for any given project, because, if it is to be considered adequately, all three ‘pillars’ of sustainability – social, environmental and economic impacts – should be taken into account,” Fergusson says.

“There are many factors within each of the pillars, but generally, social sustainability means impact on public quality of life, such as safety and level of noise, dust and disruption. Environmental considerations are the ‘green’ implications, including air pollution, emissions, energy use and ecological effects of development, whereas economic impacts equate overall to costs.”

Colas has developed an Environmental Calculator – innovative software for evaluating the green credentials of different road maintenance methods (and their materials). It is the product of Colas Group’s Campus for Science & Techniques at Magny-Les-Hameux in France. Project details are fed into the computer including the binder to be used, aggregates, the manufacturing process, transport involved and laying process; to obtain cumulative totals for energy consumed and Green House Gasses (GHG) emitted.

Energy consumption is measured as mega joules per metre squared (MJ/m2); GHG emission in kilograms per metre squared (kg/m2) of CO2 equivalent (CDE). Where different surfacing options are to be compared, details for each of the options are entered and comparative figures for energy and CDE obtained.

For instance, the Environmental Calculator has been used to compare Colas’ Retread and Repave in-situ road and footpath recycling techniques with conventional methods that first plane out then replace a road surface with new hot mix material containing virgin aggregate. For comparison, Repave, which is suitable for heavily trafficked roads and runways, involves heating and reworking the existing surface then levelling, compacting and overlaying with a new thin surface course. Similarly, Retread equipment simply pulverises the top layer of the existing pavement, before mixing in new bitumen emulsion and reprofiling and compacting the rejuvenated material. Finally, this is overlaid with a new surface dressing, microasphalt or hotmix asphalt to seal against water, provide the necessary surface friction characteristics and extend the pavement’s life.

Environmental Calculator results from a Repave project in Blackburn in Lancashire have shown that Repave – overlaid with a 25mm layer of Colas’ Colrug thin surfacing – consumed 76.3MJ/sqm of energy and emitted 5.24kg/sqm of GHG. In comparison, a conventional replacement with 40mm of SMA (Stone Mastic Asphalt) would have produced 85.8MJ/m2 of energy and 5.87kg/sqm of GHG – an additional 11% in both energy and emissions.

“Comparison between conventional methods and Retread resurfacing has shown even greater, 40% reduction in energy and GHG emission,” says Fergusson; “confirming that the most sustainable techniques in terms of short term environmental impact are those that recycle pavement in situ with cold mix processes involving very little transportation of materials.”

Consideration of energy use and emissions is not enough on its own however. Colas engineers are looking at social and economic sustainability as well. Retread and Repave are comparing favourably again, because they reduce transport of materials – and therefore noise, dust and disruption – and the time and cost involved in the maintenance of pavements. In Yorkshire, from use of Retread, Barnsley Metropolitan Borough Council has reported a 25% reduction in costs alone; and a full evaluation and value engineering exercise showed Repave to be the most sustainable solution overall for resurfacing the main runway at Duxford airfield in Cambridgeshire (see box).

Highway and airfield authorities can now access all the information they need to make fully informed choices. Their engineers need greater guidance from government on which parameters and pillars of sustainability to prioritise – if they are not to consider them equally – writes Fergusson in EU Infrastructure magazine. But with the Environmental Calculator and consideration of social and economic benefits, there are the means to measure which is the most sustainable method or material.

“The Calculator will not necessarily reveal a more environmentally friendly option as the authority may already have the best solution. Or it could be that the most sustainable solution is that which is most durable in the long term; more expensive initially perhaps, but most economic in terms of whole life cost,” Fergusson says.

“This is often the case in the airfields sector where interventions are most economically and environmentally sustainable when they are least frequent and cause the minimum of disruption to airport stakeholders. We have introduced to the UK airfields market materials developed to the French BBA (Beton Bitumineux Aeronautique) standard for this purpose and attracted a great deal of interest. BBA materials provide substantial advantages, in terms of reducing project durations and volumes of material required; and have been proven to have greater whole life cost benefits compared to the more conventional Marshall Asphalt alternative. Airport authorities need to maximise availability of their runways and are looking for sustainability benefits in the process,” Fergusson adds.

Highway and airfield authorities are now beginning to fully evaluate the sustainability of their pavement maintenance operations. Colas has risen to this challenge, measuring and developing its techniques and processes further. The company also has more options to offer, such as Vegecol, an alternative to bitumen binders (see box) manufactured from renewable agricultural plant based products. Vegecol could mean a massive reduction in energy consumption and emissions; and a decoupling of infrastructure projects from rising oil prices and diminishing crude stocks. The importance of environmental policy is growing and with it the need to fully evaluate the sustainability of the different paving options.

Duxford
Full evaluation of the options available for resurfacing the runway at Duxford airfield in Cambridgeshire showed recycling was the best method because it satisfied the required social, environmental and, most importantly, the economic parameters. The airfield operator had a finite budget available and so keeping the project programme and costs to a minimum was the highest priority; but in specifying the Repave method for recycling and resurfacing Duxford’s airfield, Colas also provided a solution high in overall sustainability.

“There was a limited budget available, but we got the project back within that limit by looking at all the options for meeting time, cost and sustainability parameters,” says Colas project manager Richard Hannah. “Repave was the perfect solution because it reduced the amount of raw materials needed and cut the number of deliveries to site by 55%, which in turn reduced emissions. The client was very keen to see its runway resurfaced in a more sustainable manner and we have since been able to verify the reductions in energy and emissions using the Colas Environmental Calculator,” says Hannah. “Additionaly, the reduction in import of new materials produced a commensurate saving on time and cost.”

Britain’s Imperial War Museum has one of its locations at Duxford and operates historic flights and airshows at the airfield during the summer months. Programming the resurfacing work prior to its July start date revealed the whole project would be “tight” in terms of available surfacing time available between air shows, but, says Hannah: “The Repave in-situ recycling process made the planning a lot easier by reducing the time and material needed by half.”

Repave recycling equipment moves continuously over an existing asphalt surface, heating it to 130oC using 15 rows of burners. Spring loaded steel tines then scarify and rework the plasticised asphalt, after which an hydraulic screed levels the material. At Duxford, a new 20mm thick, porous, high friction wearing course was then laid immediately over the top to create an ‘asphaltic weld’ and an homogenous pavement, without any need for a tack coat – and leaving no chance of separation of the two layers.

“The process is really quite straightforward and provides significant whole life cost benefit due to the hot weld resulting in a monolithic asphalt layer,” says Colas’ senior manager and business manager for airfields, Carl Fergusson. “Repave also improves the longitudinal profile of the surface because it smoothes longitudinal irregularities; and it saves on time and cost by eliminating the need to pre patch cracked areas, which would usually mean more work and further joints in the pavement. There are clearly whole life cost benefits from Repave and the Duxford project has provided a further good opportunity to evaluate them over the long term in addition to other airfield projects where the process has previously been carried out.”

Vegecol
Colas’ latest innovation, Vegecol, is an alternative to bitumen and petroleum resin binders and so carries massive potential in terms of sustainability for the highways and airfields paving sectors. The company is guarding the secrets of manufacture behind Vegecol, but the French patent gives the material’s constituents as ‘purely natural or modified natural substances of agricultural origin’.

According to Colas, anywhere bitumen is usually used the potential exists to use, Vegecol instead. This means, potentially, a massive reduction in energy consumption and ‘carbon footprints’ and a decoupling of projects from rising crude oil prices. Use of the new material also allows an asphalt mix temperature as much as 40oC lower than the norm, meaning lower fuel consumption and fume emissions at the plant; and lacking hydrocarbons, asphalt made with Vegecol is less susceptible to damage by fuel spillage.

“Colas has already carried out surface dressing in Germany with Vegecol as the binder; and surface course asphalts have been developed and laid with Vegecol in Belgium and Hungary,” says Colas senior manager Carl Fergusson. “It has also been used as the binder in an in situ asphalt recycling process in Martinique, which could be described as the ultimate in environmentally friendly resurfacing.”

Vegecol is a ‘transparent’ binder and so produces naturally aesthetic or coloured asphalts with addition of decorative aggregates or pigment. In the UK, Colas has introduced Vegecol through the company’s road maintenance PFI (Private Finance Initiative) contract with Portsmouth City Council and has received widespread interest from local authorities and architects alike.

Colas is investing in capital improvement of Portsmouth’s roads and then maintaining them for the next 25 years, which is a contractual arrangement ideal for piloting the company’s innovative products. For other UK public sector roads, approval of departure from standard may be needed because current asphalt standards refer to bitumen binders, which Vegecol is not.

“We can back up Vegecol with test data and a long list of examples of its use,” Fergusson says. “Vegecol is a product of our parent company in France. We believe it is a tremendous innovation, which has been formally recognized by several international awards for research and development. There are huge long term opportunities for its use, given the finite nature and increasing prices of oil and bitumen. If we want a sustainable business, we have got to be investing in research and developing viable alternatives to oil based binders in the transport infrastructure sector.”