A better runway performance

“Asphalt mixes for airports in Australia have, for a very long time, been designed using a methodology that was introduced some time early in the second half of the 20th century and has changed little since,” according to Australian Asphalt Pavement Association (AAPA) Director – Technology and Leadership Erik Denneman.

“The traditional approach uses a recipe-based asphalt. If we put asphalt pavement technology down on airports today, it’s the same design that was put down decades ago. Obviously, the types of aircraft using the runways have changed, and the loading impacts on the surface are different too.”

Dr. Denneman says that it is becoming increasingly challenging to confidently design durable surfacing mixes using this prescribed recipe-based approach, while managing the risk of issues, such as cracking and deformation failure. “We are starting to see some early failures – after 50 years of using the same mix design, something had to be done to improve the technology.”

With a clear need for change identified, AAPA hosted airport companies, consultants and industry to come together to develop a solution.

This February, AAPA released a national performance based model specification for asphalt surfacings on airports. The document is intended to be a standalone specification, used by reference, for asset owners in the creation of technical specification for the supply of 14-millimetre maximum sized mixes, based on performance properties.

“The intent of the specification is to allow for propriety binders. It presents the opportunity to grow beyond the currently used standard types of bitumen and PMB and give asphalt producers the chance to use propriety binders and products,” Dr. Denneman says.

It represents a step towards truly performance-based design as the specification aims to give asphalt suppliers more flexibility to innovate and test and achieve performance guarantees during the mix design process. Because the specification allows propriety binders and provides some additional freedom in other aspects of design than conventional specifications used by consultants, it allows asphalt producers more flexibility in optimising their mix for performance indicators, such as workability, durability (including water resistance), rut resistance, stiffness and fatigue.

“It will allow the contractor to better manage risk for the benefit of all. Obviously if an asphalt surfacing fails, it’s an issue for the producers and the consultants, and the main cost is to the airport companies,” Dr. Denneman says. “So, I think the main benefit is breaking away from a stalemate between producers and consultants.”

Because the specification was prepared collaboratively between contractors, design consultants and airport company representatives, Dr. Denneman said it also opened up the conversation around progressing best practice for airport pavements.

“Having all of these stakeholders in one room and taking the opportunity to discuss the challenges and come to a decision that’s acceptable for everybody involved is a major achievement,” he says.

Dr. Greg White from the University of the Sunshine Coast says the current issues in airport pavement design means the performance-based specification is a welcome one.

“The problem is that traditionally, the responsibility, quality assurance and performance of the pavements comes down to the designer. But, the contractor had to use to use the prescripted traditional mixes without the ability to innovate,” he says.

Having worked in the asphalt runway sector for over 20 years, as well as completed a PhD on aircraft-induced shear stresses and their impact on asphalt runway surfaces, Dr. White provided key insight into the development of the specification model.

“To have a specification focus on performance-based testing methodologies, it means the construction outcomes will be better – it’s a big update on what we are already doing. It gives us the opportunity to renew and update all of those technical aspects, as well as flip around the liability and responsibility for the performance of the asphalt surface from the pavement designers to the contractors,” he says.

He explains that by giving contractors more control and the ability to innovate, but also the liability for the surface performance, contractors can submit different construction proposals with alternative binders that may result in better whole of life and construction outcomes.

“Without the collaboration from the majority of stakeholders on this, we could not achieve what we achieved,” he adds.

Despite the progress made, Dr. White says there is still a way to go in reaching best practice across the Australian airport asphalt pavement sector.

“There are a few gaps in our construction surface testing methodologies, which does mean we need to take a prescriptive approach. The main one is reducing ravelling risk so stone loss from the surface can be controlled, which can be a hazard to aircraft.”

These gaps, however, are being explored. Dr. White is leading a new collaborative $2.85 million, five-year research program that aims to advance airport pavement technology and practices across the country. The program is a partnership between the University of the Sunshine Coast, Australian Airports Association, Department of Defence, Perth Airport and Sunshine Coast Council. Dr. White says it is also the first nationally funded and coordinated research program into airport pavements since Australia’s major airports were privatised in the 1990s.

Reducing ravelling on airport surfaces, for instance, is an area a PhD engineer candidate is exploring under the research. “One project, at the moment, is development of a specification – even a sister specification to the recently published AAPA document – for stone mastic asphalt (SMA). This will provide an alternative runway surface without the need to grooving, but with adequate skid resistance and durability.” The USC research program has seconded a RAAF field engineering officer to address this idea.

Dr. Denneman says the national specification – and the collaboration that led to its development – has opened up a number of opportunities in the Australian airport pavement sector. In addition to the work being explored by the industry and the research program under Dr. White, Dr. Denneman says looking at international practice in the sector has much to offer.

“Because of the shift towards performance-based specification, it’s a step up and gives us opportunity to begin looking at implementing international technologies,” he says. “Grooving, for instance, can be an issue for asphalt pavements because it can defect easily. Now, we can access some of the expertise from Europe and other places where they use non-grooved runway surfacing, as there’s a clear need to explore these.”

Dr. Denneman says China especially is making advances in non-grooved airport asphalt pavements, as well as SMA surfaces, which will be a key focus for the 2018 AAPA International Knowledge Transfer to Asia.

“Hopefully the national specification will soon be used across the board. The AAPA specification has been introduced to reflect best practice and having everyone involved from both sides of the table has helped achieve this. We’re continuing to work close with stakeholders for real change – collaboration is key.”