Extra strength surfacing specified at a Cornish airfield could lead to less use of traditional Marshall Asphalt on military runways. NCE reports.
Taxiways at one of Britain’s busiest military airfields, Royal Naval Air Station (RNAS) Culdrose, have just been resurfaced using an enhanced asphalt specification. This will provide search and rescue helicopters and jet trainers with a tough, hard wearing surface and help to prolong the operation of the runway.
Use of stone mastic asphalt (SMA) is relatively new for this type of surfacing on airfields. It contains a special polymer modified bitumen designed to increase a surfacing’s strength and ability to resist deformation under heavy loads.
Successful application at RNAS Culdrose could, say pavement specialists and asphalt specifiers, lead to SMA replacing traditional Marshall Asphalt and becoming the standard for military runways.
Recent resurfacing activity formed phase three of a contract at the airfield, and was carried out by Ministry of Defence property and services provider the Defence Infrastructure Organisation’s (DIO) prime contractor Debut Services - a Lend Lease joint venture company - and supply chain partner Bardon Contracting. Phase three was awarded to Lend Lease following successful delivery of the previous phase of work.
Completion of this final phase means that the team has totally reconstructed or resurfaced all of the taxiways on the airfield.
“Marshall Asphalt is tried and tested… But I am confident that polymer modified binder could further extend the life of runways”
Clive Darby, Bardon Contracting
Bardon Contracting airfields manager Clive Darby, who is responsible for the current renewal of surfacings at Culdrose, says: “Marshall Asphalt is a tried and tested, high strength material, which is normally renewed around every 12 to 15 years. But I am confident that the use of a polymer modified binder could further extend the life of runways.”
Polymer modified bitumen for the taxiway improvements at Culdrose was supplied by bitumen manufacturer Nynas and incorporated into asphalt designed by Aggregate Industries.
Known as Nynas Endura Z2, the binder helps to enhance workability and increase both flexibility and resistance to loading and sheer forces.
It was chosen for this contract to ensure that the surfacing resists rutting that can be caused by tightly turning aircraft using taxiways at low speed. The binder was mixed with hard wearing Cornish dolerite to produce an SMA surface course, laid to a depth of 50mm over an SMA binder course material of the same depth.
“Polymer modified binder used within an SMA ensures a surfacing material has added cohesion and adhesion - and this is particularly important for airfields,” says Nynas asphalt engineering support manager Jukka Laitinen.
“We carried out a comprehensive set of specialist tests as per the contract specification, which have included low temperature indirect tensile strength tests after ageing at -18oC. Our modified binders have been used within Marshall Asphalt at civil airports, but never before on military runways. That could be the next step.”
Aggregate Industries regional technical manager Martyn Tatlow says: “We were asked to develop a modified SMA surface course material that is fuel resisting and with an improved shear and scuffing strength, to be laid over a standard SMA binder course. With a wide range of PMBs (polymer modified bitumens) available a rigid laboratory test regime was developed to simulate higher performance levels required that could only
be delivered by the use of certain PMBs.
“We have worked with Nynas many times in the past on product development, all of the site trials were a success and the material is performing well in situ.”
“Surfacings here were in a sorry state. The airfield opened in 1947 and has received numerous treatments since, including several applications of slurry seal surfacing that were beginning to delaminate and flake off. This presented a serious risk of foreign object damage (FOD), as it is known, causing problems for the safe operation of aircraft and helicopters.”
“Proactive and efficient project management of resources and project risk helped the team to overcome extreme weather conditions and other risks”
Dave Williams, DIO
Debut originally commissioned Bardon Contracting to resurface the 1,835m long main runway at Culdrose in Marshall Asphalt, which was completed in 2011. The same contracting team has just finished improving both the northern and southern taxiways using SMA and has renewed two secondary runways with Marshall Asphalt. Pavement works to the taxiways and secondary runways began in April 2012 and were completed in November.
Other civil engineering works on site included renewal of airfield ground lighting, refurbishment of 8,000m of French drains and cutting of grooves into the Marshall Asphalt to help displace surface water. A total of 275 concrete pits that house airfield communication equipment beside the secondary runways and taxiways have also been strengthened to ensure they can withstand aircraft loading.
For the most part Culdrose’s northern taxiway was simply resurfaced. But the southern taxiway required a comprehensive rebuild; featuring 200mm sub base, 180mm of drylean concrete and 120mm base beneath two 50mm layers of SMA.
Asphalt for the base, binder and surface courses of the taxiway were produced at Bardon Asphalt’s plant at Melbur, 50km away, and bitumen was delivered by Nynas. A bond coat was sprayed between each of the asphalt layers to ensure a homogeneous layer construction to guard against delamination failures.
Torrential downpours are not uncommon at Culdrose because of the site’s close proximity to the Atlantic Ocean. But neither are strong winds, so any evidence of a heavy burst of rain in the morning can be quickly forgotten, allowing surfacing works to continue unhindered.
“This can be a strange place to work because of the weather,” says Clive Darby.
“We had our own on-site Met Office at RNAS Culdrose to give immediate forecasts and often had to stop work and be ready to start again soon after a sudden period of rain. Last year we had three days of rain in three months, but this year I don’tthink we have had three dry days in three months.”
DIO project manager for the main runway, taxiways and secondary runways projects Dave Williams says: “Proactive and efficient project management of resources and project risk helped the team to overcome extreme weather conditions and other risks such as diversion of services. A good working relationship between the project team and the operations team facilitated the continued operations of the Air Station while construction works progressed.”
In search of value
Head of airfield pavements at DIO John Cook says: “Our main objectives are to develop construction and maintenance standards and practices for airfield pavements which meet functional requirements, minimise whole life costs and disruption for maintenance and also address sustainability issues.
“As part of this strategy we continue to work with industry partners, including trade associations and our prime contractors and their supply chains, as well as participating on technical standards bodies.
“To help achieve some of these objectives it is likely that we will look more and more towards the use of polymer modified binders in surface courses on our resurfacing projects in future.”
He adds that the early indications from laboratory testing and use of modified binders in porous friction course and SMA on several resurfacing projects, including the taxiway resurfacing work at Culdrose, are that modified binders can - with judicious selection and testing - provide a substantial improvement in performance of the surface course.
“Marshall Asphalt is one of our principal materials for airfield resurfacing work, with its long track record of providing a robust and very durable surface course and meeting all of our requirements,” says Cook.
“However, if by using polymer modified binders we can improve its resistance to reflective cracking at low temperatures then this could also provide us with a significant gain in value for money.”