Asphalt Behaviour Study

Project summary

During the runway rehabilitation of a large international airport, a number of indentations in newly laid asphalt (sometimes referred to as “Elephant’s Feet”) were visible in shallow-angled lighting conditions as well as runway grooving which was showing to be closing in. Given the high-risk nature of undertaking limited possession works on active runways, a comprehensive review of likely causal factors was carried out as well as an assessment of whether the observations were as a result of any underlying issues.

Scope of assessment

The study examined several possible contributing factors including: the location of the observed depressions; the historical occurrence of similar issues; the material specification and the inherent behaviour of asphalt; airfield operational procedures; meteorological conditions and the effects of jet engine exhaust efflux.

The depressions and narrowing grooves were primarily visible in and around the runway threshold area as well as the bell mouth of the main access taxiway for departing traffic. These regions are highly susceptible to static loading from aircraft lining up and awaiting take-off clearance from air traffic control. They are also susceptible to increased shear forces from landing gear which is both pivoting and turning to align with the centre line of the runway.

Similar occurrences were noted at another major international airport where asphaltic runway holding points were ultimately reconstructed in pavement quality concrete. It is also typical for concrete runway ends to be constructed on military airfields for this very reason. Although asphalt for use on airfields is specified with a high stability and lower penetration binder, it is nonetheless a material that is susceptible to creep and deformation under elevated temperatures.

The resurfacing works were undertaken at the height of summer, albeit during night time possessions, while ambient temperatures during this period rarely dropped below 20°C. Removal of nearby AGL seating pots released a noticeable wave of warm air suggesting the pavement was retaining a significant amount of heat from direct sunlight exposure. Daytime temperatures in excess of 35°C were recorded at the nearby weather station and it was inevitable that pavement temperatures would be significantly higher, likely approaching the softening point of bitumen. Jet engine exhaust efflux was also considered as a causal factor where temperatures up to 200°C can be measured depending on aircraft type and engine model.

Benefit

The investigation sought to allay concerns regarding the pavement material specification and/or the on-site production methods. The review was able to show the employer that there were many possible contributing factors associated with the observed deformations. Almost all of these were directly attributable to the inherent behaviour of asphalt in high ambient temperatures or when subjected to concentrated static loading conditions. The report also provided some operational recommendations in areas with newly laid asphalt such as limiting waiting times for departing traffic, promoting the use of rolling departures and spreading departures amongst runway access taxiways.