An Investigation into Australian Airfield Asphalt Density Quality Control Practices

Brendan Bennett

doi:10.25907/00909

In Australia, thin (50-80 mm) dense graded asphalt mixtures are widely used as the primary type of surfacing for airfields. The primary function of the asphalt mixture is to provide a flexible, waterproof, durable, and skid-resistant surface to the pavement structure. It is well established that compaction of asphalt mixtures is critical to airfield asphalt surface performance. Since the Australian Commonwealth government privatised the large Australian airfields in the 1990s and gave responsibility for the smaller airfields to local government authorities, there has not been a nationally adopted specifications for the design and construction of airfield asphalt surfacings. Only in 2018 was a standard specification published for airfield asphalt mixtures, but this has not been adopted by all airfields and design consultants which can lead to inconsistencies in the quality of airfield pavements. This research reviewed the various approaches to the specification of airfield asphalt mixture compaction. Density test results from typical airfield asphalt surfacing projects were analysed under different acceptance criteria used by different design consultants as well as the standard specifications. Monte Carlo simulations were used to generate a dataset based on actual airfield asphalt mixture production data, to determine the variance of air voids, and the effect that production variance has on the resulting air voids, and to determine the range of air voids that should be targeted. Further Monte Carlo simulations were used to evaluate typical schemes to assess asphalt mixture compaction. These schemes were assessed against simulated scenarios that represented results conforming to the proposed compaction specifications, as well as several scenarios that represented results non-conforming to the proposed compaction specifications. The optimal scheme balances risk for both contractors (false rejections) and clients (accepting non-conforming Lots). The two standard approaches used to design airfield asphalt surfacing mixtures were analysed and compared to ascertain which is more applicable, and whether legacy performance criteria are applicable to newer mixture designs. Lastly, various alternative methods for construction of asphalt joints were trialled to determine if they would improve the performance. The different specifications in the market showed to have significant impact on the acceptance criteria for field compaction of asphalt mixtures and showed the need for a standardised approach to compaction specification to prevent inconsistencies in the quality of airfield pavements. These specifications should balance theoretical limits and practical limits. Recommendations are made for how to determine the most appropriate basis of future standard specifications. Additionally, it was found that coarse (gravel) fractions of the aggregate have the highest effect on the resultant air voids in the mixture, followed by fine aggregate and bitumen content. It was also recommended to revise the tolerances for production (Marshall) air voids in the current Australian performance specification from a tolerance that is tied to the mixture design to a fixed range. This allowed the specification to be tightened to a range that was still achievable. Industry has recommended tightening the field air voids specification and this research supported this change. All compaction assessment schemes had benefits and drawbacks, there was no scheme that stood out as the ideal solution for test frequencies in the typical range of 6 to 12 tests per Lot. However, a scheme with limits on individual results and Lot averages was preferred, because this scheme best balances the client and contractor risks, and promotes the best contractor behaviour. Additionally, the optimum number of field samples to be taken was recommended. The legacy Australian Airports Association method of undertaking mixture designs was determined to be flawed, and as such it was recommended to migrate to the Australian Standard method, and the necessary changes to the performance requirements were recommended. The various alternative joint methods did not show any significant performance benefits at the construction stage. However it was recommended that these be monitored over the life of the surfacing.

An Investigation into Australian Airfield Asphalt Density Quality Control Practices

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