Greg White

Foamed asphalt stabilized base course is a bound but flexible material, with beneficial stiffness, strength, and moisture resistance properties. When the granular material being stabilized is an existing airport pavement base, foamed asphalt stabilization is also highly sustainable and provides a more resilient pavement structure. Despite these advantages, to gain acceptance as a mature technology, a material specification and a clear basis for characterization in thickness determination software are required. A specification was published in 2024, and this research aimed to provide a basis for material characterization in layered elastic design methods. Based on a review of the nature of the materials and field performance of various airport pavements containing foamed asphalt stabilized base, it was concluded that the elastic modulus should be varied broadly from 1,000 MPa to 3,500 MPa, depending on the climatic conditions and the time until critical aircraft trafficking is expected. It was also concluded that a fatigue performance model, such as that associated with asphalt concrete or cementitiously stabilized materials, is unlikely to be applicable to foamed asphalt stabilization in airport pavements.

Low traffic local roads are often comprised of a marginal gravel base course with a thin bituminous surfacing. These pavements commonly require rehabilitation at the end of their life. The economic (financial) and environmental (embodied carbon) cost of three typical pavement rehabilitation designs were estimated and objectively compared. The rehabilitation treatments included full depth granular reconstruction with new material, an insitu cementitious stabilisation of the existing pavement to form a lightly bound gravel base, and an insitu foamed bitumen stabilised existing gravel base, each with a sprayed seal surface. The full depth reconstruction with new granular materials had the highest economic and environmental costs, by a significant margin. It is recommended that stabilisation be the preferred rehabilitation option for local roads, and that pavement reconstruction using new materials only be considered when stabilisation of the existing pavement is determined to be unviable.

The airport pavements at Merimbula airport (Australia) required strengthening to support larger aircraft. Four structurally equivalent pavement designs were developed, using standard pavement materials, except for an option to use of foamed bitumen to stabilise the existing granular pavement material, prior to surfacing with an asphalt wearing course. The sustainability and resiliency of the four pavement designs were considered and scores assigned. The sustainability score was based on the embodied carbon in the rehabilitated pavements, while the resiliency score was subjectively assigned based on the resistance of the pavement to moisture ingress. It was found that the new rigid pavement and the foamed bitumen stabilised flexible pavement were the most resilient, reflecting the bound nature of these materials. Furthermore, the foamed bitumen stabilised pavement and the structural asphalt overlay of the existing pavement were preferred in terms of sustainability. Because of its high scores in both sustainability and resiliency, the foamed bitumen stabilised pavement was preferred. It is recommended that other airports consider the potential for stabilisation of existing pavements, for sustainable and resilient pavement rehabilitation, whenever the existing pavement materials are suitable.

The construction joints between paver runs are important to the design and construction of rigid aircraft pavements. These joints are required to account for paver width and are generally dowelled to ensure load transfer to adjacent slabs through vertical shear. Typically, smooth steel round dowels are used with specifications based on providing a sufficient spacing and diameter to mitigate bearing stresses, as well as sufficient embedment length to maximise load transfer. Alternate shapes to round dowels, such as diamond-shaped plate dowels, are regularly found in industrial slab-on-ground pavement applications. However, these have generally been omitted from rigid aircraft pavement practice. Compared to round dowels, which only allow for concrete movement in one direction, diamond-shaped plate dowels allow for both transverse and lateral movement when concrete expands and contracts. However, to be included in aircraft pavement practice, diamond-shaped plate dowels must be validated against aircraft loads to ensure they provide appropriate load transfer. This research used finite element methods to analyse the effectiveness of diamond-shaped plate dowels for rigid aircraft pavements. Through a parametric analysis, the significance of load regime, dowel detailing, dowel looseness, and foundation support, on load transfer values used in contemporary slab thickness design methods, was determined. Findings from this research can be used to innovate construction joint designs for rigid aircraft pavements.

Aircraft pavements are generally strength rated through the Aircraft Classification Number—Pavement Classification Number (ACN-PCN) system, or the newer Aircraft Classification Rating—Pavement Classification Rating (ACR-PCR) system. Overload operations occur when an aircraft ACN is greater than the PCN of a pavement, and in these cases, pavement concessions are granted based on an allowable ratio of ACN to PCN. The equivalent pavement structural damage for a single operation of a given ACN-PCN ratio is well understood, with airport authorities usually allowing a maximum overload concession based on a maximum ACN-PCN overload ratio. However, the recommended frequency of overload operations is based on a somewhat arbitrary set of rules, with different airport authorities applying different frequency limits. This research analysed the effect of overload operations on the theoretical pavement life of a network of Australian military and civilian airfields when using an existing pavement concession system. It was determined that the effect of the pavement concession system on theoretical pavement life was highly dependent on the initial design traffic inputs. A new overload concession system was then developed that considers the inherent conservatism in aircraft pavement design and construction and allocates that conservatism to a lifetime overload allowance. Although this research and the proposed concession system were performed using the ACN-PCN system and in the Australian context, it can also be adapted for overseas airports and for use with the newer ACR-PCR system.

In 2020, a new system for airport pavement strength rating, known as ACR-PCR, was approved for the introduction in November 2024. The new system operates in a similar manner to the old system but uses the same mathematics and methods that are embedded in many contemporary airport pavement thickness determined software. This avoids many anomalies between pavement thickness design and strength rating, which is beneficial. This paper reviews the benefits, limitations, and some potential improvements associated with the transition to the new system. The new strength rating system is expected to be in place for many years, and it is recommended that it should be improved by allowing a rational approach to setting runway strength ratings, replacing the surface protect tire pressure limits with an index that takes into account both tire pressure and wheel load and replacing the subgrade categories with the actual subgrade characteristic value used in design and evaluation.

The performance enhancing effect of crumb rubber modified (CRM) binders has received considerable research attention, and the benefits are well understood. However, research has typically considered high dosages (>20%) in gap and open-graded asphalt mixtures, whereas the potential of low-dosage CRM binders (5%–15%) as an anti-aging modifier for local roads has received comparatively little attention. To better understand these potential benefits, low-dosage CRM binders (5%, 10%, and 15% rubber by weight of binder) were prepared using both short and long blending durations and were conditioned in the laboratory to replicate production and field aging. Rheological analysis using a dynamic shear rheometer subjected binders to temperature-frequency sweeps to better understand intermediate temperature responses post-aging. It was found that increasing crumb rubber contents improved the high-temperature performance, whilst maintaining reduced stiffness in the low-temperature test regions. Further analysis of master curves and Black space diagrams, in addition to rheological properties such as Glover-Rowe parameter, R-value (rheological index), GC (crossover modulus), and other index parameters, found that increasing rubber contents improved binder durability and reduced thermo-oxidative aging impacts. While higher dosages (15%) were observed to perform best, it was concluded that the incorporation of any amount of crumb rubber using either blending approach provided anti-aging benefits relative to the unmodified base binder.

Crumb rubber modified (CRM) bituminous binders have been used successfully in asphalt pavements to significantly enhance durability and cracking resistance properties. However, the rubber contents required (>20%) and their associated costs are generally prohibitive for use on local roads. It is therefore of interest to determine the performance and anti-aging benefits that may be realized from low-dose (≤15%) CRM dense grade asphalt (DGA) in these applications. In this study, a typical local road DGA mixture was prepared with low-dose CRM binders (5%, 10%, and 15%) using both short and long binder blending durations, to simulate field and terminal blends, respectively. Compacted asphalt specimens were oven aged for a duration of 0, 5, and 15 days, in accordance with the AASHTO R30 conditioning method. Test specimens were then subjected to mechanical performance testing, including resilient and flexural modulus, and IDEAL-CT to determine cracking resistance benefits. The relative performance and the changes to these mechanical properties were used as indicators of the resistance to thermo-oxidative aging effects imparted by the CRM binders. It was found that increasing rubber contents resulted in improved mechanical performance properties, relative to the unmodified control mixture, and that the reduction in dissolution of the rubber crumb from short-duration (field-blended) binders generally resulted in improved performance, compared to their long-duration (terminal-blended) equivalents. Similarly, with increasing rubber content, it was found that the resulting CRM DGA mixtures showed a reduced level of stiffness increase and improved retention of cracking resistance with age. Analysis of variance of the test results indicated that the CR content had a statistically significant effect on the relative aging of the CRM DGA mixtures. Consequently, it was concluded that the incorporation of low-dose CRM binders in local roads may significantly reduce the aging process and improve pavement life and should be encouraged in the future.

There is no universally adopted specification for airport asphalt compaction in Australia. Rather, various compaction schemes exist, including limits on Lot averages and/or limits for individual values, percentage of samples within working limits, and characteristic value approaches. Additionally, there is no consensus on the number of compaction tests per Lot. To address this, Monte Carlo simulations were used to evaluate typical schemes against simulated conforming and several non-conforming scenarios. The optimal scheme balances risk for both contractors (false rejections) and clients (accepting non-conforming lots). Whilst all schemes had benefits and drawbacks, there was no scheme that stood out as the ideal solution for test frequencies in the typical range of 6–12 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. It is hoped that the outcomes from the research will inform airport asphalt specification revisions in the future.

Construction and shrinkage control joints. The slabs are generally 4 m to 6 m in size and are approximately square. Although this approach to rigid airport pavement design is well established, some airports have used continuously reinforced concrete as an alternate approach. Based on a case study at a new major airport in Australia, this research theoretically compared a traditional plain jointed rigid aiport pavement to a structurally equivalent continuously reinforced concrete pavement. The two designs were compared on a whole of life cycle basis, including embodied carbonm determined based on the quantities of the different materials and published greenhouse gas emission rates associated with the different materials. The results show that the structurally equivalent continuosouly reinforced airport pavements are significant thinner and require significantly less joints than structurally equivalent plain jointed rigid airport pavements. The reduced concrete thickness was offset against the need for steel reinforcement, but still resulted in a net construction cost reduction associated with the continuously reinforced solution. The reduction in the quantity of joints in the continuosouly reinforced pavement solution flowed through the whole of life cost estimates and provided an overall whole of life benefit. The conclusions assist airports in deciding whether continuously reinforced rigid airport pavements are a viable alternate to traditional plain joints slabs in the future.