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The chapter delves into the Merimbula Airport case study, highlighting the challenges and solutions in sustainable and resilient airport pavement rehabilitation. Four distinct pavement design options are explored, including conventional flexible and rigid pavements, structural asphalt overlays, and foamed bitumen stabilisation. The study emphasizes the importance of sustainability and resiliency, evaluating each option based on embodied carbon and moisture resistance. The foamed bitumen and asphalt surface option emerges as the preferred choice, offering a balance of environmental benefits and structural performance. The chapter concludes with recommendations for other airports to consider similar sustainable practices, providing a comprehensive guide to innovative pavement rehabilitation strategies.
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Abstract
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.
1 Introduction
Merimbula airport is located on the south cost of New South Wales (Australia) approximately halfway between Sydney and Melbourne. Like many regional airports in Australia, it provides a critical link for business, tourism, medical emergencies and natural disaster response (1). Being a regional airport, it has one runway, a single taxiway and a simple parking apron (Fig. 1). Prior to the recent upgrade, the runway was 1600 m long and 30 m wide, and the total pavement area was 55,000 m2, taking into account the aircraft parking area.
Merimbula airport is located in a flat area between an ocean and mountains, just 2 m above the mean sea level and adjacent to environmentally sensitive oyster farming waterways. As a result, the site has poor drainage characteristics and a high water table, and discharging water into the adjacent waterway is environmentally unacceptable. In 2018 the runway at Merimbula airport exhibited a number of isolated structural failures, linked to water ingress into the marginal granular base course layer. At the same time, the airport owners proposed to introduce a new airline that operated larger aircraft. This required a 150 m long extension to each end of the runway and strengthening of the existing pavement structures. In light of the environmentally sensitive site and the high cost of transporting new granular materials to site, a sustainable pavement rehabilitation design was desired, maximising the reuse of the existing material, and minimisation of new granular material demand. It was also necessary to increase the resilience of the existing and new pavements by increasing the resistance to periodic moisture ingress.
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This paper presents the design of the Merimbula airport upgrade project as a case study in sustainable and resilient airport pavement rehabilitation. Four pavement design options were considered and the sustainability and resilience of each were compared. The case study provides an example of how airport pavement rehabilitation design options can be assessed, and the associated sustainability and resiliency quantified.
2 Pavement Design Options
Four different, but structurally equivalent, pavement designs for the upgrading of the existing runway pavement at Merimbula airport were developed. The designs included a conventional new flexible pavement, a new rigid pavement, a structural asphalt overlay of the existing pavement, and insitu stabilisation using foamed bitumen stabilisation of the existing pavement followed by an asphalt surface. The structural equivalence of these different pavements was defined by their theoretical ability to support the same aircraft traffic loadings over the 20-year design life (Table 1), for the existing pavement and subgrade conditions.
Table 1.
Design aircraft traffic loading
Aircraft
Mass (kg)
Tire pressure (kPa)
Total departures
Saab 340 B
13,154
379
28,000
Dash 8 Q400
29,347
1565
34,000
Fokker F100
45,813
1076
16,000
In airport pavement rehabilitation design, the existing pavement structure and materials are critical (2). Based on geotechnical investigations, it was determined that the existing pavement comprised a thin bituminous chip seal, known in Australia as a sprayed seal (3), over 500 mm of locally sourced marginal, or non-standard, natural gravel, over of a sandy clay subgrade with a characteristic bearing strength of CBR 5%.
The four pavement thicknesses were determined in FAARFIELD (4) and were based on Australian airport pavement design and construction practice (2). The only non-standard material used was the foamed bitumen stabilised base (FBB). FBB is produced by stabilising a new or existing granular material with bitumen that has been foamed by injecting cold water into a stream of the hot bitumen (5). The foamed bitumen has a temporarily lowered viscosity, allowing it to be mixed through granular materials. Once compacted, the FBB cures quickly and provides a semi-bound but flexible and moisture resistant material (6). FBB has been used at a range of airports with the main benefits being compatibility with existing local or marginal gravels, rapid construction, the ability to be produced either insitu with a stabilising machine or exsitu in a pugmil, and the relatively high elastic modulus compared to the unstabilised granular material (5). Based on the mixture design results (7) the FBB at Merimbula airport was characterised in FAARFIELD as a user defined layer with a modulus of 800 MPa and no fatigue performance criterion, which is consistent with other airport pavement rehabilitations in Australia (8). The resulting pavement rehabilitation designs are shown in Fig. 2.
The new rigid pavement was the thinnest and the structural asphalt overlay of the existing pavement structure was the thickest, when the existing pavement was also included. The FBB and asphalt surface option, as well as the new flexible pavement option, were of similar thickness, because the high modulus of the FBB offsets the low modulus of the existing granular material, resulting in a similar structural contribution to that of the comparable thickness (just 20 mm greater) of new fine crushed rock. However, because the different materials used in the four pavements have different embodied carbon and moisture resistance characteristics, the thickness of the four pavements cannot be considered as an indicator of pavement sustainability and resiliency.
3 Pavement Design Sustainability and Resiliency
3.1 Pavement Sustainability
Sustainability of different pavement options can be expressed as the embodied carbon in the new pavement materials (9). The weight of embodied carbon was expressed as the equivalent mass of carbon dioxide associated with the production, transportation, processing and construction of each material (8). The sum of the material thicknesses, when combined with the material compositions and embodied carbon rates, allowed the calculation of an environmental cost for each pavement design option. The lower the mass of embodied carbon, the higher the sustainability score for that pavement. This approach has previously been used to compare the environmental cost of different airport pavement designs and materials (10‐12).
The embodied carbon rates for the various materials were determined elsewhere (9) but are summarised in Table 2, along with the density used to convert between material mass and volume. The resulting environmental cost, over the full pavement area, is shown in Table 3, along with the sustainability score for each pavement design option.
Table 2.
Material environmental costs
Material
Density
(kg/m3)
Environmental cost
(kg.eCO2/m3)
New asphalt concrete
2450
356.5
New Portland cement concrete
2600
324.0
New crushed rock
1900
144.4
New cement treated crushed rock
2250
194.1
Retained existing gravel
1800
0.0
FBB produced from existing gravel
2100
77.4
Table 3.
Pavement environmental cost
Pavement
Environmental cost
(tonnes.eCO2)
Score
(out of 10)
FBB and asphalt overlay
3024
10.0
Asphalt over existing
4116
7.3
New rigid pavement
6244
4.8
New flexible pavement
6090
5.0
3.2 Pavement Resiliency
In contrast to the sustainability, there is no measurable or calculatable indicator of pavement resilience. Therefore, relative resilience was considered subjectively, based on the risk of moisture related pavement failures during periods of moisture exposure. Table 4 summarises the pavement resilience assessment. The rigid pavement and stabilisation with foamed bitumen design options were considered to be highly resilient, and the new flexible pavement was rated as having medium resilience. Although retaining the existing pavement and overlaying it with asphalt would better protect the existing gravel base course, the risk of further wetting of the retained marginal gravel, and failure of the base layer, is significant without replacement or treatment of this material.
Table 4.
Pavement resilience
Pavement
Assessment
Score
(out of 10)
FBB and asphalt overlay
FBB is moisture resistant and only a moderate thickness of margin gravel is retained, under the 350 mm thickness of asphalt and FBB, so the retained gravel would be well protected by the surface asphalt and the FBB
9
Asphalt over existing
The asphalt provides protection to the retained existing gravel, but lateral water movement and the water table remain a high risk without improvement of the existing granular material
4
New rigid pavement
Concrete are cement treated crushed rock sub-base are effectively waterproof and the clay subgrade is not prone to shrink-swell, minimising the risk of surface unevenness
10
New flexible pavement
New crushed rock imported to standard material requirements, including low moisture susceptibility, and protected by new asphalt surface
7
3.3 Combined Sustainability and Resiliency
Whether the sum or the product of the sustainability and resiliency scores was considered, the FBB and asphalt surfacing option was preferred (Table 5). This generally reflects the high moisture resistance and low embodied carbon associated with insitu FBB production and demonstrates the benefits that can be achieved by stabilising an existing granular pavement structure, rather than replacing it with a new pavement.
Table 5.
Combined sustainability and resiliency scores
Pavement
Sustainability
Resiliency
Sum
Product
FBB and asphalt overlay
10
9
19
90
Asphalt over existing
7
4
11
28
New rigid pavement
5
10
15
50
New flexible pavement
5
7
12
35
4 Conclusion
The new rigid pavement and the FBB and asphalt surface pavement were the most resilient design options, based on the bound nature of the all the materials in the pavement, making them resistant to the effects of water ingress. Furthermore, the FBB and asphalt surface pavement, as well as the structural asphalt overlay option, were the most sustainable design options, because they retained the existing granular pavement material, which was effectively environmentally free. Because of its good performance in terms of both resiliency and sustainability, the FBB and asphalt surface design was preferred as the most sustainable and resilient pavement rehabilitation option. This reflects the high moisture resistance and the low embodied carbon associated with FBB. It is recommended that other airports consider foamed bitumen stabilisation of existing granular pavements in the future, whenever the existing pavement materials are suitable.
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