Review of the Use of Waste Materials in Rigid Airport Pavements: Opportunities, Benefits and Implementation

Abstract

The aviation industry is under increasing pressure to reduce its environmental impact while maintaining safety and performance standards. One promising area for improvement lies in the use of sustainable materials in airport infrastructure. One of the issues preventing uptake of emerging sustainable technologies is the lack of guidance relating to the opportunities, potential benefits, associated risks and an implementation plan specific to airport pavements. This research reviewed opportunities to incorporate waste materials into rigid airport pavements, focusing on concrete base slabs. Commonly used supplementary cementitious materials (SCMs), such as fly ash and ground granulated blast furnace slag (GGBFS) were considered, as well as recycled aggregates, including recycled concrete aggregate (RCA), recycled crushed glass (RCG), and blast furnace slag (BFS). Environmental Product Declarations (EPDs) were also used to quantify the potential for environmental benefit associated with various concrete mixtures, with findings showing 23% to 50% reductions in embodied carbon are possible for selected theoretical concrete mixtures that incorporate waste materials. With considered evaluation and structured implementation, the integration of waste materials into rigid airport pavements offers a practical and effective route to improve environmental outcomes in aviation infrastructure. It was concluded that a Triple Bottom Line (TBL) framework—assessing financial, environmental, and social factors—guides material selection and can support sustainable decision-making, as does performance-based specifications that enable sustainable technologies to be incorporated into airport pavement. The study also proposed a consequence-based implementation hierarchy to facilitate responsible adoption of waste materials in airside pavements. The outcomes of this review will assist airport managers and pavement designers to implement practical changes to achieve more sustainable rigid airport pavements in the future.

1. Introduction

Airport surfaces that support movement of aircraft comprise both flexible (asphalt) and rigid (concrete) pavement structures. Rigid pavements often feature on runways, taxiways and parking aprons at major and international airports [1] and, therefore, are critical pieces of infrastructure that support the safe movement of aircraft and passengers. The criticality of this infrastructure warrants centralized, standardized guidance for practice and innovation, as occurs in the United States of America (USA) by the Federal Aviation Administration (FAA) [2]. However, some airport industries have not kept pace with available innovation and research, leading to some industries retaining conservative and outdated practices [1,3,4,5]. This contrasts with air transportation, which has grown rapidly in recent decades, including both passenger and cargo transportation [6]. For example, in Australia, the privatization of airports in the late 1980s saw the removal of a key national custodian of technical standards and best practices for infrastructure, including airport pavements [1]. This has led to inconsistent practices across jurisdictions, reduced knowledge continuity and a limited capacity for coordinated research and innovation [1], including stagnation of knowledge about sustainable pavement materials, which is a barrier to their uptake.

Rigid airport pavements generally comprise unreinforced Portland cement concrete, over a granular or bound sub-base course, on a natural, improved or imported subgrade [7]. While construction and maintenance is similar to rigid road pavements, airport pavements are generally thicker, flatter and contain higher quality materials, to cater for aircraft specific requirements [7]. These requirements include higher wheel loads and tire pressures, the lower stability of aircraft on the ground, and an intolerance to loose surface particles that could cause damage to low slung aircraft engines, a phenomenon known as foreign object debris (FOD) [3]. The resulting higher construction tolerances, and potential of high consequences if pavements fail combine to contribute to the risk-averse nature of the airport industry [7], which is a barrier to the uptake of innovative pavement technologies.

Innovation plays a critical role in the global shift towards a circular economy, which means retaining the value of resources by keeping them circulating at their highest potential value, for as long as possible [8]. Businesses and governments now recognize the opportunities that waste materials provide and their economic value, with Australia, the European Union, China and Canada all adopting circular economy policies [9]. However, this circular economy concept is not new, evolving in the 1970s and challenging the one-way model of production and consumption that saw raw materials extracted, manufactured, used, and then disposed of as waste [10]. Recycling pavement materials into new pavement construction has multiple benefits. First, it reduces the space that waste material would take up if disposed to a landfill, Second, it reduces the environmental impact of processing or producing virgin materials. Finally, it increases social equity by reducing the amount of naturally occurring virgin materials, such as aggregate and other valuable minerals, that are consumed and no longer available for future generations [11]. Using waste materials created by other industries in pavement applications can have further benefits including enhanced performance and embodied emissions reduction [12,13,14]. Decarbonization of built infrastructure is a key priority for governments and industries globally [8,15,16] and is complemented by efforts to increase re-use of waste materials. Environmental product declarations (EPDs) have emerged as an independently verified data source that enables users to objectively measure and compare the life-cycle environmental impact of different products through standardized metrics, such as total greenhouse gas emissions (GHGs) often expressed as kg CO₂ equivalent/unit [17].

Researchers and road authorities have proposed principles for incorporating more waste material into pavement systems [14,18,19,20]. Common principles include Triple Bottom Line (TBL) assessment, understanding sorting and processing requirements, understanding the limitations associated with innovative material source locations, and pavement performance impacts. These key factors are relevant to assessing and implementing waste materials in airport pavements, as well as other infrastructure. Furthermore, the pavement type, pavement strength, material performance, initial cost, durability, practicability, constructability, safety, effects of climate change, and available waste material options, should all be considered when incorporating waste materials and by-products into airport pavement construction [21]. Additionally, pavement performance is critical to the risk averse airport pavement industry. Verifying that concrete containing waste materials has a performance that is equal to or better than comparable pavements using virgin materials is essential to ensure durability and encourage change in construction practice [18].

To support change in airport pavement practice, this work reviews waste material opportunities in rigid airport pavements, addressing opportunities for the whole pavement system, but with specific attention on the concrete base slabs. Additionally, sustainability benefits of incorporating different waste materials into concrete pavement mixtures is explored through using primarily EPD data, to compare the environmental impact of those mixtures containing waste materials and those with only virgin materials. Furthermore, a consequence-based implementation framework is proposed to encourage uptake of sustainable rigid airport pavement technologies. The overall aim is to provide practitioners and researchers with a practical basis on which to consider more sustainable rigid airport pavements in the future, without adversely impacting the safety, performance or durability of airport pavement infrastructure. This work complements research undertaken by Jamieson et al. [22] that reviewed the opportunities, sustainability benefits and implementation risks for waste materials in flexible airport pavement structures. A schematic overview of the structure of this review is illustrated in Figure 1, which closely reflects the structure of this review.

2. Background

2.1. Rigid Airport Pavements

As mentioned above, rigid airport pavements generally comprise rectangular slabs of unreinforced Portland cement concrete slabs, over a granular or bound sub-base course, on a natural, improved or imported subgrade [7]. Joints are located every 4 m to 6 m to control cracking and to provide load transfer between separately constructed slabs. The joint types vary, depending on their location, with transverse shrinkage joints commonly sawn and un-dowelled, using aggregate interlock to transfer load between slabs. Longitudinal joints are generally formed and dowelled, relying on the dowels to transfer the load between slabs [4]. Slab thickness is primarily influenced by the strength of the concrete mixture and the design aircraft mass [23], with slab thickness typically in the range of 300 mm to 500 mm [3].

Figure 2 illustrates a cross section of a typical rigid pavement with each of these layers providing a different opportunity to incorporate waste materials. In contrast to flexible asphalt pavements, these pavements are referred to as rigid pavements because they are not designed to deform vertically under an aircraft load. Rather, they resist the majority of the aircraft-induced stress internally within the concrete, by slab bending, instead of transferring significant load to the underlying pavement layers [24]. Consequently, the concrete base layer provides the principal load resistance, and therefore the highest quality materials are used in this layer [25], which comprises cement concrete, sometimes referred to as Portland Cement Concrete (PCC), particularly when Portland cement is used [7,26]. Due to the primary load bearing nature of the concrete slabs, the underlying layers of a rigid pavement are not required to provide the same cascading level of support as for flexible pavements. However, the sub-base and foundation layers must provide a flat and stable platform for the concrete slabs to rest on, to prevent differential settlement and stepping between the slabs, and provide a suitable surface for construction of the slabs [27].

Rigid airport pavements generally represents a small proportion of the aircraft movement surface at many airfields, compared to flexible asphalt pavements [28]. However, cumulatively they can amount to a significant volume of concrete material. For example, in the Australian registered network of 480 airports, rigid airport pavement is estimated to equate to approximately 14.7% of pavement area [28], which, assuming an average slab thickness of 400 mm, amounts to 2.25 million m³ of concrete. Furthermore, rigid pavements are often used in areas with slow moving or stationary aircraft, or areas with high risk of fuel or hydrocarbon exposure, such as parking aprons, some taxiways and runway thresholds [5,7]. Rigid pavements are rarely used throughout the length of runways in Australia, but are common internationally, including the United States, Middle East, parts of Europe and China [5,18,25].

Concrete for rigid airport pavement differs from general construction, building and road pavement concrete. The aggregate is usually larger to ensure aggregate interlock occurs between sawn construction joints, and the strength is characterized through flexural strength, not compressive strength [4]. Current Australian practice is that airfield concrete typically consists of the elements and corresponding fractions in

Table 1. Typical airport concrete pavement components [7].

Element	Details	% by Mass
Cement	Portland type, blended with up to 20–25% fly ash	15–20
Coarse aggregate	40 mm maximum size, natural crushed rock	50–60
Fine aggregate	Natural sand	20–30
Water	Potable water often specified	5–10
Air	Entrained air volume	3–5
Additives	Such as water-reducing admixtures	<2

. A characteristic strength of 4.5 MPa to 4.8 MPa is typically specified [4]. Characteristic strength is defined statistically as the strength exceeded by 95% of test beams, meaning concrete producers often target a mixture with 5.2 MPa to 5.8 MPa average strength to achieve the 4.5 MPa characteristic strength [29]. The water/cement (w/c) ratio is generally <0.45 [2,7], with low w/c ratios increasing the strength of concrete [30] and reducing the porosity of the cement paste [31]. Reduced porosity is also related to increased durability [12,31] which is important for airport pavement that is expected to have a 40+ year design life.

The granular sub-base course is typically fine crushed rock (FCR) or a cement treated (bound or lightly bound) granular material, or lean mix concrete and is typically 150 mm to 200 mm thick [4]. The subgrade is not generally controlled by the design and can be a natural or imported material, and may need improving to ensure weakness or reactivity to moisture does not result in excessive slab thickness as compensation during pavement design [19,26,29], providing opportunity to use suitable waste materials to achieve the desired performance characteristics via chemical reaction or mechanical means [32].

2.2. Environmental Cost of Concrete

Globally, concrete is the second most widely used material after water [33] and cement production alone is estimated to account for 7% to 8% of global CO₂ emissions [15,33]. Cement is produced from clinker, a sintered material that comes from calcining limestone and clay-like materials in a kiln at high temperatures, resulting in small lumps of clinker [34]. Clinker is then milled into powder with a small amount of gypsum and other additives to form cement [34,35]. The emissions from clinker production are primarily from CO₂ released through decarbonizing the raw calcareous materials (limestone) during heating (approximately 60%), and the combustion of fuels used for heating the cement kilns and other cement plant processes (approximately 40%) [36], contributing to high levels of embodied carbon in the resultant cement product. The Global Cement and Concrete Association has published a roadmap for the global cement and concrete industry to achieve net zero carbon emissions by 2050, where efficiency gains in clinker production accounts for only 11% reduction contributing to the net zero goal [15]. This indicates that minimal opportunities exist to further reduce the impact of clinker production itself. Therefore, to significantly reduce the embodied emissions of concrete, a lower carbon alternative to traditional Portland cement—also referred to as general purpose (GP) type cement [37]—is required. Furthermore, the shipping of clinker around the world also contributes to Portland cement production emissions. International transport of cement materials is not uncommon, for example, in Australia, there are five integrated cement manufacturing facilities that produce approximately 60% of domestic demand, with the remaining 40% fulfilled by imported clinker, ground into cement at Australian facilities prior to distribution [38].

2.3. Waste Materials Commonly Used in Concrete Pavements

Concrete pavements in the road and airport context commonly feature some waste materials or by-products as part of normal practice, and this is generally supported by standards and technical specifications. The most commonly used waste materials are the supplementary cementitious materials (SCMs) of fly ash and ground granulated blast furnace slag (GGBFS), and recycled aggregates, each of which is discussed in more detail below.

2.3.1. Fly Ash

Fly ash is a by-product collected from flue gases of furnaces in coal fired power stations and has pozzolanic properties. That means they chemically react with lime in the presence of water, to form cement-like, or cementitious, compounds [35]. Fly ash is commonly combined with Portland cement to form blended cements [37]. The material properties of fly ash vary widely depending on the nature of coal (black or brown coal) and type of combustion process applied [14]. It is highly effective at protecting the concrete mixture from alkali silica reaction (ASR) if reactive siliceous aggregate is used [39]. For that reason, a major Australian state road authority, Queensland Transport and Main Roads (TMR), have mandated the use of 20% to 25% fly ash in all precast and cast in situ concrete for bridges and roads since 1993 [40]. The advantageous effects of fly ash in concrete include a slowing of hydration, reduced water demand and reduced drying shrinkage [41], all beneficial for the mass placement of concrete as occurs in pavement works. The spherical shape of fly ash particles also has the effect of increasing the workability of the concrete mixture, enabling a reduction in the water/cement (w/c) ratio, while maintaining constructability, resulting in less permeable concrete that is also sulfate resistant and more impenetrable to chloride ion attack [14,35,41], key markers for increased durability.

2.3.2. Industrial Slags

Industrial slags are the by-products of the iron and steel making process and are used in granular and fine powdered forms, with one of the most common uses being concrete mixtures [42]. Blast furnace slag (BFS), also known as rock slag, is a by-product of the iron making process using a blast furnace, where molten slag is allowed to cool in air, then can be crushed and screened into particles, similar to natural quarried aggregates [39], with a typical density only slightly lower than natural aggregate, approximately 3% lower [43]. Granulated blast furnace slag (GBFS) is sand sized material and is created by quickly quenching molten slag with water, producing a glassy granular product similar to sand [44]. Steel furnace slags (SFS) come from either electric arc furnace (EAF) or basic oxygen furnace (BOS) operations, with density approximately 25% higher than BFS [43]. However, a weathering process is required prior to use to increase volumetric stability [43] and protect against expansive reactions in pavement layers due to free lime.

Iron and steel slags are generally used uncrushed and have good particle shape which ensures good packing, with high shear strength and good mechanical interlock due the vesicular nature of slag particle faces [43]. GBFS can be ground to a fine powder, termed ground granulated blast furnace slag (GGBFS), also known as slag cement, and is highly valued as an SCM for concrete applications [39,45]. GGBFS is often specified where it is desirable to mitigate ASR [14], that can contribute to cracking, and where concrete needs to be durable and resistant to sulfate and chloride attack that causes deleterious effects, particularly in marine and other corrosive environments [35,46]. However, concrete made with GGBFS as a portion of the cementitious binder exhibits slower strength development, but higher long-term strength compared to Portland cement only concrete mixtures [47].

2.3.3. Recycled Aggregates

Recycled aggregates are used to partially or fully replace the virgin aggregate in either the concrete mixture or the granular or bound sub-base layer. The virgin aggregate that they replace is usually determined by the recycled aggregate particle size. Recycled aggregates include recycled concrete aggregate (RCA), recycled crushed glass (RCG), and recycled asphalt pavement (RAP), each of which are discussed in the following sections.

Recycled Concrete Aggregate

Recycled concrete aggregate is derived from crushing demolished or returned hardened concrete [48]. It has been commonly used in roads as unbound or stabilized materials in base course or sub-base layers, fill and drainage material [14]. Due to the varied origin of concrete that can be processed into RCA, there is potential for large variability in its characteristics, such as size available related to the size of aggregate in the original concrete [14], and the degree of contamination by timber, steel, plastic or other [18,49]. Consequently, not all sources of RCA are suitable for re-use in concrete mixture or other rigid airport pavement layers [19]. Additionally, RCA is typically 20% lighter than natural aggregates [14] due to RCA consisting of approximately 25% to 35% adhered mortar which is more porous than virgin aggregates [50]. Consequently, the increased porosity characteristic needs to be considered when performing concrete mixture design. Waste concrete requires screening, grading and washing to be used as RCA [14], each process requiring energy inputs that add to its resultant embodied emissions and financial cost.

Recycled Crushed Glass

RCG that is suitable for use as aggregate in pavement structures originates from food and beverage container glass or window and industrial glass [51], which is then processed into particles by crushing. RCG can be used as coarse or fine aggregate or in powdered form [52]. Due to the post-consumer nature of some RCG, it needs to be processed to ensure it is free of contaminants such as sugar, metal and paper which can have detrimental effects on the strength and durability if used in concrete, so some degree of washing is needed which adds to the cost [48]. Glass in fine and powdered form has a pozzolanic effect [14] making it suitable for stabilization applications or concrete when used in combination with Portland cements, and can improve mechanical and durability characteristics of concrete by refining pore structure [52]. However, powdered glass as an SCM is not well established in Australia, with RCG most frequently used as fine material in unbound granular applications, where particle size < 10 mm is often specified [14], and in asphalt mixtures as a sand replacement [53].

Recycled Asphalt Pavement

RAP is reclaimed asphalt from an existing pavement surface [18] and may be considered low, medium or high risk depending on the source, age and stockpiling controls applied in its management. The highest value and best use of RAP is incorporating it back into new asphalt surfacing [14,21]. However, it can also be used in unbound granular pavement layers, particularly when it is of lesser quality that would preclude its inclusion in new asphalt pavement applications. The value of incorporating RAP in unbound base or sub-base layers is maximized when there is an excess of RAP from other pavement works, and the resulting material can meet the performance requirements of the virgin aggregate it is replacing [14]. However, in low strength applications the high value bitumen component is wasted which is not preferred [14]. Using RAP as aggregate in concrete pavement is not identified as a valid option in the literature [14,19,20] likely due to a range of issues including varying impacts on workability, reduction in strength and detrimental effect on durability properties [54].

2.4. Sustainability Principles of Waste Material Use in Rigid Airport Pavements

While numerous waste materials have been used successfully in road and rigid airport pavements, increasing the uptake of these technologies in the airport context is desirable to support sustainable infrastructure goals [55]. Principles have been proposed for incorporation of waste materials into pavements [14,19] to verify adequate performance and importantly, a net sustainability benefit is achieved. Jamieson, White and Verstraten [18] proposed principles specific for airport pavements, including conducting a Triple Bottom Line (TBL) assessment, understanding the sorting and processing required for recycling materials, acknowledging benefits and limitations associated with their source location, and the results of performance testing.

The TBL concept involves accounting for the financial, environmental, and social costs of a system across its lifecycle. When applied to pavements, this allows for comparison to be made between pavement systems that incorporate waste materials, to conventional pavements that contain only virgin materials, supporting decision-making [19]. A TBL approach—specifically for assessing the potential of recycled materials in airport pavements—considers the financial, environmental, and social costs of pavement systems, to allow comparison of pavements both with and without waste material inclusion [22], and is shown at Figure 3.

Life cycle cost assessment (LCCA) is a financial analysis method that accounts for the total cost of a pavement system over its lifetime including construction, maintenance and rehabilitation costs, and can also factor in residual value at end of life. It has been commonly adopted as a powerful decision support tool in pavement projects to compare different pavement options [18,19,56]. LCCA is especially useful when design lives, and therefore pavement rehabilitation requirements and replacement frequency, are different. In essence the LCCA enables the considerations of durability and performance to be appraised, which is important in the case of concrete, as reduction of 50% of environmental impact is easy to achieve but may result in significant reduction in service life and performance [57] which is unacceptable in the airport pavement context. Furthermore, studies have shown that a pavement option that has a higher up-front cost, but longer life with less maintenance (i.e., more durable), may prove the most cost effective option over the long term [18]. Consequently, the financial cost component of a TBL analysis is important for both airport owner decision-making, and when comparing pavements containing various waste materials to pavements with only virgin materials, in an experimental context.

Societal impacts of concrete are the most difficult to quantify and there is no generally agreed upon metric to measure the social impact of pavements [19]. Relevant social impacts often overlap with environmental impacts and include land use change required for quarries, heavy transportation requirements, dust, noise and blasting impacts from acquiring and processing the raw resources [19]. The typical components for assessing the social impacts of road pavements, such as ride comfort, damage to vehicles, esthetics, are not appropriate for assessing airport pavements as in the airport context, these aspects are tightly controlled by regulatory safety policy [18]. Consequently, Jamieson, White and Verstraten [18] proposed that determining the amount of virgin material consumed, in combination with quantity of materials sent to landfill, is a valid way to measure intergenerational equity via access to natural resources. Other research has used this approach [11], measuring social cost benefits through the reduced volume of waste going to solid landfill.

The environmental impacts of a product, system or process can be quantified through a Life Cycle Analysis (LCA) and has been in use as a decision support tool since the 1960s [19]. An LCA considers all the environmental impacts across raw material acquisition, production, use and end of life treatment and has become the common method for quantifying environmental sustainability of pavements [18,19]. The different phases of LCA include [18,58]:

• Production Phase (modules A1–A3). This includes raw material supply, raw material transport and manufacturing. This is also known as the Cradle to Gate system boundary.
• Construction Phase (module A4–A5). This includes transport to site, and construction and installation.
• Use Phase (module B). This includes use, maintenance and repair.
• End-of-life Phase (module C). This includes demolition, transport, waste processing and disposal.
• Recovery Phase (module D). This includes material recovery, reuse and recycling.

For the purpose of comparison, defining the system boundary is important to ensure only those phases of the LCA relevant to the comparison are included. For example, Jamieson, White and Verstraten [18] identified that for the assessment of recycled materials in pavements, most assessments only included the production stage (A1–A3), because the production of the raw pavement materials is where the greatest contribution to environmental cost occurs [58]. This is particularly relevant for concrete in rigid pavement structures, as around 75% of the carbon emissions in concrete come from the raw material acquisition, transport and concrete production phases across the cradle to gate system boundary [58]. This figure is likely to be higher for airport pavements, which have a higher cement content, and because cement is the principal contributor of embodied carbon to concrete mixtures [57,59,60].

GHGs are often used as a single proxy measurement for sustainability of transport and roads as a useful starting point [19]. However, EPDs report a range of environmental impact metrics including total Global Warming Potential (GWP-t), which is reported as kg CO₂ equivalent (per unit) from an LCA assessment. Other impact categories in EPDs include acidification potential (mol H+ equivalent), water depletion potential (m³ equivalent) and ozone depletion potential (kg CFC-11 equivalent). Despite this array of potential measures, GWP-t is most commonly used as the impact metric in pavement research [61] and is an appropriate LCA output [22,62].

2.5. Application of Waste Materials in Sub-Base and Subgrade Layers

The underlying principles of sub-base construction and subgrade treatment as part of pavement structure design are the same for both flexible and rigid pavements. That is, to provide the required strength to support the loads imposed through the pavement layer above, throughout the service life of the pavement structure [7]. Additionally, in the case of rigid pavements, the sub-base needs to prevent rocking of the slab and pumping-up of material through the joints [7]. Consequently, investigations into waste material use in non-wearing course flexible pavement layers, are equally applicable to the sub-base and subgrade layers in rigid pavements.

Waste material use in flexible pavement structures, including granular and stabilized layers, has been extensively summarized by others [14,18,22], with the presence of waste materials in road and some airport specifications, indicating that their use, albeit with defined limits, is common place [20].

Table 2. Summary of recycled materials included in road and airfield rigid pavement specifications for supporting layers [22].

Application	QLD	NSW	VIC	FAA Airport	Australian Airport
Particle replacement in unbound layers (including % of total mixture)	Base RCA—100% RAP—15% Sub-base RCA–100% RAP—45% RCG—20%	RCA—100% RAP—40% RCG—10% Slag—100%	Base RCA—10% RAP—10% RCG—10% Slag—10% Sub-base RCA—100% RAP—50% RCG—50% Slag—50%	Base RCA—100% for lower base layers only. Slag 100% Sub-base RCA—100% RAP—limit not specified	No existing specification
Stabilization treatments	Fly ash GGBFS	Fly ash GGBFS Powdered glass	Fly ash GGBFS	Fly ash GGBFS	No existing specification

is adapted from work by Jamieson, Verstraten and White [22], and summarizes the presence of recycled materials, including RCA, RCG, RAP, slag aggregate, GGBFS and fly ash, in typical road and airfield pavement material and construction specifications in Australia and the USA, relevant to unbound and bound non-wearing course layers.

The higher percentage of recycled materials allowable in lower layers reflects the risk profile of this application, as well as the appetite for waste materials reuse, of the various road authorities. By allowing higher proportions of these materials in layers which are subject to less stress, the balance between material re-use and pavement performance risk is balanced [18]. This is particularly evident for RCG and RAP, where their allowable additional percentages in layers subject to less stress is up to 40%. However, the variation in allowable amounts of waste materials in base and subbase layers between road authorities likely also reflects the relative proximity of, lived experience and history of use with each material. For example, 100% slag aggregate is allowable in NSW as an unbound pavement layer, reflecting the history of joint trials between Newcastle and Wollongong-based slag suppliers (steel makers and processors) and NSW Road Transport Authority, and successful use cases [43]. This highlights that the source location of waste materials matters, because where materials are local, have the capacity for re-use, exhibit the appropriate engineering properties, and require minimal transportation, they are more readily incorporated into practice.

The 100% allowable limit of RCA in sub-base layers across pavement specifications listed in Table 2, indicates the potential for large volumes of waste RCA, effectively used in the non-wearing course pavement layers, and while having similar environmental cost to natural aggregate [22], potentially minimizing the emissions savings possible with its use, there are significant social cost savings possible through diverting the RCA away from landfill, and reducing the virgin aggregate required, which are also significant social benefits. Fly ash and GGBFS are both commonly used as stabilization treatments for pavements, in combination with Portland cements or lime in blends [32]. With cement being a highly energy intensive construction material [22,37], any cement reduction will provide significant environmental cost savings. For example, Jamieson, Verstraten and White [22] identified that replacing a 3% cement treatment for an FCR layer with a 3% blended cement containing 30% hydrated lime, 40% GGBFS and 30% fly ash, could achieve nearly 50% emission reduction, with the GWP-t reducing from 32 kg CO₂-eq/t, to 17 kg CO₂-eq/t.

3. Opportunities for Waste Materials in Airport Pavement Concrete

Some waste materials have been successfully incorporated in pavement concrete for many decades, such as fly ash [40], with other high potential materials emerging as research and technology advances. As discussed above, there is already significant literature and guidance dedicated to the use of waste materials in pavement structures [14,19,20,63,64] due to the increasing pressure on the construction industry to adopt circular economy principles and decarbonize [8]. A significant volume of research has been dedicated to investigating use of waste materials in concrete mixtures across a wide array of applications; however, not all technologies investigated show potential suitability in structural concrete pavement applications. For example, waste plastic has been investigated as an aggregate replacement in concrete, with flexural strength and density reducing, and porosity and drying shrinkage potential increasing [65]. These trade-offs may be acceptable for light weight concrete applications, but are likely the reason these materials have not been considered a viable option for heavy duty concrete pavement [14,20,64].

Despite these findings, some road and international airport specifications do include allowable limits for certain waste, as summarized in

Table 3. Summary of allowable waste materials included in road and airfield pavement specifications for concrete base slab mixtures in Australian states, USA and UK.

Jurisdiction	QLD Road	NSW Road	VIC Road	WA Road	FAA Airport	UK MoD Airport	Australian Airport
Specification name	MRTS40 [66]	D&C R83 [67]	Section 610 [68]	Specification 820 [69]	AC 150/5370-10H [2]	Specification 033 [70]	Typical specification taken from literature [7]
Material
Fly ash	15–40%	15–40%	<25%	<25%	20–30% or <10% (with GGBFS)	<25%	<25%
GGBFS	10–65%	10–65%	<40%	<65%	25–55%	<40%	No
Other waste SCMs	No	No	<10% amorphous silica	amorphous silica	raw or ultrafine fly ash	No	No
RCA aggregate	No	No	No	No	Yes	No	No
RCG aggregate	No	No	No	No	No	No	No
Slag aggregates	Yes	Yes	No	No	Yes	No	No
Non-potable water	Yes testing required	Yes testing required	Yes testing required	Yes testing required	Yes testing required	Yes testing required	No

. This likely reflects the relative market and technological maturity of the various waste materials in the respective jurisdictions. In Australia, there is currently no standardized rigid airport pavement specification [3,4,5], resulting in designers and engineering consultants adopting varied practices over time. Noting this, average Australian rigid airport pavement practice was determined from literature [7] to enable comparison of relevant specifications in Table 3. The specifications include typical Australian road specifications from New South Wales (NSW), Victoria (VIC) Queensland (QLD), Western Australia (WA), and international airport specifications.

From Table 3, the typical Australian airport practice allows the smallest proportion of waste materials across all specifications reviewed. Consequently, there is great potential to increase the amount of, and type of waste materials in Australian concrete airport pavement practice. The main opportunity to reduce the environmental impact of a concrete pavement are SCMs replacing conventional Portland cement, as cement has the greatest environmental cost of any of the concrete constituent materials [12]. Waste materials identified as suitable for inclusion in rigid airport pavement concrete mixtures are discussed in more detail in the following sections.

3.1. Portland Cement Replacement by Supplementary Cemetitious Materials

The SCMs of fly ash and GGBFS are mature technologies when it comes to their inclusion in Portland cement concrete, including the benefits they provide to plastic and hardened concrete properties. Furthermore, these waste materials are commonly used in geopolymer concrete systems, with all three technologies discussed more in the section below.

3.1.1. Fly Ash

Although up to 25% fly ash is already part of Australian practice, there is opportunity to further maximize this already adopted technology by increasing the proportion of fly ash allowed in concrete mixtures. This increase is supported by some road specifications, where maximum limits of fly ash are 20% to 40%, as shown in Table 3, in either a binary or ternary blend, with Portland cement and other SCMs. In the USA the Federal Aviation Administration (FAA) specification allows for fly ash to replace 20% to 30% weight of cement if used in a binary blend for airport concrete pavement [2].

Slower strength gain is typical of pozzolan binders [35,39,71] and needs to be considered when specifying high proportions of fly ash in concrete mixtures. However these mixtures will often exceed the long term strength obtained by cement-only concretes [72]. Furthermore, 40% fly ash mixtures have refined pore structures, indicated by increased electrical resistivity [72], with the higher resistivity considered to be an indicator of higher concrete durability [12]. As expected with a waste material, fly ash is expected to cost less than cement [64] adding to the benefits of its use. However, the reduction in embodied carbon associated with fly ash is highly dependent on the method and distance of transportation from its source, with high truck transport distances shown to negate embodied carbon benefits of fly ash in concrete [16].

3.1.2. Ground Granulated Blast Furnace Slag

GGBFS is an effective cementitious material and is noted as a substitute for Portland cement, up to 65% in some road pavement specifications, as indicated in Table 3. Silica and alumina compounds in the GGBFS react with the lime released from the cement hydration process, to form calcium silicate hydrates and calcium aluminate hydrates, adding to concrete strength and refining the pore structure of the concrete paste, enhancing durability [39,46]. GGBFS does have some slight reactivity with water, known as latent hydraulic behavior [39].

In general, GGBFS has the effect of reducing water requirements, increasing workability, lowering the heat of hydration, lowering early strength, but increasing long term strength, and significantly increasing sulfate resistance [19,46]. These are beneficial features when considering heavy duty pavement applications. Additionally, in a systematic literature review, Silva et al. [47] concluded that GGBFS was highly beneficial at a replacement rate of 60% in pavements, exhibiting a longer setting time but higher ultimate strength, with potential to save 48% CO₂ emissions and a 16% reduction in cost per m³ of the concrete. Furthermore, GGBFS is beneficial in mixtures containing RCA, with the refinement of pore structure and secondary cementitious reactions mitigating any reduction in strength associated with RCA use [73]. GGBFS is an effective cementitious material, and contextually, confidence should be found in the fact that the USA FAA specification allows for GGBFS as 25% to 55% total cementitious material mass, for airport rigid pavement concrete mixtures [2].

3.1.3. Geopolymer Cement

Geopolymer cements are primarily produced by combining aluminosilicate solids with an alkaline activating solution [74], producing a cementitious material that can be used in place of a Portland cement [75]. Aluminosilicate solids include fly ash, silica fume, metakaolin, calcined clays, waste glass, copper mine tailings, zeolite, rice husk ash and more [74]. Many of these are waste materials and are already common SCMs [75]. Common activating solutions typically include a combination of sodium hydroxide and sodium silicate, with a combination of fly ash and GGBFS as the typical binder medium [39].

The appearance of geopolymer concrete is similar to conventional concrete, with plastic and hardened properties generally tested in the same manner. However, some characteristics differ, notably a lower viscosity paste which can contribute to more difficulty in finishing due to the sticky nature of the paste, a lower w/c ratio, better response to vibration during compaction, slump that is more sensitive to water addition, and high rate of strength development, particularly at higher ambient temperatures [39]. Additionally, work by Shayan [76] noted that geopolymer concrete can be superior to Portland cement concretes, particularly with regard to lower drying shrinkage and better resistance to chemical attack, both of which are aspects that improve durability.

Toowoomba Wellcamp Airport in South East Queensland is an important test case that demonstrated geopolymer concrete could be successfully used for rigid airport pavements. Completed in 2014, the airport featured 25,000 m³ of heavy duty geopolymer concrete pavements across a turning node, taxiway and parking apron movement areas, as well as 15,000 m³ of other concrete work. This was the largest known example of geopolymer concrete use at that time [77]. The project estimated an 80% reduction in CO₂ emissions, compared to using Portland cement, which equated to a saving of 8640 tonnes of CO₂ across the project [77]. This example indicates that geopolymer concrete represents a high potential technology to replace Portland cement concretes, using waste products of fly ash and GGBFS as the binder.

3.2. Natural Aggregate Replacement with Recycled Aggregates

Replacement of natural aggregates in concrete with waste materials conserves natural resources and prevents this waste going to landfill. RCA, RCG and BFS aggregates show potential to meet performance demands when used in airport pavement concrete, and are discussed further below.

3.2.1. Recycled Concrete Aggregate

As discussed earlier, residual mortar (hardened cement and fine aggregate) in RCA contributes to a reduced aggregate density and propensity for water absorption, with some studies finding that debonding can occur at the aggregate-mortar interface, in concrete mixtures containing RCA [14]. This highlights the need to comprehensively understand preparations required for RCA (such as pre-wetting) to enable realization of the maximum potential mechanical properties and long-term concrete performance. This same factor can also contribute to a reduction in workability [19].

Due to the varied origin of concrete that can be processed into RCA, there is potential for large variability in its characteristics, such as particle size, which is related to the size of the aggregate in the original concrete [14], and the degree of contamination by timber, steel, plastic or other materials [18]. Consequently, like many waste materials, not all sources of RCA will be suitable and some may be conditionally suitable, requiring other processing or sorting [19]. For example, pavements that have been subject to ASR should not be used as RCA [78]. One protection against these variables is to source RCA that can comply with requirements of a standard, such as the Australian Standard for aggregate supplies [79], which covers the use of recycled aggregates [20]. Providing high quality source concrete is used, it is commonplace for RCA to meet the requirements of the quarried natural coarse aggregate material it is substituting [14]. The use of RCA as fine aggregate in concrete is more problematic due to the associated increase in water demand and reduced workability. However when used in low replacement levels, these issues may be overcome with appropriate mix design [13].

Research by Verian et al. [80] investigated concrete pavement mixtures containing 30%, 50% and 100% replacement of natural coarse aggregate with RCA. Findings included properties of mixtures including 30% RCA were very comparable to the control concrete (Portland type with 0% RCA), and that mixtures containing 50% and 100% RCA exhibited a reduction in mechanical properties up to 25%. However, the critical properties remained above the minimum specification requirements, notably achieving 4 MPa flexural strength at seven days. This work also found that including fly ash as a 20% replacement for Portland cement produced RCA mixtures with increased durability and later age strength, and most notably, a 50% RCA concrete demonstrated similar properties to the 0% RCA control mixture.

Changi International Airport is a notable example of RCA use in airport pavement, with a parking apron pavement featuring 20% RCA (by mass of coarse aggregate) constructed in 2010, using concrete waste from the demolished concrete pavement being replaced [81]. Stringent testing of the RCA and trial mixtures ensured that the resultant concrete conformed to all design requirements and specification criteria. The 28-day average flexural strength of 5.1 MPa to 6.3 MPa [81], indicating RCA concrete can perform similar to and better than flexural strength generally specified for airport pavement.

Furthermore, the FAA specification for airfield pavement allows for crushed recycled concrete pavement as aggregate, providing it meets aggregate properties, including reactivity, gradation, limits for deleterious substances and particle shape [2], indicating that RCA use is not seen as introducing unnecessary risk in the airport pavement context in the USA.

Waste concrete requires screening, grading and washing to become RCA, and these energy inputs add to its resultant embodied carbon and cost. However, recycled aggregate suppliers are often located close to urban areas (and demolition sites), and as transport is a governing factor in the economic viability of recycled aggregates, may increase cost effectiveness compared to natural aggregates requiring a longer haulage distance [14,19]. Therefore, significant opportunity exists where RCA is derived from a concrete airport pavement being replaced, providing it meets the physical and material requirements, reducing or eliminating transport costs, which are often a governing factor in the economic viability of recycled aggregates [14].

3.2.2. Recycled Crushed Glass

Johannesen, Xu, Garton, Rae and Roberts [48] indicated that RCG may be used to replace up to 30% natural fine aggregate, without having a detrimental impact on the strength or durability of the concrete mixture, but that RCG use as coarse aggregate was not recommended because of uncertainties of shape and strength of the coarse RCG particles. Another study indicated that 20% replacement of sand with RCG achieved higher compressive, flexural and splitting tensile strength after 28 days, without affecting the workability of the concrete mixture, when compared against the control mixture [13]. Other studies identified a varying impact on slump, either increasing or decreasing workability, and significant impact on the mechanical and durability characteristics, related to ratio of replacement and particle size of the waste glass, with around 20% RCG fines replacement being optimal [52]. This indicates significant variability of mechanical characteristics is possible in mixtures including RCG, likely reflecting the inherent variability of the material itself.

ASR may also be of concern, where the silica rich glass particles and alkali in the pore solution of concrete can cause expansion, leading to cracking. However this may be mitigated by using smaller RCG particle sizes, and the inclusion of pozzolanic materials like fly ash or GGBFS, that counteracts ASR [13]. In fact, proportions of 25% fly ash or 60% GGBFS are a suitable mitigation [48], increasing the overall environmental cost reduction of the resulting concrete.

Technical specifications that set out the requirements for the manufacture and supply of RCG as a sand aggregate replacement across a range of applications, including road pavements are available in Australia [51]. These should be used to ensure that material quality requirements for RCG are met if this is considered for use in concrete mixtures for rigid airport pavements. The relative lack of adoption generally by Australian road authorities may indicate a lack of confidence in RCG aggregate technology for structural concrete pavements, and therefore trials are recommended prior to large-scale implementation for airport pavements [48].

3.2.3. Blast Furnace Slag Aggregates

BFS aggregate has proven to be an acceptable alternative to natural aggregate resources over a wide range of applications [14], and has been in use as a coarse aggregate in concrete pavements in the USA since at least the 1930s [44]. BFS aggregates have a high percentage of fractured faces [14], and the lower particle density and higher absorption characteristics must be taken into account during mixture design. BFS aggregates do not demonstrate alkali aggregate activity [45], eliminating the risk of causing ASR.

In general, most of the properties of concrete containing BFS as coarse aggregate are similar to mixtures with natural aggregate. However fly ash inclusion and pre-wetting of the BFS is necessary to ensure adequate workability. Furthermore, strength and stiffness properties are similar to concrete mixtures with natural aggregate [44], indicating a high probability of comparable mixture performance over time.

Some international airport concrete pavement specifications already allow BFS aggregates, providing it meets key aggregate properties, including reactivity, particle gradation, limits for deleterious substances, and particle shape [2]. The Australian standard contains a specific section detailing the additional requirements for coarse slag aggregates, providing methodology to ensure slag aggregates are assessed as suitable prior to being specified in a concrete mix [79]. Use of BFS aggregates in concrete pavement has the advantage of reducing the quantity of virgin aggregates, providing significant social benefit. However, there is concern that in Australia, the use of slag as aggregate is increasing, at the same time as production is decreasing [14], with potential for lack of availability and cost increasing with demand in the future.

Overall, slag aggregate has proven to be an acceptable alternative to natural aggregate resources over a wide range of applications [14]. However, SFS (EAFS and BOS) products are specifically excluded by a number of specifications, and are therefore not considered suitable aggregate substitutions in concrete pavement mixtures most likely due volumetric instability from free lime and magnesium oxide that may cause expansion [43].

3.3. Potable Concrete Mixing Water Replaced by Non-Potable Water

While mixing water for concrete is generally specified as potable [82], non-potable sources of water may be suitable for concrete mixing water, provided the chemical composition does not impede the chemical reactions involved in the activation of cementitious compounds [32,83,84]. Sources of water that may otherwise become waste include water recovered from concrete production operations and wash water recovered from washing concrete mixers [83]. These water sources may require filtration, treatment and testing to ensure they are suitable, processes which can add to the cost and embodied emissions. The chemical composition of the mixing water for concrete needs to be understood to ensure that levels of substances that would be impactful to concrete strength, workability and durability are not used [35,39,83].

Globally a range of standards exist that specify the requirements and testing regimes of concrete mixing water, including the allowable limits of chloride, sulfate, sugar, suspended solids and other substances that are detrimental to concrete properties [83]. Specifications commonly require test results to be presented as part of the mixture design submission if a source of water other than potable water is used [67,68,69], referencing concrete production and supply standards [84]. Therefore, there is no technical barrier to the use of non-potable or waste water sources as concrete mixing water, preserving potable water resources, providing that water is tested and conforms to specified requirements. Despite this, some airport rigid pavement concrete specifications do not permit non-potable water in concrete production.

3.4. Ranking of Opportunities

The opportunities for re-use of materials in concrete mixtures for rigid airport pavement production have varied track records of use, variable effects on the workability, durability, chemical resistivity and strength of the mixture, and significantly different levels of risk. Based on review of the various opportunities, the most technically viable are (from most to least viable):

• Low carbon cements, using geopolymer cement, fly ash and GGBFS;
• Coarse aggregate replacement with RCA and BFS.

However, these preliminary rankings are based only on technical viability and industry maturity. A more robust analysis is required, taking into account the potential quantity of embodied carbon emissions reduction, as well as financial and social cost benefits. Although the financial and social costs are relatively easy to determine, the environmental cost savings require the embodied carbon to be quantified for all virgin and waste materials in any given concrete mixture.

4. Quantifying Environmental Sustainability for Rigid Pavements

As discussed earlier in Section 2.4, a TBL approach has been commonly used as a decision-making tool and method to determine overall sustainability of a pavement technology. In research reviewing sustainability of flexible pavements in Australia, Jamieson, Verstraten and White [22] proposed a cradle-to-gate assessment is an appropriate LCA boundary in the experimental, non-project specific context as the majority of environmental costs are borne through the raw material supply and pavement production phases.

4.1. Typical Embodied Carbon Rates

EPDs report a range of environmental impact metrics including total Global Warming Potential (GWP-t), which is reported as kg CO₂ equivalent (per unit) from an LCA assessment across the production stage. EPDs are a powerful sustainability language because they allow different CO₂ reducing technologies to be compared in an unbiased way, supported by ISO 14025 [85] providing robust guidelines for their development [17]. It is likely that the number of published EPDs for construction materials will continue to increase, driven by the demand for the data reported in them. For example, the Australian Government’s Environmentally Sustainable Procurement Policy (ESPP) came into effect as of July 2024 [86], aiming to ensure government procurement minimizes GHG emissions [87], whereby EPDs are an accepted data source for reporting.

In order to quantify the environmental benefits of incorporating waste materials into rigid pavements, the range of GWP-t values for concrete constituent materials have been sourced from primarily Australian and New Zealand EPDs, the literature, and life cycle inventory databases, as shown at

Table 4. Ranges of environmental cost values for concrete materials.

Material	Application	GWP-t (kg CO2-eq/t)	Statistical Properties	Reference
GP Cement	Cement binder	677–1060	n = 24 µ = 870	[64,88,89]
Fly ash	SCM	0–13.7	n = 2 µ = 6.9	[64,90]
GGBFS	SCM	149–177	n = 3 µ = 163	[64,88,91]
Natural aggregates	Coarse aggregate	2.4–11.7	n = 58 µ = 5.5	[64,88,89]
Natural sand	Fine aggregate	2.9–5.4	n = 7 µ = 3.8	[88,89]
RCA	Coarse aggregate	3.7–16.0	n = 10 µ = 5.9	[64,88]
RCG	Fine aggregate	3.1–14.9	n = 4 µ = 9.9	[64,88,89,92]
BFS aggregate	Coarse aggregate	1.98	n = 1	[91]
Potable water	Mixing water	0.41	n = 1	[93]
Admixtures	Air-entraining and water reducing types	229–2200	n = 4 µ = 1050	[16,88]

Note: n = number of samples, µ = arithmetic mean of samples.

. Some data was obtained from international EPDs and literature in the absence of other alternatives. A significant amount of GWP-t data included was adapted from recent work by Jamieson, Verstraten and White [22] due to its applicability to both flexible and rigid pavement materials.

4.2. Example of Quantification of Potential Sustainable Concrete Mixtures

In order to compare the environmental costs associated with potential sustainable concrete mixtures, a range of hypothetical concrete mixtures featuring SCMs and recycled aggregates, and the respective contribution each concrete component material makes to GWP-t. The GWP-t rate adopted for each concrete material is listed in

Table 5. GWP-t values assigned to concrete materials per kg, based on data from Table 4.

Concrete Material	kg CO2-eq/kg
GP Cement	0.8700
Fly ash	0.0069
GGBFS	0.1630
Natural aggregates	0.0055
Natural sand	0.0038
RCA	0.0059
RCG	0.0099
BFS aggregate	0.00198
Potable water	0.00041
Admixtures	1.0502

and is the mean drawn from data in Table 4, reduced to a per kg rate (rather than per tonne). For the purposes of calculating hypothetical values of GWP-t for 1 m³ of each concrete mixture, transport that would occur as part of raw material supply phase (i.e., transport of raw materials to the concrete batching plant, LCA module A2) has not been included. However, for project specific LCA where distances, and therefore environmental costs associated with transport are known, they should be included, because as previously discussed, transport can have a significant impact on the environmental cost and resultant environmental saving potential that using waste materials provide, with haulage costs estimated to be between 0.08 and 0.22 kg CO_2-eq/t per km [89]. The details of each mixture composition is in

Table 6. Details of concrete mixtures for theoretical GWP-t evaluation.

Mixture ID	GP Control	FA 1	FA 2 RCA	FA GGBFS	FA 3 RCG	FA 4 BFS
Cement	100% GP	60% GP 40% Fly ash	75% GP 25% Fly ash	40% GP 40% GGBFS 20% Fly ash	75% GP 25% Fly ash	75% GP 25% Fly ash
Coarse aggregate	100% natural	100% natural	50% natural 50% RCA	100% natural	100% natural	50% natural 50% BFS
Fine aggregate	100% natural	100% natural	100% natural	100% natural	80% natural 20% RCG	100% natural

, including a conventional reference mixture without waste materials. The concrete mixture proportions in Table 6 are based on a concrete mix design used in a recent Australian airport pavement project, with w/c ratio and proportion of admixtures held constant across all mixtures, to highlight the impact of simply replacing conventional materials with waste materials.

Table 7. Comparison of environmental costs for theoretical mixtures containing waste materials.

Material	GP Control		FA 1		FA 2 RCA		FA GGBFS		FA 3 RCG		FA 4 BFS
	kg/m3	kg CO2-eq/m3	kg/m3	kg CO2-eq/m3	kg/m3	kg CO2-eq/m3	kg/m3	kg CO2-eq/m3	kg/m3	kg CO2-eq/m3	kg/m3	kg CO2-eq/m3
GP Cement	400	348.0	240	208.8	300	261.0	160	139.2	300	261.0	300	261.0
Fly ash	0	0.0	160	1.1	100	0.7	80	0.6	100	0.7	100	0.7
GGBFS	0	0.0	0	0.0	0	0.0	160	26.1	0	0.0	0	0.0
Natural coarse aggregates	1130	6.2	1130	6.2	565	3.1	1130	6.2	1130	6.2	565	3.1
Natural sand	740	2.8	740	2.8	740	2.8	740	2.8	592	2.2	740	2.8
RCA	0	0.0	0	0.0	480	2.8	0	0.0	0	0.0	0	0.0
RCG	0	0.0	0	0.0	0	0.0	0	0.0	148	1.5	0	0.0
BFS aggregate	0	0.0	0	0.0	0	0.0	0	0.0	0	0.0	565	1.1
Water	166	0.1	166	0.1	166	0.1	166	0.1	166	0.1	166	0.1
Admixtures	4	4.2	4	4.2	4	4.2	4	4.2	4	4.2	4	4.2
A1 total	-	361.3	-	223.2	-	274.7	-	179.1	-	275.9	-	270.8
Concrete batching (A3)	-	3.0	-	3.0	-	3.0	-	3.0	-	3.0	-	3.0
A1 + A3 total	-	364.3	-	226.2	-	277.7	-	182.1	-	278.9	-	273.8
kg CO2-eq/m3 reduction compared to control (%)		-		38%		24%		50%		23%		24%

then presents the GWP-t values for different concrete mixtures across raw material supply and concrete manufacturing (batching) phases, assuming all materials are at the concrete batching plant. This approach of combining GWP-t data for each ingredient of a concrete mixture to determine GWP-t of the resultant concrete has been adopted effectively in other research [60]. Mixtures featuring waste materials have been selected based on literature discussed in this review. Additionally, the reduced density of RCA has been taken into consideration, as having 85% of the density of natural aggregates.

There is a significant variation in reported environmental savings by geopolymer concretes in the literature, from 9% savings [74] to 80% GWP-t savings [77], indicating that there is a large variation in geopolymer technology. The design of geopolymer mixtures encompasses a large range of cement chemistries [94] and often involves proprietary products. Therefore, uncertainty exists around the most appropriate geopolymer mixture for airport pavement concrete. Consequently, no geopolymer mixtures feature in Table 7 for comparison.

Due to the low GWP-t of potable water, and the absence of GWP-t data for recycled water and water recovered from concrete operations and wash water, a comparison between mixtures containing different water sources has been omitted, as contribution to overall GWP-t from water is negligible [95]. An estimated concrete batching GWP-t value of 3.0 kg CO_2-eq/m³ was obtained from literature [96], to capture concrete manufacturing (A3) phase cost.

4.3. Discussion of Results of Theoretical GWP-t Calculations for Mixtures Containing Waste Materials

The totals from Table 7 demonstrate that the highest environmental saving is achieved by the FA GGBFS mixture, where replacing 60% of GP cement resulted in a saving of 50% embodied carbon compared to the GP control mixture. The FA 1 mixture, featuring 40% fly ash, achieved the next best result with a 38% GWP-t saving. To demonstrate the influence of SCM substitution on GWP-t, all three mixtures containing 25% fly ash (FA 2, FA 3, and FA 4) and varying proportions of recycled aggregates achieved 23–24% GWP-t saving, highlighting that aggregate choice did not have a significant impact on environmental cost, and environmental cost was closely linked to the amount of GP cement replaced with SCMs. This follows the same trend as other research where mixtures featuring SCMs consistently had the lower GWP-t values compared to those without [60,97]. Additionally, these findings are consistent with a study [12] that analyzed 145 concrete mixtures and concluded that Portland cement (or GP cement) content in concrete had the greatest influence on GWP-t, and that considerable GWP-t reduction and increase in durability often occur together. This finding was observed in other work [97,98] highlighting the value of SCMs in both reducing GWP-t and increasing durability performance through refining the concrete pore structure. While the social cost of each mixture has not been specifically evaluated in Table 7, it is evident that the FA 2 RCA and FA 4 BFS mixtures show the highest potential to preserve natural aggregate resources by substituting 565 kg of natural aggregate, which is a significant benefit of incorporating waste materials.

However, as previously discussed, these finding should be taken with caution, as the GWP-t for transport of materials has not been considered in the theoretical calculations in Table 7, but literature demonstrates large transport distances can offset the befits of using waste materials such as SCMs [16,99]. While not included in theoretical calculations in Table 7, the literature shows that the use of geopolymer concretes appears to represent the greatest potential to reduce GWP-t (up to 80%) of concrete due to the complete replacement of Portland cement with SCMs, and a full-scale trial exists in Australia [77] demonstrating that geopolymer concrete is constructable as airport pavement. Monitoring of this pavement over time will add to field data on long term performance [94] and will benefit airport pavement practitioners. Proving performance is critical when using waste materials, to ensure sustainability benefits are not overridden by increased major maintenance [22], with the following section proposing a framework for implementation that factors performance history of different technologies.

Finally, it is noted that the environmental costs used in the above analysis was deterministic. To better consider the variable embodied carbon in the various materials, a stochastic analysis is required. Furthermore, the environmental cost is only one element of the TBL approach to comparing sustainable and conventional pavement materials. Consequently, the analysis should be extended to include the financial and social costs of the various concrete mixtures, and the TBL values balanced against the performance and durability risks associated with each potential opportunity.

5. Implementation of Waste Technologies

In countries like Australia, the use of recycled materials in road infrastructure is growing [20], but adoption in airside pavements remains limited. Barriers include conservative and non-contemporary design standards, a lack of long-term field data from Australian trials, and uncertainty around lifecycle performance. A risk-based approach helps bridge this gap by encouraging implementation of low-risk technologies, in low-consequence pavement areas until confidence in the technology has matured.

Firstly, confirming that the concrete pavement proposed meets the required performance criteria is essential to consider it worthy of trialing. For concrete pavements this includes the elements of constructability (encompassing how workable the mixture is, if it is suitable in its wet state to be mixed, transported, placed and finished efficiently), strength (flexural strength in its hardened state,) and durability (ability to withstand the environmental conditions it is subjected to over its 40+ year life) [7,35,39]. These are all elements that are controlled by engineering specifications, and are subject to laboratory testing regimes [2,70]. The use of prescriptive, rather than performance-based specifications, has been identified as a barrier to the efficient uptake of sustainable concrete technologies at Australian airports [18,21,77], and the issue of prescriptive specifications is not isolated to Australia [12]. Furthermore, risk aversion in the construction sector more broadly is further highlighted by Infrastructure Australia [100], noting that fear of risk exposure is a key barrier to pursuing decarbonization efforts.

Data collected on sustainable concrete airport performance through field testing and trials over time will improve the Australian based body of knowledge and support increasing adoption of sustainable concrete technologies as confidence grows. Australian road practice provides data on history of use cases, particularly the use of SCMs [101], and should provide confidence in adoption of waste material technologies. Stringent standards that apply to waste concrete materials [37,51,79,84], and lab and field trials required in concrete specifications means that the likelihood of encountering significant pavement issues is ultimately low.

Secondly, the location of the pavement on the airfield determines the likely consequence if unserviceability arises from pavement issues [22]. Consequences include potential safety hazards, operational disruption that could cause social displacement, reputational and financial costs, and excessive pavement maintenance or replacement costs [7]. As discussed above, rigid airport pavements are used across aircraft support areas, taxiways, aprons and runway thresholds, and pavement failures in each of these locations presents a different consequence to aircraft and operations. The resulting risk is also highly contextual to each individual airport, based on the aircraft type, frequency and nature of operations supported by the pavement network. Therefore, a consequence-based hierarchy that ranks implementation categories (IC) from low consequence to high consequence is proposed at

Table 8. Consequence-based hierarchy of pavement areas to implement sustainable concrete technology, adapted from Jamieson, Verstraten and White [22].

Consequence If Pavement Fails	Implementation Category	Area	Examples	Justification
LOW	IC1 Suitable for testing new technologies	Airside support areas	Aircraft hangars, ground service equipment parking areas, aprons for light aircraft and helicopters	These areas have moderate loads and are less operationally critical, allowing for early adoption and performance monitoring with lower consequences if issues arise
		Pavement shoulders	Taxiway and apron shoulders	These areas are not subject to regular aircraft loads, allowing for early adoption and performance monitoring with lower consequences if issues arise
MEDIUM	IC2 Suitable for implementing technologies with some history of good performance	Aprons	Passenger and cargo aprons, parking bays	These areas are subject to aircraft loads and may have more operational redundancy, lowering the consequence if issues arise
HIGH	IC3 Suitable for implementing technologies with significant history of good performance	Taxiways	Taxiways that connect runways to aprons and other aircraft areas	Subject to aircraft traveling at low speed, serviceability issues can have high consequences if only one taxiway available
		Runways	Runways and/or runway thresholds	Subject to aircraft traveling at high speed, low speed or stationary, serviceability critical to maximizing full length runway operations

, to enable airport owners, designers and engineers to inform risk-based decisions about where they implement sustainable concrete pavement technologies. The implementation hierarchy was adapted from Jamieson, Verstraten and White [22], and includes three implementation categories (IC); IC1, IC2 and IC3, reflecting the amount of history of good performance from low to high. This work by Jamieson, Verstraten and White [22] also includes implementation strategies. Runways and taxiways have the highest level of regulated safety requirements [102] and are most impacted by pavement unserviceability, especially at regional airports where there may be only one runway, and one connecting taxiway, providing critical transport services for a large community [28]. This contrasts with airside support areas that are subject to less loading, less frequent operations or operation of aircraft under tow, where pavement issues are unlikely to cause significant disruption.

Ultimately, applying a risk-based approach enables gradual, evidence-based integration of waste materials into rigid airport pavement practice, balancing innovation with the critical need for safety and reliability in airport pavement systems. This supports the broader circular economy in Australia, and the associated net-zero carbon goal [8] while maintaining aviation operational standards and safety.

6. Conclusions

Based on a review of current research, the literature and case studies, it is evident that the incorporation of waste material into airport rigid pavements presents a credible pathway to increasing sustainability. This research has demonstrated that incorporating waste materials into rigid airport pavement structures offers substantial environmental and social cost benefit potential. Additionally, this work demonstrates how airport industries yet to fully incorporate waste technologies in rigid pavements can look to the existing specifications used by other industries for immediately implementable options to do so. Concrete pavement as the primary load-bearing element has been shown to accommodate a range of sustainable substitutions, particularly through the use of low carbon cementitious materials (e.g., SCMs) such as fly ash and GGBFS, which significantly reduce the embodied carbon of concrete. Environmental savings of up to 50% are possible through using mature SCM technology and proportions commonly adopted by road authorities. Similarly, materials like RCA, RCG, and BFS aggregates provide viable alternatives to virgin aggregates, enabling resource conservation and landfill diversion. However, theses do not offer the environmental savings possible when using high proportions of SCMs. Importantly, data from the literature and real-world applications—such as Changi Airport and Toowoomba Wellcamp Airport in QLD—provide evidence of the successful constructability of waste materials in airport pavement contexts. The findings reinforce that sustainability in airport pavement construction must be grounded in performance-based assessments, considering not just environmental metrics like GWP-t, but also lifecycle cost and serviceability over a 40+ year design life. As a result of this research, it is recommended that a stochastic TBL analysis is conducted, to objectively compare conventional and sustainable pavement mixtures, ensuring that environmental gains do not lead to operational or financial compromise. Furthermore, a standardized, performance-based specification for rigid airport pavements should be investigated. This will enable airport owners, designers and engineers to pursue concrete pavements that feature waste materials, reducing carbon emissions and preserving natural resources in pursuit of sustainability.