1 Introduction

Aggregate is any coarse- to medium-grained particulate material, such as sand, gravel, crushed stone, or any other material used for construction purposes. Aggregates provide the necessary compressive strength while taking up a bulk of space. They may be used alone for unbound railway ballast and road pavement layers or may be mixed with cement or bituminous material to form a bound concrete, asphalt mix or mortar for construction (Christie et al. 2001). Aggregates are the most mined and most used material in the construction industry, second to water (Menegaki and Kaliampakos 2010). The aggregate market plays a major role in the economic development of New Zealand. Increased demand can also be considered a consequence of economic development while creating significant employment opportunities.

Although they are present in abundance in most countries, including New Zealand, aggregates are still considered a non-renewable mineral resource. With increased consensus about environmental effects, cultural values, and sustainability imperatives, current practices in the planning and extraction of virgin materials are not considered sustainable. The challenge is ensuring access to appropriate materials geospatially by connecting supply and demand. Failing to do so would increase emissions and congestion and accelerate infrastructure deterioration (Langer and Tucker 2003). From an environmental and social perspective, some of the problems with aggregate material extraction are noise, pollution, disturbing waterbeds, and visual hindrance. Some of the economic consequences of the current aggregate production methods include supply–demand mismatch, increasing price of infrastructure and deterioration, loss of nearby land value, and less incentives to use recycled materials.

The ultimate aim of this paper is to better understand transport sector requirements in relation to the access, supply, demand, and use of aggregates to enable sustainable sourcing of materials in a specific case study in New Zealand. This will help in the development of a national coordinated strategy and action plan to optimise material use within the transport and wider infrastructure sector. This paper sheds light on some of the practices currently employed (successful or unsuccessful) to manage the supply and demand of aggregates and to enable improved planning and use and reuse of the resource in a more sustainable manner. The specific objectives of the research are fivefold:

  • To understand the current and predicted future national picture for supply and demand of aggregates in the transport and broader construction industry to inform a national sustainable aggregate sourcing strategy for New Zealand

  • To understand how aggregate supply and demand forecast data is currently collated/reported to inform decision-making

  • To establish a baseline of current use of different aggregate materials, including recycled and re-used materials

  • To inform development of methodologies/tools to enable robust collection/forecast/reporting and geospatial representation of national supply and demand

  • To make recommendations for improving access to and supply of sustainable aggregate resources

The remainder of this paper is structured as follows. First, Section 2 reviews and discusses some of the key challenges and issues to be addressed for sustainably managing the supply and demand of aggregate in New Zealand. Section 3 analyses aggregate and land-use consents, as well as a novel telematic data to investigate a possibility of a national picture of aggregate demand and supply in New Zealand. Finally, Section 4 discusses and recommends an integrated effort across the transport authorities in New Zealand for the development of this national picture and strategy before Section 5 concludes the study.

2 Key Issues with the Sustainable Use of Aggregates in New Zealand: Evidence from the Literature

New Zealand has a large supply of quality aggregates (endowment), but it is unevenly distributed geo-spatially and not necessarily close to demand for infrastructure. New Zealand has a relatively low traffic volume road transport system when compared to some northern hemisphere countries due to having a low population density per square kilometre. This low tax base for a significant road network length (approximately 94,000 km of road network for approximately 5 million people) has meant that the road pavements are predominantly unbound flexible granular pavements. Even new expressways and motorways around the major urban centres of New Zealand are designed with relatively thin layers of asphalt mix pavements.

Although the aggregate supply is technically large nationally, current sources close to centres of demand are becoming depleted or operationally limited due to urban encroachment. The latest NZ Government Policy Statement on Land Transport (Ministry of Transport 2020) outlines record levels of investment ($48 Billion) planned over the next decade on transport-related infrastructure development throughout the country, requiring aggregates of various specifications. This places a heavy emphasis on supplying aggregate resources for roadway infrastructure and mainly in the Auckland region. According to Welvaert (2018), aggregates are mostly supplied from nearby sources because of the geographic dispersion of quarry locations (this can be seen in Freeman (2020) and also Fig. 3 in this paper). Aggregates are almost entirely transported by road. Aggregate transport accounted for only 11% of freight transport in 2012/2013, and there is rarely any aggregate freight transport between regions, with the exception of Auckland (Ministry of Transport 2017), where aggregates are mainly supplied from other regions in the North Island, making the product expensive and difficult to access. Auckland is expected to continue dominating the demand for aggregates in the future, based on residential growth driving demand for houses, roads and infrastructure to support the expanding economy. In 2018, 40% of the total construction value and 39% of new dwelling consents in New Zealand are attributed to Auckland (MBIE 2019).

Production of aggregates is also uneven, with premium grade aggregates accounting for approximately 10% of all output, and the highest grades sourced from a minority of quarries (Lane 2017). In turn, survey and case study findings suggest inadequate or unreliable supplies of recycled aggregates (either due to distance, available volumes or quality) as impediments to specifying recycled aggregates into contemporary projects (O’Donnell et al. 2018). Respondents reported that demand far outstrips supply but reported that the cost of upscaling the production of aggregates may reduce the viability in an economy of New Zealand’s size and geography. Despite the growing cost of virgin aggregates as a result of dwindling supply, consumers consistently ‘over-specify’ virgin aggregates when there are viable alternatives from technical, policy and economic standpoints (described by stakeholders as ‘unnecessary demand’ by Lane (2017)) (O’Donnell et al. 2018; Mora et al 2019).

The use of recycled materials for aggregate use is increasingly encouraged to minimize the reliance on virgin products. In New Zealand, the Cement and Concrete Association (CCANZ), in partnership with BRANZ and supported by the Aggregate and Quarry Association of New Zealand, published a guideline report for the use of recycled materials as aggregates in New Zealand (CCANZ 2013).

The challenge with the use of recycled products is the need for quality sorting, production processing, quality assurance and auditing which mandates the use of the latest technology and management systems to supply the final quality product. New Zealand road pavements are largely designed and constructed as flexible pavements, due to the country’s relatively low traffic volumes in comparison to more dense country populations and consist mostly of multiple layers of unbound granular construction. They typically consist of three layers above the subgrade. The quality of the pavement layers generally decreases with increasing depth and reduced stress and strain from induced and repetitive vehicle traffic loading.

Evidence from the literature suggests that the historical failure to account for the environmental and social costs and benefits retains the cost–benefit balance in favour of virgin materials (Lane 2017; Ministry of Transport 2014; Slaughter 2005; Wu et al. 2015). Shifting the balance towards recycled aggregates would require, for example, a more wholistic costing approach. Transport costs, for example, were identified as critical factors which could go unmeasured on a per vehicle per load basis (Baas 2012). Local and international literature thus cited the need for a life-cycle perspective that factors in key barriers: (i) perceived risk from supply chain issues as well as performance costs and working outside of standard practices, and that (ii) using alternative materials requires additional effort managing supplies and additional signoff or work to demonstrate benefits.

3 Towards a National Picture of Aggregates for Land Transport Infrastructure in New Zealand

There is currently very little literature or knowledge on the demand for aggregates for land transport infrastructure per section length of infrastructure type in New Zealand. Some countries (e.g. Canada) have undertaken studies to determine the ranges of commodities (raw materials) per building type and some ranges for infrastructure for utilities (Savoy 1996). The types of building and transport infrastructure are however very different in New Zealand to North American practices and cannot be directly related.

This section reviews a number of existing data sources in New Zealand to determine generic aggregate quantities in relation to various types of transport infrastructure and maintenance treatments. Investigated data sources include:

  • Road Asset Assessment and Maintenance Management (RAMM) database

  • Proprietary Road Controlling Authority maintenance databases, e.g. Auckland Transport

  • Infrastructure Commission Forward Works Programme (recently developed and expanded from the Christchurch Rebuild forward works programme)

  • New Zealand Petroleum and Minerals (NZP&M) data held at Ministry of Business, Innovation and Employment (MBIE)

  • Specific project As Built construction drawings and Quantity Estimates from schedules of Quantities

  • Geo-spatial modelling of mineral resources

There are a number of existing sources of data from geo-spatial data to databases that relate to the supply and demand for aggregates for infrastructure projects held within various central, regional or local government agencies. None of the data sources are currently in a form that can be easily analysed to gain cross-sector inferences from the data in regard to the national supply and demand for aggregates in relation to future infrastructure demand. Table 1 identifies the various sources of data, their type and their known advantages/disadvantages in respect to being able to use the dataset to improve resource efficiency, industry sector understanding or enable improved sustainable practices.

Table 1 Summary of existing data sources
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3.1 A Geological Map of Aggregates and Natural Endowments

In New Zealand, while most aggregates for road pavements are sourced from greywacke and volcanic rocks from crushed rock quarries, a range of different rock types can be recognised within these two large groupings. Each rock type produces aggregate with a matrix of properties which are determined by the nature of its mineral and fabric within the rock. As shown in Fig. 1, greywacke aggregates are the predominant source rock that by location forms approximately 75% of aggregates used for land transport infrastructure. However, greywackes have very variable properties, and there are five different types that can be recognised each with a distinctive matrix of engineering properties (Black 2009). Two of the greywacke types (Waipapa and Torlesse type) have very high crushing resistance values. The low contents of fines produced can mean that these aggregates sometimes have difficulties with achieving target particle size distributions within the M/4 prescribed ‘premium’ quality envelope without further production processing and can in some instances be categorised as ‘marginal’.

Fig. 1
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Aggregate geological source characteristics in New Zealand (source Black 2009)

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In some areas of New Zealand (e.g. western central north island and eastern plains of the south island), alluvial gravels are derived from the Torlesse-type greywackes and form the axial ranges of both the North and South Islands. Greywackes have variations in properties. Large areas of in situ greywacke are being eroded to shed material into the alluvial river systems. The individual pebbles/boulders show a range of grain sizes and composition although all appear to be Torlesse-type. Very small amounts of chocolate or reddish coloured chert and igneous pebbles (both found in Torlesse–terrane greywacke sequences) can appear in some gravels. Natural sorting and abrasion during river transport have largely eliminated all the weaker rocks and generally provide a very clean resource which produces aggregate with properties in the high end of Torlesse-type greywackes. The Canterbury plains around Christchurch in the South Island for example largely have aggregates sourced from alluvial sources for this reason.

Volcanic rocks that form approximately 25% of aggregates used in road construction are a major resource for production of aggregates in the North Island. The quality and nature of the aggregate resource is a function of the rock type (its chemistry and mineral content), and the environment in which it was erupted. Three different types of basaltic aggregate sources are recognised: young intraplate basalts extending from Bay of Islands to South Auckland and west Waikato, ophiolite basalts (Northland and East Cape) and arc-related basalts (includes basaltic andesites) in Northland region, Coromandel Peninsula and the Rotorua-Taupo Volcanic zone. Other volcanic rocks used in road aggregates are andesites that are commonly quarried in Northland and the Taupo region and are the major aggregate resource for the Coromandel and Bay of Plenty regions. Some dacites and rhyolites in the Taupo region are also used that have erupted as part of the arc-related volcanism along the eastern side of Northland region and in the Taupo area.

3.2 Analysis of Aggregate Extraction and Consents

Quarrying consents from Regional Councils and Territorial Authorities in New Zealand between the period of 2016 to 2018 are analysed. A total of 509 resource consent information was received from 34 Territorial Authorities relating to aggregate extraction or associated activities. Analysis of the consents revealed a lack of consistency in the information provided. The review of quarrying consents data unfortunately demonstrated that very little useful data can be currently obtained from both quarry consent applications and the responses from consenting authorities. In general, there was a very large variation in the quality of the quarry consent applications with a significant proportion not even identifying the source of aggregates that they were seeking consent to extract. This meant that aggregate quantities, quality, demand (in regards to truck movements) and the effects to local communities, environment, other resources (e.g. water) and iwi/hapu were mostly not appropriately evaluated by the consenting authority. There were however a few good examples that could be used to help create a template for applicants to use in the future.

3.3 Analysis of Infrastructure Planning and Design Construction Data

The only data source where actual quantities in relation to specific aggregate product demand could be related to various infrastructure types are from Project Infrastructure files, As Built drawings, typical cross sections and tendering schedule of quantities held within Road Controlling Authorities of New Zealand (RCA) or their consultant offices. To gain this information and relate it to variables like the subgrade strength and the design traffic volumes is a manual process, very time- and resource-intensive process and in many cases, it is challenging to gain access to project files. For this research, it was determined to be of better value to determine typical ranges of demand for aggregate quantities (basecourse and subbase) per kilometre given various design scenarios (e.g. low or high underlying subgrade strength and low, medium or high traffic volume loadings), from various road cross-sectional standards from recent and planned infrastructure projects. The estimates in demand do not include typical bulk earthwork volumes to bring the road formation up to or down to the subgrade formation level where the pavement layers will be placed above. The quantities also do not account for aggregate demand for concrete (e.g. bridges, kerb and channel, footpaths, cycleways, stormwater and other utilities, retaining walls or specific aggregate drainage/bedding materials) and are therefore a subset of total infrastructure aggregate demand. Table 2 summarises unbound pavement material layer aggregate demand data calculated from various sources for new road constructions.

Table 2 Aggregate demand in road pavement layers by various road types
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The demand for aggregates for transport infrastructure can vary significantly as a function of traffic volumes and underlying foundation strength. Premium basecourse quantities per kilometre vary from a minimum of 3250 tonnes for a local urban road to up to 13,000 tonnes per km for an urban principal arterial and 11,000 tonnes per km for a 4-lane rural divided expressway. Additionally, subbase aggregate quantities for the same road type can range from 3750 to 36,100 tonnes per km for urban roads and up to 39,100 tonnes per km for a rural 4-lane divided expressway.

Alternative pavement design strategies can be deployed to reduce the depth of pavement layers and quantities of aggregates on low-strength foundations by subgrade or aggregate improvement techniques (e.g. in situ lime or cement stabilisation). These methods can both reduce aggregate dependency and optimise pavement costs per square meter or lineal pavement length but in many existing cases, this resource use optimisation is not adequately considered through the investigation and design stage of infrastructure projects. More sustainable resource usage can be encouraged by incorporating design and selection strategies that look to minimise the carbon footprint of transport infrastructure construction and maintenance practices. However, currently, there is very little existing data available to enable this kind of design strategy analysis to be undertaken.

3.4 Analysis of Aggregate Transporting Distance

In this research, telematics data was used from EROAD to analyse truck journeys. EROAD is one of New Zealand’s leading providers of fully integrated technology, tolling and services. The term ‘telematics data’ refers to a combination of data in ‘telecommunications’ and ‘informatics’. EROAD provides the telecommunications system to send, receive and store the location and timestamps of a large proportion of commercial truck fleets in New Zealand. Figure 2 illustrates the life cycle of aggregates from suppliers of Virgin and Recycled materials to construction sites and then to recycle yards. We focus on the hauling step in this circular economy, in particular the transportation of aggregates from quarries to construction and maintenance sites.

Fig. 2
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The aggregate life cycle

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The ultimate aim of this analysis is to look at the current distances of aggregate transport journeys and to obtain insights on how they could potentially be reduced to a shorter, ideal distance to minimise the costs and negative impacts of transporting aggregates. The study initially found and visualised the locations of suppliers, i.e. aggregate quarries, on a geographical map using Land Information New Zealand (LINZ) GIS data. Since there are over 1000 quarries in the LINZ dataset, focus was made on a smaller selected list of 34 quarries across New Zealand to demonstrate the benefits of data analysis. Figure 3 illustrates the locations of these quarries spread throughout New Zealand.

Fig. 3
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Map of interested quarries across New Zealand

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The EROAD data have been collected with the following considerations:

  • Data is from 1 Jan 2019 to 31 Dec 2019

  • Only heavy vehicles were included

  • Quarries and worksite have an approximately 300-m buffer to include locations that are not perfectly inside the provided geometry

  • Trips are derived from either ignition or vehicle moving to either ignition off or vehicle stopped. This is to capture trips that do not ignition off at the worksite

  • Trips are combined into chains by grouping trips that have less than 5 min between the end of one and the start of another. Smaller trip chaining thresholds result in longer trips

  • Trip chains in this Origin–Destination matrix must start or stop in one of the supplied quarry or worksite

  • We have excluded trip chains that start and stop in the same place

We take one significantly large worksite as an example to determine how truck movements carrying aggregates are hauled. The chosen project is the Huntly Expressway site (see Fig. 3), which is a 15.2-km bypass of the Waikato Express motorway in the North Island. The Huntly Expressway is one of the recent important road infrastructure projects in New Zealand. The site is surrounded by a large number quarries. The Huntly Expressway was under construction during 2019 and the time EROAD data were analysed and prior to the site being open to general traffic. An assumption was made that all trucks that visited this worksite from a quarry would be carrying some aggregates.

Figure 4 visualises the truck movements between the quarries and especially to the Huntly Expressway worksite using a Chord diagram. A Chord diagram represents flows or connections between nodes. In this case, the nodes are either aggregate quarries or the Huntly Expressway (highlighted purple segment). The size of the fragment represents the popularity of the node in the data. The arcs between each node are proportional to the number of the journey between that pair of quarries.

Fig. 4
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Number of truck journeys between the studied quarries and the Huntly Expressway

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Figure 4 shows several large quarries with many truck movements, such as the TeKowhai, Horotiu, Waikanae and Stevenson quarries. This is expected as these quarries are among the largest in NZ. The figure also shows a Huntly Expressway worksite, which also attracts a significant number of truck trips. Figure 4 demonstrates these truck journeys between the Huntly Expressway worksite and the studied quarries in more details. It can be seen that the majority of the journeys to and from the Huntly Expressway are from quarries that are close to Huntly, such as the TeKowhai, Horotiu and Tahuna quarries. However, there are 11 different quarries that had trucks coming from them to the Huntly Expressway worksite (or the other way around). Many of these 11 quarries have a long distance to the worksite, which suggests a potential inefficiency in the hauling distance of aggregate. We further explore this fact in Fig. 5a and b. The figures show that for both travel time and travel distance, the distributions are highly skewed to the left, showing that the majority of journeys are short. Occasionally, aggregates may be transported from over 300 km away taking over 7 h to haul in.

Fig. 5
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The distribution of a travel distance and b travel time from studied quarries to the Huntly Express

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Figure 6 illustrates the distribution of journey start time at different times of the day. The figure shows that the majority of truck journeys start within the peak periods from 5:30 A.M. to 3:30 P.M., when there are higher traffic volumes. These typical construction work day time period constraints may also be due to quarry or construction site working conditions required, for example to reduce noise in communities or on specific urban roads. Some of these issues and constraints may be addressed in the near to medium future with the availability of more electric (to reduce noise) and autonomous fleets (for driver-free transport of aggregates). More advanced optimisation of truck routes carrying aggregates and truck start time may also find time windows and routes that satisfy certain requirements of noise and travel time. These routes may potentially be longer, but travelling during off-peak periods may mean shorter travel time and better overall efficiency. These specific optimisations are out of scope for this project, but can be explored in future research.

Fig. 6
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Distribution of truck start time within the time of day

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The current dataset was not originally designed for the analysis of aggregates and has the following limitations that are hindering our understanding of the aggregate life cycle:

  • The type of aggregates being transported is missing (e.g. Virgin vs Recycled aggregates)

  • Although the distance, location and trip distribution (spatially and temporally) are available, it cannot be confirmed that the trucks were transporting aggregates. This can only be an assumption, e.g. trucks coming to the Huntly Expressway worksite would likely be carrying aggregates

  • The trip-chaining procedures also used some assumptions such as the state of the trucks’ ignition and a 300-m buffer zone around the quarries or worksites. This is because the information on the trip purpose of the trucks is missing

  • There is no information on the reuse/recycle step in the dataset

A potential future telematics system that could collect dynamic data of materials logistics from place of origin to destination could become a very useful raw resource use system in the future. The ideal dataset should have detailed data of individual journeys of truck fleet carrying aggregates from quarries to construction sites and recycled material yards. The included variables can be classified into trip-related and truck-related variables:

  • Trip-related variables are specific information about the truck journey carrying aggregates, such as travel time, mean speed, distance, travel delay and the type of roads that the truck travelled on

  • Truck-related variables include information on the truck used for the above journey, such as the type of aggregates being transported, the weight/volume of aggregates, the total capacity and the type of the truck

4 Discussions on a National Picture of Aggregates

Aggregates are an important non-renewable resource for land transport and building infrastructure. While New Zealand has an abundance of rock suitable for aggregate, local supply does not always match local demand for the construction, maintenance and recycling of road infrastructure. Due to a deficit in infrastructure development for a number of decades, strong population growth in many areas and much of New Zealand’s public infrastructure is coming to the end of its useful and/or economic life — there is an increasing demand for aggregates in many regions of New Zealand. Some regions of New Zealand have difficulties sourcing appropriate materials locally for infrastructure purposes, and there are increasing sensitivities to the extraction of aggregates from communities and iwi/hapu who have experienced and seen the effects of poor industry extraction and environmental practices and the lack of monitoring or little consent conditions regulated. Very little appropriate data is currently available either nationally or within regions to sustainably plan, manage and use and reuse/recycle aggregates for various public or private infrastructure and therefore be able to forecast future aggregate demand (Wilson et al. 2022).

The ‘ownership’ of aggregates is complex. It is determined by both land ownership and mineral rights, held by a combination of public, private and iwi parties, in temporary and permanent arrangements. Limited access to information on ownership makes it difficult to identify existing and future supply options, including how difficult it might be to establish new supply in key locations. It also means that the governance of aggregate resource is unclear. We recommend expanding the Infrastructure Commission Forward Works Programme to allow future planned infrastructure to be broken down to demand by region and including primary raw materials — this can be linked to the 30-year Infrastructure Plan and the demand calculated by the Waka Kotahi resource quantity database for each region.

Alternative materials are less well documented in industry literature, compared with virgin materials. Previous research has established that alternative materials are perceived as risky and require more efforts than virgin materials, in part because information about their production and use is limited. The implication of this invisibility is that opportunities to plan for and use recycled materials are missed. Finding pathways where better information can be used to share the risk of using alternative materials and making their use a higher priority should be explored. Regions/areas should be identified and targeted where there are opportunities for increased use of recycled materials, (e.g. where there is critical mass in urban development — Auckland, Wellington and Christchurch) and correspondingly where there are significant natural aggregate supply constraints (e.g. Northland, Hawkes Bay, Horizons and Wellington regions).

Practices vary considerably across New Zealand, in terms of aggregate supply and demand. It is not currently clear whose responsibility it is to improve practices. A whole of sector approach is required to improve practices by adopting a value approach. Waka Kotahi is in a good position to provide leadership through the development and active dissemination of a complete ‘use of aggregates’ packages (O’Donnell et al. 2018). An example of these packages can be a sustainability rating scheme that promotes the use of recycled and the reuse of materials (e.g. ISCA) to prioritise low carbon emission options. An upskilling education programme is required to ensure decision makers are not only aware of the complex issues but to become aware of their own organisational biases that affect values and norms and that can reinforce poor resource use practices and outcomes. Updated guidelines and specifications on the use of all aggregates are part of reducing risk and increasing comfort levels in the sector (Ivory and Bagshaw 2020). Guidelines can provide varying levels of detail, providing ‘rules of thumb’ for sector-wide guidance down to more detailed guides and specifications for specialists. Relevant to the supply and demand issues discussed in this report, guidance can include how to determine whether materials are ‘fit for purpose’ and reducing the risk for decision-makers by providing performance measures. Guidance can also include safe stockpiling of different materials, including recycled materials, which could increase confidence in supply lines. For guidelines to be effective in changing practice in the selection and use of materials, they need to be consistent, accessible and useable across the whole aggregates sector.

There is a need to develop an aggregate data integration framework to where possible standardise/collate and improve aggregate data information at both the national and regional levels for the extraction and processing of aggregates (supply) in regard to both quantities and quality to allow wider use. This framework will enable us to investigate how aggregates and potentially other key raw resource materials could be tagged, identified and electronically tracked from place of origin to destination. Figure 7 illustrates the key elements of this ideal data framework. This would in turn allow remote data analytics and infrastructure condition monitoring to be developed throughout the various stages of the infrastructure life cycle of aggregate materials from source to place of use and to enable the more sustainable use of aggregate potential minerals and associated resources. The framework will establish a national infrastructure resource quantity and pricing database for each region and integrate into a national database so as to improve an understanding of aggregate demand by infrastructure typology and maintenance activity. This framework could extend to all RCAs for projects over a certain threshold and that receive National Land Transport Funding and be part of the currently required achievement data. The framework should include as a separate module the projected land transport future demand for aggregates by region to enable forward planning. This will require integration of data on aggregates from various systems, regions and aggregate sectors that differentiate by aggregate product quality to more sustainably manage the value and effects of aggregate resources. National usage of resources by product quality and purpose (that includes recycled materials and associated resources — e.g. Water and by product quality) should become mandatory to report to NZP&M and/or regional/local authorities on a quarterly basis (currently this is voluntary for non-Crown owned lands) for all suppliers of aggregate as part of the consenting requirements. Quarterly information will allow much better forecasting of demand within regions and nationally. It will be important to communicate the value proposition of the reporting to all stakeholders to provide context.

Fig. 7
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Aggregate data needs through the life cycle of land transport infrastructure

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5 Conclusion

The aggregate market plays a pivotal role in the economic development and well-being of New Zealand by matching the supply of construction materials with the demand for both maintaining existing infrastructure and facilitating growth of the built environment. The main demand for aggregate is in the highway construction and rehabilitation sectors. New Zealand went through a period of more than 3 decades of underinvestment in transport in the 1980s, 1990s and early 2000 decade. In the last two decades, funding investment has significantly increased, and transport taxes are now fully hypothecated to transport with additional Crown investment into capacity and safety infrastructure improvements. While transport is not the sole user of aggregate material, as aggregate is also a key raw material for building and housing, the transport sector in New Zealand accounts for up to 50% of demand. Previous studies have highlighted the need to better manage this important non-renewable mineral resource, ensuring the use of aggregates and both premium and alternative resources (that include recycled materials) are better planned and managed to ensure use is not only efficient and effective but transitions towards more sustainable practices and takes a long-term view. This paper sets out to fill a knowledge gap as there is currently no national picture of, or strategy in place for considering aggregate extraction, the ongoing and future demand and long-term supply of aggregate materials to enable sustainable aggregate sourcing and land use/environmental effects. Key issues include:

  • A lack of understanding, data and knowledge of the factors influencing supply and demand that account for quality needs of product and alternative resource options

  • Expediency of decision-making using virgin ‘tried and true’ methods alongside risk averseness of organisations and a reluctance to share risk

  • Perceptions that recycled materials are inferior products

  • Long-term planning and forecasting demand requirements

  • Community and cultural sensitivities in regard to quarrying, extraction practices and land use

New Zealand is abundant in aggregates for road transport infrastructure. However, at the regional level, scarcity becomes relevant due to unequal spatial distribution, population density and growth pressures. Recovery of aggregates impacts other stocks of natural resources, including water ways and ambient air quality. Community and cultural sensitivities especially for iwi and hapu contribute to a decline in the stock of potential aggregate resources in some areas, especially the larger urban regional areas and regions where unsustainable or poor historical practices have created increased sensitivity.

Recent advances in recycling technology provide potential partial substitutes in large urban areas where scale enables important critical mass, although they too face significant pressure from communities not wanting recycling facilities in their area in addition to the lack of experience with various technologies or quality process controls. However, there are significant barriers to the increased uptake in the use of alternative materials (both recycled and local non-premium virgin quarried materials) that have to date prevented more sustainable use of aggregate materials. Aggregate recycling can occur to produce a product for most of the pavement layers — it depends upon the cost in comparison to other available material sources. Generally, to gain the greatest gain in recycling, the higher value aggregate products need to be targeted first, but perceptions of low-quality recycled aggregates often lead to them being reused into lower value pavement layers. This may be true if processing of recycled materials is not carefully controlled. Recycled aggregate materials include ‘on-site recycling’ where maintenance interventions can reuse existing materials by including the addition of stabilisers to improve the in situ performance and/or extend the life of the asset, in comparison to the use of a transported material of higher quality.

This research highlights the multi-sector aspects and complexity in the issues surrounding the more sustainable planning, extraction and use of aggregates as non-renewable mineral resources. The effect of aggregate extraction on associated resources is not well understood, and there are many examples where historical uses have not been adequately monitored or regulated resulting in poor environmental, community and cultural outcomes. It is also clear that whilst New Zealand nationally has a combined abundance of good quality endowment in aggregates, there are various regions that have significant constraints in being able to sustainably source quality (premium) aggregates and increasing aggregate demand. This is especially so when there are large infrastructure projects that outstrip previous demand levels making the sourcing of aggregate for the various users of aggregate within a specific region (e.g. Road Controlling Authority, Government, Council or even large private sector project) for new infrastructure investment or asset maintenance needs compete against each other. Data on the demand quantity for aggregate from infrastructure construction and maintenance projects throughout the infrastructure life cycle are difficult to obtain due to the multiple agencies and industries involved and the lack of integration of asset systems. There are no simple fixes to these issues as there are significant difficulties in obtaining the appropriate data to manage aggregate resources at the National and Regional level.