ROAD SURFACING AND DESIGN
ROAD MATERIALS UNDER THE MICROSCOPE by Michelle Goldsmith, Journalist, Infrastructure magazine
The materials used in construction are of vital importance to a road’s safety, performance, maintenance requirements and longevity. They also significantly affect the construction costs of a road project and the total lifecycle cost of the asset. In the first article in a series on road construction materials, Infrastructure takes a closer look at the different layers making up a road and the varying properties required for different applications.
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urrently, the majority of Australia’s biggest and most expensive infrastructure projects underway involve constructing or upgrading our roads. According to 2017 estimates, road projects spend around $5.1 million per lane kilometre on average, varying by road class. The pavement materials contribute significantly to these costs, at an average of $233 per square metre for an urban motorway or freeway, or $114 for a major rural road. Heavily trafficked major roads are the most expensive, while more minor roads with a lower-traffic load tend to be cheaper. A wide range of materials can be incorporated into roads, with different mixes exhibiting different properties that affect their suitability for various applications and environments. Foremost, the material mixes used for this essential infrastructure must be fit-for-purpose due to the significant implications for the road safety, functionality, longevity, maintenance requirements, and total lifecycle costs. For any road project, careful consideration of the materials used is required, especially when certain materials are in short supply, or when looking to reduce costs and waste, while optimising infrastructure performance. MATCHING MATERIAL AND APPLICATION A variety of factors affect the materials that might be suitable for any given road project. For instance, a heavily trafficked road in a high-rainfall area will have different requirements to a minor local road in a dry area. Key factors affecting road material suitability include: ♦ The intrinsic physical properties of the material (e.g. structural stability, heat resistance, water permeability) ♦ The intensity of traffic loads ♦ The climate and moisture regime of the environment ♦ The availability of material sources ♦ Budget constraints
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March 2021 // Issue 18
The available road materials and their properties have significant implications for the structural design of the pavement and entirety of the road. If suboptimal materials for the application and environment must be used, additional measures will need to be taken to ensure the road meets performance standards. These may include different pavement structures to increase durability, with different material mixes in various layers, or supplementary drainage to limit moisture on the road.
LET’S GET PHYSICAL Different available materials have different intrinsic properties that will determine their performance in pavement mixes. Some of the most important physical characteristics of road materials are: ♦ Water permeability ♦ Fines content (the amount of fine soil particles) ♦ Plasticity characteristics (ability to undergo permanent deformation under stress) ♦ Stability ♦ Viscosity ♦ Durability ♦ Toxicity The relative importance of each of these factors depends on the individual application. Ideally, the more moisture a road will be exposed to, the lower the materials’ water permeability should be to reduce failure risk, and the more important the fines content and plasticity characteristics. Likewise, the higher the traffic load the road will experience, the more stable and durable the materials should be. A BIG PICTURE UNDERSTANDING OF PAVEMENT A pavement structure comprises various layers of materials. Each layer spreads the imposed traffic load to the underlying layers, meaning the stress experienced by the materials is
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