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 third and final article in our series on road construction materials, Infrastructure takes a closer look at unbound and stabilised granular materials, as well as some of the applications of recycled materials in roads.
Unbound granular materials
Unbound granular materials can feature in any layer of a road, although the exact materials, particle sizes and other properties often differ depending on the type of road and which pavement layer they comprise.
Common granular materials that may be used unbound include crushed and screen quarried products, natural sands and gravels. Generally, unbound granular materials are only used on the surface of the road as a wearing course in very low traffic applications, such as minor rural roads.
Desirable qualities include skid-resistance, load-spreading ability, wet and dry stability and low water permeability. The grades of materials used must be appropriate for the application.
While larger particles may risk damage to cars and make the road rougher and noisier to drive on, too high a proportion of finegraded silts, sands or clays can make the road overly dusty or result in material being washed away during rain.
Unbound granular materials can feature as the base course and/or subbase of sealed or unsealed roads with light traffic. Usually, the subgrade layer beneath a road is composed of unbound in-situ granular material, unless a demanding application requires further modification for stability.
To create a modified unbound granular material for any pavement layer, a second granular material can be added to the first in order to alter the particle size distribution, plasticity or other characteristics.
Additionally, modification can include adding a small amount of stabilisation binder to the mix to increase bearing capacity or stiffness, or to decrease moisture sensitivity.
At these small amounts the binder does not provide the tensile strength characteristic of a lightly bound or bound material, so the granular material is still considered unbound.
Stabilised granular materials
A granular material may be stabilised in a variety of ways to make it more suitable for use within any layer of a pavement structure. Stabilisation treatments improve the material’s performance by altering its strength, permeability, volume stability and durability.
For instance, stabilising the granular material used in the wearing course of a road will result in a more erosion-resistant road surface with improved load transfer, compared to leaving it unmodified.
Stabilisation can be achieved by methods including adding lime, bitumen, cementitious binders, chemical additives, geotextiles or geogrids to the granular material.
The amount and type of stabilisation agent used will depend on factors such as parent material composition and properties, traffic loads, climate, road importance, cost and allowable risk.
Cementitious binders are used to make the bound or lightly bound granular materials that comprise the basecourse and/or subbase layers of many roads with moderate or heavy traffic.
The cement binder increases tensile strength and decreases moisture susceptibility. Whether the material is classified as bound or lightly bound depends on the degree of tensile strength gained by adding the binder.
Lime stabilisation is another common stabilisation method. Different amounts and types of lime can be used depending on the degree of stabilisation required.
However, the tensile strength conferred by stabilising a material with lime is much less than that given by a cementitious binder. Lime stabilisation is commonly used on the subgrade level of a pavement to increase the strength of the in situ material that will provide a foundation for the layers above.
What’s old is new again – recycled materials
In recent years, using recycled materials in roads has become more popular around the world. In some cases, using an appropriate recycled alternative rather than a raw material offers an excellent opportunity to reduce material costs, meet sustainability objectives and find a use for waste materials.
However, the suitability of a given material to any given application and its properties as a construction material must remain a foremost consideration. Many industry or consumer waste materials show promise for use in road infrastructure.
Depending on their properties, they may be used as aggregates in asphalt or concrete mixes, as fillers, to provide drainage or more.
Some options include:
♦ Reclaimed asphalt pavement (RAP)
♦ Crushed recycled concrete (CRC)
♦ Construction and demolition waste (C&D)
♦ Crumb rubber from shredded tyres
♦ Commingled and crushed glass
♦ Plastic shredded
♦ Industrial wastes such as slags, fly ash and tailings
Reclaimed asphalt pavement (RAP) is commonly recycled back into construction applications. In Australia, about half of the asphalt reclaimed from old roads is re-used within hot mix asphalt (HMA) applications, as in low amounts it has little impact on the properties of the mix.
As a result, HMA including recycled asphalt can be used on the wearing course, basecourse and subbase layers of a road, or as a spray seal surfacing aggregate.
Much of the RAP not used this way is used as a granular base or subbase material. Crushed recycled concrete (CRC) from demolished infrastructure can be used as an unbound granular base or subbase material in road construction.
It can also be used as an aggregate in new concrete mixes, with appropriate consideration of particle size and the mix of other ingredients to achieve a concrete mix suited to the application.
Nevertheless, in any of these uses the traceability and potential for contamination of the recycled material must be considered. Any other materials present with the recycled material may alter its performance, and therefore must be taken into account in design.
What’s old is new again – recycled materials
Another vital consideration is whether any harmful contaminants may be present that pose a risk to workers during construction or could leach from the road into the surrounding environment.
Tight restrictions are placed on certain metals, pesticides or carcinogens (like asbestos fibres or fragments) and appropriate controls must be in place to prevent harm to people or the environment.
Recycled rubber or plastic can also potentially be incorporated into asphalt mixes, potentially providing benefits such as increasing durability and temperature resistance or reducing noise.
However, there are significant safety concerns regarding the fumes released during construction and the potential for chemicals to leach from the road under certain conditions.
Similar consideration is required for the use of certain plastics as aggregates within concrete mixes, or in spray seal mixes. Using crushed glass as an aggregate in HMA or concrete mixes, or as a granular material, has occurred for many years.
However, the performance of the mix can vary substantially with the purity of the glass and the size and shape of the particles. These must be taken into consideration in formulating a mix, as must the properties of any potential contaminants.