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 second article in a series on road construction materials, Infrastructure takes a closer look at hot mix asphalt, bituminous sprays and concrete mixes.
Hot mix asphalt (HMA) is a versatile road material made up of a blend of bituminous binder, fine or coarse rock aggregates, filler material and mix voids. The identities and volumes of the different components can be customised to achieve the structural performance, durability, functionality and workability characteristics required for the application.
HMA is suitable for use in the wearing course or base course and, in general, is a durable material which will not rut or crack with the application of traffic or environmental loads such as heat, cold or moisture. It can also provide better skid resistance and drainage than concrete, is often less expensive, enables faster construction and is easy to maintain.
The bituminous binder provides adhesive and waterproof properties to the asphalt. It holds the mix constituents together and prevents the mix from flowing and segregating. It is highly viscous (i.e. solid) at room temperature but can become liquid when heated. Various classes of bituminous binders are available, categorised by their viscosity (ability to remain solid) at 60°C.
Those with lower viscosities are suitable for applications with lower traffic loads and cooler climates, while the more solid classes are required for heavy traffic levels and warmer conditions. Binders may also be modified through the addition of polymers or other additives, typically to increase viscosity.
The aggregates within HMA provide stability and strength to withstand traffic loads and correctly spread the loads to the underlying pavement layers through the aggregate interlock and intrinsic hardness of the source rock. Aggregates also provide wearing course skid resistance properties through resistance to polishing. The particle size (or coarseness) and type of aggregate material influence the strength and performance of the mix.
Some aggregate options include crushed and screen quarried products, natural sands and gravels, or recycled materials such as commingled glass. Coarser aggregates are generally better suited to higher traffic applications and provide better skid resistance, but this also depends on the other components of the mix.
The filler material within the HMA is any particle smaller than 0.075mm, filling voids within the aggregate skeleton. Filler is incorporated into a HMA to increase its stiffness and strength by reducing mix flow. It can also increase the bitumen’s affinity to the coarse and fine aggregate and reduce the amount of binder needed. However, too much filler may reduce workability.
Sprayed bituminous materials
A sprayed layer of bituminous materials is a common option for the surface of roads (aka the wearing course). A sprayed seal consists of a thin layer of bituminous binder that is sprayed as a liquid, then covered with a layer of crushed aggregate. It can be applied onto a pavement base course, or as a ‘reseal’ over an existing bituminous surface.
Different types include single/single (one application of sprayed binder and one layer of single-sized aggregate), single/double (single application of binder and two layers of aggregate), and double/double (one application of sprayed binder with one layer of aggregate, followed by a second application of binder and a final layer of slightly smaller aggregate).
Like in HMA, the types of binder and aggregate materials affect the properties of the road surface. Various additives can also be used to modify the mix. Examples of sprayed seals customised for special purposes include:
♦ Fibre reinforced seals (FRS) – reinforcement from chopped fibres placed onto sprayed binder
♦ Geotextile reinforced seals (GRS) – reinforcement from a geotextile layer to resist cracking
♦ High stress seals (HSS1 and HSS2) – uses light to medium modified PMBs to accommodate moderate traffic stresses
♦ Extreme stress seals (XSS) – uses medium to heavily modified PMBs to accommodate extreme traffic stresses
♦ Strain alleviating membrane (SAM) – uses heavily modified PMB to resist cracking
♦ Strain alleviating membrane interlayer (SAMI) – interlayer between asphalt layers – not trafficable
Sprayed seals are a very cost-effective treatment and are the most frequently used pavement surfacing treatment in regional areas. Sprayed seals may also be used where asphalt production is not available.
These sprayed surfaces are functionally different to HMA surfaces, and do not have the same structural strength or shear stress resistance. They also have different road noise characteristics. As a result, are not usually suited for heavily trafficked intersections or built-up urban areas.
Concrete can provide an extremely durable material for the base course and sub base layers of a road. In many cases, a concrete base for a heavy traffic road can have a lifespan of 30-40 years. However, it can take longer and be more expensive to construct and repair than other options.
Concrete is also often considered more environmentally friendly than asphalt, and its components can often be recycled. Different types of concrete base can be constructed depending on the requirements of the application, with different slab dimensions, reinforcement details and minimum corner angles.
These include plain concrete pavement (PCP), jointed reinforced concrete pavement (JRCP), continuously reinforced concrete pavement (CRCP) and steel fibre reinforced concrete pavement (SFCP) One of the key factors in determining the appropriate concrete mix, structure and thickness for a pavement is the required flexural strength, or resistance to bending stress caused by traffic and environmental actions.
Various types of joints can be used within the concrete base to minimise undesired cracks in the concrete due to environmental conditions, such as temperature-induced shrinkage and expansion of the concrete. These include:
♦ Transverse contraction joints
♦ Transverse construction joints
♦ Expansion and isolation joints
♦ Longitudinal (hinge) joints
Likewise, different steel reinforcements are used to control temperature-induced shrinkage and expansion opening of cracks, restrain the separation of slabs at the longitudinal joints and provide load transfer between slabs when required.
Types of reinforcement include:
♦ Steel bar or mesh reinforcement – to restrict the opening of cracks in the concrete slabs
♦ Steel fibres – to restrict cracks in odd-shaped or acute-anglecornered slabs (e.g. roundabouts)
♦ Tiebars – to prevent longitudinal joints from opening, while allowing some rotation from curling/ warping movements
♦ Dowel bars – to transfer load across wider transverse joint spacings where the aggregate interlock is inadequate
The aggregate materials and cement binder used within the concrete are chosen to meet both the strength and workability requirements of the application. The aggregates are chosen for properties such as particle size, durability, water absorption, thermal expansion characteristics and chemical reactivity.
Options include crushed and screened quarried products, natural sands and gravels, or recycled materials. The size of the aggregate particles plays a key role in determining the strength of the concrete, with smaller particle size resulting in stronger concrete, while also generally being more expensive.
The amount and type of cement binder used in the concrete also affects the performance and durability of a concrete base or sub base. Shrinkage limited cement or general blended cement can be used as binders. A higher proportion of cement generally makes for a stronger concrete and makes the mix more workable.
This article is part two in a series looking at the different materials used in road construction. Look out for part three in the next issue of Infrastructure magazine, which will go into more detail on unbound and stabilised granular materials, as well as recycled materials. Read the first article in this series here.