What is Asphalt?

Asphalt pavement refers to any paved road surfaced with asphalt. Hot Mix Asphalt (HMA) is a combination of approximately 95% stone, sand, or aggregate/gravel bound together by asphalt cement, a product of crude oil. Asphalt cement is heated aggregate, combined, and mixed with the aggregate at an HMA/Batching facility at approximately 160 degrees C. The resulting Hot Mix Asphalt (HMA) is loaded into trucks for transport to the paving site. The trucks dump the Hot Mix Asphalt (HMA) into hoppers located at the front of the paving machines. The asphalt is spread at pre determined height and widths by paver/screed, and then compacted using a heavy duty industrial roller, which is driven over the newly laid asphalt. Traffic is generally permitted on the pavement as soon as the pavement has cooled.

Types of Asphalt

Virgin Asphalt

Asphalt is sometimes referred to as asphaltic concrete or bituminous concrete and is a mixture of coarse and fine aggregates (crushed rock), filler (dust and/or hydrated lime) and bitumen, heated and mixed together whilst hot in a mixing/batching plant. There are variants to typical asphalt where specialised materials are used in the production process to manufacture asphalt with specific functions.

Other materials used at times can include liquid adhesion agents, red oxide to colour the asphalt, polymer modified or multigrade binders, bitumen modifiers to alter the viscosity of the bitumen, fibres to increase the amount of bitumen in the asphalt and fine fibres such as Kevlar to improve the fatigue and deformation characteristics of asphalt.

Types of hot mixed asphalt used on the Main Roads WA network are:

  • Dense graded asphalt (DGA)
  • Open graded asphalt (OGA)
  • Stone mastic asphalt (SMA)

DGA, the most common type of asphalt, provides optimal structural strength and generally good resistance to deformation. OGA is designed to drain water through the asphalt layer to remove excess water from the tyre/road surface. OGA is used in high speed applications such as Mitchell and Kwinana Freeways, Roe Hwy and locations where it is important to remove water from the road surface such as hills like Greenmount on Great Eastern Hwy. SMA is similar to OGA but has a high proportion of dust and high binder contents to achieve an improved fatigue life.

SMA has a textured surface but does not drain water through its layer as does OGA.

Cold Mix Asphalt

Cold Mix Asphalt Concrete is produced by emulsifying the asphalt a blend of cutter products prior to mixing with the aggregate. Cutters are basically blends of Diesel & Jet A1 (aircraft fuel) or more recently commercially produced oils e.g. peanut oils etc. While in its emulsified state the asphalt is less viscous and the mixture is easy to work and compact.

The emulsion will break after enough cutter evaporates and the cold mix will, ideally, take on the properties of cold HMA. Cold mix is commonly used as a patching material for repairs such as pot holes etc and on lesser trafficked service roads.

What is the difference between Virgin and Recycled Asphalt?

Virgin Asphalt is a common reference for newly mixed/batched hot mix asphalt (HMA) from a mixture of coarse and fine aggregates (crushed rock), filler (dust and/or hydrated lime) and bitumen, which is heated and mixed together whilst hot in a mixing/batching plant, for use on roads, paths, car parks, driveways and airstrips.

Recycled Asphalt is asphalt that has been removed from existing paved roads and reconstituted. This dry product is crushed/broken back down into manageable particle sizes. The particles are then at a size that allows for efficient reheating of product that assist with the rejuvenation process. Depending on the age and type of asphalt being recycled, there may at times be a requirement for additives to be added that assists with the binding properties and process that are required to ensure effective compaction.

Click here for more information about Recycled Asphalt.