What is Asphaltic Concrete?Asphaltic concrete (AC), commonly referred to as asphalt, is a robust composite material used as the final layer (wearing course) in applications such as roads, car parks and footpaths. Asphalt has become the surface of choice in many applications due to its useful properties which include;
- High strength and flexibility
- Excellent traction with vehicle tyres
- Highly cost-effective
- Adaptive to various designs
- Mineral Aggregates 88-96%
- Bitumen 3-8%
- Polymers 8%
- Hydrated Lime <5%
- Additive(s) <0.1%
Asphalt CompositionThere are many different types of asphalt with various compositions. The most common types of asphalt compose of the following:
Flexible PavementsIn order to provide a strong and durable asphalt surface, it must be coupled with other components that make up road pavements. Asphalt is typically associated with flexible pavements, however, it can also be placed on top of concrete to form a rigid pavement.
Flexible pavements can contain a base course and subbase layer (both composed of roadbase) on top of a compacted subgrade (which is usually natural ground material). The example below is for a typical for a high use council road - 495mm total thickness. Note the thickness of asphalt layer relative to the overall thickness of the pavement – 40mm out of 495mm.
Typical Flexible Pavement
The pavement can include other layers such as;
- A geofabric and/or geogrid later which provides additional strength and moisture protection.
- Bitumen seal (or a tack coat), or primer seal which can provide moisture protection and additional bonding between the asphalt surface and the base course layer.
- The asphaltic concrete mix is prepared in a facility.
- The asphalt is delivered to site by truck – the temperature of the asphalt mix must not fall below the specified temperature before being laid.
- The truck reverses towards the paving machine and tips asphalt into the hopper.
- The operator guides the paving machine into the desired position, laying the asphalt to specified thickness.
- The asphalt is compacted using a vibrating roller and if required, a pneumatic tire roller. Asphalt is placed manually in situations where a paving machine cannot be used. These situations include trenches, areas with low clearance, small patches or areas with general inaccessibility for a paving machine. When placing asphalt manually, it is important to take extra care to ensure correct thickness and placement./>