Asphalt Roads: the Basic Steps for Construction
Date published 30th April 2019
Asphalt roads have several advantages over concrete. They are cheaper, more flexible when paving odd shaped areas or awkward curves, and easier to remove and replace. The material is made from a mix of about 95 per cent stone, sand and gravel and five per cent bitumen. Bitumen is a heavy, viscose crude oil that is heated and mixed with the other ingredients to produce a durable, waterproof surface when it cools.
Whether you are upgrading your driveway or involved in a major civil engineering project, several steps remain the same.
With driveways and smaller residential projects, the preparation may only involve filling existing potholes and hiring a small manual compactor. For new road projects, the first step is for the topsoil to be removed. Before this work is carried out, a survey of the sub-soil, or sub-grade, to determine its load-bearing strength will determine the specification of the road design. Sub-grade strength can be increased in several ways: by removal of the poor material which is replaced by a stabilising material; forming boreholes and other types of subsoil drainage; adding soil stabilisation materials including cement and bitumen based compounds; the compaction of the sub-grade to increase its density and force out the water content. At this stage, groundworks for drainage, culverts or soakaways are also completed.
If a road needs resurfacing, the old asphalt is normally removed using a profiler. The profiler has a series of aggressive cutting teeth, that break up the surface for removal by an industrial vacuum. Asphalt debris can be recycled for future projects if taken to specialised waste management companies.
Once the sub-grade is complete, the next stage is the formation level.
Once the road area is set out, graders are hired in to prepare the formation layer. The graders prepare the surface for the asphalt, smoothing out the sub-grade and forming the camber. Water is hazardous to motorists and contributes to potholes, cracks and heaving of the top layer. A camber is the convex shape of the road surface cross-section that is required to enable water run-off and proper drainage.
Binders and Asphalt Layer
The formation layer, whether the sub-grade or the stripped layer for resurfacing, is ready for the binding layer. The binder layer is often made from a bitumen base of 6 to 7.5 kg per 10m2. The layer binds the formation layer with the asphalt and is mixed with aggregate to provide a rough surface for improved bonding.
The thickness of the asphalt governs the times the layers are rolled. For road projects, rolling is first with an eight or 12 tonne roller, then a 15 to 30 tonne fixed-wheel pneumatic roller. Rolling speed is 5km, the wheels are kept wet, and management of the temperature influences the strength of the surface. The surface is finished with an eight or 10 tonne tandem roller.
RediPlant, based in Brisbane, has over 20 years experience meeting the needs of the road construction industry. RediPlant has a range of 10”, 12”, and 14” Grader with Topcon and Trimble machine control capabilities specifically set up for road construction. Contact us today with any plant hire or purchase enquiries.