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Macadam / Tar / Bitumen / Asphalt / Spray Seal

Macadam is a form of road-making invented by Scotsman John Loudon Macadam in the late 1700s. It was a cost effective way of making a dry road. He used a layer of crushed limestone, or even a flat dirt surface, to hold a thick 30cm layer of hard stones or ‘metal’ broken up to 35-50 mm diameter. The surface was made slightly convex to allow rain to run-off. This was then either rolled or left for the traffic to compact it down. As the surface dust was worn from the stones it fell between the cracks and combined with moisture to bind the void between the stones. Loose sand was sometimes swept into the surface to assist the process. This type of road worked well in overseas countries with a damp climate, but the dryness of South Australia caused the roads to be extremely dusty and difficult to maintain, and the surface was unsuitable for pneumatic tyres.

Tar was used from the 1880s on heavily trafficked roads and corners to make them dust free and more durable. This heavy sticky, residual by-product occurs after coal is distilled during the production of gas for use in lighting. It was covered by a layer of gravel spread over it to provide a non-sticky surface for vehicles. While having excellent adhesive qualities tar deteriorated quickly. Never used extensively in South Australia, it has not been used since the late 1920s.

Bitumen is the heavy component left after the distillation of petroleum. Normally distillation aims to produce the maximum amount of the light fractions – petrol, kerosene, etc. – leaving a small amount of bitumen. Some crudes (most Australian crudes) produce little bitumen, whereas others produce large amounts (Middle Eastern crudes).

Asphalt is a mixture of sand, gravel and bitumen mixed and laid while still hot, then rolled to a smooth finish. It was introduced for South Australian roads in the 1920s to provide a durable surfacing suitable for motor vehicles with pneumatic tyres. Natural asphalt had previously been used for some 50 years for footpaths, but was unsuitable for use as a road surfacing. In its natural state it was too slippery for animals and too soft for use by iron tyred vehicles.

Spray seal is an important Australian technology used to provide low cost sealed surfaces for low traffic roads. Hot bitumen is sprayed on the prepared road surface and immediately covered with clean stone chips, normally between 7 and 14mm nominal size, which are rolled in to ensure adhesion. Emulsified bitumen, a mixture of bitumen and water, can be used instead of hot bitumen.

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