What’s the difference between Tarmac and Asphalt?
Tarmac is a tough yet malleable substance. We use tarmac for playgrounds, driveways, and car-parks.
Asphalt is a hard and more brittle material that is more hard-wearing than tarmac. We use asphalt for roads and public footpaths.
Tarmac is comprised of 6mm or 10mm stones coated in bitumen. The bitumen coated stones along with sand bind together. It has a rougher and grainier appearance than asphalt. Tarmac is also known as Dense Bitumen Macadam (DBM).
Asphalt is comprised of sand, lime, stone, and bitumen. There is a higher percentage of bitumen in asphalt than in tarmac, and is therefore harder wearing. 15/10 HRA (Hot Rolled Asphalt) or 30/14 HRA.
Whinstone or Granite pre-coated chips (whichever stone can be sourced naturally and locally) is laid on top of the asphalt on roads for skid resistance.
White Derbyshire stone is laid on top of asphalt footpaths. This serves the joint purpose of being both skid-resistant and decorative.
The sealing of asphalt or tarmac joints can be done in two ways:
A cold-applied bitumen sealant can be brushed onto the existing vertical joints, which the new tarmac will marry into.
Alternatively, a hot bitumen sealant can be poured/applied onto the vertical joint. There is a misconception that the horizontal joint is the main joint that should be sealed. In reality, the surface area of the vertical joint is what gives the new material the best structural integrity.
Over-banding is where hot or cold bitumen sealant is applied to the horizontal surface after the tarmac has been laid. The misconception is that water will permeate the joint and lead to its breakdown if it has not been sealed horizontally.
This is a false assumption as the vertical joints have already been painted. Bearing that in mind, if there is no visual evidence for the client that this has been carried out, then over-banding can be offered as a service that will give the client more confidence in the works.
Leotac Bitumen Emulsion (Tac-Coat)
The tac-coat is used to bind together two layers of tarmac or asphalt on top of each other. It is generally sprayed and will give a brown appearance before it “breaks” and turns black with time.
The new tarmac or asphalt layer can then be laid directly on top of this. It gives a mechanical adhesion which prevents movement between the old and new layers.
Rollers and compaction plates
A new tarmac or asphalt surface must be compacted for it to gain its full engineering properties. The tarmac or asphalt will be hot when it is being laid. This makes the bitumen more likely to stick to the metal plate or roller. To counteract this, water is used to break the bond between the bitumen and the metal surface. This allows the smooth compaction and rolling of the tarmac or asphalt.
The bigger the roller or plate, the more compaction is achieved. This is not to say that a smaller roller cannot compact as effectively as a large roller, it is merely dependent on the number of passes used by the piece of plant equipment to achieve the same result. Compaction plates are often used to compact small areas, edges, and corners the roller cannot reach.
Hot boxes and asphalt duvets
The tarmac or asphalt is collected and loaded into the lorry whose sides and floor are insulated. Asphalt duvets are silver-coloured reflective tarpaulin covers used to insulate the material during transit from the quarry to the work site.
Asphalt duvets have a silver reflective coating which traps infra-red rays and the material does not stick to it. Asphalt duvets are often used in conjunction with traditional jute tarpaulin. As well as keeping the material hot, it protects it from the weather.
A hot box is an insulated container that will hold from 1 to 10 metric tonnes of hot asphalt or tarmac. It also has a gas supply which feeds burners that keep the material hot. The material can stay in the box for up to 24 hours.