Tack coat is an important step in preparing the surface for asphalt concrete or surface treatment, ensuring reliable traction between layers.
Priming is done with liquid bitumen or emulsion. The benefit of emulsion is its low viscosity. This ensures an even distribution over the surface and 20-30% economy of bitumen. The use of cationic emulsion as tack coat is questionable, since it is thought that hydrochloride acid can destroy cement stones and degrade adhesion. This issue remains to be researched scientifically, but in practice there is no significant adverse effect of emulsion on concrete. This is probably because the content of acid in the emulsion is low.
The purpose of surface treatment of roads are:
- restoration of traction between vehicle wheels and the road top;
- restoration of road waterproofing;
- creation of a wear layer.
Surface treatment is performed in dry weather with daily average temperatures above 10 °С.
Depending on the traffic load and the condition of the surface, a specific type and method of treatment is selected:
- single surface dressing (binder/macadam);
- double surface dressing (binder/macadam/binder/macadam);
- single surface dressing with double macadam application, binder/macadam/macadam;
- sandwich type treatment (macadam/binder/macadam).
The consumption of emulsion and macadam depends on the type of surface treatment, emulsion concentration and macadam size.
The machines used in the process are motorized binder spreaders, aggregate spreaders (towed or attached) and combined or pneumatic asphalt compactor rollers.
The number of compactor passes is usually three to five, and compaction speed is 3 kph for the first three passes and 10 kph subsequently.
Compaction should be started when the emulsion begins to breakdown, when the color of the emulsion changes from brown to black.
Single surface dressing with double macadam layers are compacted after each macadam layer is applied.
Double surface treatment layer is only compacted after the second macadam layer is applied.
Sandwich type layers are compacted immediately after the application of the second macadam layer.
The quality of the surface treatment depends on the weather conditions, traffic loads, traffic speeds and the timely measures taken to ensure normal settling conditions, such as:
- the road is opened for traffic one hour after final compaction, with the speed limited to 40 kph in the first seven days.
- excess macadam on the surface must be removed by mechanical brushes to prevent the destruction of the forming layer structure.
- final formation is complete in the first six weeks after making the layer at minimum midday temperature of +10 °С.
If the temperature of the surface exceeds 35°С during treatment and it is possible that macadam may be dislodged from the layer, small macadam is distributed across the surface (2-5 mm) to prevent the binder from sticking to vehicle wheels and protect the surface dressing. The amount of such macadam application is 2-4 kg/m2.