Bitumen emulsion seals road bases, sub bases and sub grades with a water resistant film. That keeps the moisture content of the layer in balance, which is what lets it reach its design strength, and it stops rainfall washing out the fines. Cement bound materials should be sealed as soon as possible after compaction, preferably within an hour of laying, and pavement quality concrete within an hour of laying.
REA Technical Data Sheet No.6, Sealing and Curing of Pavement Layers in Road Construction using Bitumen Emulsion
What sealing is for
A seal does several jobs at once. It prevents water getting in and prevents water being lost by evaporation from the surface. It maintains the moisture equilibrium of the construction course, and so its strength. It reduces the loss of fines in heavy rain. It lets cement bound materials cure to optimum strength with less surface crazing. Applied as a surface dressing it also works as a running surface, and it stops stone being displaced from the surface of bituminous materials.
Water ingress
In the winter months rainfall holds work up and site traffic is delayed by boggy conditions. A surface dressing used as a seal can keep work such as embankment and bridge construction moving in spite of the weather.
Water loss
Where the formation is cut in hot conditions, moisture evaporates from the surface and the surface may need damping to bring the water content back to optimum. Surface dressing is intended to minimise that loss, and lightly coloured aggregates reduce how much heat the surface absorbs from the sun.
Sealing the formation
Freshly cut formation should be dense, free from loose material, and sealed as soon as possible. The emulsions used should be fast breaking, such as C60B3 or C69B3.
Areas that will take no construction traffic
A single application of bitumen emulsion at 1.3 to 1.8 litres per square metre, depending on the nature of the surface and the bitumen content of the emulsion. Allow the emulsion to break, turning from brown to black, then blind the surface with dust at 2 to 4 kilograms per square metre.
Medium trafficked areas
Where light construction vehicles will run over the sealed formation, a single surface dressing is recommended, using the higher bitumen content emulsions at 1.4 to 1.8 litres per square metre. Clean 2/6mm crushed rock or slag aggregate conforming to EN 13043 is applied at 6 to 8 kilograms per square metre, before the emulsion has broken, followed by light rolling.
Heavily trafficked areas
Where construction traffic is likely to be heavy, a double surface dressing is recommended, and cement may be worth using to strengthen the sub grade. High bitumen content emulsions are preferred. The first dressing should be not less than 0.9 litres per square metre, suitable for a dense close textured surface, with 2/6mm crushed rock or slag chippings to EN 13043 applied as soon as possible at 6 to 8 kilograms per square metre and lightly rolled. Excess chippings are removed and the process repeated at 0.9 to 1.2 litres per square metre with 2/6mm chippings. Alternatively 4/10mm chippings may be used for the second dressing at 9 to 11 kilograms per square metre, with the rate of spread of emulsion increased by 0.2 litres per square metre.
Two notes on rate of spread
- Surface texture. Where the surface is open textured, porous, or under trees, work towards the higher limit, and prefer a high bitumen content emulsion. Where it is dense and close textured, work towards the lower limit. The rates should also reflect the bitumen content of the emulsion. Sealing a formation of normal texture that will take medium traffic, C60B3 could be used at 1.6 litres per square metre followed by 6mm chippings; changing to hot applied C69B3 means reducing that to 1.4.
- Gravel chippings. Where gravel chippings are used, the recommended rates of spread of emulsion must be increased by 0.2 litres per square metre.
Wet mix and other water bound materials
Water bound sub base such as wet mix depends largely on an optimum moisture content for its strength, so the surface should be sealed as soon as possible after laying and compacting. The same rates and types of emulsion as for formation sealing apply. A surface dressing is strongly recommended, because it reduces the risk of damage to the surface and the effect of temperature.
Cement bound bases, sub bases and lean mix concrete
Curing cement stabilised materials properly is what delivers the designed strength. Seal the surface as soon as possible after compaction, preferably within one hour of laying. C60B3 is normally used, though lower bitumen content emulsions such as C40B3 may sometimes be permitted on uniform, close textured surfaces. Use 1.0 to 1.5 litres per square metre depending on the bitumen content, let the film break, then blind with dust at 2 to 4 kilograms per square metre. A surface dressing is recommended on top, since it reduces the chance of the seal being damaged.
Stabilisation
A wide range of materials can be upgraded dramatically, and very economically, by stabilising the material in place, mixing it with water, rolling it and then sealing the surface with bitumen emulsion. The seal maintains the moisture equilibrium of the stabilised layer and lets it reach its maximum strength.
Kerbs, bond coats, surfacing and footways
- Kerb sealing. The concrete backing to kerb stones may be cured with bitumen emulsion, and emulsion can seal around the base of the kerb to reduce water getting in at the interface with the pavement course. It may be applied liberally, since it acts as a joint sealant, and proprietary emulsions intended as joint sealants are available.
- Bond coating. Bond coats should always be applied between asphalt and macadam courses and bases. Technical Data Sheet No.5 covers it in full.
- Surface dressing high stone content asphalt. This maximises resistance to deformation and improves skid resistance and macro texture. C69B3 is a particularly suitable binder. On high stress or high speed sites, a polymer modified emulsion is more appropriate.
- Footways. C60B3 and C69B3 are recommended. Technical Data Sheet No.7 has the detail.
Curing pavement quality concrete
Bitumen emulsions are suitable for curing pavement quality concrete. Use class C40B3 or C60B3, with the 40 per cent grade normally preferred; class A-1 anionic emulsions to BS 434-1 can also be used. Within one hour of laying, coat the concrete at 0.5 to 0.9 litres per square metre depending on the grade and the surface texture, followed soon after by sand or small grit. The road may be opened once the concrete has reached the strength the design requires.
Hot rolled asphalt and macadam base course
On large housing or industrial estates the base course is often left open for a period before the wearing course goes on. A surface dressing or slurry surfacing is an effective way of sealing the course in the meantime.
Doing the work
Agree the method of treatment at tendering stage, so that the right equipment is on site and the best practicable job can be planned for.
- Binder. Only small areas tend to be sealed at a time, which often rules out a bulk distributor, particularly where the base or formation will not take its weight. A small motorised sprayer or hand operated equipment is usually the better answer. The seal works provided a continuous film of binder is formed, and with a hand lance an even application comes from spraying with a circular action.
- Chippings. Small stockpiles placed along the site are often more convenient. Chippings should comply with EN 13043. On the final dressing, make sure the binder is well covered so that it is not picked up on tyres.
- Rolling. Roll immediately after the chippings are applied, preferably with a pneumatic tyred roller.
- Drums. Roll drums vigorously at the prescribed intervals during storage and before use, and protect them from frost.
- Bulk emulsion. Circulate thoroughly before starting work. The REA Code of Good Practice for the Use and Safety of Mobile Storage Tanks applies.
Health, safety and equipment
Follow the manufacturer’s health and safety information, and Technical Data Sheet No.2 on the safe handling and storage of bitumen road emulsions. Clean equipment after use following Technical Data Sheet No.3.
UKCA and CE marking
Construction products covered by a harmonised European standard have had to be CE marked to be placed on the European market since the Construction Products Regulation was fully implemented in June 2013. The UK introduced its own UKCA mark in January 2021 after leaving the European Union, and the transition period has been extended indefinitely, so both CE and UKCA marking can continue to be used.
References
BS EN 13043:2002, Aggregates for bituminous mixtures and surface treatments for roads, airfields and other trafficked areas. Road Note 39, Design Guide for Road Surface Dressing, published by the Transport Research Laboratory.
Related guidance
Sealing is one of the jobs emulsion does between layers. Bond coat or tack coat covers what BS 594987 requires between asphalt courses, and surface dressing with bitumen emulsion covers the treatment used here as a seal.
For the material itself, see what bitumen road emulsion is, or start at Discover REA.