Slurry and micro surfacing explained

A plain English summary of REA Technical Data Sheet No.9, revised by the Association’s Technical Committee in August 2025. The datasheet is the authority; this page exists so the guidance can be read, searched and quoted without opening a PDF.

What the treatment is for

The main purpose is to stop the surface fretting as the binder ages or is lost, to seal imperfections and repairs, and to give an even, consistent running surface.

Standard slurry seals and surfacings give excellent skid resistance for low speed traffic, but their texture depth is relatively low, so micro surfacing is usually preferred where traffic is faster. That makes slurry surfacing a natural choice in towns and at airports, where the absence of dust and flying chippings is an advantage in itself. Micro surfacing extends the range, with higher performance in texture, skid resistance and durability, and the ability to regulate a surface in a single pass.

Suitable pavements include carriageways, hard shoulders and central reservations, car parks, airport runways and taxiways, footways and playgrounds.

What is in the mix

A typical mix on site slurry surfacing is a graded aggregate, a bitumen emulsion of class C60B5 or similar to BS EN 13808, water and additives. The grading depends on the end use, and the residual binder content may run from about 6 per cent to 13 per cent depending on the substrate, the grading and the traffic.

The pattern is consistent: as expected traffic density rises, the aggregate grading is coarsened and the bitumen content reduced, and as the aggregate coarsens the surfacing is laid thicker. In every case the aggregate must be assessed so that it is not so reactive that there is no time to mix and apply the material. Water and additives are generally needed to adjust consistency, prevent segregation during spreading, or slow the set.

Where it is used

Most slurry surfacing goes on roads with an 85th percentile speed below 50 miles per hour carrying fewer than 250 commercial vehicles per lane per day. Micro surfacing of a suitable design can be used in more demanding conditions where texture depth or the polished stone value of the aggregate matter, normally with a polymer modified binder. Multiple applications are practicable where a heavier coating, extra protection over an existing treatment, or more regulating is wanted.

• Housing estate roads, where a clean, dense surface is what is wanted.
• Trenched or patched surfaces, as a uniform sealing coat, provided the reinstatements are sound and reasonably level.
• Polished surfaces, to restore skid resistance over a general area.
• Surface dressing failures, where a dressing has partially stripped.
• Crazed surfaces, where it will go some way to restoring shape after minor foundation faults, though existing crazing may reappear. Fibre reinforced treatments can help; a long term cure cannot be guaranteed.
• Motorway hard shoulders and central reservations, with properly designed systems providing texture depth where it is required.
• Airfields, including runways, taxiways, holding areas and perimeter tracks.
• Car parks and playgrounds, where the layout has to let the mixing and spreading equipment manoeuvre. The surface is finally dusted lightly so that tyres can turn.
• As a pad coat before surface dressing, where chippings would not otherwise embed well enough to be retained, on cement concrete for instance.
• Footways, cycle tracks and pedestrian areas, where coloured slurry surfacings are widely used.

Laying it

Most areas are treated by a mobile continuous mixer and spreader, metering aggregate, emulsion, water and additives into a continuous mixer and discharging into the spreader box. Batch production weighs or measures the ingredients separately. Pre packed material may need agitating on site, sometimes with a little water, before it is spread with soft brooms, squeegees or mechanical applicators.

Preparing the existing surface

The surface must be free of all dust and loose material, cleaned thoroughly with a mechanical broom or suction sweeper and by hand where necessary. Open cracks must be cleared of vegetation and loose material, and compressed air can help. All arisings are removed. A bond coat is not usually necessary, but where circumstances make one desirable it should be class C40B3, applied as REA Technical Data Sheet No.5 describes.

Afterwards

Some loss of surface aggregate particles is normal for a short period after treatment, from a few days to a few weeks depending on how heavily the road is trafficked. On the public road network routine highway sweeping is usually enough, since the particles are too small to damage windscreens and cause little nuisance to pedestrians. On airfields it may be worth increasing the sweeping frequency, depending on the aircraft and their movements.

Weather

Using emulsion classes C60B5 and A3 rapid set to BS 434-1, application should only go ahead when average road surface and air temperatures are 4°C and rising, and never during rain. Be cautious in cold, damp weather unless dry conditions can reasonably be expected soon afterwards, because full strength comes only when the material has cured completely. The surface must be free from frost, ice, snow and standing water. After a long dry, sunny spell it can help to dampen the surface before spreading, following the manufacturer’s advice.

Testing and materials

Slurry and micro surfacing is designed for end use performance, so testing belongs at the design stage and then to keeping the materials consistent. Constituent materials will normally be CE or UKCA marked.

• Aggregate must be laboratory tested for chemical and physical compatibility with the emulsion system. Wet Track Abrasion to BS EN 12274-5 and cohesion of the mix to BS EN 12274-4 are used during design, and grading is tested at the frequency the specification requires.
• Emulsion should comply with the appropriate requirements of BS EN 13808.
• Additives should comply with the relevant British or European standard, Portland cement to BS EN 197-1 for example.
• Mixed material. Samples can be put through the Wet Track Abrasion Test and analysed for binder content and grading for design purposes, but those are not acceptance tests. Acceptance uses the visual assessment test, EN 12274-8.

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 the UKCA mark in January 2021, and the transition period has been extended indefinitely, so both marks can continue to be used.

Download Technical Data Sheet No.9 (PDF)Slurry and Micro Surfacing. Revised August 2025.