A patch lasts when the hole is cut cleanly to a regular shape with vertical or slightly undercut sides, so that the edges of the new material meet sound existing material, and when the sides and base are bond coated before the repair goes in. Spray injection patching does the same job without cutting: high volume, low pressure air cleans the defect, then emulsion and clean aggregate are fed into the airstream together so coated chippings are driven into the base of the pothole and consolidated as they build. BS 10947 covers it.
REA Technical Data Sheet No.8, Patching, Grouting and Spray Injection Patching
Why bitumen emulsion suits repair work
Three properties are being used here. Emulsion can be applied cold. It penetrates the substrate. And it coats dry or damp aggregate right through the depth of a layer. Once it has cured, the residual bitumen binder gives the repair its cohesive strength.
Patching covers the repair of cracks, crazing, potholes and depressions, and it is an effective treatment before surface dressing. Where conventional macadams are used, emulsion serves as the bond coat, and proprietary brushing grades can replace hot bitumen as an edge sealant. Grouting is a different technique: emulsion applied to compacted dry or damp aggregate, where its low viscosity lets it penetrate the interstices, building a combined base and wearing course from 50mm to 75mm in one course, or up to 100mm in two.
Patching
Preparation
Cut the failed area cleanly to a regular shape with vertical or slightly undercut sides. On heavily trafficked roads a diamond shaped cut is preferred. The cut has to extend far enough that the edges of the repair material sit against good quality existing material, and all loose and unstable material must come out.
Bond coat emulsion class C40B3, C60B3 or a polymer modified grade is then applied to the sides and base of the prepared area, either sprayed or poured and brushed.
Application
On heavily trafficked roads the patching material is often a hot bituminous mix, asphalt concrete or hot rolled asphalt. Where the repair is deeper than 100mm, fill it in successive layers, levelled and compacted, with the final layer profiled to suit. The nominal aggregate size in the patching material should be no more than half the depth of the layer. A vibratory plate compactor suits the lower layers and small areas; a roller is more practical over larger ones.
If the finished repair is open textured it can be sealed with class C60B3 or C60BF3 at 0.8 to 1.0 litres per square metre and blinded with grit or a pre prepared slurry surfacing.
Alternatives on less heavily trafficked roads
- Deferred set macadams. Pay particular attention to vertical edges, so that the patch gets edge support.
- Emulsion and aggregate mixes. Several permanent cold lay surfacing materials are available.
- In situ patching. Clean aggregate of 20/31.5mm nominal size, or no larger than about two thirds of the depth of the hole, is placed slightly proud of the hole. The minimum amount of emulsion needed to coat the stone is poured on, then the patch is compacted well. Use class C60B3, C60BF3, C69B3 or C69BP3, and use the minimum quantity, or the repair will leave fat spots on the road. Seal the open textured patch with clean 6.3/10mm or 2.8/6.3mm chippings and roll again.
Grouting
Both full and semi grouted work are recognised methods. In a full grout there is enough emulsion to coat the aggregate through the full depth of the construction. In a semi grout, a lower water bound layer is formed as the aggregate is compacted, and emulsion then coats the aggregate above it. Semi grouted work suits footway construction particularly well.
Base, aggregate and compaction
- Check the foundations and drainage first. The base may be scarified, or clean sand or quarry fines applied to a total thickness of no more than 13mm.
- Any clean, angular aggregate can be used provided its crushing strength will carry the traffic. A typical grading for 50mm compacted thickness is 60 per cent 20/40mm, 30 per cent 20/31.5mm and 10 per cent 14/20mm single sized material; for 65 to 75mm it is 60 per cent 50mm, 30 per cent 20/40mm and 10 per cent 14/20mm.
- Stockpile aggregate outside the area it will be spread on and protect it from contamination. Spread it with shovels rather than forks, avoiding segregation, or mechanically with a spreader box, in which case spread the coarse aggregate first.
- For semi grouted work, apply water during rolling to form a uniform water bound layer. That lower layer should not exceed two thirds of the total thickness of the compacted aggregate.
Rates of application, up to 75mm
- 50mm course, 40mm down aggregate: full grout 5.5 to 7.0 litres per square metre, semi grout 3.0 to 5.5.
- 65mm course, 50mm down: full grout 7.0 to 9.5, semi grout 4.0 to 7.0.
- 75mm course, 50mm down: full grout 9.5 to 11.0, semi grout 5.5 to 8.0.
The emulsion is usually class C60B3, C60BF3, C69B3 or C69BP3, though other grades may be used by agreement between purchaser and supplier. For a full grout it is an advantage to apply the emulsion in two stages. Immediately after application, clean 2.8/6.3mm or 6.3/10mm chippings are spread uniformly to fill the interstices.
Above 75mm
Spread, compact and grout in two separate layers, with the bottom layer no thinner than the top and the aggregate size suited to each course. Grout the bottom course, then spread the second layer of aggregate immediately, compact and grout. Only the surface interstices of the second layer are filled with chippings. Work that will be jointed later is finished with a feathered edge of at least 300mm, cut back to a clean vertical face immediately before the new material is laid.
Spray injection patching
Spray injection patching, once called velocity patching, uses specialist proprietary machines to make an in situ repair without cutting out. It is generally used on less heavily trafficked rural roads, though it also serves for emergency and semi permanent repairs on strategic roads, hard shoulders and motorways where circumstances suit. BS 10947 gives the guidance.
The machine delivers a stream of high volume, low pressure air that removes unsound material and cleans the defect. Bitumen emulsion is fed into the airstream to prime the exposed surfaces, then clean aggregate is added to the airstream alongside the emulsion. Coated aggregate is driven into the base of the pothole, building and consolidating at the same time up to the finished level. A surface coating of clean aggregate can finish the repair, which can normally be trafficked immediately. Where traffic turns, or is heavier, or where the repair is pre patching before surface dressing, the repair benefits from additional compaction.
Materials
The emulsion is a cationic grade to BS EN 13808, chosen to suit the individual machine and the time of year, with a binder content typically between 60 and 70 per cent, formulated for optimum coating and breaking. Unmodified and modified grades are both available.
The chippings are clean, washed, surface dressing grade to BS EN 13043, typically 2.8/6.3mm. Aggregates must be assessed for compatibility with the emulsion so that they coat fully and do not break prematurely: adhesivity to BS EN 13614, and the Snowball Test in Annex A of BS 10947 to show the chipping forms a cohesive mixture. Dirty or dusty chippings, such as surface dressing sweepings, are not advised, because they coat poorly and the repair is likely to fail.
Season and conditions
BS 434-2 treated spray injection patching as a seasonal process, like surface dressing, with a season starting four to six weeks before the surface dressing season and finishing at about the same time. That still holds for maximum productivity, though seasonal grades of emulsion have extended the window for emergency and temporary repairs, and in suitable conditions for semi permanent ones. Conditions to avoid are frost or snow, whether present or forecast; road or air temperatures that are too low; prolonged wet periods; poorly drained sites; and areas of heavy shade such as trees and tall hedges.
It is good practice to run the Snowball Test before starting work each day, as a check that the conditions and the materials on the lorry will give a cohesive repair.
References
BS 10947:2019, Spray injection patching for highways and other paved areas. BS EN 13043, aggregates for bituminous mixtures and surface treatments. BS EN 13614, adhesivity. BS EN 13808, cationic bituminous emulsions. Road Note 39, Design Guide for Road Surface Dressing, Transport Research Laboratory.
Related guidance
Patching is a repair. Preventing the next pothole is a different question: does preventative road maintenance save money, and surface dressing with bitumen emulsion, which patching often precedes.
For the material itself, see what bitumen road emulsion is, or start at Discover REA.