Storage grade bituminous mixtures are the materials kept on a stockpile or in tubs for trench reinstatement and emergency patching. Made the traditional way, with hot bitumen and flux oils, they need the aggregate heated hard, carry a fire risk from the low flash point of the flux, and lose volatile oils in storage, which makes their workability unpredictable. Coating grade emulsions remove all three problems: they coat at ambient or warm temperatures, with aggregate that need not be completely dry.
REA Technical Data Sheet No.11, Bitumen Emulsions for Storage Grade Macadams
What storage grade material is for
Storage grade bituminous mixtures are produced by coating mixed aggregates with hot bitumen and flux oils, the flux modifying the binder viscosity and extending the storage life of the finished product. They are used in temporary and permanent trench reinstatements and in emergency patching, and the mixture may sit on a stockpile for some time before it is used. It has to stay workable in the pile and still be stable once it is laid.
The drawbacks of the hot method
- The aggregate has to be heated to a high temperature to make sure it is coated with bitumen.
- That elevated temperature increases the risk of fire at the mixing plant, because the flux oils have a relatively low flash point.
- Volatile oils are lost from the flux over time, which makes workability unpredictable.
What emulsion changes
Coating grade emulsions, developed in recent years, overcome those drawbacks and add benefits of their own.
- Emulsions can be used at ambient and warm temperatures, with aggregates that may not be completely dry.
- Lower temperatures mean less fuel burned drying aggregate, which cuts both cost and greenhouse gas emissions.
- Working cold gives a greater margin of safety, even though the emulsion may carry a small proportion of flux oils.
- Emulsion coated mixtures can be stockpiled for long periods, and can be packaged into small containers for storage and transport, so small areas can be treated quickly, cleanly and economically.
The now withdrawn BS 434-2 partly defined two techniques. The first produces the mixture in a single operation. The second lightly coats some of the aggregate constituents separately and then mixes them together with the rest. Which suits a given job depends on the chemical nature of the aggregate, the target grading and the type of mixing plant, and those can only be assessed in consultation with the emulsion supplier. Laboratory mixture design using representative samples of the aggregate is usually needed.
The process
Emulsion
The choice depends on the chemical nature of the aggregate, the target grading and the intended use, and should be made with the emulsion supplier. The formulation may have to be adjusted to suit the aggregate.
Aggregate
Most aggregates used in hot bituminous mixtures are suitable with emulsions, though some are not. They need not be completely dry, but excess moisture and fines may prevent coating and cause stripping. Adding water is not usually necessary, though some highly acidic or alkaline aggregates benefit from pre wetting.
Mixing
Mixtures can be produced in most types and sizes of mixer. Keep mixing times to the minimum needed for full coating: excessive mixing can strip the binder from the stone.
Specification
Agree the grading and binder content with the emulsion supplier, reflecting the characteristics of the aggregate. Most storage grade mixtures use a grading based on the routine asphalt concrete mixtures in BS EN 13108-1. Typical mixtures are an AC 20 dense binder course and an AC 6 medium or dense surface course, and AC 20 open binder course is often supplied in bags and tubs. Storage grade mixtures based on a stone mastic asphalt grading have also been produced successfully.
Laying
Lay and compact to BS 594987. Good joint preparation and the correct compaction plant, used properly, are what deliver the performance.
In short
Producing storage grade bituminous mixtures with bitumen emulsion is a proven, safe and cost effective process. Emulsions can manufacture most asphalt concrete mixtures, for immediate use or for storage. Close consultation with the emulsion manufacturer is what ensures the mixture is well coated, has the expected grading and can be made successfully.
References
BS EN 13108-1, bituminous mixtures, asphalt concrete. BS 594987, transport, laying, compaction and type testing protocols. The withdrawn BS 434-2 is referenced for context only.
Related guidance
Storage grade material is laid cold, often in a pothole or a trench. Patching, grouting and spray injection patching covers the repair methods themselves, and bond coat or tack coat what BS 594987 requires between courses.
For the material itself, see what bitumen road emulsion is, or start at Discover REA.