CONTENTS: Introduction Emulsion Approach   The Process Summary
 

The Emulsion Approach
The development of coating grade emulsions in recent years has overcome these disadvantages and given additional benefits.

(a)           Emulsions may be used at ambient and warm temperatures with aggregates that may not be completely dry.
(b)           Lower temperatures reduce the amount (and cost) of fuel used to dry the aggregate.
(c)           Operating at lower temperatures when coating with emulsion gives a greater margin of safety, even though the emulsion may contain a small proportion of flux oils.
(d)           Emulsion coated mixtures can be stockpiled for long periods.  They may also be packaged into small containers for convenient storage and transport.  This allows small areas to be treated quickly, cleanly and economically.

BS 434 Part 2 partly defines two techniques for using bitumen emulsion to produce storage grade macadams.  The first method produces a mixture in a single operation.  The second lightly coats some of the aggregate constituents separately and then mixes them together with the emulsion.

It is not possible to lay down hard and fast rules as to the method of manufacture or the choice and quantity of emulsion.  Consideration has to be given to the chemical nature of the aggregate, the target grading of the mixture and the type of mixing plant.  These factors can only be assessed in consultation with the emulsion supplier.  Laboratory mixture design using representative samples of the aggregates are usually required


The Process

1.   Emulsion

The choice of emulsion will depend on the chemical nature of the aggregate, the target grading of the mixture and the intended use.  The choice of emulsion type and grade should be made in consultation with the emulsion supplier.  The emulsion formulation may have to be adjusted to suit the characteristics of the aggregate.

2.   Aggregate

Whilst most aggregates that are currently used in hot bituminous mixtures are suitable for use with emulsions, some may not be.  Aggregate need not be completely dry but excess moisture and fines may prevent coating and cause stripping.

It is not usually necessary to add water, but some highly acidic or alkaline aggregates benefit from pre-wetting. 

3.   Mixing

Mixtures can be produced in most types and sizes of mixer.  The mixing times should be kept to the minimum required to achieve full coating.  Excessive mixing can lead to the stripping of the binder from the stone. 

4.   Specifications – grading and binder content

It is very important that specifications are agreed with the emulsion supplier and reflect the characteristics of the aggregate.

Most storage grade macadams use a grading that is based on the routinely used bitumen macadam mixtures in BS 4987-1.  Typical mixtures are 0/20 mm size dense binder course and 0/6 mm size medium graded or dense surface course.  0/20 mm size open graded binder course is often supplied in bags and tubs.  A number of storage grade mixtures based on a stone mastic asphalt (SMA) type grading have also been successfully produced. 

5.   Laying

 For further information see Summary and Reference Sheets on the Association’s website www.rea.org.uk