Bitumen Emulsions

An emulsion is an intimate mixture of two or moresubstances which normally do not mix together. Bitumen emulsion consists of a dispersion of tiny globules (4 to 10 microns in diameter) or bitumen in water, stabilized by the addition of an emulsifyingagent.

The nature and quantity of the emulsifying agentcontrols the type and stability of the emulsion. Emulsions are manufactured by forcing the required proportion of bitumen, water and emulsifying agentinto an homogenized, whereby the bitumen is broken up into the globules, dispersed and held in suspension by the emulsifying agent. The bitumen content of an emulsion can vary from 30% to 70% (usually 55-70%) by volume depending on the grade of emulsion required.

Emulsions are used primarily on maintenance work, surface enrichment, dust laying processes, slurry sealing, and in projects where weather conditions may prevent the use of hot bitumen or cutbacks.

“Breaking” of an emulsion is the term applied when the bitumen globule suspension breaks down resulting in complete separation of the bitumen and water components. Breaking normally occurs on the pavement surface after spraying whereby the water from the emulsiondrains off or evaporates leaving behind a bitumen residue.

The surfaces of the suspended bitumen globules develop an ionic (electrical) charge. The type of charge depends on the emulsifying agent employed. The type of charge depends on the emulsifying agent employed. The presence of this charge is used to advantage to secure greater adhesion of the bitumen to the aggregate.

The basic types of bitumen emulsion are available :-
• Anionic emulsions<span style="font-size:100%;">
• Cationic emulsions.

The two types of bitumen emulsion are incompatible and should not be mixed under any circumstances otherwise premature breaking will occur. Some aggregates, particularly acidic types such as granite and quartzite, develop a negative ionic surface charge when wetted. If a cationic emulsion is used with such aggregate, the bitumen globules, which have a positive ionic surface charge, will be attracted electrochemically to the aggregate and a better bond will be established between the bitumen and the aggregate.

Similarly other aggregates, such as limestone, can develop positive surface charges and under these conditions an anionic emulsion containing bitumen globules with a negative ionic surface charge is best used.

Anionic Bitumen Emulsion

The bitumen content of anionic bitumen emulsion used in Australia is not less than 55 percent but this can be increased up to 70 percent. Bitumen used may be either Class B160 or Class B80.

Anionic emulsions are classified, according to their setting (or breaking ) times, as rapid setting (ARS), or slow setting (ASS).

(a) Rapid Setting (ARS) : Rapid setting emulsion is one which breaks rapidly on application. It is suitable for maintenance patching and sealing. It may be diluted with water for light application such as surface enrichment provided the dilution does not generally exceed 1 to 1 by volume. If hard water is used a suitable detergent should be added.

(b) Slow Setting (ASS) : Slow setting emulsion is one which has sufficient mechanical and chemical stability for mixing with densely graded aggregates, soils, or finely divided
materials of small maximum size. It may be used for soil stabilisation and for light applications where a high dilution with water (up to 10 to 1 by volume) is required. Slow
setting emulsions are usually produced to meet a special need which takes into account the characteristics of the materials involved.

Cationic Bitumen Emulsion

The bitumen content of cationic bitumen emulsionused in Australia is generally between 60 and 70 percent. Bitumen used is either Class B160 or Class B80. A cationic emulsion may be used to better advantage than an anionic emulsion with acidic types of aggregate, such as granite, or in damp conditions.

Cationic emulsions are classified, according to their setting (or braking) times, into rapid setting (CRS), slow setting (CSS) and aggregate mixing (CAM).

(a) Rapid Setting (CRS) : Rapid setting emulsionbreaks rapidly on application. It is suitable for

maintenance patching and sealing.

(b) Slow Setting (CSS) : Slow setting emulsion is suitable for soil stabilization.

(c) Aggregate Mixing (CAM) : Aggregate mixing emulsion contains a proportion of oil to delay its setting time when mixed with aggregate. It is normally used for making cold mix to be stockpiled and progressively used over a period.

Bitumen emulsion used as binders seldom require any preparation other than possible warming to about 450C to facilitate spraying in cold weather. Heating above 500C should be avoided otherwise premature breaking may occur.