Bitumen emulsions were invented more than 100 years ago and are literally copied from nature. After a French physician detected the health impact of the street dust, first tests were started to keep the roads dust-free by applying road oil with liquid bitumen types and soft coal tars. During these efforts it was found out that bitumen can be applied in a cold state easily if it is emulsified before. Emulsifying means distributing fatty substances in the form of microscopic droplets in water. An effect which is used in creams and which occurs naturally in milk, for example.
There are numerous advantages of bitumen emulsions which are among the most-used raw materials in road construction:
- No risk of burns and fires
- No toxic solvents
- No risk of explosions by solvent vapors
- Release water only when hardening (liquid or steam)
- Are applied in cold state and save energy
- Don't penetrate the ground
- Great variability of characteristics, thus high flexibility
Bitumen emulsions are produced in colloid mills. The bitumen is pressed together with the "watery phase" through a narrow gap between the stator and the rotor which rotates at high speed. In this process, the raw material is torn in very fine particles. The watery phase contains emulsifiers, i.e. special chemical substances which are produced on a natural fat and oil basis and are soluble both in water and bitumen to stabilize the emulsion. That means, the border area between the bitumen droplets and the water phase is fixed and the emulsions becomes "storage-stable" as a consequence. The emulsifier lends the bitumen droplets an electrically charged surface so that the (equally) charged droplets repel mutually. Depending on the type of emulsifier used, the bitumen droplets are positive (cationic emulsions) or negative (anionic emulsions).
The bitumen content of modern bitumen emulsions can exceed 70%. One liter of emulsion contains up to one quadrillion (!) droplets with diameters between 0.1 to 50 µm. Depending on the type and quantity of the emulsifiers (the emulsifier contents are between 0.1 and 2 % depending on the type) the bitumen emulsion has different application properties.
Emulsions can be stable and unstable, with the stability reflecting the behavior when the emulsions come in contact with stone (road construction). While unstable emulsions disintegrate within a few seconds after they come into contact with stone - this process is referred to as "breaking" - and act as a glue for the stone, stable emulsions can be mixed with stone for a longer time and start their gluing effect slowly. After breaking, the "hardening" process starts, i.e. the slow evaporation of the emulsion water and the continuing filming of the bitumen, which has a weak adhesion at the beginning, to a solid bonding agent layer.
All construction methods based on bitumen emulsions depend on warm weather, because the water must evaporate gradually. For this reason, the use of bitumen emulsions is limited to the 6 warm months of the year. Surface treatment, cold-applied thin-layer covers and the production of bond coats, these are the classical, widely used construction methods using bitumen emulsions for road maintenance.
Bitumen emulsions are a standardized construction material group (requirements according to ÖNORM B 3501 through 3504 and inspections according to ÖNORM C 9230 through 9239) and are quality-protected due to their importance for economical road maintenance in Austria.
