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Milling the old, damaged pavement layers produces material of a small-sized, granulated nature that needs to be bound again. The main purpose of all types of binding agents is to ensure the cohesiveness of all particles in the mineral aggregate mixture, aiming at improving the strength of the mix, and therefore the road pavement. Three binding agents play a major role in cold recycling: cement, which is a hydraulic binding agent, and bitumen emulsion and foamed bitumen, both of which are bituminous binding agents. Pavements bound with cement tend to be fairly rigid with excellent bearing capacity, while pavements bound with bitumen are comparatively flexible and therefore less prone to cracking.
Reclaimed pavement material is most effectively stabilized with cement – even if the material was previously bound with bituminous binding agents. Roads bound with cement are more stable than bitumen-bound roads. The road may, however, become prone to cracking due to the high rigidity of the pavement material, which is why a combination of bituminous and hydraulic binding agents is increasingly applied.
Bitumen emulsion consists of two normally immiscible liquids: bitumen and water. Being an emulsion, the bitumen is dispersed in the water in minute droplets. Once the emulsion comes into contact with the aggregate mix, it “breaks”. When that happens, the bitumen droplets adhere to the aggregate, producing a film which coats the aggregate particles.
When 180°C hot bitumen is mixed with water and air, the resulting reaction can be likened to that of brewing a cup of cappuccino: The water evaporates abruptly, which causes the bitumen to foam and to increase its surface, resulting in reduced viscosity. Foamed bitumen was discovered for cold recycling at the beginning of the 1990ies. Since that time, its market share has been growing tremendously. Compared with bitumen emulsion, foamed bitumen reduces the overall consumption of bitumen, and therefore needs less supply trucks.
The functional properties of road pavements can be improved significantly by the combined use of foamed bitumen and cement. Such pavements are called semi-rigid. The combined use of foamed bitumen and cement offers the advantage that the pavement remains flexible because of its bitumen content, which counteracts the proneness to cracking, while cement increases its resistance to deformation and, therefore, its bearing capacity when compared to purely bitumen-bound roads.
