Foamed Bitumen is both a cost efficient and environmental friendly solution for the reconstruction or rehabilitation of asphalt roads.
Because of this there are now a number of, mainly, proprietary processes where suitable bituminous road planings are being plant mixed with foamed bitumens to produce proprietary bituminous products suitable for specific purposes.
- Foamed Bitumen is created by adding around 1 to 3% cold or lukewarm water to hot bitumen (at 170 to 180°C) inside an expansion chamber
- The two liquids become intimately mingled and are then forced through nozzle out of the chamber.
- At atmospheric pressure the water vaporizes instantly and the molecules of bitumen stretch to form a foam that expands to many times the original volume of the bitumen.
- The foam has a low viscosity which means it is able to mix easily with aggregates.
- The foam breaks and preferentially coats fine aggregate particles within the mix.
- Unlike hot mix and emulsion mixes; foamed bitumen mixes only partially coat large aggregate particles.
- The bitumen coated fines form a mortar that binds the mixture together.
Foamed Bitumen Recycling of Road Planings:
Road planings are especially suitable for foamed bitumen because the material is generally well granulated, allowing for an even coating of the foamed bitumen.
In some cases it may be necessary to screen the planings for any large lumps of material that could potentially hinder the laying. Any large pieces that are removed can be crushed/granulated and re-introduced into the process.
The foaming process normally takes place immediately prior to mixing, removing the need for large containers on site.
Although the process can be performed cold it is advisable in plant processing to involve low level heating, as heat aids both mixing and the subsequent workability and compaction on site.Bitumen foam mixes are normally of a less viscous nature (less stiff) and as such tend to be used on less demanding sites with respect to weight and amount of traffic. Rarely are they used as a surfacing material.
Foamed bitumen materials have been used successfully as base (roadbase) and binder course (basecourse) in road pavements that do not require a high degree of stiffness.
It is possible to improve the stiffness of materials produced using foamed bitumen by introducing a hydraulic binder such as cement, lime or pulverised fuel ash into the process. However, stiffness is not be immediate and will take considerable time to develop.