Bitumen emulsion is a cold blend of bituminous products and water. The emulsion is a liquid at room temperature, so there is no need for heating. The emulsion is a delicate system, the bitumen products and water must not separate until the material is applied to the pavement.
Many grades of emulsion are produced by special cold mix plants, depending on the varying amount of materials blended before the spray bar makes the binder. Slight ‘folding’ ensures that the emulsion coats the mineral material without separation.
Emulsion can be used to create binder layers, overlays and wearing courses for all roads, except possible the motorways with the heaviest traffic.
Cold mix can be applied just the same way as the hot asphalt, by a paving machine. The following rolling, however, is especially important for how the bitumen emulsion works. Special cold mix system of emulsion with additives (known as NYREC and NYMIX) is compacted using vibration and pneumatic tyre rollers, which cause emulsion to break; water evaporates and bitumen remains in the pavement.
A cold asphalt layer can be opened for traffic immediately after laying, but the combination becomes stronger with time. When first applied, cold mix looks and works a little differently than hot mix, which is one of the reasons why this method is not as widely applied as it, perhaps, should be. Cold mix is ofter evaluated based on the methods which are more appropriate for hot mix, but do not efficiently measure the efficiency of cold mix.
Typically, the evaluation criteria are stiffness and void content, which are not a true measure of cold mix pavement performance. This problem is now being solved by combining cold mix formulation and assessment techniques which could become the new basis for development.
Another obstacle to wide application of cold mix is financial. Many contractors invested a lot into hotmix plants and naturally, they want to maximize their return, which does not involve development of cold mix methods.
The transition from hot mix to cold can seriously reduce energy costs and CO2 emissions, since the amount of energy needed for component heating of cold mix is quite insignificant compared to that of the hot mix. Typical fuel consumption is 0,1 liter per one ton of cold mix, compared to 10-14 liters per ton of hot asphalt.
Another factor for the use of cold mix is the environmentally driven need to reuse recycled road material, or Recycled Asphalt Pavements (RAP) as much as possible. NYREC was developed specifically for the use with RAP.
Cold mix based on bitumen emulsion can be used with a much greater portion of recycled material in the mix, that hot mix. Cold mix binder layer can quite easily be made using entirely RAP. Emulsions can also be applied to surfaces infected with tar.