A cold recycling train builds new roads from old.

Cold recycling in-situ is just the right technology when it comes to building a new road from old. It is mostly carried out as mobile roadworks. What are the advantages of “in-situ” cold recycling? How does the method work?

In roads which are exposed to very high loads by heavy traffic, damages frequently extend all the way into the pavement subgrade. To repair these damages, the pavement structure needs to be rehabilitated at full depth. Cold recycling gives roads new stability and strength. In contrast to “in-plant” cold recycling, the entire “in-situ” cold recycling process is carried out in one single operation as follows: Special cold recyclers granulate the damaged road layers – usually the surface and binder courses, as well as part of the base layer –, mix the milled material with new binding agents, and place the recycled mix again immediately.

Adding precise quantities of binding agents

Milling and pulverizing of the existing pavement is carried out by a milling and mixing rotor fitted with tungsten carbide tools similar to those used for cold milling. Inside the machine, the material is then processed in a mixing chamber or compulsory mixer, in which binding agents are added by means of microprocessor control. Some recycler models feature a so-called “variable mixing chamber”: The larger the working depth, the larger is also the mixing chamber volume. The quantity of binding agents to be added increases at the same time, controlled by a computer, in accordance with the machine’s working depth and advance speed. This concept permits optimum processing of the recycling mix. Types of binding agents to be added include bitumen emulsion, foamed bitumen, and cement.

Slurry mixer meters cement quantities

Hot bitumen for the production of foamed bitumen, bitumen emulsion, and water – which is always mandatory – are delivered by tanker trucks. Cement can either be pre-spread as a powder ahead of the cold recycler or added using a so-called slurry mixer. The slurry mixer precisely meters the correct quantity of cement, mixes it with water to obtain a cement slurry, and then transfers the liquid binding agent directly into the mixing chamber of the recycler. Pre-spreading of the cement can then be dispensed with. The slurry mixer prevents the development of dust and loss of material, and helps to improve the quality of the recycled layer.

Economical and environmentally friendly

The tanker trucks supplying binding agents and water travel ahead of and are pushed by the cold recycler, forming a so-called “cold recycling train”. Some Wirtgen cold recycler models are equipped with a paving screed at the rear of the machine for placing the recycled mix. With all other Wirtgen cold recyclers, heavy tandem rollers and single-drum compactors are used for compaction of the recycled mix.

A paving screed builds the new road

A thin surface course of virgin asphalt mix is usually applied on top of the cold recycled layer by standard road pavers, and is then compacted by rollers. Short construction times and high economic efficiency are marks in favour of this method. The fact that cold recycling “in-situ” helps to save some 600 truck transports of construction material per kilometre of road built clearly shows the tremendous saving potential when compared to the conventional method of replacing the pavement, which requires a much bigger effort in terms of both logistics and personnel.