Hot-in-Place Recycling

Once the recycled pavement has been windrowed, and as a result thoroughly mixed together, it is ready to be placed in a paving process very similar to regular hot mix paving processes in use everywhere.

The windrow contains the scarified and recycled asphalt concrete in a heated and ready to be paved condition. The pavement mat is already heated which additionally contributes to the success of this process since it allows good adhesion of the recycled material. This adhesion is very important to prevent future ravelling of the existing pavement and deterioration of the pavement causing de-lamination and loss of sections of the overlay, forming potholes in the overlay. This is a common issue and is an item that affects the life of the project in a very detrimental way if overlooked during the construction phase.

When the machine was developed it was decided to add an additional 25 – 38mm (1 to 1.5 inches) of new virgin Asphaltic Concrete to provide a wearing course in the final pavement structure. In Arizona, the location where this was recently done, the addition of the wearing course is very helpful due to the high summer time temperatures and the resulting oxidation causing structural support to be compromised somewhat. The presence of new virgin asphalt allows the traffic wear and tear to be expended on the new top layer and the recycled layer is then encapsulated and further oxidation is prevented for the future life of the pavement.

The use of this HIR recycling process should be evaluated on a case by case basis. Due to the length, size and weight of the paving train, short and sinuous roadways would not be the best candidates for this process. Furthermore, the paving of subdivisions with Cul-de-sacs etc. are not recommended. However, if the section of roadway is relatively old and is paved with a minimum of 3 to 4 inches of asphaltic concrete and extends for some distance including perhaps 66,945 square meters (80,000 square yards), the project can be quite economical. The economics depend on the structural section (pavement layer types and thicknesses) layout of the pavement, and the amount of surface area to be recycled and the availability of new materials. The economy of the process is good due to the ratio of the cost of new material to recycled material. For example, a project in Arizona, USA, which used 218,095 square yards of recycled material, the estimated savings by using an HIP strategy was approximately $500,000 U.S.

The addition of new virgin material is incorporated in the recycling equipment by adding the new virgin asphaltic mix within the paving process. This requires a double screed to be used to allow the placement of the recycled material in the bottom layer (over any existing remaining asphaltic concrete that is still in place after the milling operation). The first screed is used to apply the recycled mat. Then, the new material is added on top and the entire mass passes under a second screed that incorporates the new AC on top of the just laid down recycled material. The results are generally very serviceable. Since they are both heated, they bond and are passed under the screed to generate a monolithic thick layer that has the recycled material on the bottom and the new Asphaltic Concrete on the top. The illustration below shows the various parts of the process and how they work together with the double screed system to effectively laminate the two layers together and form a monolithic layer with the new material on top as a wearing course and the recycle material on the bottom as a structural layer.

The picture above shows the process and how it proceeds to give a good re-paving solution. Notice the comment about the recycle layer. It mentions that it is a leveling course. This is true due to the tendency of the long paving machine to integrate out minor fluctuations in grade as the paver passes over it. This coupled with the new hot mix (or warm mix) on top, results in a very serviceable and somewhat more economical paving process when the caveats mentioned above are observed. For instance, due to the size of the paving train, it is not practical to pave roadways that are not conducive to the long turning radius of such large equipment. However, the large size does contribute to excellent screed compaction and good paving quality for the appropriate paving runs. It should be noted that the good paving does not obviate the need for rolling of the mat after placement. However, the heated mat will facilitate the rolling process and leads to better overall compaction if good rolling practices are observed.

The use of HIR is advantageous for longer pavement sections that do not contain unusually difficult short radius turns due to the turning radii of the equipment used. In addition, the need for at least three inches thick Asphalt Concrete is of paramount importance to allow milling to take place in the scarfication process. The results are about as good as a new pavement and the expected life of the recycled pavement is good, particularly when a new wearing course is added on top of the recycled layer.