Milling Pavements Containing Fiberglass Reinforcement

When an asphalt reinforced pavement structure reaches the end of its designed service life, the layers, including the reinforcement, needs to be milled and recycled.

Asphalt Interlayers are composed of different materials, and which consequently leads to different behavior when milling and recycling. Synthetic fiberglass reinforcement products can be milled and recycled together with the asphalt granular. This document gives an overview over items which should be considered when milling asphalt pavements that contain high strength fiberglass reinforcement.

If it is planned to mill and recycle reinforced asphalt layers, the specification should outline the type and location of the interlayer for an adequate consideration in the construction process.

Milling

If an asphalt interlayer is installed, the exact location should be recorded for consideration of future maintenance and rehabilitation works. If asphalt layers are planned to mill down, cores should be taken in advance to identify the layer composition including an identification of an existing asphalt interlayer. Once an interlayer is identified in the structure it should be considered for further procedure.

If the project circumstances allow the existing asphalt to be milled down just above the asphalt interlayer followed by paving the new asphalt, this option should be considered to maintain the benefits of the interlayer. In these cases, it`s necessary to ensure an adequate distance between the interlayer and the cutting tool of the milling machine to avoid any damage to the interlayer. Further, the layer bonding in the level of the interlayer should be adequate to allow milling without debonding the upper remaining asphalt to the interlayer. Due to the general use of an increased amount of bitumen for the installation for most asphalt interlayers and therefore an increased adhesion to the asphalt layers, the risk is of debonding relatively low.

When milling reinforced asphalt layers, the composition of the interlayer has less impact on the milling procedure and later reuse, rather the milling procedure itself and the handling of the asphalt granular is decisive for both, milling and recycling. Following aspects mainly influence the results and should be considered to optimise a proper milling process and the particle size of the interlayer in the (RAP) recycled asphalt pavement:

Milling Depth

It`s recommended to mill at a minimum depth of ½ inch below the interlayer, so that the angle of the milling tool is steep and cuts the interlayer into small pieces. The deeper the cutting tool mills below the interlayer, the smaller are the remaining strands and/or pieces of the interlayer. Good results were achieved in a depth of 1 inch below. Milling at the level of the interlayer results generally in cutting out larger strands and/or pieces of the interlayer.

Driving Speed

The driving speed of the milling machine has an influence on the particle size left in the asphalt granular. The slower the milling machine drives, the smaller the strands and/or pieces of the fiberglass reinforcement. 

Milling Drum Speed

The speed of the milling drum has an influence on the particle size left in the asphalt granular. The faster the milling drum rotates, the smaller the strands and/or pieces of the interlayer.

Distance of Cutting Tools

The distance of the cutting tools at the milling drum has an influence on the particle size left in the asphalt granular. The less distance they have, the smaller the strands and/or pieces of the interlayer.

Layer Bonding

The better the adhesion of the layers below and above the interlayer, the smaller the strands and/or pieces of the interlayer when milled. 

Recycling

Research and testing have shown that RAP containing high strength fiberglass reinforcement can be recycled and used in new asphalt mixes. 

A full-scale study conducted by the National Center for Asphalt Technology (NCAT) show that GlasPave high strength fiberglass reinforcement can be milled and recycled back into recycled asphalt pavement without any significant impact to the performance of the new mix.

Conclusion

High strength fiberglass asphalt reinforcement products can be successfully milled and recycled. When milling asphalt pavements containing pavement reinforcement care should be taken to identify the location of the reinforcement in the existing pavement structure. To insure a proper milling operation it is important to take into consideration the milling depth, speed, drum speed and spacing of teeth on the cutting tool.

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