PAVEMENT RECYCLING By WILLIAM T. SUNLEY, Engineer of Local Roads and Streets

The 1992 roadway improvement season is rapidly drawing to a conclusion. It is time to take stock of your highway or street system and begin to plan for next year's improvement program. With today's climate of increasing costs and decreasing revenues, public officials are consistently being asked to make their highway dollars stretch farther than in the past. An increasingly attractive approach to roadway rehabilitation involves pavement recycling. Recycling offers an opportunity to economize on energy and material costs by re-using some or all of the existing roadway components. There are a number of recycling alternatives that will aid in extending the service life of the road or street.

Recycling of Bituminous Mat Surface. An existing bituminous mat may be recycled by cold milling some or all of the pavement surface and re-using the milled material, recycled asphalt pavement (RAP), in the new surface. Most hot mix asphalt producers have the capability to process the RAP in their plants and re-use it as part of the new pavement structure. Some highway agencies and contractors have developed procedures to combine RAP with emulsified asphalt or rejuvenating agents to produce cold-applied base or surface material. Your District Local Roads Engineer will be able to help you obtain information about these procedures.

Bituminous mats may also be reclaimed by the hot surface recycling process. In this process, the existing surface is heated to a depth between 3/4 and 1 1/2 inches. The heated surface is then scarified to loosen the heated material. An approved rejuvenating agent is added to the scarified material, which is then placed as a leveling course or mixed with new surface and laid as a new surface.

Full-Depth Recycling of Flexible Pavements. Flexible base pavements whose wearing surface is beginning to deteriorate may be recycled in-place. The existing base and surface are pulverized and mixed together. Some type of stabilizing agent, the most common of which is emulsified asphalt, is then mixed with this material and compacted to form a stabilized base which is usually stronger than the previously existing base material. A new wearing surface, either a seal coat or mat surface, is then applied to the recycled base.

This procedure is highly suited to a roadway where an adequate thickness of base material is present. If the base thickness is inadequate or the base is contaminated with soil, emulsified asphalt or pozzolanic material cannot be used as stabilizing agents. In those situations, Portland cement could be used to create a soil cement base.

Recycled Portland Cement Concrete Pavements. Existing deteriorated concrete pavement may be reused as a base for a bituminous overlay by cracking and seating or by rubbleizing. Cracking and seating relieves the stresses and faulting of large pavement slabs by cracking the pavement into smaller slabs. The cracked pavement is seated firmly on the subgrade with a heavy roller prior to resurfacing.

Rubbleizing creates an aggregate base by fracturing the concrete pavement into rubble-size fragments and resurfacing. The most efficient method of fragmentation utilizes a resonant pavement breaker which fractures the pavement with high frequency vibrations. This process is suitable for reinforced pavements while cracking and seating is only suitable for non-reinforced pavement.

All of these recycling options have been used successfully in Illinois. Each roadway rehabilitation project should be evaluated to determine if it is a candidate for recycling and if so, which option would be most suitable. •

Credits to: Haldor C. Lowrey, Project Development Engineer

September 1992 / Illinois Municipal Review / Page 15