The Recycling Process

Design and construction: Simple and fast

The basic procedure is simple. The complete recycling process can be finished in one day, and local traffic can return almost immediately. The procedure includes the following steps:

Thickness Design. Pavement thickness can be determined by using PCA’s Thickness Design for Soil-Cement Pavements (EB068). Other methods, such as the American Association of State Highway and Transportation Officials (AASHTO) Guide for Design of Pavement Structures can also be used.

Site Investigation. The site should be investigated to determine the cause of failure. Cores or test holes should be used to determine layer thicknesses and to obtain samples of the material to be recycled (which can include asphalt surface, base course aggregate, and subgrade).

Lab Evaluation. Material samples from the site should be pulverized in the lab to create an aggregate-soil mix that will be similar to that expected from the recycling process. The mix design procedure is the same as that performed for soil-cement. Refer to PCA publication EB052 Soil-Cement Laboratory Handbook. This includes the determination of maximum dry density and optimum moisture content. If unconfined compressive strength is used to determine cement content, a 7-day strength of 300 to 400 psi (2.1 to 2.8 MPa) is recommended.

Scarification and Pulverization. Depending on the construction equipment available, and the thickness of the existing pavement, the roadway may need to be scarified (ripped) before it can be pulverized. Some equipment, however, is capable of pulverization without scarification first. Quality full-depth recycling is usually performed using equipment especially designed for this purpose. The depth of pulverization is usually 6 to 10 in. (150 to 300 mm), which on secondary roads will typically include all of the surface and base, plus some part of the subgrade. To achieve the proper gradation after pulverization, more than one pass of the equipment may be necessary. The particle distribution should have 100% smaller than 2 in. (50 mm) and 55% passing a No. 4 (6-mm) sieve.

Shaping and Grading. The pulverized material is shaped to the desired cross-section and grade. This could involve additional earthwork in order to widen the roadway. Final base elevation requirements may necessitate a small amount of material removal or addition.

Spreading Cement. Cement is spread in a measured amount on the surface of the shaped roadway, in either dry or slurry form.
Spreading dry cement
Spreading cement (slurry)

Water Application. Water is added to bring the aggregate-soil mixture to optimum moisture content (water content at maximum dry density as determined by ASTM D558), either in front of the pulverizer/reclaimer or in the mixing chamber.

Mixing. The aggregate-soil-cement-water mixture is combined and blended with the pulverizing/mixing machinery. Multiple passes of the mixer may be required to achieve a uniform blend of materials.

Compaction. The mixture is compacted to the required density of at least 96% of standard Proctor density (ASTM D558). The compaction is usually performed with smooth-wheeled vibratory rollers. A pneumatic-tired roller may follow to finish the surface. Final compaction should take place no more than 3 hours past initial mixing of the cement. The field density and moisture are monitored for quality control purposes.

Curing. The surface is kept moist by periodically applying water to the surface, to make sure it does not become dry. This is done continuously through the curing period until the base can support traffic without deforming. The application of the prime coat should occur as soon as possible to ensure that moisture is sealed inside the base.

Pavement Surface. The new pavement surface consisting of a chip seal, hot-mix asphalt, or concrete is constructed to complete the recycling process.
Applying tack coat prior to paving
Paving

Quality Control. Recycling with cement follows the same basic procedures used for normal soil-cement operations. The success of a recycling project depends upon the careful attention to the following control factors:

 

Text and Photos courtesy of Portland Cement Association www.portcement.org

 

©2005 Road Recycling Council
P.O. Box 17362 - Cambridge, MA 02141
phone: (857) 998-0119 fax: (617) 547-0042
www.roadrecycling.org