It needs to be green” is the popular phrase heard these days. Agencies in search of environmentally friendly technologies and needing to stretch ever-shrinking budget dollars are turning to recycling and reuse now more than ever. Some of the most effective techniques that satisfy both criteria—saving green while being green—are cold in-place recycling (CIR), hot in-place recycling (HIR) and full-depth reclamation (FDR).
Since its inception in 1976, the Asphalt Recycling & Reclamation Association (ARRA), the largest construction recycling organization in the world, has promoted these pavement recycling technologies with respect to their economic and environmental benefit. Apparently it has taken ARRA 33 years to become an overnight success, but that is not to imply that there is not much more work that still needs to be done.
An ARRA-promoted recycling process, though not as well-known, is cold central plant recycling (CCPR). In this process, concrete, reclaimed asphalt pavement (RAP) and/or base is removed from one area or site and trucked to a separate central processing location.
At the processing location, the imported materials are crushed, screened and blended in a pugmill-type mixing plant with additives and water. Typical additives include asphalt emulsions and/or portland cement.
The recycled mixture is then trucked to the end-use location back onsite or at an alternative project location, where it is laid down and compacted as a new pavement. End-use conditions, including traffic volume and loads as well as environmental susceptibility, determine the design of the CCPR pavement with respect to structure, thickness, additives and type of surface seal used. By incorporating some innovation and sound engineering principles, almost every pavement reconstruction project is a potential candidate for CCPR, as illustrated by the following project examples.
Reuse is key
In July 2003 Tim Kretschmeyer, owner of Kretschmeyer Steel, a steel fabrication company located in Rialto, Calif., was fed up with maintaining an unpaved yard, as he had done for many years before. After the sticker shock from cost estimates incorporating conventional hot-mix asphalt (HMA) options for paving the surface wore off, Kretschmeyer decided to use Pavement Recycling Systems Inc.’s (PRSI) proposed CCPR option. The PRSI-designed structural section called for 3 in. of CCPR recycled pavement over 4 in. of crushed slag base. The slag base was a recycled product manufactured from the waste of a local steel foundry. PRSI manufactured the CCPR pavement onsite using RAP millings imported from a nearby city of Riverside street resurfacing project.
PRSI used their Nescon recycling unit, comprising a crushing and screening deck integrated and computer interlocked to a dual-shaft counter-rotating pugmill and mass flow emulsion meter, to process the recycled pavement. Half the yard was mixed with a recycling emulsion provided by SemMaterials LP (previously Koch Materials) and the other half was mixed with a recycling emulsion provided by Western Emulsions Inc. Initially, a fog seal was applied as the only surface seal. Approximately a year later, half the site was resealed with Preservation Seal (a seal coat incorporating recycled tire rubber) and the other half with a Type I rubberized emulsion asphalt slurry (REAS), both provided by Petrochem Manufacturing Inc. The steel yard pavement surface needed to be durable, since it was subject to large forklifts with tight turning radii and high shear forces.
In 2007, Kretschmeyer purchased the adjacent unimproved property. This time, he insisted the new site be paved with CCPR pavement. It was more cost-effective than the HMA option, and the original site pavement had shown excellent performance. The adjacent site was paved with a CCPR pavement manufactured by Reclaimed Aggregates Inc. under their trade name ReNew. ReNew Pavement is a 100% RAP product that requires a job-specific mix design. For the anticipated steel-yard loadings, the RAP aggregate grading called for a blend of three distinct RAP screenings: 1 in. x 3?8 in., 3?8 in. x No. 4 and No. 4 minus. The graded RAP aggregate was then blended at prescribed proportions in RAI’s Colton, Calif., facility and mixed with water and a specially formulated emulsion provided by SemMaterials in accordance with the mix design.
RAI, a Nevada company, was established in 2004 for the express purpose of developing urban quarries. Urban quarries are nonpit, nonmining processing facilities strategically positioned to accept asphalt and concrete rubble for recycling into pavement and other aggregate products. At each of its three facilities located in Colton, Chula Vista and Lancaster, Calif., RAI carefully monitors and segregates the rubble into separate piles. The piles are composed of RAP millings, chunked asphalt concrete, clean portland cement concrete and mixed product unable to be segregated effectively. Depending on the end product to be produced, the rubble is fractionated and reconstituted as necessary.
RAI’s engineering, testing and a rigorous QA/QC program have successfully demonstrated that much of the best aggregate currently available exists in the pavements and slabs of the country’s deteriorating infrastructure.
“Not to reuse the aggregates, residual asphalt and/or cement available within these discarded products in the highest engineered option possible truly is a waste,” said Steve Gibb, RAI’s Colton facility manager.
More than a supplier, RAI put their practice to their own use by paving each of their facilities with variations of recycled pavements. The main driveway at the Colton facility, traversed by an average of 125 fully loaded trucks per day, is paved with 3 in. of ReNew sealed with a thin fog seal and supported by 6 in. of a cement-treated soil.
The city of Colton used a similar product in February 2007 on a pilot project for Key Street. Key Street is an industrial road that was heavily fatigued and in desperate need of rehabilitation. Because of a lack of working area, Key Street’s existing pavement was milled to a depth of 3 in. and hauled to RAI’s Colton facility where it was recycled and then returned, paved and sealed. The CCPR pavement was sealed on one side of the roadway with a central plant-manufactured Type I REAS and the other with a Type II RAP slurry. RAP slurry is typical road slurry that uses aggregate 100% recycled from RAP.
Pavement Coatings Co. of Cypress, Calif., installed both slurries. The two wearing surfaces are monitored to this day with respect to durability and performance. Both surfaces appear to be performing well, with the primary difference being in the surface texture between the Type I and Type II aggregate used.
The County of San Bernardino Special Service District was not sure what they were going to do when bids came in over budget to improve the roads of an unpaved desert/mountain residential community in the Wrightwood area of California. For years the county was responsible for maintaining the dirt roads through hot, dusty summers and cold, snowy winters. Finally, funds to pave the project were generated via assessment of the homeowners, but increasing the available budget was not an option.
The county was able to solve the dilemma by rebidding the project specifying a more cost-effective CCPR recycled pavement alternative. PRSI, the low bidder on the project, completed the main construction portion of the project in October 2006, just as early winter storms threatened cooler temperatures and significant precipitation. PRSI processed RAP millings obtained from a nearby highway reconstruction project, again processing them with a recycling emulsion provided by SemMaterials. PRSI was faced with several inherent challenges including a late season start to the construction, relatively steep hillsides and the necessity to provide many homeowners access over the freshly laid recycled pavement because of limited alternative routes and the narrowness of many of the roads.
PRSI added 0.75% of portland cement to the CCPR mix to facilitate curing and provide early strength. Care was taken to ensure the ideal proportion of emulsion and cement in order to achieve sufficient coating and bonding. Upon completion of construction, a few isolated areas required minor repair of raveling of the pavement surface. The areas of shallow raveling appeared confined to steeper slopes and perpetual shade areas. After initial construction, a fog seal was applied as a sealer. Approximately a year later, after the pavement was subjected to winter snows and freeze-thaw cycles, Type II RAP slurry was applied.
Paul Von Norman, a Caltrans maintenance superintendent, took advantage of Caltrans’ Pavement Preservation Task Group Innovation program to construct a CCPR project on Highway 79 in April 2006. The 2-mile project consisted of milling the upper 3 in. of a 1-mile-long stretch of highway in the early morning. The milled RAP was trucked to a nearby central processing plant where it was recycled. It was trucked back and then repaved that afternoon. The second mile was accomplished in nearly the identical way, with the exception that the compaction was improved based upon lessons learned from the first day. The project included several variations including using a rubberized recycling emulsion provided by PMI along with the addition of portland cement as a curing agent for the recycled pavement. In addition, Caltrans performed a great deal of work with their own work force, including the loading, trucking, sweeping and a portion of the lay down and compaction.
Unfortunately the utilization of an undersized roller for compaction the first day led to that section of pavement experiencing excessive raveling when swept. The proper-sized roller in accordance with the project specifications was utilized the second day. The proper compaction minimized raveling on the second mile of highway. Because of the rough surface associated with the raveling, the first mile was sealed with a scrub chip seal by Western. The scrub seal was covered as a cape with Type I REAS. The second mile’s nonraveled, tightly compacted surface was sealed with PMI’s Preservation Seal.
ReCap
Portland cement alone also can be effectively utilized in CCPR pavements. The parking lot demand of a large trucking facility prompted RAI engineers to develop their ReCrete product, a cross between cement-treated base and roller-compacted concrete (RCC). ReCrete is composed of aggregates generated from recycled concrete with limited RAP blended with portland cement and water. Like RCC, it is placed with a paver and compacted by heavy rollers. RAI has improved the process to the point that minimal cement and fog sealing, and mircocracking at the opportune time, results in a hard, durable, puncture-resistant surface. ReCrete has been used effectively in heavy truck and loading areas.
For most projects a fog seal proves an adequate surface sealant. However, when additional skid resistance is required, the ReCrete has had a RAP chip seal applied. A RAP chip seal uses chip aggregate produced 100% from RAP. After traffic and absorption of the sun’s energy, the RAP chip surface molds into a dark, coarse asphalt concrete appearance that is both aesthetically pleasing and aids in chip retention.
In times of economic and environmental pressures, CCPR should be considered for most construction projects. With the variations of aggregates, emulsions, additives and surface seals that are both cost effective and incorporate recycled products, an innovative designer has virtually unlimited options with respect to final pavement structure. By proper engineering, QA/QC and construction practices, the road to the future can and should be paved with recycled materials.
