Smithfield DPW Tests New Use for Recycled Asphalt Shingles
In its ongoing efforts to reduce road maintenance costs, the Town of Smithfield, Rhode Island is currently testing a novel application for discarded asphalt roofing shingles - using them to enhance the aggregate base produced during full-depth reclamation (FDR).
This differs from the industry practice that has emerged over the past decade whereby crushed recycled asphalt shingles (RAS) are added to hot mix asphalt and warm mix asphalt for pavement applications. In 2014, an estimated 2 million tons of asphalt shingles were recycled, mostly for use in pavement.
Smithfield, however, has taken a different approach. The town is studying the feasibility of adding RAS not to the asphalt pavement overlay but to the underlying aggregate base, to see if this can reduce the required thickness of the asphalt pavement overlay.
A Timely Idea
Director of Public Works Seth Lemoine said the idea was proposed to him by consultant R. Paul Montenegro. Providence, Rhode Island-based Montenegro specializes in pavement preservation, recycling, rehabilitation and reclamation.
"I was having discussions with Paul on various mix designs and pavement management techniques, and he told me about a project he had been developing on the use of recycled asphalt shingles blended with a roadway sub-base material," said Lemoine.
"I was intrigued by Paul's theories, and since Smithfield was in the process of reclaiming some local streets, we thought this would be a great opportunity to try it out," he said.
Montenegro explained that this use of RAS makes economic common sense.
"We're trying to stabilize and strengthen the base with the recycled shingles," said Montenegro. "There's about 25 percent asphalt content in recycled shingles. Therefore every 4 tons of shingles gives us 1 ton of residual asphalt. At present, four tons of RAS cost about $80, so for $80 we get $500 worth of asphalt content.
"It makes more sense to add inexpensive structure to the less costly base than to build structure with expensive asphalt hot mix. And by building up the base we can reduce the lift thickness of the asphalt and increase the life cycle of the road."
While the asphalt content of RAS is relatively high, the recoverable asphalt is harder than virgin asphalt binder. Consequently RAS is usually added in small amounts, say 5 percent of the total weight of hot mix asphalt. In addition, asphalt rejuvenator may be needed to improve binder quality. For the Smithfield trial, no asphalt rejuvenator was added to the RAS.
Never Tried Before
Montenegro said that to his knowledge, adding RAS to an FDR base has never been done in the U.S. or anywhere else. A search of technical literature seems to corroborate this.
There is evidence that the industry practice of adding small amounts of RAS to asphalt pavement is on the rise, but no signs of RAS being added to FDR bases. As if to prove this point, at the October 2015 Shingle Recycling Forum in Chicago, which brought together all stakeholders in the asphalt shingle recycling industry, nearly two dozen presentations by industry experts offered the very latest information on using RAS in hot mix or warm mix asphalt pavement projects. But evidently, none of them referenced adding RAS to FDR base.
Ideal FDR Candidates
Sections of Smithfield local streets, Hughes Drive and Swan Road, were chosen for the reclamation with RAS trial. Both roads were ideal candidates for FDR. They were in poor condition, had originally been constructed with substandard specifications and had utilized native materials for roadway subbase. Furthermore, the existing 1-1/2- to 2-inch asphalt pavements showed alligator cracks and sinkholes, and were rated with a PCI of only 40.
PCI refers to Pavement Condition Index, a pavement management tool. Developed by the U.S. Army Corps of Engineers and adopted by the Department of Defense and the American Public Works Association, the PCI is a numerical rating of pavement condition that ranges from 0 to 100 with 0 being the worst possible condition and 100 being the best possible condition. It is based on distress observed on the pavement surface, which indicates structural integrity and surface operational condition but not structural capacity.
For poor pavements having such defects as the two chosen for the Smithfield RAS trial, the Federal Highway Administration recommends full-depth reclamation. In this process, all of the asphalt pavement layer and a predetermined amount of underlying aggregate undergo pulverization, introduction of additive such as calcium chloride, shaping of the mixed material,
compaction, and application of an asphalt wearing or surface course. FHWA says this method results in improvement of pavement structure, restoration of desired profile, and elimination of cracks. Furthermore, these results can be accomplished without changing the geometry of the pavement or shoulder reconstruction.
It was decided to have the two trial sections of roads undergo full depth reclamation to between 8 and 10 inches, then carry out a second run with the top 4 to 5 inches of reclaimed base mixed with pulverized RAS. Four inches of asphalt pavement would then be installed over the reclaimed base.
T. Miozzi Inc. of Coventry, Rhode Island, was selected to be general contractor as well as paving contractor and hot mix asphalt supplier. Murray Paving and Reclamation Inc. of Holliston, Massachusetts, performed the reclamation. C. Carney Environmental of Raynham, Massachusetts, who produced the material using a CBI Shingle Pro XL 406 Shingle Shredder, donated the crushed shingles.
Town forces prepared the roads for reclamation, which included removing hazardous trees and damaged portions of existing New England (asphalt) berms, and protecting a few existing manhole structures from the reclaiming machine by covering them with steel plates. Since the finish grade of the asphalt overlay would be the same as the existing pavement grade, no curb reveal would be lost. The reclamation process also allowed the DPW crew to rebuild the crown of the roadway before the roads underwent finish paving.
Lemoine empirically determined the depth of the RAS layer and method of spreading the material.
"Paul Montenegro recommended an application rate of approximately 20 pounds per square yard of recycled asphalt shingles. After measuring the unit weight of the shingles delivered, I determined that approximately 3/8 inches of loose recycled shingles spread evenly on the surface would yield the desired amount.
"Initially on Hughes Drive we tried a couple of methods for distribution: (1) use a truck-mounted sanding unit (2) spreading RAS from the tailgate of a dump truck and (3) dump and spread the RAS with a grader.
"Spreading the material with the sanding unit provided the most even application, although it took multiple passes to achieve the 3/8-inch thickness.
"On Swan Road, we used several sanding trucks and were able to spread the material evenly in two passes, with no interruption of the reclamation process."
Work crews reclaimed Hugh's Drive on October 19 and paved over the reclaimed base on October 26, with a total area of 1840 square yards. Swan Road was reclaimed on October 21, with base course of asphalt installed October 23, and the wearing course installed on October 30, with total coverage of 5816 square yards. (See accompanying sidebar for step-by-step operations.)
Montenegro will work in conjunction with DPW Director Lemoine over the coming months to evaluate the performance of the pavement structure and its RAS-enhanced base. In connection with this, Dr. Walaa Mogawer, P.E., Head of the UMass Dartmouth Highway Sustainability Research Center, has expressed interest in the Smithfield trial, and is studying samples of the RAS-enhanced base materials as part of a research project to quantify the value of RAS in full-depth reclamation.