Indiana's Recycled Shingle Road Test Offers Iowa Paving Insights
When a roof is redone, old shingles often end up in a landfill. But a 2009 Indiana road trial showed that recycled shingles can be mixed into asphalt pavement, offering a real-world durability test that could inform Iowa's own road-building decisions. The project, which ran under real traffic conditions on US Route 6, revealed both promise and pitfalls for using tear-off shingles in pavement.
What the Indiana trial tested
Indiana's Department of Transportation (INDOT) launched a field demonstration in 2009 to see how recycled shingles performed in asphalt under actual traffic. Contractor Phend & Brown completed the renovation in late summer 2009, milling 1.5 inches of old asphalt and replacing it with one of three mixes: a standard Indiana mix with 15% recycled asphalt pavement (RAP) and no shingles, a hot mix with 3% post-consumer shingles, and a warm mix with the same 3% shingles using foaming technology.
The shingles were ground to pass through a half-inch sieve and contained 26.8% asphalt by weight. Engineers wanted to answer two questions: Would warm mix technology hurt a shingle mix? And how would a shingle recipe compare to Indiana's usual recycled pavement mix?
Lab results showed trade-offs
Indiana's project was part of a larger national pooled fund study (2009-2013) led by Missouri and including Iowa, California, Colorado, Illinois, Indiana, Minnesota, Wisconsin, and the Federal Highway Administration. Iowa State University researchers coordinated the effort, with each state testing different aspects of shingle recycling.
Lab tests found that old shingle asphalt is stiffer than fresh paving asphalt, which changes the binder's performance grade. Mixes with shingles had a higher high-temperature grade (PG 76-10) compared to the RAP-only mix (PG 70-16), meaning better resistance to summer rutting but a warmer low-temperature grade, indicating a potential trade-off in cold-weather performance. Foaming did not affect the grade.
Performance tests were encouraging: shingle mixes showed better rutting resistance in flow number tests, and bending beam fatigue and low-temperature cracking tests showed no substantial differences across the mixes.
Real road performance told a mixed story
Engineers surveyed the road one, two, and three years after construction. By the end, all three sections showed transverse, longitudinal, and fatigue cracking. The RAP-only section had the least distress, while the warm-mix shingle section had the most. INDOT attributed much of the transverse cracking to movement between the concrete slab underneath and the new asphalt overlay, noting that heavy truck traffic may accelerate damage in wheel tracks.
The national study's final report (September 2013) found no rutting, wheel-path fatigue cracking, or thermal cracking across demonstration projects after two years, though transverse reflective cracking was recorded in several states, including Indiana. Indiana's more detailed three-year observations showed progressive cracking, but the lack of a pre-construction distress survey meant researchers couldn't conclusively blame the shingles rather than the existing pavement beneath.
What this means for Iowa roads
Iowa's participation in the national study means local engineers have direct access to these findings. The trial showed that asphalt with 3% recycled shingles can meet state quality-assurance requirements, even with warm mix technology. INDOT found the results promising enough to share with other agencies creating their own rules.
For Iowa, where agriculture and transportation depend on durable roads, the key takeaway is that recycled shingles can be a viable option, but pavement condition underneath matters just as much as the mix recipe. As Iowa looks to stretch road budgets, this real-world data offers a practical starting point for evaluating shingle recycling in local projects.
Frequently asked questions about recycled shingles in pavement
Can recycled shingles be used in Iowa asphalt?
Yes, the national study included Iowa and found that mixes with 3% recycled shingles can meet state specifications, though performance depends on underlying pavement conditions.
Do recycled shingles make roads more prone to cracking?
The Indiana trial showed cracking in all test sections, but researchers couldn't separate shingle effects from pre-existing pavement movement. The national study found no significant cracking in most states after two years.
What are the cost benefits of using recycled shingles?
Recycling shingles diverts waste from landfills and can reduce the amount of virgin asphalt needed, potentially lowering material costs for road projects.