By Jason Wielinski, P.E.
Major infrastructure projects receive most of the fanfare when it comes to publicity and attention for their sheer size, funding levels and significant scoping challenges. These are the interstate or other high-traffic volume projects where agencies are adding pavement structural capacity with added travel lanes or constructing brand new roadways to serve the traveling public.
Solutions to the unique challenges associated with rural two-lane highways are celebrated less frequently. These rural roadways exist everywhere across our nation’s system of highways. They are the two-lane highways serving rural America; critical farm-to-market corridors, connecting rural America’s people and goods. Agencies recognize these roadways have their own unique challenges. These challenges typically include a lack of proper subgrade support, drainage issues, insufficient pavement thickness, and, in some cases, narrow travel lanes. Often these challenges stem from the fact that these roadways have been in service for many decades, originating as low volume, gravel or thin pavements that have evolved over time with demand and increased traffic.
State Route 427 in Northeast Indiana is one of these critically valuable rural roadways. It connects the communities of Hamilton, Indiana and Eaton, Ohio in Steuben County. The pavement, despite carrying much lower traffic volumes than typical interstate pavements, was showing signs of its age. It was plagued with fatigue-related cracking in the outer wheel path from repeated loads and lack of edge support. The pavement edges were further subjected to concentrated damage from the narrow 10-foot travel lanes.
Despite preserving and maintaining this roadway for many years in its current geometry, the Indiana Department of Transportation (INDOT) recognized it was time to rebuild. Like many two-lane roadways, where funding is limited and traffic must have access to homes and businesses, reconstruction in the form of complete removal and replacement is not an ideal option. In these cases, INDOT has been turning to a new form of pavement reconstruction using pavement recycling technologies including full-depth reclamation (FDR) and cold central plant recycling (CCPR).
New design approach using FDR and CCPR
The process INDOT is using to reconstruct rural highways is a multi-step solution that uses recycling technologies to rebuild the roadways. The process has unique scenarios when it comes to thicknesses and final pavement widths, but generally, they have been completed following these similar steps.
Step 1. Removal and stockpiling of existing asphalt pavement
The first step is to remove and stockpile the existing asphalt pavement. Depending on how much existing asphalt structure is available for reuse, the milling depth varies as the material to be used for cold central plant recycling should be free from subgrade and base materials. The material is transported to a nearby location for stockpiling and processing, which oftentimes reduces the trucking distances and costs associated with transporting the existing material.
Step 2. Full-depth reclamation of remaining materials and subgrade
The second step involves creating a stable, solid foundation for the new flexible pavement. The FDR process involves the blending and shaping of any remaining pavement, aggregate base and subgrade materials. This blend of material is then stabilized with cement to provide strength to the foundation. Typically, the FDR is performed wider than the existing pavement structure. In the case of SR 427, FDR was executed at a total of 28 feet in width at a depth of 12 inches, adding four feet of width to the pavement foundation. This practice increases lateral support to the newly widened pavement.
Step 3. Paving CCPR
The existing asphalt material that was removed from the pavement is processed through crushing and screening units to a desirable maximum aggregate size and consistency. The processed reclaimed asphalt pavement (RAP) material is then blended with an engineered asphalt emulsion in a portable mixing unit and delivered to the nearby paving site. It is placed with conventional paving equipment in two three-inch layers for a total of six inches. The CCPR layer serves as a flexible, durable layer in the pavement structure. It is also placed wider than the original width of 20 feet to add width and shoulders to the pavement. In the SR 427 project, it was placed 13 feet from the centerline in each lane, providing 26 feet of flexible pavement structure from edge to edge.
Step 4. Finishing with asphalt surface layer
The final step in the construction process is the placement of the two-inch asphalt surface layer. The new surface is also placed at a width of 26 feet, which includes widened travel lanes of 11 feet in width and a two-foot paved shoulder. The added shoulder provides both a performance and safety benefit for the newly constructed pavement.
Applying the technique across Indiana
INDOT has used this technique on six contracts across four districts since 2018.
“At INDOT we have enthusiastically embraced combining multiple pavement recycling processes within a single reconstruction project. Each completed project represents a journey of learning, partnership, and commitment, to establish a process where a road can be substantially reconstructed by only using the materials already present in the existing roadway. By recycling the roadway, INDOT can transform a deficient pavement structure into a new uniform cross-section with increased structural capacity, while conserving non-renewable resources and providing a cost-effective alternative to other rehabilitation methods. What was once an experimental endeavor, has been integrated into a regular pavement treatment for reconstruction of our rural roadways,” said INDOT Pavement Design Engineer Nick Cosenza, P.E.
1. State Route 101 in Allen County
SR 101 in Allen County was the first CCPR/FDR combination treatment used by INDOT in 2018. A total of 119k square yards of CCPR and cement stabilized FDR were specified to reconstruct approximately 9 miles of the two-lane roadway. A 1.5-inch-thick surface asphalt was placed on the widened 12-foot travel lanes.
2. State Route 236 in Hendricks and Putnam Counties
SR 236 is the largest of the six CCPR/FDR combination projects. 193k square yards of CCPR and 207k square yards of FDR were used to reconstruct the narrow roadway with subgrade-related distress in 2021 and 2022. The resulting pavement included a two-inch asphalt surface; 11-foot travel lanes and a two-foot paved shoulder spanning approximately 12.5 miles across Putnam and Hendricks County.
3. State Route 38 in Clinton County
Approximately nine miles of the FDR/CCPR combination was used to enhance the structural capacity of SR 38 in Clinton County. Four total inches of asphalt were placed on top of the recycled pavement layers.
4. State Route 55 in Benton County
Approximately 9.6 miles of SR 55 were reconstructed in 2023 with 130k sq yds of CCPR and FDR. The pavement was surfaced with two inches of asphalt.
5. State Route 236 in Parke County
SR 236 in Parke County is the smallest project on the list. In this 2023 project, 2.6 miles were reconstructed with 32k sq yds of CCPR and 35k sq yds of FDR. This project called for a 3-inch base lift of CCPR to support the 4.0 inches of asphalt placed on top.
6. State Route 427 in Steuben County
Approximately 5.5 miles of roadway was reconstructed and widened using 87k sq yds of CCPR and FDR. The travel lanes were widened from 10 feet to 11 feet and included a two-foot paved shoulder. The entire length of the project was surfaced with two inches of asphalt.
How does the FDR/CCPR reconstruction option perform?
As with any new pavement technology or design, it takes some time to understand how effective they are to withstanding the effects of weather and traffic. The 2018 reconstruction of State Route 101 in Allen County was the first of the six CCPR/FDR combination projects and since then has endured a few Indiana winters and heavy traffic. Cosenza presented some pavement condition data from this project at the Asphalt Recycling and Reclaiming Association’s 2023 Pavement Recycling Summit held in Indianapolis. Very similar to the SR 427 example, SR 101 was reconstructed using 12 inches of cement stabilized FDR, six total inches of CCPR and two inches of asphalt surface and widened from 10-foot travel lanes with no shoulder to 11-foot travel lanes with two-foot paved shoulders.
When it comes to metrics to characterize pavements, there are three that are typically used by agencies to measure performance: smoothness, rutting and structural capacity. Cosenza shared information related to all three in his presentation.
• First, Cosenza shared that after four years of traffic, the pavement’s smoothness represented by the average International Roughness Index, or IRI, across the entire length of the project was measured at 56.0 inches per mile. That is a stark difference to the significantly rougher 132.8 in/mi IRI the traveling public was enduring before the 2018 reconstruction.
• Rutting measured is also very low. The average mean rut depth across the entire length of the roadway is 0.10 inches with a couple of slightly higher anomalies near bridge approaches and intersections.
• Perhaps the most telling of performance so far is the structural response. Through Falling Weight Deflectometer (FWD) testing, Cosenza shared that “as constructed” the Structural Number (a measure of pavement strength or stiffness) remains higher than the design Structural Number of the project. Deflection data shows that at both the surface and at the subgrade level, the pavement is performing very well, aside for one or two anomalies across the entire nearly nine miles of pavement.
• He mentioned that there are no signs of wheel path or fatigue cracking. The only visible surface distresses include a few thermal cracks in the surface layer asphalt and some load-related distress localized to the outside six inches of the shoulders.
• Cosenza further stressed the important practice of building the foundation with FDR and CCPR wider than the surface to help minimize the risk of edge-related distress.
How cost-effective is the CCPR/FDR option?
INDOT has shared engineering estimates in various presentations showing the economic benefit of the FDR/ CCPR option versus conventional remove-and-replace options. In the 2023 PRS presentation, Cosenza shared that FDR/CCPR option was up to 20 percent lower in costs as compared to what the traditional reconstruction would have cost based on INDOT’s engineering estimates for the SR 236 project constructed in 2021 and 2022. As for earlier projects, Cosenza (2019 ARRA PRS) shared that the State Route 101 project was approximately 30 percent more cost-effective than the traditional design option based on 2017 engineering estimates. Whether it is 20 percent or 30 percent, the cost savings are significant, especially on these larger rural projects coupled with tighter budgets.
Additional benefits
There are additional benefits realized by INDOT and other agencies when using pavement recycling processes for pavement rehabilitation. There are environmental considerations that accompany the use of pavement recycling technologies. With this process, INDOT is reusing materials already on site, or transported and processed close to the project. This significantly reduces the demand and energy associated with extraction, processing and delivery of virgin materials (primarily aggregates). The reconstruction processes, both FDR and CCPR, consume less energy than the production and installation of comparable base-type materials. There are also the social components realized by the traveling public including the reduction in road closure duration and enhanced safety benefits with the widening of these pavements.
“INDOT has found pavement recycling as a suitable treatment for various types of roadways, including rural roads, divided highways and urban streets, providing a versatile solution for different infrastructure needs. Combined, our pavement recycling projects have resulted in the use of five million square yards of recycled pavement. Besides reducing environmental impacts, recycling provides cost savings that include a reduction in trucking and energy consumption,” Cosenza added.
Ultimately, time will provide a clearer picture of the overall effectiveness of these reconstruction processes as these roadways continue to serve Indiana and its roadway needs.
“Recycling pavement not only preserves our natural resources but paves the way for a sustainable future, where environmental responsibility meets the road to progress,” said INDOT Pavement Engineering Manager Kumar Dave.
Wielinski is an Asphalt Institute Regional Engineer based in Ohio.