By Kristin Dispenza, AOE

A five-year research project in Minnesota will determine the efficacy of one type of preservation treatment for thin concrete overlay pavements. Less than one year into the study, reports on the successes of diamond grinding a faulted concrete overlay are confirming its value as a preservation method.

A concrete overlay can extend the life of existing concrete, asphalt, or composite pavements. Properly constructed, the overlay will deliver the durability and excellent ride quality associated with concrete construction, lasting 15-35 years or more with little required maintenance. 

When an overlay is less than or equal to six inches thick, it is generally referred to as a “thin” overlay. Joint spacing on thin overlays is closer than on traditional pavement, with panels typically measuring six feet long by six feet wide. Thin overlays can be designed and constructed to be bonded or unbonded to the underlying pavement; in unbonded conditions, some degree of movement between the overlay and existing pavement may be designed in or an interface layer may be placed between the existing pavement and the overlay. Bonded solutions utilize the underlying layer to share stresses with the concrete overlay, and are therefore better suited to applications where the underlying pavement is in reasonable condition prior to the overlay. 

For an overlay to function optimally during its service life, road owners must thoroughly evaluate the structural integrity of existing pavement prior to commencement of a project. During the design phase, careful consideration must be given to the various options for overlay thickness, panel dimensions and bonding conditions. Once an overlay is in place, it should be regularly inspected and maintained in order to fully reap the benefits of its longevity. 

Preservation of Thin Concrete Overlay on Asphalt in Minnesota

Minnesota’s Washington County Public Works department wanted to ensure the continued success of a concrete overlay on an asphalt section of County State Aid Highway (CSAH) 16, known locally as Valley Creek Road. The overlay was originally installed in response to distresses in the existing asphalt pavement that had manifested by 2011. Average daily traffic levels on the road at the time averaged 25,200 vehicles per day and the pavement was in need of significant repair. The Washington County Public Works department performed a life-cycle cost analysis and chose a concrete overlay in preference to traditional bituminous pavement rehabilitation methods such as remove-and-replace and full-depth reclamation. The analysis showed that a concrete overlay would offer high durability for comparatively little maintenance. The longer frequency between maintenance projects, which permitted less overall disruption for the driving public and the neighborhoods served by Valley Creek Road, was a deciding factor in favor of concrete for the Public Works department.

“After completing the cost analysis, we felt that the concrete overlay was the most cost-effective alternative to resurface the roadway while reducing future disturbances to the traveling public,” said Andrew Giesen, Engineer, Washington County Public Works.

In all, 8,300 cubic yards of concrete was installed on a 1.42-mile stretch of CSAH 16, using a proprietary blend of ready-mix concrete that cured within 12 hours. Of this 1.42-mile stretch, 1.02 miles received a concrete overlay placed on a milled asphalt layer, while the other 0.40 miles received a full-depth concrete reconstruction. The overlay was five inches thick, with panels sized at six feet long by six feet wide, placed over the four-inch-thick remaining asphalt that had been in place since 1970. This stretch of pavement is a four-lane divided highway that includes turn lanes. The project cost was $3.268 million, with the concrete overlay expected to have a service life of 30 years.

Heavy traffic and its associated vehicular weight, however, pose a risk of breaking the bond between a concrete overlay and underlying asphalt. To guard against this, the Washington County Public Works department monitors the overlay’s rideability, because rough pavement is known to exacerbate vehicular “bounce,” increasing impact loading and intensifying the risk of bond damage. By 2020, while the overall condition of the overlay was very good, roughness had increased between the intersections of Radio Drive and Bielenberg Drive, with many transverse joints showing some faulting. Therefore, the department deemed it time to provide preservation treatments. 

Fortunately, distresses such as faulting or roughness in a concrete overlay can be repaired using the same patching and diamond grinding techniques that work for traditional concrete pavement. Since concrete panels on Valley Creek Road had only limited cracking, the Washington County Public Works considered panel replacement unnecessary and identified diamond grinding as the preservation solution for the full 1.02-mile length of the Valley Creek Road overlay. Diamond grinding was performed by Interstate Improvement in 2021 for a cost of $5.90 per square yard.

“When undertaking a preventive maintenance or preservation project, the key to success is to discuss the available options with the contractors. In this way, the team can arrive at the most efficient process, minimizing costs and delays. Washington County did this prework, and has great results to show for it,” said Matthew J. Zeller, Executive Director, Concrete Paving Association of Minnesota. 

Sam Gramling, President and Chief Operations Officer, Interstate Improvement, Inc., agrees. “I credit the team at Washington County with giving our crews adequate space to accomplish the work; grinding machines are very large and, on a divided highway like this one, with curb and gutter on either side, single lane closures would likely have been too tight. To compensate for the additional lanes that were closed to motorists, we performed the work in a shorter time window than we otherwise would have,” said Gramling. 

Research into Optimal Preservation Approach

More than 30 concrete-over-asphalt (COA) projects have been built in Minnesota, most of them on the county highway system. Even though there are fewer COA projects conducted on larger road systems, the Minnesota Department of Transportation (MnDOT) has been actively studying the performance of COAs, both at their MnROAD facility and on highway installations across the state. Monitoring the post-grinding performance of local concrete overlays on asphalt, especially those exhibiting joint faulting, is of special interest to MnDOT officials, because while guidelines exist for managing transverse joint faulting on thicker concrete pavements with base layers, extensive data for faulting of thin COAs is not currently available. As part of their research, MnDOT partnered with Washington County to conduct a five-year post-grinding evaluation of Valley Creek Road, with annual testing to monitor the performance of the diamond-grinding. An area of focus is the time it takes for transverse joint faulting to redevelop. During the summer of 2022, similar grinding efforts and monitoring will be performed at the MnROAD facility and on I-35 near North Branch, MN; for more information visit MnDOT’s “Road Research: NRRA Rigid Team Project” webpage.

Knowing that Valley Creek Road was about to undergo diamond grinding, MnDOT conducted pre- and post-grind surveys, taking place in September and October 2021. The surveys included visual inspection and photographic documentation, falling weight deflectometer testing, ultrasonic tomography (MIRA) testing, fault-meter testing, and surface profiling. Load transfer efficiency and joint deflection was measured on 100 randomly selected transverse joints in the right wheel path of the right lane of the eastbound direction of Valley Creek Road between Bielenberg Drive and Tower Drive. Faulting measurements were also taken in the right and left wheel paths of the same 100 joints. Ride quality profiling of the entire segment containing the 100 joints was determined using a laser-based lightweight profiler.

“Establishing the condition of the concrete overlay before and after grinding is critical in understanding the long term efficacy of diamond grinding to mitigate joint faulting,” said Tom Burnham, MnDOT Senior Road Research Engineer and co-principal investigator on the study. 

Diamond grinding was shown to substantially improve the ride quality of the thin concrete overlay. Pavement roughness went from a pre-grind average International Roughness Index (IRI) value of 128 to a post-grinding average of 54—a dramatic improvement in rideability. Faulting was substantially reduced, with pre-grind fault values ranging from 0.9 mm to 4.5 mm, with an average value of 2.2 mm, to post-grind values ranging from -1 mm to 1.9 mm, with an average 0.5 mm. The visual condition of the overlay remained very good after diamond grinding was performed, as well.

“Washington County Public Works is pleased with the initial results of diamond grinding and we, along with many others, are interested in the long-term performance results. We are fortunate to have MnDOT as a partner and are able to thoroughly monitor the long-term performance of diamond grinding over the next five-years,” said Giesen.

“For every job, we like to achieve a 30-50 percent improvement in IRI,” said Gramling. “For this overlay, the post-grind IRI was much better than 50 percent.”

“There can be a real benefit to diamond grinding a pavement before its IRI measurements become too high,” said Zeller. “When IRI values are over 90 inches per mile, decreasing roughness through diamond grinding offers a real opportunity to reduce vehicle impact loading and therefore minimize further pavement damage.”

“Another consideration,” says Gramling, “is that faulting on panels that are spaced only six feet apart can sound louder to drivers than joint-faulted areas that are spaced 15 to 20 feet apart, which is the panel distance drivers are more accustomed to. Therefore, it’s advisable to address faulting before it becomes too noticeable to drivers.”

Concrete pavement preservation is critical for achieving durable, long-lasting pavement—on thin concrete overlays as well as on traditional concrete pavement. Preservation treatments including diamond grinding provide a sustainable, cost-effective solution that has the added benefit of reducing travel interruptions for the driving public.

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