many transverse joints showing some faulting. Therefore, the depart - ment 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 Washing - ton 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 contrac- tors. In this way, the team can arrive at the most efficient process, mini - mizing 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 of - ficials, 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 grind - ing, 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 grind - ing provide a sustainable, cost-effective solution that has the added benefit of reducing travel interruptions for the driving public.
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August 2022 csengineermag.com
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