In recent years, MnDOT has seen an increase in cracking of low slump overlays. Cracking of overlays allows chlorides to get into the bridge deck which leads to deterioration of the reinforcement and eventual delamination or spalling. This means that the bridge deck needs to be repaired or even replaced before the service life has been reached.
What is a low slump overlay? A low slump overlay is a technique used by DOTs to extend the life of bridge decks. They are typically 2″ thick and designed to provide low permeability.
Well-documented efforts undertaken two decades ago to mitigate corrosion of a Highway 394 reinforced concrete bridge have given researchers the perfect scenario for evaluating the treatments’ long-term effectiveness. The test results are mixed: State-of-the-art methods for electrochemical chloride extraction and fiber-reinforced polymer wrapping of bridge elements performed well in combination, but poorly in isolation.
The Minnesota Department of Transportation (MnDOT) had suspended the use of post-installed epoxy-coated rebar for concrete barrier repairs as a precautionary measure because chemical adhesives used in the process are not designed for use with coated bars. But laboratory testing (conducted in a recent MnDOT-sponsored research study) has now shown that using these adhesives with coated rebar for post-installation works well and provides a safety level 200 to 300 times that predicted by manufacturer specifications. MnDOT is considering research recommendations to modify the installation process in order to resume using coated rebar in post-installed concrete crash barriers.
A new MnDOT research study determined that depositing concrete grinding residue (CGR) slurry at specific rates on roadside vegetation and soil may not cause lasting harm to plant growth and soil quality; however, follow-up research is recommended.
Using recycled pavement as aggregate in new concrete mixes can save money and promote environmental sustainability. New design methods published in a new research report allow engineers to create more durable mixes from recycled aggregate than in the past, reducing the need for virgin aggregate, a diminishing and expensive resource.
“This report shows that a lot can be done with recycled aggregate,” said Matt Zeller, Executive Director, Concrete Paving Association of Minnesota. “We can get the strength up to that of concrete with virgin aggregate by bumping our mix design and lowering our water-to-cement ratio.”
“Concrete pavement made with RCA can be beneficial both economically and environmentally,” said Farhad Reza, Professor, Minnesota State University, Mankato, Department of Mechanical and Civil Engineering.
Reza served as the project’s principal investigator.
What Was the Need?
When pavements are due for reconstruction, the old pavement is frequently crushed to aggregate-sized particles and used as the base course for new pavement. In the 1980s, MnDOT and other state transportation agencies began using such recycled aggregate in the concrete course as well. But this latter practice was discontinued by the early 1990s due to mid-slab cracking observed in pavements constructed with such concrete. Using recycled concrete aggregate (RCA) in the base course has continued, however.
Newer mechanistic-empirical design methods and performance engineered mixtures have led to improved RCA mixtures. For example, concrete mixtures now have lower water-to-cement ratios. These advances present an opportunity to re-evaluate the use of recycled aggregates in concrete mixes, which aligns with two important trends: the diminishing availability of virgin, high-quality aggregate, and the growing federal emphasis on sustainable design. Using recycled concrete as aggregate fulfills the three basic principles of sustainability: performance, environmental stewardship and cost-effectiveness.
What Was Our Goal?
Researchers sought to evaluate the performance of selected sections of concrete pavement in Minnesota that had been constructed with RCA; examine field samples and lab mixes; and develop guidelines for successful use of recycled aggregate in new concrete pavements.
What Did We Do?
After a literature search on the use of RCA in new concrete pavements, investigators examined the following issues:
Historical Performance. The research team gathered and compared data on performance, ride quality and durability for 212 miles of RCA pavement and for 212 miles of regular concrete pavement in the state. Both pavement samples had been built in the same time period and had had similar traffic levels.
Materials and Constructability. Investigators analyzed the ride quality of two-lift (or two-layer) concrete pavement test sections built in 2010 at the MnROAD test facility, using modeling to project long-term performance based on the historical evaluation. They conducted tests on nine cores pulled from the RCA pavements and tested new mixes made with recycled aggregate from Olmsted County, Minnesota. For comparison, they tested virgin aggregates from a Mankato, Minnesota, plant and fines from a Henderson, Minnesota, site.
Life-Cycle Cost Analysis. The research team conducted a life-cycle cost analysis of new RCA mixes and traditional concrete mixes, comparing their performance and cost-effectiveness.
RCA Guidelines. Based on the historical analysis, laboratory testing and modeling, and life-cycle cost analysis, the researchers developed new guidelines for the design and construction of pavements containing RCA in concrete mixes.
What Did We Learn?
Results showed that using RCA in concrete pavements can save money and is a sustain-able practice that produces durable concrete pavement.
Historical Performance. Most of the existing pavement studied had not reached the terminal ride quality index of 2.5—the level that generally indicates a major pavement rehabilitation must be performed. Analysis showed that rehabilitation is required, on average, at about 27 years of service for RCA pavements and at 32 years for standard concrete pavements.
Materials and Constructability. Mix design can be adjusted to achieve traditional strength levels that older RCA mixes did not reach. Elimination of fines and stricter adherence to gradation specifications for concrete aggregate can achieve workable and durable mixes that are less likely to suffer excess drying shrinkage. Pavements designed in this way meet the standards of the Federal Highway Administration’s INVEST program for sustain-ability in highway construction.
Life-Cycle Cost Analysis. Long-life RCA pavements are more economical in cost-benefit terms than are thinner, shorter-life RCA pavements.
RCA Guidelines. Researchers developed specification recommendations and design guidelines for the use of RCA in new pavement construction. Trial mixes are critical, and absorption and compressive strength must be examined before use. Recycled fines are not recommended, but otherwise RCA can be used in the full range of aggregate sizes between minimum and maximum. Recycled concrete pavement may not produce enough aggregate for both pavement and base course, but acquiring extra RCA to make the base course 70 percent recycled and 30 percent virgin makes the new pavement economical and sustainable.
Keeping detailed records on mix designs used and tracking mix performance over time will help MnDOT to further refine its use of recycled aggregate in concrete mixes and will provide robust data on the performance of more sophisticated RCA mixes. A research team may want to consider using lower-quality recycled concrete as a bottom lift and higher-quality recycled concrete with virgin aggregate in the top lift. Methods for managing water input with recycled aggregate to achieve proper water-to-cement ratios warrant further study.
Just how long will it be before a bridge deck needs to be rehabilitated? Why not look to history to find out?
Researchers have put several decades of MnDOT bridge inspection records to good use by analyzing old bridge deck condition reports to calculate how quickly similar bridge decks will deteriorate.
MnDOT inspects bridges regularly, but had never used this historical data to help determine the rate of bridge deck deterioration and what factors influence it.
“We’re always trying to improve the timing of bridge deck repair projects and improve our understanding of what contributors affect the way our bridge decks deteriorate,” said Dustin Thomas, MnDOT’s South Region Bridge Construction Engineer.
From their analysis, researchers created deterioration tables that can be used to better predict the timing and costs of repairs and maintenance.
Researchers looked at the inspection history and construction details of 2,601 bridges to determine the impact of factors such as type of deck reinforcement, depth of reinforcement below the driving surface, traffic levels and bridge location.
Using the inspection data, researchers developed curves that show how long a bridge deck is likely to stay at a given condition before dropping to the next. They developed separate curves for each variable that had a significant impact on deck deterioration rates.
What They Found
Several factors were found to have a notable impact on how quickly bridge decks deteriorate:
Decks without epoxy-coated bars built between 1975 and 1989 deteriorate more quickly than other bridge decks.
Bridges with less traffic showed slightly slower rates of deterioration than highly traveled bridges.
Metro area bridges drop to a condition code of 7 (good) more quickly than bridges in other parts of the state. This may be due to increased chemical deicer usage or because maintenance activities like crack-sealing are more likely to be delayed on larger metro bridges because of the difficulty accessing middle lanes.
When a new deck is installed on an existing bridge, the deck performs like a brand-new bridge and so MnDOT should use the deterioration table for the re-decking year, rather than the year the bridge was originally constructed.
MnDOT plans to incorporate future bridge inspections into the dataset to enhance the predictive value of the deterioration tables.
Researchers from around the world rely on Minnesota’s pavement testing center, MnROAD.
Minnesota alone saves at least $33 million each year, thanks to quantifiable advances made at MnROAD. The annual nation-wide savings is thought to be even larger: $749 million.
Established in 1994, MnROAD partners with the FHWA, industry and dozens of other states and countries to conduct research on two live test tracks in rural Albertville.
No other cold-weather facility offers such an array of pavement types with thousands of electronic sensors recording both environmental changes and dynamic truck testing.
“If not for MnROAD, many of our projects wouldn’t be nearly as successful,” said Highway Research Engineer Larry Wiser of the Federal Highway Administration.
At an Aug. 6 open house, this one-of-a-kind research facility celebrated 20 years of finding ways to make roads last longer, perform better and cost less.
Two separate road segments contain 51 test cells, with different combinations of surface materials, aggregate bases and subgrades, as well as variations in structural design and drainage features.
MnROAD’s initial research on pavement life and performance (from 1994 to 2006) reduced maintenance costs, repairs and motorist delay.
In the second phase of research, MnROAD reconstructed almost 40 test cells for more than 20 different studies. The benefits derived from this work is estimated to be worth nearly nine times what the studies cost – and that’s just the benefit for Minnesota.
“We’re excited for the third phase of research, which will be mainly focused on maintenance and rehabilitation,” said MnROAD Operations Engineer Ben Worel. “We’ve seen the benefits of our past research and expect the same in the future.”
MnROAD’s facility includes:
– A test section of I-94 carrying live traffic
– A low-volume roadway that simulates rural road conditions
– Thousands of sensors that record load response and environmental data.
Staff from MnROAD, the Minnesota Department of Transportation’s cold weather road research facility in Albertville, Minn., were presented with the Marlin J. Knutson Award for Technical Achievement by the American Concrete Pavement Association in December.
The award cites the facility’s well-deserved reputation for being a place where both agency and industry ideas are put to the test. This award was presented as a tribute to the agency’s commitment to learning and putting ideas into practice.
The Marlin J. Knutson Award for Technical Achievement is presented to an individual or group who has made significant contributions to advance the development and implementation of technical innovations and best practices in the design and construction of concrete pavements.
“MnROAD is helping to make roads last longer, perform better, cost less, construct faster, and have minimal impact on the environment,” said Gerald Voigt, ACPA president and CEO. “It is a model for other agencies to follow.”
MnROAD is a pavement test track initially constructed between 1991-1993. It uses various research materials and pavements and finds ways to make roads last longer, perform better, cost less to build and maintain, be built faster and have minimal impact on the environment. MnROAD consists of two unique road segments located next to Interstate 94.
This article, authored by Rich Kemp, originally appeared in Newsline, MnDOT’s employee newsletter.
Placing concrete streets, sidewalks, curbs, and gutters just got a lot easier for cities and counties—and the inspectors, engineers, and contractors who work on them in Minnesota. Locals no longer have to adapt to the rigorous Minnesota Department of Transportation (MnDOT) specifications for trunk highways.
The Minnesota Concrete Flatwork Specifications for Local Government Agencies tech memo was issued by the MnDOT Office of State Aid in 2012. These specifications guide all State-Aid-funded local concrete projects and should reduce the confusion and misunderstanding that arose when engineers and contractors used different interpretations of the highway specifications.
The new specs require two people to hold a current ACI Concrete Flatwork Technician certification, with at least one on-site for all concrete pours.