Tag Archives: Concrete

Using Chemical Adhesives to Post-Install Epoxy-Coated Rebar in Concrete

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.

What Was the Need?

As bridges age, local and state agencies are called on to repair and upgrade elements. Local agencies may install new light posts and replace chain-link rails, while state agencies may replace slabs on wing walls, deck and crash barriers, and other non-hanging bridge components. Work like this requires replacing concrete elements and installing new reinforcement bars. Until recently, MnDOT and local agencies used epoxy-coated steel rebar for these post-installed concrete panels and barriers.

Epoxy-coated rebar resists corrosion from salt and water that penetrate concrete members, especially at seams and cracks. Post-installed rebar requires chemical adhesives to secure the bar in place to effectively transfer loads from one concrete slab to another. Manufacturers test these adhesives with standard uncoated steel rebar to provide users with application guidance and to give engineers data on the tensile pullout strength of the rebar in concrete, a key property in the design of concrete bridge components.

As with many bridge materials, specifications for adhesives are conservative; the tensile strength of chemically adhered rebar can be assumed to be much higher than specified. Despite the almost certain safety of the practice, the MnDOT Bridge Office suspended the use of epoxy-coated rebar in post-installation applications because manufacturer specifications are based only on results from testing with uncoated rebar.

What Was Our Goal?

The goal of this project was to determine the effect of the epoxy coating on the tensile strength of rebar that is post-installed with a chemical adhesive. To achieve this goal, researchers surveyed other state transportation agencies to determine how these agencies use epoxy-coated rebar in certain post-installation practices. In addition, the research team conducted laboratory evaluations of common adhesives and epoxy-coated rebar installed in hardened concrete.

“The point of epoxy-coated rebar is to gain a longer life cycle. While MnDOT practices were not unsafe, there are ways the agency could be more accurate.”—Ben Dymond, Assistant Professor, University of Minnesota Duluth Department of Civil Engineering

What Did We Do?

Two post-installation applications were studied: crash barriers between bridge support piers and crash barriers on the edges of bridge decks. Investigators distributed a survey to all 50 state transportation agencies to learn the state of the practice related to postinstalling epoxy-coated rebar with chemical adhesives in hardened concrete bridge elements. The research team also studied MnDOT procedures for using adhesives with epoxy-coated and uncoated rebar during post-installation of rebar in crash barriers.

Investigators then conducted a laboratory study of the four most common adhesives used in Minnesota. They installed six uncoated lengths of rebar for each adhesive in one concrete slab and six epoxy-coated lengths of rebar for each adhesive in another slab. Rebar was pulled from the concrete to determine the tensile strength. 

In tests of adhesives and
reinforcement steel,
epoxy-coated rebar and
uncoated rebar showed
high pullout strength
when extracted from a
concrete test slab.
Two lengths of uncoated rebar (black) and two lengths of epoxy-coated rebar (green).

What Did We Learn?

Thirty states responded to the survey. Twelve of these state agencies do not use epoxy-coated rebar post-installed with chemical adhesives in concrete. Eleven of the 18 agencies that do use epoxy-coated rebar in these applications employ manufacturers’ data on bond strength; another six use a standard, national calculation method. 

Tensile pullout strength of the bars varied by adhesive type. In all cases, the manufacturer specifications for strength were very conservative and safe, identifying strength values 200 to 300 times lower than shown in laboratory tests. MnDOT’s approach to adhering epoxy-coated and uncoated reinforcement steel in hardened concrete barriers results in safe, reliable bonding and load transfer. 

The difference in strength between chemically adhered epoxy-coated steel and uncoated steel bars was on the order of 10 percent; in some cases, coated rebar was slightly stronger with the tested adhesive. Below are the pullout strength ratios of epoxy-coated bars to uncoated bars (reduced by three standard deviations) for the four adhesives:

  • Powers AC100+ Gold: 0.89.
  • Red Head A7+:1.02.
  • ATC Ultrabond 365CC: 0.98.
  • Hilti HIT-RE 500 V3: 1.11. (In this case, however, bars ruptured, and pullout strength was not definitively established.) 

Researchers recommended a modification factor for calculating bond strength to reflect the findings. Investigators also recommended changing a MnDOT bond stress specification or adopting manufacturer values, which would better meet national specifications for bridge design. 

“After exploring the epoxy-coated rebar practices of other states, we identified a difference in the installation design with epoxy-coated bars and uncoated bars.”—Joe Black, Senior Engineer, MnDOT Bridge Office 

What’s Next?

Although MnDOT is not currently considering the use of epoxy-coated rebar in post-installed concrete, this study suggests what steps may be necessary to reauthorize its use in these applications. One solution would be to specify longer epoxy-coated rebar to mitigate the minor potential loss in pullout strength, allowing bridge managers to leverage the corrosion-resistance benefit of epoxy-coated rebar in these applications. 

A researcher uses a dust-collecting drill to ensure clean insertion of rebar in concrete.
A researcher uses a dust-collecting drill to ensure clean insertion of rebar in concrete. 

This post pertains to Report 2019-07, “Anchorage of Epoxy-Coated Rear Using Chemical Adhesives,”  published February 2019. For more information, visit MnDOT’s Office of Research & Innovation project page.

Concrete Grinding Residue Doesn’t Appear to Negatively Affect Roadside Vegetation and Soil

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.

Study results showed that CGR did not appear to hinder vegetation growth or soil quality, but did change soil chemistry. At some roadside areas, the increase in soil pH enhanced plant growth. Results cannot be generalized for all soil types, plant communities, concrete residues or water sources in Minnesota. Access to real-time slurry disposal activity is needed for a thorough investigation.

Study background

Construction crews use diamond grinders to level newly cured concrete with adjacent slabs of older pavement and to smooth new pavement surfaces for improved friction and tire traction. Diamond grinders are fitted with hoses for rinsing grinding burrs with water to keep the burrs clean and prevent overheating. Vacuum lines then collect the residual dust and rinsing fluids, generating a slurry of concrete grinding residue (CGR) that is frequently discarded on roadside slopes and vegetation. 

When slurry dries, it leaves pale gray patches on roadside vegetation and other features, lightening the soil surface for a season or more. The effect of this slurry on vegetation, soil and drainage was unknown. Engineers and researchers presumed that the concrete dust temporarily coats roadside turf and plants, raises the soil pH, clogs soil pores and inhibits water drainage, invites invasive species to take root, and may infiltrate storm drains and waterways. 

What Was Our Goal?

MnDOT needed to study the impact of CGR on roadside vegetation and soil. Research would evaluate sites where residue has been deposited and determine its impact on vegetation and soils common to state roadsides. 

What Did We Do?

A literature review indicated that related research has been limited and that vegetation samples of only one or two species have been examined. Researchers developed two approaches for investigating the impact of CGR on plant density, plant growth and soil properties. 

First, researchers collected CGR slurry from a slurry tank at a Minnesota construction site to replicate residue application at the Kelly Farm, an Iowa State University research site near Ames, Iowa, that features prairie vegetation similar to that found along Minnesota roadsides. They applied slurry at application rates of zero, 10, 20 and 40 dry tons per acre. Plant cover, soil chemistry and soil structure properties, such as plant biomass, density, hydraulic conductivity, infiltration and pH, were measured before the slurry was applied and again at one-, six- and 12-month intervals after application. 

Second, researchers visited two roadside locations along Interstate 90 near Austin, Minnesota, where CGR had been applied. The research team evaluated vegetation content and cover, took soil samples and compared survey results to neighboring roadside environments that had not received CGR slurry.

The infiltrometer system setup at the Kelly Farm site in Ames, Iowa.
This water infiltrometer measured infiltration of water at the roadside environment test site.

What Did We Learn?

Statistical analyses established that at the Kelly Farm, CGR did not significantly impact soil physical properties and plant biomass, but did alter soil chemistry. Levels of soil pH, electrical conductivity, metals content and other properties rose significantly after CGR application. These effects increased with increases in application rate and decreased at increased soil depths. These changes did not reduce soil quality, and higher pH levels did not persist after one month. For certain warm-season grasses and legumes, increased pH improved plant growth. Some nutrients such as calcium and magnesium leached from CGR could benefit plant growth as well.

“Concrete grinding residue or slurry can, under certain conditions, be a benefit. It can act as a liming agent, changing soil pH in a positive manner.” —David Hanson, Integrated Roadside Vegetation Manager, MnDOT Roadside Vegetation Management

The two roadside environments yielded differing results. Slurries had been deposited in 2009 at the first site and in 2013 at the second. At the first site, soil bulk density and hydraulic conductivity in the slurried areas did not differ significantly from measures at the nonslurried areas; at the second site, the levels differed significantly. At both sites, electrical conductivity, calcium content and base saturation values were higher at the areas with CGR than the areas without CGR. 

Researchers concluded that at the Kelly Farm and at the roadside locations, slurry applications at a rate of up to 40 tons per acre did not reduce soil quality and vegetation growth for longer than three years. 

What’s Next?

Efforts to access grinding operations and CGR deposits in real time were not embraced by Minnesota’s concrete industry, and researchers were unable to properly assess residue composition and rates, and volumes of slurry deposition on roadway environments. A thorough investigation of residue impact will require such access and follow-up on site conditions after established periods of time. 

Researchers noted that findings cannot be easily generalized since CGR compositions may vary depending on source and water quality, influencing soil and vegetation differently, and soil and plant communities may differ in response to comparable CGR applications. Investigators recommended that MnDOT develop quick field measures of slurry pH, electrical conductivity and alkalinity to use in adjusting slurry spreading rates at grinding sites.

“This study was a great start to this topic. Follow-up research is recommended to evaluate live projects, field demonstrations and data collection.” —Halil Ceylan, Professor, Iowa State University Department of Civil, Construction and Environmental Engineering

This technical summary pertains to Report 2019-06, “Concrete Grinding Residue: Its Effect on Roadside Vegetation and Soil Properties,” published January 2019. Visit the MnDOT research project page for more information.

Using Recycled Concrete Aggregate in New Concrete Pavement Mixes

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?

Researcher vibrates RCA mix samples in a box.
Investigators vibrated RCA mixes in sample boxes to prepare the mixes for mechanical analysis.

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.

What’s Next?

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.


This Technical Summary pertains to Report 2017-06, “Evaluation of Recycled Aggregates Test Section Performance,” published February 2017.

Using History to Predict Bridge Deck Deterioration

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.

Data-Crunching

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.

Related Resources

The impact of overlays on bridge deck deterioration in Minnesota was not clear, but redecked bridges were found to perform similarly as brand-new decks.

MnROAD celebrates 20th anniversary, prepares for next research phase

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 consists of two unique road segments located next to Interstate 94.
MnROAD consists of two unique road segments located next to Interstate 94.

Annual Savings

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.

MnROAD earns concrete pavement association award

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.

(far right) Gerald Voigt, ACPA president and CEO, presented MnDOT with the Marlin J. Knutson Award for Technical Achievement during a ceremony in December. Receiving the award are (from left) Luke Johanneck, Bernard Izevbekhai, Roger Olson, Tom Burnham, Glenn Engstrom, Maureen Jensen and Sue Mulvihill. (Photo courtesy of the ACPA)
(Far right) Gerald Voigt, ACPA president and CEO, presented MnDOT with the Marlin J. Knutson Award for Technical Achievement. Receiving the award are (from left) Luke Johanneck, Bernard Izevbekhai, Roger Olson, Tom Burnham, Glenn Engstrom, Maureen Jensen and Sue Mulvihill. (Photo courtesy of the ACPA)

“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.

Staff from the MnROAD facility in Albertville were recognized during the ACPA’s Distinguished Service and Recognition Awards ceremony in December. (Photo by David Gonzalez)
Staff from the MnROAD facility in Albertville were recognized during the ACPA’s Distinguished Service and Recognition Awards ceremony in December. (Photo by David Gonzalez)

This article, authored by Rich Kemp, originally appeared in Newsline, MnDOT’s employee newsletter. 

New flatwork specs ensure higher quality for local concrete projects

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.

Read more in the May 2013 issue of the TERRA E-News