Tag Archives: rebar

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.

Epoxy-Coated Rebar Bridge Decks Outperform Mixed Rebar Decks

Bridge decks reinforced with one layer of epoxy-coated rebar and a bottom layer of uncoated steel rebar show corrosion damage sooner than decks constructed with all epoxy-coated rebar. Inspection methods should be enhanced to add a rating for cracking density on the underside of bridge decks. Repairs to mixed rebar decks should be conducted once a key deck surface inspection element has received a condition rating of two and held that rating for seven years, which is sooner than the average repair time of 8.5 years.

What Was the Need?

In concrete bridge decks, steel reinforcing bars are necessary to add tensile strength and transfer loads to beams. Additionally, steel reinforcement in concrete bridge decks is designed to control cracking, which will extend the service life of the bridge.  

Steel also corrodes in salt environments, even when embedded in concrete. Water and road deicing chemicals can reach the steel and damage its strength and integrity. Between 1973 and 1990, MnDOT built approximately 660 bridges with more expensive, epoxy-coated rebar in the top layer of reinforcing matting and standard black rebar in the bottom layer. The coated top layer, only 3 inches below the deck surface, was expected to resist corrosion, and damage from salt and water would not reach the next layer of rebar, another 3 inches down in a 9-inch deck. 

In recent years, MnDOT has used another reinforcing strategy: mixing noncorrosive fibers into concrete mixes to help prevent or minimize cracking and resist corrosion. The older, mixed reinforcement bridges remain in service and few have been redecked. Performance of mixed reinforcement and fiber reinforcement in Minnesota bridge decks has not been compared to the performance of bridge decks constructed with only epoxy-coated rebar.

What Was Our Goal?

MnDOT sought to compare the performance of mixed rebar decks with all epoxy-coated rebar decks, and the performance of fiber-reinforced decks with no-fiber concrete decks. MnDOT also wanted to learn how to plan preventive maintenance efforts for mixed rebar decks.

What Did We Do?

Researchers reviewed reports from inspections, conducted every two years, for bridges with mixed reinforcement decks and decks with 3-inch strips of fiber reinforcement mixed into the concrete. They narrowed their review to bridge inspection data from 506 bridges with epoxy-coated rebar (including 35 control decks with all-epoxy rebar) built between 1973 and 1990, and 22 bridges with fiber-reinforced concrete and epoxy-coated rebar (including four controls with no rebar) built between 2012 and 2017. All of the bridges were inspected through 2017.  

Investigators then conducted site evaluations of 75 mixed rebar decks and 25 all-epoxy rebar decks, as well as 11 fiber-reinforced concrete decks with epoxy rebar and four without rebar. Site surveys focused on confirming the accuracy of recent inspection reports and recording signs of cracking, spalling and other deterioration conditions. 

Spalling has occurred because deicing chemicals and water have seeped into the concrete deck through cracks in the deck surface, causing uncoated steel reinforcement to corrode and concrete to spall.

What Did We Learn?

All-epoxy rebar decks outperformed mixed rebar decks, showing less cracking on the top and underside of the decks. Mixed rebar decks deteriorated at a quicker rate on bridges with steel beams than on bridges with prestressed concrete beams. Traffic levels and surface cracking did not appear to affect deterioration of decks in any group. 

“There’s not really a good visual inspection standard for quantifying cracking under the bridge deck, and that’s especially important for these types of bridges with mixed rebar. Using only epoxy-coated rebar in decks was a good idea.” —Ben Dymond, Assistant Professor, University of Minnesota Duluth Department of Civil Engineering

Data sets were too small to draw any conclusions about possible differences in performance of fiber-reinforced decks compared to bridge decks that were not built with fibers. 

Individual bridge elements, such as bridge deck surfaces, have historically been rated from one (best condition) to five (worst condition). Mixed rebar decks earned element ratings of three to four more frequently than all-epoxy rebar decks, and visual surveys identified more deterioration on the underside of mixed rebar decks than all-epoxy rebar decks. 

The research team recommended amending bridge inspection procedures to add a new rating element for quantifying crack density on the underside of decks to anticipate and prevent spalling and delamination on the underside of mixed rebar decks. The team also recommended that once the deck condition element in mixed-bar decks holds a rating of two for seven years, more robust cracking sealing techniques should be considered to prevent it from reaching a rating of three. (For most bridges, that repair typically occurs after 8.5 years of service.)

Finally, the team recommended continued evaluation of fiber-reinforced decks as inspection data is collected over time. 

What’s Next?

“These findings may help us shift some priorities for repairing or replacing mixed rebar bridges. We will continue to advocate for the use of all epoxy-coated rebar wherever we anticipate high levels of chlorides.” —Nick Haltvick, North Region Bridge Construction Engineer, MnDOT Bridge Office

This research confirms that MnDOT’s current practice of using only epoxy-coated rebar in bridge decks remains a durable solution and offers the best long-term value in terms of repair needs. MnDOT will continue to evaluate fiber-reinforced concrete deck behavior and may adopt a rating method for identifying crack density on the underside of concrete decks. 

A crack underneath a bridge deck, revealing signs of leakage and corrosion of steel reinforcement bars.
Cracks on the undersides of a bridge deck can leak as chlorides corrode steel and seep through the crack. 

This Technical Summary pertains to Report 2019-09, “Deterioration of Mixed Rebar and Fiber-Reinforced Concrete Bridge Decks,”  published February 2019. Visit the MnDOT research project page for more information.