A new MnDOT-funded research study has found that most agencies in states with weather similar to Minnesota’s use debonded strands in prestressed concrete bridge beams. MnDOT may begin piloting debonding as an alternative to draping, which manufacturers claim is time-consuming, challenging to worker safety and expensive.
What Was the Need?
Bridge designers often prestress concrete beams with steel strands to improve the performance of the beams. The strands precompress the beams so that when external loads like vehicle traffic are applied, the concrete is less likely to crack under loading.
When the beams are fabricated, the strands are stretched from one end of the concrete form to the other, and then concrete is poured and hardens around the stretched cable. Once the concrete is cured, the cables are released from the precasting bed. When the cables shorten, they shorten or squeeze the concrete they are bonded to in the beam, precompressing it.
Because concrete is effective in compression and poor in tension (it cracks), precompressing the concrete leads to beams that may not crack in service conditions. It also leads to less deflection of the beams under loading. Both outcomes improve the strength, serviceability and durability of the system.
“We were assessing the current state of the practice of debonding strands in prestressed concrete, learning what other agencies have done and how much success they’ve had,” said Catherine French, CSE distinguished professor, University of Minnesota Department of Civil, Environmental and Geo-Engineering.
Prestressing causes high stress at beam ends, which is conventionally mitigated with some combination of two common design approaches. Cables can be debonded at the ends, typically by using a sleeve of a limited length that prevents concrete from directly bonding with the strands where covered. Draping strands can also help with end stresses by reducing the eccentricity of the strands. While regular strands run parallel to the length of the beam, draped strands are pulled in a somewhat V-shape, from the top of the beam at each end to the bottom of the beam in the middle.
MnDOT currently uses draping to relieve the end stresses, but it does not allow debonding due to its potential to provide a path for chlorides to enter the concrete along the debonding tubes, which could lead to corrosion. Manufacturers would like to rely less on draping, which requires more time, cost and care to safely fabricate. Local agencies are interested in debonding because draping requires thicker and more expensive concrete beams. National standards offer the choice of draping, debonding or a combination of both.
What Was Our Goal?
Researchers investigated the state of the practice for using debonded strands in prestressed concrete beams. MnDOT and the Local Road Research Board (LRRB) needed recommendations for using debonded strands to position the agency to adopt current and imminent national debonding standards for prestressed beams and to use debonding as an alternative to draping, where appropriate.
What Did We Do?
Researchers studied current MnDOT prestressing specifications, prestressing and debonding guidelines established by the American Association of State Highway and Transportation Officials (AASHTO), and research on debonding and draping. They also surveyed 11 agencies in 10 states with climates similar to Minnesota’s about their use of debonding and its performance in terms of reducing beam end stresses and resisting corrosion. They followed up with some respondents to gather more detail on respondents’ practices and experience.
Debonding sleeves protect prestressing strands from bonding with concrete, reducing stress and cracking at beam ends.
What Did We Learn?
Debonding appears to reduce cracking at beam ends. Currently, AASHTO allows debonding of up to 25 percent of prestressing strands in concrete beams, though this may soon be revised to allow a higher debonding limit. AASHTO’s T-10 Technical Committee proposes allowing up to 45 percent debonding, while NCHRP Research Report 849 recommends allowing up to 60 percent of strands be debonded.
“Researchers did not find that there was any excessive corrosion with debonded strands. The team is recommending we start at debonding 40 percent of prestressed strands,” said Brian Homan, State Aid bridge plans engineer, MnDOT Bridge Office.
Ten of the 11 responding agencies use debonding, typically in coordination with sealing beam ends with silicone or similar material to protect sleeved cables from water and salt intrusion. Five of the 10 reported debonding as their primary method for reducing end stresses, and three indicated draping as their favored approach. Six limit debonding to 25 percent of strands, though others allow a higher percentage, including Michigan DOT, which allows up to 40 percent of the strands to be debonded. Respondents reported few problems with debonded strands.
Researchers recommend that MnDOT begin debonding up to 40 percent of its strands to refine the practice before it considers adopting the 60 percent standard. Two split sheath tubes, one over the other, should be applied to the strands to achieve debonding in the end regions.
Concrete ends should be sealed as MnDOT currently requires, and silicone sealant should be applied to exposed strand ends. The research team recommended a sequence for releasing prestressing cable to minimize cracking at the beam ends—bonded before debonded and shorter debonded lengths before longer lengths.
Debonding strands costs less than draping and is favored by local agencies; the practice will reduce prestressed beam fabrication costs for MnDOT and the LRRB. Draping procedures present safety concerns that will be relieved by a reduction in draping, and debonding is expected to reduce end cracking.
MnDOT is developing two pilot projects in which up to 45 percent debonding may be used. Future research may be warranted to identify the best percentage of strands that should be debonded and evaluate debonding sleeve materials, designs and performance.
This post pertains to Report 2019-30, “Debonded Strands in Prestressed Concrete Bridge Girders,” published July 2019. For more information, visit the MnDOT project page.