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Three different methods of blanketing or sheathing the strands to obtain debonding were evaluated based on the experiences of fabricators and other state highway agencies as well as research findings. Split-sheathing tube is preferred by fabricators due to the ease of fabrication in using this material over rigid sheathing tube. Split-sheathing tubes can achieve debonding with either a single split-sheathing tube or double split-sheathing tube method. Figure 5.3 and Figure 5.4 depict a single split-sheathing and double split-sheathing tube method, respectively. A concern with the single split-sheathing tube is that it allows concrete to seep through the sheathing and form a bond with the strand. Some states specify that single sheathing be taped or tied along its length to preclude the entry of concrete (e.g., WisDOT).

Figure 5-3: Single split-sheathing tube (ALP Supply)

Figure 5-4: Double split-sheathing tube method

This is not preferred as a tight contact between the debonded strand, and single-split sheathing tube can lead to cracking along entire debonded length due to radial expansion of strand at prestress release (Burgueno & Sun, 2011). Burgueno and Sun (2011) suggest that oversized rigid sheathing tubes should be used instead. However, the fabricators surveyed expressed concern with this solid/rigid sheathing tube because it is more difficult to work with and requires that the strands be fed through the tube. Thus, an oversized double split-sheathing tube method is recommended, which is the use of two single split-sheathing tubes. By placing the two slits/openings of the sheathing tubes on opposite sides, concrete entry will be prevented without having to tape or tie along the sheathing length. Thus, a double split-sheathing tube method will provide sufficient room for the strand to dilate, all while maintaining ease of construction in the fabrication process compared with the other two alternatives. This method of strand sheathing is also currently used by MDOT, SDDOT, SCDOT, and NDOR. The end of the double split-sheathing tubes inside the beam form must also be tied with suitable material (e.g., rebar tie wires) or taped with waterproof material to prevent concrete entry. Alternatively, Figure 5-5 shows application of a silicone sealant within the beam forms between the sheathing and strand to prevent concrete entry. This alternative method is currently utilized by IDOT.

Figure 5-5: Silicone sealant applied within the beam form (Photo courtesy of IDOT)

5.4 STRAND RELEASE PATTERN

Most state highway agencies rely on the experience and best practices of bridge fabricators regarding strand release patterns and de-tensioning of prestressed girders.

Of the ten highway agencies that were surveyed that allow debonded strands, no indication or information was gathered regarding changes to their specification as a result of debonded strands, except for one state which adds a note on its beam sheets regarding the release of debonded strands (SDDOT).

The research team expanded the scope of the study beyond the eleven surveyed agencies in an effort to find information on release methodology specified with the use of debonded strands. Some standard detail sheets of additional state highway agencies that use debonded strands were perused. The South Carolina Department of Transportation (SCDOT), similar to SDDOT, adds a note on its standard beam sheets as an exception to the fabricator’s method of strand release pattern when debonded strands are used. SCDOT and SDDOT notes state that fully bonded strands are to be released first, then debonded strands are to be released after all fully bonded strands have been released. The debonded strands are to be released in sequence from shortest debonding length to maximum debonding length. The release symmetry that MnDOT requires will be maintained because strands with equal debonding lengths will be placed symmetrically about the beam vertical centerline. It is recommended that symmetry be maintained in the strand release pattern.

It is recommended that these exceptions (i.e., SCDOT and SDDOT notes on releasing fully bonded strands first) be incorporated into standard beam sheets or in the special provisions. There are no apparent risks associated with these notes, but they do offer a few benefits. One benefit is that because debonding cannot be placed in the outermost strands, the last few strands to be de-tensioned will likely be away from the surface. This will reduce the risks of spalling of corner concrete that fabricators have reported as a result of releasing outermost strands last. The other benefit of incorporating this

exception is that the final strands released (i.e., longest debonded strands) will introduce less restraining stress in the beam before they are cut because they have the longest debonded length. As the girder shortens, the reduction in length of the girders causes an increase in the free length of the uncut strands. Because the debonded strands have a longer free length than the bonded strands, the

restraining stresses in the free length portion of those strands will be smaller than they would be in the bonded strands.

For these reasons, it is recommended that fabricators use the MnDOT strand release patterns method or their preferred method with the one exception listed previously. Fully bonded strands should be released first, and debonded strands are to be released after all fully bonded strands have been released, in sequence from the strand with the shortest debonding length to the strand with the maximum debonding length.