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The photograph shows a side view of a girder with inclined cracks, horizontal cracks, and a Y crack 

Fig. 1. An example of girder cracks
            

Controlling Prestress Release Cracks in High Performance Concrete Bridge Girders
Pinar Okumus, University at Buffalo, & Michael Oliva, University of Wisconsin

Very efficient new pretensioned concrete highway bridge girders have taken advantage of high strength concrete and unique shapes to allow the application of much higher levels of prestress. These newer girders often exhibit unexpected end cracking upon prestress release, a concern for designers and manufacturers. Cracks that are near prestressing strands may be particularly harmful if deck joints above the girder ends leak and allow chloride solutions to run along the girder ends. This could create a situation where chloride solutions penetrate the concrete surrounding the strands and cause accelerated corrosion.

Bulb-Tees are a prime example, often with deep sections, slender webs, and large amounts of prestress. The 54-inch deep girder shown in Figure 1 exhibits three types of typical end cracks: inclined cracks that run parallel to draped strands near the top of the web, horizontal cracks over the web depth, and bottom-flange Y cracks. The inclined cracks and horizontal web cracks may close as loads are applied to the girder and compressive end reactions develop. The Y or T shaped cracks, however, propagate into the bottom flange where strands are located and can increase in width to 0.05 inches as loads are applied.

End cracking is an endemic problem in prestressed concrete due to the highly localized force applied to the concrete at the release of pretensioned forces. Two mechanisms are involved: shear lag and localized bending. Both of these mechanisms can cause very high principal tension stresses in the concrete in the development length region at the girder ends, particularly with the slender cross sections of newer type girders.

Recent research conducted by the authors for the Wisconsin Department of Transportation (WisDOT) was aimed at controlling the tension stresses and strains developed in the concrete surrounding the strands to values below the tensile cracking strain. Both field testing and non-linear finite element analysis that simulated cracking were used to quantitatively examine various means of end crack control.

Crack control methods include: 1) providing vertical steel in the webs or spiral confinement reinforcing in the girder flange over the end transfer length region, 2) changing the drape of strands, 3) controlling the pretensioned strand cutting sequence during de-tensioning, and 4) debonding strands near the girder end.

Two types of steel reinforcing were examined for their effect in reducing concrete tension strains. Vertical steel bars, in addition to normal stirrup reinforcing, placed in the web of the beams are typically used to control the inclined cracks and horizontal web cracks. Results proved that only vertical bars very near the girder end (first two bars) are effective in controlling horizontal cracks. Using a bar size that would be difficult to fit in a web, 2 - #10 bars at 3 inches, reduced concrete tension strains in the horizontal crack region by 50% compared to typical girder reinforcing, but still allowed strains of more than double the concrete cracking strain to develop. Vertical reinforcing is effective in reducing crack widths for horizontal cracks, but not in eliminating them. Spiral confinement reinforcing in the bottom flange, likewise, is ineffective at controlling the Y cracks when placed around the bottom flange strands.

Reducing the strand drape or fanning the draped strands apart at the end of the girder can eliminate the inclined cracks, but does not eliminate the horizontal cracks. Changing the strand drape makes girders less efficient since it also requires a reduction in bottom flange strands to control initial end stresses.

Changing the sequence of stand cutting reduces concrete tension strains, but not enough to prevent Y cracks from developing.

Debonding selected strands at the girder ends is effective in controlling or even totally eliminating girder end cracking. In examining WisDOT 54 inch, 72 inch and 82 inch deep girders with the full possible complement of strands, even debonding only 25% of the strands removed nearly all the concrete end cracks. Debonding 50% of the strand at the girder end completely eliminated cracking, but care must be taken to insure sufficient shear strength remains. AASHTO LRFD Specifications currently suggest that the total amount of debonding should be limited to 25% and that no more than 40% of the strands shall be debonded. Yet the AASHTO commentary acknowledges that some states have had success with greater debonding. An alternate solution: debonding all of the bottom straight strands for only 12 inches at the girder end also proved to be very effective in eliminating cracking and could be considered to satisfy AASHTO criteria if it occurred outside of the end bearing region. A simple solution to eliminate end cracking might be to move the girder bearing pads in 12 inches at the ends (using slightly longer girders) and debonding all the strand for 12 inches.

Color graphic shows Tensile strains for a 54-in. deep girder with standard strand configuration are compared to a girder where all strands are debonded for 12 inches


Fig. 2. Tensile strains for a 54-in. deep girder with standard strand configuration are compared to a girder where all strands are debonded for 12 inches
            

The results summarized below are aimed to help bridge engineers and manufacturers pick the best practices to control end cracking to extend the life of high performance prestressed bridge girders. In all of the scenarios below, except with debonded strands where draping is not needed, draped strands are assumed to be present.

Control method
Inclined Cracks
Web Cracks
Y Cracks
Increase in reinforcement area of Two bars closest to girder end
Mild
Moderate
None
Bars farther away from girder end
None
None
None
Bottom flange stirrups
None
None
None
Debonding some strands at girder end
High
Moderate
High
Debonding all strands up to 12 in. from the end
Mild
High
High
Change in strand cutting sequence
None
None
Moderate
Draped strands Removed
High
None
None
Lowered
None
Moderate
None
Lowered and spread
High
Moderate
None

Table 1. Practices to control end cracking
            

Further Information

Please contact Pinar Okumus at okumuspinar@gmail.com or Michael Oliva at Michael Oliva at oliva@engr.wisc.edu.

References

Okumus, P., Oliva, M. G, “Evaluation of Crack Control Methods for End Zone Cracking in Prestressed Concrete Bridge Girders”, PCI Journal, volume 58, Issue 2, pp 91-105. Okumus, P., Oliva, M.G., “Finite Element Analysis of Deep Wide-Flanged Pre-stressed Girders to Understand and Control End Cracking”, Wisconsin Highway Research Program, Final Report, Report No. WHRP 11-06, June 2011.