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7 (a) Above the Strands (b) At or Near the Strands Figure 1.1. End-of-member cracks for hollow-core slabs. Figure 1.2. End-of-member reduce shear capacity, but it does not give any criteria on cracks for double tees. when to reject the product. The report gives some repair procedures that range from epoxy injection for small cracks to solid grouting of the voids. The report recognizes the fact that this type of cracking For double tees the report recognizes horizontal end crack- does not grow once the beam is installed on a bridge. On ing in the stem during prestress release, as shown in Fig- the contrary, the cracks will close to some extent due to ure 1.2. It states that the crack length can extend horizontally applied dead and live loads, as end reactions provide a for a distance of from several inches to a few feet. However, clamping force. The PCI report does not give any guidelines on when to the report does not give any guidelines regarding the crack width or when to reject the product. The report states that reject a beam with end cracks. if the crack plane coincides with a strand, it may affect the More information on the permissible crack width is pro- bond between the strand and concrete and increase trans- vided in Appendix A, Literature Review, of this report. fer and development length. In 2006, the Precast/Prestressed Concrete Institute (PCI) 1.2.2 Sources of End Zone Cracking published the Manual for the Evaluation and Repair of Pre- Longitudinal end zone cracking occurs in pretensioned cast, Prestressed Concrete Bridge Products (11). The objective girders during release of the pretensioned strands. The draped of the report is to achieve a greater degree of uniformity among strands are usually released first using flame cutting at the owners, engineers, and precast producers with respect to ends and then by removing the hold-down anchorage devices the evaluation and repair of precast, prestressed concrete at the harp points. The straight strands are then released by bridge beams. The report recognizes end-of-beam cracking in one of the following two methods (1) flame cutting, which "Troubleshooting, Item #4." A summary of the report find- is a practice used by a large number of precast producers, ings and recommendations are as follows: or (2) gradual release (jack down) in which the abutment of the prestressing bed is equipped with a hydraulic system For cracks that intercept or are collinear with strands but that allows it to move gradually towards the concrete member. without evidence of strand slippage (significant retraction During release, the strands grip against the concrete, grad- of strand into the beam end), the report recommends inject- ually transferring their force to the concrete girder through a ing the cracks with epoxy. distance known as the transfer length. The force transferred The report uses the crack width values developed in ACI from the strands causes member shortening. The member 224R-01 as guidelines whether or not to inject cracks. These slides on the bottom pallet, dragging the ends at the bottom. values are shown in Table 1.4. The horizontal sliding is accompanied by upward camber, For cracks that intercept or are collinear with strands with and the precast member becomes supported at its ends only. evidence of strand slippage (significant retraction of strand The release process is typically accompanied with forma- into the beam end), the report recommends injecting the tion of longitudinal cracks at the girder ends. These cracks cracks with epoxy and re-computation of stresses after shift- may occur in the web or at the junction between the web and ing the transfer and development length of affected strands. the bottom flange. There are many possible sources that may Table 1.4. End-of-beam cracks that should be injected (11). Exposure Condition Crack Width, in. (mm) 1. Concrete exposed to humidity > 0.012 (0.30) 2. Concrete subject to deicing chemicals > 0.007 (0.18) 3. Concrete exposed to seawater and seawater spray, wetting and drying > 0.006 (0.15) cycles

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8 increase or decrease the likelihood of this longitudinal end Length of the free strand in the prestressing bed: As the first zone cracking in pretensioned girders. Within the literature strands are cut and the precast member is compressed caus- search and the survey responses, the following multiple sources ing elastic shortening, the remaining uncut strands must were suggested: lengthen to accommodate the shortening of the member. The resulting tensile force in the uncut strands causes ver- Method of detensioning: As previously explained, the tical cracks to form near the ends of the member, where bottom strands can either be flame cut manually while the compression from the cut strands has not been fully still fully tensioned, or they can be slowly jacked down by imparted on the section. This source can be very detrimen- a hydraulic release before being cut. Since flame cutting tal in cases where more than one precast member is cast is done manually, the strands are released individually, on a single prestressing bed. In a study conducted in 1978 which creates uneven forces throughout the beam and (12), researchers found that this source of cracking can be presents a more localized aggressive introduction of force eliminated by making the free strand length between the to the beam. Slowly jacking down the strands prevents the abutment and the concrete member or between adjacent sudden introduction of force that flame cutting causes members as short as needed for fabrication. and gives the concrete girder more time to accommodate Friction with the bottom form of the prestressing bed the transformed compressive force. Although hydraulic (11, 14): In cases where the bottom form of the prestress- release is preferred to reduce end zone cracking, very few ing bed is not properly oiled or has indentions, horizontal state departments of transportation (DOTs) mandate its cracks are developed in the bottom flange. When cutting use because it requires the precast plants to restructure the strands the beam may be moved horizontally along the the existing prestressing beds. bed floor, causing friction on the surface between the con- Release of the top straight or draped strands before the crete and steel. bottom straight strands: This sequence puts the bottom Heat concentration during flame cutting (11, 15): It was flange in tension (especially with deep precast members), reported that concentrated heating of a strand leads to high trying to stretch it out. Since the beam at this stage is in full sudden shock of the released prestress force. It is always contact with the bottom form of the prestressing bed, and recommended to heat the strand over a long distance to its bottom flange is restrained by the straight strands that allow slow elongation (annealing), and that flame cutting are not released yet, the frictional force produced at the bot- of strands be done by trained and experienced workers. tom surface of the member resists this movement and may Lifting the precast member from the bed (16): The pre- produce a vertical crack at the side of the bottom flange that stressing force causes the girder to camber so that the cen- extends vertically towards the web/bottom flange junction. ter of the beam is forced higher than the ends. Shortly after In order to treat this problem, some state DOTs require not prestress release, the precast member is lifted from the bed to fully tension strands located in the top flange, reduce the and moved to the storage area. In most cases where the height of the draped strands to the level that makes release member is relatively long, the lifting points are generally stresses within their allowable limits, and/or uniformly dis- recessed by as much as 15 to 20 ft from the member ends, tribute the draped strands across the web height rather than at camber raised locations. The lifting point locations are concentrating them close to, or in, the top flange. subject to negative moments not only from the prestress but Order of release of bottom strands with the flame cut- also from the self weight. This latter effect is often ignored ting method: Due to limited accessibility of interior strands, by designers. It is a major contributor to the temporary the edge strands on each layer are generally released before crack widening that occurs at the time of lifting. At this the interior strands. This order puts the tips of the bottom initial lifting of the beam, the prestress force has not yet flange in compression and makes them act as free cantilevers, diminished and is at its highest while the concrete has not which initiates horizontal cracking at the web/bottom flange yet reached its full strength. It has been known to con- junction or sloped cracks in the web close to its junction tribute to downward diagonal cracks in the upper part of with the bottom flange. A specific pattern must be fol- the web. lowed in order not to increase cracking. Angular cracks Hoyer Effect (17): Upon release of the prestress, the diam- can occur from the stress difference of cut and uncut pre- eter of the strand expands and pushes against the surround- stressed strands if the cutting pattern is not idealized. ing concrete. This action, which is known as the Hoyer Both ends of the same prestressing strand should also be Effect, improves the bond between the strand and the con- cut simultaneously to prevent uneven forces. However, crete and helps in transferring the prestress force to concrete. researchers found that the sudden introduction of stress into However, it creates radian tensile stresses in the concrete the girder from flame cutting of the strands is conducive to volume, which leads to a radial crack that extends from cracking, even with a planned pattern (1213). the strand to the nearest concrete surface at the end sur-