Question:
How can vertical pre-release cracks in prestressed concrete beams be avoided?
Answer:
Vertical cracks have been observed in prestressed concrete beams before the prestressing strands are released. Their occurrence appears to be more frequent in deep, long-span members. These pre-release cracks, usually two to four in number, are generally located within the middle third of the beam length. They run transversely across the top flange of the beam, extend downward through the beam web, and may extend into the bottom flange of the beam. As soon as the strands are released, the cracks are closed by the prestressing force and become virtually invisible.
Research(1,2) conducted in North Carolina determined that the vertical pre-release cracks are caused by the restraining effect of the prestressing strands when the newly cast beams begin to cool and shorten before the strands are released. After the beams have been heat cured overnight, the temperature of the prestressed concrete beams is generally much higher than the ambient temperature. This temperature differential causes the beams to contract while the prestressing strands are still anchored to the ends of the casting bed. In addition, the lower ambient temperature causes the exposed strands to contract.
These thermal contractions are restrained by the anchored prestressing strands before they are released. The restraining force exerts a tensile force on the beams, causing pre-release vertical cracks when the tensile stress in the beams exceeds the concrete tensile strength, which is relatively low since the concrete is usually less than one day old.
The tensile stress induced in the concrete by the restrained thermal contraction increases with longer spans, larger sizes of the beams, greater temperature differential, and reduced length of exposed strands. The occurrence of the cracks can be minimized by two approaches. The first is to release the strands immediately following the curing period as required by the PCI Manual for Quality Control.(3) The second approach is to increase the exposed strand length between adjacent beams and between the end beams and the anchorages. For a temperature drop of 60°F (33°C), the exposed tendon length should be about 28 percent of the beam length.(2) The first approach is preferable since the second approach reduces the efficiency of production. It is also beneficial to protect the exposed strands from low ambient temperatures until the strands are released.
Since the problem of beams with pre-release vertical cracks occurs from time to time, outright rejection of the beams with such cracks is not a practical approach. North Carolina Department of Transportation has developed a set of acceptance criteria for beams with pre-release vertical cracks and the criteria can be found in Reference 1.
References
- Zia, P. and Caner, A., “Cracking in Long-Span Prestressed Concrete AASHTO Girders During Production,” Symposium Proceedings, PCI/FHWA/FIB International Symposium on High Performance Concrete, Orlando, Florida, Precast/Prestressed Concrete Institute, Chicago, IL, 2000, pp. 459-469.
- Zia, P. and Caner, A., “Cracking in Large-Sized Long-Span Prestressed Concrete AASHTO Girders,” Report No. FHWA/NC/94-003, NCDOT, Raleigh, NC, 1994, 98 pp.
- Manual for Quality Control for Plants and Production of Structural Precast Concrete Products, MNL-116-99, Precast/Prestressed Concrete Institute, Chicago, IL, 1999.
Answer contributed by Paul Zia of North Carolina State University. He may be contacted at [email protected].