ASPIRE is a quarterly magazine published by PCI in cooperation with the associations of the National Concrete Bridge Council. The editorial content focuses on the latest technology and key issues in the Concrete Bridge Industry.
Issue link: http://www.aspiremagazinebyengineers.com/i/485551
CONCRETE BRIDGE TECHNOLOGY Bearing Zone Tearing and Spalling in Precast, Prestressed Concrete Girders by Hugh Ronald, Reynolds, Smith and Hills If there is one thing you can count on: it’s that engineers will push the envelope to achieve longer spans with fewer members. What used to be an industry dominated by standard American Association of State Highway and Transportation Officials (AASHTO) girder shapes, is now an industry that has a range of new bulb tees designed to more effectively utilize higher concrete strengths and larger-diameter strands. From Florida to Washington s tate, new standard wide flange I-girders have been developed and are growing in use. These girders may be as deep as 10 ft and can use not only more strand, but larger (0.6-in.-diameter) strand, than was typically used in the standard AASHTO shapes. Use of these new shapes is leading to new challenges in the casting process and may require new details to produce precast concrete girders that are both strong and durable, particularly given the need to increase the service life of bridges to 75 or 100 years. One of the challenges to girder fabrication has been the advent of tearing (cracking) and spalling of the concrete in the girder bearing zones, a phenomenon that occurs during prestress transfer. Girders that experience tearing are generally longer, heavier, and/or more highly prestressed than their older counter parts. Bearing Zone Tearing and Spalling The primary causes of tearing and spalling are girder shortening and camber that occur as the prestressing force is transferred to the concrete during detensioning. As a highly prestressed girder cambers, the bearing reactions concentrate at the ends of the girder, producing a knife-edge loading condition. At the same time, the girder contracts and slides due to elastic shortening. If the girder is long, the bearing reactions will be proportionately high, and the friction that develops between the girder and casting bed soffit will produce tension along the longitudinal axis of the girder. The magnitude of this tension is directly proportional to the friction force that develops at the ends of the girder. If not adequately addressed, this force may crack and possibly spall the concrete at the ends of the girder. Unfortunately, the strands in the bearing zone are not fully developed so they cannot mitigate the problem. Remedial Countermeasures To address the problem, a number of countermeasures have been employed with varying levels of success. The first countermeasure employed has generally been to lubricate the soffit pan using oil or wax at the ends of the girder (over a distance roughly equal to the strand transfer length), or to place polytetrafluoroethylene (PTFE) or plywood sheets, compression tape, or steel sheets under the bearing zone. The intent is to lower the coefficient of friction between the girder and soffit, thus reducing the magnitude of the tension associated with girder camber and shortening. The table shows that the friction coefficient is reduced from as high as 0.51 on an unlubricated soffit to 0.23 for a wax lubricant. Still, when soffits are lubricated, cracking and spalling are generally not eliminated, but are only reduced in frequency and magnitude. The second approach has been to embed and anchor steel plates or angles in the bearing zone at the ends of the girder where the knife-edge loading occurs. Use of embedded angles anchored to the girder with headed studs decreases the frequency and magnitude of the cracking and spalling more than use of lubricants alone, but test results show that cracking may still occur. Embedded plates, adequately sized to extend from the end of girder across the bearing zone, have been used successfully to virtually eliminate bearing zone tearing. This detail has been adopted by several owner agencies. The Florida Department of Transportation employs galvanized bearing plates in all of their new Florida I-beams, one of the new generation of wide flange girders designed for long-span, pretensioned concrete girder construction. Galvanized plates are less susceptible to corrosion and will provide the durability needed for a long service life.