End Zone Reinforcement for Pretensioned Concrete Girders
In this study, a literature review was conducted to establish the background of current specifications and to evaluate the applicability of various theories and methods for design of end zone reinforcement. Analytical methods reviewed in this paper include finite element analysis, strut- and-tie modeling, and the Gergely-Sozen equivalent beam method. Previous experimental work combined with work conducted as part of this study were used to correlate between various theoretical and experimental results. This paper illustrates that no single theoretical method adequately represents the complex behavior at the end of a pretensioned concrete member. A general semi-empirical design procedure is proposed here. It is based on theoretical behavior and experimental observation. Standard reinforcement details are given. The proposed procedure could result in reduction in the congestion of reinforcement while maintaining acceptable crack control at the member end. Application of the proposed procedure to highly pretensioned bridge girders is demonstrated.
Field and Laboratory Study of the Mn/DOT Precast Slab Span System
The Minnesota Department of Transportation (Mn/DOT) Precast Slab Span System was initially designed by Mn/DOT with input from University of Minnesota researchers and local fabricators. The bridge system consisted of a series of precast, prestressed concrete inverted tee bridge elements which also served as stay-in-place formwork for the cast-in-place portion of the deck placed in the field. One of the Mn/DOT implementations, located in Center City, MN, was instrumented. The bridge has been monitored for reflective cracking and continuity over the piers since the deck was cast. Transverse load distribution was evaluated with a static truck test. In addition, a two-span test specimen was constructed to investigate effects of variations in flange thickness, bursting reinforcement, horizontal shear reinforcement, and flange surface treatment. The data obtained from the field study indicated that cracking had initiated in the bridge at the locations of some of the gages at midspan and near the support. The cracking was determined to be the result of environmental loads and shrinkage rather than due to vehicular loads. The data from the truck tests indicated that the design assumption of a monolithic slab system was valid for the determination of load distribution factors. The results of the laboratory study showed that positive restraint moments developed in the precast system for which continuity was made at a young age (i.e., seven days), and that these moments could be reasonably well predicted by existing models. It has also been found that current American Association of State Highway and Transportation Officials bursting requirements require unnecessary transverse reinforcement in the end zones of slab span systems.
BEHAVIOR OF POST-TENSIONED GIRDER ANCHORAGE ZONES
Several large, thin-webbed box girder bridges, with post-tensioned anchorage zones designed in accordance with AASHTO and ACI requirements, have experienced large cracks along the tendon path in the anchorage zones at the design stressing load. Cracking of this nature provides a path for penetration of moisture and salts and thus presents a potential corrosion and frost damage threat. In addition, such cracking negates a major reason for the use of prestressed concrete, the minimization of service load cracking. This report summarizes the observations noted in an extensive physical test program. A simplified test specimen was developed to accurately simulate the behavior of the post-tensioned box girder web. The experimental program investigated the primary variables affecting the formation of the tendon path crack: tendon inclination and eccentricity, section height and width, tensile splitting strength of the concrete, anchor width and geometry, and the effect of supplementary anchorage zone reinforcement, both active and passive. Behavioral trends are presented as determined from three sources. These include physical tests of 40 quarter-scale microconcrete models, physical tests of 9 full-scale prototype box girder web sections, and an extensive series of three-dimensional linear elastic finite element computer analyses. (FHWA)