End Zone Design for Alabama Deep Pre-Stressed Girders

Deep prestressed concrete bridge girders are becoming increasingly popular over previous superstructure members, for their better structural efficiency to allow higher prestress level and span longer distance. Florida-I-Beam (max. span 210ft), Nebraska NU girder (max. span 240ft), and the Washington WF100G (max. span 220ft) all belong to this category. However, when designed according to the current American Association of State Highway and Transportation Officials (AASHTO) Specifications, cracking is frequently observed at the end zones of these pretensioned concrete members at the time of prestress transfer. Previously, Alabama Department of Transportation (ALDOT) implemented deep girders that can span 165ft. During the fabrication process of these girders, some cracking issues have been observed at the end zones of the girders. Based on experience and engineering judgment, additional longitudinal reinforcement was added into the end zones. This prescriptive solution was able to reduce the cracking to certain level but not completely solve the problem. The goal of this proposed research project is to provide a practical engineering solution to the end zone cracking problem. The project proposes to conduct the following tasks: 1. Work with an Alabama precast concrete fabricator to provide a standard long span girder design (aiming at approximately 180ft span) by fully utilizing the shape that is readily available at the fabricator's facility, and determine three (3) modified end zone details as potential candidates to mitigate end zone cracking issues; 2. Instrument the end zones of four (4) experimental deep girders and measure steel and concrete strains at prestress transfer. The girders will include the standard end zone detail and modified end zone details; 3. Load test the girders at University of Alabama (UA) Large Scale Structures Laboratory (LSSL) to determine the effects of modified end zone details (for example strand debonding and self-consolidating concrete) on girder strength; and 4. Evaluate the performance of the experimental girders based on strain and load test data, and propose a prototype end zone detail suitable for both ALDOT and Alabama precast concrete fabricators.