Virginia Polytechnic Institute and State University
Concrete; Durability; Highway bridges; Mix design; T beams
This research project will develop design tables and new shapes for span lengths from 20 to 60 ft. The current Inverted-T shape works for span lengths around 45 ft, but changes in the cross-sectional geometry are needed to allow the system to work for span lengths from 20 to 60 ft. In this task, at least two additional cross-sectional shapes will be developed. Preliminary design tables will be prepared indicating number of strands required for various size strands, concrete release and 28 day strengths, and concrete unit weights. Typical end zone details will also be prepared for each cross-section and maximum number of prestressing strands. This project will also investigate additional mix designs and develop recommendations for topping mixture. The key to the success of the Inverted T-Beam System is the enhanced durability which results from the control of reflective cracking specifically, and reduction of all types of topping cracking, as compared to conventional adjacent member bridges. The reduction in cracking is a function of the cross-sectional shape of the precast elements, but is also due to careful design of the topping concrete to reduce the stresses that develop in the topping due to restrained shrinkage. Work was performed in the first study to develop a mix design which had relatively low shrinkage, and high creep. Creep allows the relaxation of stresses which develop from restraint. In the first test program, seven mixtures were compared based on ASTM standard tests for compressive and tensile strength, modulus of elasticity, shrinkage and creep. The best performing mixtures were a normal weight concrete with fly ash, and a normal weight concrete with saturated lightweight fine aggregate. Work will be performed to further refine the mixtures for both normal weight and lightweight concrete. Innovative materials will be investigated as possible constituents in the new mix designs. In addition to the ASTM tests, mock up tests will be developed to better represent the interaction between the precast concrete and the topping. These tests will be designed to gather data on the stresses that develop due to restrained shrinkage, and the time to onset of cracking. Tests to characterize tensile creep will also be performed. This research project will also perform further investigations of the elimination of horizontal shear connectors In the first project, one proof test was performed which illustrated that horizontal shear connectors were not required between the precast inverted T-beams and the cast-in-place topping. This work will be expanded to further investigate the horizontal shear strength provided by surfaces roughened by undulating formwork, a raked finish and an exposed aggregate finish. The horizontal shear capacities will be compared using simple push-off tests. Both normal weight and lightweight concrete will be used in the testing.