Link to Latest Report: Coming Soon!
Background:
Precast, prestressed concrete bridge girders are used extensively in conventional and accelerated bridge construction. As longer span girders are desired to reduce the number of supports and improve speed of construction the impacts of high prestressing force on the end regions of the beams become more significant. End region behavior of prestressed concrete girders has been a significant concern warranting numerous studies over the years focused on stress limits, prestress transfer length, cracking caused by the prestress, and shear capacity. Ultra-high performance concrete (UHPC) is a relatively recent advancement in cementitious composite materials with mechanical and durability properties far exceeding those of conventional concrete. These improved mechanical properties have the potential to mitigate the impacts of high stresses in prestressed girder end regions and to provide greater overall girder capacity. In addition, UHPC has the potential to increase the overall durability of these prestressed girders if used in areas of high exposure. However, little research has been conducted on the behavior of hybrid girders
using UHPC in the end region or as a stay-in-place formwork shell. The proposed project will leverage results obtained through Oklahoma DOT support on long term behavior of full UHPC prestressed girders to design and evaluate time dependent behavior and strength of hybrid conventional self-consolidating concrete and UHPC girders. A total of 10 prestressed girders will be cast, instrumented, and tested and the results used to develop predictions for prestress loss behavior and recommendations for girder detailing.
Objective:
There are three objectives of the proposed research. The first objective is to measure prestress losses for full UHPC prestressed bridge girders. The second objective is to evaluate the effect of a hybrid UHPC-conventional concrete girder design on prestress loss behavior. The third objective is to evaluate the effectiveness of different design details for mitigating stresses in the girder end region and their effect on hybrid girder capacity.
Scope:
Task 1 – Design, Construct, and Monitor Full UHPC Girder Specimens (ODOT funding)
A literature review will be conducted to identify previous research on UHPC prestressed concrete girders to determine the maximum stresses applied in both tension and compression. Information identified from this review will also be used to identify required reinforcing details to reduce potential for end region cracking, camber, and prestress losses. Girder specimens will be designed as approximately half-scale elements to utilize existing formwork, stay within the constraints of the OU prestressing bed, and to obtain the required stress states. Four girder designs will be developed to vary end region stress in tension and compression and to vary end region reinforcement detailing. Girders will be cast at the OU Fears Structural Engineering Laboratory using the OU-developed J3 UHPC mix design and will be instrumented with vibrating wire strain gages at mid-span to monitor prestress losses and in the end regions to evaluate stresses resulting from the prestress force. Specimens will be evaluated for cracking at
the point of prestress release and over time and will be monitored to evaluate prestress losses over time. The girders will be monitored over time with readings taken continually using a dedicated data logger.
Task 2 – Design, Construct, and Monitor Hybrid Girder Specimens (IBT/ABC-UTC
funding)
Six hybrid UHPC-SCC precast, prestressed concrete beams will be cast at Fears Lab on the OU campus using designs developed based on the results of Task 1. The different designs will be based on the maximum stresses measured in the full UHPC beams tested in Task 1 and will result in variations in reinforcing details and combination of SCC and UHPC. At least one specimen will consist of a UHPC stay-in-place formwork shell that will be filled with SCC and left in place to provide protection of the conventional concrete against corrosion. Interfaces between the UHPC and SCC will be based on results of research completed by the co-PI. All specimens will be instrumented with strain gages placed at midspan and within the UHPC portion to evaluate strain development along the length of the girder and prestress losses over time. The girders will be monitored over time with readings taken continually with a dedicated data logger.
Task 3 – Prediction of Time-Dependent Behavior (IBT/ABC-UTC funding)
A prestress loss prediction model using existing model structures for creep and shrinkage will be developed utilizing data from material property testing at OU and in the literature and will be compared to experimentally measured prestress losses for both the full UHPC and hybrid girder specimens.
Task 4 – Shear Testing (IBT/ABC-UTC funding)
All girder specimens will tested in shear to evaluate strength of the girder end regions and to evaluate effectiveness of the composite behavior of the UHPC-SCC hybrid sections. These tests will be conducted near the end of the project, after the prestress losses have stopped changing significantly.
Task 5 – Final Report and Training Materials (IBT/ABC-UTC and matching funding)
A final report will be prepared compiling the results of all work in the project and a one hour pre-recorded training module will be developed to disseminate the results of this research.
Research Team:
Principal Investigator: Royce W. Floyd, Ph.D., P.E., S.E.
Co-Principal Investigator: Jeffery S. Volz, Ph.D., P.E., S.E.