Relief of reinforcing congestion in beams and bent caps of concrete bridges

Project Category: OTCREOS 9.1 Projects

Principal Investigator:

• Thomas Kang, University of Oklahoma

Project Description: Dr. Thomas Kang, Assistant Professor of Civil Engineering and Environmental Science at the University of Oklahoma, aims to conduct research into methods of reducing steel congestion of reinforced and prestressed concrete infrastructures without affecting overall strength. The first aspect of the research will focus on relief of reinforcing congestion by utilizing headed bars. Closely spaced headed bars (with clear bar spacing less than 4db) are not permitted by the current ACI 318-08 provisions. His research will be used to assist updating ACI 318 provisions and 352 recommendations. The second aspect of Dr. Kang’s investigations will involve use of steel fibers for reinforced and prestressed concrete members to reduce or eliminate stirrups. The proposed project will help determine a better minimum steel fiber volume ratio resulting in a mix that is both workable and durable. The current ACI 318-08 minimum ratio has been deemed unsatisfactory by ODOT engineers.

Both aspects of the research will require specialized equipment, personnel, and facilities to conduct large-scale monotonic and reversed cyclic testing. In this proposed project, a total of 30 large-scale beams, girders, and cap beam-bridge column joints will be tested. Computer-based finite element modeling will also be performed to help evaluate bond and bearing of headed reinforcement in concrete and behavior of steel fiber-reinforced concrete, as well as to assess seismic performance of bridge concrete systems using headed bars and/or steel fibers.

Overall, this project directly addresses one of the major concerns of OTC and ODOT: congestion due to reinforcing details. Some bridge reinforcing details (e.g., joint region or overlay bridge deck where adequate shear resistance is also necessary) cannot be produced within the tolerance limits in the field or can cause problems with concrete placement and quality. Requirements for bar anchorage and development have been frequently neglected in actual construction simply because there was no space for re-bars at the joint and connection region. Since the lack of anchorage and development can lead to a catastrophic failure and this issue has still not been practically solved, great attention should be paid by DOTs. The current minimum required dosage rate of steel fibers (ACI 318-08, §5.6.6.2) causes even more concrete workability problems, which in fact does not help in concrete placement. The results and outcomes of this project will impact the quality control of cast-in-place bridge construction. More specifically, the optimal use of steel fibers and headed bars corresponding to well-detailed guidelines that will be developed based on the proposed research will improve all new bridge construction and rehabilitation projects in the nation.