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Project 2013-03

Project Title:

Use of Headed Reinforcing Bars to Develop High-Strength Reinforcing Steel

Institution:

Kansas State University
Lawrence, Kansas

Researchers:

Dr. David Darwin, PE
Dr. Adolfo Matamoros

Award Period:

2012-2013 school year

Status:

Ongoing

Description:

A headed reinforcing bar consists of a conventional reinforcing bar with a head attached to one or both ends. The head, which must have a bearing area equal to at least four times the bar area to meet the requirements of the ACI Building Code, provides mechanical anchorage that allows the bar to be developed (anchored) using a significantly shorter length than straight reinforcement and, in many cases, in a shorter length than a hook. The prime advantage is reduced congestion and easier concrete placement. The force in headed bars is provided by a combination of bearing force beneath the head and bond force developed along the length of the bar.

Headed reinforcement is permitted under the provisions of ACI 318. Those provisions, however, limit the value of yield strength that may be used with headed bars to 60 ksi. Further, concrete strengths used in design are limited to 6 ksi. These limitations are based on the lack of data for higher-strength materials. Thus, heads cannot be used to anchor Grades 75 or 80 headed bars. Headed bars can be used in concretes with compressive strengths above 6 ksi, but any advantage provided by the higher strength concrete cannot be included in the design. The formulation of design criteria for high-strength headed bars will require tests that develop bars to at least 80 ksi. Those tests will need to address the size and geometry of the head, spacing and clustering effects, and test configurations to cover the range of potential failure modes of headed bars in reinforced concrete members. Confinement provided by transverse reinforcement has been shown to have little if any effect on the capacity of headed bars loaded to stresses up to 60 ksi and subjected to static loading, but it has improved the performance of some configurations of headed bars under cyclic loading and should be considered.

Objectives – The purposed study will develop basic data on the anchorage of high-strength headed reinforcing bars and use those results to formulate design criteria for reinforced concrete structures. The study will include configurations and material properties that, while being necessary or advantageous for construction, are not addressed by the current design codes due to a lack of experimental results. Tests will include high-strength reinforcing bars, high-strength concrete, a broader range of head geometries than tested to date, group effects, and the effects of staggering the heads.