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SUMMARY
Design of Concrete Structures Using
High-Strength Steel Reinforcement
Recent revisions to §9.2 of the AASHTO LRFD Bridge Construction Specifications and to
AASHTO MP 18 Standard Specification for Uncoated, Corrosion-Resistant, Deformed and
Plain Alloy, Billet-Steel Bars for Concrete Reinforcement and Dowels permit the specification
of ASTM A1035 reinforcing steel. A1035 reinforcing bars are low carbon, chromium steel
bars characterized by a high tensile strength (minimum yield strength of 100 or 120 ksi
determined using the 0.2% offset method) and a stress-strain relationship having no yield
plateau. Because of their high chromium content, A1035 bars are reported to have superior
corrosion resistance when compared to conventional reinforcing steel grades. For this rea-
son, designers have specified A1035 as a direct, one-to-one, replacement for conventional
reinforcing steel as an alternative to stainless steel or epoxy-coated bars. The specifications,
however, limit the yield strength of reinforcing steel to 75 ksi for most applications. There-
fore, although A1035 steel is being specified for its corrosion resistance, the benefits of its
higher yield strength cannot be utilized.
A number of types and grades of steel reinforcement with yield strengths exceeding 80 ksi
are commercially available in the United States. If allowed, using steel with this higher capac-
ity could provide various benefits to the concrete construction industry by reducing mem-
ber cross sections and reinforcement quantities, leading to savings in material, shipping, and
placement costs. Reducing reinforcement quantities also would reduce congestion problems
leading to better quality of construction. Finally, coupling high-strength steel reinforcement
with high-performance concrete should result in a much more efficient use of both materials.
This report provides an evaluation of existing AASHTO LRFD Bridge Design Specifications
relevant to the use of high-strength reinforcing steel and other grades of reinforcing steel
having no discernable yield plateau. The report identifies aspects of reinforced-concrete
design and of the specifications that may be affected by the use of high-strength reinforcing
steel. An integrated experimental and analytical program intended to develop the data
required to permit the integration of high-strength reinforcement into the LRFD specifica-
tion is presented. In addition, a number of "proof tests" intended to validate existing spec-
ifications provisions applied to higher strength reinforcing steel are presented. The focus of
the experimental phase of this study is the use of ASTM A1035 reinforcing steel since it cap-
tures both behavioral aspects of interest (i.e., it has a very high strength and has no discern-
able yield plateau). In addition, this study specifically considers the use of higher strength
concrete. The experimental and analytical studies include concrete having compressive
strengths of 5, 10, and 15 ksi.
The primary deliverable of NCHRP Project 12-77 was to provide recommendations for
changes to the specifications necessary for the use of high-strength reinforcing steel. This
report provides the background and engineering basis, in the form of experimental and ana-
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lytical studies, supporting these recommendations. Although summarized in Chapter 3, the
recommendations forwarded to the Project Panel, and eventually to the AASHTO Techni-
cal Committee for Concrete Design (T-10), are not presented in this document. In all cases,
recommended language was proposed that specifically permits the use of high-strength rein-
forcing steel with specified yield strengths not greater than 100 ksi when the specific article
permits it. This methodology is consistent with the manner by which the specifications handle
high-strength concrete, allowing its use only when a specific article permits it. Specifications
Sections 3, 5, and 9 were identified as having articles potentially requiring changes. Although
considered in its entirety, no changes were identified in the AASHTO LRFD Bridge Construc-
tion Specifications. The 2009 revisions to §9.2 of the Construction Specifications permit the
use of A1035 reinforcing steel.
Yield Strength
A critical objective of the present work was to identify an appropriate steel strength and/or
behavior model to adequately capture the behavior of high-strength reinforcing steel while
respecting the tenets of design and the needs of the designer. A value of yield strength, fy, not
exceeding 100 ksi was found to be permissible without requiring significant changes to the
specifications.
Flexure
The current specifications design methodology for flexure, that is, a simple plane sections
analysis using stress block factors to model concrete behavior and an elastic-perfectly plastic
steel behavior (having E s = 29,000 ksi), is shown to be appropriate for values of fy 100 ksi.
To ensure ductility, steel strains corresponding to tension- and compression-controlled lim-
its (defined in §5.7.2.1 of the specifications) are recommended as follows:
Current §5.7.2.1; No Recommended Limits for
Recommended Changes High-Strength Reinforcement
fy 60 ksi fy = 100 ksi
Tension-Controlled Section t 0.005 t 0.008
Compression-Controlled Section t 0.002 t 0.004
Values may be interpolated between limits
These strain limits were developed through a rigorous analytical study of 286 cases, which
included seven different grades of reinforcing steel, three concrete strengths, and multiple
section geometries. Six large-scale beam specimens reinforced with A1035 reinforcing steel
confirmed the appropriateness of the proposed tension- and compression-controlled lim-
its. All beam specimens met and exceeded their designed-for strength and ductility criteria
and exhibited predictable behavior and performance similar to beams having conventional
reinforcing steel.
Fatigue
Two large-scale proof tests conducted as part of this study and a review of available pub-
lished data demonstrate that presently accepted values for the fatigue or "endurance" limit
for reinforcing steel are applicable--and likely conservative--when applied to higher
strength bars. Additionally, it is shown that fatigue considerations will rarely affect the design
of typical reinforced-concrete members having fy 100 ksi.
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Shear
Five large-scale reinforced-concrete beams and four AASHTO Type I prestressed girders
were tested to evaluate the performance of high-strength A1035 steel as shear reinforcement
in comparison to that of the commonly used A615 steel. Test specimens were designed using
the specifications approach of summing concrete and steel contributions to shear resistance
(i.e., Vc + Vs). All beams exhibited good performance with little difference noted between
the behavior of spans reinforced with A1035 or A615 transverse steel. The use of current
specifications procedures for calculating shear capacity were found to be acceptable for val-
ues of shear reinforcement yield fy 100 ksi.
Shear Friction
A series of eight push-off (direct shear) proof tests of "cold construction joint" interfaces rein-
forced with either A1035 or A615 bars demonstrated that current specifications requirements
for such joints are adequate. Significantly, the restriction that fy be limited to 60 ksi when calcu-
lating shear friction capacity must be maintained regardless of the reinforcing steel used. This
limit is, in fact, calibrated to limit strain (and therefore interface crack opening) to ensure ade-
quate aggregate interlock capacity across the interface and is, hence, a function of steel modu-
lus rather than strength. It is noted that steel modulus does not vary with reinforcing bar grade.
Compression
Analytical parametric studies were performed to examine behavior of columns reinforced
with A1035 longitudinal and transverse reinforcement. Results indicate the current specifi-
cations requirements for both longitudinal and transverse reinforcement design in compres-
sion members are applicable for fy 100 ksi.
Bond and Development
The applicability of current specifications requirements for straight bar and hooked bar
development lengths was confirmed through a series of spliced-bar beam tests and pull-out
tests, respectively. "Proof test" spliced-bar beam specimens, having development lengths
that were shorter than those required by the present specifications equations (with all appro-
priate reduction factors applied), were tested. All developed bar stresses exceeding fy and
approaching the ultimate bar capacity, fu, prior to the splice slipping, and in one case bar
fracture. Tests of hooked bar anchorage resulted in bar rupture outside of the anchorage
region with very little slip, clearly indicating the efficacy of the hooked bar development
requirements in the specifications. Significantly, it is recommended that development,
splice, and anchorage regions be provided with cover and confining reinforcement--based
on current design requirements--when high-strength bars are used. Existing equations for
development where no confinement is present are demonstrated to be unconservative. The
presence of confining reinforcement effectively mitigates potential splitting failures and
results in suitably conservative development, splice, and anchorage capacities.
Serviceability--Deflections and Crack Widths
A fundamental issue in using A1035, or any other high-strength reinforcing steel, is that
the stress at service load (fs; assumed to be on the order of 0.6fy) is expected to be greater than
when conventional Grade 60 steel is used. Consequently, the service-load reinforcing strains
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(i.e., s = fs/Es) are greater than those for conventional Grade 60 steel. The large strains affect
deflection and crack widths at service loads. Based on the results of the flexural tests con-
ducted in this study, deflections and crack widths at service load levels were evaluated. Both
metrics of serviceability were found to be within presently accepted limits and were pre-
dictable using current specifications provisions. A limitation on service-level stresses of fs
60 ksi is recommended; this is consistent with the recommendation that fy 100 ksi.
Summary
The extension of present AASHTO LRFD Bridge Design Specifications to permit reinforc-
ing bar yield strengths not exceeding 100 ksi was investigated and, for the most part, vali-
dated for concrete strengths up to 10 ksi, and, in some instances, 15 ksi. This study did not
address seismic applications, and no such increase in permitted yield strength is addressed
for Seismic Zones 2 through 4. Other limitations to the use of high-strength reinforcing steel
also are identified. Recommended specifications language was proposed to the Project Panel
that specifically permits the use of high-strength reinforcing steel with specified yield
strengths not greater than 100 ksi when the specific article permits it. This report provides
the necessary background and engineering basis, in the form of experimental and analytical
studies, supporting these recommendations.
