Bond Behavior of Reinforcing Steel in Ultra-High Performance Concrete

CHAPTER 5. CONCLUSIONS

INTRODUCTION

The research discussed herein focused on assessing the bond strength of deformed reinforcing steel in UHPC. Deformed reinforcing steel, including ASTM A615 Grade 60 uncoated No. 5 bar and epoxy coated No. 5 and No.8 bars and ASTM A1035 Grade 120 No.4, No.5, and No.7 bars, were tested. The specific UHPC material used in this study had a steel fiber content of two percent by volume and an average compressive strength of 13.5 ksi (93 MPa) at one day, 17.0 ksi (117 MPa) at three days, 19.4 ksi (133 MPa) at seven days, and 21.3 ksi (147 MPa) at 14 days.

The main factors affecting bond performance, including the structural characteristics like the embedment length, concrete side cover, bar spacing, bar size, and bar type, and materials properties such as UHPC compressive strength and bar yield strength are investigated. The summary of the findings are presented in this chapter. Design details for using deformed reinforcing steel in UHPC are then recommended. At the end, the proposed future research on this topic is included.

CONCLUSIONS

The following conclusions are based on the research presented in this report for deformed reinforcing steel embedded in UHPC.

• Increasing the embedment length of the bar increases bond strength.
• The relationship between the bond strength and the bonded length for reinforcing bar embedded in UHPC is nearly linear, indicating that UHPC exhibits enhanced performance as compared to conventional high strength concrete.
• Bond strength increases as the side cover increases.
• Non-contact lap splice specimens, where the bar spacing is less than l_stan(θ)^†, exhibit higher bond strength than contact lap splice specimens; when the bar clear spacing is bigger than l_stan(θ), the bond strength decreases as compared to those having lesser spacing.
• The decrease in bond stress for contact lap splice specimens is probably due to decreased contact area between the reinforcing bar and UHPC materials. Tight spacing between bars limits the ability of the fiber reinforcement to locally enhance the mechanical resistance of the UHPC.
• When the bar clear spacing is bigger than l_stan(θ), the induced diagonal cracks from the pullout force will not intersect with the adjacent bar. The adjacent bar will not help stop the propagation of the diagonal cracks. The bond strength becomes a function of the mechanical properties of the UHPC.
• Models that use bar spacing and bar cover to predict bond stress may need to be reevaluated in consideration of the added crack propagation resistance provided by fiber reinforcement in UHPC.
• An increase on the compressive strength of the UHPC results in an increased bond strength.
• The effect of UHPC properties on bond strength cannot be effectively represented by the compressive strength f’_c, or the square root of its compressive strength f’_c^1/2. Other UHPC mechanical properties, particularly those relevant to the post-cracking tensile behavior of UHPC, may be more appropriate for evaluating the bond strength of reinforcing bar in UHPC.
• For bars with larger diameter, the bond strength decreases.
• Bars that yield before bond failure have less ultimate bond strength than high strength bars that do not yield before bond failure; the reduction in bond strength is amplified as the ultimate bond strength increases.
• The epoxy coated bar had lower bond strength than uncoated bar; the reduction was minimized when there is a sufficient embedment length that the bar yields before bond failure.

RECOMMENDED DESIGN

One of the main goals of the research is to develop design recommendations for reinforcing bar embedded in UHPC, thus providing guidance for designers using reinforced UHPC in innovative applications. This study focused on a widely available UHPC product containing 2% steel fiber (by volume). Reinforcing bar sizes ranging from No. 4 to No. 8 and bar type including A615 Grade 60 uncoated and epoxy coated bar and A1035 Grade 120 bar were included in the study.

Deformed reinforcing bar embedded in UHPC can attain the lesser of the bar yield strength or 75 ksi (517 MPa) at bond failure when the following conditions are met:

• Bar size from No. 4 to No. 8,
• Uncoated or epoxy coated bar,
• Minimum embedment length of 8d_b,
• Minimum side cover of 3d_b,
• Bar clear spacing between 2d_b and l_s, and
• Minimum UHPC compressive strength of 13.5 ksi (93 MPa).

For lap splice reinforcement configurations, a minimum lap splice length of 75 percent of the embedment length is suggested, which is the range into which most of tests in this study fell. Note that d_b is the bar diameter and l_s is the lap splice length.

For situations wherein the above conditions are met except that the minimum side cover is between 2d_b and 3d_b, the minimum embedment length should be increased to 10d_b.

Refinements of the recommended design can be made for specific applications. For example, if a larger side cover is provided or/and UHPC has gained higher compressive strength, an embedment length reduction may be possible; if a longer embedment length is provided, the side cover can be correspondingly reduced with caution. Figure 32, Figure 33, and Figure 34 provide supporting information.

FUTURE RESEARCH

The research in this study mainly focuses on one specific UHPC material. Future research in this topic area may also consider the following:

• Bond performance in other UHPC materials and other grout materials, such as traditional non-shrinkage grout and epoxy grout.
• Investigation of the mechanical properties that are predictive of the bond performance of UHPC materials.
• Development of a general expression to estimate the bond strength of deformed steel reinforcing bars in UHPC materials with different fiber content.

ACKNOWLEDGMENTS

The research which is the subject of this document was funded by the U.S. Federal Highway Administration. This support is gratefully acknowledged.

This research project could not have been completed were it not for the dedicated support of the federal and contract staff associated with the FHWA Structural Concrete Research Program. Recognition also goes to the technical staff who assisted with specimen fabrication and testing. PSI, Inc. provided laboratory support to FHWA under contract DTFH61-10-D-00017 through the duration of this research project.

^† l_stan(θ): refer to Figure 20

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