Transfer, Development, and Splice Length for Strand/Reinforcement in High-Strength Concrete (2008) / Chapter Skim
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Pages 6-25
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From page 6... ...
Some of these variables are currently included in the AASHTO LRFD Bridge Design Specifications while some are not. In the area of transfer length and development length of prestressing strand, specifications do not account for variables such as concrete strength, strand size, "top bar" effects, epoxy coating, bond quality of individual strand samples, and structural behavior issues (e.g., the interaction of shear and bond)
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From page 7... ...
The experimental work supporting the current requirements in the AASHTO LRFD Bridge Design Specifications and ACI 318-05: Building Code Requirements for Structural Concrete and Commentary (2005) for development of standard hooks in tension was conducted using a test setup representing an exterior beam column joint.
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From page 8... ...
In this study, a comprehensive and critical literature review was undertaken to gather and synthesize existing data and information related to the transfer length and development length of prestressing strand with diameters up to 0.6 in., and development and splice length in tension and compression of individual bars, bundled bars, and welded wire reinforcement and development length of standard. The literature review centered on collecting information on testing protocols for determining surface bond characteristics of strand, performance of members containing transverse reinforcement, bond and transfer length, and tests addressing deformation capacity.
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From page 9... ...
The results of the blind trial testing indicated that the NASP Bond Test provided the best repeatability. Based on these results and on as yet unpublished results from NASP Round III testing, the NASP recommended the use of the NASP Bond Test as the standardized test to assess the bond characteristics of prestressing strands.
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From page 10... ...
1993) was one of the more recent works dedicated to investigating the effects of strand spacing on transfer and development lengths of pretensioned strands.
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From page 11... ...
In several of the projects, strand transfer length was measured, and development length tests were conducted to ensure adequate bonding properties from the pretensioned strands and to add to the body of knowledge regarding the transfer and development of pretensioned strands in high-strength concrete. Perhaps the first of these tests was performed in Texas by Gross and Burns (1995)
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From page 12... ...
Concrete strength at the time of development length testing was 11,000 psi. For embedment lengths in excess of 60 in., the strands demonstrated the ability to develop adequate tension force to support the flexural capacity of the beams.
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From page 13... ...
Data comparing transfer lengths to concrete strength at release (Barnes and Burns 2000)
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From page 14... ...
The other database consists of 33 specimens with uncoated bars terminated with standard hooks and 13 specimens with epoxy-coated bars, for a total of 46 specimens. The provisions for development length of reinforcement in Section 5 of the AASHTO LRFD Bridge Design Specifications are based on the provisions of ACI 318-89 (ACI 1989)
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From page 15... ...
The experimental work conducted in the mild steel phase of NCHRP Project 12-60 was focused on filling the gaps identified in order to extend the applicability of the present AASHTO LRFD Bridge Design Specifications to normalweight concrete with compressive strengths up to 15 ksi. The 639-specimen database is shown in Figures 2.7 through 2.10 by plotting the bond strength, utest, versus the concrete compressive strength, f ′c.
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From page 16... ...
Bond stress at failure (utest) versus the concrete compressive strength ( )
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From page 17... ...
reported on the results of 24 tests to evaluate the anchorage performance of epoxy-coated hooked bars. Based on these results, a 20-percent increase on the basic development length was recommended for epoxy-coated hooked bars.
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From page 18... ...
2.3 Identification of Issues and Needs The work described in the previous section was used to assemble a comprehensive list of issues pertaining to transfer length, development length, and splice length of strand/ reinforcement to normal-weight concrete with compressive strengths in excess of 10 ksi and up to 15 ksi. In this section, a discussion of the main issues related to bond performance of reinforcement is presented, and gaps found in the existing database are addressed.
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From page 19... ...
The mechanical interlocking bond stresses are derived by the helical windings of the 7-wire prestressing strand, which act similarly to the mechanical deformations found on rolled, mild reinforcement. Development length testing of pretensioned beams indicates that splitting occurs less frequently than in conventionally reinforced beams (although splitting cracks have been observed in pretensioned bond failures)
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From page 20... ...
This observation supports the need to modify the current AASHTO LRFD Bridge Design Specifications for bond and development length of mild reinforcement to incorporate the effects of cover, bar spacing, and transverse reinforcement. 2.3.2 Material Properties 2.3.2.1 Reinforcement Properties For a given bonded length required to achieve a given steel stress level, reinforcement of different areas will achieve different levels of force at the onset of splitting failure, with the larger area reinforcement achieving higher forces.
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From page 21... ...
have emphasized the paucity of experimental data on the bond strength of reinforced concrete elements made with lightweight aggregate concrete. The AASHTO LRFD Bridge Design Specifications includes a factor of 1.3 for development length to reflect the lower tensile strength of lightweight aggregate concrete and allows that factor to be taken as 0.22 if the average splitting strength, fct, of the lightweight aggregate concrete is specified.
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From page 22... ...
Therefore, the testing program for NCHRP Project 12-60 employed the NASP Bond Test as the standard test to assess the relative "bond-ability" of prestressing strands. Previous experience with research on strand bond demonstrates the importance of quantifying the strand bonding properties prior to or concurrent with testing programs for transfer and development length of strands.
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From page 23... ...
. Splice tests have been realistic simulations of real conditions in structures, but development length tests have been conducted largely using pull-out tests in which splitting failures are purposely avoided.
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From page 24... ...
Review of experimental data on anchorage of bars terminated using standard hooks indicates the need for additional testing to extend the current AASHTO LRFD specifications to concrete strengths up to 15 ksi (see Section 2.2.2)
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From page 25... ...
More information on the local behavior of the splice or development length can be obtained by placing strain gages on the bar itself. The strain gage instrumentation provides information on the changes in bar force along its length.
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