Transfer, Development, and Splice Length for Strand/Reinforcement in High-Strength Concrete (2008) / Chapter Skim
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Pages 110-118
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From page 110... ...
The recommended code expression is founded on the same principles as prior editions of the AASHTO LRFD Bridge Design Specifications, i.e., the development length is the sum of a transfer length expression plus a flexural bond expression. At "normal" concrete strengths, the development length expression requires 60 strand diameters for transfer length and approximately 90 strand diameters for the flexural bond length, for a total of 150 strand diameters.
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From page 111... ...
The recommendation from this report is that the Standard Test Method for the Bond of Prestressing Strands be incorporated into the AASHTO LRFD Bridge Design Specifications in Section 5.4.4. The LRFD specifications text should state that the material supplier is required to provide certification that the bonding ability of the prestressing strand is acceptable for use in pretensioned and prestressed concrete applications and that the Standard Test Method for the Bond of Prestressing Strands "shall be permitted" to provide acceptability of the prestressing strand product.
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From page 112... ...
In other words, the performance of Strand HH would be minimally acceptable, and the value of the NASP Bond Test of 10,700 lb can be rounded to 10,500 lb for simplification. Further, the data support the overall conclusion that the development length performance improves for strand with improving NASP Bond Test values and therefore support the recommendation that the Standard Test Method for the Bond of Prestressing Strands be adopted into the AASHTO LRFD Bridge Design Specifications.
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From page 113... ...
The results from both transfer and development length testing support the conclusion that concrete strength is an important factor. These results are supported independently by the results from NASP Bond Tests in varying concrete strengths.
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From page 114... ...
Following is an outline of the rationale for developing the recommendation for development length: 1. The current AASHTO development length equation can be used to adequately predict required development lengths for "normal strength concrete" with release strengths in the range of 4 ksi to 6 ksi, provided that the strand itself is qualified by the Standard Test Method for the Bond of Prestressing Strands.
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From page 115... ...
Figure 3.49 shows the results of development length tests on Strand D Strand D demonstrated below average bond performance with a relatively low NASP Bond Test value of 6,890 lb.
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From page 116... ...
an upper limit of 100 psi on the term in the anchorage and development length provisions is imposed; in Section 5.4.2.1 of the Interim 2008 AASHTO LRFD Bridge Design Specifications, it is stated that design concrete strengths above 10 ksi shall be used only when allowed by specific articles or when physical tests are made to establish the relationships between concrete strength and other properties. The experimental program on mild reinforcement described in NCHRP Project 12-60 was designed to determine, in conjunction with the data already available in the literature, whether these limitations can be removed for concrete compressive strengths up to 15 ksi.
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From page 117... ...
The development length of the hooked bar was calculated as the product of a basic development length and appropriate modification factors. In the 1995 edition of the 318 Code, a factor of 1.2 was introduced in the hooked anchorage requirements (ACI 1995)
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From page 118... ...
The researchers note that additional testing of bottom cast uncoated splices is justified with higher strength concretes. 4.3.3.2 Anchorage in Tension of Uncoated and Coated Mild Reinforcement Using Standard Hooks Article 5.11.2.4 of the AASHTO LRFD Bridge Design Specifications was verified for high-strength concrete in the proposed experimental Work Plan for NCHRP Project 12-60 with the exception of the lightweight aggregate factor.
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