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Q & A

Question: Can I use the AASHTO LRFD Bridge Design Specifications for specified concrete compressive strengths above 10 ksi (69 MPa)?

Answer: The scope of the Fifth Edition of the American Association of State Highway and Transportation Officials (AASHTO) LRFD Bridge Design Specifications states that the provisions of Section 5: Concrete Structures are based on concrete strengths ranging from 2.4 to 10.0 ksi (17 to 69 MPa), except where higher strengths are allowed for normal weight concrete. Article 5.4.2.1—Compressive Strength of the specifications states that design concrete strengths above 10.0 ksi (69 MPa) for normal weight concrete shall be used only when allowed by specific articles or when physical tests are made to establish the relationships between the concrete strength and other properties. Appendix C5 of the specifications contains a table showing the articles for which strengths above 10.0 ksi (69 MPa) are currently permitted. These include Articles 5.4.2.3—Shrinkage and Creep; 5.4.2.4—Modulus of Elasticity; 5.4.2.6—Modulus of Rupture; and 5.9.5—Loss of Prestress.

Three National Cooperative Highway Research Program (NCHRP) projects have been completed to address other design provisions.(1-3)

NCHRP Report No. 579(1) addresses design for shear and confirms that the existing provisions of Article 5.8.3—Sectional Design Method are applicable for normal weight concrete with specified concrete compressive strengths up to 18.0 ksi (124 MPa).

NCHRP Report No. 595(2) addresses design for flexure and compression and recommends some fine tuning of the existing provisions to make them applicable for normal weight concrete with specified compressive strengths up to 18.0 ksi (124 MPa). The fine tuning includes modifications to Equation 5.4.2.3.2-5 for time-development factor; Equation 5.4.2.4-1 for modulus of elasticity; Article 5.4.2.6 for modulus of rupture; Article 5.7.2.2 for equivalent rectangular stress distribution; a new article 5.7.3.2.6 on nominal flexural resistance; and Equation 5.7.4.2-3 for minimum area of longitudinal reinforcement.

NCHRP Report No. 603(3) addresses transfer length and development length for prestressing strand and development length and splice length for non-prestressed deformed reinforcement. For prestressing strands, the research confirmed that existing provisions for transfer length and development length are applicable for normal weight concrete with specified compressive strengths up to 15.0 ksi (103 MPa) but overestimate the required lengths. Modifications to the existing provisions are proposed in the report to reflect shorter lengths as concrete strength increases.

For development length and splice length of non-prestressed reinforcement, the report recommends that the provisions be extended up to a specified concrete compressive strength of 15.0 ksi (103 MPa) using a format similar to the American Concrete Institute's (ACI) building code ACI 318-05.(4)

The three research projects provide revisions to allow more provisions to be extended to specified concrete compressive strengths above 10.0 ksi (69 MPa) for normal weight concrete. These revisions, however, do not become the official specification articles until approved by the AASHTO Highway Subcommittee on Bridges and Structures and published by AASHTO.

References
1. Hawkins, N. M. and Kuchma, D. A., "Application of LRFD Bridge Design Specifications to High-Strength Concrete: Shear Provisions," Transportation Research Board, NCHRP Report 579, Washington, DC, 2007, 197 pp.

2. Rizkalla, S., Mirmiran, A., Zia, P., Russell, H., and Mast, R., "Application of the LRFD Bridge Design Specifications to High-Strength Structural Concrete: Flexure and Compression Provisions," Transportation Research Board, NCHRP Report 595, Washington, DC, 2007, 28 pp.

3. Ramirez, J. A. and Russell, B. W., "Transfer, Development, and Splice Length for Strand/Reinforcement in High-Strength Concrete," Transportation Research Board, NCHRP Report 603, Washington, DC, 2008, 122 pp.

4. ACI Committee 318, "Building Code Requirements for Structural Concrete (ACI 318-05) and Commentary (ACI 318R-05)," American Concrete Institute, Farmington Hills, MI, 2005.



The answer to this question was provided by Henry G. Russell, Editor of HPC Bridge Views.


HPC Bridge Views, Issue 65, Jan/Feb 2011