Question:
Can I use the LRFD Bridge Design Specifications for concrete strengths above 10 ksi (69 MPa)?
Answer:
The recently published Third Edition of the 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. Article 5.4.2.1 – Compressive Strength states that design concrete strengths above 10.0 ksi (69 MPa) 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. The wording to allow the use of higher strength concretes in specific articles was introduced so that the results of ongoing research on high strength concrete can be introduced as the research projects are completed and revisions to the Specifications are approved. There are currently three National Cooperative Highway Research Program projects underway to address design for shear; transfer, development, and splice lengths; and flexure and compression. It is anticipated that results from these projects will allow the respective provisions to be extended for design concrete compressive strengths of at least 15.0 ksi (103 MPa) and possibly 18.0 ksi (124 MPa).
In the meantime, the only article in the Third Edition that specifically references a concrete compressive strength above 10.0 ksi (69 MPa) is 3.5.1 – Dead Loads for the unit weight of concrete. This new revision requires that the unit weight of normal weight concrete be increased when the specified concrete strength used in design exceeds 5.0 ksi (35 MPa). The revision is applicable to concrete compressive strengths up to 15.0 ksi (103 MPa).
At the 2004 Annual Meeting of the AASHTO Subcommittee on Bridges and Structures, several revisions were approved to existing articles to allow their use for concrete compressive strengths up to 15.0 ksi (103 MPa). The articles are 5.4.2.3 – Shrinkage and Creep, 5.4.2.4 – Modulus of Elasticity, 5.4.2.6 – Modulus of Rupture, 5.7.1 – Modular Ratio, and 5.9.5.1 through 5.9.5.4 – Prestress Losses. In addition to allowing these articles to be used for higher compressive strengths, other revisions to the articles were made to make them more applicable to the higher strength concretes. These revisions, however, do not become the official specification articles until they are published by AASHTO.