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Chapter 1. Introduction - HIGH-PERFORMANCE CONCRETE BRIDGES

HIGH-PERFORMANCE CONCRETE BRIDGES

In 1993, the Federal Highway Administration (FHWA) initiated a national program to implement the use of high-performance concrete (HPC) in bridges. The program included the construction of demonstration bridges in each of the FHWA regions and the dissemination of the technology and results at showcase workshops. Initially, a total of 18 bridges in 13 States were included in the national program. In addition, other States have implemented the use of HPC in various bridge elements.

The bridges were located in different climatic regions of the United States and used different types of superstructures. The bridges demonstrated practical applications of HPC. In addition, construction of these bridges provided opportunities to learn more about the placement and actual behavior of HPC in bridges. Consequently, many of the bridges were instrumented to monitor their short- and long-term performances. Additionally, concrete material properties were measured for most of the bridges.

SPECIFICATIONS OF THE AMERICAN ASSOCIATION OF STATE HIGHWAY AND TRANSPORTATION OFFICIALS (AASHTO)

The AASHTO Standard Specifications for Transportation Materials and Methods of Sampling and Testing consists of specifications and test methods for materials commonly used in the construction of highway facilities.⁽¹⁾ Part I contains specifications for materials. The relevant specifications for this project are indexed under the three general subject areas of aggregates; concrete, curing materials, and admixtures; and hydraulic cement. Many of the specifications are similar to the equivalent specification published by the American Society for Testing and Materials (ASTM). However, equivalent documents are frequently not identical. When the AASHTO document does not contain a particular specification, bridge owners will reference the ASTM specification. Part II of the AASHTO Standard Specifications contains the test methods. The relevant specifications for this project are indexed under the three general subject areas of aggregates; concrete, curing materials, and admixtures; and hydraulic cement. As with the specifications for materials, many of the test methods are similar, but not identical, to their equivalent ASTM method.

The AASHTO Standard Specifications, like the ASTM specifications are generally based on conventional concrete and have proved to be reliable over the years. However, with the rapidly changing pace of technology, it is difficult for consensus standards to maintain pace with the technology and new information. This is particularly true with the many aspects of HPC. HPC has unique characteristics such as high strength, improved workability, and low permeability. These require closer attention to quality control and quality assurance. HPC is not as forgiving as conventional concrete. It is engineered concrete and must be produced with great care and attention. Performance specifications are highly desirable for HPC. Yet, many specifications still remain prescriptive in their approach and need to be revised to make them more appropriate for use with HPC.

The first edition of the AASHTO Standard Specifications for Highway Bridges was published in 1931. Since that time, the AASHTO Standard Specifications have been continuously updated to the 16th edition in 1996, plus interim revisions published in 1997, 1998, 1999, and 2000. See references 2 through 6, respectively. Generally, these updates reflect changes in the state-of-the-art and state-of-the-practice in bridge engineering. However, article 9.15 states that the design of precast, prestressed members ordinarily shall be based on the compressive strength of 34 megapascals (MPa) (5,000pounds force per square inch (psi)). "An increase to 6000 psi is permissible where, in the Engineer's judgment, it is reasonable to expect that this strength will be obtained consistently. Still higher concrete strengths may be considered on an individual area basis. In such cases, the Engineer shall satisfy himself that the controls over materials and fabrication procedures will provide the required strengths." Article 9.15, which affects all precast, prestressed concrete members, appears to be out of date in light of the consistently higher strengths being achieved in practice and represents a barrier to the use of higher strength concretes.

The design philosophy of the AASHTO Standard Specifications is being slowly replaced by the newer design philosophy of load and resistance factor design (LRFD) as published in the AASHTO LRFD Bridge Design Specifications. Since many States are still using the AASHTO Standard Specifications, it is important to address advances in technology that may impact the provisions of the AASHTO Standard Specifications. This includes the use of higher strength concretes.

The first edition of the AASHTO LRFD Bridge Design Specifications was published in 1994. The second edition was published in 1998, with interim revisions in 1999, 2000, and 2001. See references 7 through 10, respectively. The AASHTO LRFD Specifications introduced the design philosophy of LRFD for all materials. In this approach, variability in the behavior of structural elements is taken into account in an explicit manner. The AASHTO LRFD Specifications rely on the use of statistical methods, but set forth the results in a readily usable manner. Design of concrete structures in the AASHTO LRFD Specifications is addressed in one section that contains all provisions for design of reinforced, prestressed, and partially prestressed concrete. This is in contrast to the AASHTO Standard Specifications, which has reinforced concrete and prestressed concrete in separate sections.

Article 5.4.2.1 of the AASHTO LRFD Bridge Design Specifications limits the applicability of the specifications to a maximum concrete compressive strength of 69 MPa (10,000 psi ) unless the physical tests are made to establish the relationship between concrete strength and other properties. Hence, the AASHTO LRFD Specifications have extended the implied limit from 41MPa (6000psi) in the AASHTO Standard Specifications to 69 MPa (10,000 psi). With the greater use of higher strength concrete and its economical and technical advantages, consideration needs to be given to raising the limit above 69 MPa (10,000 psi).

The first edition of the AASHTO LRFD Bridge Construction Specifications was published in 1998.⁽¹¹⁾ Interim Editions were published in 1999, 2000, and 2001. See references 12 through 14, respectively. Section 8 of the specifications deals with concrete structures and is essentially the same as the AASHTO Standard Specifications for Highway Bridges, division II, section 8, "Concrete Structures."

OBJECTIVES

Based on the above introduction, the following objectives for the project were established:

1. Collect and compile information on concrete mixtures, concrete properties, research projects, girder fabrication, bridge construction, live-load tests, and specifications from each of the joint State-FHWA HPC bridge projects and other HPC bridge projects underway or completed. This includes all information related to material properties and structural performance.

2. Analyze and evaluate the compiled information in comparison with existing AASHTO specifications and guidelines for materials, testing, and structural design. For topics where the AASHTO specifications and guidelines need to be revised or do not exist, evaluate the compiled information in comparison with specifications and guidelines from ASTM; the American Concrete Institute (ACI); the Expanded Shale, Clay and Slate Institute (ESCSI); the Precast/Prestressed Concrete Institute (PCI^®); the Post-Tensioning Institute (PTI); the Concrete Reinforcing Steel Institute (CRSI); the American Segmental Bridge Institute (ASBI); the Ontario Highway Bridge Design Code (OHBDC); and the Fédération Internationale du Béton (fib).

3. Where sufficient research results exist, recommend equations, specifications with commentary, and guidelines for material and structural properties. These are for use by the AASHTO Highway Subcommittee on Bridges and Structures in the AASHTO Standard Specifications for Highway Bridges, the AASHTO LRFD Bridge Design Specifications, and the AASHTO LRFD Bridge Construction Specifications, and for use by the AASHTO Highway Subcommittee on Materials in the AASHTO Standard Specifications for Transportation Materials and Methods of Sampling and Testing.

4. Where insufficient research results exist, produce specific recommendations for needed research.

The above objectives were accomplished through the following tasks:

• Task A: Compilation of Information and Specifications from HPC Bridges.
• Task B: Compilation and Review of AASHTO Specifications and Other Publications.
• Task C: Preparation of an Interim Report.
• Task D: Development of Recommended Equations, Specifications, and Guidelines.
• Task E: Development of Specific Recommendations for Needed Research.
• Task F: Preparation of Final Report and Technical Summary.

This final report constitutes the end product from task F. A summary of the results from task A is given in chapter2. The detailed compilation from task A is included in a separate CD-ROM. The review of the AASHTO specifications from task B is given in chapter 3. The interim report from task C was completed in May 2001. The proposed revisions to the AASHTO specifications developed in task D are included in appendixes A through E. The proposed changes have been developed for direct use by the AASHTO highway subcommittees. The research problem statements from task E are included in appendix F. The research problem statements have been prepared using the format of the National Cooperative Highway Research Program (NCHRP) so that they may be submitted directly to the appropriate subcommittees of the Transportation Research Board (TRB) and AASHTO.

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FHWA-HRT-05-056