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Q & A

Question: What is the latest definition of high performance concrete for bridges?

Answer: Ever since the term "high performance concrete" (HPC) was introduced into bridge industry terminology, numerous definitions have been created and published.

SHRP Definition
The first definition was developed as part of the first Strategic Highway Research Program (SHRP). It defined HPC by the following three requirements:(1)
  1. Maximum water-cementitious materials ratio of 0.35
  2. Minimum durability factor of 80% as determined by ASTM C666 Method A
  3. Minimum compressive strengths of
    a. 3000 psi (21 MPa) within 4 hours after placement,
    b. 5000 psi (34 MPa) within 24 hours, or
    c. 10,000 psi (69 MPa) within 28 days.
FHWA Definition
In 1996, Goodspeed et al. published a proposed definition for HPC that the Federal Highway Administration (FHWA) developed for bridges.(2) The proposed definition consisted of four strength-related performance characteristics (compressive strength, modulus of elasticity, shrinkage, and creep) and four durability-related performance characteristics (freeze-thaw resistance, scaling resistance, abrasion resistance, and chloride penetration). For each characteristic, a standard test method was listed and various performance grades established. Consequently, the selection of performance characteristics and performance grades became a decision to be made by the owner for the intended application. For example, a precast, prestressed concrete bridge beam could be specified to have a high concrete compressive strength and normal chloride permeability whereas a bridge deck could have a low chloride permeability and normal concrete compressive strength. Both concretes would be HPC but with different requirements.

The intent of the FHWA definition was to stimulate the use of higher quality concrete in highway structures. Based on lessons learned from the FHWA implementation of HPC in bridges, Russell and Ozyildirim proposed that alkali-silica reactivity, sulfate resistance, and workability be added to the performance characteristics.(3) The last characteristic became important because of the introduction of self-consolidating concrete.

ACI Definition
Although not intended specifically for bridges, the American Concrete Institute (ACI) defines HPC as concrete meeting special combinations of performance and uniformity requirements that cannot always be achieved routinely using conventional constituents, and normal mixing, placing, and curing practices.(4) ACI has a separate definition for high strength concrete: concrete that has a specified compressive strength for design of 8000 psi (55 MPa) or greater.

One of the misconceptions that has developed over the years is that HPC is always high strength concrete. Whereas high strength concrete is generally considered as HPC, the reverse is not true. High performance concretes exist that are not high strength concretes but many concretes that are developed to be durable HPCs turn out to have a high compressive strength. A reduction in the water-cementitious materials ratio required to produce a low chloride penetration or high abrasion resistance results in a higher compressive strength even though the higher strength may not be desirable or necessary, such as in bridge decks.

Bridge Specifications
The American Association of State Highway and Transportation Officials (AASHTO) LRFD Bridge Construction Specifications includes two classes of HPC. Class P(HPC) is intended for use in prestressed concrete members with a specified concrete compressive strength greater than 6.0 ksi (41 MPa). Class A(HPC) is intended for use in cast-in-place construction with a specified concrete compressive strength less than or equal to 6.0 ksi (41 MPa) and where performance criteria in addition to concrete compressive strength are specified. Maximum water-cementitious material ratios of 0.40 and 0.45 are specified for Class P(HPC) and Class A(HPC) concretes, respectively. The Commentary to the AASHTO LRFD Bridge Design Specifications includes a Class P(HPC) concrete intended for use when concrete compressive strengths in excess of 4.0 ksi (28 MPa) are required. A maximum water-cement ratio of 0.49 is specified along with a minimum cement content of 564 lb/yd3 (335 kg/m3). The maximum water-cement ratio is reduced to 0.45 for concrete used in or over salt water.

The Answer
The ACI definition is a qualitative definition that has lasted 12 years and has accommodated new concretes such as self-consolidating concrete and ultra-high performance concrete without change. The FHWA definition is quantitative and needs to be updated with time as new products come along and the technology improves. It is, however, more practical and can be used in performance specifications for bridges, while the ACI definition cannot. The AASHTO Specifications provides a combination of performance and prescriptive criteria.

References
1. Zia, P., Leming, M. L., and Ahmad, S. H., High Performance Concrete, A State-of-the Art Report, Report No. SHRP-C/FR-91-103, Strategic Highway Research Program, National Research Council, Washington, DC, 1991.

2. Goodspeed, C. H., Vanikar, S., and Cook, R., "High Performance Concrete Defined for Highway Structures," Concrete International, Vol. 18, No. 2, February 1996, pp. 62-67.

3. Russell, H. G. and Ozyildirim, H. C., "Revising High-Performance Concrete Classifications," Concrete International, Vol. 28, No. 8, August 2006, pp. 43-49.

4. Russell, H. G., "ACI Defines High-Performance Concrete," Concrete International, Vol. 21, No. 2, February 1999, pp. 56-57.



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


HPC Bridge Views, Issue 68, July/Aug 2011