Paul Finnerty, MDSHA, Vicki Stewart, MDSHA, and Rodney Meyers, Master Builders , Inc.
In September 1996, Mr. Samuel Miller Jr., Deputy Chief Engineer of the Maryland State Highway’s Office of Materials and Technology authorized the formation of a high performance concrete (HPC) committee. The committee membership represented the State Highway’s Bridge and Materials Offices, the Federal Highway Administration, and members of the Maryland Ready Mixed Concrete Association. The committee’s objective was to develop a specification for high performance portland cement concrete to achieve a 75-year service life in Maryland State Highway Administration (MDSHA) bridge decks.
Implementation
The specification was implemented last year with an HPC bridge deck on MD Route 64 over the CSX railroad in Washington County. The superstructure has a span length of 100 ft (30.5 m) and consists of six precast, prestressed concrete AASHTO Type IV girders spaced at 7 ft 8 in. (2.34 m) centers supporting an 8-in. (203-mm) thick cast-in-place concrete deck. The specified concrete compressive strength of the girders was 7000 psi (48 MPa) at 28 days.
Deck Concrete Specifications
The deck concrete mix design used the following criteria:
• Maximum cement content of 550 lb/cu yd (326 kg/cu m) to reduce early age thermal stresses
• Maximum water-cementitious materials ratio of 0.45
• A 28-day specified compressive strength of 4200 psi (29 MPa), compared to the standard strength of 4500 psi (31 MPa), to provide ductile behavior and to reduce cracking
• Air content of 6.5 ± 1.5% to provide freezethaw resistance
• Pozzolans at 35 percent of the total cementitious materials to reduce chloride permeability and mitigate against alkali-silica reactivity
• Average charge passed per AASHTO 277 of 2000 coulombs or less with no individual value greater than 2500 coulombs
• Corrosion inhibitor at 2 gal/cu yd (10 L/cu m) to inhibit corrosion of the reinforcing steel
• Polypropylene fibers to provide resistance to plastic shrinkage cracking
• Maximum 28-day drying shrinkage of 400 microstrain to reduce drying shrinkage cracking
Life Expectancy
The use of HPC is expected to increase the time until corrosion initiation to 50 years calculated using Fick’s second law of diffusion. The proper use of epoxy-coated reinforcement should inhibit corrosion for another 25 years. Consequently, the first significant repair for the HPC bridge deck is not expected for at least 75 years.
Cost
Implementation of the specified quality control standards increased the in-place cost of the bridge deck concrete by approximately $50/cu yd ($65/cu m) for this particular project compared to a cost of $75/cu yd ($98/cu m) for conventional concrete. The normal range of cost increase is $40 to $80/cu yd ($52 to $105/cu m) and is largely dependent on the risk associated with higher quality standards and competitive market forces. Currently, the time to rehabilitate a bridge deck is less than 40 years. Therefore, the added cost of HPC is a sound investment for the anticipated service life of 75 years.