Kenneth F. Dunker, Iowa Department of Transportation*
In 1999, a committee from the Iowa Department of Transportation (DOT) and the Iowa FHWA office began exploring the use of high performance concrete (HPC) for bridge projects in the reconstruction of I-235—an urban interstate loop around Des Moines, IA.
The initial expectations for HPC were reduced permeability, increased durability, increased strength when needed, and reduced cracking. To achieve these expectations, the committee evaluated concrete mix proportions, materials selection, timing and duration of curing, winter placement practices, and placement size and sequence. The committee also identified specific performance criteria for the substructure components; precast, prestressed concrete beams; deck; and barrier rails.
In 2000, the Office of Materials began evaluating the standard Iowa DOT mix designs for cast-in-place concrete and precast, prestressed concrete. The standard mixes were improved by reducing the water-cementitious materials ratio, adding supplementary cementing materials, and improving aggregate gradation, while testing for strength, permeability, and cracking tendency.
An early bridge deck experiment off the I-235 corridor included the use of silica fume and a high-range water-reducing admixture (HRWR). Although trial mix testing gave favorable results, the on-site slump and air content were inconsistent, and pumping reduced the air content.
After this experience, the Office of Materials revised the HPC deck mix to eliminate silica fume and the HRWR, to make ground granulated blastfurnace slag (GGBFS) the primary supplementary cementing material, and to use a water-reducing and retarding admixture. By late 2002, the HPC mix design and construction specifications had matured, and test data had stabilized to the values shown in the table.
Curing of deck concrete takes place immediately after finishing, with longitudinal grooving for velvet ride and skid resistance delayed until the concrete has hardened. The curing specifications include prohibition of curing compound, two layers of pre-wetted burlap to be placed less than 10 minutes after final finishing, and continuous wet sprinkling for 7 days. Deck cracking has not been a problem, although the 42nd Street Bridge did develop a transverse crack above the central pier shortly after construction.
Use of GGBFS and Class C or F fly ash in the HPC mix for substructure components has reduced the heat of hydration and slowed strength gain. For hot weather concreting, the reduced heat has been an advantage, but it is a disadvantage for cold weather concreting.
The Iowa DOT found an opportunity in a major corridor project to adjust standard concrete mixes for improved performance. The agency worked with the local ready-mix concrete suppliers and precast, prestressing plants; developed changes; and tested trial mixes to meet the expectations for HPC. The final results are three durable concretes for all bridge components above the foundations.
*Contributors to the article were Ahmad Abu-Hawash, Todd Hanson, Kimball Olson, and Wayne Sunday.