James M. Parry, Wisconsin Department of Transportation
The first development by the Wisconsin Department of Transportation (WisDOT) toward what could be considered high performance concrete (HPC) for bridge decks came in the mid to late 1990s with the development of the first Quality Management Program (QMP) specifications. It was not called HPC at that time, but certainly led toward improvement in concrete properties. The principal motivation for these changes was to improve concrete quality and durability and decrease bridge deck cracking. The air content used in all QMP and HPC specifications described below has been 6 ± 1.5%. Components of the early QMP specifications and the associated benefits were as follows:
- Introduced percent within limits (PWL) requirements for compressive strength with incentive/disincentive payments. This solved an age-old problem of excessive water addition (retempering) of concrete mixes in the field. It also rewarded uniformity of production for producers with good quality control. Reduced water content also led to reduced shrinkage.
- Reduced the minimum cementitious materials content from 610 to 565 lb/yd3 (362 to 335 kg/m3). This reduced shrinkage.
- Increased maximum nominal size of aggregate from ¾ to 1½ in. (19 to 38 mm). This also reduced shrinkage.
- Required 7-day continuous wet cure with burlap cover. This decreased concrete permeability and cracking potential.
First Generation of HPC Specifications
In 1998 and 1999, WisDOT programmed the use of an HPC specification as a pilot program on 22 bridge decks across the state. WisDOT followed a traditional approach to HPC that equated high performance concrete with high strength concrete and low water-cementitious materials ratio (w/cm), with the following mix requirements supplementing the QMP mix requirements listed previously.
- 5000 psi (34 MPa) compressive strength requirement at 28 days
- High-range water-reducing admixture required
- Maximum w/cm of 0.40
This approach resulted in mixes that generated high temperatures, very high early strength gain, and an extremely large amount of deck cracking on several structures. This cracking problem was judged to be of sufficient severity that this specification was removed from several of the projects scheduled for the second construction season.
Second Generation of HPC Specifications
The Marquette Interchange in downtown Milwaukee, constructed from 2004 to 2008, was the first of several megaprojects scheduled for reconstruction of aging interstate highways in southeastern Wisconsin. The Federal Highway Administration was pushing for a 75-year service life for the structures; so the use of HPC was essential in the harsh Wisconsin environmental conditions. A holistic approach was taken to address all properties of the HPC. This included the following:
- Aggregate quality specifications tightened to allow only the best of locally available materials
- Crushed limestone with 100% fractured faces and low coefficient of thermal expansion
- Mandatory use of supplementary cementitious materials (SCM’s)
- Cementitious materials content between 565 and 660 lbs/yd3 (335 and 392 kg/m3)
- Central mixed concrete
- 5000 psi (34 MPa) minimum compressive strength at 28 days
- Rapid chloride permeability (RCP) of 2000 coulombs maximum using standard 28-day curing
- 80°F (27°C) maximum concrete temperature at placement
- 0.15 lb/ft2/hr (0.73 kg/m2/hr) maximum evaporation rate during deck placements
- 10-day continuous wet curing using soaker hoses and two layers of burlap
- Wet burlap placed within 10 minutes of strike-off by finishing machine
- Longitudinal grooving texture applied later to the hardened concrete
- Silane sealer applied to the final textured deck
Third Generation of HPC Specifications
The 5000 psi (34 MPa) concrete compressive strength on the Marquette Interchange was required for structural reasons and was not considered to be optimum for minimization of deck cracking. Many decks turned out well on that project, but a few had excessive cracking. It was decided at the conclusion of the project to use a 4000 psi (28 MPa) compressive strength requirement for future HPC bridge decks. Contractors also commented that it was difficult to meet the 28-day RCP requirement using fly ash as the locally preferred SCM. The fly ash did not have sufficient time to impart benefits to the concrete in 28 days at the standard curing temperature. However, it was desired not to extend the curing period beyond 28 days because the test was being used for acceptance and monitoring during construction of projects with very tight schedules. It was decided to adopt the accelerated curing method developed by the Virginia Transportation Research Council* (VTRC), in which the specimens are cured at 73°F (23°C) for the first 7 days and 100°F (38°C) for the last 21 days. This method is reported to give an equivalent test result to a 90-day standard curing period.(1)
In 2009, construction began on two additional megaproject corridors. These were the north-south stretch of I-94 between Milwaukee and the Illinois state line, and the U.S. Highway 41 corridor south of Green Bay. This construction is still ongoing. The HPC for these corridors has utilized the Marquette HPC specifications with the following modifications:
- Maximum cementitious materials content of 610 lb/yd3 (362 kg/m3)
- 4000 psi (38 MPa) minimum compressive strength at 28 days
- RCP of 1500 coulombs maximum at 28 days using VTRC accelerated curing procedure
- 14-day continuous wet curing
WisDOT will continue to move forward with refinements to our HPC specifications.
Reference
1. Ozyildirim, C., “Permeability Specifications for High-Performance Concrete Decks,” Transportation Research Record No. 1610, Concrete in Construction, Transportation Research Board, Washington, DC, 1998, pp. 1-5.
Further Information
For further information, please contact the author at [email protected] or (608) 246-7939.
* VTRC is now the Virginia Center for Transportation Innovation and Research (VCTIR).