Sergio Rodriguez, Alabama Department of Transportation
The Alabama Department of Transportation’s (ALDOT) first high performance concrete (HPC) bridge was let in April 1998. There was a lot of excitement in Alabama because we were going to play in the majors. We wanted to win the World Series title for the best HPC bridge. Two years before the letting, we started revising the plan of those who played the game before us. In Alabama, everybody played the game; there was no bench. ALDOT, FHWA, academia, contractors, and suppliers all played an important role in drafting what later became our first specification for HPC bridges.
For conventional concrete, ALDOT provides a master proportions table for all mixes used in a project. The responsibility for the HPC mix design fell to the contractor. The only parameters specified were the materials, water-cementitious materials ratio, temperature of the concrete, air content, slump, and compressive strength. The contractor was permitted to use Class C fly ash at 20 to 30 percent, Class F fly ash at 15 to 25 percent, or microsilica at 7 to 15 percent by weight of the total cementitious materials. A combination of one of the fly ashes with microsilica was allowed provided that the microsilica percentage was considered as additive. Crushed limestone was specified for the coarse aggregate and natural sand for the fine aggregate. Chemical admixtures were allowed based on the manufacturer’s recommended dosages.
The maximum specified water-cementitious materials ratios were 0.40 and 0.32 for cast-in-place concrete and precast, prestressed concrete, respectively. The minimum temperature of the concrete at time of placement was set at 50°F (10°C) and the maximum at 95°F (35°C). The range of air content allowed was from 3.5 to 6 percent. There were three criteria for slump of the concrete: for non-prestressed concrete, the maximum slump was set at 5 in. (125 mm) for superstructure concrete and 8 in. (200 mm) for substructure concrete. For the 54-in. (1.37-m) deep bulbtees, the maximum slump was set at 9 in. (230 mm). The compressive strength was specified at 6000 psi (41 MPa) at 28 days for non-prestressed concrete. For precast, prestressed concrete, compressive strengths of 10,000 psi (69 MPa) at 28 days and 8,000 psi (55 MPa) at release of the strands were specified.
Additionally, for acceptance of the concrete mix, test pours were required to provide evidence of the contractor’s ability to mix, transport, place, consolidate, finish, and cure the concrete properly. For the bridge deck, the test pour required a minimum slab area of 400 sq ft (37 sq m) with a minimum thickness of 4 in. (100 mm). The test pour for the precast, prestressed concrete girders consisted of a full crosssection of a girder at least 10-ft (3.1-m) long and using the planned casting bed.
We made it through spring training. Then, we had to prepare ourselves for those unexpected injuries during the season. To prevent plastic shrinkage cracking of the bridge deck concrete, the maximum evaporation rate was specified not to exceed 0.1 lb/sq ft/hr (0.5 kg/sq m/hr) as determined by an “Evaporation Rate of Surface Moisture” chart. Moist curing was the only curing method allowed for bridge decks. Match curing of the quality control cylinders was required for the precast, prestressed concrete bridge members. The contractor was responsible for providing a protected environment for field curing of concrete cylinders. This protected environment consisted of a curing box equipped with heating and cooling capabilities, and high/low temperature readout. Concrete testing was required on the first load of concrete delivered every day and then on every 50 cu yd (38 cu m). Concrete samples were obtained for testing of slump, air content, temperature, unit weight, and compressive strength.
As simple as it sounds, that was our game plan. We were ready to play ball! We had a few bad innings, but also hit a few home runs. We found out that playing in the majors is no different than playing minor league. The stakes may be higher but the game is played the same way. Yes, we won the World Series title for the best HPC bridge, but so does everybody else who plays. We are all winners in this game. The experience is great and the challenge so intense that our learning is enriched and we can hardly wait for the next game.
I don’t want to end this article without some words of advice for those who are about to bat for HPC. Make sure that everybody is involved (no bench allowed), keep your specifications as simple as possible, specify only what is needed but make sure your goals are attainable. Then, don’t settle for less. Remember that the ultimate goal is to produce a structure that is safe for the public, durable, and as economical as possible.
Further Information
A complete copy of ALDOT’s game plan is available from the author at 334-206-2410 or at [email protected].