David Hand, Wando Concrete, LLC
Mix design, batching, transporting, and testing of the concrete for the Cooper River Bridge project posed some challenges from a producer’s standpoint. The first challenge was to develop cost-effective mixes that met the required specifications and could be modified to meet the non-specified challenges. As concrete supplier, our team had a real place at the partnering table with the contractor, designer, and the South Carolina Department of Transportation. The real story in the success of this project was everyone’s commitment to construct the very best bridge possible and the designbuild process allowed us the flexibility to achieve that goal.
The project required the production of over 320,000 cu yd (245,000 cu m) of concrete involving high strength concrete, high early strength concrete, low permeability concrete, extended set times, extended transportation times, and control of initial concrete temperatures.
Slump life and initial set were critical in the construction of the drilled shafts. A 6- to 9-in. (105- to 225-mm) slump and initial set durations in excess of 11 hours were accomplished with the use of hydration stabilizing admixtures. In effect, the hydration process was stopped for various durations with the use of these chemicals. This method was preferred over using retarders, which can become unstable at higher dosage rates. This also worked well for the bridge deck concrete, which was placed transversely, because differential displacement from loading the deck beams would cause cracking in deck mixes with normal setting times. The potential for surface drying and plastic shrinkage cracking due to extended set times of about 4 hours for the deck concrete was a significant potential problem. Specific combinations of admixtures were used to provide the desired set time while achieving the workability, paste, and bleeding characteristics consistent with normal bridge deck concrete.
Permeability requirements led to the use of cementitious materials containing 400 lb/cu yd (237 kg/cu m) of cement and 300 lb/cu yd (178 kg/cu m) of fly ash for the substructures. Cost and material supply issues precluded the use of slag or silica fume. Temperature control and finishability were added benefits of the high amount of fly ash.
Strength requirements were met using a high cementitious materials content and a low water-cementitious materials ratios. Design strengths of 7000 and 8000 psi (48 and 55 MPa) were often accompanied by additional requirements. For example, portions of the cable-stayed bridge deck infill concrete with a specified 28-day compressive strength of 8000 psi (55 MPa) needed to remain at a 7- to 9-in. (175- to 225-mm) slump for 4 hours but achieve 3000 psi (21 MPa) compressive strength at 18 hours. In addition, the maximum aggregate size was limited to 1/2 in. (13 mm) to ensure passage through the closely spaced reinforcement.
All testing was initially performed in the laboratory followed by full-scale batches of 10 cu yd (6 cu m) or larger to ensure success in the field. Several full-scale dry runs were made on critical placements to ensure the concrete behaved as expected.
Concrete temperatures were important to allow for year-round production as well as for mass concrete placement. Several methods were involved in lowering concrete temperatures. Cement was readily available from a local mill, but due to the high demand during this period, the temperature of the cement as delivered was relatively hot. With the help of the supplier, cement was imported from Greece, which allowed a 14-day cooling period during shipment. Because the water-cementitious materials ratios were low and the cementitious materials content high, the use of ice was discounted because rates of 100 percent ice would be necessary. In addition, there were concerns about achieving full hydration and proper mixing when all the batch water was added as ice. Our solution was to immerse and chill the coarse aggregate in very large pits filled with near-freezing water. This, accompanied by chilled water and, at times, a small amount of ice allowed us to produce extremely cool concrete while maintaining a homogeneous product.
