By law, casinos in St. Louis must float. And for that reason, the Pinnacle Casino and Hotel now under construction in St. Louis — but well inland from the mighty Mississippi — has some notable similarities with the story of Noah: it’sa really big boat being built on dry land that will eventually float right where it is. But unlike the ark of old, this boat is being built of structural concrete.
The keel slab that is the bottom of the barge is approximately 70,000 square feet of lightweight concrete 8 inches thick. At 1700 cubic yards, it is one of the largest lightweight posttensioned concrete placements ever poured monolithically.
But the size of the pour wasn’t the only challenge. Before it could take place, more than 220 8-foot tall precast concrete walls were installed atop the casino barge to form 25-foot by 12-foot cells. Some but not all of the cells are watertight and will serve various purposes as machinery rooms, electrical rooms, data rooms, ballast tanks, and void spaces. After the cell walls were in place, walkways and ladders were constructed over the entire pour area to provide construction access into and out of the cells.
Teamwork and planning were essential to the success of this job. St. Louis-based Vee-Jay Cement, the flatwork contractor, used three concrete pumps, one placing boom, and one back-up pump. Thirty finishers, 40 laborers, and six operators were on the job for the pour. Because the quantity of lightweight aggregate required for this pour was so large, concrete was supplied by two different ready-mix companies, Five Star and Raineri. Pumping began at 3:30 a.m. on August 4, 2006 and the pour was completed 11 hours later.
Unit weight was probably the most important characteristic of the slab concrete. The target dry unit weight of the mix was 114 to 118 pounds per cubic foot. Staying within that range was important for the barge to float properly in the basin. The project team used mock-ups to gain experience in working with Gravelite, the expanded shale lightweight aggregate selected for the mix. These trials also allowed them to see how the concrete mixes would perform and pump, and to test the shrinkage, weight, and strength of the various mixes.
The ready-mix suppliers and Vee-Jay Cement conducted numerous field trials of the concrete mix designed for this slab, batching loads in the morning, at midday, and in the afternoon to determine the strength gain they could expect throughout the pour and to ensure pumpability of the mix during extreme conditions. On the day of the pour, ice and chilled water were used to keep concrete delivery temperatures below 90° F. The slab was subsequently wet cured for seven days to minimize shrinkage and cracks.
The prepour trials also allowed the concrete contractor to perfect a plan to place and finish the slab while working around the cell walls. Using foam sheets to mock-up the precast walls, the bottoms of which were to be about an inch into the slab, crews learned to vibrate the concrete just enough to consolidate it without allowing too much to flow out of one cell and into the next.
Because the keel slab was to be post-tensioned, the pour had to be continuous; no cold joints could develop. There were 16 cells across the slab, so each of the three pumps was designated to handle either five or six cells across. Working from north to south along the long axis of the slab, 16 teams placed and finished the concrete. The casino is expected to be floated in early 2007.