In 1999, Congress authorized the U.S. Army Corps of Engineers (COE) to design and construct modifications to the 1950s' era Folsom Dam near Sacramento, Calif., to increase the hydraulic performance of the existing outlets. A study of alternatives led the COE to decide to enlarge eight gates and add two new gates as the most cost-effective design. This approach would reduce the risk to this heavily populated area from one chance in 100 of flooding in any given year to one chance in 130.
The U.S. Bureau of Reclamation (USBR) and COE are working jointly to design and to award the bid to a qualified contractor. The contractor will remove concrete from the existing gates and will use controlled demolition to create spaces for the two new gates. The plan is to widen the gates from 5 to 9 feet and increase the height from 9 to 14 feet. Finally the contractor will anchor the new gate bodies with concrete. This past fall, at the Denver Federal Center, engineers from the USBR Materials Engineering and Research Lab conducted placement tests to determine the best concrete mix for modifying the outlet works.
The 9-inch space between the new gate body and the existing concrete will have a series of steel reinforcing ribs running in both directions spaced approximately 2 feet apart. Because of the confined space at the bottom of the gate, vibrating the concrete to fill the spaces between ribs would be very difficult. The solution to this problem was self-consolidation concrete (SCC).
During construction the contractor will pump from one side of the form so air is pushed out the other side. The concrete has to flow down one side and then turn a 90-degree angle to flow across the 9-foot-wide horizontal bottom and then come partially up the other side under its own weight.
“We won't have a second opportunity in the field [to get it right] so we're conducting up to three model placement tests to make sure we have the best performing mix and placement technique,” says John LaBoon, USBR's manager of waterways and concrete dams. The model, a square U-shaped mock-up, represents a cross-sectional slice of the bottom and sides of the 29-foot-deep gates. To approximate the field conditions at the bottom of the gate, USBR engineers, Walt Heyder and Ernie Hall designed the gate formwork model with 3-foot-high sides and a 9-foot-wide bottom. The larger outer box simulates the excavated concrete surface of the dam with split concrete block walls and floor. The plywood gate body model, 4 feet deep, contains a waffle-like bottom of plywood panels in both directions to model the prototype steel plate ribs. There are also two plexiglass panels in the formwork to allow observation of the concrete flow adjacent to the ribs.
“Normally we design our own mix, but in this case the ready-mix company has already done R&D so we decided to start with Lafarge's Agilia architectural mix,” explains Erin Gleason, USBR civil engineer materials specialist.
The removable form on what would be the downstream side of the gate was lined with DuPont's Zemdrain, a polyurethane liner that has a textured side facing the concrete formwork and a smooth side facing the concrete; it is designed to reduce pinholes. Brundage Bone pumped the concrete to deliver it at a rate of 106 cubic yards an hour. During the 9-cubic-yard placement, engineers climbed ladders to access a platform on the back of the model to watch the concrete flow and air bubbles emerging from the far side of the horizontal section.
“Our main concern was the horizontal placement,” says Tim Dolen, USBR civil engineer at the materials lab. “Would the concrete continue across the bottom of the gate and around embedments without voids? Would it fill the areas between the ribs? Would it push out the entrapped air bubbles as it advanced under the flat form? Would it segregate as it rose up the opposite side?”
As the placement continued, Gleason measured the concrete depth on either side from the form top. On both model placements, the concrete in the far side was just 6 inches lower than the concrete on the access side. When the actual dam gates are constructed, concrete will be placed in this manner along the entire 29-foot length of the gate body. An additional three or four placements totaling about another 27 vertical feet will encase the remainder of the gate body and gate bonnets.
Besides the large gate form, the two truckloads of Agilia were also used to fill two L-shaped test forms, a test cylinder 2 feet in diameter by 4 feet high, a box with a steel divider to test adherence to the steel, a shallow rectangular test box with a steel bottom, and 24 standard 6-inch compression test cylinders. USBR staff removed the small L-shaped forms the afternoon of the model test to check the setting process and the surface smoothness.
The lab's 5-million-pound Universal testing machine broke the 4-foot-high cylinder in demonstration classes at the Fed Center. One of only two testing machines of this capacity in the United States, it is five stories high, three stories above ground and two stories below. The concrete tested at 6200 psi after 35 days and will attain 7000 psi, according to Tom Cummings, Lafarge North America.
At the end of September, after 28 days, USBR engineers stripped the first model. Gregg Day took seven cores and found no voids and no segregation. According to Heyder, the SCC filled the entire void between the ribs and traveled well under the gate body and between the ribs. Visual checks found the apparent bond between the concrete and the concrete block acceptable.
The engineers oversaw the second test placement on October 5. “The second test is to verify that we didn't accidentally get it right the first time, that the design is working. Also we're doing some refinement such as working out bug holes,” says Heyder.
On this second placement, Day filled the air vent caps that protrude from the top of the gate bottom model with rubber foam to release air while keeping the concrete out. Several hours after the placement, technicians and engineers pressure-pumped grout at 10 psi between the form and the concrete to seal any bug holes and to bleed water channels. One section of the form upstream of the gate body will be removed to become the upstream conduit and must be finished to extremely high tolerance to resist high-velocity water releases that can flow at 90 to 100 feet per second.
The COE is preparing performance specifications for the SCC the contractor will place in the field. The contractor will be required to conduct a test for flowability before actually placing the concrete for the gates.
“SCC presents unique finishing challenges due to the lack of bleed water,” Dolen says. “The contractor won't have a second chance with schedule constraints so will have to coordinate closely with the ready-mix producer.”
COE finalized the design in late November 2004 and put the project out to bid in early December 2004. The project will require approximately 14,000 cubic yards of SCC and is scheduled for completion in October 2011.