Engineering innovation: Olmsted outside envelope placing concrete in congested shells
April 23, 2013
OLMSTED, Ill. (April 23, 2013) -- The "warm joint," in tandem with the application of self-consolidating concrete, gave the Olmsted team the time to beat the rising Ohio River by making placement in a tainter-gate pier in 72 hours rather than 44 or more days.
And members of the Olmsted Dam construction team may have coined a term that will ever after be used to describe the special joint between two lifts of self-consolidating concrete (SCC).
The warm joint is created when the lower lift has stiffened sufficiently to support a successive lift yet has not reached initial set and therefore achieves adequate joint strength. For a warm joint the lower lift has a penetration resistance of 15 to 45 pounds per square inch and this allows it to properly support the weight of the next concrete lift. A true cold joint, by contrast, is fully hardened and must be prepared by removing surface laitance prior to the next concrete lift.
"We wanted something to distinguish it from the conventional cold joint that is formed only after the lower lift is internally vibrated and allowed to cure for days before another lift is placed on top," explained Dave Kiefer, Louisville District, Corps of Engineers' geotechnical and concrete engineer with the Lakes and Rivers Division Regional Technical Specialist program. "The term 'warm joint' is used in roller-compacted-concrete construction but it has a slightly different meaning."
The use of SCC to infill the tainter-gate shells was an ideal application for this type of concrete. Some areas within the 102 by 66 by 14-foot, 2,300-ton monoliths were congested with steel reinforcement bars, guide frames and post tensioning ducts that would have made conventional placement very inefficient.
"It was the perfect application for the use of self-consolidating concrete," Kiefer said, referring to the shells' cores crowded with steel to the point it was neither safe nor practical for man and machine.
According to Mick Awbrey, deputy chief of the Olmsted Division, the current contract method requires cold joints, curing conditions, joint preparation, removal of material after joint preparation and before the next placement, and internal vibration to fully consolidate the concrete.
"With the amount per placement limited by lift height due to internal pressure, we were looking at a minimum of 11 lifts requiring four days per lift, cure, prep and cleanup," Awbrey said. "However, with the tension rod ducts, guide frames and reinforcing steel we would have had to drop the concrete up to 25 feet through all the congestion, causing segregation. And there was no way we could get any vibration devices down through the steel to properly consolidate, no way to water blast for joint prep, and no way to snake vacuum hoses down to remove waste material."
Necessity also became the midwife for developing the warm joint methodology.
"If the Ohio River overtopped the pier shells before we finished placing the concrete it would have been catastrophic," said Awbrey. "There would have been no way of ever completely cleaning the bar or sediment out without cutting the entire trunnion anchorage out and beginning over."
So in 2012 the Olmsted team developed the SCC mix and the warm joint process to be able to continuously place, allowing it to stiffen enough to eliminate pressures but not reach initial set and create a cold joint, Awbrey said.
Since the congested cores were not accessible to measure the concrete's stiffness with a standard time-of-set device, the team had to develop a test procedure to know when another lift could be placed, explained Joe Kissel, a materials engineering technician at the Olmsted concrete laboratory.
"We came up with a four-foot length of number nine rebar with a one-inch cross-section weighing 15 pounds so that it exerts 15 psi on the tip," Kissel said. "The bar is then lowered on a rope and if it sinks in it's not ready to support the next lift. When penetration was about one inch we knew we were about an hour away from the next lift and we called the on-site batch plant for more concrete."
Kissel said slump and joint-strength testing were done extensively before the first placement in a lower pier shell. To increase slump without decreasing strength, about 55 ounces or four soda cans of a high-range water reducing admix were added per cubic yard of concrete.
Specimens for testing the warm-joint strength were prepared at the project site and cores were taken of test blocks that were also made at the site, Kiefer explained. Cold-joint and no-joint specimens were also fabricated. The cores were tested by splitting tension and the prepared samples were tested in direct shear. Kiefer said the testing showed the warm joint was equivalent in strength to the prepared cold joint and the warm-joint methodology was therefore adopted.