Q.: I need to place concrete in four 30-inch-diameter, 37-foot-long steel-shell piles that are driven at the bent lines of a river bridge. River depth varies, but is 5 feet in the deepest spot. After the steel shell is driven, the water level is about 1 foot below the top of the pile. The water moves swiftly and its temperature ranges from 35° F to 44° F.
Specifications require me to keep the concrete temperature above 45° F for 72° hours and above 40° F for the next 96 hours. The concrete is a seven-sack, 3500-psi mix with no changes allowed in the mix proportions. Each pile will only take about 7 cubic yards of concrete, so I don't think heat of hydration will be sufficient to maintain the minimum temperatures. How can I handle this problem? The job schedule won't permit us to wait until the weather is warmer.
A.: Since the embedded part of the pile will probably retain enough heat to meet the minimums, your main concern is the 4-foot section that's exposed to the cold water. Perhaps you could drive a slightly larger-diameter (and shorter) steel-shell pile around each pile to form a cofferdam. Then you could pump out the water and wrap the inner pile with a closed-cell insulating foam. Another possibility: Leave the water in the annular space between the two sheet piles and heat it by using an electric immersion heater or, more safely, by circulating hot liquid through tubing placed in the annular space.
Note: The specifying agency wouldn't permit driving another pile, so the contractor built a wooden box around the pile and sealed the bottom of the box. At press time, he was still deciding whether to insulate the pile form or heat the water in the box.