Cold weather is defined by ACI Committee 306, “Cold Weather Concreting,” as “a period when for more than three consecutive days the following conditions exist:
- The average daily air temperature is less than 40° F (5° C) and,
- The air temperature is not greater than 50° F (10° C) for more than one-half of any 24-hour period.”
In New Brunswick, Canada, this condition sometimes extends from early December to mid-April, which creates a challenge on a fast-track project with over 50,000 cubic yards of concrete to place and budget restraints.
The Coleson Cove Refurbishment Project was the renovation of an existing oil-fired power plant and the addition of a flue gas desulfurization (FGD) system. The FGD system is housed inside a 214-foot-high building and contains two 56-foot-diameter absorbers. The FGD building sits atop a massive concrete foundation that ranges from 1 foot to 45 feet thick, depending on the existing topography. All excavation was required to go to bedrock, or, in another sense, all foundations were to be founded on bedrock. Rock quality was approved by the engineer prior to concrete placement. This was due to the heavy loads and vibrations from the equipment. The fast-paced and compacted schedule dictated that the project be completed by November 2004, which meant that the placement of concrete for the foundations of the FGD building had to start in early December 2002 to ensure that structural steel erection could begin in April 2003.
The contract specified that the temperature of the concrete be maintained at 10° C (50° F) for five days after placing, or until 70 percent of the 28-day compressive strength was reached. Adding to this challenge, there was no allowance for accelerators in the concrete. The allowable range of concrete temperature at the time of placement was 15° to 25° C (59° to 77° F).
Concrete placement for the foundations started on December 7, 2002. Since the temperature was already as low as +5° C (41° F) during the daytime and -18° C (0° F) at night, the contractor used a glycol-based ground heater to preheat the rock surface before placement and had the concrete delivered at a range of 17° to 25° C (63° to 77° F). Immediately after the concrete was placed, the area was completely covered with insulated tarpaulins and, in some cases, propane-fired heaters were used.
Unfortunately, during the 12 hours after the first concrete placement the insulated blankets were blown off by high winds. Test cores taken at the area of exposure, however, proved that the concrete was not structurally damaged.
The winter of 2002–2003 proved to be a challenging concrete season with several days where the temperature never rose above 0° C (32° F) and several days in February where the high was -10° C to -20° C (14° to -4° F) with wind chills as low as -42° C (-43° F). Despite this, the contractor placed more than 28,000 cubic yards of concrete between December 6, 2002, and March 31, 2003. This was done with the aid of hot water in the mixture, steam lines installed in the aggregate piles, glycol lines to heat the ground, and propane heaters and insulated tarpaulins to maintain a warm environment for the concrete before, during, and after placement and during the curing period.
As shown in the table, placement of concrete was scheduled on 62 days between January 1 and March 31,2003, and was cancelled on only four days due to weather-related issues. Maintaining adequate temperatures for curing was, of course, an ongoing task.
Due to the profile of the rock in the area of the main FGD building, the thickness of the concrete foundation ranged from 1 foot in some areas to 44 feet at the north end. Concrete at the north end was placed in four lifts, three of which were 10 feet or more in thickness, with one side placed against rock and the other sides against either concrete or plywood forms. The surface area was very large and irregular, so the contractor chose to preheat the rock surface with propane heaters, under the cover of temporary enclosures. Where possible, the heaters and enclosures were left in place during the concrete placement, and in all cases the concrete was covered with insulated tarps after placement.
On February 24, 2003, the outside temperature was -9° C (16° F) at 8 a.m. Fortunately, the contractor had enclosed the entire area with insulated tarps and had several propane-fired heaters in place to maintain a comfortable working environment while the workers placed 2404 cubic yards of concrete.