When temperatures dip toward freezing, building officials routinely shut down foundation work—or require costly protective measures, including insulating blankets and heated shelters. Residential foundation contractors have long chafed against these restrictions, and now they say they've finally got the research to back up their gut beliefs. According to a report from the Mount Vernon, Iowa–based Concrete Foundations Association (CFA), freezing weather is no reason to stop pouring basement walls. And at least for residential foundation wall pours, the CFA maintains, shelters or even blankets are usually overkill. Success, its findings indicate, depends mostly on what goes into the mix.
The story of the CFA's research project goes back to the fall of 2000, when concrete foundation contractors in Medina County, Ohio, found themselves with a problem. They had received notice from the county building department that cold weather was coming and that they would all need to adjust their practices accordingly. In freezing temperatures, no concrete could be placed except inside heated enclosures.
“Poured-wall contractors make up about half of our business in the winter,” says Rick Buccini of Osborne-Medina, a ready-mix supplier in Medina. “If they were shut down, we knew we would be shut down too. But we also knew that we had some high-performance concrete mixes that could perform very well in cold weather without the need to provide tents or heat.”
The problem was convincing the building department. So when Buccini and a group of foundation contractors went to meet with the county building official, they brought along engineer Brad Barnes, who runs a concrete testing lab called North Central Engineering, in Canton, Ohio. Barnes already had credibility with the county official—the two had worked together on commercial projects when the official was employed in the private sector.
“We asked the county what they needed in order to be satisfied with our mix performance,” says Buccini. “They said they wanted to see 1,200 psi overnight. So we went and mixed a batch that we thought could do that. It's nothing that we haven't done for the last 10 years. We make a mix with plenty of Type III [high-early-strength] cement and accelerators that we were quite comfortable would have the strength we needed 24 hours after the pour.” Barnes and Buccini placed a test cylinder in a freezing room overnight and fractured the cylinder in a standard test apparatus the next morning to determine its compressive strength. “We got 1,800 psi,” says Buccini, “and they accepted that.”
It could have stopped there; Medina building officials were satisfied. But several of the contractors, who were members of the CFA, wanted more-comprehensive data—something their trade association could show building officials all over the Northern United States whose code interpretations frequently curtail the wall contractors' cold weather work. Says Barnes, “They started talking within their association, and it ballooned into a whole research project.”A MATTER OF DEGREE
“It didn't take long for me to realize that I needed to know the temperature of the concrete in the walls,” says Barnes. Then, at the 2002 World of Concrete show in New Orleans, CFA executive Ed Sauter, along with CFA member Terry Lavy of Lavy Concrete Construction in Piqua, Ohio, happened to bump into John Gnaedinger, president of Con-Cure Corp. Based in Chesterfield, Mo., Con-Cure supplies a “concrete maturity system” that couples a thermistor (temperature sensor), buried in the fresh concrete at the time of the pour, with a solid-state black box that keeps a continuous record of the concrete's temperature as the concrete sets and hardens.
Combining each mix's precise time and temperature record with its known strength based on cylinder fracture tests, maturity software can establish a “maturity signature” for each mix design. Then, using well-known equations for the heat released in the chemical reactions involved in concrete curing, the software can estimate the strength gain for the same mix when cured at some other temperature, even at a fluctuating temperature in the outdoors. That information can be especially valuable for concrete placed in cold weather. Writes Lavy in a CFA report, “When Ed and I explained to John what we were trying to accomplish, the light bulbs went off in everybody's head.”
Maturity testing is a standard practice in the concrete industry. Usually, the signature curves for any mix are created at ideal curing conditions—about 70°F and 100 percent relative humidity. But, says Gnaedinger, “We needed to design an experiment where people would not question the result of a cold weather study, just because the maturity curves were created at a warm temperature. We wanted to create a maturity curve at or close to freezing.” So the CFA decided to measure the temperature and strength gain over time of a full range of mixes, cured in a freezing cold room. “Now,” Gnaedinger says, “we have real data that shows concrete, even very lean mixes, curing out at this very cold temperature.”
In the end, the CFA had designed and tested 44 different mix formulas. It had also poured test walls outdoors in sub-freezing weather with six of the mixes and tested core samples of the walls for compressive strength at various ages. To learn more about how the concrete had fared in the freezing conditions, the CFA sent core samples from the test walls for a petrographic analysis (an expert microscopic inspection of the concrete's physical condition). Finally, it put the cores through a 300-cycle freeze-thaw exposure test to assess the concrete's durability.