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.
Using maturity software, says CFA technical director Jim Baty, a contractor can now predict the cold weather strength development of any CFA mix within 5 percent or 10 percent, as long as the curing temperature is known. The most aggressive, or “hottest,” of the mixes have been shown to reach freeze-proof strength in less than one day, even in freezing weather—without heated shelters or even full blanketing.
A CFA report on the testing, “Cold Weather Research Report 2004,” is available from the association. The CFA's research is referenced in the newly drafted American Concrete Institute (ACI) Standard 334, “Residential Concrete,” which is in the final stages of acceptance by the ACI as an industry consensus standard. In Medina, the CFA study has gained full credence with contractors, suppliers, and code officials alike. “One contractor I know has told his foremen to look on every delivery ticket for the CFA mix number,” says Barnes. “If it's not on there, they reject the load.” As for the code officials, says Barnes, “If the contractor's using a CFA cold weather mix, they don't question it any more. They've moved on. They worry about other things.” Says Buccini, “We've been using that cold weather mix for three years now, and we haven't had any problems with it.”
Now, the CFA is working to spread the news to other areas. “There are a lot of people using our mix designs now,” says Baty. “Probably our biggest success story is Anchorage, Alaska. They picked two of our mixes from the outdoor phase of our study, and they've got it written into their code to follow our recommended practices.” Barnes traveled to Alaska to talk to builders and code officials. “They were requiring heated enclosures if the weather was colder than 40 degrees Fahrenheit,” says Barnes. “Now, the heated enclosures are only required if it's below 20.”
As structural engineer and foundation expert Brent Anderson, principal of Minneapolis-based Brent Anderson and Associates, puts it, “What the CFA cold weather research proves is just this: What these contractors have all been doing for the last 30 years works. If a contractor wants to work with a ready-mix supplier to develop a custom mix for 20°F or 25°F weather, there is no reason they shouldn't place concrete at those temperatures right through the winter.”
Contractors have been quick to adopt the recommendations of the CFA study because it coincides with their years of practical experience. (As Anderson notes, “The guys in the CFA have way, way more years of experience placing concrete under these adverse conditions than many of the people sitting on the ACI committees.”) But in convincing the more technically minded people in the industry, CFA contractors have found it extremely helpful to be able to support their experience-based knowledge with technical data from an accepted engineering technique such as maturity testing.
THE CFA EXPERIMENTS
Using a maturity system to gauge concrete curing is a two-step process. First, the “maturity signature” of the specific concrete mix formula has to be established. This is done by pouring a set of test cylinders with the mix in question and placing the temperature probe into one of the cylinders. Then, the cylinders are cured under controlled conditions and fractured in a test apparatus one by one at specified intervals. The strength of the curing concrete is plotted against its time and temperature record to create a maturity curve.
The second step requires monitoring of the actual concrete placement in the field. A probe is placed in the structure that is being cast, and its temperature data is recorded. Then, the software can compare the temperature experience of the field-cured concrete at any moment to the curve generated by the lab-cured concrete samples and estimate the strength of the concrete in the wall, slab, or other structure.
In the first phase of its testing project, the CFA team designed 44 different concrete recipes that they knew would cure at different rates. One obvious variable was the amount of cement in the mix; quantities ranged from a “five-sack mix” containing 459 pounds of cement per cubic yard of concrete up to a “six-and-a-half-sack mix” with 599 pounds per yard. Cement type is also important: Type III cement, which is more finely ground and reacts faster than standard Type I cement, was used in some of the mixes. Calcium chloride, a common accelerating admixture, was also added to some of the mixes at doses of 1 percent or 2 percent by weight of cement; some mixes incorporated a different, nonchloride accelerator, and some included a water-reducing admixture.
At a Cleveland test facility of admixture supplier Master Builders, the CFA team mixed sample batches with each formula, inserted temperature probes, and placed the batches in a freezing cold curing chamber. Although the room temperature was kept within a degree or two of freezing, the heat of hydration of some of the mixes kept the concrete warmer than the air in the room for some time. The curves generated for the sample mixes show concrete temperatures that averaged slightly above freezing (from about 8 to 25 degrees Fahrenheit warmer than the ambient air in the test chamber).
“Every one of those mixes—even the leanest Type I mix with no accelerator at all—reached 3,000 psi strength in the 28-day curing period,” says Gnaedinger. “A couple of the top-performing mixes were at 3,000 psi after two days in that freezer.”
OUTDOOR WALL ORDEAL
Armed with maturity curves from the refrigerated batches, the team proceeded to outdoor testing. At the Osborne-Medina batch plant, crews set up aluminum forms for a dozen test walls. “We did everything we could to make that concrete fail,” says Gnaedinger. “It was 25°F and falling the day we poured. It went down to 17°F that night. We covered only the tops of the walls with blankets that hung down 3 feet on either side. And we stripped forms the very next morning and took the blankets off. Ask any of those contractors if they would have stripped forms on a real job the next day, and they will all tell you no—they would leave the forms on. And the weather did not get above freezing for the whole month of the test.”
Despite the brutal conditions, says the CFA's research report, core sample tests show that all the concrete walls came up to minimum required compressive strength within 28 days. “Even the leanest mix, Mix 3, reached 3,000 psi,” says Gnaedinger. “That is a mix that no CFA member contractor would ever use in winter—five-bag mix, Type I cement, no accelerator. But it came up to strength.”
The unblanketed portions of some test walls—particularly the walls poured with lean, slow mixes—did show signs of surface freezing in the petrographic analysis. But cores taken from the blanketed tops of the walls had no frost damage at all. “The guy who did the petrographic analysis of the cores from the covered portion of the walls said that if I had told him this concrete was placed in July, he would have believed it,” says Barnes.
RACE AGAINST FREEZING
The data about freezing raises some eyebrows in the concrete engineering community. Charles Korhonen, chairman of the ACI committee that maintains ACI 306, a guidance document for cold weather concrete work, says, “Preventing freezing is what this industry is all about. When concrete freezes, the part that freezes is not good. You have damaged goods. You have to start looking at durability issues. If you want to rationalize somehow that the freezing was OK, that's one thing, but you shouldn't give the impression that the concrete didn't freeze if it did freeze.”
Indeed, it's a common assumption that freshly placed concrete should never be allowed to freeze. “Hydration” of concrete is a complex set of chemical reactions between the calcium and aluminosilicates in cement powder and the hydrogen and oxygen supplied by water in the mix. As hydration progresses, tiny crystalline fingers of hydrated cement slowly grow and interlock to produce a uniform, hard mass. If water in the mix freezes too early in the process, expanding ice can rupture the fragile crystalline matrix before it isfully developed—and once destroyed, the interlocking structure can't repair itself. “When concrete freezes, the matrix of cement and sand and aggregate gets disrupted, and durability suffers,” says Anderson.
After the concrete is hard enough—500 psi compressive strength is the official limit—much of the free water in the mix has already been used up by hydration reactions, and the developing concrete matrix is strong enough to resist the expansion of the ice formed when remaining water freezes. Also, concrete formulas for cold conditions typically include “air entraining admixtures,” which form tiny air bubbles in the wet concrete paste that end up as microscopic air pockets in the hardened concrete. These mini-cavities provide a pressure-relieving space for water to move into under the pressure of expanding ice, helping the concrete resist freezing damage.
But when a concrete mass freezes solid before the 500 psi limit is reached, the results can be disastrous. Engineer Barnes recalls a case where a contractor inadvertently used the wrong mix formula for a cold weather pour: “This guy wanted to pour on Saturday in March, and his usual supplier wasn't open. So he turned to another supplier, and they had already started using a summer mix.” Acold front came through the area: “It was 60 degrees that morning, but by dark it was in the 20s, and it got blue cold that Saturday night and Sunday. I got a call Monday to come look at that wall—they tried to strip forms. Well, if you've ever seen concrete that has been frozen hard enough to damage, you know it. It's just ugly. You can take a screwdriver or something and just dig a hole right through it. I told them, ‘Boys, that wall's gone.' When they went to knock it down, it crumbled. It just fell over. And it was all because he used the wrong mix.”
But the CFA test walls didn't freeze solid—even the unblanketed walls had only surface freezing. And Anderson argues that for a residential foundation wall, a minor amount of surface freezing is not a significant defect. “For flatwork like a slab or a driveway, durability is important—probably more important than strength, because everything is supported. But these CFA tests are not applicable to flatwork, only to vertical walls.” Freezing damage to the skin of a basement wall isn't a major concern, says Anderson: “If you get a little freezing—1/32 inch or 1/8 inch—the surface will be dusty. Dust will always come off on your hand or your clothes if you rub against it. And if you want to paint it or put a coating on it, that could be a problem. But if you're going to put steel or wood studs over it and insulate it, or if it's going to be buried outside, it's really not an issue.”
Even so, Anderson favors covering newly poured walls with full blanketing in freezing weather. “It doesn't take that long to do it,” he says. “Remember, our greatest stresses on a wall are from the middle to the bottom of the wall, where the lateral pressure of the soil is the greatest. So it makes sense to blanket the whole wall, even though you can probably get away with covering just part of it.”
Barnes notes that the cores from covered portions of the CFA test walls, as well as the sample cylinders taken from that day's pour (which were covered during curing), showed no frost damage at all and reached very high strengths. “That really shows what an advantage it can be to cover the wall,” he observes.
For builders who need a foundation poured in midwinter, the CFA research offers a basis for making informed choices. The major lessons can be easily summed up:
Use an appropriate mix. The CFA offers 44 standard mix designs, but it's wise for local ready-mix suppliers to do their own maturity testing of their own batches, so that differences in cement or aggregates can be taken into account.
Be prepared to cover walls with blankets. If the ambient temperature is above about 30°F, says Anderson, “You don't need to blanket anything.” (Water in fresh concrete freezes at a temperature slightly below the freezing point of pure water.) Between 20°F and 30°F, covering all or part of the wall is a judgment call, based on knowledge of the behavior of the mix; but covering the whole wall can't hurt, and it's good insurance.
Brace walls before backfilling, and backfill with extra care. Damage to walls from “abusive backfilling,” notes Anderson, is far more common than freezing damage. “The code now requires all walls to be braced before backfilling.” His advice: In warm weather, a brace every 12 feet between wall corners is sufficient, but walls gain strength more slowly in cold weather and spacing of braces should be reduced accordingly. Taller walls also require better bracing, says Anderson: “A 10-foot wall should have double braces, with one brace centered vertically and a second brace about 2 feet from the top of the wall.” Walls should reach at least 1,750 psi before backfilling starts, advises Anderson.
Do not add water to the mix on site. Maturity testing data is only valid for the mix as designed, and additional water will change the mix properties. If a contractor needs to use extra water on site, the maturity curves should be re-created first using the actual water-cement ratio the concrete will have when placed.
Monitor concrete delivery temperatures. Concrete should be placed at a minimum temperature of 60°F. Says Gnaedinger, “At the end of a cold day, ready-mix plants may have a hard time keeping the water and aggregate hot enough—sometimes their boilers just can't keep up. And a five-degree difference in delivery temperature can be the difference between freezing or not freezing that concrete overnight.” Says CFA technical director Baty, “One of our members has an infrared, point-and-shoot thermometer on each job, and he takes a reading from the bottom of the drum. If it reads colder than 60°F, he rejects the load.”
For advice and support with cold weather concrete work, says Baty, foundation contractors are welcome to contact CFA. And he's asking them for feedback: Says the CFA report, “Please document and report any problems you see to the CFA. The research in this report is not finished; we will continue to study the effects of cold weather on concrete mixes as they pertain to residential concrete foundation walls.”
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