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CFA Cold Weather Research Transitions to Contractos

Jim Baty, CFA Technical Director

“You can feel it in the air, you can feel it on the breeze…” isn’t that how the song goes? For many contractors throughout the middle to upper states, cold weather is not only on the horizon, it is approaching rapidly. What better time for the CFA to finish production of its long-awaited Cold Weather Research Final Report available to the Association.

Those who attended the CFA’s Summer Meeting in Breckenridge, Colorado this past July were able to receive the very latest presentation based on this research. John Gnaedinger of Con-Cure and CFA President, Terry Lavy, conducted a seminar during the opening day of education in Breckenridge presenting the results of the third phase—Durability: Freeze/Thaw Performance and wrapping up the current program with a concise summary of the impact it will have on the market. Attendees were provided with a draft copy of the final report to gauge the readability and the interpretation of the data.

“Everyone that I talked with in Breckenridge was satisfied with the results that we presented and discussed in the draft form of the final report,” said President Lavy. “The editorial committee that I worked with on this including Brad Barnes, John Gnaedinger and Jim Baty gave a tireless commitment to making this document not only presentable to the Association and inspectors, but immediately useable for this next cold weather season.”

The information contained throughout the report is a thorough representation of the scope and success of this two-year study. The size of the report is far too significant to reproduce in this publication; however, the key components of where you, as a foundation contractor, concrete supplier or building inspector should go from here can best be summarized with the following information:

RESULTS FROM THIS RESEARCH PROJECT

The data resulting from this research suggests the following insights:

1. Ambient temperatures, although having an affect on the drop in concrete temperature, should not set the restrictive condition. All concrete mixes tested displayed a significant lag in internal temperature in relation to ambient temperatures— remaining above the freezing point much longer than the ambient temperature.

2. All the concrete continued to gain strength even at early ages and at low temperatures which would suggest that cement hydration does not stop at 40°F (4.4°C).

3. The strengths determined using the maturity curves created with prediction software very closely match the actual strengths obtained from testing the cylinders and cores in both phases. This relationship gives further support to the theory that in-place strength can be accurately and adequately determined using maturity meters for prediction purposes—making it easier to adjust mix designs to suit individual and regional differences and requirements.

4. Admixtures that reduce water content enhance strength gain at lower temperatures.

5. Many references state that concrete must not freeze before it reaches 500 psi. While this may or may not be a valid benchmark for concrete in general, our research indicates it may be less important as a target for residential foundation walls. The amount of “free” water in the mix has a direct relationship to the affect freezing has on concrete. Concrete produced with modern technologies can continue to gain strength even if the internal temperature drops below 32°F (0°C) before it reaches the 500-psi level. The “frozen state”, as predicted by the ambient temperature, did not negatively affect the strength of the concrete. All samples reached or exceeded their designed ultimate strength. Mix designs did cause variable time intervals for gaining target strengths. This provides contractors with information to use with their own mixes to achieve specific performance requirements.

6. Wall samples in Phase II that were uncovered developed similar strengths to those that were covered for the first 18 hours. Petrographic examination showed improvements in the surface characteristics and reduction in micro-fracturing in the cores taken from the walls which were covered for the first 18 hours. Samples subjected to 300 freeze-thaw cycles were proven to be durable, but the data suggests better durability can be achieved with higher cement contents and/or Type III cement.

7. Mix designs using Type III cement were shown to gain strength much faster than those with Type I cement, and usually with less accelerating admixture.

8. The incorporation of calcium chloride up to 2% as an accelerating admixture provides faster strength gain at a low cost. Nonchloride accelerators (NCA) provide a similar benefit without corrosive affect to steel reinforcement, but at a higher cost.

RECOMMENDATIONS AS A RESULT OF THE RESEARCH

• The CFA cautions the perception that all concrete practices and mixes are appropriate for cold weather. Casting a wall is very different than a four-inch thick driveway. Good common sense and concrete procedures should be observed.

• Rules and Regulations based on ambient temperatures have little or no validity.

• Contractors should work with their local ready-mixed concrete producer to design concrete mixes that will perform well based on the expected variables for a placement. The mix designs used in this research provide a sound basis for your own mix development but should be used after localized testing.

• There is no single mix design that works best for cold weather concreting. Results vary as mixes vary but all are predictable and economy can be kept in mind when designing and selecting a mix.

• Use maturity meters or other instrumentation on your cold weather projects to evaluate how your concrete mixes perform and provide a documented history of their maturity.

• There is a significant impact resulting from adding excessive water to concrete under cold weather conditions—the greater the water content, the greater the affect those freezing conditions will have on the concrete. A six-inch slump may be acceptable, but a high water to cement ratio is not.

• Placing concrete early in the day will normally give you an advantage by capitalizing on solar gain to dampen the affect that ambient temperature has on internal concrete temperatures.

• Finally, cold temperatures do cause slower strength gains than normal, so don’t remove structural supports from a wall too early—top and bottom restraint must be in place prior to applying any lateral loads to the walls.

A NOTE TO CONTRACTORS AND SUPPLIERS

It should come as no surprise that it is to everyone’s benefit to provide our customers with the best possible product, at the most economical price. In today’s business climate, the economic impact of stopping construction unnecessarily is huge. Listed below are a few notes and suggestions as to what each of us can do to help.

CONTRACTORS:

Keep in mind that this study discusses cast-in-place concrete walls only. While many of the findings will apply to other types of pours, there are many variables that should be considered.

The CFA wants to make it absolutely clear that we are not saying you can pour any ol’ mix design in any kind of weather, this would be irresponsible thinking. If you have been doing that and see no need to improve your procedures, file this report away and forget it.

For the rest of you – either you just have a desire to learn to do things better, or you are seeing pressure from customers and/or code officials to change your ways. The CFA is seeing, what we believe to be, overly restrictive codes being adopted all over North America. The problem is growing like a cancer. Chances are good that you will see the effect of this in your area sooner or later.

Work with your local Ready- Mix Producer to develop a few different mix designs for the conditions you are likely to encounter. Some of our suggested recipes may be a starting point; however, consideration should be made to using some form of instrumentation and/or maturity testing on each selected mix design to validate its performance. The goal is to get the hydration process “kickstarted” so that the material can generate its own heat. Keep in mind that if anything in the mix changes, ie; water/cement ratio, type or brand of cement, admixtures, etc, the performance will change as well. (Note:It is not essential to wait until the first cold day to perform testing. However, the mix design used during cold weather must be identical to that tested.)

Pay attention to possible shrinkage problems with some of the hotter mixes. We have the shrinkage data from our test mixes, but the potential will vary greatly from local differences.

Type III cement, where available, really does set much faster and usually requires less accelerator.

The temperature of the concrete when it hits the forms is of great importance. Material that is 60° to 70°F (15.5° to 21.1°F) is much preferable, and usually not to hard to accomplish.

Water/cement ratio is also a huge factor, eliminate as much water as you can.

Type E, (accelerating) Mid-Range water reducers are a great tool, talk to your supplier about them.

Good old Calcium Chloride is probably the best “bang for your buck” accelerator, as long as steel corrosion is not a big concern. You may want to consider adding your own dry calcium at the job-site as opposed to the liquid calcium from the producer. Non-Chloride Accelerators do a wonderful job, they do not promote corrosion, and may work in even colder weather, but they do cost more. Use them where applicable.

Make sure that all of your employees and anyone else involved understand that concrete does indeed set slower in colder weather. Things to consider are the safety of stripping a cantilevered haunch, or door openings, or backfilling before the wall has gained enough strength to support itself. Also, damage can be done by applying torque to the anchor bolts prematurely.

Working in a freshly excavated hole has large benefits. Capitalize on the heat of the earth. Try to get the foundation in quickly and backfill a couple of feet around it ASAP.

Making your cold weather pours early in the day can help a lot. The solar heat gain on the walls and forms will boost the concrete temperatures substantially, even on a dreary day.

Help to promote the idea that rules and codes based solely on weather forecast, are out of step with reality.

Please document and report any problems you do see to the CFA. The research contained in this report is not finished; we will continue to study the affects of cold weather on concrete mixes as they pertain to residential concrete foundation walls.

SUPPLIERS:

Learn about Maturity testing and make an investment in it. You will find it becoming more and more a part of the specs on government and commercial jobs. Be pro-active.

Depending on your location, making Type III cement available in the winter could be a huge benefit to all concerned.

Take a good look at your material heating systems, you need to be able to provide concrete to the job site at a consistent 60° to 70°F (15.5° to 21.1°F). Some plants are installing concrete storage pads with hydronic heat systems.

Work with your Contractors to provide the Maturity Testing required to assure the customers and code officials what your products can do. Several Ready-Mix suppliers around the country market special mix designs which have been proven.

It appears to most members of this committee that dry (flake or powder) calcium added at the job site works better than the liquid calcium typically added at the plants. Consider stocking calcium in dry form, and teach your contractors how to be responsible in its use· Please document and report any problems you do see to the CFA. The research contained in this report is not finished; we will continue to study the affects of cold weather on concrete mixes as they pertain to residential concrete foundation walls.

A NOTE TO CODE OFFICIALS

Understand that the American Concrete Institute 306 (Cold Weather Concrete) Committee, is currently re-writing the entire cold weather code, including the definition of what is cold weather. While it will be several years before the new language reaches your code books, it is important to understand that the experts agree big changes are necessary. Please do not pass or go overboard in enforcement of codes that we already know are outdated. Please do not ignore the fact that we currently cast basement walls in extremely cold weather and with a very, very low incidence of problems.

If the code says “concrete should achieve 3,000 psi in 28 days” request that the contractor provide the assurance (maturity prediction) as to how this concrete will reach this strength and make this your area of enforcement.

Please document and report any problems you do see to the CFA. The research contained in this report is not finished; we will continue to study the affects of cold weather on concrete mixes as they pertain to residential concrete foundation walls.

As you can see from the details above, the effects this research should have on the foundation environment are quite significant. However, this research also reinforces a lot of the information many of the CFA contractor members already know…good sense makes good practice. The main thing to remember going forward is that the CFA needs your communication and assistance in making this a success. Reports will be available for supplemental purchase. We encourage you to distribute this information to the entities that you work with throughout the year to help them prepare for working with you. Finally, we encourage you to contact us periodically with a report on the successes and shortcomings you are experiencing as a result of this research. If you begin concrete monitoring to supplement this data as presented, we’d love to receive copies of these logs to further supplement our data records. If you experience differing results or take photos of interesting notations, please send us these as well. The entire industry will continue to benefit from us recognizing that the work isn’t finished…it’s just beginning. Contact Jim Baty at CFA for more information at phone number should be 866-CFA-WALLS or jbaty@cfawalls.org.

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Concrete FACTS, a publication of the Concrete Foundations Association, is THE voice for residential concrete industry news, market intelligence, business strategies, technical solutions, product information, and other resources for professionals in the cast-in-place concrete industry. Subscriptions to Concrete FACTS is available to anyone involved or interested in the residential concrete industry as a service to your industry. Please contact CFA Headquarters to find out more about your free subscription or Email Us