The American Concrete Institute (ACI) recently updated its Guide to Cold Weather Concrete specification, giving contractors guidance for properly protecting concrete. Published in the fall of 2010, the ACI 306R-10 publication includes new equipment and tactics, based on updated field experience and new technologies introduced since the 2002 edition.
Guides to temperature uniformity, refined cold-weather concreting definitions and more clearly defined approaches to temperature protection are but a few of the updates found in the revised spec. For this first time, the latest spec lists hydronic heating systems as an approach to protect concrete during cold weather.
Clear & Uniform
“Contractors will notice that the committee clarified the definition of cold-weather concreting,” says Tim Lickel, voting member on the ACI Board and strategic business development manager for Wacker Neuson. Cold weather, according to the revised spec, exists when the air temperature has fallen, or is expected to fall below 40° F (4° C) during the protection period.
Concrete’s critical protection period is the time required to prevent concrete from being freezing. “Generally, concrete cannot freeze before it reaches a compressive strength of 500 PSI (3.5 MPa),” adds Lickel.
Several factors affect this time period, including ambient temperature, the type of concrete, size of the pour and how soon the concrete will be exposed to load conditions. Fresh concrete under no load and not exposed to freeze-thaw cycles will have a minimum protection period of 2 days. As anticipated load increases and concrete is exposed to freeze-thaw cycles, the protection period can be as many as 6 days.
“When concrete is exposed to colder temperatures and freeze-thaw cycles, compressive strength of 500 PSI (3.5 MPa) may not be enough,” explains Ed Jaroszewicz, director of Wacker Neuson Climate Technology. “Concrete structures exposed to future freeze-thaw cycles or roads and bridges exposed to salt should achieve at least 3,500 PSI (24.5 MPa) compressive strength before removing heat and protection.”
The revised specification advises against extreme temperature swings when placing concrete in cold weather. “Forms should be warmed to within a minimal temperature differential to that of the concrete poured,” adds Lickel. On the back end, ACI 306R-10 recommends that the heat from external heating sources be reduced gradually to ambient temperatures before removing the protection.
Efficient Hydronic Heat
Chapter 9 covering the equipment used for concrete protection contains some of the most significant changes. With regards to external heating sources, it more clearly defines combustion heaters by breaking them into direct fired and indirect fired types.
Because of the combustion by-products generated by direct fired heaters, they are not suitable for cold-weather concreting applications without first protecting the concrete. “These by-products can react with the concrete and cause it to ‘chalk’,” mentions Ken Cannella, product manager for Wacker Neuson Climate Technology.
The use of air heat requires construction of costly enclosures, which must be heated in addition to protecting the concrete. “On average, it costs $0.19 per ft2 to build and heat an enclosure, and this method often delivers uneven temperatures and the possibility of cracking if the heat is not controlled properly,” adds Cannella.
The newly added hydronic heating method offers contractors a much more efficient and cost-effective solution for cold-weather concreting. “Using a Wacker Neuson hydronic heater will typically cost 50% less than traditional heat protection methods,” says Jaroszewicz.
With Wacker Neuson hydronic heaters, a heated glycol/water solution pumps through flexible hoses placed either on top of a vapor barrier, attached to formwork of walls and columns, or positioned inside the concrete. These flexible heaters also thaw frozen ground and warm forms prior to concrete placement.
The heaters come standard with a digital temperature controller and push-button adjustment for easy control over fluid output temperature. This allows contractors to adjust heating temperatures throughout the curing process to avoid temperature extremes.
Insulating blankets are used to direct heat directly into the concrete. Hydronic system heat transfer fluids are 970 times denser than and offer more than 6 times the specific heat of air. BTUs are not wasted heating enclosures and, therefore, hydronic systems deliver high BTUs at very low temperature differentials between the transfer hose and concrete.
The revised ACI 306R-10 specification recommends that temperatures throughout the concrete remain as uniform as possible. “The specification can be interpreted that concrete structures should not have differentials greater than 25° F (14° C) anywhere within the structure,” comments Jaroszewicz. “Hydronic heat produces more even temperatures with differentials in the range of 10° F (5.5° C).”
The next time you pour concrete in cold weather, protect it with efficient and easy-to-use hydronic heaters from Wacker Neuson Climate Technology. The line includes 6 models offering curing capacities from 2,200 to 30,000 ft2.