Winter has arrived with a vengeance this year and as with any construction season, colder temperatures deserve respect. But in general, concrete contractors shouldn’t overreact. Unfortunately, though, a strong response to the cold is an annual reality as the fear of freezing temperatures affecting concrete performance sets in.
The American Concrete Institute provides guidance for construction practices for cold weather concreting in ACI 306R-10, Guide to Cold Weather Concreting. A general document of recommended practices, ACI 306 covers the entire spectrum of applications for concrete elements and conditions that may be generally described as cold weather. What this document does not provide is specific recommendations for particular concrete elements or regional conditions that make cold weather such a challenging condition to specify.
Cold weather performance
ACI 306 Section 10.4.3 states, “Take advantage of the opportunity provided by cold weather to place low temperature concrete,” then goes on to note that concrete placed at ambient air temperatures of 55° F down to around 40° F has the potential to develop higher ultimate strengths and durability than concrete placed at what might be termed more “normal” or higher temperatures. In this same section, ACI 306 states that this concrete must be, “protected from freezing and properly cured for sufficient length of time,” which infers that responsibility lies with the construction team to achieve this greater performance during cold weather.
So, the primary questions are: When is protection needed, what is proper protection, and for how long does protection/curing need to be provided? This is where specifiers and design professionals overreact, largely due to outdated and ineffective research or misunderstood references, to create overly conservative requirements.
For starters, the introductory statements found in ACI 306R-10 state, “Reference to the document shall not be made in contract documents. If items found in this document are desired by the Architect/Engineer to be part of the contract documents, they shall be restated in mandatory language for incorporation by the Architect/Engineer.” The statement intends to make sure that contractors are not burdened with all of the potential options for cold weather protection that are in ACI 306, but only with the specific requirements that are necessary for the project. A blanket application of the full breadth of guidance options in ACI 306 as mandatory requirements results in the most restrictive possible approach and not necessarily the requirements that are the most appropriate to enforce for a particular project. In most cases, this methodology not only results in excessive construction costs, it can also adversely impact the natural behavior of the concrete that the requirements were intended to protect.
Overly conservative
A primary example of this overly conservative approach would be the discussion in section 4.3 of a suggested temperature range of 40° F to 55° F. Would the conclusion then be that below 40° F, the concrete is subject to freezing and certain damage or to insufficient strength gain if left unprotected? Not only is this assumption unfounded, research in the last 15 years by the Concrete Foundations Association in 2004 shows that the internal concrete temperature must drop to the mid-to-upper 20s before hydration stops and freezing starts.
Every day during winter construction issues arise in the field where a concrete temperature or a curing temperature has dropped a few degrees below the recommended temperature in a table from ACI 306. Somehow, this always seems to become a concern, even when in section 3.2 of ACI 306R-10 the last paragraph states, “Times and temperatures in this guide are not exact values for all situations and should not be used as such. The end-user should consider the primary intent of the recommendations and use adequate judgment in deciding what is adequate for each particular circumstance.”
This is a primary example of how ACI 306 allows for variance in tables that are only recommended tables and encourages intelligent construction practice and experience to determine the appropriate response. But in the field, quality control personnel often believe the table provisions to be absolute and any disregard of such a recommended condition to be an immediate infraction that will lead to imminent failure of the concrete.
ACI 306R-10 is the only construction industry document referenced by specifications, standards, and construction documents for cold weather practice for concrete — a very diverse sector of the construction industry. Within the 26 pages of ACI 306 are multiple tables and recommended practices, some easy to understand, some confusing, and some outdated. Contractors, architects, engineers, suppliers, inspectors, and lab technicians must work together to effectively determine that an actual need to modify a concrete design or implement a certain procedure or protection measure exists before reacting to a sudden concern about colder air temperatures.
