Balancing Risk in Cold Weather Concreting

The goal of the American Concrete Institute (ACI) 306 Guide to Cold Weather Concreting is to give guidance so that concrete placed in cold temperatures will develop sufficient strength and durability to satisfy the intended service requirements when properly produced, placed, and protected.

The concrete industry rule of thumb is that cold weather concrete provisions start when the air temperature is expected to fall below 40°F.

A microscopic view of frozen concrete. Source: Mark Lukkarila.

Objectively, cold weather concreting practices prevent damage due to early age freezing, ensure that concrete develops the required strength, and limit rapid temperature changes.  Even if concrete hydration is not disrupted due to the freezing of water, concrete durability can still be compromised due to a loss of water that causes a contraction of the paste.  The necessary degree of concrete protection increases as the ambient temperature decreases.  This seems obvious but is often neglected.

ACI 306 is used for guidance on how long to maintain concrete at a certain temperature and curing condition to protect it from freezing and rapid loss of water that would undermine its durability.  At some point, concrete strength is high enough to take one freeze/thaw cycle.  Industry standard is 500 psi, but this was likely an off-the-cuff estimation by T.C. Powers in the 1940s.

Guidance in ACI 306 about cold weather concreting must be extracted from tables and equations using inputs of total cement (developed before pozzolans were in widespread use), size of member, and ambient conditions.  The values in these tables are based on work published in 1951 and performed in 1948.  In 1976, curves were changed to reflect a change that concrete should be kept above 40°F instead of 32°F.  Modeling for heat concrete/formwork heat loss was done with Heisler charts before computers.

Contractors, engineers, and owners all want to manage risk.  To have certain success in managing concrete in cold weather, we could enclose all concrete, heat to 80°F, and water cure.  The concrete would never freeze.  To have certain failure, we could place concrete in -25°F with metal forms and no insulation.  Ideally, we want to build somewhere in the middle.

1940s ACI 306 needs to be updated to factor in today’s materials

The problem we have with ACI 306 is that if we follow the rules, we always have good concrete.  If we don’t follow the rules, we often still have good concrete.  This occurs so often that ACI 306 is held in contempt. It is a consequence of the data used to formulate the tables in ACI 306.

There needs to be an update to the cold weather concreting practices recommended in the ACI 306 documents.  Unfortunately, this would require research that attracts little academic interest because the research required to update the recommendations of cold weather construction does not lead to tenure.

How can we change ACI 306?  First, the guidance for duration of covering concrete and minimum incoming concrete temperature to the formwork should reflect the materials we use today.  Cement today produces 6 times as much heat as the cement used in 1948.

Next, ACI 306 should give guidance on heating enclosed areas to avoid dusting, drying, and carbonating of concrete surfaces and monitoring carbon monoxide and dioxide levels.

Third, ACI 306 is full of tables focused on estimating the concrete temp.  These assumptions are based upon 100% cement concretes from the 1940’s that do not include pozzolans or admixtures. Instead, we should have guidance on heat development based on water content in concretes, modern admixtures, and concretes with proportions of fly ash, slag, and silica fume.

For example, water has a high heat capacity (5 times larger than the heat capacity of the aggregate and cementitious material).  It takes more heat to change the temperature of a pound of water by 1 degree than to change the temperature of a pound of aggregate by 1 degree.  Concrete low in water content is more susceptible to cooling because the water is not there to moderate heat loss.

Some of the data to revise 306 guidance is available but has not been collated and made into a form useable by practitioners. Some of the data needs to be developed in order to facilitate ACI 306’s rewriting.

This post is based on a Minnesota Concrete Council (MCC) presentation given by Kevin MacDonald on 9-13-18.