Cold weather concreting is becoming a topic that has the potential to substantially affect nearly every contractor in the CFA. This past year alone, members in Ohio experienced the frustration that can develop when misinterpretation of the ACI requirements for cold weather occurs. Based on this and similar occurrences over the past few years, the CFA has taken an aggressive position on analyzing, testing and producing guidelines for the poured wall foundation industry. This position is one that seeks to answer the age old question, “When is it actually to cold to pour concrete?”

In 2000, the CFA and a team from The Aberdeen Group commissioned a study of concrete cylinders cured at varying temperatures to attempt to answer this question. After many months of testing and analyzing the data from this study, justifiable conclusions were not definitively apparent. In short, it was concluded that additional research on actual wall specimens were required to further substantiate the data that was produced from the laboratory tests.

Recently, the CFA Board of Directors appointed an ADHOC Cold Weather Committee consisting of CFA contractor members, industry professionals, ready-mix suppliers and CFA executive staff to carry out further steps towards a resolution for our industry. The mission for this committee, as read by committee chairman Terry Lavy at the first meeting in Chicago on 6-15-02 is “To investigate the causes, effects, and remediation of extreme weather conditions on the residential Cast-in-place wall industry while working to establish the methods and parameters needed to decide When Is It Too Cold for poured concrete foundations.” The committee is seeking additional members that are interested in the progress towards resolution of this issue on an Association wide basis and feel that they can and wish to contribute to the direction of the committee. Please contact Terry Lavy at 937-773-3963 or terry@lavyconcrete.com for more information.

This committee, however, is not the only effort that is currently being undertaken within the CFA. Many members have committed to developing research and information that can solve this problem on a local level. Scott Smith, Modern Foundations and Frank Ramey, Tri County Excavating recently banded together to prevent inspectors from shutting down pouring of foundations this past winter. Their efforts, centered on mix designs by their local supplier and analysis by a registered engineer, produced immediate results that permitted them to continue with only a mild setback in construction schedules.

The article on cold weather concrete that follows was written by CFA member Kirby Justesen, owner of Formco Foundations in West Jordan, Utah. Kirby has been a member of the CFA since 1978 and has served on the Board of Directors. Kirby does commercial and residential concrete construction.

Objective:

The primary objective in cold weather concreting is to design a concrete mix design with admixtures that will achieve early strength gains to sufficiently protect against damage due to freezing and freeze thaw cycles. The resulting benefit is less effort and expense than with traditional tenting and heating.

Cost effectiveness and productivity can be achieved through better knowledge but the knowledge to accomplish these goals needs to defy conventional understanding. The application of this information will benefit the customer with lower cost yet will maintain the desired results.

Definition:

The definition of cold weather as stated by ACI 306R-78 1.1 is: Cold weather is defined as a period when more than three successive days the mean temperature drops below 40°F. When temperatures above 50°F occur during more than half of any 24 hour period, the concrete should no longer be regarded as winter concrete.

Thesis:

ASSESSING WHEN TO PROTECT THE CONCRETE:

Obviously concrete can resist freezing temperatures and freeze thaw cycles; the question is when does it have the ability to do so. To find an answer we can refer to ACI Cold Weather Concreting 306R-78 1.10.1.

Prevent damage to concrete from freezing at early ages. The degree of saturation off newly placed concrete, which has no access to external sources of water, will be reduced as the concrete hardens and water is combined in the hydration process. Under such conditions, the time at which the degree of saturation becomes reduced below that level which would cause damage by freezing corresponds roughly at the time at which the concrete attains a compressive strength of 500 psi (3.5 Mpa).

At temperatures of 50°F (10°C) most well proportioned concrete will reach this strength during the second day.

This tells us the resistence to early freezing is determined by saturation of water concrete mix. The degree of saturation to be able to withstand damage corresponds roughly to 500 psi which can be achieved quickly by well proportioned concrete.

Further ACI provides a table to understand what application of concrete, compared to the general mix design, will require how many days to protect the concrete against frost damage.

The left column, Service Category, refers to the type of use and whether load capability is needed now or will come later. For footing, foundation, and retaining wall, Category 1, “No load, no exposure,” would apply. See section 6.1.1.

To first includes such items as foundation and substructures which will not be subject to early load, and due to their deep or backfield location, will be subjected to little or no freezing and thawing service. In these circumstances of construction and service, conditions will be favorable to continued natural curing. This concrete would require only the protection time recommended for Category 1 in Table 1.4.2.

In the vertical columns of table 1.4.2 “damage from freezing” and “safe strengths” both right hand columns would apply. “Type III accelerator or extra cement.”

The conclusion is, after one day, footing and foundations cast in place with a mix design containing extra cement and accelerator admixture will be strong enough to resist freezing and freeze cycles. We see the well proportioned concrete mix is most vulnerable to frost damage from it’s earliest age and continues to decrease in vulnerability with time, until 24 hours, at which point conditions will be favorable to continued natural curing.

FIRST 24 HOUR PERIOD:

Understanding that the first 24 hours are critical and that reducing saturation is the key to frost resistant concrete we examine appropriate methods to cold weather protection. ACI 306R-78 5.1.

During the winter, however, when air temperature is below 50°F, atmospheric conditions in most areas will not cause undesirable drying; but, new concrete is vulnerable to freezing in a saturated condition, and therefore, should be allowed to undergo some drying before being exposed to freezing temperatures.

In contemplating the type of protection for the first 24 hours, first, consider drying to reduce saturation and second, allowing for adequate protection against the forecast low temperature. ACI further states in chapter 4 entitled “Protection” ACI 306R-78 4.2

Since most of the heat of hydration of the hardening cement in developed during the first three days, no heat from outside sources may be required to maintain concrete at correct temperatures if heat generated in the concrete is suitably conserved.

Emphasis should be placed on conserving the heat of hydration with out introducing heat from outside sources. Outside heat sources can produce carbon dioxide which reacts with calcium hydroxide to form a chalky calcium carbonate on the concrete surface. Forced air may also cause excessive below.

TYPES OF PROTECTION

Types of cold weather protection found in ACI 30678-R Chapter 4 include polyethylene plastic, foamed vinyl blankets, and straw. Advantages and disadvantages of each type of protection are listed below.

Cold Weather Protection Plan

MIX DESIGN

Mix Design should include 6 bag to 7 bag full sack mix with hot mixing water, + % to 1 + % non-chlorine calcium, and 6% to 7% air entertainment. The range in bag mix and calcium is increased with expected overnight low temperatures.

CALCIUM CHLORIDE

The best known and accepted accelerator is calcium chloride, but it cannot be used in prestressed or in concrete in which aluminum or galvanized metal is to be embedded. ACI 212 2.2

Accelerating admixtures are useful for modifying the properties of concrete, particularly in cold weather, to: (a) expedite the start of finishing operations and, when necessary, the application of insulation for protection; (b) reduce the time required for proper curing and protection; © increase the rate of early strength development so as to permit earlier removal of forms and earlier opening of the construction for service; and (d) permit more efficient plugging of leaks against hydrostatic pressure.

Some long term strength is sacrificed when using calcium chloride, but is more than offset by additional cement. ACI 212 2.3.4

Compressive strength is increased substantially at early ages. The ultimate strength may be reduced slightly. The increase in flexural strength is usually less than that of the compressive strength.

AIR ENTRAINMENT

Normally footings and foundations are designed with low air entrainment 3% to 4%. ACI 306 R-78 tells us in chapter 1.5, “If it is likely that any concrete will be exposed to freezing in a saturated condition during construction it should be properly air entrained event though it will not be exposed to freezing in service.” Therefore 6% to 7% air entrainment should be used during cold weather.

PROTECTION MATERIAL

Since there will be no load or service required, protection need only be for the first 24 hours. For forecast temperature lows 32°F to 18°F cover unformed surfaces with 4″ loose straw. When precipitation is forecast cover straw with polyethylene plastic. For forecast temperature 18°F to 10°F use foam filled blankets to cover formed and unformed surfaces.

TESTING

Cast one each concrete cylinder, each pour, leave on site under similar protection to represent conditions of concrete pour. Break cylinder at 24 hours, test results should or exceed 500 psi.

Through efforts such as these, and the work of the newly-formed CFA Cold Weather Committee, it is estimated that poured foundations in the extremes of winter will become as common nature if not preferred to pouring during the schedule-packed months of April through September. For more information on coldweather concreting, please contact Jim Baty, Technical Director of the CFA at jbaty@cfawalls.org or toll free at 866-CFAWALL.