MRT | PRODUCTS | GUIDE FOR COLD WEATHER CONCRETING

Products | Guide for Cold Weather Concreting

Cold Weather concrete placement should proceed according to the "cold-weather concreting" guidelines published by the American Concrete Institute (ACI) and the Portland Cement Association (PCA). These guidelines are applicable when air temperature is not above 50º F for at least 12 hr during any 24-hr period and the average daily air temperature is less than 40º F. Concrete must be protected against early freezing until it has attained at least 500 psi to assure strength development and job progress.

Objectives of cold-weather concreting practice are to:

Prevent damage to the concrete due to freezing at early age
Assure that the concrete develops the required strength
Maintain curing that assures normal strength development
Limit effects of rapid temperature changes

Principles of cold-weather concreting practice are:

Protect concrete from freezing until it has attained sufficient strength. Concrete with less than 500 psi is subject to damage by exposure to a single freezing cycle.¹
Concrete protected from freezing will mature to its potential strength despite subsequent exposure to cold weather.
Where a specified strength must be attained in a few days or weeks, protection at exposure temperatures below 50º F is required.

ACI 306R-88¹ defines cold weather for concreting as: " A period when, for more than 3 consecutive days, the following conditions exist: 1) the average daily air temperature is less than 40ºF (5ºC) and 2) the air temperature is not greater than 50ºF (10ºC) for more than one-half of any 24-hr period". Cold weather is any combination of conditions causing concrete temperature to dip to 40ºF (5ºC) or below.

At low temperatures, concrete gains strength slowly. At 50º F, the rate of hydration and strength gain is approximately half the rate of the same mixture at 73º F.

ACI prescribes minimum temperatures for fresh concrete as placed and maintained:

55º F - most slabs, pavements, sections less than 12 in. minimum dimension
50º F - most beams, columns, walls, sections 12 to 36 in. minimum dimension
45º F - large columns, footings, pedestals, mats, sections 36 to 72 in. minimum dimension
40º F - sections over 72 in. minimum dimension

Concrete slump should be lower than normal, particularly for flatwork, to avoid retarding setting. Due to slowed cementing reactions, setting will be slowed. While initial set at 73º F may occur in about 180 minutes, at 50º F, initial set will require over two times as long. Similarly, final set will double. Lower temperatures will delay setting even more. Make sure that the temperature of the sub base and any other surfaces that come in contact with the concrete are not below freezing. Use hot (160 –180º F) water for batching concrete. Never begin final finishing operations while bleed water is present. The lower bleed with high fly ash content mixes may cause the uninformed contractor to begin finishing prematurely.

Curing is one of the most important -- and most neglected -- steps in concrete construction.²

Concrete hardens through the chemical reactions between cement and water. The idea is to keep moisture available, at a favorable temperature, to promote the chemical reactions. Both fresh and newly hardened concrete lose moisture and heat rapidly in cold-weather conditions. Heated insulating blankets over plastic sheeting or chemical curing compounds keep the concrete moist combined with thermal protection keep the temperature above 50º F for 5 to 7 days will generally provide effective curing.

Protectionagainst freezing must be provided immediately after placement. Covering, insulating, housing, or heating of all sections of the concrete must be provided. Freeze protection should be continued for at least 3 days, unless otherwise specified. The length of the protection period depends on the type and amount of cement, whether an accelerating admixture is used, and the service category of the concrete. ACI 306.1R³ provides additional recommendations on the length of the protection period.

Critical areas of edges and corners require extra protection because they are exposed to heat loss in two or more directions.

Acceleration of setting and strength development can be accomplished by careful use of accelerating admixtures or by the use of additional cementitious (cement plus fly ash). Calcium chloride should not be used in conjunction with unprotected reinforcing steel.^4,5 The maximum chloride ion content of concrete is prescribed in ACI 318⁶, and these limits govern where the Code is in use. Some water-reducing accelerating admixtures, conforming to Type E in ASTM C 494, can accelerate setting and strength gain at ambient temperatures of 50º F and below, and also can reduce the required water content of the mixture. Some Type E admixtures contain small percentages of calcium chloride.

Temperature records are desirable and may be required for cold-weather concreting as part of the permanent records on the job.

Concrete test samples (cylinders and beams) must be maintained at a temperature between 60 and 80º F in or on the structure as near to the point of deposit of the concrete represented as possible until they are taken to the laboratory for standard curing. In the initial period, cylinders should be kept in a temperature controlled curing box, or in a temperature-controlled building⁷.

Curing and temperature protection from start to finish should be continuous and uninterrupted until the concrete develops its desired strength. After attainment of strength, remove the protection in such a manner that the temperature of the concrete will not drop faster than 40º F in 24 hours.

References:

Cold Weather Concreting, ACI 306R-88, ACI Committee 306-88 Report, American Concrete Institute, Detroit, MI, 1988
Kosmatka, Steven H. and William C. Panarese, Design and Control of Concrete Mixtures, 13th Ed., Portland Cement Association, Skokie, IL, 1990
Standard Specification for Cold Weather Concreting, ACI 306.1-90
Chemical Admixtures for Concrete, ACI 212.3R-91, ACI Committee 212 Report, American Concrete Institute, Detroit, MI, 1991
Guide to Durable Concrete, ACI 201.2R-92, ACI Committee 201 Report, American Concrete Institute, Detroit, MI, 1992
Building Code Requirements for Reinforced Concrete, ACI 318-89 (Revised 1992, ACI Committee 318 Report, American Concrete Institute, Detroit, MI, 1992
American Society for Testing of Materials (ASTM), Volume 4.02, C 31 Standard Practice for Making and Curing Test Specimens in the field

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