1. The average daily temperature is less than 40 F (5 C), and
2. The air temperature is not greater than 50 F (10 C) for more than one-half of any 24-hour period.

This definition may tempt you to define 40 F (5 C) as the cold-weather concreting threshold temperature. But the conservative precaster will focus on Part 2 of the ACI text. After a careful read, you will note that cold weather can occur even if the temperatures rise above 50 F (10C). And this is when cold-weather problems can start.

Cold weather problems
It is important to understand how cold-weather conditions affect concrete, because after cold-weather damage, concrete can never be fully restored. The main problems during cold weather are:

• Inadequate strength gain associated with low concrete temperature
• Delayed initial set
• Fluctuations in workability
• Concrete damage due to freezing at early ages
• Surface evaporation and heat loss

Concrete Temperature. Recall from Part 1 in this series that during hot weather, concrete can experience a detrimental rise in temperature and result in lower-than-expected ultimate strengths. Since the hydration process, like many chemical reactions, inherently produces heat (contributing to hot-weather damage), it may seem that cold weather does not adversely affect concrete at all. Although intuitive, the notion is deceptive.

Figure 2 shows the effect of temperature conditions on the strength development of concrete. The lower curve represents concrete cast at 25 F (-4 C). Both curves represent concrete cured at 100 percent relative humidity for 28 days followed by 50 percent relative humidity curing. At 28 days (along the dashed vertical line), the concrete cast at the higher temperature reached nearly four times the strength as the concrete cast at the lower temperature. Although a later increase in curing temperature can help increase concrete strength, ultimate concrete strengths can never increase to levels achieved by concrete cured at ideal temperatures (approximately 70 F) at early ages.

Initial Set. Temperature affects the rate at which hydration of cement occurs – that is, at low temperatures hydration occurs very slowly. Consequently, low ambient temperatures retard the hardening (set time) and strength gain of concrete (see Figure 3).

Recent research has confirmed the effects of casting temperature on initial set for varying cement types. The time for concrete to reach initial set increases as the casting temperature decreases. In cold weather, an approximate 30 percent to 35 percent increase in set time can be expected for each 10-degree F (5.5-degree C) drop in ambient temperature, as seen in Figure 4. This, in turn, can lead to problems with handling, consolidating and/or finishing.

Workability. Decreases in temperature will greatly affect the workability of a mix, which is indicated by changes in slump. There is an approximately 0.8-inch (20-mm) increase in slump for every 20-degree F (11-degree C) decrease in concrete temperature. Figure 5 illustrates this point. It shows a mix designed with a known slump at 73 F (23 C). If cured at 50 F (10C), the same mix, according to the graph, will exhibit a slump 1.5 to 2 times greater at 50 F (10 C), depending on cement type.

Freezing at early age. Cold weather effects on concrete temperature, initial set and/or slump will be the least of your worries if fresh concrete is allowed to freeze. The ultimate strength of concrete can be reduced by up to 50 percent if it freezes soon after placement, usually before 24 hours and before reaching a strength of 500 psi (3.5 MPa). Surprisingly, studies have shown that strengths can be restored if fresh concrete undergoes only one freeze-thaw cycle and proper curing is immediately restored and maintained. However, restored strength is not an indication of restored quality. When the water within a plastic mix freezes, the ice leaves impressions in the paste (see Figure 6), disturbing the matrix and increasing porosity. Consequently, freezing also results in decreased watertightness regardless of subsequent temperature increases and curing methods. Concrete frozen at an early age will also exhibit less resistance to weathering and the product’s durability will be compromised.

Evaporation. Although ACI does not include wind in its definition for cold weather, concrete exposed to drafts can still be compromised. Similar to the effects of wind during hot weather, wind during cold weather can encourage evaporation of mix water. During cold weather, the heat of hydration of cement helps to heat and humidify the air immediately surrounding the surface, especially before initial set. Wind will blow this heat and humidity away, causing surface drying and potential cracking.

The detrimental effects of cold-weather on concrete cannot be refuted. Obviously, you must take precautions to preserve and maintain the integrity of the concrete. However, preventive efforts to combat cold weather problems should start long before you begin batching and mixing.

Precautions during cold weather
Having a strong understanding of fresh concrete’s behavior under extreme conditions will help you prepare for cold-weather conditions. There are a number of precautions and techniques you can use to ensure success when cold weather strikes.

Control Mixing Temperature. The colder the ambient temperature, the more difficult it is to maintain a constant concrete temperature. Although it is possible to produce quality concrete at lower temperatures, Figure 7 offers some temperature guidelines.

Minimum mixing concrete temperatures are listed on Lines 2-4 for various ranges of ambient air temperature. The actual temperatures should not exceed the listed values by more than 15 F (8 C). The mixing temperature can be increased by heating the aggregates and/or heating the mix water. Typically, it is difficult to heat aggregates evenly – consequently, when aggregates are free of ice and snow, it is usually sufficient to heat only the mixing water. Ideally, aggregates should be stored underground or in a building where temperature and humidity can be controlled.

Since the temperature of concrete affects the rate of slump loss and the efficacy of various admixtures, use caution when increasing concrete mixing temperature. When using heated mixing water, the water temperature should be the same from batch to batch to ensure consistency and performance predictability. Only if absolutely necessary, adjust the batching sequence to introduce the cement after adding hot water. Also, rather than adding hot and cold water intermittently, regulate the heated mixing water to prevent the addition of very hot water that can cause flash set.

ACI 306 recommends heating only the mixing water unless ambient temperatures consistently fall below 25 F (-4 C). Typically, if coarse aggregates are free of ice and snow, the only the sand’s temperature needs to be increased.

Preparations Before Concreting. All surfaces in contact with concrete should be protected from freezing for at least 24 hours before placement, including embedded items. Embedded items, especially reinforcement, at temperatures below freezing can cause localized freezing of concrete. All surfaces in contact with concrete should be kept between 35 F (2 C) and 10 F (5 C) higher than the minimum placement temperatures listed on Table 3.1 (Figure 7). Consequently, embedded items should be kept indoors for as long as possible, or heated enclosures should be prepared to heat the items in contact with concrete before the concrete arrives. In certain instances, insulating blankets are also useful.

Control Placement Temperatures. Line 1 on ACI 306 Table 3.1 (Figure 7) lists recommended concrete temperatures at the time of placement (after mixing). If the as-placed values (Line 1) are impractical, it is important to not exceed these temperatures by more that 20 F (11 C). Of course, it is best to keep concrete at the recommended temperatures, because concrete that is placed at the temperatures listed on Table 3.1, protected from freezing and receives long-time curing, has been shown to develop higher strengths and better durability. The concrete temperature should be controlled for a length of time (called the protection period) specified by ACI 306 Table 5.1 (see Figure 8).

As indicated, the protection period can be reduced by using Type III cement, using an accelerating admixture or increasing cement content. The recommended protection period assumes that the concrete is air entrained – that is, all concrete cast during cold weather (as defined by ACI 306) should be air entrained even if the product will not be exposed to freeze/thaw cycles during service. This is because if cast during cold weather, precast concrete products may potentially experience freeze-thaw cycles before or during installation. Concrete that is not air-entrained should only be cast in cold weather if it is specifically required by specification. In such cases, protection periods should be doubled at a minimum.

At the end of the protection period, allow the concrete to cool gradually to ambient air temperature in order to prevent shrinkage cracking. Slowly reduce the heat source and remove insulation in layers, observing the maximum temperature drops in ACI 306 Table 5.5 (see Figure 9).

Heaters. During the protection period, it is common to use heaters when concreting outdoors. Heated enclosures should also be used and heaters should not point directly onto formwork as this will cause local drying of the product. Flue gases should be vented outside of the enclosure, since carbon dioxide can cause carbonation of exposed concrete surfaces. This can result in a porous, sponge-like surface and may pose a safety hazard to workers. This means that direct-fired heaters should not be used for heating products during the curing process.

You should not wait for record-setting cold to make adjustments at the plant. Controlling concrete temperature in cold weather is tricky. Since cold weather conditions can affect slump, initial set and concrete temperature, you should plan ahead. And don’t forget to prevent wind from drying out the concrete surface. Install wind breaks if necessary – your production employees would likely appreciate this as well.