Cold Weather Concrete

What is Cold weather concrete?

 Cold weather is define as a period  when for more than three consecutive days the average daily temperature is less than 40 F and the air temperature is not 50 F for more than one-half  of many 24-hour period these condition warrant special   precaution when placing, finishing, curing and protecting concrete against the effect of cold weather. Since weather condition can change rapidly in the winter months, good concrete practices and proper planning are critical conditions at a jobsite – hot or cold, windy or calm, dry or humid – may be vastly different from the optimum conditions assumed at the time a concrete mix is specified, designed, or selected – or from laboratory ..

Explanation 

Cold-weather concreting” as the operations concerning the placing, finishing, curing and protection of concrete during cold weather. More specifically, it defines “cold weather” as a period of three or more successive days during which the average daily outdoor temperature drops below 40 degrees F (4 degrees C)and the air temperature is not greater than 50 F (10 C)for more than one-half of any 24-hour period.

Cold-weather concreting practices need to be addressed in order for precast manufacturers to produce quality products that meet specifications. Problems associated with cold-weather concreting are freezing of concrete at an early age; lack of required strength; improper curing procedures; rapid temperature changes; and improper protection of the structure consistent with its serviceability.

By observing a few principles, these problems can be avoided. Use discretion when deciding what is sufficient for dissimilar applications. What works for one application may not be the best for another, but generally these principles will help you make quality precast products during cold weather. The main principles that should be defined are as follows:

Concrete that has attained a compressive strength of 500 psi or more and has been protected from freezing during this period will not be affected by a single freezing cycle.

Concrete described above will be able to establish its potential design strength even if exposed to further cold weather. This also means that there is no need for further protection of the concrete.

Design strengths which must be attained in a short time span (a few days or weeks) must be sheltered at temperatures above 50 F (10 C).

Little or no added external moisture is needed for curing during cold weather, unless located in a heated enclosure.

Take special precautions when using calcium chloride as an accelerator (hardening and setting); especially when the concrete contains embedded metals. (See ACI 318 for acceptable limits of calcium chloride in reinforced and non-reinforced concrete members.)

When faced with cold-weather concreting situations, the manufacturer must decide whether it is profitable to operate during this period of time or whether it makes more sense to wait until warmer weather. Statistics have shown that the cost of adequate cold-weather concreting is not extreme when considering the products manufactured. If one decides to follow through and manufacture precast products during cold weather conditions, do not take any shortcuts. This will ensure that the products are of the highest quality.

Preparation before concreting

To make a quality precast concrete product during cold-weather conditions, preparation is key.

Temperatures of surfaces in contact with fresh concrete

Preparation for cold-weather concreting primarily consists of ensuring that all surfaces in contact with early aged concrete are above temperatures that will cause early freezing or draw out the setting time. As long as the surfaces that come in contact with the early aged concrete are a few degrees above freezing and within 10 F (-12 C) of the minimum required placement temperatures, you should be OK.

Removal of ice and snow

Remove all ice from the aggregates. Failure to do so can ultimately disrupt the water content in the mix design. If the concrete’s temperature is too high (in cold weather, this is hard to accomplish), then crushed ice may be added to the mix. The volume of the ice should not replace more than 75 percent of the batch water. The maximum temperature reduction from the use of ice is limited to about 20 F (-7 C). If you add ice to the mixing water, then the ice must be completely melted by the time mixing is complete.

Structural concrete requires a higher level of design strength than non-reinforced concrete. Cold-weather concreting requires more protection beyond the minimum requirements shown in Table 5.1. For structural members, the requirements change in the duration of the removal of forms and shores. The difference is that removal of forms from structural concrete is now solely based on the in-place strength and not the duration for which they are secured.

Testing of field-cured specimens

To ensure that the concrete is holding up to standards, testing should be administered to concrete cured in the field. Test for in-place strength before the forms are removed and curing takes place. Testing should conform to ASTM C 31 standards.

In-place testing

Non-destructive strength testing can be performed on concrete that is cured in place and in the field using hand-held, portable instruments. Methods such as the pullout test (ASTM C 900) and the probe penetration test (ASTM C 803) is the most common.

Attainment of design strength

Ultimate design strength is attained when certain factors come together to produce a structurally sound member. One of the main reasons that concrete does not meet its specified design strength during cold weather is due to improper or lack of curing. Concrete must be cured for a specified amount of time in order to meet design strength. Tests have shown that when concrete specimens have been removed from the curing process before the required duration, full design strength is jeopardized. In cold-weather concreting, precast structural members need to gain a sufficient high early strength so that they will be protected from exposure to freezing weather.

Holding early strength

The most important factors when determining when to remove forms and shores are those that affect strength development. These are concrete initial placement temperature, type of cement, temperature at which the concrete is maintained after placing, type of admixtures and accelerator admixtures (if any), and curing and protection. There are many instances when accelerated manufacturing of precast members is essential to running an efficient plant, and in this case the duration of protection may be reduced.

Acceleration of setting and strength development

It is permissible to expect shortening of the required strength setting times by the addition of cement, type of cement or admixtures. This is often used in cold-weather concreting because it shortens the protection period, allows forms to be reused more quickly and requires less labor to finish the work. Accelerated setting can also raise the heat of hydration, which could come in handy when keeping the concrete’s temperature up to the required temperature. Additional information on accelerated setting can be found in ACI 212.

Admixtures

 There are many accelerating admixtures available. A couple of the most common accelerating admixtures used today are calcium chloride and those that conform to Type E (ASTM C 494). Calcium chloride, which reduces the setting time and increases the rate of early-age strength development, is a very popular accelerating admixture. It must be limited in use, since too much can be detrimental to the integrity of the concrete. ACI 318 defines the calcium chloride limits and regulations. Some Type E accelerating admixtures have also been found to increase strength gain and accelerate setting time.

Types of heater using in cold water concrete:

Three types of heaters are used in cold-weather concrete construction:

1.              Direct fired
2.               Indirect fired
3.               Hydronic systems

To avoid carbonation of fresh concrete surfaces, indirect-fired heaters should be used. If the concrete is not exposed to the heater or exhaust directly, then a direct-fired heater is suitable. Caution should be taken to ensure that workers are not overexposed to carbon monoxide anytime a heater is used inside an enclosure. Hydronic systems transfer heat by circulating a glycol/water solution in a closed system of pipes or hoses. Typical applications for hydronic systems include thawing and preheating subgrades and heating areas that are too large to be practical for an enclosure