By Phillip Cutler, P.E., and Evan Gurley
For precast concrete producers in the northern latitudes, it is once again that time of the year when Mother Nature blows in her frigid temperatures. It is also the time when
smart precasters are prepared with NPCA-recommended cold-weather procedures, requirements and practices to counteract any adverse effects of cold weather on production.
For manufacturers in the more temperate lower latitudes, cold-weather concreting is not so much of a concern, but it is always better to be prepared than to be caught out in the cold.
What is “cold” for concrete?
Precast producers can find valuable information on cold-weather concreting in ACI 306 and the “NPCA Quality Control Manual for Precast and Prestressed Concrete Plants,” which define the conditions and operational procedures concerning the placing, finishing, curing and protection of concrete during winter weather. Cold weather is defined as a period of three or more successive days during which the average daily outdoor temperature drops below 40 F and the air temperature is not greater than 50 F for more than one-half of any 24-hour period. Remember, the referred temperature is the ambient temperature of the casting environment, and the temperature of concrete at the time of placement must not be less than 45 F.
Cold-weather concreting practices need to be implemented in order for precast manufacturers to ensure quality production. The most common problems associated with cold-weather concreting are:
1. Freezing of concrete at an early age;
2. Lack of required concrete compressive strength;
3. Improper curing procedures;
4. Rapid temperature changes; and
5. Improper protection of the structure consistent with its serviceability.
Concrete that freezes before its compressive strength reaches the minimum of 500 psi shall be discarded. Suitable precautions shall be taken in cold weather to prevent concrete from freezing, including:
• Heat the mixing water, but not >180 degrees F.
• Avoid using frozen aggregates.
• Heat the forms prior to and after casting.
• If the concrete does not freeze and no heat is applied, do not strip the product until adequate strength is attained.
• Monitor and record concrete temperatures during curing.
Most important cold-weather considerations
Among the most important cold-weather production considerations are the following:
• Concrete must attain a compressive strength of 500 psi or greater and must be protected from freezing until this minimum strength level is met. Concrete attaining ≥500 psi in strength will not be adversely affected by a single freezing cycle according to industry literature.
• Concrete strength described above will be able to attain its potential design strength even if the curing concrete is subsequently exposed to cold weather. This also means that there is no need for additional cold-weather protection of the concrete.
• When design strengths must be attained in a short time span (a few days or weeks) concrete must be adequately protected at temperatures ≤50 degrees F.
• Little or no added external moisture is needed for concrete cured during severe weather conditions, unless fabrication takes place in a heated enclosure.
• Take special precautions when using calcium chloride (CaCl2) as an accelerator (a chemical found in some hardening and setting agents), especially when the concrete contains embedded reinforcement metals (see ACI 318 for acceptable limits of calcium chloride in reinforced and non-reinforced concrete members). Note that other accelerating admixtures are readily available that do not contain CaCl2.
The following are excerpts from Chapter 4.5, Curing Concrete, of the “NPCA Quality Control Manual.” This chapter presents the required curing procedures, temperatures and plant requirements for curing in cold weather. The specifics of your production facility and project requirements will play a major part in the curing procedures that are applicable to the products you manufacture.
4.5.1 – General
Effective curing must begin as soon as casting is completed. If concrete is cured with steam or radiant heat, curing procedures must be established and records kept of the temperature of the concrete and the ambient environment during the curing period (see Section 4.5.3).
4.5.2 – Curing by Moisture Retention
Preventing moisture from evaporating from the exposed surfaces of precast and prestressed concrete elements shall be considered an effective method of curing, provided the concrete temperature is ≥55 F. If the concrete temperature is <55 degrees F and >35 degrees F and moisture evaporation is prevented, the curing period must be extended. Forms shall be considered effective in preventing evaporation from the contact surfaces. The use of a membrane-curing compound applied thick enough to prevent evaporation of moisture shall also be considered an effective curing method. Local regions and ambient temperature and humidity conditions will influence the need for curing with heat combined with moisture.
4.5.3 – Curing with Heat and Moisture
Concrete shall not be subjected to steam, hot air, or other means of accelerated curing until after the concrete has attained its initial set. Record the initial set of the concrete (according to ASTM C403 ) at a minimum of once per month when heat-curing. Steam, if used, shall be applied within a suitable enclosure that permits free circulation of the steam. If hot air is used for curing, precautions shall be taken to prevent moisture loss from the concrete; these requirements do not apply to products cured with steam under pressure in an autoclave.
The ambient curing temperature (for both wet-cast and dry-cast products) shall be monitored and documented at a minimum of once per week, when using accelerated curing with heat and moisture. The plant shall then establish an ambient curing cycle that ensures that the ambient curing temperature does not exceed 150 degrees F unless measures are taken to prevent delayed ettringite formation (DEF). In addition, the rise in ambient curing temperature shall be limited to ≤40 F per hour.
Gas-fired heaters shall not be used to directly heat exposed concrete surfaces due to the risk of severe carbonation of the concrete.
188.8.131.52 – Curing with Heat and Moisture – Prestressed Elements
• The initial set of concrete must be established in accordance with ASTM C403. Accelerated curing shall not commence until concrete has achieved its initial set. See Section 4.5.3 of the NPCA QC Manual for requirements for monitoring concrete temperature. The maximum curing temperature for prestressed concrete must be ≤150 degrees F. In addition, the maximum rise (change) in ambient curing temperature must be ≤40 degrees F per hour, and there must be no temperature gradient (change) >40 degrees F within the structural member.
• Maintain curing until the required transfer strength is achieved.
• Do not direct steam jets or other heated air directly onto the forms or the prestressed member. Ensure full circulation of induced heat between the forms and the enclosure.
4.5.4 – Plant Requirements
1. If products are cured with heat and moisture to accelerate concrete strength gain, the ambient curing temperature shall be monitored during the curing period at least once per week. Plants shall maintain temperature records.
2. If heat curing is used, the necessary initial-set period shall be determined per Section 4.5.3 of the NPCA QC Manual.
3. Products cast outdoors or in dry conditions shall be protected from moisture loss by application of a curing compound, moist curing or impervious sheeting.
4. The QC Inspector shall ensure that proper curing procedures are followed and inspect the exposed concrete surfaces of stripped products for evidence of plastic cracking; any observed damage shall be documented.
When we think of curing concrete, the first thing that usually comes to mind is that we need to prevent excessive moisture loss. In cold weather, however, drying is not as critical a concern as preventing the saturated concrete mix from freezing before reaching the 500-psi-strength threshold. The best approach is to carefully consider both temperature and moisture retention.
Use discretion when deciding which cold-weather precautions are sufficient for dissimilar applications; what works for one application may not fit another. In general, the procedures and prescriptions presented in this article will help you manufacture quality precast products during colder weather.
Phillip Cutler, P.E., is director of Technical Services, and Evan Gurley is a technical services engineer with NPCA.