11.2 Cold Weather Concreting Practices & Thermal Protection (ACI 306R)

11.2 Cold Weather Concreting Practices & Thermal Protection (ACI 306R)

Cold concrete generally sets and gains strength more slowly. Protection keeps fresh concrete from freezing and supports strength development. ACI 306R-16 provides guidance for planning, while the governing contract and applicable code set the requirements. Model IBC Table 1705.3 calls for periodic verification of specified curing temperature and techniques.

Identify the applicable cold-weather provision

ACI 306R-16 Section 2.2 describes cold weather by air temperature that has fallen to 40°F or is expected to fall below it during the protection period. An overnight forecast of 35°F while new concrete is protected warrants planning under that definition even during an otherwise warm week.

ACI 301-20 Section 4.2.2.5(a) has a separate default for concrete delivered to meet minimum temperatures immediately after placement when the daily average ambient temperature is expected below 40°F for more than three successive days. It calculates the daily average using the midnight-to-midnight high and low. ACI 306.1-90, reapproved in 2002, also uses a multi-day definition and separately ends that defined period when temperatures exceed 50°F for more than half of a 24-hour duration. Read the particular specification cited. ACI 301-20 also permits its delivery minima to end under that same above-50°F condition. Ending a delivery provision does not establish that already placed concrete can be exposed or loaded.

Select the temperature row and verify site conditions

The minimum dimension of a slab is typically its thickness. Thin members lose heat more readily than larger members. ACI 306R-16 Table 5.1 distinguishes temperatures as mixed, temperatures as placed and maintained, and the recommended limit for gradual cooling in the first 24 hours after protection ends. Values below are in °F. Consult the governing provision for range boundaries and any specified modifications.

Table 5.1 recommends limiting placement temperature to 20°F above the listed placement minimum. Its as-mixed rows have a separate 15°F upper margin. The cold-weather delivery provisions in ACI 301-20 Section 4.2.2.5(a) and the applicable C94 delivery temperature provisions also distinguish mixture temperature and the temperature required at placement. Use the specified conditions and ASTM C1064/C1064M to measure the fresh concrete temperature. A delivery temperature reading does not establish the in-place surface history.

ACI 301-20 Sections 5.3.1.3 and 5.3.2.1(b) give preparation and temperature requirements. Remove snow, ice, frost, water and foreign matter from the relevant surfaces, reinforcement and embedments. Maintain ground, subbase or mud-mat contact surfaces above 32°F where concrete is placed against them. For formwork contact surfaces use the specification's separate above-10°F limit and insulate or heat the formwork as needed to prevent freezing. Check any special metallic-embedment requirements in the governing documents. Frozen supporting material can cool the concrete and may settle as it thaws.

For an 8-inch slab with air at 25°F, Table 5.1 recommends at least 65°F as mixed and at least 55°F as placed and maintained. Before the pour, confirm the protection is available for the slab, including its faster-cooling edges and corners. Reconcile measured conditions with the approved procedures if a receiving surface is frozen or the specified concrete temperature is missed.

Freeze resistance, heat and moisture

ACI 306R-16 explains that under its stated conditions, concrete with strength of at least 500 psi can resist a single early freezing-and-thawing cycle. Section 7.2 gives the example of air-entrained concrete without an external water source. Repeated freezing-and-thawing while critically saturated can still damage concrete below about 3500 psi, and durability conditions can require further protection. The 500 psi benchmark is not a general release strength for formwork, construction loads or termination of all curing. Under ACI 301-20 Section 5.3.6.7(a), the default thermal protection duration is at least three days or until a condition in Section 5.3.6.6(b) is met. Check the separate code, contract, construction load and curing criteria for the planned operation.

Protection methods include insulating blankets or forms, heated enclosures, and hydronic heat. Pay particular attention to exposed edges, corners, closures and gaps where heat escapes. ACI 306R-16 Section 4.5 recommends venting combustion heaters and avoiding direct heating or drying of fresh concrete. ACI 301-20 Section 5.3.1.6 prohibits unvented combustion heaters in the stated cold-weather conditions. Carbon dioxide from exhaust can carbonate the fresh concrete surface, producing softness or crazing. Warm enclosure air can also dry the concrete. Verify the curing moisture is maintained and the heat source does not dry the surface.

Verify cooling and readiness

Measure and record concrete surface temperatures at the prescribed locations and intervals. Use the approved thermal control plan and compare edge and corner temperatures with temperatures in better-insulated areas. At the end of the protection period cool gradually. Under the ACI 301-20 Section 5.3.6.7(b) default, apply the least-dimension cooling limits until the concrete surface is within 20°F of its surroundings. A specified mass-concrete thermal control plan can set a different removal procedure. For an 8-inch slab, a drop from 65°F to 25°F in 24 hours is 40°F, below the 50°F limit. If the surroundings are then 0°F, the surface is still 25°F warmer. Check that condition before removing the rest of the protection.

Standard-cured cylinders represent the concrete's strength potential for acceptance. Follow the edition and the curing conditions covered in Section 3.3. Where the applicable method allows field-cured specimens, they represent their exposure and use case. ACI 306R-16 Section 8.2 cautions against relying on field-cured cylinders to estimate cold-weather in-place strength for removal of construction supports. Use the accepted strength evidence for that operation, such as a calibrated maturity method with the other required verification.

ASTM C1074 maturity uses a laboratory strength relationship for the mixture and the measured concrete temperature history in the member. For the temperature-time factor, multiply each interval length by the mean concrete temperature above the selected datum and add the results. For a mixture calibrated with a 0°C datum, 12 hours averaging 12°C and 12 hours averaging 8°C yield $12(12-0)+12(8-0)=240$ °C·h. Use the corresponding calibrated curve to estimate strength and follow the approved verification process. The index has temperature-time units. It cannot be read directly as psi or applied to a different mixture without supporting evidence.

Record the mix and member IDs, temperature measurements, heating and moisture protection, and supporting strength evidence. Compare the record with the specified release criteria and report any deviation before the affected work proceeds.