Cold Weather Requirements for Structural Concrete

Cold weather concreting can be challenging. In many parts of the world, cold weather concreting occurs 4-8 months each year. In these regions batch plants and contractors simply adjust their procedures to the cold weather as a part of doing business where they work.

I received a call from a contractor in a southern state that had a tight production schedule. The crew was faced with an unexpected cold snap requiring them to implement cold weather concreting procedures. In this case, the contractor had to scramble to find curing blankets to protect the placed concrete during curing. This resulted in unexpected cost overrun and costly delays.

Fortunately there’s help preparing for a cold weather pour. ACI 306 (Guide to Cold Weather Concreting, ACI 306R) provides the best practices based on information from experts. The Guide is not written in mandatory language, thus does not provide the requirements that each party involved in a cold weather placement must do.

So, in this paper, I will discuss the codes (ACI 318-19 “Building Requirements for Structural Concrete”) and specifications (ACI 301-20 “Specifications for Concrete Construction”) that are required for cold weather concreting. These documents are written in mandatory language and describe what the batch plant operator, contractor(s) and the testing laboratory/ inspection agency are required to do. I will discuss each one separately.

What Is Cold Weather Concreting

ACI provides a definition of the ambient conditions for cold weather concreting in ACI 306R. “The conditions of cold weather concreting exist when the air temperature has fallen to, or is expected to fall below, 40°F (4°C) during the protection period. The protection period is defined as the amount of time recommended to prevent concrete from being adversely affected by exposure to cold weather during construction.”

This definition is very straightforward and eliminates much of the guesswork out of knowing if cold weather concreting conditions apply. It does require that the contractors and batch plant operators must monitor the weather conditions. If the batch plant or the contractor are not prepared to do cold weather concrete, it is best to delay the concrete placements until the weather stays above 40°F.

Batch Plant Operator Requirements

ACI 301.4.1.4.2 Aggregates—Store and handle aggregate in a manner that avoids segregation and prevents contamination by other materials or other sizes of aggregates. Store aggregates in locations that permit them to drain freely. Do not use aggregates that contain frozen lumps.

AC1 318.26.5.5.2(a) Adequate equipment shall be provided for heating concrete materials and protecting the concrete from freezing or near-freezing weather

AC1 318.26.5.5.2(b) Frozen materials or materials containing ice shall not be used.

Cold weather batching issues often revolve around the fine aggregates. Sand will normally have a moisture content from 2-6 % and when it freezes it can have frozen lumps. The batch plant can solve these issues by storing the aggregates in heated areas. In Mongolia (with cold weather about 8 months a year), they will cover the stockpiles with tarps and blow heat under the tarps to keep the sand from freezing. In extremely cold weather (below 0°F) most Mongolian batch plants close.

ACI 301.4.2.2.5(a) Minimum temperature—If the average of highest and lowest ambient temperature from midnight to midnight is expected to be less than 40°F for more than 3 successive days, deliver concrete to meet the following minimum temperatures immediately after placement unless otherwise specified:

Temperature of concrete as placed shall not exceed these values by more than 20°F. These minimum temperature requirements may be terminated if temperatures above 50°F occur for more than half of any 24-hour duration.

This requirement means the batch plant must coordinate the placement with the contractor, the weather forecasters, and maybe even a crystal ball. The batch plant must look 3 days into the future and determine if the weather is going to have an average highest and lowest temperatures that are likely to be less than 40°F. The batch plant must have accurate weather forecasting information and luck that the weather forecasting information is accurate.

This specification also requires the batch plant to know exactly what members the contractor is casting since the required minimum temperatures depend on the size of the member being cast. This can present a difficulty for the batch plant since they are batching concrete from several different projects with different size members. It would be difficult to adjust the temperatures of each load to meet the contractors need.

An alternative approach is for the batch plant to deliver all batched concrete at temperatures between 55-60 °F. The batch plant would not exceed the above temperature limits by over 20°F for any placements.

ACI 306R also provides formulas that can be used to calculate the temperatures of the concrete mixture at the batch plant. This is a very complex formula, and most batch plants will simply use their previous cold weather experiences to estimate the concrete temperatures. I have written a computer program that can estimate the batched concrete temperatures plus calculate heat loss in transit to the jobsite. If you want this program, please contact me by email.

Once the temperature improves (ACI 301 defines this when over ½ of the day is over 50 °F), the temperature limits for the fresh concrete may be terminated. It should be noted that the word may was used instead of shall. This is likely due to the uncertainty of knowing if the temperatures will remain above 50 °F. Rather than changing procedures for a few warm days, the batch plant may decide to maintain their cold weather batching procedures until temperatures above 50°F are routine.

Contractor Requirements

ACI 301. 3.3.2.8(a) Cold bending—Reinforcing bar sizes No. 3 through No. 5 may be bent cold one time, provided reinforcing bar temperature is above 32°F.

This requires that the contractor measures the temperature of the reinforcement in cold weather that must be field adjusted prior to placement or if they are making stirrups or ties on the jobsite. In some cases, they may need to heat the reinforcement prior to bending or bringing it back to its correct location. Note: this requirement only applies to reinforcements No. 3 thought No 5. I recommend that they use the infrared thermometer for these temperature measurements. It is simple and easy to use. WOC 360 has previously discuss the details of selecting and using an infrared thermometer.

ACI 301.5.3.1.3 (b) Remove snow, ice, frost, water, and other foreign materials from surfaces against which concrete will be cast, and from reinforcement and embedded items.

If snow and ice are allowed to remain in the concrete placement, it would slow the strength gain, increase the amount of water in the mixture and possibility create voids in the concrete member. All these situations should be avoided.

ACI 301. .3.2.1(b) Cold weather—Concrete temperatures at delivery shall meet the requirements of 4.2.2.5 (a). Unless otherwise specified, massive metallic embedded items in concrete, including piping and conduit equal to or less than 1/4 in. wall thickness and individual or bundled metallic embedded items with a cumulative cross-sectional area less than 4 in.2, shall be above 10°F. Maintain formwork contact surface temperature above 10°F and insulate or heat formwork as required to protect concrete from freezing. Maintain contact surface temperature above 32°F if placing concrete in contact with ground, subbase, or mud mat.

AC1 318.26.5.5.2 (c) Forms, fillers, and ground with which the concrete is to come in contact shall be free of frost and ice.

AC1 318.26.5.5.2(a) Adequate equipment shall be provided for heating concrete materials and protecting the concrete from freezing or near - freezing weather

The reference ACI 301.4.2.2.5(a) minimum temperature also applies to the contractor as well as the batch plant. This requires the concrete as placed also meets the minimum temperature requirements. The contractor needs adequate equipment to handle the concrete and keep it within the required temperature ranges during the placement.

The contractor is also required to measure the temperatures of most metallic embedded items to be more than 10°F plus make sure the ground subbase or mud slab (anything the fresh concrete come in contact with) will be above 32°F and free of frost. The infrared thermometer can provide the needed temperature information for compliance to this specification. The contractor would need to do a visual inspection to make sure there is no frost or ice in the placement areas.

ACI 301.1.9.3 Protection from environmental conditions—During the curing period, protect concrete from damage due to weather.

ACI 318.26.5.3.2(a) Concrete, other than high-early -strengths shall be maintained at a temperature of at least 50°F and in a moist condition for at least 7 days after placement except if accelerated curing is used.

ACI 301.5.3.6.6 Duration of curing

ACI 301.5.3.6.7 Thermal protection against cold weather—Maintain in-place concrete temperature to prevent freezing of concrete and to ensure strength development.

This specification is to provide details about the length of time for curing plus when the contractor can be terminated their curing procedure. In cold weather, maintaining the concrete temperature above 50 °F and keeping the moistures in the concrete may be a challenge. These requirements also point out that the termination of curing before 7 days is not an arbitrary decision. It must be based on test results that the concrete has achieved some verifiable concrete strength.

ACI 301.5.3.6.2(a) Fogging—Provide fogging equipment for complete coverage of area to be cured. Maintain visible water sheen without accumulation of standing water on concrete surface until final setting of concrete. Do not use fogging if temperature of air in contact with concrete is at or below 32°F.

Fogging will lower the temperature of the concrete surface thus concrete strength gain would be slower and the surface of the concrete could freeze. Obviously, fogging would not be allowed if the air temperatures are below 32 °F.

Testing Laboratory/ Inspection Agency Requirements

ACI 301.1.7.3.3(d) For each concrete mixture cast in one day, obtain samples of fresh concrete in accordance with ASTM C172/ C172M. Truckloads or batches of concrete will be sampled on a random basis. Unless otherwise specified, at least one composite sample will be obtained for consecutive 150 yd3 of concrete or 5000 ft2 of surface area of slabs or walls, or fractions thereof. At least five strength tests for a given concrete mixture will be required for Work. If total quantity of a given concrete mixture is less than 50 yd3 , strength tests may be waived. Sampled concrete used to mold strength test specimens (ASTM C31/C31M) will be tested for slump (ASTM C143/ C143M) or slump flow (ASTM C1611/C1611M), air content (ASTM C231/C231M or ASTM C173/C173M), temperature (ASTM C1064/C1064M), and density (ASTM C138/ C138M).

This specification incorporates ASTM C31 (Standard Practice for Making and Curing Concrete Test Specimens in the Field). This requires that concrete cylinders with design strength below 6000 psi are to be maintained between 60-80 °F; concreted cylinders with design strengths above 6000 psi are to be maintained between 68-78 °F while in the field. The cylinders are also to be protected from direct exposure to sunlight. The maximum and minimum temperatures of the concrete cylinders while in the field is to be recorded. This is best accomplished by a max/min thermometer placed with the cylinders.

I often hear complaints that the cylinders are not being maintained within the required temperatures on the jobsite and that testing laboratories are not using max/min thermometers. The testing laboratories in these cases are nor providing documentation that the cylinders receive proper curing. This lack of attention of keeping the cylinders within the required temperature ranges can have reduce the 28-day cylinder strength and in some cases initiate costly and unnecessary additional strength evaluations and project delays.

There are commercially available curing boxes that make sure the concrete is maintained at the correct temperatures. In some cases, the laboratory maintains the required temperatures by moving the cylinders into a heated space or an on- site laboratory immediately after they are cast.

The laboratory should be careful about using homemade curing boxes. On one project, the laboratory used an insulated box with a light bud as a heat source. The following day when they removed the cylinders, the max/min thermometer was broken (temperatures in the curing box exceeded 120 °F). These cylinders had 30% lower compressive strength that companion cylinders that had received proper curing.

ACI 318.26.13.2.2(d) Inspection reports shall document: Concrete temperatures and protection given to concrete during placement and curing when ambient temperatures fall below 40°F or rises above 95°F.

The testing laboratory or the inspection agency that prepares the inspection reports are required to document what the contractor is doing when the ambient( air) temperatures are below 40 °F. This provides a record of what exactly what the contractor has done during cold weather concreting.

As I stated in the beginning of this paper, cold weather concreting can be challenging for all parties involved. A successful placement requires all people to work together.

Luke M. Snell is a Concrete Consultant Concrete and an Emeritus Professor of Construction from Southern Illinois University Edwardsville. He has done extensive consulting work on construction and concrete problems throughout the U.S. and internationally. He has also written over 400 articles on concrete, construction materials, and construction education. Snell is an Honorary Member of ACI. He is a licensed Professional Engineer in Missouri and Illinois.

References:

ACI Committee 318, “Building Code Requirements for Structural Concrete (ACI 318-19)”
American Concrete Institute, Farmington Hills, MI, 2019

ACI Committee 301 “Specifications for Concrete Construction (ACI 301-20)”
American Concrete Institute, Farmington Hills, MI, 2020

ASTM C31 “Standard Practice for Making and Curing Concrete Test Specimens in the Field”
ASTM International, West Conshohocken, PA, 2019

Committee 306 “Guide to Cold Weather Concreting (ACI 30R-10)”
American Concrete Institute, Farmington Hills, MI, 2020