With news of polar arctic weather sweeping through the Midwest and Eastern part of the country, one can’t help thinking about the movie The Day After and wondering if this is just a passing thing or something more serious! With record low temperatures (-20!) bringing many cities to a grinding halt, we thought it would be interesting to find out what are some of the challenges of pouring concrete in cold weather and recommended mitigation methods.
So how cold is cold? According to the American Concrete Institute (ACI) the definition of cold-weather concreting, as stated in ACI 306 is, “a period when for more than three successive days the average daily air temperature drops below 40 degrees Fahrenheit and stays below 50 degrees Fahrenheit for more than one-half of any 24 hour period.” Based on this information and this week’s nationwide temperatures, no construction is happening in most of the country! Or is it?
SPECIAL PLANNING FOR LOW TEMPERATURES
To ensure that concrete placed in cold weather will last a long time, proper planning is required. This includes: selection of cement mix, proper mixing, placing, curing time, finishing techniques and overall protection of the site and concrete. In cold weather, the curing period takes longer because the lower rate of strength gain. Make sure to take into account the transportation time from the plant to the point of placement as it can have a major impact on the temperature of the mix.
If at the time of pouring, temperatures drop below 40-50°F, you can heat the water or the aggregates to reach the desired temperature. Heating cement is not an option!
Concrete operators can also use a low slump concrete for flatwork and lower water/cement ratio mixes because it will reduce the setting time and bleeding water. Selecting a concrete mix that contains admixtures that accelerate or a Type III Portland cement, Hi-Early cement that will greatly help reducing the protection time from freezing. It is especially important to increase the quantity of accelerated admixtures (PolarSet®, DCI®, Daraccel®, Gilco® or Lubricon®) towards the end of the pour to produce a more consistent set and avoid the results of cooler mixing water. It will also speed up the setting time of the last concrete batches so that the entire concrete area can set at the same time.
THE MAIN RISK: PREVENT THERMAL CRACKING
It is essential that every surface that will enter in contact with the concrete is free of frost, ice and snow. Whether it is reinforcement, embeds, forms, fillers, or ground
The first 24 hours after the concrete is poured is the most critical period because it is the usual amount of time that is required to reach its minimal strength of 500 pounds per square inch. If freezing occurs while the concrete is fresh or has not reached this minimal strength, ice will form in the frozen water altering the cement mix which in turn will damage its overall strength. So at 500 psi, the cement is able to resist the expansion caused by freezing water and be safely removed from the forms. As a rule of thumb, an 18°F (10°C) drop in concrete temperature will double the setting time and make it more vulnerable to freezing. Use the maturity method to verify that the concrete has reached the proper strengthening level.
PROTECTIONS TO KEEP TEMPERATURES ABOVE 50F
Some of the most commonly used methods to keep temperatures above 50° Fahrenheit are: evaporation reducers, curing compounds, polyethylene sheeting and insulating blankets. Cover protruding rebars and make sure that they don’t blow away at night when the temperatures are even lower. If the temperatures are very low, you can use a combination of electric heated blankets and insulated blankets. If the site is subject to cold winds, a 6-foot wind breakers is recommended to reduce evaporation and drop in temperature. A more costly method involves heated enclosures that can be made of wood, canvas tarpaulins, or polyethylene. Heaters can be direct-fire, indirect-fire or hydronic systems (use glycol/water solution to produce heat in a closed system of pipes). Adequate venting to the outside must be included to protect workers from inhaling carbon monoxide gas. Operators should make sure that the heaters are properly fueled to last through the night.
Two elements will affect the quality of curing: moisture and temperature. In cold weather, little to no moisture is required for curing in cold weather conditions. It is recommended to keep the concrete temperature above 40° degrees Fahrenheit for three to seven days.
WHAT IF TEMPERATURES DROP BELOW 50F?
If during the three to seven days that followed the pouring, the temperatures drop below 50 degrees Fahrenheit or are near freezing, then the time has come for a good curing compound to create a protective liquid membrane (usually made of a water based hydrocarbon resin).
MAINTENANCE DO’s AND DON’Ts
Do make sure that no extra water or bleed water is on the surface of the concrete.
Do prevent ice from forming at all costs (this will cause an immediate drop in hydration and strength).
Do watch for hardened concrete area and make sure that they don’t occur.
Do leave forms in place as long as possible because they distribute heat more evenly during the setting phase.
Don’t overworked areas that seem to be setting more slowing.
Don’t seal freshly placed concrete or if bleed water is visible.
Don’t turn off heat too quickly as the difference of temperature between the enclosed area and the outside could cause thermal cracking (days and weeks may be required in case of large structures)
Have more questions about cold weather concrete or concrete pumping? Call Dick at (503)283-2105.
Additional information can be found in ACI 306.1, Standard Specification for Cold Weather Concreting, ACI 306R, Cold Weather Concreting