In most places in the world, there are only a few days a year that can be counted on for perfect weather for placing concrete. As luck would have it, you will more than likely be on vacation on those days.
So when you need vital information on the subject of hot weather and cold weather concreting, take advantage of these two very good sources: ACI 305 Hot Weather Concreting and ACI 306 Cold Weather Concreting. These two documents yield detailed and updated information about the subjects.
Hot Weather Concreting: ACI 305
ACI 305 defines hot weather as "any combination of the following conditions that tends to impair the quality of freshly mixed or hardened concrete by accelerating the rate of moisture loss and the rate of cement hydration or otherwise causing detrimental results." ACI 305 lists the following conditions as important factors in hot weather concrete placements: high ambient temperature, high concrete temperature, low relative humidity, wind speed, and solar radiation.
Effects of Hot Weather
It is a well known fact that concrete which is placed and cured at 95ºF or higher will have lower long term strength than concrete cured at room temperature (70ºF). This is the result of not only higher temperatures, but also the frequent addition of water to maintain a workable slump. When cured at higher temperatures, the cement will lose moisture faster. This reduces the amount of cement that will hydrate. Hydration only takes place when water is present in the cement paste. It is common to have the rate of slump loss twice as much at 90ºF as compared to concrete that has temperatures of around 70ºF. Once concrete temperatures reach 95ºF the rate of hydration greatly increases, producing exothermic heat. Under these conditions, flash setting is more than likely.
When placing concrete in hot weather, the rate of evaporation is greater. This results in plastic shrinkage cracks and a surface condition known as crusting. Plastic shrinkage cracks can be easily addressed by floating or troweling the cracks closed, then applying an evaporation retardant such as L&M's E-Con to the surface of the concrete.
Controlling Concrete Temperatures
Water has a specific heat index of about four to five times that of cement and aggregates. Therefore, the temperature of the mixing water will have a great effect on the concrete temperature. It has been reported that cool water (40 – 50ºF) can reduce the fresh concrete temperature up to 10ºF.
Using crushed or shaved ice is one of the most efficient methods of reducing concrete temperature. Melting ice will absorb heat at the rate of 144 Btu/lb. In order to produce a well-mixed concrete at the batch plant, no more that 75% of the total mixing water should be replaced by ice at any time. At these levels the concrete temperature will be reduced approximately 20ºF. All of the ice must melt before the concrete is placed-otherwise the un-melted ice will leave voids in the concrete.
Another method of controlling concrete temperatures is painting the mixer drum white, which will radiate heat away from the concrete inside the mixer drum. Along this line, in some cases concrete producers have erected a pipe over the mixing drum. Holes are drilled into the pipe about one foot on center. Water is then forced though the pipe and through the holes in the pipe onto the top of the mixing drum to keep it cool.
Concrete producers should keep the coarse aggregate pile wet at all times. This will cool the aggregate. It will also help cool the concrete even though the aggregate has only about 25% the cooling power of water.
Replacing some of the cement with fly ash will reduce the concrete temperature only 1- 2ºF and will greatly reduce the rate of hydration. With the replacement of up to 20% of portland cement with fly ash, the concrete will take longer to reach design strength, in most cases 56 days with fly ash vs. 28 days without it.
The greatest temperature reduction is achieved by injecting liquid nitrogen into the concrete. In this procedure, the temperature of the fresh concrete can be lowered close to the freezing point. Caution should be exercised when using this procedure.
Cold Weather Concreting: ACI 306
ACI 306 defines cold weather as "a period when, for more than 3 consecutive days, the following conditions exist: 1) the average daily air temperature is less than 40ºF and 2) the air temperature is not greater than 50ºF for more than one-half of any 24-hour period."
The Cold Facts
Depending on the type of admixture used and the mix design proportion, the freezing temperature of concrete is approximately 28ºF (-2º C). Strength gain is a function of time and temperature. The rate of hydration at 50ºF is approximately one half the rate of concrete with a concrete temperature of 70ºF. As the temperature of concrete approaches 40ºF, the rate of hydration slows almost to the point of stopping.
Some admixtures accelerate the rate of setting but not the rate of strength gain. Others accelerate the rate of setting and the rate of strength gain. The setting time relates to the finishing properties of the concrete while the rate of strength relates to the length of time the concrete must be protected from low temperatures and freezing. All concrete subjected to a wet environment and freezing temperature should be air entrained and should be protected from freezing and thawing until the compressive strength has reached at least 2500 psi. Air entrained concrete that is low in compressive strength can be damaged by exposure to repeated wet freezing and thawing cycles.
Cold Weather Precaution
Concrete should not be placed over frozen substrate. If outdoors, it has been found that covering the substrate with black polyethylene sheets can draw solar heat and thaw the substrate. If indoors and the substrate is heated using fossil fuel heaters, a heat exchanger must be used. The heat exchanger and flue gases must be vented to the outdoors. If the flue gases are allowed to come in contact with freshly placed concrete, carbonation damage will occur. If carbonation damage occurs, the surface of the concrete will be covered with a white powder. This white powder is chemically identified as calcium carbonate. Severe carbonation can destroy the surface of a concrete floor to a degree requiring the concrete floor to be replaced. In less severe cases of carbonation, the concrete surface may be restored to its intended service life by treating the surface with a chemical hardener or densifier. Concrete should never be placed over frozen reinforcing bars or other metallic objects. This can result in local freezing of the concrete at the interface.
Removal of Thermal Protection for Mass Concrete Pours
After a sufficient period of thermal protection, new mass concrete should be allowed to gradually cool and to reduce thermal stress related cracking. I am referring to ACI 306, table 5.5 entitled Maximum Allowable Temperature Drop During First 24 hours After End of Protection Period. In this table you will find reference to maximum allowable temperature drop following removal of thermal protection for concrete given for various thicknesses. For example: for concrete pours of less than 12 inches in thickness, up to 50ºF change can be tolerated; 12 to 36 inches thick, up to 40ºF; from 36 to 72 inches thick, up to 30ºF; and for concrete pours of greater than 72 inches in thickness, no more than 20ºF change should be allowed in 24 hours.
Heating Up the Mix
The specific heat capacity of water is 4 to 5 times that of cement and the aggregates. Using these criteria, ACI 306 gives the following guidelines for aggregates and mixing water temperatures: "Heating aggregates to a temperature higher that 60ºF is rarely necessary if the mixing water is heated to 140ºF."
ACI 305 and ACI 306: two standards that can help concrete professionals in temperature extremes.
ACI 305 and ACI 306 should be in the library of anyone who produces, specifies or places concrete. In fact, not only should they be in the library, they should be read from cover to cover and referred to often. These two reports by ACI can help to fix the problems of concreting in extreme weather before they become a problem. These two documents can be purchased online at www.aci-int.org.
© 2008 L&M Construction Chemicals, Inc. | ConcreteNews January 2008.