Hot Weather Concreting
Is your hot weather concreting out of control?
Placing concrete in "the good old summer time" can be painful for both the workmen and for the concrete.
Not only do the workmen have to guard against heat stroke-so does the concrete. Both the workmen and concrete have similar methods for cooling down, too. Workmen sweat. Concrete has water that migrates to the surface (bleed water). When the sweat or bleed water evaporates, the surface of both the workmen and concrete cool down somewhat. But this is where the similarity between men and concrete ends.
Workmen do not undergo plastic shrinkage cracking, but concrete does. Plastic shrinkage cracks occur before the concrete hardens. Cracks that occur after the concrete hardens are called drying shrinkage cracks. Plastic shrinkage cracks, unlike drying shrinkage cracks, can be fixed if they are attended to during the finishing operation. In most cases they can be floated or troweled out of the surface of the concrete.
The rapid loss of volume when water evaporates produces the plastic shrinkage cracks. In other words, the concrete is drying up like mud. (Could it be that is why some people call concrete "mud"?)
It has often been said that an ounce of prevention is worth a pound of cure. In this case the "ounce of prevention" is to prevent the rapid evaporating of water from the concrete. There are a number of ways to accomplish this, including the following:
- Plastic sheets have been tried, but they interfere with the finishing process.
- Fogging the slab has been tried with some success. Fogging is defined as spraying a mist of water over the concrete slab at the rate of ¼ to ½ gallon per minute per hundred square feet. This cloud (fog) increases the relative humidity over the concrete which, as a result, reduces the evaporation rate.
Technically, fogging is a good method. Yet fogging may not always be practical. For example, maintaining a stable fogging cloud over the concrete slab in a moderate wind can become problematic.
What is needed is a miracle in a drum. The product must be able to be sprayed on the surface of the concrete at anytime during the finishing process and control the evaporation. In addition, it must be able to be applied a number of times and not interfere with the finishing process, and when no longer needed, evaporate without leaving a film or residue.
Our miracle in a drum is E-con. E-con is an evaporation retardant. E-con consists of a polar molecule, which means it has a positive and negative end.
Water is a polar molecule and the surface of the concrete slab has negative and positive sites. E-con, when misted onto the surface of plastic concrete, becomes chemically bonded to the surface of the concrete. Then it captures any passing water molecule trying to leave the concrete through evaporation. This action increases the relative humidity and greatly reduces the evaporation rate.
E-con has two advantages over the fogging method:
- the ease of application, and
- its stability, even in high wind conditions.
When should E-con be used? As can be seen in the chart, evaporation is driven by three factors:
- air temperature,
- relative humidity, and
- wind velocity.
So the answer is...anytime the evaporation rate approaches 0.2 Lbs /Sq. Ft./hour. (See the chart to the right.)
Concrete temperature does have a profound effect on how concrete is placed and finished. When the concrete temperature reaches the 90°F and above, the working time for concrete can be very short. At 100°F, flash set is not uncommon.
In the middle of the last century, when I was a young concrete technician, I would occasionally get a call from a concrete truck asking for help with a hot load of concrete. I would rush to the truck in trouble and add 15 to 20 pounds of sugar to the hot and rapidly setting concrete. The sugar would stop the hydration of the cement in the concrete. (In fact, the concrete with that much sugar in it would never set up. I would lose the concrete but save the mixing drum.)
Little wonder, then, that a type of sugar is used to produce ASTM C 494 type B and D chemical retarders. These sugars come from wood sugars such as lignins; corn sweeteners are also used. Used in small amounts, these chemical retardants can extend the working time and setting time, allowing the placement of concrete in the hottest weather.
A fact worth knowing is that concrete placed at 90°F will set in one half the time as concrete placed at 70°F, and concrete placed at 50°F will take twice as long to set as concrete placed at 70°F.
Most ready mixed concrete producers do a number of things to lower the temperature of the concrete leaving the plant. It is a common practice to use water sprinklers to keep the aggregates wet. This helps in two ways. First, the evaporation of water from the aggregate pile lowers the temperature of the aggregate. Second, the aggregate being filled with water throughout will not be taking up mixing water from the mix.
Some ready mix producers use liquid nitrogen to cool concrete. However, the most practical way is to use cool water or crushed ice. As ice returns to its water state 80 calories per gram of heat is consumed, making ice a very effective method of lowering concrete temperature.
Another effective method of controlling concrete temperature is to replace some of the cement with fly ash. Portland cement, during the early stages of hydration, will give off heat. However, fly ash does not start to react for several days after the concrete has set.
Concrete that has had its temperature lowered at the ready mix plant can develop air entrainment problems at the job. The amount of air entrainment in a concrete mix can and will be affected by the concrete temperature. It is common for concrete to leave the plant with 6% air content at 50°F, be placed at the job site at 95°F, and end up with an air content of only 4%. In the summer months, the air content must be monitored very closely.
If water curing is to be used, the temperature of the curing water should not be more that 20°F cooler than the concrete to prevent thermal stresses. Concrete that has undergone thermal stress can suffer internal cracking and often severe strength loss. It should be noted that water from the tap can often be 20°F to 30°F cooler than the concrete.
As can be seen in the chart below, concrete under stress can suffer severe strength loss and may never obtain its designed 28-day strength.
So, how do you prevent concrete heat stroke? L&M's E-Con is certainly one solution. Find out more at www.lmcc.com
The American Concrete Institute (ACI) has published a report on hot weather concreting (ACI 305) which covers this subject in great detail. ACI 305 can be found online at www.aci.org.
© 2005 L&M Construction Chemicals, Inc. | ConcreteNews Summer 2005.