ASTM Standard C1064: Temperature of Freshly Mixed Hydraulic Cement Concrete
How Does Temperature Affect Concrete?
ASTM C1064 is an ASTM standard that allows you to measure the temperature of freshly mixed concrete in order to make sure it is conforming to specifications, as temperature at mixing time will affect the way the concrete cures later on.
Concrete that is poured at too high of an internal temperature may show false high values during compressive strength testing, and will set faster, requiring rapid finishing to get the required appearance and strength. The heat from high temperature concrete can also cause a loss of entrained air. Concrete with a high internal temperature will also be prone to cracking because quick evaporation will cause shrinking of patches on the surface.
Concrete that is poured at too low of an internal temperature may freeze if it falls below 25 degrees Fahrenheit, which can cut its compressive strength in half and make it unusually brittle. Cold concrete cures more slowly and gains strength at a slower pace than concrete cured within a normal temperature range. If cold concrete is not air entrained, it will also be prone to a freeze-thaw cycle, creating huge cracks in the concrete.
Having the temperature information will help site engineers figure out how they will protect the concrete from extreme temperatures, and how to cure the concrete so that it reaches its optimum strength.
Concrete Temperature Equipment
Container - Your container needs to be big enough to have a distance of at least three inches between your thermometer and any side of the container. You also need to make sure it is three times as deep as the length of the largest, or nominal, aggregate size in the concrete. For example, if your aggregate is 2 inches in diameter, the container must be at least 6 inches deep. The wheelbarrow you sample your concrete in when you make cylinders should serve this purpose.
Thermometer - Your concrete thermometer must be able to measure from 30°F to 120°F, and needs to accurately measure the temperature of the concrete to ±1°F. It also needs to be able to be submerged at least 3 inches into the concrete, so the stem of the thermometer must be at least 3 inches long. At the company I work for, we calibrate our field thermometers annually with a reference thermometer that is traceable to the National Institute of Standards and Technology, so please let your lab technician know if your thermometer is up for calibration, or if there are any problems with your thermometer.
Scoop or shovel - The scoop you use for making cylinders should be fine to mix the concrete up, but it may be easier to mix a large wheelbarrow full of concrete with a shovel. Whatever you use should be large enough to scoop a representative amount of concrete, as it will be easier to mix the concrete with something that will move a lot of it at once.
ASTM C1064 Procedure
- Sample the concrete according to ASTM C172, and mix it thoroughly with your scoop or shovel to get a temperature reading that is representative of the entire sample. Once the concrete is mixed, put your thermometer in the center of the sample, making sure that there is at least 3 inches of concrete between the sides and bottom of the container and your thermometer. Try to put your thermometer in so that the stem is covered as much as possible.
- Close any spaces around the thermometer by gently pressing the concrete around the stem at the surface of the sample. You are doing this to keep the nearby air temperature from affecting the reading.
- If the nominal aggregate size of your concrete is less than 3 inches, wait at least two minutes but not more than 5 minutes, then read and record the temperature to the nearest 1°. If the nominal aggregate size is over 3 inches, then it can take up to 20 minutes to stabilize the temperature. When you are reading the temperature do not take the thermometer out.
- Record your temperature data on your paperwork and wipe your thermometer with a wet rag or sponge. Make sure to get everything off of the stem as that is what reads the temperature and you want it to be clean for the next test.
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