Last week, we learned about the importance of sampling aggregates and the appropriate way to do it. This Friday, our Fun Fact is about testing the moisture content of that aggregate. Why do we need to do this? Without accounting for the amount of moisture in the aggregate when it is mixed into concrete, the appropriate amount of water might not be added to the concrete mix, resulting in it being too wet or too dry.
In years past, rather unscientific methods were used to estimate the moisture content of a sample – squeezing it, throwing it at a wall and looking at the damp mark left behind – none of which were consistent or very accurate. There are now ASTM guidelines, moisture probes, and other tools at our disposal to maintain consistent testing results. There are a few different ways to determine the moisture content of aggregate, including the drying method, displacement method, calcium carbide method, electrical meter method, and automatic measurement.
The drying method is fairly simple, and commonplace in the materials testing industry. There are some instances where it is not feasible to use the drying method, such as when the aggregate contains gypsum or salt, but it works much of the time. The aggregate sample is weighted, then dried in a vented, thermostatically-controlled drying oven and weighed again. This allows the technician to determine how much moisture was in the aggregate, with corrections made for the saturated and surface dry condition. While drying aggregate in an oven is too slow a method for field use (drying time is usually 12-16 hours), variations exists where the aggregate is heated quickly in an open pan using a hot plate, burner, or even microwave oven.
The displacement method uses a pycnometer or the Siphon-Can Method. Because the specific gravity of normal aggregate is higher than that of water, and a given weight of wet aggregate will occupy a greater volume than the same weight of dry aggregate, the specific gravity of the wet aggregate can be calculated based on the known specific gravity of the dry aggregate. The moisture content can be calculated based on the difference between the wet and dry specific gravities.
The calcium carbide method works because calcium carbide reacts with surface moisture in the aggregate to form acetylene gas, the pressure of which depends on the moisture content of the aggregate. By mixing aggregate with calcium chloride in an air-tight vessel equipped with a calibrated pressure gauge, the moisture content can be calculated based on the pressure. This method takes less than five minutes per test, making it useful on a jobsite.
The electrical meter and automatic measurement methods of moisture content testing are generally used in batching plants rather than field applications. Electrical meters have been developed to measure instantaneous or continuous reading of the moisture content of the aggregate, using the principle that the resistance changes as the aggregate is used and its moisture content changes. Automatic measurement uses sensors to record and adjust the moisture content of the aggregate mixture.
Periodically testing the moisture content of aggregate and adjusting the batch as necessary saves time, money, and product waste as well as helps ensure the safety of the final product. Better to catch an out-of-tolerance value before the concrete is poured and the whole batch is rendered unusable!
If you have any questions about aggregate moisture content testing, or would like to schedule testing, contact Jeremy Lake at 716.592.3980, ext. 133 or [email protected].
