Moisture Content Test
A moisture content test on aggregate refers to the testing method used to determine the amount of moisture present in a sample of aggregate. Aggregate is a material made up of a collection of particles, such as gravel, crushed stone, or sand, that is used in construction projects, including road building, foundations, and concrete production.
There are a few ways to perform a moisture content test on aggregate. One of the most common methods is called the oven-dry method, which involves taking a sample of the aggregate, drying it in an oven at a specific temperature and weight it before and after the drying process. Moisture content is then calculated as the percentage of the weight of the sample that is lost due to evaporation during the drying process. Other methods include, using microwave, infrared or using a moisture meter.
Test Method for Determining Moisture Content of Aggregate Sample
Governing Standards (US)
Three common test methods are used to determine the moisture content of both normal-weight and lightweight aggregates:
ASTM C566: Standard Test Method for Total Evaporable Moisture Content of Aggregate by Drying. Applicable to both fine and coarse aggregate, this is the most commonly used procedure.
ASTM C70: Standard Test Method for Surface Moisture in Fine Aggregate. Primarily for fine aggregate, it is less commonly used due to its complex procedure reliant on measuring displaced volumes.
AASHTO T255: Standard Method of Test for Determining Total Moisture and Free Moisture in Aggregate (Coarse and Fine). Similar to ASTM C566, often used in preparation for hot-mix asphalt applications.
Frequency (example)
In accordance with NPCA Quality Control Manual for Precast Concrete Plants:
Moisture content of each aggregate needs to be determined at least once daily following ASTM C566 or ASTM C70.
For plants using moisture probes or automatic mixing water adjustment systems, an aggregate moisture content test must be conducted on each aggregate at least once per week to validate the accuracy and performance of the moisture probe.
Equipment
The following equipment is necessary for conducting aggregate moisture content tests:
Accurate balance or scale (within 0.1% accuracy of the test load).
Source of heat (electric hot plate, gas burner, ventilated microwave oven, or ventilated oven).
Heat-resistant sample container.
Stirrer (metal spoon or spatula).
Heat-resistant glove or hot pad.
Procedure
Preparation
Ensure all required equipment and personal protective gear are available and functioning properly.
Sample Collection: a. Collect a representative aggregate sample from the stockpile following ASTM D75. b. Ensure the collected sample represents the entire stockpile in size, gradation, and moisture content. c. Protect the collected aggregate sample from moisture loss or gain before the test.
Determine Sample Weight: a. Weigh the empty sample container and record its weight to the nearest 0.1%. b. Weigh the sample container with the aggregate and record the combined weight. c. Subtract the weight of the sample container to obtain the weight of the aggregate sample.
Drying the Aggregate Sample
Gradually heat the aggregate sample using the selected method, ensuring consistent heating and preventing overheating.
Stir the sample periodically to expedite moisture evaporation, except for microwaving or oven drying (optional for fine aggregate).
Monitor the sample carefully during the heating and drying process.
Weighing the Dried Sample
Weigh the dried aggregate sample and record the weight to the nearest 0.1%.
If further drying is needed, repeat the drying process and weigh again until less than 0.1% additional loss in weight is achieved.
Determining Moisture Content
Subtract the weight of the sufficiently dried sample from the weight of the original representative aggregate sample to determine the weight of moisture held by the aggregate.
Calculate the aggregate moisture content as a percentage using the weight of moisture and the weight of the sufficiently dried sample.
Note:
Ensure thorough adherence to safety protocols during the entire testing process.
Maintain consistency and accuracy in measurements and recording throughout the procedure.