Q. The last few seasons we've had difficulty controlling our water demand and our yields due to the sand's variability in absorption and specific gravity. The quarry claims there's little it can do. Its primary working area is on the fringe of a large glacial till deposit that now has several lenses of differing sands with gradations and minerologies.
Our quality control problem is compounded because our stockpile area is rather small. So by the time the lab sends us the sample results, the plant has already used the tested material in shipped concrete. Are there tests that can provide specific gravity and absorption of fine aggregate in a reasonable time period?
A. The need for prompt and accurate results in determining the specific gravity and absorption of construction aggregates has driven researchers to develop two new testing procedures. Many concrete producers may not be aware of these developments. These new procedures, primarily developed for the asphalt paving industry, have been monitored by the ASTM committee focused on road and paving materials.
Either could be incorporated into your quality control program to reduce product variability. ASTM D 7172, Standard Test Method for Determining the Relative Density (Specific Gravity) and Absorption of Fine Aggregates Using Infrared, was approved in 2006. ASTM D 7370, Test Method for Determination of Relative Density and Absorption of Fine, Coarse, and Blended Aggregate using Combined Vacuum Saturation and Rapid Submersion, was approved in October 2008.
Both methods outline procedures that use a specific proprietary automated testing system. In D 7172, technicians can determine a fine aggregate's bulk specific gravity (dry), apparent specific gravity, and absorption using a system developed by Barnstead International, of Dubuque, Iowa. Using D 7370, the methodology allows testing fines, coarse and combined aggregate for specific gravity and absorption using an instrument developed by InstroTek Inc., of Raleigh, N.C.
Both tests were initially developed to aid asphalt producers in decreasing the 15- to 18-hour testing interval AASHTO T84, Specific Gravity and Absorption of Fine Aggregates, requires. D 7172 takes about one and a half hours to complete; D 7370 takes less than a hour.
Both tests have also undergone thorough, independent technical evaluations as part of the ASTM standard drafting process. For example, in 2004 National Center for Asphalt Technology representatives presented a technical paper evaluating the D 7172 test method compared to T84.
The D 7172 test method offered improved precision compared to T84. The ASTM document references this research in its precision and bias section. But the document warns that D 7172 results may differ from those of T-84, and may vary by material type.
The D 7370 procedure was tested for the Federal Highway Administration by National Center of Asphalt Technology (NCAT). Researchers reported that the repeatability and reproducibility of the test method were similar to AASHTO T166.
D 7172 centers on using an automated system with two specialized devices: an automatic volumetric mixer (AVM) and an infrared unit.
The technician divides a sample of fine aggregate, dried to a constant mass, into two sample splits. He places one sample in the special volumetric flask filled with water to the calibrated line and then weighed. The technician mounts the flask to the mixing platform of the AVM, which automatically mixes the water and material with vacuum air from the sample chamber until the sample has reached the Apparent Specific Gravity condition. The flask is removed, refilled with water to the calibration line, and weighed.
While the first sample is being tested, the second is placed in the special bowl the manufacturers provide. This sample and bowl are weighed, then mounted to the mixing platform of the SSD device. The apparatus injects small amounts of water into the sample, mixes the material, and absorbs water until reaching the saturated surface dry (SSD) condition.
An infrared detector monitors the sample during the mixing/water injection process for trace amounts of water to begin building on the aggregate surface. The unit will automatically stop when a stable SSD condition is reached. The technician weighs the test bowl to determine the net weight of the sample at SSD.
The D 7370 procedure uses a vacuum to calculate the rapid saturation of dry aggregates. Each test involves two representative dry aggregate samples of the same material. One sample is evacuated inside the vacuum chamber inside a plastic bag and opened under water for rapid saturation of the aggregate. The dry mass and submerged mass of the sample are used to calculate apparent relative density.
The second sample of the same aggregate is tested in a known volume metal pycnometer. The known mass of the pycnometer with water, mass of the dry aggregate, and mass of the aggregate and pycnometer filled with water is used to calculate unsaturated density. Results from the samples tested are used to calculate absorption and specific gravity.
To buy either test procedure, visit ASTM at www.astm.org.