Test for Undesirable Lightweight Material

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Test for Material Finer Than No. 200 Sieve

Intermediate Engineering

Specific Gravity and Absorption, Coarse Aggregate (ASTM C 127)

Test for Undesirable Lightweight Material. Soft, laminated pieces of aggregate, such as chert or shale, are harmful to concrete. Coal and lignite, also, are harmful and are distinguished from the lightweight materials by the brownish black or black color of the particles. Visual examination of the coarse aggregate will often show these minerals. The amount of these minerals in an aggregate can be determined by submersing the aggregate in a liquid with a specific gravity that will allow the shale or other light particles to float and heavier particles to sink.

For specific testing procedures, you should refer to ASTM C 123 or to NAVFAC MO-330. Briefly, you perform the test by first sieving the dried sample over a No. 50 sieve for fine aggregate and a No. 4 sieve for coarse aggregate. Weigh the sample and then place it into a heavy liquid, such as zinc chloride (specific gravity of 1.95 at 78F). Agitate the mixture to allow the lightweight particles to rise to the surface and skim them off. When repeated agitation causes no further particles to rise, you wash the sample in alcohol and then dry and weigh it. The percentage of undesirable, lightweight particles can then be calculated using the following formula:

Where:

L= percentage of lightweight material

W1 = dry weight of lightweight material

W 2 = dry weight of initial sample retained on No. 50 sieve for fine aggregate or No. 4 sieve for coarse aggregate

Color Test for Organic Matter. Any sand that gives a color darker than the standard of this test probably contains an excess of organic matter that will reduce the strength of the concrete in which the sand is used. If you determine that organic matter is present, it is possible that it can be removed by washing; if not, better sand should be obtained. If neither of these things can be done, it will be necessary to use a lower water-cement ratio and control the concrete production carefully to obtain the desired strength.

For the color test, you will need the items illustrated in figure 13-17. To perform the test, you add the sample being tested to a solution of sodium hydroxide and water. After allowing the sample to stand for 24 hours, you then compare the color of the liquid above the sample to a standard color solution consisting of tannic acid, alcohol, and sodium hydroxide. If the liquid above the sample is darker in color than the standard solution, the sand may contain organic impurities. NAVFAC MO-330 provides full details for preparing the solutions and performing the test.

As an alternative, the color of the liquid above the sample can be compared with the colors given in the ASTM standard color plate. Then decide whether the sand contains an excessive amount of organic matter.

SPECIFIC GRAVITY, ABSORPTION, AND SURFACE MOISTURE. These tests must be performed on the aggregate before the necessary calculations can be made to design a concrete mixture. For aggregates used in portland cement concrete, measurements are made to determine the bulk specific gravity of the aggregates in a saturated, surface-dry (SSD) condition. Specific gravity is thus based on determining the total volume occupied by the aggregate particles, including the permeable pore space.

Absorption and surface moisture determinations are necessary to calculate the amount of mixing water used in a concrete mixture. Absorption, determined as a percentage, represents the moisture content of the aggregate when the aggregate is in a SSD condition. Surface moisture is the water that is present in both fine and coarse aggregate, exceeding that which corresponds to a SSD condition.

Concrete-mixture design is discussed at the EA1 level in Part 2 of this TRAMAN.