For aggregate tests to be worthwhile, the sample secured for testing must be representative of the supply of aggregates. If possible, samples should represent the processed and ready-for-use aggregate. Sufficient samples should be taken from the processing plant discharge to represent the material in the stockpile. The sample should contain at least four times as much material as is needed for testing and should be reduced to the size desired for testing by quartering. The standard method for sampling aggregate can be found in ASTM D 75.

Stockpile samples should be taken at or near the base, at about the middle, and at the top of the pile. Three or four such samples should be taken and recombined. A board shoved into the pile just above the point of sampling will prevent the material above the sampling point from falling or sifting into the sample.

TESTS FOR GRADATION. A sieve analysis indicates whether an aggregate is coarse or fine and whether it is evenly or unequally distributed between the larger and smaller sizes. This information is useful in deciding whether the aggregate will make good concrete. Analysis methods can be found in ASTM C 136. Table 13-2 shows the acceptable gradation limits.

Fine aggregate grading limits are specified by the American Society for Testing and Materials; however, since many gradings of fine aggregate can produce a good quality concrete, the engineer should compute the fineness modulus of the aggregate and compare it to the specified concrete class requirements.

 The fineness modulus is an empirical factor that gives a relative measure of the proportions of fine and coarse particles in an aggregate. It is a value widely used to indicate the relative fineness or coarseness of a fine aggregate. To obtain the fineness modulus, sieve a 500-gram sample of sand through a series of sieves (Nos. 4, 8, 16, 30, 50, and 100). Convert the weight retained on each sieve into a cumulative percentage retained, starting with the No. 4 sieve. Divide the sum of the six percentages by 100. The resulting answer is the fineness modulus. Typical fineness modulus values are as follows:

TESTS FOR SOUNDNESS. Soundness is the property of aggregate to resist disintegration when subjected to freezing and thawing. Two methods are used to test for soundness. In the freeze-thaw test method (ASTM C 666), concrete specimens, made with the aggregate in question, are placed in water and then subjected to alternate cycles of freezing and thawing. An alternate method, requiring considerably less equipment and time, is the salt test. This procedure involves solutions of special salts (sodium sulfate or magnesium sulfate) in which the aggregate is immersed and saturated. The crystals of these salts are permitted to grow that creates a disruptive force similar to freezing water. You can find the procedures to perform the salt test in ASTM C 88 and in NAVFAC MO-330.

TESTS FOR IMPURITIES. The quality of aggregate is another important consideration. The presence of organic material, excessive quantities of silt or clay and shale, or other water-absorbing particles can be detrimental to the concrete strength, watertightness, and durability. Tables showing the recommended limits of deleterious materials in fine and coarse aggregate can be found in NAVFAC MO-330.