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Tests for Aggregates - 14071_290
Test for Undesirable Lightweight Material - 14071_292

Engineering Aid 2 - Intermediate Structural engineering guide book
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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: Fine sand = 2.20 to 2.60 Medium  sand   = 2.60 to 2.90 Coarse  sand = 2.90 to 3.20 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. Test for Material Finer Than No. 200 Sieve.— The extremely fine mineral material (clay, silt, dust, or loam) occurring in most aggregates can affect concrete in two ways. The added surface area of the fine particles picks up the cement paste and reduces the amount available to bind and hold the aggregate. The small particles also tend to float up to the surface when the concrete is finished (especially when wet mixes are used). That results in a surface covered by hairline cracks and a tendency for the fines to dust off when dry. For some purposes a small amount of fine material may improve workability;  however,  amounts  in  excess  of  3  to  5 percent of the total weight of the aggregate are generally considered harmful to the concrete. The  specific  procedures  that  you  should  follow when testing for these fine materials can be found in ASTM C 117 and in NAVFAC MO-330. Briefly, you oven-dry and weigh a sample of the aggregate. Then the dried sample is brought to suspension in water and is carefully poured through a nest of sieves (No. 16 and No. 200) until the wash water is clear. Finally, the material remaining on the sieves is oven-dried and weighed. The percent of material finer than the No. 200 sieve is then calculated using the following formula: Where: P   = wd = Wdw  = Percentage of fines original dry weight of sample dry  weight  after  washing The  method  described  above  is  accurate,  but time-consuming. When time is critical and less accurate results  are  acceptable,  you  can  place  a  1,000-gram sample into a quart mason jar to a depth of about 2 inches. Then fill the jar three-fourths full of water, shake the mixture vigorously, and allow it to stand for 1 hour. The silt and clay will form a layer at the top of the sand. If the layer is more than 1/8 inch thick, the material has more than 3 percent fines and should be washed before using. Test for Clay Lumps and Friable Particles.— This test is performed on the material remaining after you determine the material finer than the No. 200 sieve. The size of samples needed and the specific test procedures that you should use are in ASTM C 142 or in NAVFAC MO-330. Spread the sample in a thin layer on the bottom of a flat pan and cover it with distilled water. After about 24 hours, you break up all particles by crushing them between the fingers. Next, you remove the broken clay lumps and friable particles by wet sieving over the appropriate  one  of  the  following  sieves:  aggregate—No. 20;  No.  4  to  3/8  inch—No.  8;  over  3/8  inch—No.  4. 13-23







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