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Aggregates
TESTS FOR SOUNDNESS - 14071_291

Engineering Aid 2 - Intermediate Structural engineering guide book
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economy of the mix. Too much fine aggregate, besides preventing  a  good  bonding,  also  increases  the  surface area  that  must  be  coated  with  cement  paste.  That weakens the concrete. Good gradation results in a dense mass of concrete with a minimum volume of voids, an economical  mix,  and  a  strong  structure.  Optimum strength, watertightness, and durability in the hardened concrete require careful control of aggregate gradation. DURABILITY.— Durability is the ability to resist the elements of weathering and the load pressures. Weak or easily crushed rock or other mineral particles that break down under the applied loads will cause changes in the internal stresses and a breakdown of the concrete. Rocks  or  mineral  particles  that  are  absorptive  or susceptible  to  swelling  when  saturated  will  disintegrate when  acted  upon  by  different  weather  conditions. Freezing  moisture  causes  expansion  stresses  that  can easily rupture absorptive rocks. Radiant heat from the sun causes rocks to swell. If the heat is then followed by sudden  cooling  because  of  a  shower  and  temperature drop,  shrinkage  and  a  breakdown  of  some  rocks frequently  occur.  The  aggregate  must  be  chosen  to withstand  these  forces  of  nature. DETERIORATION.—  Deterioration  of  concrete, in  many  cases,  can  be  traced  to  the  aggregate.  An excessive  amount  of  organic  material  in  or  on  the aggregate  prevents  the  cement  paste  from  forming  an adequate bond with the aggregate particles. A large percentage of clay or fine silts adhering to the aggregate may  prevent  the  cement  paste  from  reaching  the particles. That results in a structurally weak concrete that also is susceptible to breakdown by weathering. Washing  the  aggregate  to  remove  the  silts,  clays,  and organic  material  prevents  this  problem. CHEMICAL COMPOSITION.—  Chemical  com- position  of  the  aggregate  is  also  important.  Any chemical reaction between aggregate and cement in the presence of water reduces the hardening and cementing process. Any reduction in the amount of water-cement paste caused by a chemical reaction reduces the amount available to bond to the aggregate. This result is similar to one caused by an insufficient amount of cement. Tests for Aggregates 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. Unprocessed sources of sand and gravel should be taken  by  channeling  exposed  faces,  or  they  should  be taken from pits if exposures are not available. Take care to ensure that the samples include only those materials that are below the overburden or strip zone. 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. Table  13-2.—Desirable  Gradation  for  Aggregates  in  Concrete 13-22







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