economy of the mix. Too much fine aggregate, besidespreventing a good bonding, also increases the surfacearea that must be coated with cement paste. Thatweakens the concrete. Good gradation results in a densemass of concrete with a minimum volume of voids, aneconomical mix, and a strong structure. Optimumstrength, watertightness, and durability in the hardenedconcrete require careful control of aggregate gradation.DURABILITY.— Durability is the ability to resistthe elements of weathering and the load pressures. Weakor easily crushed rock or other mineral particles thatbreak down under the applied loads will cause changesin the internal stresses and a breakdown of the concrete.Rocks or mineral particles that are absorptive orsusceptible to swelling when saturated will disintegratewhen acted upon by different weather conditions.Freezing moisture causes expansion stresses that caneasily rupture absorptive rocks. Radiant heat from thesun causes rocks to swell. If the heat is then followed bysudden cooling because of a shower and temperaturedrop, shrinkage and a breakdown of some rocksfrequently occur. The aggregate must be chosen towithstand these forces of nature.DETERIORATION.— Deterioration of concrete,in many cases, can be traced to the aggregate. Anexcessive amount of organic material in or on theaggregate prevents the cement paste from forming anadequate bond with the aggregate particles. A largepercentage of clay or fine silts adhering to the aggregatemay prevent the cement paste from reaching theparticles. That results in a structurally weak concretethat also is susceptible to breakdown by weathering.Washing the aggregate to remove the silts, clays, andorganic material prevents this problem.CHEMICAL COMPOSITION.— Chemical com-position of the aggregate is also important. Anychemical reaction between aggregate and cement in thepresence of water reduces the hardening and cementingprocess. Any reduction in the amount of water-cementpaste caused by a chemical reaction reduces the amountavailable to bond to the aggregate. This result is similarto one caused by an insufficient amount of cement.Tests for AggregatesFor aggregate tests to be worthwhile, the samplesecured for testing must be representative of the supplyof aggregates. If possible, samples should represent theprocessed and ready-for-use aggregate. Sufficientsamples should be taken from the processing plantdischarge to represent the material in the stockpile. Thesample should contain at least four times as muchmaterial as is needed for testing and should be reducedto the size desired for testing by quartering. The standardmethod for sampling aggregate can be found in ASTMD 75.Stockpile samples should be taken at or near thebase, at about the middle, and at the top of the pile. Threeor four such samples should be taken and recombined.A board shoved into the pile just above the point ofsampling will prevent the material above the samplingpoint from falling or sifting into the sample.Unprocessed sources of sand and gravel should betaken by channeling exposed faces, or they should betaken from pits if exposures are not available. Take careto ensure that the samples include only those materialsthat are below the overburden or strip zone.TESTS FOR GRADATION.— A sieve analysisindicates whether an aggregate is coarse or fine andwhether it is evenly or unequally distributed between thelarger and smaller sizes. This information is useful indeciding whether the aggregate will make goodconcrete. Analysis methods can be found in ASTM C136. Table 13-2 shows the acceptable gradation limits.Table 13-2.—Desirable Gradation for Aggregates in Concrete13-22
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