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.:' of strength development..4s a rule of thumb, the 7-day strength is about 60% to 75% of the 28-day strength, depending upon the type and amount of cement, water- cement ratio, curing temperature. and o[her variables. TESTING AGGREGATES Sampling Aggregates btethods for obtaining representative samples of aggre- gates are given in ASTht D 75. Accurate sampling is important. Reducing large field samples to small quan- tities for individual tests must be done with care so that the final samples will be truly representative. For coarse aggregate, this is usually done by the quartering method: The sample, thoroughly mixed, is spread on a piece of canvas in an even layer 3 or 4 in. thick. It is divided into four equal parts. Two opposite parts are then discarded. This process is repeated until the de- sired size of sample remains. A similar procedure is sometimes used for moist, fine aggregate. Sample sput- ters are desirable for dry fine aggregate (Fig. 14-I ). Organic Impurities Organic impurities in fine aggregate should be deter- mined in accordance with .45TM C40. A sample of fine aggregate is placed in a sodium hydroxide solution and shaken. The following day the color of the solution is compared with a standard color solution. If the color is darker than the standard, the fine aggregate should 164 not be used for important work without further inves- tigation. Some fine aggregates contain small quantities of coal or lignite that give the liquid a dark color. The quantity may be insufficient to reduce the strength of the concrete appreciably and the fine aggregate may be acceptable o[herwise. In such can's, mortar strength tests (.45TM C 87) using the fine atgregate in question will indicate the effect of the impurities present. It should be noted that appreciable quantities of coal or lignite in aggregates can cause popouts and staining of the concrete and can reduce durability when concrete is exposed to weathering. Local experience is often the best indication o(the durability ofconcrete made with such aggregates. Objectionable Fine Material Large amounts of clay and silt in .aggregates can ad- versely affect durability, increase water requirements. and increase shrinkage. Specifications usually limit the amount of material passing the No. 200 sieve to Z%or 3% in fine aggregate and to I % or less in coarse aggre- gate. Testing for material finer than the No. 200 sieve should be done in accordance with ~\STM C I17. Test- ing far clay lumps should be in accordance with ASTM1t C 142. Grading Gradation of aggregates significantly affects concrete mixture proportioning and workability Hence, grada- tion tests are an important element in the assurance of concrete quality. The grading of an aggregate is deter- mined by a sieve analysis test in which the particles are divided into their various sizes by standard sieves. The analysis should be made in accordance with ASTbI C 136. Results of sieve analyses are used in three ways: (I ) to determine whether or not the materials meet speci- fications; (2) to select the most suitable material i( several aggregates are available; and (3) to detect variations in grading that are sufhcient to warrant blending selected sizes or an adjustment of concrete mix proportions. The grading requirements for contr.^te aggregate arc shown in Chapter 4 and ASTM C 33. Materials con- raining too much or too little of any one size should be avoided. Some specifications require that mixture pro- portions be adjusted if the average fineness modulus of fine aggregate changes by more than 0.20.Other speci- ficationsrequire anadjustment in mixture proportions if the amount retained on any two consecutive sieves changes by more than 10% by weight of the total fine- aggregate sample. A small quantit}• of clean particles that pass a No. 100 sieve but are retained on a No. 200 sieve is desirable for workability. For [Iris reason most specifications permit up to 10% of this material in fine aggregate. Moisture Content of Aggregates Several methods can be used Tor determining the 1 Fig. 14-1. Sample sputter commonly used to reduce sand samples.