suspension. The manner in which the correction factorsare determined and applied to obtain corrected readingscan be found in ASTM D 422 and NAVFAC MO-330.The corrected readings are used to determine the percentpassing and the particle sizes corresponding to eachreading.As you know, the larger particles in a soil-watersuspension settle more rapidly than the smallerparticles; therefore, for each hydrometer reading, thepercent passing is the percentage of soil remaining insuspension at the level at which the hydrometermeasures the density of the soil-water suspension. Thatpercentage, for each hydrometer reading, can becalculated using the formulas given in NAVFACMO-330 or ASTM D 422. The particle sizescorresponding to each of those percentages is calculatedon the basis of Stokes’ law, that relates the terminalvelocity of a free-falling sphere in a liquid to itsdiameter. To calculate the sizes, use the formulas givenin ASTM D 422 or use the nomograph procedurediscussed in NAVFAC MO-330.CONCRETE AND CONCRETE TESTINGAs you learned in the EA3 TRAMAN, concrete isone of the most economical, versatile, and universallyused construction materials. It is one of the few buildingmaterials that can be produced directly on the jobsite tomeet specific requirements. In this section, you willstudy the materials used to produce concrete and youwill further your knowledge of concrete testing.COMPOSITIONConcrete is a synthetic construction material madeby mixing cement, fine aggregate, coarse aggregate, andwater together in proper proportions. The followingparagraphs discuss each of these materials:CementCement is a substance that hardens with time andholds or entraps objects or particles in a definiterelationship to each other. For concrete, portlandcement is usually used. Portland cement ismanufactured by a standardized process consisting ofgrinding limestone and clay, mixing them inproportions, heating the mixture to a high temperatureto form clinkers, and then pulverizing the clinkers sothat 95 percent of the material will pass through a No.200 sieve. The following paragraphs describe thevarious types of Portland cement:. TYPE I. Normal portland cement is anall-purpose type used to make ordinary concretepavements, buildings, bridges, masonry units, and thelike.. TYPE II. Modified portland cement is a typethat generates less heat during the curing process thanType I. The hydration process generates heat that, in alarge mass of concrete, can become high enough toaffect the concrete adversely. Type II is also moresulphur-resistant than Type I. Sulphur exists in water orsoil having a high alkali content and has an adverseeffect on the concrete.. TYPE III. High-early-strength portland cement, as the name suggests, is used where a highstrength is needed quickly. That maybe due to a demandfor early use, or in cold-weather construction to reducethe period of protection against low or freezingtemperatures.. TYPE IV. Low-heat portland cement has theheat-resistant quality of Type II, but to a higher degree.It develops strength at a slower rate than Type I but helpsprevent the development of high temperatures in thestructure with the attendant danger of thermal crackingupon later cooling.. TYPE V. Sulphate-resistant portland cementhas a higher degree of sulphate resistance than Type IIand is for use where high sulphate resistance is desired.Other types of cements maybe variations of the fivetypes above or may be special types. Some of these typesare as follows:. TYPE IS. Portland blast-furnace slag cementuses granulated slag, which is rapidly chilled orquenched from its molten state in water, steam, or air.The slag (from 25 to 65 percent of the total weight ofthe cement) is interground with cement clinker. Thiscement is for general use in concrete construction.. TYPE IP. Pozzolan cement uses a mixture offrom 15 to 40 percent of pozzolan with the cementclinker. Pozzolan is a siliceous or siliceous andaluminous material, such as fly ash, volcanic ash,diatomaceous earth, or calcined shale. The strength ofconcrete made with pozzolanic cements is not as greatas concrete made with the same amount of Portlandcement, but its workability may be better for some uses.. AIR-ENTRAINED CEMENT. Concrete madewith air-entrained cement is resistant to severe frostaction and to salts used for ice and snow removal. It isproduced by adding air-releasing materials to the13-19
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