Figure 5-5.—Compactive soil.Another method, more commonly used, is toexpress the gradation in terms of the TOTALPERCENTAGE that passes each sieve. This isdetermined by adding the listed percentage that passedeach sieve and was retained on the next finer sieve tothe percentages listed beside each of the sieves belowit.For example, in the sample just analyzed, 24%passed the 1/2-inch sieve and was retained on the3/8-inch sieve. The total percentage that passed the1/2-inch sieve is 24% + 20% + 15% + 19% + 14%+ 8% = 100%The total percentage that passed the 3/8-inch sieveis 20% + 15% + 19% + 14% + 8% = 76%.Continuing the calculation as indicated, the results of thetest in terms of total percentage passing each sieve areas follows:Aggregate passing 1/2-inchsieve= 3,000 grams = 100%Aggregate prosing 3/8-inchsieve= 2,280 grams = 76%Aggregate passing No. 4sieve= 1,680 grams = 56%Aggregate passing No. 10sieve= 1,230 grams = 41%Aggregate passing No. 40sieve= 660 grams = 22%Aggregate passing No. 200sieve=240 grams = 8%Figure 5-4 shows the aggregate gradation curve forthis particular example, plotted by using the abovepercentages.For bituminous aggregate, anything that passes theNo. 4 sieve and is retained on the No. 200 sieve isconsidered FINE aggregate. Anything retained on theNo. 4 sieve is considered COARSE aggregate.Material that can pass a No. 200 sieve is sometimesused in bituminous paving; this material is known asMINERAL FILLER, DUST, OR FINES. It consists offinely powdered rock, portland cement, hydrated lime,or some other artificially or naturally powdered dust.All testing sieves larger than three-sixteenths of aninch are identified by the actual clear opening in inches.All testing sieves smaller than three-sixteenth of an inchare identified by mesh number. (See fig. 5-4.)Compaction Qualities of SoilCompaction is the process of physically densifyingor packing the soil. The strength of any given soil canbe increased by the densification obtained fromcompaction.Three factors affect compaction: materialgradation, moisture content, and compactive effort.Material gradation(fig. 5-1) refers to thedistribution of different sizes of particles within a givensoil sample. A well-graded sample contains a good, evendistribution of particle sizes. When the composition ofa soil sample is made up mostly of one size of particles,this soil would be poorly graded. A well-graded soil willcompact more easily than one that is poorly graded. Inwell-graded material, the smaller particles tend to fill theempty spaces (voids) between the larger particles,leaving fewer voids after compaction (fig. 5-5).Moisture content is the amount of water present inthe soil. Because water acts as a lubricant that helpsparticles slide into a denser position, the moisturecontent of a soil is very important to compaction. Wateralso helps bond clay particles, giving cohesive materialstheir sticky qualities.5-7
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