Purpose of AggregatesThe large, solid coarse aggregate particles formthe basic structural members of the concrete. Thevoids between the larger coarse aggregate particlesare filled by smaller particles. The voids between thesmaller particles are filled by still smaller particles.Finally, the voids between the smallest coarseaggregate particles are filled by the largest fineaggregate particles.In turn, the voids between thelargest fine aggregate particles are filled by smallerfine aggregate particles, the voids between the smallerfine aggregate particles by still smaller particles, andsoon. Finally, the voids between the finest grains arefilled with cement. You can see from this that thebetter the aggregate is graded (that is, the better thedistribution of particles sizes), the more solidly allvoids will be filled, and the denser and stronger willbe the concrete.The cement and water form a paste that binds theaggregate particles solidly together when it hardens.In a well-graded, well-designed, and well-mixedbatch, each aggregate particle is thoroughly coatedwith the cement-water paste. Each particle is solidlybound to adjacent particles when the cement-waterpaste hardens.AGGREGATE SIEVES.— The size of an aggre-gate sieve is designated by the number of meshes tothe linear inch in that sieve. The higher the number,the finer the sieve. Any material retained on the No. 4sieve can be considered either coarse or fine.Aggregates huger than No. 4 are all course; thosesmaller are all fines. No. 4 aggregates are thedividing point. The finest coarse-aggregate sieve isthe same No. 4 used as the coarsest fine-aggregatesieve. With this exception, a coarse-aggregate sieveis designated by the size of one of its openings. Thesieves commonly used are 1 1/2 inches, 3/4 inch,1/2 inch, 3/8 inch, and No. 4. Any material thatpasses through the No. 200 sieve is too fine to be usedin making concrete.PARTICLE DISTRIBUTION.— Experienceand experiments show that for ordinary buildingconcrete, certain particle distributions consistentlyseem to produce the best results. For tine aggregate,the recommended distribution of particle sizes fromNo. 4 to No. 100 is shown in table 6-1.The distribution of particle sizes in aggregate isdetermined by extracting a representative sample ofthe material, screening the sample through a series ofsieves ranging in size from coarse to fine, anddetermining the percentage of the sample retained oneach sieve. This procedure is called making a sieveanalysis. For example, suppose the total sampleweighs 1 pound. Place this on the No. 4 sieve, andshake the sieve until nothing more goes through. Ifwhat is left on the sieve weighs 0.05 pound, then5 percent of the total sample is retained on the No. 4sieve. Place what passes through on the No. 8 sieveand shake it. Suppose you find that what stays on thissieve weighs 0.1 pound. Since 0.1 pound is 10percent of 1 pound, 10 percent of the total sample wasretained on the No. 8 sieve. The cumulative retainedweight is 0.15 pound. By dividing 0.15 by 1.0 pound,you will find that the total retained weight is 15percent.The size of coarse aggregate is usually specifiedas a range between a minimum and a maximum size;for example, 2 inches to No. 4, 1 inch to No. 4,Table 6-1.—Recommended Distribution of Particle Sizes6-5