The desired characteristics of aggregates used for bituminous paving include angular shape, rough surface, hardness, and gradation. These qualities and grain distribution largely affect the quality of the resulting pavement. The larger, coarse aggregate particles are the main structural members of the pavement; however, if there were nothing but large particles, there would be many unfilled voids between adjacent particles. The fewer voids the mixture has, the more dense the pavement and, therefore, the more

durable it is. Ideal density is obtained by filling the voids between the largest particles with smaller particles, and soon, right down through the whole range of sizes from coarsest to finest.

Common practice divides the aggregates used for bituminous paving into coarse aggregate (retained on the No. 10 sieve), fine aggregate (retained on the No. 200 sieve), and mineral filler. Usually 65 percent or more of the mineral filler will pass the No. 200 sieve. The distribution of the sizes determines how many voids will remain and aids in determining how much bitumen will be needed.

The EA must be able to perform a number of tests on aggregates to determine their acceptability y for bituminous construction. One test is the grain size distribution test that is done by sieve analysis, using the same procedure as described for soil in the EA3 TRAMAN. Tests for mineral dust and specific gravity are also needed. Instructions for conducting both these tests are contained in the following paragraphs.

Figure 13-34.-Dunagan apparatus for specific gravity test.

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