AGGREGATES - Materials for Civil and Construction Engineers - page 189

Note that the percentage of coarse aggregate in each stockpile was calculated by
subtracting the percentage passing the 4.75 mm sieve from 100.

Asphalt concrete mix design requires that the engineer knows the com-
posite specific gravity of all aggregates in the mix. The composite specific
gravity of a mix of different aggregates is obtained by the formula

1

(5.18)

G =

P 1
G 1

P 2
G 2

P 3
G 3

+ + + . Á

where

specific gravity
gravities of fractions 1, 2, and 3
fractions by weight of aggregates 1, 2, and 3 used in
the blend, where the total is 1.00

G = composite

G 1 , G 2 , G 3 = specific

P 1 , P 2 , P 3 = decimal

Note that Equation 5.18 is used only to obtain the combined specific
gravity and density of the blend, whereas Equation 5.17 is used to obtain
other combined properties.

Sample Problem 5.7

Aggregates from three sources having bulk specific gravities of 2.753, 2.649, and
2.689 were blended at a ratio of 70:20:10 by weight, respectively. What is the bulk
specific gravity of the aggregate blend?

Solution

1

G = = 2.725

0.7
2.753 +

0.2
2.649 +

0.1
2.689

■ 5.5.9 Deleterious Substances in Aggregate

A deleterious substance is any material that adversely affects the quality of
portland cement or asphalt concrete made with the aggregate. Table 5.10
identifies the main deleterious substances in aggregates and their effects on
portland cement concrete. In asphalt concrete, deleterious substances are
clay lumps, soft or friable particles, and coatings. These substances decrease
the adhesion between asphalt and aggregate particles.