AGGREGATES - Materials for Civil and Construction Engineers - page 169

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Specific Gravity
The weight-volume characteristics of aggregates are not an important indi-
cator of aggregate quality, but they are important for concrete mix design.
Density , the mass per unit volume, could be used for these calculations.
However, specific gravity (Sp. Gr.), the mass of a material divided by the mass
of an equal volume of distilled water, is more commonly used. Four types of
specific gravity are defined based on how voids in the aggregate particles are
considered. Three of these types— bulk-dry, bulk-saturated surface-dry, and
apparent specific gravity—are widely accepted and used in portland cement
and asphalt concrete mix design. These are defined as

5.5.5

Dry Weight

W s

Bulk Dry Sp. Gr. = = (5.1)

1 Total Particle Volume 2 g w 1 V s + V i + V p 2 g w

W s + W p

SSD Weight

Bulk SSD Sp. Gr. = = (5.2)

1 Total Particle Volume 2 g w 1 V s + V i + V p 2 g w

Dry Weight W s

Apparent Sp. Gr. = = (5.3)

1 Volume Not Accessible to Water 2 g w 1 V s + V i 2 g w

where

W s = weight of solids

of solids
of water impermeable voids
of water permeable voids
of water in the permeable voids when the aggregate is in
the SSD condition
weight of water

V s = volume

V i = volume

V p = volume

W p = weight

g w = unit

Figure 5.8 shows that, when aggregates are mixed with asphalt binder,
only a portion of the water-permeable voids are filled with asphalt. Hence,
a fourth type of specific gravity—the effective specific gravity —is defined as

Dry weight W s

Effective Sp.Gr. = = (5.4)

1 2 1 2

Volume not accessible to asphalt g w V s + V c g w

where V c is volume of voids not filled with asphalt cement.

Asphalt binder

Voids not filled
with asphalt

FIGURE 5.8 Aggregate particle
submerged in asphalt cement; not all
voids are filled with asphalt.