Section 3: Specific Gravity, Absorption and Unit Weight

Section 3:
Specific Gravity,
Absorption and Unit
Weight
Specific Gravity
S.G. is a measure of relative density
• How much heavier is an aggregate particle
compared to an equal volume of water?
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Specific Gravity
Water: SG = 1
Steel: SG = 7.85
• Since SG is greater than water, steel
sinks in water
Plywood: SG = 0.5 to 1
• Since SG is less than water, plywood
floats in water
Specific Gravity
Common Values of SG for SC Aggregates
Granite: SG = 2.65
Limestone: SG = 2.66
Gneiss: SG = 2.74
Lightweight: SG = 1.41 (Carolina Stalite Salisbury, NC)
Steel Slag: SG = 3.43 (International Mill
Service - Georgetown, SC)
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Specific Gravity
Specific Gravity can be measured
three ways:
• Bulk
• Bulk, saturated surface dry (SSD)
• Apparent
The test procedure for Specific
Gravity and Absorption is given in
ASTM C127
Bulk Specific Gravity
Bulk specific
gravity includes
volume of pores
‘Real’ specific
gravity
stone
pore
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Bulk SSD Specific Gravity
Saturated Surface Dry (SSD)
Assumes all permeable pores filled with water
(saturated)
Particles appear moist but not shiny (surface dry)
X
Apparent Specific Gravity
Aggregate has
tiny pores
Apparent specific
gravity does not
include volume
of pores (only
volume of solid
stone)
stone
X
pore
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Absorption
Because of the void
spaces, aggregates are
like sponges
Absorption is a measure
of the amount of water
absorbed into pores of
aggregate particles
Absorption is expressed
as percentage of water
in the pores to the dry
weight of the aggregate
Absorption
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Absorption
Common Values
Absorption is typically less than 2%
• if dry aggregate weighs 1 pound, water in pores
weighs 0.02 pounds
Granite: 0.4% (Hanson – Jefferson, SC)
Limestone: 0.5% (Martin Marietta – Kings
Mountain, NC)
Sandstone: 1.8%
Marine Limestone: 3.1% (Grand Strand
Aggregates - Goretown, SC)
Unit Weight
Significance and Use
Often used to determine bulk density
values used in selecting proportions
for concrete mixes
Also used for mass/volume
relationships for conversions in
purchase agreements
Also used annually for inventory
control and verification
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Unit Weight
Unit Weight is the weight per unit
volume of a material.
Unit Weight = Weight of Material
Volume of Material
Example:
U.W. = 103.2 lbs = 103.2 pcf
1 cf
Unit Weight
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