Civil Engineering Reference
In-Depth Information
Saturated,
surface dry
Damp
or wet
water. It is used in certain computations for mixture
proportioning and control, such as the volume occupied by
the aggregate in the absolute volume method of mix
design. It is not generally used as a measure of aggregate
quality, though some porous aggregates that exhibit accel-
erated freeze-thaw deterioration do have low specific grav-
ities. Most natural aggregates have relative densities
between 2.4 and 2.9 with corresponding particle (mass)
densities of 2400 and 2900 kg/m
3
(150 and 181 lb/ft
3
).
Test methods for determining relative densities for
coarse and fine aggregates are described in ASTM C 127
(AASHTO T 85) and ASTM C 128 (AASHTO T 84), respec-
tively. The relative density of an aggregate may be deter-
mined on an ovendry basis or a saturated surface-dry (SSD)
basis. Both the ovendry and saturated surface-dry relative
densities may be used in concrete mixture proportioning
calculations. Ovendry aggregates do not contain any
absorbed or free water. They are dried in an oven to con-
stant weight. Saturated surface-dry aggregates are those in
which the pores in each aggregate particle are filled with
water but there is no excess water on the particle surface.
State
Ovendry
Air dry
Total
moisture:
None
Less than
potential
absorption
Equal to
potential
absorption
Greater
than
absorption
Fig. 5-12. Moisture conditions of aggregate.
aggregates in order to accurately meet the water require-
ment of the mix design. If the water content of the concrete
mixture is not kept constant, the water-cement ratio will
vary from batch to batch causing other properties, such as
the compressive strength and workability to vary from
batch to batch.
Coarse and fine aggregate will generally have absorp-
tion levels (moisture contents at SSD) in the range of 0.2%
to 4% and 0.2% to 2%, respectively. Free-water contents will
usually range from 0.5% to 2% for coarse aggregate and 2%
to 6% for fine aggregate. The maximum water content of
drained coarse aggregate is usually less than that of fine
aggregate. Most fine aggregates can maintain a maximum
drained moisture content of about 3% to 8% whereas coarse
aggregates can maintain only about 1% to 6%.
Bulking.
Bulking is the increase in total volume of moist
fine aggregate over the same mass dry. Surface tension in
the moisture holds the particles apart, causing an increase
in volume. Bulking of a fine aggregate (such as sand)
occurs when it is shoveled or otherwise moved in a damp
condition, even though it may have been fully consoli-
Density
The density of aggregate particles used in mixture propor-
tioning computations (not including voids between parti-
cles) is determined by multiplying the relative density
(specific gravity) of the aggregate times the density of
water. An approximate value of 1000 kg/m
3
(62.4 lb/ft
3
) is
often used for the density of water. The density of aggre-
gate, along with more accurate values for water density,
are provided in ASTM C 127 (AASHTO T 85) and ASTM
C 128 (AASHTO T 84). Most natural aggregates have
particle densities of between 2400 and 2900 kg/m
3
(150
40
and 181 lb/ft
3
).
Fine grading
Absorption and Surface Moisture
30
Medium
grading
The absorption and surface moisture of aggregates should
be determined according to ASTM C 70, C 127, C 128, and
C 566 (AASHTO T 255) so that the total water content of
the concrete can be controlled and correct batch weights
determined. The internal structure of an aggregate particle
is made up of solid matter and voids that may or may not
contain water.
The moisture conditions of aggregates are shown in
Fig. 5-12. They are designated as:
1.
Ovendry
—fully absorbent
2.
Air dry
—dry at the particle surface but containing
some interior moisture, thus still somewhat absorbent
3.
Saturated surface dry (SSD)
—neither absorbing water
from nor contributing water to the concrete mixture
4.
Damp or wet
—containing an excess of moisture on
the surface (free water)
The amount of water added at the concrete batch
plant must be adjusted for the moisture conditions of the
20
Coarse
grading
10
0
0
5
10
15
20
Percent of moisture added by mass
to dry, rodded fine aggregate
Fig. 5-13. Surface moisture on fine aggregate can cause
considerable bulking; the amount varies with the amount of
moisture and the aggregate grading (PCA Major Series 172
and PCA ST20).
