Civil Engineering Reference
In-Depth Information
1. One-fifth the narrowest dimension of a concrete
member
2. Three-quarters the clear spacing between reinforcing
bars and between the reinforcing bars and forms
3. One-third the depth of slabs
These requirements may be waived if, in the judg-
ment of the engineer, the mixture possesses sufficient
workability that the concrete can be properly placed with-
out honeycomb or voids.
• The optimum mixture has the least particle interfer-
ence and responds best to a high frequency, high
amplitude vibrator.
However, this optimum mixture cannot be used for
all construction due to variations in placing and finishing
needs and availability.
Crouch (2000)
found in his studies
on air-entrained concrete that the water-cement ratio
could be reduced by over 8% using combined aggregate
gradation.
Shilstone (1990)
also analyzes aggregate grada-
tion by coarseness and workability factors to improve
aggregate gradation.
Combined Aggregate Grading
Aggregate is sometimes analyzed using the combined
grading of fine and coarse aggregate together, as they exist
in a concrete mixture. This provides a more thorough
analysis of how the aggregates will perform in concrete.
Sometimes mid-sized aggregate, around the 9.5 mm (
3
⁄
8
in.) size, is lacking in an aggregate supply, resulting in a
concrete with high shrinkage properties, high water
demand, poor workability, poor pumpability, and poor
placeability. Strength and durability may also be affected.
Fig. 5-10 illustrates an ideal gradation; however, a
perfect gradation does not exist in the field—but we can
try to approach it. If problems develop due to a poor
gradation, alternative aggregates, blending, or special
screening of existing aggregates, should be considered.
Refer to
Shilstone (1990)
for options on obtaining optimal
grading of aggregate.
The combined gradation can be used to better control
workability, pumpability, shrinkage, and other properties
of concrete.
Abrams (1918)
and
Shilstone (1990)
demon-
strate the benefits of a combined aggregate analysis:
• With constant cement content and constant consis-
tency, there is an optimum for every combination of
aggregates that will produce the most effective water
to cement ratio and highest strength.
Gap-Graded Aggregates
In gap-graded aggregates certain particle sizes are inten-
tionally omitted. For cast-in-place concrete, typical gap-
graded aggregates consist of only one size of coarse
aggregate with all the particles of fine aggregate able to
pass through the voids in the compacted coarse aggregate.
Gap-graded mixes are used in architectural concrete to
obtain uniform textures in exposed-aggregate finishes.
They can also used in normal structural concrete because
of possible improvements in some concrete properties, and
to permit the use of local aggregate gradations (
Houston
1962
and
Litvin and Pfeifer 1965
).
For an aggregate of 19-mm (
3
⁄
4
-in.) maximum size, the
4.75 mm to 9.5 mm (No. 4 to
3
⁄
8
in.) particles can be omit-
ted without making the concrete unduly harsh or subject
to segregation. In the case of 37.5 mm (1
1
⁄
2
in.) aggregate,
usually the 4.75 mm to 19 mm (No. 4 to
3
⁄
4
in.) sizes are
omitted.
Care must be taken in choosing the percentage of fine
aggregate in a gap-graded mixture. A wrong choice can
result in concrete that is likely to segregate or honeycomb
because of an excess of coarse aggregate. Also, concrete
with an excess of fine aggregate could have a high water
demand resulting in a low-density concrete. Fine aggregate
is usually 25% to 35% by volume of the total aggregate. The
lower percentage is used with rounded aggregates and the
higher with crushed material. For a smooth off-the-form
finish, a somewhat higher percentage of fine aggregate to
total aggregate may be used than for an exposed-aggregate
finish, but both use a lower fine aggregate content than
continuously graded mixtures. Fine aggregate content also
depends upon cement content, type of aggregate, and
workability.
Air entrainment is usually required for workability
since low-slump, gap-graded mixes use a low fine aggre-
gate percentage and produce harsh mixes without
entrained air.
Segregation of gap-graded mixes must be prevented
by restricting the slump to the lowest value consistent with
good consolidation. This may vary from zero to 75 mm (to
3 in.) depending on the thickness of the section, amount of
reinforcement, and height of casting. Close control of grad-
ing and water content is also required because variations
might cause segregation. If a stiff mixture is required, gap-
25
20
15
10
5
0
Sieve size in mm
Fig. 5-10. Optimum combined aggregate grading for
concrete.
