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Purpose of Aggregates | Up
Builder 3&2 Volume 01 - Construction manual for building structures | Next
Handling and Storage |
Table 6-2.—Recommended Maximum and Minimum Particle Sizes
2 inches to 1 inch, and so on. The recommended
particle size distributions vary with maximum and
minimum nominal size limits, as shown in table 6-2.
A blank space in table 6-2 indicates a sieve that is
not required in the analysis. For example, for the
2 inch to No. 4 nominal size, there are no values listed
under the 4-inch, the 3 1/2-inch, the 3-inch, and the
2 1/2-inch sieves. Since 100 percent of this material
should pass through a 2 1/2-inch sieve, there is no
need to use a sieve coarser than that size. For the
same size designation (that is, 2 inch size aggregate),
there are no values listed under the 1 1/2-inch, the
3/4-inch, and the 3/8-inch sieves. Experience has
shown that it is not necessary to use these sieves in
making this particular analysis.
Quality Standards
Since 66 to 78 percent of the volume of the
finished concrete consists of aggregate, it is
imperative that the aggregate meet certain minimum
quality standards.
It should consist of clean, hard,
strong, durable particles free of chemicals that might
interfere with hydration. The aggregate should also
be free of any superfine material, which might
prevent a bond between the aggregate and the
cement-water paste. The undesirable substances most
frequently found in aggregate are dirt, silt, clay, coal,
mica, salts, and organic matter. Most of these can be
removed by washing. Aggregate can be field-tested
for an excess of silt, clay, and the like, using the
following procedure:
1. Fill a quart jar with the aggregate to a depth of
2 inches.
2.
3.
4.
Add water until the jar is about three-fourths
full.
Shake the jar for 1 minute, then allow it to
stand for 1 hour.
If, at the end of 1 hour, more than 1/8 inch of
sediment has settled on top of the aggregate,
as shown in figure 6-2, the material should be
washed.
An easily constructed rig for washing a small
amount of aggregate is shown in figure 6-3.
Weak, friable (easily pulverized), or laminated
(layered) aggregate particles are undesirable.
Especially avoid shale, stones laminated with shale,
and most varieties of chart (impure flint-like rock).
For most ordinary concrete work, visual inspection is
enough to reveal any weaknesses in the coarse
Figure 6-2.—Quart jar method of determining silt content of
sand.
6-6
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