Oven-dry and weigh the material retained. Then you can
calculate the percentage of clay lumps and friable
particles using the following equation:
Where:
P = percent of clay lumps and friable
particles
w1 = original weight of test sample
W2 = weight of retained sample after wet
sieving and drying
Test for Undesirable Lightweight Material.—
Soft, laminated pieces of aggregate, such as chert or
shale, are harmful to concrete. Coal and lignite, also, are
harmful and are distinguished from the lightweight
materials by the brownish black or black color of the
particles. Visual examination of the coarse aggregate
will often show these minerals. The amount of these
minerals in an aggregate can be determined by
submersing the aggregate in a liquid with a specific
gravity that will allow the shale or other light particles
to float and heavier particles to sink.
For specific testing procedures, you should refer to
ASTM C 123 or to NAVFAC MO-330. Briefly, you
perform the test by first sieving the dried sample over a
No. 50 sieve for fine aggregate and a No. 4 sieve for
coarse aggregate. Weigh the sample and then place it
into a heavy liquid, such as zinc chloride (specific
gravity of 1.95 at 78°F). Agitate the mixture to allow the
lightweight particles to rise to the surface and skim them
off. When repeated agitation causes no further particles
to rise, you wash the sample in alcohol and then dry and
weigh it. The percentage of undesirable, lightweight
particles can then be calculated using the following
formula:
Where:
L = percentage of lightweight material
W1 = dry weight of lightweight material
W2 =
dry weight of initial sample retained
on No. 50 sieve for fine aggregate or
No. 4 sieve for coarse aggregate
Color Test for Organic Matter.— Any sand that
gives a color darker than the standard of this test
probably contains an excess of organic matter that will
reduce the strength of the concrete in which the sand is
used. If you determine that organic matter is present, it
is possible that it can be removed by washing; if not,
better sand should be obtained. If neither of these things
can be done, it will be necessary to use a lower
water-cement ratio and control the concrete production
carefully to obtain the desired strength.
For the color test, you will need the items illustrated
in figure 13-17. To perform the test, you add the sample
being tested to a solution of sodium hydroxide and
water. After allowing the sample to stand for 24 hours,
you then compare the color of the liquid above the
sample to a standard color solution consisting of tannic
acid, alcohol, and sodium hydroxide. If the liquid above
the sample is darker in color than the standard solution,
the sand may contain organic impurities. NAVFAC
MO-330 provides full details for preparing the solutions
and performing the test.
As an alternative, the color of the liquid above the
sample can be compared with the colors given in the
ASTM standard color plate. Then decide whether the
sand contains an excessive amount of organic matter.
SPECIFIC GRAVITY, ABSORPTION, AND
SURFACE MOISTURE.— These tests must be
performed on the aggregate before the necessary
calculations can be made to design a concrete mixture.
For aggregates used in portland cement concrete,
measurements are made to determine the bulk specific
gravity of the aggregates in a saturated, surface-dry
(SSD) condition. Specific gravity is thus based on
determining the total volume occupied by the aggregate
particles, including the permeable pore space.
Absorption and surface moisture determinations are
necessary to calculate the amount of mixing water used
in a concrete mixture. Absorption, determined as a
percentage, represents the moisture content of the
aggregate when the aggregate is in a SSD condition.
Surface moisture is the water that is present in both fine
and coarse aggregate, exceeding that which corresponds
to a SSD condition.
Concrete-mixture design is discussed at the EA1
level in Part 2 of this TRAMAN.
Specific Gravity and Absorption, Coarse
Aggregate (ASTM C 127).— The summarized steps in
determining the bulk specific gravity of SSD coarse
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