heating causes, or would cause, less than 0.1 percent
additional loss in weight.
3. Weigh and record the weight of the oven-dried
sample.
40 Calculate the total moisture content
following formula:
using the
Where:
P
= total moisture content (percent)
W = weight of original sample (in grams)
D = weight of oven-dried sample (in grams)
The surface moisture is equal to the difference
between the total moisture content and the absorption.
An alternate determination of surface moisture in
fine aggregate is obtained by displacement as follows:
1. Select a representative sample of the fine
aggregate weighing not less than 200 grams.
2. Weigh a pycnometer filled to the calibration
mark with water.
3. Place the sample in the pycnometer half filled
with water. Add additional water to the calibration mark
and remove all entrapped air. Weigh the pycnometer,
water, and sample.
4. Calculate the weight of the water displaced by
the sample using the following formula:
Where:
Vs = weight of displaced water (in grams)
Wc = weight of water-filled pycnometer (in
grams)
Ws = weight of sample (in grams)
W = weight of pycnometer, water, and sample
(in grams)
5. Calculate the percent of surface moisture using
the following formula:
Where:
P = percent of surface moisture
Vs = weight of displaced water (in grams)
Ws = Weight of sample (in grams)
Vd = weight of sample in grams divided by the
bulk specific gravity of the sample
ADMIXTURES
Several chemical agents, or admixtures, are
available to improve workability, increase resistance to
freezing and thawing, and compensate for inadequate
curing time and conditions.
Accelerators
Sometimes it is desirable to accelerate the hydration
reactions. The result is a high-early strength and a higher
rate of heat production. This combination can be useful
in winter operations. The addition of a chemical
accelerator (generally calcium chloride) to the mix will
produce the desired conditions. The amount specified is
usually 2 percent of the weight of cement and rarely
more than 3 percent. The main reaction with calcium
chloride occurs within the first 3 days. The ultimate
strength of concrete is not affected by the use of this
chemical.
Retarders
Retarders are used when excessively high heat and
too rapid setting of concrete would prevent full
hydration. Many materials retard setting of concrete.
Basically, these materials are types of fatty acids,
starches, or sugars.
Workability Agents or Plasticizers
The workability of concrete is governed by the
amount of aggregate in the mix. Where reduction of
aggregate (or increase in cement) is impractical,
workability is increased by adding a plasticizer.
Air-entraining agents, when used, are plasticizers. Other
substances include calcium chloride, lime, fly ash, and
other pozzolans. Calcium chloride is also an accelerator.
Lime increases the cementing properties of cement, as
do pozzolans combined with lime. Fly ash is
inexpensive compared to cement and is used as a partial
replacement (up to as much as 50 percent) of the cement.
It changes both the plastic and the hardened properties
of concrete. Fly ash improves workability and reduces
segregation, bleeding, and the heat of hydration. The
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