Grading refers to the determination of
the particle-size distribution for aggregate. Grading limits and maximum
aggregate size are specified because grading and size affect the amount of
aggregate used as well as cement and water requirements, workability,
pumpability, and durability of concrete. In general, if the water-cement ratio
is chosen correctly, a wide range in grading can be used without a major effect
on strength. When gap-graded aggregate is specified, certain particle sizes of
aggregate are omitted from the size continuum. Gap-graded aggregate are used to
obtain uniform textures in exposed aggregate concrete. Close control of mix
proportions is necessary to avoid segregation.
AIM:
To determine particle size distribution
of coarse aggregates by sieving or screening.
Apparatus:
·
Test Sieves conforming to IS: 460-1962
o
80 mm,
o
40 mm,
o
20 mm,
o
10 mm,
o
4.75 mm,
·
Balance,
·
Stop Watch.
FIGURE:
Procedure:
1.
Take a sample of coarse
aggregate in pan and placed it in dry oven at a temperature of 100 – 110oC.
2.
Take the sample weight to
nearest 0.1 gram.
3. Place the set of sieves, with the largest
opening on top, into a pan and pour the aggregate onto the top sieve.
4.
The use a mechanical sieve shaker is the
preferred method for separating the materials into the specified sizes. If a
mechanical shaker is not available, proceed with hand shaking.
5.
Establish a shaking time for different types
of aggregates that will assure proper sieving of the material without
degradation.
6.
When the required mechanical shaking time is
complete, hand sieve by lateral and vertical motion of the sieves with a
“jarring” action that keeps the material moving continuously over the surface
of the sieves.
7.
Continue hand sieving until no material
passes through the sieve.
8.
Brush particles clinging to each sieve into
the next lower sieve with a bristle brush. Ensure no material is lost.
9.
Determine the individual weights to the
nearest 0.1 g of aggregate retained on each sieve and record these weights.
10.
Report the percentages to the nearest 0.1%
for each size of aggregate retained on each sieve.
OBSERVATION:
IS Sieve
|
Weight Retained
on Sieve (gms)
|
Percentage of
Weight Retained
(%)
|
Percentage of
Weight Passing
(%)
|
Cumulative
Percentage of
Passing (%)
|
Remark
|
80 mm
|
|||||
40 mm
|
|||||
20 mm
|
|||||
10 mm
|
|||||
4.75 mm
|
|||||
Total
|
CALCULATION:
The
percent retained (%), Cumulative retained (%) & percent finer (%) is
calculated.
Percent
retained on each sieve = Weight of retained sample in each sieve / Total weight
of sample
The
cumulative percent retained is calculated by adding percent retained on each
sieve as a cumulative procedure.
The
percent finer is calculated by subtracting the cumulative percent retained from
100 percent.
Conclusion / Result:
The result of the sieve analysis is reported
graphically on a semi log graph, taking sieve sizes on log scale and % finer in
arithmetic scale.
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