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SIEVE ANALYSIS AND FINENESS MODULUS
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Sampling
Since the reason for sampling aggregates is to determine the gradation (particle
size) of the aggregate, it is necessary that they be sampled correctly.
The results of testing will reflect the condition and characteristics of the aggregate
from which the sample is obtained. Therefore, when sampling, it is important to obtain a
representative sample that is representative of the source being tested. Unless it is truly
representative, the test results apply to the sample only and not to the entire aggregate
shipment or stockpile. Without accuracy in sampling, test results are worthless.
In many cases, representative sampling cannot be achieved by a single sample. It
may be necessary to take a number of samples to obtain a true picture of the properties
of a stockpile or source of material. Also, as the maximum particle size in the aggregate
increases, the size of the sample must increase to maintain accuracy in testing.
How To Take a Sample
There are three principal aggregate sampling points that are of concern at a concrete
plant. These are:
1) source of material, (quarry, gravel pit, etc.)
2) the stockpile, and
3) the storage bin.
The first two will be discussed in this section and the third in the section on moisture
content and batch weight adjustments.
When sampling at the source of materials, it would be well to remember one
general rule. It is easier to obtain a representative sample from the production stream,
such as from the conveyor belt, than from trucks, storage bins or stockpiles. If the sample
is taken from the conveyor belt, take the entire cross-section of the belt. The same is true
when sampling from the chutes or bins.
Getting a sample from a stockpile is not easy, and great care must be taken to
obtain a truly representative sample. Segregation usually occurs when the material is
stockpiled, because the coarse particles will roll to the base of the pile while the fine
particles stay on top. When sampling coarse aggregates from stockpiles, samples should
be taken at or near the top and base, and at some intermediate point. To prevent further
segregation while sampling, a board may be shoved into the pile just above the sampling
area. A second method of sampling coarse materials would be to expose the face of the
stockpile from the top to the bottom, with a front end loader. The samples could then be
taken from the exposed face. A third method would be to have the overhead loader take
a scoop from bottom to top and dump the material in a convenient location for sampling.
The sample bag could then be filled from various locations around the scoop of material.
Fine aggregate may be sampled with a sampling tube approximately 1 ¼ inches (30 mm)
in diameter and 6 feet (1.8 meters) in length. Or if sampling a stockpile of sand, or fine
aggregate, it is usually necessary to remove the dry layers where the segregation occurs
and sample the damp material below.
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Figure 1. Sampling locations
Sieve Analysis
Aggregate gradation (sieve analysis) is the distribution of particle sizes expressed
as a percent of the total dry weight. Gradation is determined by passing the material
through a series of sieves stacked with progressively smaller openings from top to bottom
and weighing the material retained on each sieve. Sieve numbers and sizes most often
used in grading aggregates for Hydraulic Concrete paving mixtures are as follows:
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Nominal Dimensions of U.S. Standard Sieves - AASHTO M 92
Sieve Designation Nominal sieve
Openings
metric standard Metric inches
50.0 mm 2 50.0 mm 2.00
37.5 mm 1 ½ 37.5 mm 1.50
25.0 mm 1 25.0 mm 1.00
19.0 mm 3/4 19.0 mm 0.750
12.5 mm 1/2 12.5 mm 0.500
9.5 mm 3/8 9.5 mm 0.375
4.75 mm No. 4 4.75 mm 0.187
2.36 mm No. 8 2.36 mm 0.093
1.18 mm No. 16 1.18 mm 0.0469
600 μm No. 30 0.600 mm 0.0234
300 μm No. 50 0.300 mm 0.0117
150 μm No. 100 0.150 mm 0.0059
75 μm No. 200 0.075 mm 0.0029
Sieve sizes to be used for the various mixtures are designated in the Road and
Bridge Specifications. Gradations are expressed on the basis of total percent dry weight
passing, which indicates the total percent of aggregate by weight that will pass a given
size sieve.
Some of the descriptive terms used in referring to aggregate gradations are:
Coarse Aggregate: All the materials retained on and above the No. 8 (2.36 mm) sieve
Fine Aggregate: All the material passing the No. 8 (2.36 mm) sieve.
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Procedure For Sieve Analysis
Dry sieve analysis and washed sieve analysis are two methods of determining
proportions of various particle sizes in a mineral aggregate. In Virginia, however, the
WASHED SIEVE ANALYSIS is used, and will be discussed in this section. Standard
procedures for running the sieve analysis are given in AASHTO T 27 and AASHTO T
11.
Regardless of the size of the aggregate, the procedure for running a sieve analysis
is basically the same. The steps for this procedure are outlined as follows:
1. Obtain a representative sample of the material from the original sample by either a
sample splitter or the quartering method. (See paragraphs a and b below.) Reduce
to a size that can be handled on the balance and sieves, also, according to maximum
stone size. Reference AASHTO T 27.
a. Sample Splitter - Sample splitters shall have even number of equal width chutes,
but not less than a total of eight for coarse aggregates, or twelve for fine aggregates,
which discharge alternately to each side of the splitter. The splitter shall be
equipped with two receptacles to hold the two halves of the sample following
splitting. It shall be equipped with a hopper or straightedge pan which has a
width of the assembly of chutes, by which the sample may be fed at a controlled
rate to the chutes. The splitter and accessory equipment shall be so designed
that the sample will flow smoothly without restriction or loss of material. Place
the field sample in the hopper or pan and uniformly distribute it from edge to
edge, so that when it is introduced into the chutes, approximately equal amounts
will flow through each chute. The rate at which the sample is introduced shall
be such as to allow free flowing through the chutes into the receptacles below.
Reintroduce the portion of the sample in one of the receptacles into the splitter
as many times as necessary to reduce the sample to the size specified for the
intended test. The portion of the material collected in the other receptacle may
be reserved for reduction in size for other tests.
b. Quartering Method - The following method for size reduction by quartering is
outlined for use when a conventional sample splitter is not available.
1. Distribute a shovel full of the aggregate as uniformly as possible over a
wide, flat area on a tight weave canvas or other smooth surface. Continue
to distribute shovels full of material in layers until all the sample is used
to make a wide, flat pile that is reasonably uniform in thickness and
diameter. Do not permit coning of the aggregate.
2. Divide the pile cleanly into equal quarters with a square-ended shovel
or straight piece of sheet metal. When a canvas is used, the division may
be conveniently made by inserting a thin stick (or rod) under the canvas
and raising it to divide the sample equally, first into halves, then into
quarters.
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3. Remove two opposite quarters, including all fine materials, and set
aside.
4. Repeat the foregoing procedure with the remaining portion of the
aggregate until a test sample of desired size is obtained.
5. If desired, store the portion that has been set aside for possible check
testing.
2. Dry aggregate sample thoroughly. The samples are dried to constant weight on a
hot plate or in an oven at a temperature of 230°F (110ºC).
3. Accurately weigh the dried sample. When weighing and handling the sample,
extreme care must be taken to avoid any loss of the material, as this will affect the
accuracy of the results. Also, do not adjust the weight of the split sample to an
even figure, such as 500 grams, 1000 grams, etc. Use the entire reduced and dried
sample.
4. Record the total dry weight on the worksheet. For example, assume the total dry
weight of the sample is 506.4 grams.
5. Wash the sample over a nest of two sieves, the upper or top sieve being the No.
16 (1.18 mm) mesh sieve and the lower or the bottom sieve being the No. 200 (75
μm) mesh sieve. Doing this, you would take your sample, add water, to cover the
material completely, add a drop of soap and wash it thoroughly being careful not to
lose any of the material. Pour the water and material over the nest of sieves, the No.
16 (1.18 mm) and No. 200 (75 μm), being careful not to lose any of the material, then
repeat the procedure until the water is clear.
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Sample Splitter
(a) Large Riffle Samplers for Coarse Aggregate
(b) Small Riffle Sampler for fine aggregate
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Quartering Method
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6. Dry the sample again to a constant weight on a hot plate or in an oven at a temperature
of 230°F (110 ºC), then accurately weigh and record.
7. Separate the sample into individual sizes using the proper sieves. The sieves
normally used are the standard 8 inch (200 mm) sieves. With Fine Aggregate the
coarse sieve or the 3/8” (9.5 mm) sieve at the top; each sieve below is finer; and the
finest sieve, a No. 200 (75 μm) is at the bottom. A pan is placed below to retain any
fine material that may pass this sieve.
The dried sample is placed on the top sieve, and the entire nest of sieves is placed in
a shaker that produces a circular and tapping motion, or in other approved shaking
devices. This motion assists gravity in settling the individual aggregate particles on
the sieve which will properly identify the size of that particular particle. It will take
approximately 7 to 10 minutes of shaking to separate the material. Always refer to
the AASHTO T-27 procedure to assure shaking for the proper amount of time.
8. Weigh and record the weights retained on each sieve cumulatively, increasing by
successive additions to the nearest 0.1 grams. For example, suppose a particular
gradation required a maximum size of 3/8” (9.5 mm). Upon examination of this
sieve no material was retained. This would then be recorded in the “Cumulative
Grams Retained” column of the worksheet, as “0” for this sieve. The No. 4 (4.75 mm)
sieve is then checked, the material carefully removed, placed on the balance and a
weight of 14.8 grams is recorded. The No. 8 (2.36 mm) sieve is removed from the
nest of sieves, the material is emptied into the pan with the No. 4 (4.75 mm) material
and the cumulative weight is recorded on the worksheet beside the No.8 (2.36 mm)
sieve, example 47.6 grams. Repeat the procedure until all of the sieves are weighed
and recorded cumulatively. (Note: In a fine aggregate sieve analysis, the test sample
is washed over the No. 200 (75 μm) sieve and the portion retained on the No. 200 (75
μm) sieve is dried and the loss recorded.)
Sieve Analysis - Example
The following sieve analysis is for a sample of natural sand for use in concrete
subject to abrasion and meets Virginia Department of Transportation requirements for
Grading “A” Sand.
Sieve
Size
Cumulative
Grams Retained
Cumulative
%Retained %Passing
VDOT Specs.
(%Passing)
3/8 inch 0.0 0.0 100.0 100.0
No. 4 14.8 2.9 97.1 95-100
No. 8 47.6 9.4 90.6 80-100
No. 16 117.3 23.2 76.8 50-85
No. 30 274.2 54.1 45.9 25-60
No. 50 418.8 82.7 17.3 5-30
No. 100 467.4 92.3 7.7 0-10
No. 200 494.2 97.6 2.4 0-3
PAN 506.4 100.0 0.0
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In this example, the cumulative grams retained on each sieve was determined
in the sieve analysis.
9. Calculate the cumulative percent retained on each sieve. (Answer to the nearest
0.1%) This is determined by the following formula:
Cumulative % Retained: Cumulative Grams Retained X 100
Total Weight of Sample
For Example: Weight on the No. 4 sieve = 14.8 grams
Total Dry Weight of Sample = 506.4 grams
%Retained = 14.8 = .029 x 100 = 2.9%
506.4
This is performed for each sieve size and the end figures entered in the “Cumulative
% Retained” column of the worksheet.
10. Calculate the percent passing each sieve. (Answer to the nearest 0.1%). To determine
this figure, subtract the percent retained on each sieve from 100.
Example:
1.) % Retained on 3/8 inch sieve = 0
100 - 0 = 100.0% passing 3/8 inch sieve
2.) % Retained on No. 4 sieve = 2.9
100 - 2.9 = 97.1% passing No. 4 sieve
3.) % Retained on No. 8 sieve = 9.4
100 - 9.4 = 90.6% passing No. 8 sieve
This is performed for each sieve and entered in the % Passing Column of the
worksheet. Check to see if the % Passing complies with the Virginia Department of
Transportation Specifications (Table II-1, page 2-12).