Sampling of Aggregates AASHTO T-2-91 - New Mexico Department ...

Aggregate T-2 January 2012
Sampling of Aggregates
AASHTO T-2-91 (2010)
(Modified)
Prior to performing any of the tests required for aggregate, it is necessary to obtain a sample.
Samples can be obtained by a variety of methods. The preferred method is from a conveyor belt
or bin discharge, and the third best is from the roadway. Windrow sampling is a distant fourth in
order of preference, and stockpile sampling is the least preferred method, although relatively
common. State guidelines will dictate point of acceptance.
The most important consideration in sampling is to obtain a sample that represents the
material to be tested. This can be greatly facilitated by combining several incremental samples
selected using a systematic random sampling plan, by sampling the full cross-section at the
sampling point where ever possible, by using methods which minimize segregation, and by
avoiding obviously unrepresentative material.
For NMDOT construction projects, the methods in AASHTO T-2 will apply primarily to base
course and to aggregates for hot-mix asphalt (HMA), open-graded friction course (OGFC) and
Portland cement concrete (PCC).
Modification: 1. Windrow and Roadway samples require one increment.
2. Windrow sampling is permitted and described.
Key Elements:
1. Select sample locations by random method.
This requires use of random number table or
random number generator applied
systematically to the whole of material
represented by sample to determine specific
sampling points. Samples tested for quality
shall be obtained from the finished product
unless otherwise specified (4.1). Random
sampling does not mean haphazard sampling.
2. Combine sample mass to comply with
Table 1, AASHTO T-2. This table provides
minimum sample masses as a function of the
maximum nominal aggregate size of the
material. The values in the table apply to
processed aggregates only. For combined
materials (e.g. base course) AASHTO
recommends that the values in the table be
increased by at least 10 kg (22 lb). In New
Mexico, this will be considered optional for
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AASHTO T-2 (2005 version – English / Metric Units)
TABLE 1 – Size of Samples
Approximate Minimum
Maximum Nominal Size Mass of Field Samples,
of Aggregates A lb (kg) B
Fine Aggregate
No. 8 (2.36 mm) 25 (10)
No. 4 (4.75 mm) 25 (10)
Coarse Aggregate
3/8 in. (9.5 mm) 25 (10)
½ in. (12.5 mm) 35 (15)
AASHTO T-2 (2005 version – English /Metric Units)
¾ in. (19.0 mm) 55 (25)
1 in. (25.0 mm) 110 (50)
1 ½ in. (37.5 mm) 165 (75)
2 in. (50 mm) 220 (100)
2 ½ in. (63 mm) 275 (125)
3 in. (75 mm) 330 (180)
3 ½ in. (80 mm) 385 (175)
A
For processed aggregate the nominal maximum size of particles is the largest sieve
size listed in the applicable specification, upon which any material is permitted to be
retained.
B
For combined coarse and fine aggregates (for example, base or subbase) minimum
weight shall be coarse aggregate minimum plus 25 lb (10 kg).
practical reasons, except when sample is a referee sample. This is a minor deviation from
AASHTO. The values are minimum masses to ensure a reasonably representative sample.
Larger samples should be obtained if necessary for tests to be performed (e.g. mix designs).

Aggregate T-2 January 2012
3. Obtain increments as required. The increments combined shall meet or exceed the
minimum mass described in No. 2 above. Again, locations for incremental samples should
be determined using random number methodology. Windrow and Roadway samples
require one (1) increment each. Flowing Stream/Bin, Conveyor Belt, and Stockpile
samples require three (3) increments each.
4. Place sample in suitable container. Transport container shall be constructed to prevent
contamination, loss of material, loss of moisture, and contain fines or dust and prevent
damage to contents from mishandling during shipment. Cloth bags are commonly used for
aggregate. For material containing significant quantities for fines or dust, an alternate
container such as a can or box may be more appropriate, in order to minimize loss of
material.
5. Label sample properly. Minimum information shall include project I.D., name of sampler,
sample location, date sampled, description of material and purpose of sample (tests
required). Labels shall not smudge or run when wet and be affixed to the container in a way
that it will not become detached or lost.
Methods:
A. Flowing Aggregate Stream (Belt or Bin Discharge): (4.3.1)
1. Sample entire cross-section of discharge. It will probably be necessary to use a
sampling box or a pan on rails beneath the discharge to intercept the flowing aggregate
stream. Never attempt to intercept the stream with a shovel or a scoop, as this can cause
serious injury. Obtain three approximately equal increments.
2. Avoid sampling the initial discharge or final few tons of aggregates (Note 3).
Sampling the initial and/or the final portion of material increases the chances of
obtaining a segregated sample. Additionally, bins should be kept as full as possible
during the process to minimize segregation.
B. Conveyor Belt: (4.3.2)
1. Stop Conveyor. This is necessary for safety as well as for proper sampling technique.
2. Insert a template conforming to the shape of the conveyor belt. A template shall be
inserted perpendicular to the belt at a spacing that yields the desired increment of
material. Three increments are required to meet the mass in Table 1.
3. Collect all material inside the template. Avoid spilling any material and collect all
fines with a scoop, shovel, dustpan, and brush into a suitable container.
C. Roadway: (4.3.4)
1. Use template to mark area to be sampled. Obtain one sample, at a random location,
which totals the minimum mass in Table 1.
2. Sample the full depth of the material. This helps ensure representative material with
minimal segregation.
3. Avoid contaminating sample with underlying material.
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Aggregate T-2 January 2012
D. Windrow (not specifically described in AASHTO T-2):
1. Avoid windrow sampling unless no alternative exists. There is a strong potential for
obtaining a segregated sample from a windrow. However, circumstances may arise or
specifications may dictate that this method be used. An example would be certain base
course gradation samples where material has been sweetened and reprocessed on the
roadway due to a previous out-of-specification test.
2. Remove outer layer of material at sampling points. Material on the outside of the
windrow is more likely to be segregated. Boards shall be driven into windrow around
sampling location to prevent adjacent material from sloughing into area cleared of
surface material.
3. Sample the full height of the remaining material. Material collected for the sample
shall be obtained from the full height of the exposed face, after clearing the surface
material.
4. Obtain one (1) sample of sufficient size.
E. Stockpile: (4.3.3)
1. Avoid sampling stockpiles, if an alternative exists. It is difficult to obtain unbiased
samples from stockpiles due to segregation (X1.2.1) that occurs when material is
stockpiled, with coarser material rolling to the outside base of the stockpile.
2. Develop smaller sampling pile from large pile using power equipment. Obtain
increments from various levels of pile, and combine to form sampling pile (X1.2.1). For
extremely large piles and where practical, consider development of a random sampling
plan to create sampling stockpile.
3. Sample from bottom third, middle third and top third by volume. Combine to form
the minimum sample size from Table 1 (X1.2.2). Obtain at least three increments.
4. Avoid sampling the outer layer of the pile or obviously segregated material. A board
driven into the stockpile above the sampling point will help prevent further segregation
(X1.2.2).
Definition to Remember:
Table 1, Note A
Maximum Nominal Aggregate Size:
For processed aggregate, the maximum nominal size of particles is the largest sieve size listed in
the applicable specifications upon which any material is permitted to be retained.
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