How to Sample Coarse and Fine Aggregate According to ASTM Standards?

The aggregates used for material testing need proper sampling and resizing to meet the test specifications of aggregates. Three methods are recommended by ASTM C702, Method A, B, and C in " Standard Practice for Reducing Samples of Aggregates to Test Size". Method A is by using a Mechanical Splitter, Method B is Quartering and Method C is Miniature Stockpile Sampling ( for damp fine aggregate only).

The large aggregate samples either obtained from the field or made in the laboratory are reduced to a convenient size for performing a number of tests and therefore describe their quality and properties with utmost efficiency. The test methods are conducted in such a manner that a smaller test sample portion represents a larger sample and thus the total supply of material. 

This article discusses the three methods used to sample and reduce the size of aggregates for material testing for construction applications. 

Method A - Mechanical Splitter

A mechanical splitter called the sample splitter is used to sample coarse and fine aggregates. The splitter shall have an even number of chutes with equal width as shown in figure-2 below.

Figure.2. (a) Riffle Sample Splitter for Coarse Aggregates

 The number of chutes differs based on whether the sample aggregates are coarse or fine.

1. Coarse aggregate: Minimum 8 chutes, 
2. Coarse and mixed aggregate: Minimum width = 50 % larger than the largest particles in the sample to the split
3. Fine aggregate: Minimum of 12 chutes
4. Dry fine aggregate: The entire sample must pass through a 9.5 mm sieve, the minimum width of individual chutes = is 50 % larger than the largest particles in the sample, and the maximum width of the chute = 19 mm.

Figure.2. (b) Sample Splitter for Fine Aggregate

The sample splitter is equipped with 2 acceptable to hold two halves of the sample undergoing splitting. To feed the sample at a controlled rate to the chutes, a hopper or a straightened pan is provided. 

Mechanical splitters are commonly available in sizes adequate for coarse aggregate having the largest particle, not over 37.5 mm (11/2 in.).

Procedure

Take the original sample and place it on a hopper or a pan. Distribute it from edge to edge uniformly. Now start introducing it to the chutes such that an equal amount flow through each chute. The rate of pouring shall be similar to a free flow of aggregates into the chutes and then to the receptacles below. 

The sample in one of the receptacles can be reintroduced to the splitter as many times as required to achieve the desired size for the intended test. The portion of the sample collected in other receptacles is saved for the other tests.

Suitability of Mechanical Sample Splitter

1. For Fine Aggregates
• Samples of fine aggregate that are drier than the saturated surface-dry condition
• If the sample has free moisture on the particle surfaces, the entire sample may be dried to at least the surface-dry condition, using temperatures that do not exceed those specified for any of the tests contemplated, and then the sample reduction performed. 
• If the moist sample is very large, a preliminary split may be made using a mechanical splitter having wide chute openings 38 mm (11/2 in.) or more to reduce the sample to not less than 5000 g. The portion so obtained is then dried, and reduction to test sample size is completed using Method A
2. For Coarse Aggregates
• Mechanical splitters are commonly available in sizes adequate for coarse aggregate having the largest particle, not over 37.5 mm (11/2 in.).
3. Combined Coarse and Fine Aggregates
• Samples that are in a dry condition
• If the sample is damp or shows free water, dry the sample until it appears dry or until clumps can be easily broken by hand. Dry the entire sample to this condition, using temperatures that do not exceed those specified for any of the tests contemplated, and then reduce the sample. The dryness of the sample can be tested by tightly squeezing a small portion of the sample in the palm of the hand. If the cast crumbles readily, the correct moisture range has been obtained.

Method B- Quartering

The quartering method of sampling aggregates is followed by two techniques or by their combination. The apparatus includes a straightedge, straightened scoop, shovel or trowel, broom or brush, and a tear-resistant tarp or canvas blanket ( 2 by 2.5 m in dimension).

Technique 1: Quartering on a Hard, Clean, and Level Surface

Fig.3. Quartering on a Hard, Clean, and Level Surface

1. Place the original sample on a hard, clean, level surface where there will be neither loss of material nor the accidental addition of foreign material.                                                 
2. Mix the material by turning the entire sample over at least three times until the material is thoroughly mixed. With the last turning, form the entire sample into a conical pile by depositing individual lifts on top of the preceding lift. 
3. Carefully flatten the conical pile to a uniform thickness and diameter by pressing down the apex with a shovel or trowel so that each quarter sector of the resulting pile will contain the material originally in it. The diameter should be approximately four to eight times the thickness. 
4. Divide the flattened mass into four equal quarters with a shovel or trowel and remove two diagonally opposite quarters, including all fine material, and brush the cleared spaces clean. 
5. The two unused quarters may be set aside for later use or testing if desired. Successively mix and quarter the remaining material until the sample is reduced to the desired size

Technique 2: Quartering on a Canvas Blanket or Tear-Resistant Tarp

Fig.4. Quartering Aggregates on a Canvas Blanket or Tarp

1. When the floor surface is uneven, the field sample may be placed on a canvas blanket or tear-resistant tarp and mixed with a shovel or trowel as described before, leaving the sample in a conical pile.
2.  As an alternative to mixing with the shovel or trowel, lift each corner of the blanket or tarp and pull it over the sample toward the diagonally opposite corner, causing the material to be rolled.
3.  After the material has been rolled a sufficient number of times (a minimum of four times), so that it is thoroughly mixed, pull each corner of the blanket or tarp toward the center of the pile so the material will be left in a conical pile. 
4. Flatten the pile as described in technique 1 and also divide the sample as described or insert a stick or pipe beneath the blanket or tarp and under the center of the pile, then lift both ends of the stick, dividing the sample into two equal parts. 
5. Remove the stick, leaving a fold of the blanket between the divided portions. Insert the stick under the center of the pile at right angles to the first division and again lift both ends of the stick, dividing the sample into four equal parts. 
6. Remove two diagonally opposite quarters, being careful to clean the fines from the blanket or tarp. The two unused quarters may be set aside for later use or testing if desired. Successively mix and quarter the remaining material until the sample is reduced to the desired size 

Suitability of Quartering Method for Aggregate Sampling

1. Fine Aggregates
• Samples having free moisture on the particle surfaces may be reduced in size by quartering.
• If the sample has no free moisture on the particle surfaces, the sample may be moistened to achieve this condition, thoroughly mixed, and then the sample reduction performed.
2. Coarse Aggregates
• Reduce the sample by quartering
3. Combined Coarse and Fine Aggregates
• Samples that are in a dry condition may be reduced in size
•  Samples having free moisture on the particle surfaces may be reduced in size by quartering

Method C- Miniature Stockpile Sampling

The miniature stockpile sampling method is only for sampling damp fine aggregate and is not permitted for sampling coarse aggregates or mixtures of coarse and fine aggregates.

The apparatus for the miniature stockpile sampling method shall consist of a straight-edged scoop, shovel, or trowel for mixing the aggregate, and either a small scoop or spoon for sampling.

Procedure

1. Place the original sample of damp fine aggregate on a hard clean, level surface where the potential for material loss or the accidental addition of foreign material is minimized.
2. Mix the material thoroughly by turning the entire sample over three times turning the entire sample into a conical pile.
3. Flatten the conical pile to a uniform thickness and diameter by pressing down the apex of the pile with the scoop, shovel, or trowel.
4. Obtain a test sample by selecting at least five increments of material at random locations from the miniature stockpile using the small scoop or spoon.

The miniature stockpile Method C is not permitted for coarse aggregates or combined aggregates.

Reference

ASTM C702/C702M-11

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