2.3 ASTM C702 / AASHTO R 76: Reducing Field Samples to Testing Size

Once a representative field sample of aggregate has been obtained in accordance with ASTM D75, the laboratory technician faces a secondary logistical and technical challenge: field samples typically weigh between 10 kg and 175 kg (22 to 385 lb), whereas individual laboratory test methods require portions weighing as little as 100 g to 15 kg. A technician cannot simply stick a hand or scoop into a 50-kg sample bag and pull out 5 kg of stone for a test. Doing so introduces catastrophic operator bias, selectively gathering either surface coarse rocks or bottom fines.

ASTM C702 / C702M (and AASHTO R 76) standardizes the mechanical and manual procedures for reducing large field samples to the exact sample size required for testing. The objective is to produce a test portion that is statistically indistinguishable from the parent field sample in gradation, specific gravity, cleanliness, and particle composition.

1. The Cardinal Law of Sample Reduction: Zero Hand-Selection

The most critical operating principle of ASTM C702 is that reduction must always be achieved through proportional physical splitting.

The Cardinal Rule: Under no circumstances may an operator hand-pick, discard, or selectively add individual coarse aggregate particles or handfuls of sand to hit an exact target mass on a balance. If a reduction split yields 5,240 grams and the target test specification calls for 5,000 grams, the technician must test the entire 5,240-gram split or split it again proportionally. Selectively removing larger stones alters the gradation, spikes the percent passing smaller sieves, and invalidates the test.

2. Selection of Reduction Method

ASTM C702 provides three distinct methods for sample reduction. Selection depends upon aggregate particle size, moisture condition, and whether the test is for routine quality control or official specification acceptance/dispute resolution.

The Three Reduction Methods:

• Method A — Mechanical Riffle Splitter: Gravity feed through an assembly of alternating chutes.
• Method B — Quartering: Manual coning, flattening into a circular disc, and quartering on a clean floor or canvas.
• Method C — Miniature Stockpile Sampling: Manual extraction of multiple spoon increments from a damp fine aggregate pad.

Method Selection Matrix (ASTM C702 Section 5)

Read Section 5 as a pair of conversions, not a list of bans. ASTM C702 does not forbid Method C for dry sand outright; it requires the sample to be in the right moisture state for the method:

• Section 5.1.1 — dry sand, want Method B or C? Moisten the sample to obtain free moisture on the particle surfaces, mix thoroughly, and then reduce the sample size.
• Section 5.1.2 — moist sand, want Method A? Dry the entire sample to at least the saturated-surface-dry condition, using temperatures that do not exceed those specified for any of the tests contemplated, and then reduce the sample size. Alternatively, if the moist sample is very large, make a preliminary split using a mechanical splitter having chute openings of 38 mm (1-1/2 in.) or more in width to reduce the sample to not less than 5000 g, dry that portion, and reduce it to test size using Method A.

The one flat prohibition is in Section 5.2: the miniature stockpile Method C is not permitted for coarse aggregates or mixtures of coarse and fine aggregates. That one has no moisture-state escape hatch.

[!NOTE] A field check for "wetter than SSD." ASTM C702 Note 1 gives a quick approximation that needs no apparatus: if the fine aggregate will retain its shape when molded in the hand, it may be considered to be wetter than saturated-surface-dry. The formal method for determining the SSD condition is the cone-and-tamper test of ASTM C128, covered in Section 6.2 of this guide.

[!WARNING] ASTM C702 publishes no acceptance, dispute, or referee hierarchy. The standard selects a method from particle size and moisture condition only. It does not designate Method A as mandatory for acceptance testing, and it does not bar Method C from acceptance testing. If a project specification or agency procedure imposes such a rule, that rule comes from the specification, not from C702 — do not carry it into a written-exam answer about the standard itself.

3. Method A — Mechanical Riffle Splitter

Method A is the fastest, most precise, and most objective reduction method. It utilizes gravity to divide an aggregate mass into two statistically identical halves.

Apparatus Requirements & Chute Geometry

A mechanical splitter consists of a rigid frame supporting a series of chutes discharging alternately to opposite sides into two collecting pans. The apparatus must comply with rigorous dimensional rules:

• Number of Chutes:
  • Coarse Aggregate: Minimum of 8 equal-width chutes.
  • Fine Aggregate: Minimum of 12 equal-width chutes.
• Chute Width Clearances:
  • Coarse Aggregate Splitter: Individual chute openings must be at least 50% larger than the maximum aggregate particle size (Chute Width ≥ 1.5 × Max Particle Size). For example, splitting 1-inch (25 mm) aggregate requires chutes at least 1-1/2 inches (37.5 mm) wide. If chutes are too narrow, larger particles bridge across the openings, choking flow and deflecting aggregate into adjacent chutes.
  • Fine Aggregate Splitter: For dry fine aggregate in which the entire sample will pass the 9.5-mm (3/8-in.) sieve, the splitter shall have chutes 12.5 to 20 mm (1/2 to 3/4 in.) wide.
• Leveling: The splitter assembly must be leveled precisely on a sturdy workbench or floor. An unlevel splitter creates unequal gravity flow, biasing the split ratio away from 50/50.
• Feed Hopper / Pan: The hopper or straightedge feed pan must span the full width of the chute assembly, allowing the operator to introduce aggregate across all chutes simultaneously.

Step-by-Step Splitting Execution:

1. Position clean receiving pans beneath both chute discharge banks.
2. Distribute the field sample evenly along the entire length of the feed hopper or straightedge pan from end to end.
3. Pour the aggregate into the chutes in a continuous, uniform stream across the full assembly width at a rate that allows free flow without choking or banking up over the chute dividers.
4. Material divides cleanly into two approximately equal fractions in the bottom pans.
5. Set aside one pan and retain the other. Re-split the retained portion as many times as necessary until the desired test mass is obtained.
6. Store the unselected portions in sealed containers until all testing and potential referee retests are completed.

4. Method B — Quartering

Quartering is an established manual method ideal for field sites lacking mechanical splitters, or for damp aggregate mixtures containing both coarse stone and sand.

Option 1: Hard, Clean, Level Surface Protocol

1. Surface Preparation: Place the sample on a clean, non-absorbent, flat floor, steel plate, or smooth concrete pad.
2. Thorough Mixing: Mix the material thoroughly by turning the entire sample over three times (ASTM C702 Section 10.1.1 specifies three).
3. Coning: Form a conical pile by depositing each shovelful directly on top of the preceding one. This forces particles to tumble radially down the slope in all directions, achieving rotational symmetry.
4. Flattening into a Disc: Carefully press down the apex of the cone with a shovel so that a circular disc of uniform thickness and diameter is formed and each quarter sector of the resulting pile contains the material originally in it. Current editions of C702/C702M and AASHTO R 76 add the practical target that the diameter should be approximately four to eight times the thickness; a pile flatter or thicker than that band quarters unevenly.
5. Quartering: Divide the flattened disc into four equal quarters using a shovel blade, straightedge, or specialized metal cross-cutter driven down through the center.
6. Removal of Opposite Quarters: Remove two diagonally opposite quarters and set them aside. Use a broom, whisk, or brush to sweep clean all fine dust and sand from the cleared quadrants.
7. Recombination: Thoroughly mix the two remaining diagonally opposite quarters, form a new cone, flatten, and quarter again until the target mass is reached.

Option 2: The Quartering Canvas Protocol

For field soils or aggregate mixtures on unpaved ground, a clean canvas or heavy polyethylene tarp (approx. 6 ft × 8 ft or 2 m × 2.5 m) is utilized:

1. Place the field sample in the center of the canvas.
2. Mix by lifting one corner of the canvas and pulling it diagonally across toward the opposite corner, causing the aggregate to roll and tumble over itself. Repeat with each of the other three corners.
3. Roll the aggregate into the center to form a cone and flatten it into a circular pad.
4. Insert a straight wooden stick, broom handle, or pipe under the canvas beneath the exact center of the pile. Lift both ends of the stick to cleanly divide the pile in half.
5. Lower the canvas, withdraw the stick, rotate it 90 degrees, pass it under the center of the remaining halves, and lift again to divide the pile into four equal quarters.
6. Discard two diagonally opposite quarters and retain the remaining pair.

5. Method C — Miniature Stockpile Sampling (Damp Fine Aggregate Only)

Method C is a specialized manual technique permitted solely for damp fine aggregate. It relies on capillary moisture tension to prevent segregation.

The Physical Principle of Surface Moisture

When fine sand contains free surface moisture (damp condition), surface tension forces hold fine dust and sand particles together in cohesive clusters. Because the particles cling together, coning and scooping do not cause the segregation observed in dry sand. If dry sand were coned, coarser sand grains would roll to the perimeter, leaving fines in the center.

What ASTM C702 actually restricts about Method C:

• Not permitted for coarse aggregate, or for mixtures of coarse and fine aggregate (Section 5.2). This restriction is absolute.
• For fine aggregate that is drier than SSD, Section 5.1 routes you to Method A — but Section 5.1.1 permits Method C if you first moisten the sample to obtain free moisture on the particle surfaces and mix thoroughly.
• The standard imposes no acceptance, dispute, or referee restriction on Method C. Any such rule in your laboratory comes from a project specification or agency procedure, not from ASTM C702.

Step-by-Step Miniature Stockpile Protocol:

1. Place the damp fine aggregate sample on a clean, hard, non-absorbent level surface.
2. Mix the material thoroughly by turning the entire sample over three times.
3. With the last turning, shovel the entire sample into a conical pile by depositing each shovelful on top of the preceding one.
4. If desired, flatten the conical pile to a uniform thickness and diameter by pressing down the apex with a shovel, so that each quarter sector of the resulting pile contains the material originally in it. ASTM C702 Section 12.1 makes this flattening step optional, not mandatory — the miniature stockpile may be sampled as a cone.
5. Obtain the sample for each test by selecting at least five increments of material at random locations from the miniature stockpile, using a small sampling thief, small scoop, or spoon (Section 11.1).
6. Combine the five or more increments to form the final laboratory test specimen.

6. Performance Exam Checklists, Examiner Cues, and Critical Failure Points

Examiners evaluating ASTM C702 on the ACI performance examination look for specific physical demonstrations and verbal explanations. Any procedural shortcut results in a failing score.

Step-by-Step Performance Exam Checkpoints:

• Method Selection Rationale: Candidate must correctly explain why a given method is chosen based on material size, moisture state, and dispute criteria.
• Splitter Inspection: Before pouring, the candidate must visually inspect the splitter to verify that the chutes are clean, free of wedged rocks, level, and that chute widths exceed 1.5 × maximum particle size for coarse aggregate (or 12.5 to 20 mm chutes for dry fine aggregate).
• Uniform Feed Across Hopper: The candidate must fill the feed pan along its entire length and discharge across all chutes uniformly. Dumping aggregate into the center three chutes of an 8-chute splitter is an automatic failure.
• Opposite Quarter Retention: In Method B, the candidate must discard two diagonally opposite quarters and retain the other two. Discarding two adjacent quarters (e.g., left and top) destroys radial symmetry and causes an immediate test failure.
• Sweeping Discarded Quadrants: In Method B, the candidate must brush clean the surface of the discarded quadrants, capturing all residual fines before recombining the retained quarters.
• Method C Increment Count: In Method C, the candidate must take at least five increments at random locations using a sampling thief, small scoop, or spoon. Taking only two or three scoops fails the step.