2.2 ASTM D75 Sampling Procedures by Source: Belts, Streams, Stockpiles, Transportation Units & Roadways

1. Sampling Sources and Detailed Step-by-Step Procedures

ASTM D75 / AASHTO R 90 identifies five primary sampling locations. Each source presents unique physical challenges and demands strict procedural adherence.

Source A: Stopped Conveyor Belts (The Industry Reference Standard)

Sampling from a stopped conveyor belt is the method that best preserves the true gradation, because halting the belt freezes kinetic segregation and lets the technician capture every particle size in its exact proportion across a full cross-section of the stream. ASTM D75 treats stopped-belt, flowing-stream, stockpile, transportation-unit, and roadway sampling as procedures for different situations rather than ranking one as a formal referee method, but where a stopped belt is available it is the least biased option on offer.

Step-by-Step Protocol:

1. Lock-Out / Tag-Out: Ensure the conveyor power drive is fully locked out and tagged out in strict compliance with safety regulations before approaching the belt.
2. Template Positioning: Insert a specialized metal sampling template onto the belt. The template consists of two parallel curved blades spaced sufficiently apart to capture the required increment mass. The shape of the blades must conform exactly to the transverse contour and trough of the belt rubber.
3. Coarse Material Removal: Carefully scoop out all aggregate particles trapped between the two template blades and deposit them into a clean sample container.
4. The Critical Sweeping Step: Use a stiff-bristle brush, whisk broom, and dustpan to meticulously sweep all fine dust, mineral filler, and clinging particles off the belt rubber between the template blades into the container. Leaving fine dust on the belt is an automatic failure on the performance examination.
5. Replication: Repeat this procedure at a minimum of three separate locations along the belt to obtain three distinct increments, combining them into a single composite sample.

---

Source B: Flowing Aggregate Streams (Bins and Belt Discharges)

Sampling from a flowing stream requires cutting across a falling curtain of aggregate as it discharges from a conveyor head pulley, chute, or batch bin.

Step-by-Step Protocol:

1. Apparatus Design: The sampling container or mechanical cutter must be engineered to span the entire width of the stream. It must possess sufficient depth and volume to prevent any material from splashing out or overflowing.
2. Transverse Cut: Pass the container entirely through the falling stream in a single, continuous, uniform motion from one side to the other. The speed must remain steady throughout the cut.
3. Zero Overflow Rule: The pan must be of sufficient size to intercept the entire cross-section of the discharge stream and hold the required quantity without overflowing. If aggregate spills over the rim, coarse rock bounces out preferentially and the cut is biased. Discard an overflowing cut and take another with a larger pan.
4. Increment Collection: Collect a minimum of three approximately equal increments at spaced time intervals throughout the production run and combine them into a composite sample.

---

Source C: Aggregate Stockpiles

Stockpiles are the most common yet most hazardous sampling locations due to extreme gravity segregation. Direct sampling from the sloping face or toe of a stockpile with a handheld shovel is strictly prohibited unless barrier boards are deployed.

Method 1: Power Equipment (Preferred Standard Method)

1. Equipment Selection: A front-end loader is the preferred apparatus for stockpile sampling.
2. Entering the Pile: Drive the loader bucket into the stockpile at the bottom third of the pile height.
3. Lifting and Discharging: Roll back the bucket, back away, and deposit the aggregate onto a clean, level surface to form a separate miniature stockpile (or test pad).
4. Back-Blading: Gently back-drag or back-blade the top of this miniature pile with the loader bucket to flatten it into a uniform pad roughly 1 to 2 feet (0.3 to 0.6 m) in thickness.
5. Increment Extraction: Sample from the flattened pad by taking scoops from at least three randomly distributed points across the pad, avoiding the outer edges.

Method 2: Manual Sampling from Stockpile Faces (Board / Shield Method)

1. Barrier Placement: If power equipment is unavailable, shove a board vertically into the pile just above the sampling point. Note the orientation carefully — ASTM D75 and AASHTO R 90 both describe a board driven vertically down into the pile immediately above where the increment will be taken, so that loose material on the upper slope cannot cascade into the sampling zone. A board laid horizontally against the face does not hold the slope back and is a standing examiner trap.
2. Clearing Surface Material: When sampling fine aggregate stockpiles, remove the outer layer that has become segregated or weathered and take the sample from the material beneath. The standards call for removing the outer layer without fixing a numeric depth — judge it by exposing fresh, unsegregated material.
3. Three-Elevation Increments: Extract equal increments from three distinct elevations:
   • Top third of the stockpile height
   • Middle third of the stockpile height
   • Bottom third of the stockpile height
4. Combination: Combine the three increments in equal proportions to create the composite field sample.

Method 3: Sampling Tubes for Damp Fine Aggregate

For fine aggregate stockpiles, ASTM D75 offers an optional sampling tube of approximately 30 mm (1-1/4 in.) minimum diameter and about 2 m (6 ft) minimum length, which may be inserted into the pile at random locations to extract a minimum of five increments of fine material. Cohesion from surface moisture holds the sand in the tube. The tube option is offered for fine aggregate only; coarse aggregate stockpiles are sampled by the power-equipment or board methods above. The same tube may be used in place of trenching when sampling fine aggregate from a transportation unit.

---

Source D: Transportation Units (Truck Beds, Railcars, and Barges)

Aggregates delivered in haul trucks or railcars segregate during transit as road vibrations shake fines to the bottom and coarse stones roll outward.

The Trenching Protocol:

1. Excavation: Excavate three or more trenches across the transportation unit at random points that will represent the entire load.
2. Trench Dimensions: Each trench must be level, and at least 1 ft (0.3 m) in width and 1 ft (0.3 m) in depth below the surface. A shallow scrape samples only the segregated crust.
3. Extraction: Take a minimum of three increments from equally spaced points along each trench. Three trenches at three points each is therefore nine increments, not three.
4. Fine aggregate option: For fine aggregate, the sampling tube described above may be used in place of trenching.
5. Practical caution: Avoid material in direct contact with the metal sidewalls or bed floor, where fines and road grime collect.

---

Source E: Roadway Bases and Subbases

When sampling aggregate from an in-place roadway base or subbase before or after compaction:

1. Clearly mark the areas from which each increment is to be removed, and place a rigid, full-depth metal template over the layer.
2. Take all increments for the full depth of the material, extracting everything within the template boundaries.
3. Take care to exclude any underlying material; gouging into the subgrade contaminates the sample with clay or silt and biases both the gradation and the minus-No. 200 result.
4. Sweep all fine material from the base interface into the sample container.
5. Combine at least three equal increments, selected at random from the unit being sampled.

2. Performance Examination Checklists & Common Examiner Traps

On both the ACI written exam and the hands-on performance evaluation, examiners closely monitor candidates for specific procedural omissions. Below are the primary failure points:

Critical Performance Traps to Avoid:

1. Failing to Brush the Belt: When sampling a stopped conveyor, candidates often scoop out the rocks between the template blades but neglect to sweep the fine dust with a brush and dustpan. This omission constitutes an immediate failure on the performance exam.
2. Overflowing the Flowing Stream Pan: Permitting a sampling pan to overflow while cutting a discharge stream invalidates the increment. The examiner expects the candidate to state or demonstrate that the cutter has sufficient volume and moves at a uniform speed without spilling.
3. Direct Stockpile Face Scooping: Shoveling aggregate from the toe or surface of a stockpile without inserting a horizontal barrier board or utilizing a front-end loader test pad demonstrates a fundamental failure to understand segregation.
4. Confusing Maximum Size with NMAS: Written exam questions frequently present both Maximum Size and Nominal Maximum Size. Candidates who base their minimum sample mass on Maximum Size rather than NMAS will select the wrong mass from Table 1.
5. Inadequate Increment Count: Combining only one or two scoops into a field sample violates ASTM D75. The standard requires a minimum of three approximately equal increments for all composite samples.
6. Moisture Loss in Transport: If the sample is intended for moisture content determination (ASTM C566), packaging it in permeable burlap bags or open buckets allows evaporative loss. Moisture samples must be sealed immediately in vapor-tight, non-absorbent containers or heavy plastic bags.