NETTCP Sampling Methods 3 — Questions and Answers
Question 1: What is the minimum number of grab samples typically combined to form a composite sample of Portland cement concrete during placement?
- 2
- 3 (Correct answer)
- 5
- 10
Correct answer: 3
AASHTO R 60 specifies that a composite concrete sample is made from at least three grab samples taken at regular intervals during discharge.
Question 2: A field sample of aggregates taken from a stockpile must avoid the outer surface layer because:
- Surface material is too wet to test
- Segregated or degraded material collects at the surface and edges (Correct answer)
- Surface particles are always oversized
- AASHTO prohibits surface sampling
Correct answer: Segregated or degraded material collects at the surface and edges
The outer layer of a stockpile accumulates segregated coarse particles and may contain contamination, making it unrepresentative.
Question 3: For HMA acceptance testing, which sampling method involves collecting material directly from the paver's hopper?
- Roadway core method
- Behind-the-screed method
- Hopper sampling (Correct answer)
- Windrow sampling
Correct answer: Hopper sampling
Hopper sampling involves collecting HMA directly from the paver's hopper before it is laid, capturing a sample representative of incoming material.
Question 4: When sampling soil from a test pit for density testing purposes, which layer is sampled first?
- Deepest layer to avoid disturbance from upper layers (Correct answer)
- Surface layer before excavation proceeds
- The layer closest to the groundwater table
- The most visually uniform layer
Correct answer: Deepest layer to avoid disturbance from upper layers
Sampling begins at the bottom of the excavation to prevent contamination from material falling from disturbed upper layers.
Question 5: A lot acceptance plan for pavement base course uses systematic sampling, taking one sample every 500 tons. If a lot is 3,200 tons, how many samples are collected?
- 5
- 6
- 7 (Correct answer)
- 8
Correct answer: 7
3,200 tons ÷ 500 tons/sample = 6.4, which rounds up to 7 samples to cover the entire lot.
Question 6: In the context of NETTCP testing, what does 'representative sample' mean?
- The largest single piece taken from the material
- A portion that reflects the same properties as the entire material mass (Correct answer)
- A sample taken only from the center of a shipment
- A sample collected by the contractor without technician oversight
Correct answer: A portion that reflects the same properties as the entire material mass
A representative sample accurately reflects the composition, gradation, and properties of the total material being evaluated.
Question 7: Which tool is specifically recommended for sampling fine aggregate from a flowing stream (belt conveyor) rather than from a static stockpile?
- Thin-walled tube sampler
- Sampling pan passed through the full stream cross-section (Correct answer)
- Auger sampler
- Split-spoon sampler
Correct answer: Sampling pan passed through the full stream cross-section
A sampling pan or trough passed through the full width of a moving stream ensures all size fractions are captured equally.
NETTCP Sampling Methods 3
What is the minimum number of grab samples typically combined to form a composite sample of Portland cement concrete during placement?