ATT Specific Gravity & Absorption 3 — Questions and Answers
Question 1: A coarse aggregate sample has an OD mass of 4,856 g, SSD mass of 4,921 g, and submerged mass of 3,057 g. What is the absorption (%)?
- 1.14%
- 1.34% (Correct answer)
- 1.54%
- 1.74%
Correct answer: 1.34%
Absorption = ((4921 - 4856) / 4856) × 100 = (65 / 4856) × 100 ≈ 1.34%.
Question 2: What is the primary reason that aggregates with high absorption values can be problematic in concrete?
- They increase the unit weight of the mix
- They may absorb mixing water, effectively lowering the water-cement ratio (Correct answer)
- They always cause alkali-silica reactivity
- They reduce workability by increasing aggregate surface area
Correct answer: They may absorb mixing water, effectively lowering the water-cement ratio
High-absorption aggregates can draw water from the paste, reducing effective w/c ratio and potentially causing workability loss.
Question 3: In the wire basket method for coarse aggregate (ASTM C127), what is the purpose of the wire basket?
- To dry the aggregate rapidly by allowing air circulation
- To hold the aggregate submerged while its mass is determined in water (Correct answer)
- To separate fine particles from coarse aggregate
- To calibrate the balance against a known mass
Correct answer: To hold the aggregate submerged while its mass is determined in water
The wire basket is suspended in water on a balance to obtain the submerged (buoyant) mass of the aggregate.
Question 4: If apparent specific gravity is 2.71 and bulk OD specific gravity is 2.61 for the same aggregate, what can be inferred?
- The aggregate is non-absorptive
- The permeable pore volume is significant (Correct answer)
- The aggregate contains no impermeable solids
- The test was performed incorrectly
Correct answer: The permeable pore volume is significant
A larger difference between apparent and bulk OD specific gravity indicates a greater permeable pore volume within the particles.
Question 5: Which aggregate characteristic directly affects the calculation of batch weights in mix design using specific gravity?
- Aggregate color and texture
- Volume of aggregate per unit mass (reciprocal of specific gravity) (Correct answer)
- LA abrasion loss percentage
- Aggregate shape index
Correct answer: Volume of aggregate per unit mass (reciprocal of specific gravity)
Mix design uses specific gravity to convert between aggregate mass and volume, so it directly affects batch weight calculations.
Question 6: What action is taken if air bubbles are observed in the pycnometer during the fine aggregate specific gravity test?
- The test is immediately discarded
- Agitate or roll the pycnometer to dislodge the air bubbles (Correct answer)
- Add a surfactant to eliminate bubbles
- Heat the pycnometer to release trapped air faster
Correct answer: Agitate or roll the pycnometer to dislodge the air bubbles
ASTM C128 requires agitating or rolling the pycnometer to remove entrapped air, which would otherwise cause low mass readings.
Question 7: Why is the pycnometer method preferred over the wire basket method for fine aggregates?
- Fine particles would fall through the openings of a wire basket (Correct answer)
- The pycnometer provides faster results
- Fine aggregates have higher specific gravity than coarse aggregates
- Wire baskets cannot be submerged fully in water
Correct answer: Fine particles would fall through the openings of a wire basket
Fine aggregate particles are too small to be retained in a wire basket, so a volumetric pycnometer is used instead.
ATT Specific Gravity & Absorption 3
A coarse aggregate sample has an OD mass of 4,856 g, SSD mass of 4,921 g, and submerged mass of 3,057 g.
What is the absorption (%)?