ATT Moisture Content Testing 3 — Questions and Answers
Question 1: The 'free moisture' (surface moisture) of aggregate is calculated as:
- Total moisture content minus absorption capacity (Correct answer)
- Absorption capacity minus total moisture content
- Oven-dry mass divided by wet mass
- Total moisture content plus absorption capacity
Correct answer: Total moisture content minus absorption capacity
Free moisture = total moisture content − absorption capacity; it represents the water that will add to the mix water in a concrete batch.
Question 2: Which ASTM standard governs the test method for total evaporable moisture content of aggregate by drying?
- ASTM C127
- ASTM C128
- ASTM C566 (Correct answer)
- ASTM C70
Correct answer: ASTM C566
ASTM C566 is the standard test method specifically for total evaporable moisture content of aggregate by drying.
Question 3: A stockpile of coarse aggregate is sampled at three locations: top, middle, and side. Why are multiple locations sampled?
- To average out scale inaccuracies
- Because moisture is uniform in all stockpiles
- Because moisture content can vary significantly at different points in a stockpile (Correct answer)
- To comply with aggregate gradation requirements
Correct answer: Because moisture content can vary significantly at different points in a stockpile
Moisture tends to drain downward, making lower or interior portions of a stockpile wetter than the top or sides.
Question 4: When calculating the batch water for a concrete mix, the free moisture in the aggregate must be:
- Added to the design water content
- Subtracted from the design water content (Correct answer)
- Ignored if it is less than 3%
- Converted to SSD weight before any adjustment
Correct answer: Subtracted from the design water content
Free moisture in aggregates will contribute water to the mix, so it must be subtracted from the design (added) water to maintain the correct water-cement ratio.
Question 5: An oven used for moisture content testing of aggregate must be checked for temperature uniformity. What tool is used to verify oven temperature?
- A hygrometer
- A calibrated thermometer or thermocouple (Correct answer)
- A Speedy moisture tester
- A slump cone
Correct answer: A calibrated thermometer or thermocouple
A calibrated thermometer or thermocouple is placed inside the oven to verify it maintains 110 ± 5°C throughout the drying cycle.
Question 6: When is aggregate considered 'oven-dry' during a moisture content test?
- After exactly 24 hours in the oven at 110°C
- When successive weighings at 1-hour intervals show less than 0.1% change in mass (Correct answer)
- When the surface of the aggregate appears dry to the touch
- After the aggregate reaches ambient temperature
Correct answer: When successive weighings at 1-hour intervals show less than 0.1% change in mass
ASTM C566 requires drying until successive weighings (after additional drying intervals) differ by less than 0.1% of the specimen mass, confirming constant mass.
Question 7: What effect does high aggregate moisture content have on concrete workability if no adjustment is made to the batch water?
- Workability decreases because aggregate absorbs water
- Workability increases because total water content is higher than designed (Correct answer)
- Workability remains unchanged since aggregate moisture is inert
- Workability decreases because cement sets faster when aggregate is wet
Correct answer: Workability increases because total water content is higher than designed
Unadjusted high aggregate moisture adds excess water to the mix, increasing the water-cement ratio and workability but reducing strength.
ATT Moisture Content Testing 3
The 'free moisture' (surface moisture) of aggregate is calculated as: