Moisture Content & Specific Gravity Flashcards
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Read the first 7 Moisture Content & Specific Gravity flashcards as text
A technician notices the aggregate sample mass is still decreasing after 24 hours in the oven. Per ASTM C566, when should drying be considered complete?
Answer: When successive weighings at 2-hour intervals show less than 0.1% mass change
ASTM C566 defines constant mass as successive weighings (after cooling) differing by no more than 0.1% of the preceding mass.
If the wire basket used in coarse aggregate specific gravity testing has rust deposits that add 15 g to its mass, how does this affect results if the technician fails to account for the discrepancy?
Answer: Submerged mass is too high, resulting in an understated specific gravity
A heavier basket increases the measured submerged mass of the sample, which decreases the calculated volume (buoyant force), causing bulk Gs to be understated.
Which condition results in 'free moisture' on aggregate surface that contributes directly to mix water?
Answer: Moisture content is greater than absorption capacity
Free (surface) moisture = total moisture − absorption; it only exists when moisture content exceeds absorption, meaning the aggregate is wetter than SSD.
During pycnometer calibration for fine aggregate testing, the flask is filled with water and weighed. This calibration mass (C) is used to:
Answer: Establish the volume of the pycnometer at the test temperature
The calibration mass C represents the mass of water the pycnometer holds at a given temperature, establishing flask volume for Archimedes-based Gs calculation.
Aggregate from a stockpile exposed to rain is tested and shows 5.2% total moisture with an absorption of 1.8%. What surface moisture correction should be applied to the mix water for 500 lb of aggregate (OD basis)?
Answer: Reduce mix water by 17 lb
Free moisture = 5.2% − 1.8% = 3.4%; water contributed = 0.034 × 500 = 17 lb; this free water reduces the required mix water addition by 17 lb.
A specific gravity test on a lightweight aggregate yields a bulk OD Gs of 1.65. How does this affect concrete unit weight compared to using normal-weight aggregate (Gs ≈ 2.65)?
Answer: Concrete unit weight decreases significantly due to lower aggregate density
Lower aggregate specific gravity means less mass per unit volume, directly reducing the unit weight of fresh and hardened concrete.
What is the primary reason technicians must record the temperature of water used in the pycnometer specific gravity test?
Answer: Water temperature determines the calibration mass of the pycnometer at that temperature
Water density varies with temperature, affecting the pycnometer calibration mass C; tests must be conducted at the same temperature used for calibration, or corrections applied.