Fresh Concrete Air Content Flashcards
6 cards from real ACI Certification practice questions. Tap to flip, then mark Knew It or Still Learning — missed cards come back until you master them.
Read the first 6 Fresh Concrete Air Content flashcards as text
What is the purpose of air entrainment in concrete exposed to freezing and thawing cycles?
Answer: Air voids provide pressure relief for freezing pore water, preventing internal cracking and surface scaling
Entrained air bubbles act as expansion chambers for freezing water in the paste; without them, hydraulic pressure from freezing builds until the concrete cracks or scales.
According to ACI 318 Table 19.3.3.1, what is the required total air content for concrete with 3/4-inch maximum aggregate size in Exposure Class F2 (severe freeze-thaw)?
Answer: 6.0%
ACI 318 Table 19.3.3.1 specifies 6.0% total air content for 3/4-inch aggregate in F2 (severe) exposure; larger aggregate allows lower air content because aggregate particles don't contribute air voids.
When testing air content of fresh concrete using the pressure method (ASTM C231), what type of aggregate requires using the volumetric method (ASTM C173) instead?
Answer: Lightweight aggregate or highly porous natural aggregate such as vesicular basalt
ASTM C231 (pressure method) is not accurate for lightweight or porous aggregates because the aggregate pores absorb air under pressure, giving falsely high air readings; ASTM C173 (volumetric method) is the correct alternative.
How does the addition of a polycarboxylate-based superplasticizer affect the air content of a mix that already contains an air-entraining admixture?
Answer: Some superplasticizers can significantly reduce entrained air content; the effect depends on the superplasticizer chemistry and requires air content to be checked after both admixtures are added
Certain polycarboxylate-based superplasticizers can destabilize the air void system, reducing measured air content or changing void size distribution, so air must be tested after adding all admixtures.
What is the correct procedure for charging the pressure meter bowl when performing an ASTM C231 Type B pressure meter test?
Answer: Fill the bowl in three equal layers, rodding each layer 25 times and tapping the sides 10–15 times to close voids, then strike off and clamp the lid
ASTM C231 requires filling the bowl in three equal layers, rodding 25 times per layer, tapping the bowl sides 10–15 times per layer to close rod holes, striking off the top, and clamping the lid air-tight.
What effect does increasing the temperature of fresh concrete have on its measured air content?
Answer: Higher concrete temperature generally reduces air content because warmer concrete has lower surface tension and air bubbles coalesce and escape more easily
As concrete temperature rises, the viscosity of the paste decreases and surface tension of pore water drops, making it easier for air bubbles to merge and escape, reducing measured air content.