ACI Certification Fresh Concrete Air Content 2 — Questions and Answers
Question 1: What is the purpose of air entrainment in concrete exposed to freezing and thawing cycles?
- Air voids provide pressure relief for freezing pore water, preventing internal cracking and surface scaling (Correct answer)
- Air reduces concrete density, making the mix easier to pump
- Air increases the water-cement ratio, improving workability
- Air replaces fine aggregate in low-strength mixes
Correct 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.
When water in concrete pores freezes, it expands about 9% in volume, generating hydraulic pressures that can exceed the tensile strength of the paste. Properly spaced entrained air voids — typically 200–600 µm in diameter — are close enough that freezing water can flow into a void before pressure builds to destructive levels. ACI 318 and ACI 308 specify air content requirements based on aggregate size and exposure severity.
Question 2: 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)?
- 6.0% (Correct answer)
- 3.5%
- 4.5%
- 7.5%
Correct 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.
As maximum aggregate size increases, the paste volume fraction decreases. For 3/8-inch aggregate in F2 exposure, 7.5% is required; for 3/4-inch, 6.0%; for 1.5-inch, 5.0%. The specification ensures adequate air void spacing factor regardless of aggregate size.
Question 3: 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?
- Lightweight aggregate or highly porous natural aggregate such as vesicular basalt (Correct answer)
- Dense natural gravel
- Crushed granite aggregate
- Recycled concrete aggregate with low absorption
Correct 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.
The pressure meter operates by applying a known pressure to the concrete surface and measuring the volume change due to air compression. If the aggregate itself is porous, it compresses under the applied pressure, and the meter cannot distinguish between aggregate porosity and entrained air. ASTM C173 avoids this problem by using isopropyl alcohol to dissolve air from the concrete in a measuring column — a volumetric method unaffected by aggregate porosity.
Question 4: How does the addition of a polycarboxylate-based superplasticizer affect the air content of a mix that already contains an air-entraining admixture?
- 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 (Correct answer)
- Superplasticizers always increase air content uniformly
- Superplasticizers have no interaction with air-entraining admixtures
- Superplasticizers seal entrained air voids permanently, improving freeze-thaw resistance
Correct 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.
Polycarboxylate ether (PCE) superplasticizers have high molecular weight side chains that can compete with air-entraining agents at the water-air interface, reducing foam stability and air content. Pre-qualifying admixture combinations in the mix design phase is essential. The air-entraining admixture dosage may need to be increased when PCE superplasticizers are used.
Question 5: What is the correct procedure for charging the pressure meter bowl when performing an ASTM C231 Type B pressure meter test?
- 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 (Correct answer)
- Fill the bowl in one lift and vibrate externally for 30 seconds
- Fill the bowl and allow it to self-consolidate without rodding
- Fill the bowl in five layers with no rodding, relying on tamping only
Correct 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.
Under-consolidation leaves macro voids that are not true entrained air and inflates the reading. The three-layer rodding procedure matches the cylinder-making protocol in ASTM C31: divide the sample into thirds, rod 25 times per layer, and tap the outside of the bowl 10–15 times with a rubber mallet. The assembly plate is cleaned and the lid clamped before taking the pressure measurement.
Question 6: What effect does increasing the temperature of fresh concrete have on its measured air content?
- Higher concrete temperature generally reduces air content because warmer concrete has lower surface tension and air bubbles coalesce and escape more easily (Correct answer)
- Higher temperature increases air content by accelerating cement hydration gas production
- Temperature has no effect on air content in properly air-entrained mixes
- Higher temperature always increases air content by reducing concrete viscosity
Correct 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.
For every 10°F (5.5°C) increase in concrete temperature above about 70°F, air content typically decreases 0.5–1.0%. This means that hot-weather concrete requires increased air-entraining admixture dosage to achieve the specified air content at the point of discharge. ACI 305R and ACI 306R address admixture dosage adjustments for hot and cold weather concreting respectively.
ACI Certification Fresh Concrete Air Content 2
What is the purpose of air entrainment in concrete exposed to freezing and thawing cycles?