ACI Certification Fresh Concrete Air Content 2

Question 1

What is the purpose of air entrainment in concrete exposed to freezing and thawing cycles?

Accepted Answer

Air voids provide pressure relief for freezing pore water, preventing internal cracking and surface scaling

Answer

Air reduces concrete density, making the mix easier to pump

Answer

Air increases the water-cement ratio, improving workability

Answer

Air replaces fine aggregate in low-strength mixes

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)?

Accepted Answer

6.0%

Answer

3.5%

Answer

4.5%

Answer

7.5%

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?

Accepted Answer

Lightweight aggregate or highly porous natural aggregate such as vesicular basalt

Answer

Dense natural gravel

Answer

Crushed granite aggregate

Answer

Recycled concrete aggregate with low absorption

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?

Accepted 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

Answer

Superplasticizers always increase air content uniformly

Answer

Superplasticizers have no interaction with air-entraining admixtures

Answer

Superplasticizers seal entrained air voids permanently, improving freeze-thaw resistance

Question 5

What is the correct procedure for charging the pressure meter bowl when performing an ASTM C231 Type B pressure meter test?

Accepted 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

Answer

Fill the bowl in one lift and vibrate externally for 30 seconds

Answer

Fill the bowl and allow it to self-consolidate without rodding

Answer

Fill the bowl in five layers with no rodding, relying on tamping only

Question 6

What effect does increasing the temperature of fresh concrete have on its measured air content?

Accepted Answer

Higher concrete temperature generally reduces air content because warmer concrete has lower surface tension and air bubbles coalesce and escape more easily

Answer

Higher temperature increases air content by accelerating cement hydration gas production

Answer

Temperature has no effect on air content in properly air-entrained mixes

Answer

Higher temperature always increases air content by reducing concrete viscosity