ACI Fresh Concrete Air Content Questions and Answers

Question 1

A technician performing an air content test using the pressure method (ASTM C231) has pressurized the meter, opened the main air valve, tapped the sides of the bowl, and stabilized the gauge. The gauge reading is 5.8%. What is the next critical step to determine the final air content percentage?

Accepted Answer

Subtract the aggregate correction factor from the 5.8% reading.

Answer

According to ASTM C231, the apparent air content is the final, stable reading on the pressure gauge after the pressure has been released into the bowl. To determine the correct air content of the concrete, the pre-determined aggregate correction factor must be subtracted from this apparent reading. This accounts for air voids within the aggregate particles themselves.

Question 2

When using the volumetric method (ASTM C173) for determining air content, what is the primary purpose of adding a measured amount of isopropyl alcohol?

Accepted Answer

To break down the foam and ensure a stable, accurate reading of the liquid level.

Answer

In the volumetric method, agitation causes foam to develop on the surface of the water. Isopropyl alcohol is used as a defoaming agent to break the surface tension and dissipate these bubbles, allowing for a clear and accurate reading of the new liquid level in the neck of the meter.

Question 3

A project's concrete specification requires an air content of 6.0% ± 1.5%. A technician performs a test according to ASTM C231 and gets a final, stable gauge reading of 5.2%. The aggregate correction factor for the mix has been previously determined to be 0.8%. What is the reported air content and does it meet the specification?

Accepted Answer

4.4%; It does not meet the specification.

Answer

The final reported air content is the gauge reading minus the aggregate correction factor (5.2% - 0.8% = 4.4%). The allowable range per the specification is 4.5% to 7.5%. Since 4.4% is below the lower limit of 4.5%, the concrete does not meet the specification.

Question 4

Which of the following situations would require the use of the volumetric method (ASTM C173) instead of the pressure method (ASTM C231) for determining the air content of fresh concrete?

Accepted Answer

The concrete is made with lightweight aggregate.

Answer

The pressure method (ASTM C231) is not applicable for concrete made with lightweight or other highly porous aggregates. The pressure applied during the test would compress the air within the pores of the aggregate, leading to an inaccurate, artificially high air content reading. The volumetric method (ASTM C173) must be used in these cases.

Question 5

According to ASTM C231, after placing and rodding each of the three layers of concrete in the measuring bowl, what is the next immediate step before adding the subsequent layer?

Accepted Answer

Tap the sides of the bowl smartly 10 to 15 times with a mallet.

Answer

The standard procedure in ASTM C231 requires that after each layer is rodded, the technician must tap the sides of the measuring bowl 10 to 15 times with the specified mallet. This action closes any voids left by the tamping rod and helps release large, trapped air bubbles.

ACI (American Concrete Institute Certification) Practice Test

ACI (American Concrete Institute Certification) Practice Test

ACI Fresh Concrete Air Content Questions and Answers