ACI Fresh Concrete Air Content Questions and Answers 1 — 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?
- Record 5.8% as the final air content.
- Release the pressure and read the new value on the gauge.
- Subtract the aggregate correction factor from the 5.8% reading. (Correct answer)
- Add the manufacturer's calibration factor to the 5.8% reading.
Correct answer: Subtract the aggregate correction factor from the 5.8% reading.
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?
- To accelerate the chemical reaction and speed up the test.
- To break down the foam and ensure a stable, accurate reading of the liquid level. (Correct answer)
- To act as a calibration agent for the measurement marks on the meter.
- To increase the density of the water, making the air bubbles rise faster.
Correct answer: To break down the foam and ensure a stable, accurate reading of the liquid level.
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?
- 4.4%; It does not meet the specification. (Correct answer)
- 5.2%; It meets the specification.
- 6.0%; It meets the specification.
- 4.4%; It meets the specification.
Correct answer: 4.4%; It does not meet the specification.
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?
- The concrete has a very high slump (e.g., 8 inches).
- The concrete is being placed in a high-temperature environment.
- The concrete contains a high-range water-reducing admixture.
- The concrete is made with lightweight aggregate. (Correct answer)
Correct answer: The concrete is made with lightweight aggregate.
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?
- Weigh the bowl and concrete to check the density.
- Tap the sides of the bowl smartly 10 to 15 times with a mallet. (Correct answer)
- Use a trowel to level the surface of the layer.
- Insert a thermometer to measure the concrete temperature.
Correct answer: Tap the sides of the bowl smartly 10 to 15 times with a mallet.
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.
Question 6: While performing a test with a Type B pressure meter (ASTM C231), the technician pumps air into the chamber until the gauge hand is exactly on the initial pressure line. What is the purpose of then lightly tapping the gauge?
- To ensure the main air valve is properly seated.
- To check if the concrete sample is fully consolidated.
- To verify that the petcocks are fully closed.
- To stabilize the gauge hand and ensure an accurate starting pressure. (Correct answer)
Correct answer: To stabilize the gauge hand and ensure an accurate starting pressure.
After pumping to the initial pressure line, the gauge needle may be slightly affected by friction or stickiness in its mechanical parts. Lightly tapping the gauge helps to overcome this friction and allows the needle to settle at the true pressure reading, ensuring the test begins from the correct, stable initial pressure.
ACI Fresh Concrete Air Content Questions and Answers 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?