22 Air Content: Pressure Method (ASTM C231) Practice Questions & Answers

Every Air Content: Pressure Method (ASTM C231) practice question from the ACI Concrete Field Testing Technician Grade I Practice Test, with the correct answer and a short explanation.

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1. A project calls for structural lightweight-aggregate concrete, and the inspector asks for an air content test on the fresh concrete. Which approach is correct?
- A.Use the pressure method (C231/C231M), but raise the aggregate correction factor to offset the porous particles.
- B.Use the volumetric method (C173/C173M), because the pressure method would count air held inside the aggregate pores.✓ Answer
- C.Use the pressure method (C231/C231M), but run the Type B meter twice and average the two gauge readings for accuracy.
- D.Use the pressure method (C231/C231M), but presoak the lightweight aggregate so its pores are already full of water.
The pressure method compresses every air void in the sample, including air in aggregate pores, so it is limited to concrete made with relatively dense aggregates where a small correction factor covers that pore air. Lightweight, air-cooled slag and highly porous aggregates hold too much compressible pore air for a correction to be reliable, so the volumetric (roll-a-meter) method, which displaces only the air in the concrete, is used instead.
Ref QB-3GNN54Source: ASTM C231/C231M, Standard Test Method for Air Content of Freshly Mixed Concrete by the Pressure Method — scope limited to relatively dense aggregatesReport a problem with this question
2. When a technician reports the air content measured on fresh concrete with a pressure meter, what does that value represent?
- A.The total air in the concrete, entrained and entrapped together, with no split between the two types✓ Answer
- B.Only the entrapped air, since entrained bubbles are too small to respond to the test pressure
- C.The entrained air plus pore air in the aggregate, with entrapped air removed by mallet tapping
- D.Only the entrained air, since entrapped voids are too large to compress at the test pressure
Every air void, large or small, shrinks when pressure is applied, so the meter responds to all of them at once. The method therefore gives total air content and cannot distinguish entrained bubbles from entrapped voids; telling them apart requires a hardened-concrete air-void analysis.
Ref QB-MY66W5Source: ASTM C231/C231M, Standard Test Method for Air Content of Freshly Mixed Concrete by the Pressure Method — significance and useReport a problem with this question
3. On what physical behavior does the pressure method rely to determine air content?
- A.Pressure forces water into the aggregate pores, and the volume of water absorbed is read as air content.
- B.Air voids shrink as pressure rises while paste, water and dense aggregate barely compress (Boyle's law).✓ Answer
- C.Air escapes through the petcocks as pressure rises, and the mass lost by the concrete is converted to air.
- D.Air dissolves into the mix water as pressure rises, and the drop in gauge reading equals the air absorbed.
By Boyle's law, at constant temperature the volume of a gas falls in proportion as its pressure rises. Because the solids and water are nearly incompressible, the volume change observed under a known pressure change comes from the air voids, and the meter is calibrated to express it as percent air.
Ref QB-4GZYA5Source: ASTM C231/C231M, Standard Test Method for Air Content of Freshly Mixed Concrete by the Pressure Method — principle of the method (Boyle's law)Report a problem with this question
4. What does a technician observe on a Type B pressure meter that differs from a Type A meter?
- A.Type B: water is added to a graduated standpipe, and the fall of the water column gives percent air.
- B.Type B: the bowl is weighed before and after pressurizing, and the mass change is converted to air.
- C.Type B: a known volume of alcohol is added, and the bowl is rolled until the level stops moving.
- D.Type B: an air chamber at a known pressure is released into the bowl, and a dial gauge reads percent air.✓ Answer
A Type B meter equalizes a known volume of air at a known pressure in a sealed chamber with the unknown volume of air in the sample; the resulting pressure is read on a dial gauge calibrated in percent air. The water-column standpipe describes a Type A meter, weighing describes the gravimetric method, and adding alcohol and rolling describes the volumetric method.
Ref QB-NYHG37Source: ASTM C231/C231M, Standard Test Method for Air Content of Freshly Mixed Concrete by the Pressure Method — apparatus, Type A and Type B metersReport a problem with this question
5. A technician's pressure-meter kit contains a steel hammer for tapping the bowl. What does the method expect of the mallet instead?
- A.No mallet is needed, because the sides are tapped with the tamping rod handle once each layer is rodded.
- B.A rubber or rawhide head, sized to the bowl, so taps close rod holes and free large bubbles without damage.✓ Answer
- C.A steel head is fine if it is light, so the taps are gentle enough not to drive entrained air out of the paste.
- D.A wooden head is required, so that the same tool can also serve as the strike-off bar for leveling the bowl.
The method specifies a mallet with a rubber or rawhide head, with a mass matched to the bowl size, so that firm taps on the bowl close the voids left by the rod and release large entrapped bubbles while the head absorbs the impact rather than denting the bowl. A steel hammer, a wooden strike-off substitute or the rod handle does not meet that requirement.
Ref QB-5GCBG6Source: ASTM C231/C231M, Standard Test Method for Air Content of Freshly Mixed Concrete by the Pressure Method — apparatus, malletReport a problem with this question
6. During a calibration check of a Type B meter with the bowl full of water and no air introduced, the gauge consistently fails to return to the expected reading. What should the technician do?
- A.Keep testing, since a water-only check does not reflect how the meter acts with concrete.
- B.Recalibrate the meter and reset the initial pressure line before testing any concrete.✓ Answer
- C.Record the offset, then add it to every later concrete reading as a standing correction.
- D.Record the offset, then fold it into the aggregate correction factor for the day's tests.
A consistent error in the water-filled calibration check means the initial pressure setting no longer corresponds to the meter's calibration, so the meter must be recalibrated, which usually means re-establishing the initial pressure line, before it is used on concrete. Adding a homemade offset or folding it into the aggregate correction factor hides an instrument error instead of fixing it.
Ref QB-PYVT88Source: ASTM C231/C231M, Standard Test Method for Air Content of Freshly Mixed Concrete by the Pressure Method — annex, calibration of apparatusReport a problem with this question
7. A crew calibrated a Type A meter at a plant near sea level and now takes it to a job at a much higher elevation. What should be done with the meter?
- A.Redetermine only the aggregate correction factor at the new site, since altitude changes pore air.
- B.Leave the calibration as is, because the meter reads a pressure difference that elevation cancels out.
- C.Recalibrate it at the new site, because its operating pressure depends on the atmospheric pressure.✓ Answer
- D.Apply a fixed elevation correction to each apparent reading in place of a new calibration at the site.
In a Type A meter the water column is open to the atmosphere, and the operating pressure that produces correct readings is set by calibration under local atmospheric pressure. A substantial change in elevation changes atmospheric pressure, so the meter must be recalibrated at the new location; a Type B meter, which compares two sealed volumes, is not affected in the same way.
Ref QB-6GQNN7Source: ASTM C231/C231M, Standard Test Method for Air Content of Freshly Mixed Concrete by the Pressure Method — annex, calibration of Type A meter and change in elevationReport a problem with this question
8. Assume the method requires the air test to begin within 5 minutes after the composite sample is obtained. The composite was completed at 10:00, but the meter was not ready until 10:12. What should the technician do?
- A.Test the aged sample after adding water to restore the workability it lost while waiting.
- B.Test the aged sample anyway, and note the 12-minute delay on the report for the engineer.
- C.Remix the aged sample thoroughly and test it, since remixing restores its original condition.
- D.Obtain a fresh composite sample and start the air test on it within the allowed time window.✓ Answer
Fresh concrete loses air and changes consistency as it sits, so a sample held beyond the allowed window no longer represents the concrete being placed. Remixing does not restore lost air and adding water changes the concrete itself; the valid course is a new sample tested within the time limit.
Ref QB-FYDG2YSource: ASTM C231/C231M, Standard Test Method for Air Content of Freshly Mixed Concrete by the Pressure Method — sampling per ASTM C172/C172M and time to start the testReport a problem with this question
9. Fresh concrete contains coarse aggregate larger than the maximum size the pressure method allows to be tested directly. What should the technician do?
- A.Switch to the volumetric method, since large aggregate rules out any test with a pressure meter.
- B.Test the full mix as delivered, and raise the correction factor to cover the large stones.
- C.Hand-pick the oversize stones out of the bowl after filling, then top off the bowl with mortar.
- D.Wet-sieve a portion over the specified sieve, then test the sieved concrete in the pressure meter.✓ Answer
When aggregate exceeds the size limit, a representative portion is wet-sieved per C172/C172M to remove the oversize particles, and the sieved concrete is tested. Because wet-sieving removes aggregate along with some mortar, the result applies to the sieved fraction; picking stones out of a filled bowl disturbs consolidation, and the correction factor cannot account for oversize stones.
Ref QB-XF8BFXSource: ASTM C231/C231M, Standard Test Method for Air Content of Freshly Mixed Concrete by the Pressure Method — maximum aggregate size and wet-sieving per ASTM C172/C172MReport a problem with this question
10. A low-slump mix is being consolidated in the pressure-meter bowl with an internal vibrator. Which practice is correct?
- A.Vibrate each layer and then also rod it, since vibration alone leaves entrapped voids in the bowl.
- B.Vibrate each layer well beyond a smooth surface, so that every entrained bubble rises and escapes.
- C.Hold the vibrator against the bowl wall, so its energy spreads evenly through the whole sample.
- D.Keep the vibrator off the bowl walls and bottom, and stop before over-vibration drives out air.✓ Answer
Vibration is the consolidation method for stiff, low-slump concrete, but the vibrator must not touch the bowl, and vibration stops once the surface becomes relatively smooth. Over-vibrating drives entrained air out of the sample and gives a falsely low air content, which is why vibrating far past a smooth surface or adding rodding on top of vibration is wrong.
Ref QB-25HJ6DSource: ASTM C231/C231M, Standard Test Method for Air Content of Freshly Mixed Concrete by the Pressure Method — placement and consolidation by vibrationReport a problem with this question
11. After rodding each layer in the pressure-meter bowl, the technician taps the outside of the bowl with the mallet. What is the purpose of this tapping?
- A.To close the rod holes and release large entrapped bubbles that the rodding has left behind.✓ Answer
- B.To settle coarse aggregate to the bottom, so the top layer is mortar for a smooth strike-off.
- C.To check that the bowl is seated level before the cover is clamped and the meter is pressurized.
- D.To drive entrained air out of the paste, so that the reading reflects only the entrapped air.
Rodding leaves holes and can trap large air pockets that are not part of the concrete's normal void system. Tapping the bowl sides closes those holes and lets the large bubbles escape, so the sample in the bowl is uniformly consolidated; it is not meant to remove entrained air or segregate the mix.
Ref QB-JMPQRDSource: ASTM C231/C231M, Standard Test Method for Air Content of Freshly Mixed Concrete by the Pressure Method — consolidation by rodding and tappingReport a problem with this question
12. The bowl of a Type B meter has been filled, struck off, the rim cleaned and the cover clamped. Which sequence of the next steps is correct?
- A.Close main valve; open petcocks; pump to initial line; inject water; open main valve; close petcocks.
- B.Open main valve; close petcocks; inject water; pump to initial line; open petcocks; close main valve.
- C.Close main valve; open petcocks; inject water; pump to initial line; close petcocks; open main valve.✓ Answer
- D.Close main valve; close petcocks; pump to initial line; open main valve; inject water; open petcocks.
The main air valve stays closed so the chamber is isolated while water is injected through the open petcocks to fill the space under the cover. The chamber is then pumped to the initial pressure line, the petcocks are closed so the bowl is sealed, and only then is the main air valve opened to release the chamber air into the sample. Opening the main valve while the petcocks are open lets the pressure escape and ruins the reading.
Ref QB-35VVBESource: ASTM C231/C231M, Standard Test Method for Air Content of Freshly Mixed Concrete by the Pressure Method — procedure for Type B meterReport a problem with this question
13. On a Type B meter, why is water injected through one petcock until it flows out of the other before the test pressure is applied?
- A.To dilute the top of the sample, so the gauge reading settles faster once the main valve is opened.
- B.To fill the space between the concrete and the cover with water, so no unmeasured air stays there.✓ Answer
- C.To moisten the concrete surface, so the paste does not stick to the cover while it is pressurized.
- D.To cool the air chamber after pumping, so that the gauge does not drift while pressure is being set.
The meter is calibrated on the assumption that everything under the cover except the concrete's own air is incompressible water. Any air pocket left between the struck-off surface and the cover would be compressed along with the sample's air and inflate the reading, so water is pushed through until it flows out the other petcock, with gentle jarring to expel trapped air.
Ref QB-KM23XESource: ASTM C231/C231M, Standard Test Method for Air Content of Freshly Mixed Concrete by the Pressure Method — procedure for Type B meter, adding water through petcocksReport a problem with this question
14. On a Type B meter, pumping has just brought the gauge hand to the initial pressure line. What should happen before the petcocks are closed and the main air valve is opened?
- A.Let the compressed air cool briefly, then reset the hand on the initial line by pumping or bleeding.✓ Answer
- B.Open the main air valve at once, before the heat from pumping escapes and the pressure drops off.
- C.Open both petcocks briefly to vent any air that the pump forced into the water above the sample.
- D.Pump a few extra strokes past the initial line, so the reading is not short when the valve opens.
Pumping warms the air in the chamber, and as it cools to normal temperature its pressure falls. Waiting a few seconds and then stabilizing the hand exactly on the initial pressure line, by pumping or bleeding while lightly tapping the gauge, ensures the chamber holds the calibrated starting pressure; overshooting or opening the valve while the air is still warm builds in an error.
Ref QB-4588HFSource: ASTM C231/C231M, Standard Test Method for Air Content of Freshly Mixed Concrete by the Pressure Method — procedure for Type B meter, establishing initial pressureReport a problem with this question
15. When a Type A meter is used, how is the apparent air content of the sample obtained?
- A.From the water column: the level read at operating pressure minus the level read after its release.✓ Answer
- B.From the pressure needed to push the water column down to the zero mark on the graduated standpipe.
- C.From the rise in the water column when operating pressure is applied, read without releasing pressure.
- D.From the dial gauge, which reads percent air directly once the air chamber is released into the bowl.
In a Type A meter the water is filled to the zero mark, the calibrated operating pressure is applied, and the column falls as the sample's air compresses; that level is h1. After the pressure is gradually released, the level h2 is read, and the apparent air content is h1 minus h2. A dial reading percent air directly describes a Type B meter.
Ref QB-MMDD4GSource: ASTM C231/C231M, Standard Test Method for Air Content of Freshly Mixed Concrete by the Pressure Method — procedure for Type A meterReport a problem with this question
16. A Type B air test has just been read. Which sequence for releasing pressure and taking the meter apart is correct?
- A.Open the petcocks to vent the bowl, close the main air valve, and then unclamp the cover.
- B.Unclamp the cover to vent the bowl, close the main air valve, and then open the petcocks.
- C.Close the main air valve, open the petcocks to vent the bowl, and then unclamp the cover.✓ Answer
- D.Open the air bleeder valve to vent the chamber, unclamp the cover, then open the petcocks.
Closing the main air valve first isolates the air chamber; if pressure is released from the bowl while that valve is still open, water can be drawn back into the air chamber and upset the meter's calibration. With the chamber isolated, opening the petcocks vents the bowl, and the cover is unclamped only after the bowl is no longer under pressure.
Ref QB-55KJPGSource: ASTM C231/C231M, Standard Test Method for Air Content of Freshly Mixed Concrete by the Pressure Method — procedure for Type B meter, releasing pressure and removing the coverReport a problem with this question
17. How is the aggregate correction factor for a pressure-meter air test determined?
- A.On a sample of the fresh concrete tested twice in a row, with the difference taken as the factor.
- B.On fine and coarse aggregate in the concrete's proportions and moisture state, inundated in water.✓ Answer
- C.On a standard reference aggregate kept with the meter, whatever the job's aggregate source is.
- D.On coarse aggregate alone in a dry state, since fine aggregate pores are too small to hold air.
The correction factor measures the air held inside the aggregate particles, so it must be found on the job's own fine and coarse aggregate, combined in the same proportions and moisture condition as in the concrete. The aggregates are placed in a bowl partly filled with water, inundated for a period, worked to remove entrapped air, and then run through the same meter procedure.
Ref QB-NMRQ9HSource: ASTM C231/C231M, Standard Test Method for Air Content of Freshly Mixed Concrete by the Pressure Method — determination of aggregate correction factorReport a problem with this question
18. A technician prepares an aggregate correction factor test. The bowl holds S = 0.25 ft3 (7.1 L) of concrete, one batch produces B = 27 ft3 (0.76 m3), and each batch contains 1350 lb (612 kg) of fine aggregate and 1800 lb (816 kg) of coarse aggregate. Which aggregate masses go into the test?
- A.Fine 337.5 lb and coarse 450 lb, found by multiplying each batch mass by S
- B.Fine 50.0 lb and coarse 66.7 lb, found by dividing each batch mass by B
- C.Fine 12.5 lb and coarse 16.7 lb, found by scaling each batch mass by S/B✓ Answer
- D.Fine 16.7 lb and coarse 12.5 lb, found by scaling each batch mass by S/B
The aggregate in the test must equal what the bowl's volume of concrete actually contains, so each batch mass is scaled by S/B: Fs = (0.25/27) x 1350 = 12.5 lb and Cs = (0.25/27) x 1800 = 16.7 lb. Swapping the two, multiplying by S alone, or dividing by B alone does not give the aggregate content of one bowl of concrete.
Ref QB-W448G8Source: ASTM C231/C231M, Standard Test Method for Air Content of Freshly Mixed Concrete by the Pressure Method — aggregate correction factor, Fs = (S/B) x Fb and Cs = (S/B) x CbReport a problem with this question
19. A Type B meter gives an apparent air content of 5.9% on a concrete sample. The aggregate correction factor for this mix is 0.4%. What air content should be recorded?
- A.5.5%, because the correction factor is subtracted from the apparent reading✓ Answer
- B.5.1%, because the factor is subtracted once for fine and once for coarse
- C.6.3%, because the correction factor is added back to the apparent reading
- D.5.9%, because the correction factor is already built into the gauge setting
The apparent reading includes air in the aggregate pores, which is not part of the concrete's air-void system, so the correction factor is subtracted: As = A1 - G = 5.9 - 0.4 = 5.5%. The factor is determined on the combined fine and coarse aggregate, so it is subtracted once, not once per aggregate.
Ref QB-EM9D39Source: ASTM C231/C231M, Standard Test Method for Air Content of Freshly Mixed Concrete by the Pressure Method — calculation, As = A1 - GReport a problem with this question
20. On a Type A meter, the water level reads h1 = 7.2 at the operating pressure and h2 = 0.6 after the pressure is released. The aggregate correction factor is 0.5%. What is the air content of the concrete?
- A.6.7%: apparent 7.2% from h1 alone, with the correction factor then subtracted
- B.7.1%: apparent 6.6% from h1 minus h2, with the correction factor then added on
- C.7.3%: apparent 7.8% from h1 plus h2, with the correction factor then subtracted
- D.6.1%: apparent 6.6% from h1 minus h2, with the correction factor then subtracted✓ Answer
For a Type A meter the apparent air content is the difference between the reading at operating pressure and the reading after release, A1 = h1 - h2 = 7.2 - 0.6 = 6.6%. The aggregate correction factor is then subtracted, As = 6.6 - 0.5 = 6.1%; using h1 alone ignores the residual level, and adding the factor counts pore air as concrete air.
Ref QB-RKKEYPSource: ASTM C231/C231M, Standard Test Method for Air Content of Freshly Mixed Concrete by the Pressure Method — calculation for Type A meter, A1 = h1 - h2 and As = A1 - GReport a problem with this question
21. Which value should a technician enter on the report as the air content from a pressure-meter test?
- A.The apparent reading plus the correction factor, so that air held in the aggregate is also counted.
- B.The apparent reading as shown on the gauge, with the correction factor left for the engineer to apply.
- C.The apparent reading minus the aggregate correction factor that applies to the mix.✓ Answer
- D.The average of the apparent reading and the aggregate correction factor for the mix.
The reported air content is the concrete's own air, so the technician subtracts the aggregate correction factor from the apparent reading before reporting. Reporting the raw gauge value or adding the factor misstates the air in the concrete. Averaging the two values has no basis in the method.
Ref QB-84E9DPSource: ASTM C231/C231M, Standard Test Method for Air Content of Freshly Mixed Concrete by the Pressure Method — reportReport a problem with this question
22. A Type B gauge reads far below what the mix design and earlier tests suggest, and the technician realizes the petcocks were still open when the main air valve was opened. What should be done?
- A.Discard the reading and retest a fresh portion of the sample, following the correct valve order.✓ Answer
- B.Correct the reading by adding the usual gap from the mix design, and report the adjusted value.
- C.Report the reading as measured, since a retest would use concrete that has lost air over time.
- D.Reopen the main air valve on the same bowl, re-pump, and report the higher of the two readings.
With the petcocks open, chamber air escaped instead of equalizing with the sealed sample, so the reading is invalid. A broken procedure cannot be fixed by adjusting the number or re-pressurizing the same disturbed bowl; the reading is discarded and a fresh portion of the sample is tested following the correct sequence.
Ref QB-SKXR5RSource: ASTM C231/C231M, Standard Test Method for Air Content of Freshly Mixed Concrete by the Pressure Method — procedure for Type B meter and validity of the testReport a problem with this question

1. A project calls for structural lightweight-aggregate concrete, and the inspector asks for an air content test on the fresh concrete. Which approach is correct?

2. When a technician reports the air content measured on fresh concrete with a pressure meter, what does that value represent?

3. On what physical behavior does the pressure method rely to determine air content?

4. What does a technician observe on a Type B pressure meter that differs from a Type A meter?

5. A technician's pressure-meter kit contains a steel hammer for tapping the bowl. What does the method expect of the mallet instead?

6. During a calibration check of a Type B meter with the bowl full of water and no air introduced, the gauge consistently fails to return to the expected reading. What should the technician do?

7. A crew calibrated a Type A meter at a plant near sea level and now takes it to a job at a much higher elevation. What should be done with the meter?

8. Assume the method requires the air test to begin within 5 minutes after the composite sample is obtained. The composite was completed at 10:00, but the meter was not ready until 10:12. What should the technician do?

9. Fresh concrete contains coarse aggregate larger than the maximum size the pressure method allows to be tested directly. What should the technician do?

10. A low-slump mix is being consolidated in the pressure-meter bowl with an internal vibrator. Which practice is correct?

11. After rodding each layer in the pressure-meter bowl, the technician taps the outside of the bowl with the mallet. What is the purpose of this tapping?

12. The bowl of a Type B meter has been filled, struck off, the rim cleaned and the cover clamped. Which sequence of the next steps is correct?

13. On a Type B meter, why is water injected through one petcock until it flows out of the other before the test pressure is applied?

14. On a Type B meter, pumping has just brought the gauge hand to the initial pressure line. What should happen before the petcocks are closed and the main air valve is opened?

15. When a Type A meter is used, how is the apparent air content of the sample obtained?

16. A Type B air test has just been read. Which sequence for releasing pressure and taking the meter apart is correct?

17. How is the aggregate correction factor for a pressure-meter air test determined?

18. A technician prepares an aggregate correction factor test. The bowl holds S = 0.25 ft3 (7.1 L) of concrete, one batch produces B = 27 ft3 (0.76 m3), and each batch contains 1350 lb (612 kg) of fine aggregate and 1800 lb (816 kg) of coarse aggregate. Which aggregate masses go into the test?

19. A Type B meter gives an apparent air content of 5.9% on a concrete sample. The aggregate correction factor for this mix is 0.4%. What air content should be recorded?

20. On a Type A meter, the water level reads h1 = 7.2 at the operating pressure and h2 = 0.6 after the pressure is released. The aggregate correction factor is 0.5%. What is the air content of the concrete?

21. Which value should a technician enter on the report as the air content from a pressure-meter test?

22. A Type B gauge reads far below what the mix design and earlier tests suggest, and the technician realizes the petcocks were still open when the main air valve was opened. What should be done?

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