Pressure Versus Volumetric Air Content With Lightweight Aggregate
A volumetric air meter is used on a 0.25 ft³ sample of concrete containing lightweight coarse aggregate. The technician fills the meter in three layers, rods 25 strokes per layer, strikes off, adds water, rolls the meter, and reads 7.5% after the water level stabilizes. The same concrete gives 5.5% on a pressure meter. The specification requires 5 to 7% air. The pressure meter’s aggregate correction factor was last determined for a different lightweight aggregate, and the volumetric meter was calibrated six months ago. The technician must decide which result to report.
1. Which air-content method is more appropriate for this concrete, and why?
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The pressure method is less reliable for lightweight aggregate concrete. It assumes that the volume change under pressure is due to air in the concrete, but lightweight aggregate particles can be compressible or porous and can distort the pressure reading. The aggregate correction factor must be determined for the actual aggregate combination; a factor from a different lightweight aggregate is not acceptable. Therefore the 5.5% pressure result is not trustworthy as reported.
The volumetric method measures air by water displacement and is commonly used where the pressure method is unsuitable, including lightweight concrete. If the meter is properly calibrated, clean, watertight, and operated correctly, the 7.5% volumetric result is more credible. However, lightweight aggregate absorption and operator technique can also affect the volumetric method, so the result should be verified with a fresh sample rather than accepted solely because the method is preferred.
2. Identify the procedural and equipment problems that could explain the 2% disagreement.
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Several factors could explain the disagreement:
- The pressure meter uses an aggregate correction factor from a different lightweight aggregate, so its reading may be biased.
- The pressure meter calibration and gauge condition are unknown; a damaged or uncalibrated gauge can shift the result.
- The volumetric meter’s calibration status should be confirmed, and the meter must be clean, level, and watertight.
- The two tests may have used different portions of concrete, or one sample may not be representative.
- Filling and rodding technique, lightweight aggregate absorption, and the time between sampling and testing can affect air content.
- The volumetric reading must be taken after proper rolling and stabilization, with the meter at the correct temperature.
The technician should not simply choose the result that falls within specification. The equipment and aggregate correction must be verified, and a fresh sample should be tested.
3. The volumetric result is 7.5%, just above the 7% limit. What should the technician do before rejecting the load?
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The technician should not reject the load on one unverified volumetric result, especially when the pressure result differs by 2 percentage points. First, confirm that the volumetric meter is calibrated, clean, level, and watertight, and that the sample was representative and properly consolidated. Check the pressure meter’s aggregate correction factor and calibration as well.
Then obtain a fresh representative sample and repeat the air test by the volumetric method. If possible, verify with a gravimetric check using accurate batch weights, or another accepted method. If the repeat result is still above 7%, the load does not meet the air specification and should be reported to the supplier and contractor. Document both results, the methods, and the corrective actions. If the repeat result is within 5 to 7%, the earlier 7.5% reading should be investigated as an equipment or sampling error.