7.6 Alcohol Correction, Calculation, Reporting, and Performance Habits
Key Takeaways
- The reported air content is the final stable meter reading, plus any measured water additions for off-scale readings, minus the alcohol correction when required.
- No aggregate correction factor is ever applied in C173 — that is unique to the C231 pressure method.
- When 2.5 pt (1.2 L) or more isopropyl alcohol is used, subtract the Table 1 alcohol correction; below that, read air directly.
- Air content is reported to the nearest 0.25%, with sample identification, method, corrections, and any unusual conditions noted.
Building the Reported Air Content
The reported air content is assembled from up to three pieces:
Air % = (final stable meter reading) + (any measured water additions made to return an off-scale level to the scale) − (alcohol correction, if required)
Notice what is not in the equation: there is no aggregate correction factor. That term belongs only to the C231 pressure method. The most common calculation trap on the exam is bolting an aggregate correction onto a C173 result — it never applies.
| Component | When Applied |
|---|---|
| Final meter reading | Always — the stable, agreed reading on the neck |
| + Water addition | Only when the level went off scale and water was added back |
| − Alcohol correction | Only when 2.5 pt (1.2 L) or more alcohol was used |
| − Aggregate correction | Never in C173 |
A worked example: the stable reading is 8.0 %; the level had gone off scale so 2 % of water was added back; 3 pt of alcohol were used, giving a Table 1 correction of, say, 0.5 %. Reported air = 8.0 + 2.0 − 0.5 = 9.5 %, rounded to the nearest 0.25 %. The sign of each term is worth memorizing: water additions add (the level was artificially high because you put water back in), while the alcohol correction subtracts (the alcohol made the reading look airier than it was).
The Alcohol Correction in Detail
Isopropyl alcohol occupies volume and, in quantity, slightly inflates the apparent air, so the standard provides a Table 1 correction. The threshold the candidate must memorize:
- Up to ~2.4 pt of alcohol used → no correction, read air directly.
- 2.5 pt (1.2 L) or more used → subtract the published correction from the meter reading.
The correction is not optional when the quantity triggers it — applying it is part of producing a valid number. The practical defense is to add only as much alcohol as needed to clear foam, keeping most tests in the no-correction zone.
Reporting and Documentation
A defensible C173 report includes more than a single number:
- Air content to the nearest 0.25 %.
- Sample/specimen identification (project, location, batch/truck, time).
- Method: ASTM C173/C173M, volumetric.
- Corrections applied: water additions and/or alcohol correction.
- Unusual conditions: excess foam, repeated rolls, ambient issues, or an invalidated first attempt.
Performance-Exam Habits
The ACI Grade I performance exam grades you on a checklist while you physically run the test. Habits that earn points:
- Verbalize the validity checks as you go — seal is tight, level set to zero, alcohol amount logged, readings agree within 0.25 %.
- Track the alcohol out loud so the correction decision is obvious.
- State the final-bowl check — confirm no concrete is caked in the bowl before declaring the result valid.
- Report to the nearest 0.25 % and never invent an aggregate correction.
Success on this section is turning a stable reading into a correctly corrected, clearly documented, defensible air content — and being able to say why each correction was or was not applied.
C173 vs. C231 on the Calculation Side
The two air methods diverge most sharply at the calculation, and the exam exploits the contrast. With C231 (pressure) you read an apparent air, then subtract an aggregate correction factor (G) determined separately, because the aggregate's own voids compressed under pressure. With C173 (volumetric) there is no aggregate correction at all — instead the only adjustments are the optional water add-back for off-scale levels and the alcohol correction for heavy alcohol use.
| Calculation Element | C173 Volumetric | C231 Pressure |
|---|---|---|
| Base value | Stable neck reading | Apparent air on gauge |
| Aggregate correction | None | Subtract factor G |
| Alcohol correction | Subtract when ≥2.5 pt | Not applicable |
| Off-scale water add-back | Add measured water | Not applicable |
| Reporting precision | Nearest 0.25 % | Nearest 0.1–0.25 % |
A Defensible Record
The final number lives or dies on documentation. A report that says only "7 % air" cannot be defended if challenged; one that records the method (C173), the stable reading, the alcohol quantity and any correction, any water add-back, the sample identity and time, and any anomalies can be reconstructed and trusted. On the performance exam, the proctor is effectively that future reader: narrate the seal check, the alcohol count, the agreement of readings, and the clean-bowl confirmation, and the checklist points follow. The discipline that produces a defensible record is the same discipline that produces an accurate one.
Putting the Whole Method Together
5 pt or more was used, and a clean-bowl confirmation. 25 %, and never add an aggregate correction. 5 %, the corrected C173 value is what you compare against that band to accept or reject the load. Master this number and you have mastered the most demanding of the seven Grade I field tests.
Alcohol-correction quick recap
- No correction is needed up to about 2 pints of alcohol; beyond that, subtract the Table 1 alcohol correction.
- Report air content to the nearest 0.25 %.
- Unlike the pressure method, C173 needs no aggregate correction factor because it physically removes the air rather than compressing it.
Which correction is NEVER applied to an ASTM C173 volumetric air result?
A C173 test used 3 pt of isopropyl alcohol. How does this affect the reported result?
A C173 test gives a stable reading of 6.0%, with no off-scale water additions and only 2 pt of alcohol used. What is the reported air content?
To what precision is C173 air content reported?