6.1 Scope, Principle, and When Pressure Air Fits

What C231 Measures And Why

ASTM C231/C231M is the Standard Test Method for Air Content of Freshly Mixed Concrete by the Pressure Method. It determines the air content of fresh, normal-weight concrete by sealing a known volume of concrete in a meter, applying a calibrated pressure, and reading how much the entrained air compresses. In ACI Field Testing Technician Grade I, it is taught almost exclusively with the Type B pressure meter, the kind whose dial gauge reads percent air directly.

The governing physics is Boyle's Law: at constant temperature, a fixed mass of gas decreases in volume as pressure increases. Concrete solids, aggregate, water, and cement paste are essentially incompressible, but the air voids are not. When the meter applies a known pressure, only the air shrinks, and the meter is calibrated so that the resulting pressure-volume response maps to a percent-air value on the gauge.

Because the method responds to compressible air wherever it sits, the meter cannot distinguish entrained air in the paste from air trapped in the pores of a porous aggregate. That single fact drives the method's biggest scope limit and the entire reason an aggregate correction factor exists.

Scope Limit: Dense Aggregate Only

C231 applies to concrete made with relatively dense, normal-weight aggregate. It is explicitly not appropriate for concrete made with lightweight aggregate, air-cooled blast-furnace slag, or other highly porous aggregates. In those mixtures the aggregate pores respond to pressure so strongly that the apparent reading is unreliable and the correction becomes too large to trust. For lightweight or porous-aggregate concrete, use the volumetric method, ASTM C173, instead.

C231 also answers only one narrow question: the air content of this fresh sample by pressure. It is not a slump test (C143), a density/yield test (C138), or a strength test. In a field workflow it runs alongside slump, temperature (C1064), and density, but each has its own equipment, sequence, and invalid-result traps.

If an exam scenario hands you lightweight or slag-aggregate concrete and asks the right air method, the answer is the volumetric method, not C231.

Apparent Versus Actual Air

The Type B gauge value is the apparent air content, symbol A1. It is never the final reported number on its own. The actual air content of the sample is:

As = A1 - G

where G is the aggregate correction factor, the share of the apparent reading attributable to air in (and around) the aggregate rather than to entrained air in the paste. G is determined separately, on the same aggregates in the same proportions, and is subtracted from A1. Forgetting to subtract G, or adding it instead of subtracting, is one of the most common written-exam traps.

Keep three numbers straight:

• A1 (apparent air): what the gauge shows after the pressure sequence.
• G (aggregate correction factor): a separately measured, mixture-specific value, usually small.
• As (actual air): the reported result, equal to A1 minus G.

C231 is also a field-performance test. A correct number depends on a representative sample, correct consolidation, a flush strike-off, a leak-free seal, free air vented from the cover with water, pressure pumped to the initial line, and a stable gauge read at the right moment. A shortcut in any one step can produce a believable but wrong A1, and no correction will rescue a bad fill.

Why Air Content Matters

Air content is not measured for its own sake; it is the field control for freeze-thaw durability and workability. Properly entrained air creates millions of microscopic, closely spaced bubbles that give freezing water somewhere to expand, protecting the paste from internal cracking through repeated freeze-thaw cycles. Too little air leaves the concrete vulnerable; too much air sharply reduces strength, roughly 5 percent loss in compressive strength for each 1 percent of added air. That trade-off is why specifications set a target band rather than a single value.

A typical specification for exterior, freeze-thaw concrete might call for 6.0 percent air, plus or minus 1.5 percent, so the technician's job is to deliver a fast, accurate field number that tells the placing crew whether the load is acceptable, needs adjustment, or must be rejected before it goes into the structure. Because the result drives accept/reject decisions in real time, the method is built for speed and repeatability in the field rather than laboratory precision.

Where C231 fits in the field test suite:

• Slump (C143): consistency and water control.
• Temperature (C1064): concrete temperature limits.
• Air content (C231 or C173): durability and workability control.
• Density and yield (C138): unit weight, yield, and a check on air.
• Cylinders/beams (C31): strength specimens for later testing.

Each test draws from the same representative sample but answers a different acceptance question, and the technician must not let one test compromise another, such as letting concrete sit too long before running air.