7.4 Inversion, Shaking, Rolling, and Air Release

Why C173 Is Called the Rollameter Test

After the meter is sealed, filled with water, and dosed with alcohol, the agitation sequence physically separates air from the concrete. This is the defining step of the method — the apparatus is literally rolled, which is why the meter is nicknamed the rollameter. The goal is to free every air bubble from the concrete mass and let it rise into the neck, while simultaneously breaking the concrete apart so no air stays trapped in clumps.

The Inversion and Shaking Sequence

The operator first inverts and shakes the sealed meter to loosen the concrete that has settled against the bowl base. A representative sequence:

Inversion alone does not finish the job — it loosens the mass so the subsequent rolling can wash air out. The candidate must keep a firm grip and keep the cap secured; a meter that comes apart mid-inversion spills concrete and invalidates the test.

The full agitation sequence in order:

1. Invert and shake in ~5-second cycles, at least 45 seconds total, to free concrete from the base.
2. Roll on the side at ~45°, rocking a quarter to half turn, for about 1 minute.
3. Stand upright and let settle (~2 minutes) so foam clears.
4. Read the neck, then repeat rolling and reading until the level stops changing.

Rolling at 45 Degrees

With the concrete loosened, the meter is laid on its side and rolled at about a 45° tilt for roughly 1 minute, rocking it back and forth a quarter to a half turn vigorously. Rolling tumbles the concrete through the water so trapped air is scrubbed loose and floats to the neck. The operator then stands the meter upright and lets the foam settle (about 2 minutes) before reading.

The sequence is iterative, not one-and-done:

• Roll for ~1 minute, let settle, read the level.
• Roll again and read again. If the level dropped, air is still being released.
• Repeat rolling until the liquid level stops changing — i.e., two successive readings agree (within 0.25 %). Only then is all the air out.

A reading taken after a single roll, before the concrete has fully released its air, is not a valid final result; it will read low because air remains trapped.

Controlled Agitation and Common Faults

Movement must be purposeful and controlled. A short fault list the exam likes:

• Cap loosens / leaks during rolling → liquid lost → invalid; restart with fresh sample.
• Rolling too gently → air not fully released → false low reading; keep rolling until stable.
• Concrete sticks in a wad → some air stays trapped; inversion/shaking must break the mass first.
• Foam not settled before reading → unreadable meniscus; add a little alcohol next cycle and let it settle.

The takeaway: inversion frees the concrete from the base, rolling washes the air out, and the level keeps dropping until — and only until — the concrete has given up all its air.

Timing discipline matters because air release is time-dependent. If the liquid level fails to stabilize within about 6 minutes of total settling, the test is discarded — the concrete is either stiffening or foam is not clearing, and continued rolling will not produce a trustworthy number. Conversely, a technician who reads after one quick roll and reports it has almost certainly under-measured the air, because trapped pockets had no chance to wash free. The rule of thumb: roll, settle, read, repeat — and trust only the reading that two cycles confirm.

What the Liquid Level Is Telling You

It helps to picture the physics. Inside the sealed meter sit three things: the concrete solids, the water, and the air that was in the concrete. Before rolling, that air is still locked in bubbles inside the paste, so the liquid sits at zero. As rolling tumbles the mix, each freed bubble rises and occupies space in the neck that liquid used to fill, so the liquid level drops. The total drop, in percent, is the air content. When no more bubbles are being freed, the level stops dropping — which is exactly why the agreement of two successive readings signals completion.

A short troubleshooting list ties the mechanics to field symptoms:

• Level not dropping at all → seal failed or no air to release; check the seal and that admixture was actually used.
• Level still dropping after several rolls → keep rolling; air is not fully out.
• Level dropped below the neck → off scale; add measured water back (covered in 7.5).
• Foam capping the column → add a little alcohol, settle longer.

Reading the level correctly therefore is not a clerical step — it is the entire measurement, and every part of the inversion-and-rolling routine exists to make that one number trustworthy.