ASTM C173: Air Content of Freshly Mixed Concrete by the Volumetric Method

What the ASTM C173 test measures

ASTM C173 measures the air content of fresh concrete by the volumetric method, using the tall rolling meter the field calls a roll-a-meter. Like the pressure method, it reports the percentage of the concrete that is air. Unlike the pressure method, it physically washes the air out of the sample with water and alcohol and measures the volume that comes out. That makes it immune to a problem that breaks pressure meters: porous aggregate.

That is the whole reason this method exists on your project. Lightweight aggregates, expanded shales and clays, are full of internal pores. A pressure meter squeezes the air in those pores and reports it as if it were air in the paste, reading falsely high. The volumetric meter only counts air that can be rolled and floated out of the mix, which is the air that actually matters to the concrete.

This page explains where the volumetric method is required and how the rolling procedure reaches its answer. It also covers how we staff it so the slower meter never slows the pour.

Why the right meter matters

Specify lightweight concrete on a deck and the air requirement is still there, usually for the same durability and pumpability reasons as normal-weight work. Test it with the wrong meter and you get numbers that fail perfectly good concrete. Or the producer ends up chasing an air target that is only a measurement artifact. The standards are explicit: lightweight aggregate concrete gets the volumetric method. Knowing which meter comes off the truck for which mix is a technician competence question, and it is one we train hard.

The method also serves as the referee when pressure readings on any mix look suspicious. That is because its answer does not depend on aggregate correction factors or gauge calibration in the same way.

It matters most exactly where lightweight concrete is most common: elevated decks on steel framing. There, the mix is pumped high and the air is working hardest. Coring the structure later to answer a durability doubt is the expensive alternative to five extra minutes of rolling at the hose end.

How the ASTM C173 test works

From a composite sample per ASTM C172, the technician fills the meter's bowl in two equal layers, not the three used elsewhere. Each layer is rodded 25 times and the sides are tapped. The top section clamps on, water is added to the calibrated mark, then a measured dose of isopropyl alcohol to control foaming.

Then comes the part that gives the meter its nickname. The sealed unit is inverted, agitated, and rolled, so the water scrubs through the concrete and collects the air. The technician reads the liquid level, rolls again, and reads again, repeating until consecutive readings agree within the standard's tight tolerance. That is because one roll never gets all the air and stopping early always under-reports. The stabilized reading, adjusted for the alcohol added, is the air content.

The last step is teardown discipline: the bowl is opened and checked. Concrete still stuck to the bottom means the agitation never freed it, and the test, however stable its readings looked, is invalid and repeated. It is a humbling little rule and a perfect example of why procedure beats confidence.

A small but consequential detail: the alcohol dose is recorded, because the correction applied to the final reading depends on it. A high-foam mix that needed extra alcohol carries a correspondingly larger correction. It is the kind of line item nobody notices until an audit, which is exactly why it is on our form.

What the number means

The result reads against the same specification band as any air requirement, target plus tolerance, judged at the point of placement. On lightweight pumped decks, the aggregate and the pump both work to change the air between plant and hose tip. The placement-end volumetric reading is the number the durability of the deck actually rides on.

The method takes longer than a pressure test, several minutes of rolling versus one squeeze of a pump. So coverage planning matters. On high-volume lightweight pours we stagger sampling so the meter's cycle time never gates the trucks.

Reported results carry the same traceability as every fresh property we run: the meter identification, its calibration status, the alcohol correction applied. The sampling time, so the number can be reconstructed years later without anyone's memory involved.

Who needs ASTM C173 testing

Contractors on lightweight structural decks, which is most elevated composite deck work, need it on essentially every sampled truck. Owners need it because entrained air in a lightweight deck is a durability property they cannot see and will live with for fifty years. Engineers specify it directly, and producers of lightweight mixes prefer it for the same reason everyone should: it measures their concrete, not their aggregate's pore structure.

Public agency work adds one more constituency. Transportation specifications frequently require volumetric air on lightweight bridge elements outright. The meter, and the trained hands behind it, are a bid-day requirement, not a preference.

Common questions about ASTM C173

How we help with ASTM C173 testing

Both air meters ride on our trucks. The mix design review before your pour determines which one governs which mix, so the lightweight deck never gets the wrong instrument on placement day. Volumetric results are reported per load with location and companion fresh properties. On lightweight pump work we test at the hose end where the specification points, staffed so the meter's cycle time is our problem to manage, not your pour's.