7.2 Sample, Bowl Filling, Consolidation, and Strike-Off

Obtaining and Placing the Sample

The air sample must come from a representative composite sample obtained per ASTM C172 — not from the first or last of the discharge, and remixed in the wheelbarrow before any test specimen is taken. The same composite that feeds slump (C143) and unit weight (C138) feeds the air test. If the concrete has visibly segregated, it is remixed; testing partially consolidated or segregated material gives an air result that describes nothing on the project.

The rollameter bowl is filled with concrete in two equal layers. "Equal" means roughly half the bowl depth per layer, eyeballed at placement — not weighed. Concrete is placed with a scoop moved around the perimeter so the bowl loads evenly and the sample does not heap to one side. Filling in two layers (rather than one) is what makes the 25-stroke rodding reach the full depth without leaving a poorly consolidated bottom.

Consolidation: Rodding and Tapping

Each layer is consolidated by rodding 25 times with the standard 5/8 in. (16 mm) rounded-tip tamping rod, distributing the strokes uniformly over the surface:

After rodding each layer, tap the sides of the bowl 10 to 15 times with the 1.25 ± 0.50 lb mallet, distributing the blows around the perimeter. Tapping closes the holes left by the rod and releases large entrapped air pockets trapped against the wall. The candidate must say and do both actions per layer; skipping the taps leaves voids that read as false air. A common performance-exam deduction is rodding the first layer all the way through and striking the bottom of the bowl — the rod should reach the full depth of the layer but not forcibly hammer the base, which can dent the calibrated bowl.

Why Consolidation Must Be Controlled

The number one conceptual trap on the exam is the difference between entrapped and entrained air. Rodding and tapping are designed to drive out the large, irregular entrapped voids — the accidental pockets from placement — while leaving the small, deliberate, uniformly distributed entrained air bubbles that the admixture created and that the test is meant to measure. Over-vibrating or over-rodding would expel entrained air and report an artificially low result; under-consolidating leaves entrapped voids and reports an artificially high result.

C173 specifies rodding (not internal vibration) so the operator does not accidentally drive out entrained air. Internal vibration is generally not used in the C173 fill — the controlled 25-stroke rodding plus mallet tapping is the prescribed consolidation for this bowl size.

Entrained vs. entrapped air, side by side:

Strike-Off and Cleaning the Rim

After the second layer is rodded and tapped, the surface is struck off flush and flat with the top rim of the bowl using the strike-off bar in a sawing, then a leveling, motion. A short list governs a clean strike-off:

• Remove any excess concrete heaped above the rim.
• Fill any low spots, then re-level so the surface is planar with the rim — not crowned, not dished.
• Wipe the flange/rim completely clean of all concrete, grout, and grit before the top section is set on.

Why this matters: the calibrated bowl volume is the basis of the air percentage. If the concrete is struck off high, the true concrete volume is unknown and the air reads low; if struck off low, air reads high. A dirty rim prevents a watertight seal, which causes leakage and an invalid test. Strike-off, not the later rolling, defines the measured volume — the rolling step can never recover a bad fill.

Sequencing the Fill Within the Field Tests

Timing matters. Air, slump, and temperature are all time-sensitive because fresh concrete is changing from the moment it leaves the mixer. ASTM limits how long you may wait: air testing should begin promptly after sampling, generally completing the measurement within the window allowed after obtaining the composite sample, because air content can drift as the concrete stiffens and as entrained bubbles coalesce or escape. A technician who lets the wheelbarrow sit for twenty minutes and then fills the bowl is testing a different concrete than the one being placed.

A disciplined fill sequence the candidate should be able to recite in order:

1. Remix the composite sample in the wheelbarrow.
2. Place the first equal layer with a scoop moved around the perimeter.
3. Rod 25 times, then tap the sides 10–15 times with the mallet.
4. Place the second equal layer to slightly overfill the bowl.
5. Rod 25 times (penetrating ~1 in. into layer one), then tap 10–15 times.
6. Strike off flush, fill low spots, re-level, and wipe the rim clean.

Doing these steps out of order — for example, striking off before tapping the second layer — leaves the surface voids open and changes the measured volume, so the candidate must keep the sequence intact.