5.3 Strike-Off, Cleaning, Mass, and Density
Key Takeaways
- Strike-off creates the final measured volume, so the surface must be flush with the rim rather than heaped or scooped.
- Concrete on the outside of the measure must be removed before weighing because exterior paste adds mass outside the calibrated volume.
- Density is calculated from net concrete mass divided by calibrated measure volume, with units kept consistent.
- A density value should be checked for reasonableness before it is used for yield or gravimetric air calculations.
Turning A Filled Measure Into Density
Once the concrete is placed and consolidated, the top of the measure defines the final volume used in the C138 calculation. Strike-off is the step that makes that volume real. The technician removes excess concrete and finishes the surface flush with the rim. A mound above the rim means extra concrete mass outside the calibrated volume. A hollow or scooped surface means the measure is not truly full.
The strike-off tool must bridge the rim and create a plane surface. The action should remove excess concrete without pulling coarse aggregate out and leaving depressions. If large aggregate is dragged out, the surface may need careful correction with representative concrete from the sample, following the method. The target is not a polished slab finish. The target is a full, level container of known volume.
After strike-off, the outside of the measure must be cleaned. Paste on the rim, side, handles, base, or bottom is not part of the calibrated volume, but it will be included by the balance. That extra mass can make density too high. The same problem happens when the measure sits on a dirty scale pan or when loose concrete is trapped under the base.
Density calculation map:
| Symbol or value | Meaning | Common mistake |
|---|---|---|
| Full mass | Measure plus concrete | Treating it as concrete mass |
| Empty mass | Clean empty measure | Using an old tare for another measure |
| Net mass | Full mass minus empty mass | Forgetting to subtract tare |
| Volume | Calibrated measure volume | Using nominal bucket size without calibration |
| Density | Net mass divided by volume | Mixing lb, kg, ft3, and m3 units |
The core formula is D = M / V, where D is density, M is net mass of concrete, and V is calibrated volume of the measure. In inch-pound work, density is commonly expressed as pounds per cubic foot. In SI work, density is commonly expressed as kilograms per cubic meter. The formula is simple, but unit discipline is strict.
Reasonableness checks matter. Normal-weight concrete density is often near the familiar range taught in concrete courses, while lightweight and heavyweight concretes can differ. The exam will not require you to diagnose every mix, but it may ask you to spot impossible logic. A negative net mass, a density based on gallons mixed with cubic feet, or a value that changes because the wrong tare was used should be rejected.
A density result also has field meaning. If the density is lower than expected, possible causes include high air content, high water content, missing aggregate mass, poor consolidation, or an unrepresentative mortar-heavy sample. If density is higher than expected, possible causes include lower air, heavier aggregate, extra coarse aggregate in the sample, or batching differences. C138 tells you where to look; it does not replace project acceptance rules.
Do not rush the scale reading. The balance should be on a stable surface and have capacity for the filled measure. The reading should be recorded with the correct unit and enough precision for the method and report. A clean scale and stable number are part of the test, not clerical details.
Exam-ready strike-off checklist:
- Slightly overfill the final layer before strike-off.
- Strike off to a flush surface at the rim.
- Avoid leaving aggregate pop-outs or mortar depressions.
- Clean the exterior, rim, base, and scale contact area.
- Weigh the filled measure and subtract the empty measure mass.
- Divide net mass by calibrated volume and check units.
Why must concrete paste be cleaned from the outside of the C138 measure before weighing?
Which formula represents the basic C138 density calculation?
A struck-off surface is visibly below the rim in one area. What is the concern?