5.3 Strike-Off, Cleaning, Mass, and Density
Key Takeaways
- After consolidation, slightly overfill the measure, then strike off with a flat plate using a sawing/pressing motion to make the surface flush and smooth.
- Clean all concrete from the exterior and rim of the measure before weighing, or the recorded mass — and density — will be too high.
- Density D = (Mc − Mm) / Vm, reported to the nearest 0.1 lb/ft³ (1 kg/m³).
- Normal-weight concrete typically falls between 140 and 150 lb/ft³; results far outside this signal a procedural error.
- The balance must be accurate to 0.3% of the test load to keep the density reliable.
Strike-Off With a Flat Plate
After the final layer is consolidated, the concrete should slightly overfill the measure — about 1/8 in. (3 mm) above the rim. Using the flat strike-off plate (not the round tamping rod, a frequent distractor), strike the surface flush in two motions:
- A sawing motion across the top with the plate held at a slight angle, working off the excess concrete.
- A pressing/finishing pass with the plate nearly flat, using light pressure to leave a smooth, flush surface level with the rim.
The goal is that the volume of concrete in the measure exactly equals the calibrated volume Vm — no heaping (which inflates mass and density) and no depression below the rim (which deflates them). A few light taps with the mallet during strike-off can help close any small surface voids the plate exposes.
Clean the Measure Before Weighing
Before taking the mass, clean all excess concrete from the exterior, the rim, and the base of the measure. Any concrete clinging to the outside adds mass that the calculation will wrongly attribute to the concrete inside, pushing the density too high. This is one of the most-cited field errors:
| Error | Effect on Mass | Effect on Density | Effect on Yield |
|---|---|---|---|
| Concrete left on the exterior | Too high | Too high | Too low |
| Heaped (not struck flush) | Too high | Too high | Too low |
| Depressed surface / under-fill | Too low | Too low | Too high |
Note the chain: anything that raises measured density lowers calculated yield, because yield = batch mass ÷ density. The exam routinely tests this inverse relationship by describing a sloppy strike-off and asking which way the yield error goes.
Weighing and Computing Density
Weigh the filled, cleaned measure on a balance accurate to 0.3% of the test load. Record this as Mc. Subtract the previously recorded empty-measure mass Mm to get the net concrete mass, then divide by the calibrated volume Vm:
D = (Mc − Mm) / Vm
Density is reported to the nearest 0.1 lb/ft³ (or 1 kg/m³).
Worked density example
Empty measure Mm = 9.10 lb; filled measure Mc = 45.60 lb; volume Vm = 0.250 ft³.
- Net mass = 45.60 − 9.10 = 36.50 lb
- Density = 36.50 / 0.250 = 146.0 lb/ft³
This sits squarely in the 140–150 lb/ft³ normal-weight range, so it passes the field sanity check. Lightweight concrete will read well below 140, and heavyweight (e.g., barite or magnetite) well above 150 — so 'reasonable' depends on the mix being tested, not a fixed number.
Putting the Procedure in Order
The correct sequence is worth memorizing because the exam may scramble it:
- Obtain and remix the C172 sample.
- Select and confirm the clean, calibrated, empty-weighed measure.
- Fill and consolidate (3 layers rodded / 2 layers vibrated) per slump.
- Tap the sides 10–15 times per rodded layer.
- Strike off flush with the flat plate.
- Clean the exterior of the measure.
- Weigh the filled measure (Mc).
- Compute D = (Mc − Mm) / Vm.
Skipping or reordering the clean-then-weigh steps is the classic trap — cleaning must come before weighing, and striking off must come before cleaning.
Reading the Balance Correctly
The balance must be accurate to 0.3% of the test load. For a filled small measure weighing roughly 45 lb, 0.3% is about 0.14 lb — so a scale that reads only to the nearest pound is not adequate for C138. On a field digital scale, allow the reading to stabilize before recording, keep the scale level, and protect it from wind that can make a hanging or platform reading drift. Re-zero (tare) the scale before each test sequence.
A subtle field point: weigh the same measure you calibrated and emptied. If two similar measures are in use, mixing them up pairs the wrong Mm and Vm with the filled mass — an error that can be several pounds and is easy to miss. Labeling measures and recording their empty mass and calibrated volume together on the data sheet prevents this.
Worked Check: Tracing an Error Through the Numbers
Suppose two technicians test the same concrete. Technician A follows procedure: Mm = 9.10 lb, Mc = 45.60 lb, Vm = 0.250 ft³ → D = 36.50/0.250 = 146.0 lb/ft³.
Technician B leaves a film of concrete on the exterior, adding 0.50 lb, so Mc reads 46.10 lb → D = (46.10 − 9.10)/0.250 = 37.00/0.250 = 148.0 lb/ft³.
The 2.0 lb/ft³ inflation looks small, but trace it forward: for a 3,886 lb batch, A computes yield = 3,886/146.0 = 26.62 ft³, while B computes 3,886/148.0 = 26.26 ft³ — a difference of 0.36 ft³ per batch, which across a day's deliveries becomes a meaningful volume and dollar discrepancy. The exam rewards technicians who understand that small density errors are not harmless because of this leverage on yield.
The same logic explains why heaping the measure is so damaging: a surface mounded 1/4 in. above the rim on a 6-in.-diameter measure adds roughly half a pound of concrete, inflating density just as the exterior film did. Strike-off and cleaning are not cosmetic — they are how the measure ends up holding exactly Vm of concrete and nothing more.
Which tool is used to strike off the surface of the concrete in the C138 measure?
A technician forgets to clean a film of concrete off the outside of the measure before weighing. What is the effect on the reported density and yield?
Empty measure mass is 8.95 lb, filled mass is 45.20 lb, and the calibrated volume is 0.250 ft³. What is the density to the nearest 0.1 lb/ft³?