4.6 Invalid Slump Results, Reporting, and Integrated Habits
Key Takeaways
- If a portion of the concrete shears off or falls away, the slump test is disregarded and repeated on another portion of the sample.
- If repeated slump tests shear or collapse, the concrete may lack the plasticity or cohesiveness needed for C143.
- The slump value is reported to the required increment, commonly the nearest 1/4 in. or 5 mm.
- C143 should be started within the C172 sample-control window and completed without interruption once filling begins.
Knowing When a Slump Test Is Valid
The slump test does not end when the cone is lifted. The technician must look at the concrete shape and decide whether the result is valid. A normal slump settles downward and remains generally cohesive. If a portion of the concrete falls away or shears from one side, the test is disregarded and a new test is made on another portion of the sample. Measuring the remaining pile and reporting it as if it were normal is a common exam error.
If a second test on another portion also shears or collapses, the issue may be the concrete's lack of plasticity or cohesiveness rather than the technician's first attempt. C143 is not intended to force a number out of concrete that cannot produce a meaningful slump. Highly flowable concrete may require another workability method, and very stiff or noncohesive concrete may also be outside the useful range of C143.
Reporting requires accuracy without overprecision. Slump is measured immediately as the vertical difference to the displaced original center of the specimen and is commonly reported to the nearest 1/4 in. or 5 mm. Do not average a shear slump, estimate from the side of the pile, or report extra decimals. The report should include the units and connect the result to the sampled load.
| Condition after lift | Technician action | Reason |
|---|---|---|
| Cohesive vertical settlement | Measure and report slump | Valid slump shape |
| One side shears away | Disregard and retest another portion | Shape is not valid |
| Repeated shearing or collapse | Recognize C143 may not apply | Concrete lacks required behavior |
| Delayed measurement | Do not treat as normal | Shape changes with time |
| Twisted or bumped cone | Repeat if procedure disturbed specimen | Technician caused invalid result |
Timing links C143 back to C172. Slump testing must be started within 5 minutes after the final sample portion is obtained, and the C143 test itself must be completed without interruption within its own limit after filling starts. In a full field set, this means the slump cone cannot be an afterthought. The technician should have the base, mold, rod, scoop, and ruler ready before sampling.
Performance-exam habits are concrete and observable. Keep both feet on the foot pieces until the mold is ready to lift. Count rod strokes consistently. Maintain enough concrete above the top layer before strike-off. Lift in one smooth vertical motion. Measure at the correct point. Speak results clearly and do not argue a bad shape into a valid result.
Final field checklist:
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Start slump within the C172 fresh-property window.
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Complete the C143 procedure without interruption.
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Watch the specimen shape after lifting the cone.
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Repeat the test when shearing or falling away invalidates the result.
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Report the measured slump with the proper units and increment.
What should the technician do if a portion of the concrete shears off during the slump test?
To what increment is slump commonly reported in U.S. customary units?
Which timing statement correctly links C172 and C143?