Concrete can be accepted or rejected on the basis of slump. NRMCA estimates that 43% of concrete rejected at the jobsite is due to slump. So, how accurate is this test?
How much should two tests vary?
According to ASTM C143, when the same person is testing the same concrete, results from two “properly conducted” tests can vary as much as 1.13 in. for a concrete with a slump of 6.5 in. When two people perform the test, the range can be as high as 1.5 in.
Slump Test Precision and Bias in ASTM C143
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Is the concrete sampled properly?
Further variation can be introduced when sampling from the truck to the wheelbarrow to the slump cone. ASTM C172 requires that concrete be sampled from the middle portion of the discharge. When evaluating concrete for acceptance, ASTM C94 allows samples for slump and air to be taken after ¼ yd3 or ¼ m3 is discharged. The first portion of concrete can misrepresent the remaining portion if the concrete is not fully mixed, segregated, or the top of the drum and chute are not clean and damp.
What about improperly conducted tests?
The number and vigor of stokes for compaction (3 layers, 25 strokes per layer), the time to perform the test (less than 2.5 min), and the time to lift the cone (5+-/2 s) all affect test results. The test should be conducted on a flat, rigid, level, and moist surface.
Is slump even applicable?
ASTM C143 indicates that slump may not be applicable for concrete with slump less than ½ in. or greater than 9 in. If a portion of the concrete shears off from the sample, the test is considered not applicable.
How does Verifi help?
Verifi automates the determination of slump by using sophisticated algorithms to process hydraulic pressure and drum speed sensor data from the truck. See how.
These measurements are made on the full load in the drum to avoid sampling errors. Human error in performing the test is eliminated. Verifi typically measures slump 95% of the time within ASTM limits. Much of this difference is due to sampling and testing errors with the manual slump test.