4.6 Invalid Slump Results, Reporting, and Integrated Habits

Recognizing a Valid Slump

The slump test does not end when the cone is lifted — the technician must read the shape and decide whether the result is valid. A true slump settles evenly and stays generally cohesive. A shear slump occurs when one side slides down and off, leaving an asymmetric mass. A collapse slump occurs when the concrete falls apart completely, typical of a very wet or harsh mix.

If a portion of the concrete shears or falls away, the test is disregarded and a new test is made on another portion of the sample. Measuring the remaining pile and reporting it as a normal slump is a frequent exam error. If a second consecutive test also shears or collapses, the concrete probably lacks the plasticity and cohesiveness the method requires, and that condition is noted rather than forced into a number. Highly flowable concrete may need slump flow; very stiff concrete may be outside C143's useful range.

Distinguishing the three shapes is a frequent performance-exam checkpoint. In a true slump the mass settles fairly symmetrically and you measure to the displaced original center. In a shear slump roughly half the cone slips down and to one side, leaving a slanted top; the result is invalid even though a ruler could still reach the concrete. In a collapse slump the concrete subsides completely and spreads, typical of a high-water or low-cohesion mix. The exam wants the candidate to name the shape, not just a number, and to act on shear or collapse by retesting rather than reporting.

Reporting Without Overprecision

Reporting requires accuracy without false precision. Slump is measured immediately as the vertical difference to the displaced original center of the specimen and reported to the nearest 1/4 in. (5 mm). Do not average a shear slump, estimate from the side of the pile, or add decimals beyond the 1/4 in. increment. The field report includes units and ties the result to the sampled load along with the C1064 temperature, air content, and density from the same sample.

Timing, Coordination, and Performance Habits

Timing links C143 back to C172. Slump must be started within 5 minutes after obtaining the final portion of the composite sample, and the C143 test itself must be completed within 2.5 minutes without interruption once filling begins. In a full field set, the slump cone cannot be an afterthought; the base, mold, rod, scoop, and ruler are staged before sampling so temperature, slump, and air all happen inside their respective windows.

It helps to memorize the two clocks separately. The 5-minute clock starts when the final portion of the composite sample is obtained and governs when slump (and temperature) must begin. The 2.5-minute clock starts when filling the cone begins and governs how long the slump procedure itself may take. Mixing these up is a common written-exam error: candidates wrongly say the whole test must finish 5 minutes after sampling, or that there is no internal limit once filling starts.

Performance-exam habits are concrete and observable: keep both feet on the foot pieces until the mold is ready to lift, count rod strokes to 25 consistently, keep surplus concrete above the top layer before strike-off, lift 12 in. in 5 plus or minus 2 seconds straight up, measure at the displaced original center, and speak the result clearly without arguing a bad shape into a valid one.

Slump also coordinates with the other fresh tests sharing the same composite sample. After remixing the C172 sample, many technicians run slump and temperature first because both have short windows, then proceed to air content (C231 or C173) and density (C138), and finally mold strength specimens (C31). Because slump must start within 5 minutes of sampling and finish within 2.5 minutes, it is rarely the last test performed. Planning the order before concrete arrives keeps every method inside its own time limit and prevents a rushed, invalid slump caused by doing it after slower tests.

A last reporting reminder: never adjust a reported slump to make a load look acceptable, and never combine a discarded shear test with a later valid test by averaging. Report only the valid, single measurement to the nearest 1/4 in., note any retest, and let the data stand.

Final field checklist:

• Start slump within 5 minutes of the final C172 sample portion.

• Complete the C143 procedure without interruption within 2.5 minutes.

• Read the shape: true, shear, or collapse.

• Retest a fresh portion when shearing or collapse invalidates the result.

• Report the measured slump to the nearest 1/4 in. at the displaced original center.