What is a Slump Test and Why Perform One

We have just completed pouring an 8-inch-thick pavement for a truck terminal. The specification called for 4000-psi concrete to be poured at a 4 inch slump. The owners inspector made a slump test and molded cylinders for ever 40 cubic yards of 1000 cubic yards poured. The strength tests averaged over 5000 psi with the lowest test over 4400 psi. Slump was never questioned. Now the owner wants an extension of the warranty or a discount because the slumps were reported at 5 to 5 inches. He claims the higher slump shows that the concrete wasnt of the quality specified. What can I tell him?

Solution:

Slump isnt described as a measure of the quality of concrete in any standard of the American Concrete Institute or in information from any source such as the American Concrete Institute or the National Ready Mixed Concrete Association. Slump is a control test that indicates the uniformity of the concrete mix from one batch to the next. It is also an indication of workability or placeability of the concrete. It is not a measure of strength, durability, or future performance.

In your situation it appears the concrete contained enough cement to produce higher strengths than specified even when proportioned for a higher slump than specified. In designing concrete mixes by either the American Concrete Institute or the Portland Cement Association method, strength and durability are governed by the water-cement ratio (pounds of water per pound of cement), not by slump. It is possible to meet the same standards for concrete through a wide range of slumps as long as cement is added in the same proportion as water is added to increase slump.

Overly-wet mixes should be avoided, of course, because they can lead to finishing problems and problem surfaces. Although 4 inches is a reasonable slump to specify, 5 to 5 inches is not indicative of problem concrete. Slumps vary considerably in paving work to meet the requirements of the equipment used for placing and finishing. The lowest slumps are used for slipform paving and the highest slumps for hand placing and finishing. The concrete mix is designed accordingly. The point you will have to make is that the water-cement ratio in the mix you used produced the required quality of concrete at the slump at which it was poured.

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