The slump test is the most widely used test for concrete workability worldwide. The slump test, however, is not applicable to all concrete. ASTM C143, which is the standard for slump, defines three conditions where the slump test should not be used.
Non-plastic concrete: When slump is less than ½ inch, concrete may not be adequately plastic for the slump test. The slump test is not effective in distinguishing these concrete mixtures: two concretes with a zero slump can have drastically different workability, especially in their response to vibration.
Non-cohesive concrete: When slump is greater than 9 inches, concrete may not be adequately cohesive for the slump test. Non-cohesive concrete is highly susceptible to segregation and should be redesigned, such as by adding a viscosity-modifying admixture or adjusting aggregate grading. Not all concrete with slump greater than 9 inches is non-cohesive. For example, self-consolidating concrete can flow under its own mass with adequate cohesion to resist segregation.
Shear slump concrete: If during the slump test, a portion of the concrete shears from the rest of the concrete, the slump cannot be evaluated. It is difficult to find the true displaced center of the slump specimen. ASTM C143 advises that two consecutive shear slumps indicate the concrete should not be evaluated with the slump test.
It’s important to ensure that manual reading is of a valid slump. If the manual slump test indicates the concrete is non-plastic, non-cohesive, or results in a shear slump, the manual slump test results should not be used. A measurement tool like Verifi, however, can measure, manage, and record slumps from 0 to 10 inches in the truck mixing drum and can distinguish between workability levels even if the slump test cannot.
Submitted by Eric Koehler, Vice President of Verifi. Verifi helps concrete producers control slump and provide documentation to inspectors, engineers, and owners. Learn more at VerifiConcrete.com.