Title: Time-Dependency of the Slump Test, or “What Slump is it Now?”

Author(s): Kenneth Hover

Publication: Web Session

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Date: 10/23/2022

Abstract:
Slump is specified, evaluated for compliance, and argued about as if it were an intrinsic property of concrete, with a desired fixed value for any given batch of fresh concrete. More correctly, slump is a time-dependent property of concrete, and its value at any moment and its rate of change depends on mixture ingredients, mixing, and temperature. Rate of slump loss may therefore be a more reliable characteristic of a given mixture in a given environment. This time-dependency is important when attempting to correlate slump with various fresh or hardened concrete behaviors, or even when using relatively uniform values of slump as an indicator of relatively uniform concrete composition. Multiple records of slump vs. time for the same mix-design, commercially batched, sampled from the truck chute, and tested over more than 10 years are used to present simple linear and non-linear ways to characterize time dependency to include “Slump Half-Life,” i.e., the time required for the slump to diminish to ½ of its current value. This is not to be confused with the “Double-Slump,” i.e., the amount of added water required to double the current slump. This concept may be more useful that the traditional rule-of-thumb that the addition of 1 gallon of water per CY will increase the slump by 1 inch. (adding 5 liters of water/m 3 will increase slump by 25 mm).