What is slump? How is it tested? And what are its effects?

Slump is the workability, consistency, and the plasticity of concrete.  Workability is a measure of how easy or difficult it is to place, consolidate, and finish concrete. Consistency is the ability of freshly mixed concrete to flow. And Plasticity determines concrete’s ease of molding.  

The slump test is the most generally accepted method used to measure the consistency of concrete. The test equipment consists of a slump cone 12” high by 8” in diameter at the base, 4” in diameter at the top, and a 5/8” steel rod, 24” long with a hemispherically shaped tip.  The slump cone is filled in 3 equal layers, each layer being rodded 25 times. Following rodding, the last layer is struck off at the top, and the cone is then slowly raised vertically in 5 + - 2 seconds. As the concrete subsides
or settles to a new height, the empty slump cone is then inverted and gently placed next to the settled concrete and measured down to the nearest 1/4” from the original center. The higher slump value is indicative of a more fluid concrete.

The ready mixed concrete supplier designs the concrete mixture according to industry standards to provide the intended performance.  Addition of water in excess of the design mixing water or slumps higher than that of the design will affect concrete properties, such as reducing strength, increasing its susceptibility to cracking, and extending set times.

Job Site Water Addition

• Establish the maximum allowable slump and water content permitted.
• Estimate or determine the concrete slump from the first portion of concrete discharged from the truck.
• Add an amount of water such that the maximum slump or water cement ratio is not exceeded.
• Measure and record the amount of water added.
• Mix the concrete for 30 revolutions at mixing speed.

Do not add water if:

a. The maximum water-cement ratio has been reached.
b. The maximum slump is obtained, or
c. More than 1/4 cu. yd. has been discharged from the mixer.