Slump: What Does it Mean?

Slump: What Does it Mean?

The first question that must be asked is, "What is slump?" Simply defined, slump is a measure of the consistency of fresh concrete.

The slump test is a very simple test. The slump cone is a right circular cone that is 12 inches high. The base of the cone is 8 inches in diameter and the top of the cone is 4 inches in diameter. The cone is filled with fresh concrete in three layers of equal volume. Each layer is stroked 25 times with a rod that is inch in diameter. The end of the rod is bullet shaped. After the cone has been filled with concrete and the concrete has been cut off level with the top of the cone, the cone is raised vertically allowing the concrete to fall or slump. The distance that the concrete falls or slumps from the original height is the slump of the concrete. Slump is measured in inches and is reported to the nearest inch.

In the early days of concreting when concrete was composed of cement, aggregate and water, the coarse aggregates determined the water content and the water determined the slump. During this time a lower slump value meant a lower water content, which also meant a higher quality of concrete.

Today, concrete is not only a blend of the three primary ingredients. Today's typical concrete may also contain admixtures, fibers and polymers. Therefore, the coarse aggregate may not be the lone factor determining the water content of today's concrete. That means a high or low slump may not be a clear indication of the quality of the concrete. In other words, the slump cannot be used to directly determine the water content of a concrete mix.

Using modern technologies, a traditional concrete mix design with a natural 2-inch slump may actually have a higher water content than that of a modern concrete mix design with a 9-inch slump that contains a chemical superplasticizer. (A superplasticizer is a chemical that is added to a concrete in order to increase the slump without adding additional water.)

A mixture of stone, sand and water without cement will not flow like concrete, no matter how much water you add. The reason for this is that it is the cement that gives concrete its flowing properties. By chemically dispersing the cement grains a superplasticizer enables cement to become a more effective lubricant, thereby increasing the slump without adding additional water.

With the advent of chemical admixtures, slump can now be seen as either a water slump or as a plasticized slump. If the slump is determined only by the water content, it is said that the slump is a water slump. If the slump is determined by the water content and the effects of a chemical admixture, it is said to be a plasticized slump.

Almost everything that we put into concrete can affect the slump. As the air content of concrete is increased, the slump will also increase. Placing fibers into concrete will decrease the slump. All factors have to be taken into account when designing a concrete mix in order to produce concrete with a workable slump. The best slump from the standpoint of workability and consolidation is 4 to 5 inches. Slump values in the 1 to 2-inch range are not only hard to place, they are also hard to consolidate. On the other hand, slump values above 6 inches may be prone to segregate and produce excessive bleed water.

It should be pointed out that the slump test can only be used to determine the quality of concrete from batch to batch within a given mix design. It cannot be used to determine the quality of concrete from mix design to mix design. The slump does not determine the strength of the concrete. That determination is made by the water-to-cement ratio.

With the use of modern admixtures in the mix designs of today, we can set the slump of the concrete at any practical range we choose and still have good quality concrete.

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© 2008 L&M Construction Chemicals, Inc. | ConcreteNews January 2008.

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