What is a "slump test" for concrete?


This is a frequently asked question. The purpose and process of a slump test may be the best way to answer the question. The purpose of a slump test is to determine the consistency or texture of fresh concrete and to check its uniformity. Uniformity or consistency of concrete is important to a successful concrete project. The "slump" test is based on American Society for Testing and Materials C 143 "Standard Test Method for Slump of Portland Cement Concrete."

The visual part of a slump test should be part of this answer. There is a sturdy metal cone, the cone is open at the top and bottom. Cone is a good description for this equipment, because the top is more narrow in diameter than the bottom. The entire cone is twelve inches high. There are simple handles on the sides of the cone. The handles allow the tester to hold the cone down or lift it up with ease.

The hollow metal cone is filled with fresh concrete in a very precise set of steps. In general the cone is filled in three lifts and each lift is rodded 25 times to consolidate the lifts of concrete. All this is done while the cone is held firmly against a flat firm surface. The term rodding refers to a stabbing motion performed with a long metal rod, twenty-four inches in length and 5/8" in diameter. The use of a chunk of rebar rod is not considered proper.

The cone is full of compacted or rodded concrete and now begins the delicate part of the "slump" test. The person performing the slump test must lift the cone off the fresh concrete in a manner not to disturb the concrete. Anyone that has used a drinking cup to mold a mound of damp sand knows the cup or cone must be lifted straight up and avoid bumping the molded sand. The same care is required in lifting the slump cone up off the fresh concrete.

Slump is then observed by the person performing the test. The top of the fresh concrete "slumps" down towards the ground and the slump cone is placed next to the pile of concrete. The slump cone is used as pedestal or support for a straight edge to bridge over the pile of fresh concrete. A ruler is extended down from the straight edge to the original top and center of the concrete pile. The distance between the bottom of the straight edge and the top of the concrete is reported to the nearest one-quarter inch. The term "a four inch slump" means the distance between the bottom of the straight edge and the top of the original center of the pile of concrete is 4 inches.

Since the metal cone is twelve inches tall the slump could never be more than twelve inches. Typical slumps are three inches, four inches, to as much as nine inches for concrete with special additives to make it very plastic in "slump."

Thus, slump is used to give a person a reference to the consistency of the concrete, terms like wet, dry, runny, etc., are not precise enough, slump is a much better way to describe the consistency of the concrete.