The relevancy of the slump test

The relevancy of the slump test

The slump test is one of the oldest and most frequently used tests to measure the consistency of fresh concrete. Consistency refers to the relative mobility or ability of freshly mixed concrete to flow. Common terminology used to describe the consistency of fresh concrete include stiff, plastic, normal, flowable, and fluid. Workability refers to the relative ease at which freshly mixed concrete can be placed, consolidated, and finished. Though frequently used inter changeably, the terms consistency and workability are independent concrete properties. This misconception is most likely based on the false presumption that as the concrete slump increases, so does workability. Whether or not increasing slump improves or worsens workability depends on several factors, including aggregate grading, cementitious materials content, and W/B ratio.

 By definition, slump is a measure of the relative stiffness, or consistency of fresh concrete. It is not a measure of workability, water content, or W/B ratio. Procedures for performing the slump test are described in ASTM C143.4 A very popular misconception within the industry is that a strong correlation exists between slump and water content. Slump is influenced by many factors in addition to water content. Even in concrete, where consistency is not produced with the aid of water-reducing admixtures, there is no reliable correlation between slump and water content. Other factors influencing slump, include, aggregate cleanliness and aggregate particle grading. For example, measures taken to improve aggregate grading will usually result in a reduction in water demand. If the same quantity of water were used to produce the concrete, the consequence of using aggregates having better grading uniformity would be an increase in measured slump. If, on the other hand, the water content was not adjusted, the increased slump might exceed the maximum prescribed limit when tested at the job site, and forming a basis for rejection, even though the W/B ratio remained unchanged.

 For the slump test to be relevant, the concrete must be of a plastic and cohesive consistency. Lean concretes often lack enough cohesion to prevent the slump test sample from shearing off to one side. The slump test is not suitable for measuring the consistency of very stiff or flowing and self-consolidating concretes. High-strength concrete is a cohesive material, and most modern high-strength concrete is placed at flowing or fluidized consistencies. High-strength concrete produced using well-graded aggregates usually do not exhibit segregation at measured slumps below 250 mm(10 in) or below. Measuring the diameter of spread of the slump sample rather than the vertical drop distance is a more relevant method for determining the consistency of flowing and self-consolidating concretes.

 The slump test has little relevancy with super plasticized flowing concretes, and it is not recommended that the slump test be used as an acceptance test. If the slump test is used for these types of concrete, caution should be exercised when interpreting the results. Emphasis should be placed on controlling the W/B ratio, not slump.