Abrams cone

As measures of workability of the fresh mix, several different tests are proposed, but each of them gives a certain indication which is not comparable with the others. The slump test (Abrams cone) and the Vebe test give only single values by which the mix is characterized, in millimeters and seconds, respectively. Therefore, nothing more complicated than the Newton model is used and the test results do not characterize the mix without ambiguity. However, these and other similar one-point tests are used frequently, mainly for their simplicity and, in most cases, acceptable repeatability of results. The required workability is expressed as minimum slump, maximum Vebe time, or any other measure, and in many cases such a simple result is sufficient however, it is necessary to understand its limitations. In certain compositions the results are ambiguous or even impossible to execute very stiff mixes cannot be tested simply and compared with valid results the same concerns very fluid mixes with superplasticizers. 

In the case of fibre reinforced composites a special measure of workability was proposed by ACI 544.2R-89 called inverted slump cone. A standard Abrams cone is suspended upside down and filled with tested material without any compaction. The internal vibrator is activated and the bottom cover of the cone is removed, then the time of flow out of the material is recorded, which is usually between 10 and 30 seconds. This method is still in use, but is subject to some criticisms. Other methods of the flowability of the fresh mix with dispersed fibres are proposed by Laboratoire des Fonts et Chaussees in France (cf. Section 12.5). 

The main reason why high performance concrete is accepted and fully supported by contractors and investors is the excellent workability of the fresh mix obtained by the appropriate application of superplasticizers, which are either added to the water before mixing or, when the mix is transported over a longer distance, is added in two portions before mixing and after transportation, directly before casting. The facility to fill the moulds or forms by the fresh mix and its pumpability is ensured when sufficient slump of the Abrams cone and the coherence of the mix are maintained during all the time required for both transportation and casting. 

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