The word ―workability or workable concrete signifies much wider and deeper meaning
than the other terminology ―consistency often used loosely for workability. Consistency
is a general term to indicate the degree of fluidity or the degree of mobility.
The factors helping concrete to have more lubricating effect to reduce internal friction
for helping easy compaction are :
(a) Water Content (b) Mix Proportions (c) Size of Aggregates (d) Shape of Aggregates
(e) Surface Texture of Aggregate (f) Grading of Aggregate (g) Use of Admixtures.
Slump test is the most commonly used method of measuring consistency of concrete which can be employed either in laboratory or at site of work. It is not a suitable method for very wet or very dry concrete. It does not measure all factors contributing to workability, nor it is always representative of the placeability of the concrete.
It indicates the characteristic of concrete in addition to the slump value. If the concrete slumps evenly it is called true slump. If one half of the cone slides down, it is called shear slump. In case of a shear slump, the slump value is measured as the difference in height between the height of the mould and the average value of the subsidence.
Fig. 1: Schematic Slump Cone Apparatus
The Slump Cone apparatus for conducting the slump test essentially consists of a metallic mould in the form of a frustum of a cone having the internal dimensions as : Bottom diameter : 20 cm, Top diameter : 10 cm, Height : 30 cm and the thickness of the metallic sheet for the mould should not be thinner than 1.6 mm.
Fig. 2: Slump Cone Apparatus
The Slump Cone apparatus along with Tamper (16 mm in diameter and
600 mm length. The tamping end of rod shall be rounded to a hemispherical
tip), Ruler, etc.
IS 1199:1959 Methods of Sampling and Analysis of Concrete. Reaffirmed- Dec 2013.
|Workability||Compaction Factor||Slump (mm)|
|Very Low||0.78||0 - 25|
|Low||0.85||25 - 50|
|Medium||0.92||50 - 100|
|High||0.95||100 - 175|
Table 1 :Relation between Workability and Slump
Fig. 3: Type of Slump
The slump measured shall be recorded in terms of millimetres of subsidence of the specimen during the test. Any slump specimen which collapses or shears off laterally gives incorrect result and if this occurs the test shall be repeated with another sample. If, in the repeat test also, the specimen should shear, the slump shall be measured and the fact that the specimen sheared, shall be recorded.