Table of Contents
Introduction
Slump test is the most commonly used method of measuring consistency or workability of fresh concrete which can be employed either in laboratory or at the site of work. It is not a suitable method for very wet or very dry concrete. It is used conveniently as a control test and gives an indication of the uniformity of the concrete from batch to batch.
Read my post on How To Calculate Workability Of A Concrete Mix for more details.
Objective
To determine the workability or consistency of a concrete mix of given proportion by slump test.
Apparatus
Iron pan to mix concrete, weighing machine, trowel slump, cone, scale, and tamping rod. The slump cone is a hollow frustum made of thin steel sheet with internal dimensions, as the top diameter 10 cm. The bottom diameter 20 cm, and height 30 cm. It stands on a plane’s non-porous surface. To facilitate vertical lifting from moulded concrete it is provided with a suitable guide attachment and suitable foot pieces and handles. The tamping rod is 16 mm dia. 60 cm long and is bullet-pointed at the lower end.
Theory
Unsupported concrete, when it is fresh, will flow to the sides and sinking in height will take place. This vertical settlement is called slump. Slump is a measu6, 0.7 and 0.8. For each mix take 10 Kg. C.A., 5 Kg., FA and 2.5 Kg. Cement.
Procedure
- Mix the dry constituents thoroughly to get a uniform colour and then add water.
- The internal surface of the mould is to be thoroughly cleaned and placed on a smooth, horizontal, rigid and non-absorbent surface.
- Place the mixed concrete in the cleaned slump cone in 4 layers each approximately 1/4 in height of the mould. Tamp each layer 25 times with a tamping rod.
- Remove the cone immediately, rising it slowly and carefully in the vertical direction.
- As soon as the concrete settlement comes to a stop, measure the subsistence of the concrete in cm or mm, which gives the slump.
Note: Slump test is adopted in the Laboratory or during the progress of the work in the field for determining the consistency of concrete where nominal max., size of aggregates does not exceed 40 mm. Any slump specimen which collapses or shears off laterally gives incorrect results and at this juncture, the test is repeated only true slump should be measured.
Observation Table
| SL No | Water Cement Ratio | Slump value in mm |
|---|---|---|
| 1 | 0.5 | |
| 2 | 0. 6 | |
| 3 | 0. 7 | |
| 4 | 0. 8 |
Precautions
- The strokes are to be uniformly applied through the entire area of the concrete section.
- The cone should be removed very slowly by lifting it upwards without disturbing the concrete.
- During filling the mould must be firmly pressed against the base.
- Vibrations from nearly machinery might also increase subsidence; hence test should be made beyond the range of ground vibrations.
Comment: This test is not a true guide to workability. For example, a harsh coarse mix cannot be said to have the same workability as one with a large portion of sand even though they have the same slump.
Recommended Slump Test Values
| SL No | Description of Works | Recommended Slump Value (mm) |
|---|---|---|
| 1 | Road Works | 25 – 50 |
| 2 | Ordinary Beam, Slab | 50 – 100 |
| 3 | Column of Thin Vertical Section, Retaining Wall etc. | 75 – 125 |
| 4 | Mass Concrete (Dam, Runway, Pavement etc.) | 25 – 50 |
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