To prepare the concrete mix and determine the workability of fresh concrete by slump test method.

Object: To prepare the concrete mix and determine the workability of fresh concrete by slump test method.

Apparatus :

• Slump cone,
  
• Scale for measurement,
  
• Temping rod (steel)
  

THEORY:

Concrete Slump Test is a measurement of concrete’s workability, or fluidity. It’s an indirect measurement of concrete consistency or stiffness. A slump test is a method used to determine the consistency of concrete. The consistency, or stiffness, indicates how much water has been used in the mix. The stiffness of the concrete mix should be matched to the requirements for the finished product quality. The test is popular due to the simplicity of the apparatus used and the simple procedure

Principle of Slump Test :

The slump test result is a measure of the behavior of a compacted inverted cone of concrete under the action of gravity. It measures the consistency or the wetness of concrete which then gives an idea about the workability condition of concrete mix.

Types Of Concrete Slump :

The slumped concrete takes various shapes, and according to the profile of slumped concrete, the slump is termed as;

1. Collapse Slump
   
2. Shear Slump
   
3. True Slump
   

• Collapse Slump
  

In a collapse slump the concrete collapses completely. A collapse slump will generally mean that the mix is too wet or that it is a high workability mix, for which slump test is not appropriate. It means the water-cement ratio is too high, i.e. concrete mix is too wet or it is a high workability mix, for which a slump test is not appropriate.

• Shear Slump
  

In a shear slump the top portion of the concrete shears off and slips sideways. OR.If one-half of the cone slides down an inclined plane, the slump is said to be a shear slump. The shear slump indicates that the result is incomplete, and concrete needs to be retested for valid results.

1. If a shear or collapse slump is achieved, a fresh sample should be taken and the test is repeated.
   
2. If the shear slump persists, as may the case with harsh mixes, this is an indication of lack of cohesion of the mix.
   

• True Slump
  

In a true slump the concrete simply subsides, keeping more or less to shape

1. This is the only slump which is used in various tests.
   
2. Mixes of stiff consistence have a Zero slump, so that in the rather dry range no variation can be detected between mixes of different workability.
   

However, in a lean mix with a tendency to harshness, a true slump can easily change to the shear slump type or even to collapse, and widely different values of slump can be obtained in different samples from the same mix; thus, the slump test is unreliable for lean mixes.

Procedure

• Do the batching of concrete ingredients properly according to the design.
  
• Mix the ingredients (fine aggregates, coarse ingredients & cement) properly.
  
• Pour the water in the mixed ingredients according to consistency, and then mix them properly to get a uniform paste.
  
• The mold for the concrete slump test is a frustum of a cone, 300 mm (12 in) of height. The base is 200 mm (8in) in diameter and it has a smaller opening at the top of 100 mm (4 in).
  
• The base is placed on a smooth surface and the container is filled with concrete in three layers, whose workability is to be tested.
  
• Each layer is temped 25 times with a standard 16 mm (5/8 in) diameter steel rod, rounded at the end.
  
• When the mold is completely filled with concrete, the top surface is struck off (leveled with mould top opening) by means of screening and rolling motion of the temping rod.
  
• The mould must be firmly held against its base during the entire operation so that it cannot move due to the pouring of concrete and this can be done by means of handles or footrests brazed to the mold.
  
• Immediately after filling is completed and the concrete is leveled, the cone is slowly and carefully lifted vertically, an unsupported concrete will now slump.
  
• The decrease in the height of the center of the slumped concrete is called a slump.
  
• The slump is measured by placing the cone just beside the slumped concrete and the temping rod is placed over the cone so that it should also come over the area of slumped concrete.
  
• The decrease in height of concrete to that of mold is noted with scale. (usually measured to the nearest 5 mm (1/4 in).
  

Result:

Slump of the concrete………………

The tested concrete fall in ……………degree of workability

Precautions in Slump Test

In order to reduce the influence on the slump of the variation in the surface friction, the inside of the mould and its base should be moistened at the beginning of every test, and prior to lifting of the mould the area immediately around the base of the cone should be cleaned from concrete which may have dropped accidentally.