Concrete Slump Test | Civil Engineering Practicals

Definition of Slump Test:

The concrete slump test measures the consistency of fresh concrete before it sets. It is performed to check the workability of freshly made concrete and therefore the ease with which concrete flows. It can also be used as an indicator of an improperly mixed batch. The test is popular due to the simplicity of apparatus used and simple procedure. The slump test is used to ensure uniformity for different loads of concrete under field conditions.

Point to be noted:

• The slump test is the most simple workability test for concrete, involves low cost and provides immediate results. Due to this fact, it has been widely used for workability tests since 1922. The slump is carried out as per procedures mentioned in ASTM C143 in the United States, IS:1199 – 1959 in India and EN 12350-2 in Europe.
• Generally, concrete slump value is used to find the workability, which indicates water-cement ratio, but there are various factors including properties of materials, mixing methods, dosage, admixtures etc. also affect the concrete slump value.

Types of equipment Required for Concrete Slump Test:

1. Mold for slump test.
2.  Nonporous base plate, 
3. Measuring scale, 
4. Tamping rod. 
5. The mold for the test is in the form of the frustum of a cone having height 30 cm, bottom diameter 20 cm and top diameter 10 cm. 
6. The tamping rod is of steel 16 mm diameter and 60cm long and rounded at one end.

A sampling of Materials for Slump Test:

• A concrete mix (M15 or other) by weight with suitable water/ cement ratio is prepaid in the laboratory similar to that explained in 5.9 and required for casting 6 cubes after conducting Slump test.

Procedure of Slump Test

1. Clean the internal surface of the mold and apply oil.
2. Place the mold on a smooth horizontal non-porous base plate.
3. Fill the mold with the prepared concrete mix in 4 approximately equal layers.
4. Tamp each layer with 25 strokes of the rounded end of the tamping rod in a uniform manner over the cross section of the mold. For the subsequent layers, the tamping should penetrate into the underlying layer.
5. Remove the excess concrete and level the surface with a trowel.
6. Clean away the mortar or water leaked out between the mold and the base plate.
7. Raise the mold from the concrete immediately and slowly in the vertical direction.
8. Measure the slump as the difference between the height of the mold and that of height point of the specimen being tested.

Factors which influence the concrete slump test:

1. Material properties like chemistry, fineness, particle size distribution, moisture content, and temperature of cementitious materials. Size, texture, combined grading, cleanliness and moisture content of the aggregates.
2. Chemical admixtures dosage, type, combination, interaction, sequence of addition and its effectiveness,
3. Air content of concrete,
4. Concrete batching, mixing and transporting methods and equipment,
5. The temperature of the concrete.
6. Sampling of concrete, slump-testing technique and the condition of test equipment,
7. The amount of free water in the concrete, and
8. Time since mixing of concrete at the time of testing.

NOTE: 

The above operation should be carried out at a place free from Vibrations or shock and within a period of 2 minutes after sampling.

Slump Value Observation:

The slump (Vertical settlement) measured shall be recorded in terms of millimeters of subsidence of the specimen during the test.

RESULT OF CONCRETE SLUMP TEST:

• Slump for the given sample= _____mm
• When the slump test is carried out, following are the shape of the concrete slump that can be observed:

• True Slump – True slump is the only slump that can be measured in the test. The measurement is taken from the top of the cone and the top of the concrete after the cone has been removed as shown in figure-1.
• Zero Slump – Zero slump is the indication of very low water-cement ratio, which results in dry mixes. These type of concrete is generally used for road construction.
• Collapsed Slump – This is an indication that 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 – The shear slump indicates that the result is incomplete, and concrete to be retested.