This test is performed to measure the stability or workability of fresh concrete, where the nominal maximum size of the aggregate does not exceed 38 mm, using a slump test apparatus.
IS: 1199 – 1959 – Method of sampling and analysis of concrete.
tools and equipment
- Dim cone (height = 30 cm, base diameter = 20 cm, top diameter = 10 cm)
- Tamping Rod (Length=60cm, Dia=16mm)
- The inner surface of the mold is thoroughly cleaned and freed from excess moisture before starting the test. And if the cone is completely dry, wet it with a damp cloth.
- The mold is then placed on a smooth, horizontally level hard and non-absorbent surface such as a rigid plate. It is kept firmly in place during filling by the operator by standing on two foot slits provided in the slump cone.
- The mold is filled with concrete in four layers, each about one-quarter the height of the mold, and each layer is pressed down in a similar manner with 25 strokes of the tamping rod with the pointed end.
- After tamping the top layer, the concrete is leveled with a trowel and cleaned of any mortar leaking between the mold and the base plate.
- The mold is then immediately removed from the concrete by lifting it slowly and carefully in a vertical direction.
The deceleration is measured immediately by determining the difference between the height of the mold and the highest point of the sample.
Report the fall down to the nearest 5 mm of the specimen during the test in millimeters.
notes to remember
- If the concrete contains aggregates of size larger than 38 mm, that concrete should first be sieved over a 38 mm sieve. Concrete passing through a 38 mm sieve is tested for workability using a deceleration taper mechanism of the dimensions mentioned above.
- If the deceleration test shows shearing of the solid mass or a complete collapse, the test results are invalid and the deceleration test is repeated. (see picture below)
- After the slump test is complete, slightly tap the outer perimeter of the slumped concrete to get an idea of the concrete’s cohesion. If it decreases further, it is a sign of good quality concrete that has the required cohesion. But if it collapses or wears away then the concrete lacks cohesion and is a sign of poor quality concrete.
- Use hand gloves and shoes during the test.
- The equipment should be thoroughly cleaned before and after testing.
- The equipment must remain free from vibration during the test.
- Petroleum jelly should be applied to the mold.