What is slump in concrete

 

Slump is vertical settlement of the concrete after the mould has been withdrawn and the vertical difference between height of mould and the highest point of subsided concrete.

Following slump test apparatus is used to do slump test.

The apparatus for determining the slump includes following items 

1- Slump Cone ( Steel mould in form of a truncated cone).

2- Tamping Rod (Is used to tamp the slump cone and for Roding of concrete

3-Measuring Tape (measure vertical displacement of concrete after removal of cone)

4-Smooth Surface Sheet (Provides smooth surface to keep slump cone vertical).

5-Trowel. (Used to Fill the concrete in cone).

6-Hand Level (after removal of cone Hand level is placed horizontally over the Slump cone to measure vertical displacement of concrete.)

Testing Procedure

Place the slump cone on leveled sheet and fill the cone with concrete in 3" consecutive layers with Roding to compact concrete until it fills up to the top of cone. During filling of concrete cone should be in hold with placing of feet on lower pedestals and may be lifted above holding the hooks fixed in the middle of the cone. Now after removal of cone place the cone near settled concrete and place the hand level on the top of cone extending up to the top of concrete and then measure vertical displacement with measuring tape.

What should be the slump of concrete.

Although the slump test is not entirely satisfactory since it gives widely varying results and also does not give a true measure of workability but it is of value in the field as a control test and is useful in comparing the consistence of successive batches of concrete made with the same ingredients ,and is one of the simplest tests to carry out at site. Provided no change is made in the aggregate or it's grading ,slump tests will indicate whether correct water and cement contents are being maintained. For a given slump  and aggregate grading, water required for the unit volume of concrete is constant irrespective of the change of the cement content.

The amount of the slump depends not only on the amount of water in the mix but also on the nature of aggregates ; rounded stones give a greater slump than angular stones for the same mixture.  

Recommended values for slump in Millimeters.
S No	Types of Work	With Vibrations	Without Vibrations
1	Mass Concrete, Large sections, roads	10 to 25	50 to 75
2	Foundations ,Footings ,Sub-Structures, Walls and other heavy sections.	26 to 50	40 to 115
3	Thin Sections such as slabs ,columns, beams with congested reinforcements	40 to 80	100 to 175

 

How slump is recommended ?

This is very important to understand about the practical recommendations of slump and here is the details which includes Selections of materials and proceeding with job mix formula with number of hit and trial tests to obtained required strength of concrete. During each test of job mix slump test has been conducted and recorded in laboratory and when that slump is recommended on which designed strength of concrete has been obtained.  

Types of Slump

There are three types of Slumps .

True slump refers to common drop of the concrete mass evenly all around without collapse.

Shear slump indicates that the concrete lacks cohesion. It may undergo segregation and bleeding and thus is undesirable for the durability of concrete.

Collapse slump indicates that concrete mix is too wet and the mix is regarded as harsh and lean.

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