To get the durable concrete, the ingredients of concrete should be mixed properly and in required proportion. Minor fluctuation in mixing of concrete ingredients or in concrete mix design can leads to the poor workability of concrete. To check whether the concrete is in workable condition or not, concrete experts carry out concrete Slump Test.
The concrete slump test is a measure of the workability of fresh concrete. More specifically, this test is performed to measure the consistency of the fresh concrete.
Concrete slump test can either be carried out in the lab or on site during the progress of work. Concrete slump test is only carried out where the nominal maximum size of aggregate does not exceed 38 mm using slump test apparatus.
To get the best concrete, proportion of concrete ingredients should be decided first by trial and error method which is also known as ‘concrete mix design’. Though the weight (quantity) or proportion of cement, sand, course aggregates, water and admixture is already specified in approved mix design report, occasionally the proportioning of concrete ingredients is compromised.
Why Slump Test?
Due to the Faulty batching plant, the proportions or the weight of cement, aggregates, water are not properly maintained as specified in the concrete mix design report. Sometimes worker mix the extra water at site to get the flexibility in on going concrete pouring and it makes the concrete poor. Hence poor knowledge of workers may lead to the poor workability of concrete. In both the case, the improper batching of concrete results into the poor workability of concrete.
To get the durable concrete during the whole construction work, workability of concrete is being measured at regular interval with the help of concrete slump test.
Here we discuss the apparatuses of slump test and its procedure.
Slump Test Apparatuses
Following is the brief introduction of slump test apparatuses which are used to perform the concrete slump test in laboratory or at the construction site.
01. Slump Test Cone (Mould)
The slump test Cone or mould for the test specimen shall be in the form of the frustum of a cone. The slump cone has the following internal dimensions:
|Slump Test Cone (Mould)
The slump test cone (mould) shall be made of metal (brass or aluminum shall not be used) of at least 1.6 mm (or 16 BG) thickness. The top and bottom of the mould shall be open and at right angles to the axis of the cone.
The slump test cone shall have a smooth internal surface. It shall be provided with suitable foot pieces and handles to facilitate lifting it from the moulded concrete test specimen in a vertical direction as required by the test. A mould provided with a suitable guide attachment may be used.
02. Tamping rod
The tamping rod shall be of steel or other suitable material, 16 mm in diameter, 0.6 m long and rounded at one end. The tamping rod is graduated in mm for measuring the slump.
03. Scale for measurement
A good quality of scale is used to measure the difference of the slump.
Two or more mechanisms of slump test apparatuses are available in the market. Don’t get confused with type of slump test apparatuses, the main idea is to observe the failure of slump or measure the workability of concrete in mm. You can measure the slump of concrete with measure tape or use tamping rod in place of measure tape.
Slump Test Procedure
The internal surface of the mould shall be thoroughly cleaned and freed from superfluous moisture and any set concrete before testing.
The mould shall be placed on a smooth, horizontal, rigid and non-absorbent surface. The mould should be firmly held in place while it is being filled.
The mould shall be filled in four layers; each layer shall be approximately one-quarter of the height of the mould. Each layer shall be tamped with 25 strokes of the rounded end of the tamping rod. The strokes shall be distributed in a uniform manner over the cross-section of the mould.
The bottom layer shall be tamped throughout its depth. After the top layer has been rodded, the concrete shall be struck off level with a trowel or the tamping rod, so that the mould is exactly filled.
If mortar may have leaked out between the mould and the base plate, it shall be cleaned away. Then, the mould shall be removed from the concrete immediately by raising it carefully and slowly in a vertical direction. This allows the concrete to drop and the slump shall be measured immediately by tamping rod shall be placed over the cone to determine the difference between the height of the mould and that of the highest point of the specimen being tested. The decrease in height of slump is measured with a scale. The above test shall be carried out at a place free from vibration or shock, and within a period of two minutes after sampling.
The concrete slump measured shall be recorded in terms of milli-metres. Any concrete slump specimen, which collapses laterally and gives incorrect result; and if this happens, the slump test shall be repeated with another sample. If, in the repeat test, the specimen should shear, the slump shall be measured and the measurement of that sheared specimen shall be recorded.
Types of Slump
Full Collapse slump or full failure of slump:
In Full collapse, the concrete collapse completely. It indicates the higher amount of water is mixed to make the concrete.
01. Shear Slump or Shear Failure of Slump
In shear failure, the top portion of concrete slump slips away from side. It indicates fall results and it is necessary to test the concrete again.
02. True Slump
In true slump, the slump moderately changes its shape. It indicates that the proportion of water cement is as specified in concrete mix design.
03. Zero Slump
In zero slump, the fresh concrete does not change its shape at the time of concrete testing. It indicates that the small amount of water is used to make the concrete.
|Degree of Workability
|Use to construct / Placing Conditions
|0 – 25 mm
|Very low Workability
|Road, PCC, building RCC foundation in water logged area, blinding concrete, shallow sections, pavements using pavers.
|10 – 40 mm
|Foundations with light reinforcement, Mass concrete, lightly reinforced sections in slabs, beams, walls, columns, floors, Hand placed pavements, canal lining, strip footings
|50 – 90 mm
|Normal reinforced concrete placed with vibration
|100 mm -120 mm
|Where reinforcing has tight spacing, and/or the concrete has to flow a great distance, heavily reinforced sections in slabs, beams, walls, columns, slipform work, pumped concrete
|120 mm – 150 mm
|Very High Workability
|Trench fill, in-situ piling, tremie concrete
In conclusion, depending upon the aforementioned working condition, the degree of workability or slum value of concrete should be maintained to do the good quality concrete work.