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Workability

 

The term workability is used to describe the ease or difficulty with which the concrete is handled, transported and placed between the forms with minimum loss of homogeneity. However this gives a very loose description of this vital property of concrete which also depends on the means of compaction available. For instance, the workability suitable for mass concrete is not necessarily sufficient for thin, inaccessible or heavily reinforced sections. The compaction is achieved either by ramming or by vibrating.

The workability, as a physical property of concrete alone irrespective of a particular type of construction, can be defined as the amount of useful internal work necessary to produce full compaction.

If the concrete mixture is too wet, the coarse aggregates settle at the bottom of concrete mass and the resulting concrete becomes of non-uniform composition. On the other hand, if the concrete mixture is too dry, it will be difficult to handle and place it in position. Both these conflicting conditions should be correlated by proportioning carefully various components of concrete mixture. The important facts in connection with workability are as follows:

(1) If more water is added to attain the required degree of workmanship, it results into concrete of low strength and poor durability.

(2) If the strength of concrete is not to be affected, the degree of workability can be obtained:

    (i) by slightly changing the proportions of fine and coarse aggregates, in case the concrete mixture is too wet; and

    (ii) by adding a small quantity of water cement paste in the proportion of original mix, in case the concrete mixture is too dry.


(3) A concrete mixture for one work may prove to be too stiff or too wet for another work. For instance, the stiff concrete mixture will be required in case of vibrated concrete work while wet concrete mixture will be required for thin sections containing reinforcing bars.

(4) The workability of concrete is affected mainly by water content, water-cement ratio and aggregate-cement rati

The cone is filled with about one-fourth portion and then rammed with a rod which is provided with bullet nose at the lower end. The diameter of the rod which is provided with bullet nose at the lower end. The diameter of the rod is 16 mm and its length is 60 mm. The strokes to be given for ramming vary from 20 to 30. the remaining portion of the cone is filled in with similar layers and then the top of concrete surface is truck off so that the cone is completely full of concrete. The cone is the gradually raised vertically and removed. The concrete is allowed to subside and then the height of concrete is measured. The slump of concrete is obtained by deducting height of concrete after subsidence fro 30 cm.

Following are the advantages of slump test:

(1) It grants the facility to easily detect the difference in water content of successive batches of concrete of the same identical mix.

(2) The apparatus is cheap, portable and convenient to be used at site.

Following are the limitations of slump test:

(1) As such, there is no direct relationship between the workability and the value of slump.

(2) It is not suitable for a concrete in which maximum size of the aggregate exceeds 40 mm.

(3) There are chances of many shapes of slump to occur and it is difficult to decide which is the correct value.

(4) The slump occurs only in case of plastic mixes. It does not occur in case of dry mixes.

 
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