Workability of Concrete
Workability of Concrete
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 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:
(i) If more water is added to attain the required degree of workmanship, it results into concrete of low strength and poor durability.
(ii) If the strength of concrete is not to be affected, the degree of workability can be obtained:
(a) by slightly changing the proportions of fine and coarse aggregates, in case the concrete mixture is too wet
(b) by adding a small quantity of water cement paste in the proportion of original mix, in case the concrete mixture is too dry.
(iii) 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.
(iv) ) The workability of concrete is affected mainly by water content, water-cement ratio and aggregate –cement ratio.
(v) The workability of concrete is also affected by the grading, shape, text and maximum size of the coarse aggregates to be used in the mix.
In order to measure the workability of concrete mixture, the various tests developed. The tests such as flow test and compacting test are used in great extent laboratory. The slump test, which is commonly used test to determine the workablility of concrete in the field.
It should however be remembered that numerous attempts have been made correlate workability with some easily determinable physical measurement. But none of these tests is fully satisfactory although they may provide useful information within a range of variation in workability. At the same time, the slump test not measure the workability of concrete. It is simply useful in detecting variation in the uniformity of a mix of given nominal proportions.
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