Workability of concrete is the ability of a fresh concrete mix to properly fill the desired formwork or mould with the desired operations such as pouring, pumping, spreading, tamping, vibrating without affecting the concrete quality. Furthermore, the desired workability in any particular case would depend on the means of compaction available, likewise, a workability suitable for mass concrete is not necessarily sufficient for thin, inaccessible, or heavily reinforced sections. For the above said reasons workability should be defined as a physical property of concrete alone without reference to the circumstances of a particular type of use.
Workability depends on water content, aggregate (shape and size distribution), cementitious content and age (degree of hydration) and can be modified by adding chemical additives such as superplasticizers. Increasing the water content or adding chemical additives increases the workability of the concrete. Excess water leads to increased bleeding or aggregate run-off (when cement and aggregate begin to separate), with the resulting concrete being of reduced quality.
Changes in the gradation of aggregates can also affect the workability of the concrete even though a wide variety of gradings can be used for different applications of construction. Undesirable grading may mean using a large aggregate that is too large for the size of the formwork, or using too few of the smaller aggregates to fill the gaps between the larger aggregates, or too little for the same reason or using too much sand, or using small quantity of water, or too much cement. Any combination of these and other factors can result in a mix that is too harsh, does not flow or spread smoothly, is difficult to get into formwork, and is difficult to finish.
Most of the time workability will be discussed as a property of fresh concrete but it is a vital and very important property as far as a finished surface or product is concerned because concrete must have a workability proper compaction with a reasonable amount of work.