Workability - Factors affecting workability

 Workability  : 

It is the ease and homogeneity with which the concrete can be mixed, transported, placed, compacted and finished easily at site without segregation or bleeding.

Workability: Factors affecting

the concrete is said to be workable when it has a least value of internal friction between intermediate particles or between the particles and the surrounding form work surface and the reinforcement contained in the concrete with a suitable amount of compacting effort. There are various interacting factors which help concrete to reduced internal friction and providing the lubricating effort, which are as follows:

          1. Water content : Water is the easist way of increasing the workability of concrete as when the water content increases in the concrete mix, more lubricating effect is provided by it to the surrounding concrete. But it should be increased within limits so that the segregation and bleeding doesn't occur and also more water content result in more serious problems of shrinkage and  cheep of hardened concrete. Water is added to concrete keeping in view of target design strength  of concrete and the water to cement ratio of  resulting concrete mix. Although it is easiest way for increasing workability, it should be adopted only when  all others steps to improve workability fails.

          2. Mix Proportions : As the aggregate constiutes 70-75% of total volume of concrete, so the aggregates/cement ratio is an important factors which influences the workability. The lower aggregates/cement ratio, the richer is the concrete and vice-versa for lean concrete. The more amount of cement paste per unit surface area of aggregates, the more will be lubricating effect so the rich concrete, which makes the mix cohesive, gives the better workability than the lean concrete.

          3. Aggregates Properties : Various aggregates properties like size, shape, surface texture,  grading or aggregates influence greatly the workability of concrete.

       i.            Size of aggregates : As the size of aggregates increases the  workability also increases due to the  lesser surface area provided by  the bigger size aggregates which required lesser amount of water/paste  for melting of the surface or for lubricating the surface to reduced internal friction. So, bigger size aggregates gives more workability for a given amount of water and paste.

     ii.            Shape of aggregates : Round and smooth shape aggregates increases the workability of concrete when used in place of angular, elongated or flaky aggregates which  results in the low workability.

  iii.            Surface texture : This is the same as the shape of aggregates as the smooth surface aggregates gives better workability than rough aggregates.

          4. Grading of aggregates: This factor greatly influence the workability of plastic aggregates.

There are basically three type of aggregates

a)    Well graded aggregates

b)    Uniformly graded aggregates

c)     Gap graded aggregates.

          The well graded aggregate constitutes the aggregate of different sizes while uniformly grades constitutes only a particular size of aggregate and the Gap Grades has a deficiency of particular size of aggregates. So, That the well graded aggregates provides better workability than uniformly and gap graded aggregates as they contain almost all the size of aggregate which reduces the voids in the aggregates and results in greater compaction, hence increases the workability.

          5. Admixtures : Concrete inn more workable when the admixture are used than the concrete in which admixtures are not used. The admixtures such as air-entraining agents (e.g. viusol resin) produces the air bubbles which acts as ball-bearings and reduces the internal friction between the particles and increases the workability.

          6. Environmental Conditions : Environmental conditions greatly affects the workability of concrete as on a hot climate the water required in the concrete is more to produce the same water to cement ratio in the concrete mix. As increases in water content increases the workability. Hence, increase  in temperature also increases the workability.

          7. Time : Moisture is lost due to evaporation, so the workability of fresh concrete loses as the time  increase.