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Why is Workability Required in Fresh Concrete?

When a concrete mix is readily compactable, it is said to be workable mix. Workability defines ease in compaction and resistance against segregation. But expected workability for a specific case depends on available means of compaction, as an example, workability sufficient in case of mass concrete is found quite insufficient for placing in inaccessible, narrow or section with reinforcement congestion.

Thus workability should have a definition irrespective of reference about circumstances prevail for a particular construction. To define this physical property, it is essential to take consideration about mechanism of compaction.

Mechanism of compaction:


The compactive effort is delivered to concrete by whether ramming or vibration. Whatever the method, this process consists of removal of air trapped within concrete until mix has reached close configuration as much as possible for particular concrete mix. The purpose is to overcome two type of friction namely:



• Internal friction

• Friction between constituent particles of concrete

• Surface friction; friction between surface of reinforcement, mold or formwork and concrete itself.

Thus compacting effort should be enough to overcome both types of frictions, additionally some of work done has been utilized to vibrate mold, or to shock and, actually, to vibrate that portion of concrete mix has already been consolidated completely.

Workability:


Interestingly, the effort applied has some portion that is wasted and some portion is useful that are used to overcome surface friction and internal friction as mentioned earlier. Among these, only internal friction is inherent property of concrete mix; so the best definition of workability is amount of internal useful work that are required to achieve full compaction(defined by Glanville et al.)

According to ASTM C125 (Standard Terminology Relating to Concrete and Concrete Aggregate):

Property that determines effort required to use/place a particular quantity of fresh concrete mix least loss of homogeneity.

According to ACI 116R (Cement and Concrete Terminology):

The property of fresh concrete mix or mortar that determines ease, homogeneity or uniformity with which mixing, placing, compaction and finishing of concrete can be done. 



Consistency:


Consistency is another term that defines state of concrete in fresh condition. The ordinary meaning of this word is firmness of structure of a material or ease to flow. Consistency of concrete is often taken as degree of wetness; to some extent, wetness is a measure of workability as wet concrete is more workable than dry one.

According to ACI code:

Consistency is ability of fresh concrete mix or mortar to flow or in one word relative mobility of mix.

Consistency Vs workability:


Concrete having same consistency may not have same workability.

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