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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 concrete 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. A concrete having high consistency essentially more mobile, may not have right workability for particular job. The workabilities for a particular job say roof construction may differ depending on thickness of section; even for same section different workability may require when vibrator is applied instead of hand compaction. Thus instead of only mobility workability is based on assumption of

• Construction type
• Thickness of concrete section
• Compaction techniques
• Obstruction of reinforcement against mobility of concrete

In technical terms we can present many differences between workability and consistency, but by definition they are all qualitative by nature; reflection of personal point of view rather than technical. Same comments can be addressed for additional terms like

• Mobility
• Flowability
• Pumpability


Besides these, a term stability is also used in concrete technology which means cohesion of concrete mix i.e. its resistance against segregation.These have separate meaning but under a group of specific circumstances. They are seldom used from quantitative description of concrete mix.

Necessity of adequate workability:


So far we presented workability only as a property of concrete in fresh state; however, it is an important property when our concern is finished product as workability of concrete should be adequate to achieve maximum density by compaction with the amount of work that are expected to use for a given condition or with reasonable amount of effort.

Necessity of compaction is now clear from present study on relation between compaction effort and resulting strength. The compaction can be expressed as density ratio for convenience of discussion and strength as ratio of concrete strength.

Density ratio means ratio of exact density of a given concrete specimen to that density of identical mix which is fully compacted. Similarly ratio of strength means ration of the actual strength of concrete (compacted partially) to strength of concrete of identical mix which is fully compacted. The relation between density ratio and strength ratio takes in the form of following figure.

The voids present in hardened concrete significantly reduces strength; a volume of 5 percent voids can yield concrete having strength as low as 30 percent and a more lower amount of voids like 2 percent can produce concrete of more than 10 percent less strength. This, obviously, follow the Feret’s rule which provided an expression relating strength with sum of volumes of air, water 
in hydrated cement paste.


Rene Fe'ret formulated this general rule in 1896; which was like this form:

expression relating strength with sum of volumes of air, water in hydrated cement paste
Where

fc = strength of concrete
c = proportion of absolute volume of cement in mix
w = proportion of absolute volume of water in mix
a = proportion of absolute air of cement in mix
and k is a constant

Voids remain in concrete are in the form of either spaces left at the end of removal of excess water or entrapped air as bubbles. The volume of former depends mainly on water/cement ration of concrete mix; but to some extent on spaces arrived from accumulation of water underneath large aggregate particles or under reinforcement.

The air bubbles discussed earlier can be said accidental air. The voids of air originated actually within loose granular particles, in this regard grading of finer particles of the mix dominates, which can be more easily driven away from wetter mix than a drier one.

Thus it is evident that for desired compaction technique, there may have a particular water content at which total volume of water space and air bubbles will be reach minimum value. At this water content, concrete with maximum density ratio can be obtained. This optimum value of water content for a mix many vary significantly depending on methods of compaction.

For concrete engineer; extensive knowledge about workability is essential for concrete mix design. A designer should specify workability as design parameter in mix design having full understanding about

• Type of work
• Distance to be haul
Slump loss
• Placing techniques
• Many other considerations involved in this process.

Assumption about proper workability having proper understanding based on experiences will result concreting operation durable and economical.

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