Workability of Concrete:-
The behavior of green or
fresh concrete from mixing up to compaction depends mainly on the
property called “Workability
of concrete”.
Workability of concrete is a term which consists of the following
four partial properties of concrete namely, Mixability, Transportability,
Mouldability and
Compactibility.
In general terms, workability represents the amount of work which is to be done to compact the concrete in a given mold. The desired workability for a particular mix depends upon the type of compaction adopted and the complicated nature of reinforcement used in reinforced concrete. A workable mix should not segregate. Following are the partial properties of workability :
1.) Mixability:
It
is the ability of the mix to produce a homogeneous green concrete from
the constituent materials of the batch, under the action of the mixing forces. A less mixable concrete mix requires more time of mixing to produce a homogeneous and uniform mix.
the constituent materials of the batch, under the action of the mixing forces. A less mixable concrete mix requires more time of mixing to produce a homogeneous and uniform mix.
2.) Transportability:
Transportability
is the capacity of the concrete mix to keep it homogeneous.
To keep the homogeneous concrete mix from segregating during a limited
time period of transportation of concrete, when forces due to handling
operations of limited nature act. Any segregation that is caused during the remaining operations that follow. In most of the countries, general recommendations for practice exist for transporting the concrete, which fact highlights the importance of this property.
operations of limited nature act. Any segregation that is caused during the remaining operations that follow. In most of the countries, general recommendations for practice exist for transporting the concrete, which fact highlights the importance of this property.
3.) Mouldability:
It
is the ability of the fresh concrete mix to fill completely the forms or moulds
without losing continuity or homogeneity under the available techniques of
placing the concrete at a particular job. This property is complex, since the behavior of concrete is to be considered under dynamic conditions.
placing the concrete at a particular job. This property is complex, since the behavior of concrete is to be considered under dynamic conditions.
4.) Compactibility:
Compactibility
is the ability of concrete mix to be compacted into a
dense, compact concrete, with minimum voids, under the existing means of compaction at the site. The best mix from the point of view of compactibility should close the voids to an extent of 99% of the original voids present, when the concrete was placed in the moulds.
dense, compact concrete, with minimum voids, under the existing means of compaction at the site. The best mix from the point of view of compactibility should close the voids to an extent of 99% of the original voids present, when the concrete was placed in the moulds.
Factors affecting workability:-
Workable concrete is the one which exhibits very little internal friction between particle and particle or which overcomes the frictional resistance offered by the formwork surface or reinforcement contained in the concrete with just the amount of compacting efforts forthcoming. The factors helping concrete to have more lubricating effect to reduce internal friction for helping easy compaction are given below:
Water content:-
Water content in a given volume of concrete, will have
significant influences on the workability. The higher
the water content per cubic meter of concrete, the higher will be
the fluidity of concrete, which is one of the important factors affecting
workability.
At the work site, supervisors who are not well versed
with the practice of making good concrete resort to adding more water for
increasing workability. This practice is often
resorted to because this is one of the easiest corrective measures that can
be taken at the
site.
It should be noted that from the desirability point of
view, increase of water content is the last recourse to be
taken for improving the workability even in the case of
uncontrolled concrete.
For controlled concrete one cannot arbitrarily
increase the water content. In case all other steps to
improve workability fail, only as last recourse the addition of more
water can be considered. More water can be added, provided a correspondingly
higher quantity of cement is also added to keep the water/cement ratio
constant, so that
the strength remains the same.
Mix proportions:-
Aggregate/ cement ratio is an important factor influencing workability.
The higher the aggregate/cement ratio, the leaner is the concrete. In lean
concrete, less
quantity of paste is available for providing lubrication, per unit surface area
of aggregate and
hence the mobility of aggregate is restrained. On the other hand, in case
of rich concrete
with lower aggregate/cement ratio, more paste is available to make the
mix cohesive and
fatty to give better workability.
Size of aggregate:-
The bigger the size of the aggregate, the less the
surface area and hence less amount of water is required for
wetting the surface and less matrix or paste is required for
lubricating the surface to reduce internal friction. For a given quantity of
water and paste,
bigger size of aggregates will give higher workability. The above of
course will be
true within certain limits.
Shape of aggregate:-
The shape of the aggregate influences the workability
in good measure. Angular, elongated or flaky
aggregate makes the concrete very harsh when compared to
rounded aggregates or cubical shaped aggregates.
Contribution to better workability
to rounded aggregate will come from the fact that for the given volume or
weight it will
have less surface area and less voids than angular or flaky aggregate. Not
only that, being
round in shape, the frictional resistance is also greatly reduced. This
explains the
reason why river sand and gravel provide greater workability to concrete
than crushed sand
and aggregate.
The importance of shape of the aggregate will be of
great significance in the case of present day high
strength and high performance concrete when we use very low w/c in the order of about
0.25. Shape of crushed sand as available today is unsuitable but the
modern crushers are designed to yield well shaped and
well graded aggregates.
Surface texture:-
The influence of surface texture on workability is
again due to the fact that the total surface area of rough
textured aggregate is more than the surface area of smooth rounded aggregate of
same volume. From the earlier discussions it can be inferred that
rough textured aggregate will show poor workability and smooth or glassy
textured aggregate
will give better workability. A reduction of inter particle frictional
resistance offered
by smooth aggregates also contributes to higher workability.
Grading of aggregates:-
This is one of the factors which will have maximum influence on workability. A well graded aggregate is the one which has least amount of voids in a given volume. Other factors being constant, when the total voids are less, excess paste is available to give better lubricating effect.
With excess amount of paste, the mixture becomes
cohesive and fatty which prevents segregation of particles. Aggregate particles
will slide past each other with the least amount of compacting efforts. The
better the grading,
the less is the void content and higher the workability. The above is true for
the given
amount of paste volume.
Use of admixtures:-
Of all the factors mentioned above, the most important
factor which affects the workability is the use of
admixtures. It is to be noted that initial slump of concrete mix or
what is called slump of reference mix should be about 2 – 3 cm to
enhance the slump
many fold at a minimum doze. Without initial slump of 2-3 cm, the
workability can be
increased to higher level but it requires higher dosage – hence uneconomical.
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