Definition of Workability
It
is property of freshly prepared concrete or mortar.
It
is easiness with which it can be mixed, placed, compacted and finished
In
simple terms, if you don’t find difficulty to lay concrete, or mix or finish
then workability is good
Definition In terms of internal energy
The
property of concrete which determines the amount of useful internal work
necessary to produce full compaction i.e. workability is the amount of energy
to overcome Friction while compacting. Sometimes plasticity and consistency are
used to denote workability of concrete
Thus,
concrete is said to be workable if it has
•
Lubrication
•
Fluidity
•
Movability
•
Stability
•
Flowability
•
mixiability
Factors affecting workability
1.
Water content: if water content increases then lubrication increases
and hence workability increases.
2.
Size of aggregate: The
concrete having larger size of aggregate is more workable than that of smaller
size because larger size of aggregate requires less water for lubrication than
smaller size as surface area of larger aggregate is less than that of smaller
aggregate.
3.
Shape of aggregate: The
round shape increases the workability because it gives minimum ratio of surface
area to volume thus requiring minimum cement paste to make good concrete
whereas angular, flaky, elongated aggregates reduce workability as they need
more cement paste to make workable concrete. Also, round aggregate has less
friction and thus is more workable.
4.
Surface texture of aggregate: The
smooth surface aggregates require less quantity of water for lubrication than
that of rough surface aggregate. So smooth surface aggregate increases
workability.
5.
Grading of aggregate: If
proper grading of aggregate is done then the amount of void is less and less
cement paste is required to fill the void. The excess cement paste is now
available for lubrication.
6.
Admixture:
1.
Plasticizer and super plasticizer improve
workability.
2.
Pozzolanic material also increase the
workability.
3.
Air entraining
agent create large number of minute air bubbles distributed throughout the mass
and acts as roller and increase workability.
7.
Mix
proportions: Aggregate/ cement ratio is the measure of how lean or
rich the concrete is. If aggregate/ cement ratio is higher, the concrete
becomes leaner. In lean concrete, less paste is available for the lubrication
of the aggregate, while in rich concrete with low a/c ratio, more paste is
available which makes the mix more cohesive and hence provides better
workability.
8.
Temperature: As
temperature increases workability of concrete decreases.
Measurement of workability
Ø Slump test
•
Slump is loss in height of concrete after placing concrete
•
As slum increases, workability increases
•
Suitable for concrete of medium to high workability
•
Generally used as field test
•
Its unit is millimeter (mm)
Ø Compaction factor test
•
It is ratio of partially compacted to fully compacted concrete
•
As compaction factor increases, workability increases
•
Suitable for concrete of low to medium workability concrete
•
More accurate than slum test
•
Generally performed in laboratories
•
It has no unit
Ø Vee-Bee test
•
Suitable for stiff concrete mixes having low and very low workability
•
Its unit is time (Vee-Bee seconds)
•
AS Vee-Bee seconds decreases, workability increases
Recommended workability values
|
Degree
of
workability |
slump
value
(mm) |
Compaciton
factor |
Vee-Bee
time
in seconds |
|
Very
low
|
0-25
|
0.75-0.8
|
20-10
|
|
low
|
25-50
|
0.8-0.85
|
10-5
|
|
medium
|
50-75
|
0.85-0.92
|
5-2
|
|
high
|
75-150
|
>0.92
|
2-0
|
How to improve workability of concrete?
ü use optimum water cement ratio
ü use of round
and smooth aggregate
ü use of lower
aggregate-cement ratio
ü proper grading
of mix
ü Use of larger
size of aggregate
ü Use of
pozzolanic materials
ü Use of air
entraining agent
ü Use of
plasticizer and super plasticizer
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