Workability
Workability is that property of
concrete which determines the case with which it can be placed in position and
compacted. Workability is normally measured by the ‘slump test’ as a guide. In
order to obtain concrete of maximum strength, good compaction is essential and
this can only be achieved if the concrete has adequate degree of workability in
relation to the method of compaction to be used. The workability if concrete
should be just sufficient to enable the concrete to be compacted fully by
whatever method is employed. Concrete that is to be placed in narrow forms
congested with reinforcement will require a much higher degree of workability
(i.e. fluidity) than that for unreinforced mass concrete. Concrete which is to
be compacted by mechanical vibration may be much drier than that which is to be
tamped by hand. This concrete needs about 20 percent less water and about 15
percent less cement. As such, a drier concrete which has been compacted by
vibrations gives more strength and density for the same quantity of materials.
The principle factors which effect the workability of concrete are :
Consistency
For RC works, concrete which
will flow sluggishly into the forms and around the reinforcement without any
segregation of coarse aggregate from the mortar, has to be used. The degree of
consistency, which shall depend on the nature of the work and weather the
concrete is to be vibrated or hand tamped, shall be determined by slump tests.
The quantities of materials may
be regulated by carrying out regular slump tests. More water will be needed for
dry conditions and less for wet and cold conditions. Structures in contact with
water should be made of drier cement.
Rapid hardening cement needs
about 4.5 liters more water per 50 kg of cement than ordinary cement.
Water cement ratio or water
content of a concrete mix
Water cement ratio of the water
in a mix (excluding water already absorbed by the aggregate) to the weight of
cement therein, and this is the most important factor governing the strength of
a concrete. The strength of a concrete depends mainly upon the amount of cement
and the amount of water in it. The (the quantity of water varies with the size
and shape of both the fine and coarse aggregate), method of compaction applied
(whether with vibrations or hand tamping), and the weather conditions.
Water content and workability
The workability of a concrete
increases as the water content of the mix is increased, water lubricates the
mixture. But increase in water content would cause a decrease in strength. Excess
of water weakens a concrete, produces shrinkage, cracks (shrinkage under
“contraction and expansion joints”). Water occupies space in concrete and as it
evaporates it leaves voids and cracks. The volume of water voids may be as much
as 10 percent of the total volume of concrete. An excess of 10 percent of water
may reduce the strength by about 15 percent and an excess of 30 percent of
water may reduce the strength by half. Generally speaking, lower the water
content the stronger the concrete but the quantity of water must be sufficient
to produce a workable mix required for the particular method of compaction to
be adopted.
Concrete made with low
water/cement ratio is un workable. If stiff or dry concrete is used
honey-combing will result decreasing density and strength. An unworkable
concrete results in incomplete compaction giving rise to air voids. Presence of
5 percent air voids will cause a 30 percent strength loss and 10 percent air
voids may cause as much as 50 percent strength loss. Therefore, there is an
optimum value of the water/cement ratio for every mix. The quantity of water
has to be restricted within certain minimum limits. Concrete should be just
plastic enough to be worked around the reinforcement rods.
Sometimes strength has to be
sacrificed by adding more water to obtain a higher degree of workability where
concrete has to be place in narrow and thin sections. The best mix is the one
which gives the maximum workability with the minimum amount of water. An
increase in water content must be accompanied by a proportionate increase of
cement if strength is to be maintained.
Grading of aggregate
Other things being equal the
workability of concrete is greater with aggregate of larger maximum sizes. For
dry mixes workability is generally greater with rather coarse aggregate
gradings but for wet mixes better results are often obtained with finer
gradings.
Shape of aggregate
A smooth and rounded aggregate
will produce a more workable concrete than sharp angular aggregate (crushed
rock or crushed gravel). A flaky aggregate produces the harshest or most
unworkable concrete. (aggregate producing more workable concrete need less
water and hence give higher strength).
Cement content
The higher the cement the
greater the workability and the less the effect of grading. As such, much
greater latitude in grading can be permitted with a rich mix (high cement
content) than with a lean mix (low cement content), but that is uneconomical. A
slight increase in the quantity of cement increases correspondingly the concrete
strength provided water/cement ratio is kept constant.
Segregation
Segregation is the separating
of the coarse aggregate from the rest of the mix or the separating of the
cement water paste from the aggregate. Segregation generally indicates poor aggregate
grading or mix design. Segregation may occur in mixes which are too wet or too
dry, and most frequently in under-sanded mixes.
Segregation can generally be
reduced by altering the water or sand content or by using a finer sand. Even
with a mix of satisfactory design, segregation may be caused by mishandling
during transport, faulty placing or over-compaction. Segregation leads to lack
of uniformity causing honey-combing which reduces the strength and durability
of the structure.
If segregation occurs the
larger particles of aggregate tend to move to the bottom and this causes
undesirable variation of strength through the thickness of the slab.
Bleeding
Bleeding is the appearance of a
watery scum (also called laitance) on the surface of a concrete after
compaction. It is an indication that there is too much water or deficiency of
fine material in the mix, or that too much tamping, floating or toweling has
been done. The result is a porous, dusty and weak surface. This scum should be
removed. Bleeding makes weak joints between successive lifts in structural
work. Bleeding can be reduced by using less water, a finer sand, or by adding a
finely ground inert material (stone dust).
The aggregate commonly used are
seldom found in a perfectly dry state in the field. Moreover, aggregates have
to be washed very often for removing impurities which further add to the
moisture content. The weather conditions, and this is especially so in the case
of sand. The aggregate when dry will absorb water from the concrete and when
wet at the surface the mixture will have excess of water. Therefore, while
computing the quantity of water due consideration must be given to the surface
conditions of the aggregate that would exist at the time of preparing the mix.
Small size of aggregate need
more water than big size and angular aggregate need more than rounded
aggregate. In other words, a concrete containing a finely graded aggregate will
require more water for a given workability than one containing an aggregate
with a coarse grading. Consequently, the more finely graded aggregate, or that
containing a larger proportion of fine aggregate (and similarly a concrete with
angular aggregate) will produce a weaker concrete.
Hydration of cement
When a water is added to
cement, the cement hydrates, calcium hydroxide or hydrated lime is liberated.
During the chemical reaction which takes place while cement is setting and
hardening an increase in temperature occurs and considerable quantity of heat
is evolved. Shrinkage occurs on subsequent cooling resulting in cracks.
Hydration of cement is incomplete without an adequate quantity of water. Less
water impedes complete setting of cement and decreases strength. The amount of
water required to hydrate cement is about 25 percent of the weight of the
cement. The amount of mixing water is rarely less than twice this quantity.
If water/cement ratio is less
than 0.4 to 0.5 complete hydration of cement will not occur. Roughly
water/cement ratio is 0.60 for a 1:2:4 mix, 0.5 for 1:1.5:3 mix, and 0.45 for
1:1:2 mix.
Quantity of water and aggregate
per 50 kg (one bag) – about 35 liters of cement for Hand Compaction.
Mix
|
Water
litres
|
Fine
Aggregate litres
|
Coarse
Aggregate
litres
|
|
1:1:2
|
21 to
27
|
35.0
|
70.0
|
For
vibrated concrete, the quantity of water can be reduced by about 20 percent
|
1:1.5:3
|
26 to
30
|
52.5
|
105.0
|
|
1:2:4
|
29 to
32
|
70.0
|
140.0
|
|
1:3:6
|
34 to
36
|
105.0
|
210.0
|
|
1:4:8
|
45 to
47
|
140.0
|
280.0
|
If sand is wet, increase its
quantity by 25 percent and reduce quantity of water by 20 percent.
Litres of water per 50 kg of
cement
|
Water/cement Ratio by Weight
|
Cubes Crushing Strength at 28
days in kg/sq.cm.
|
|
17.5
|
0.35
|
530
|
Mix too dry for hand
compaction.
|
20.0
|
0.40
|
470
|
|
22.5
|
0.45
|
420
|
|
25.0
|
0.50
|
370
|
|
27.5
|
0.55
|
320
|
Mix workable for hand
compaction.
|
30.0
|
0.60
|
280
|
|
32.5
|
0.65
|
250
|
|
35.0
|
0.70
|
220
|
|
37.5
|
0.75
|
200
|
|
40.0
|
0.80
|
180
|
