Design mix concrete – Economy & Environmental issues
Design mix concrete – Economy & Environmental issues
(Nirmal Goel, Technical Examiner, Central Vigilance Commission, N. Delhi)
1. Concrete is most commonly used material in civil construction work all over
the country. There is hardly any major original civil construction work where
structural concrete is not used.
2. Nowadays concrete is produced in batch mixing plants located either at site
of construction or away from the site in a location from where concrete is carried
in transit mixers to the site. The later one is commonly called Ready Mix
Concrete (RMC).
3. The proportion of various ingredients of concrete made in batch mixing
plants mentioned above is usually determined in laboratory. This process is
called designing (proportioning) of concrete mix and such a concrete is called
design mix concrete. The designing process is a trial and error method in which
right proportion of ingredients is sought to be determined so as to achieve
targeted mean strength which is kept somewhat higher than the characteristic
compressive strength of the concrete. Besides achieving the targeted strength,
the workability and durability requirements are also required to be ensured while
designing the concrete mix. All this has to be done keeping in mind the objective
of achieving overall economy by reducing the content of costliest material in the
concrete, i.e. the cement.
4. The designing process in most of the major projects is usually carried out
through reputed laboratories. IS 10262:2009 is the relevant Indian standard
stipulating guidelines for concrete mix proportioning.
5. Some important economy and environmental issues pertaining to design
mix concrete are discussed hereunder a) Use of fly ash: Fly ash is a waste product in thermal power generation.
Besides occupying a large space in coal based thermal power plants, fly ash
pollutes the air. Extremely fine particles of fly ash are a source of
respiratory disease as these particles settle on the human lungs. Disposal
of fly ash is a cause of concern from environmental consideration.
Fortunately this harmful material is a very useful material in civil
construction since it has got very good pozzolanic properties and up to 35%
of the cement content in concrete can be substituted with fly ash without
sacrificing strength and achieving durability and economy in the
construction. The quality parameters of flyash for use in concrete are laid
down in IS 3812(part 1). There should be uniform blending of flyash with
ordinary Portland cement. In all civil construction in the vicinity of thermal
power plants, the fly ash should be used as environmental friendly measure
and for economizing construction cost. While selecting concrete items at
the time of preparing estimates for construction works, provision should be
kept for use of fly ash along with ordinary Portland cement. The tender
documents should not discourage use of fly ash in concrete as the same
approach is not backed by any scientific reasoning. The use of fly ash in
concrete should be promoted not only from environmental consideration
but also for economizing construction cost.
b) Use of PPC: IS Code permits use of flyash based Portland Pozzolana
Cement (PPC) conforming to IS1489 (part 1) in concrete. The PPC many a
times is available at lesser cost than OPC. Besides it is more environmental
friendly compared to OPC since it utilizes the fly ash in its manufacturing.
Use of PPC in concrete helps in environmental conservation and for
economizing construction cost without sacrificing strength and achieving
durability and economy in the construction.
c) Specifying high quantity of cement in tender documents: Quantity of
cement in design mix concrete depends upon several factors, chiefly on
workability requirement. Workability requirement (slump values) should
not, therefore, be prescribed unnecessarily high but as given in table under
Para 7.1 of IS 456-2000. Further, higher workability requirement should be
met by use of suitable chemical admixtures (super plasticizers / water
reducing admixtures) to reduce water requirement in the concrete. The
design process takes care of various factors affecting strength, durability
and workability of concrete.
(Nirmal Goel, Technical Examiner, Central Vigilance Commission, N. Delhi)
1. Concrete is most commonly used material in civil construction work all over
the country. There is hardly any major original civil construction work where
structural concrete is not used.
2. Nowadays concrete is produced in batch mixing plants located either at site
of construction or away from the site in a location from where concrete is carried
in transit mixers to the site. The later one is commonly called Ready Mix
Concrete (RMC).
3. The proportion of various ingredients of concrete made in batch mixing
plants mentioned above is usually determined in laboratory. This process is
called designing (proportioning) of concrete mix and such a concrete is called
design mix concrete. The designing process is a trial and error method in which
right proportion of ingredients is sought to be determined so as to achieve
targeted mean strength which is kept somewhat higher than the characteristic
compressive strength of the concrete. Besides achieving the targeted strength,
the workability and durability requirements are also required to be ensured while
designing the concrete mix. All this has to be done keeping in mind the objective
of achieving overall economy by reducing the content of costliest material in the
concrete, i.e. the cement.
4. The designing process in most of the major projects is usually carried out
through reputed laboratories. IS 10262:2009 is the relevant Indian standard
stipulating guidelines for concrete mix proportioning.
5. Some important economy and environmental issues pertaining to design
mix concrete are discussed hereunder a) Use of fly ash: Fly ash is a waste product in thermal power generation.
Besides occupying a large space in coal based thermal power plants, fly ash
pollutes the air. Extremely fine particles of fly ash are a source of
respiratory disease as these particles settle on the human lungs. Disposal
of fly ash is a cause of concern from environmental consideration.
Fortunately this harmful material is a very useful material in civil
construction since it has got very good pozzolanic properties and up to 35%
of the cement content in concrete can be substituted with fly ash without
sacrificing strength and achieving durability and economy in the
construction. The quality parameters of flyash for use in concrete are laid
down in IS 3812(part 1). There should be uniform blending of flyash with
ordinary Portland cement. In all civil construction in the vicinity of thermal
power plants, the fly ash should be used as environmental friendly measure
and for economizing construction cost. While selecting concrete items at
the time of preparing estimates for construction works, provision should be
kept for use of fly ash along with ordinary Portland cement. The tender
documents should not discourage use of fly ash in concrete as the same
approach is not backed by any scientific reasoning. The use of fly ash in
concrete should be promoted not only from environmental consideration
but also for economizing construction cost.
b) Use of PPC: IS Code permits use of flyash based Portland Pozzolana
Cement (PPC) conforming to IS1489 (part 1) in concrete. The PPC many a
times is available at lesser cost than OPC. Besides it is more environmental
friendly compared to OPC since it utilizes the fly ash in its manufacturing.
Use of PPC in concrete helps in environmental conservation and for
economizing construction cost without sacrificing strength and achieving
durability and economy in the construction.
c) Specifying high quantity of cement in tender documents: Quantity of
cement in design mix concrete depends upon several factors, chiefly on
workability requirement. Workability requirement (slump values) should
not, therefore, be prescribed unnecessarily high but as given in table under
Para 7.1 of IS 456-2000. Further, higher workability requirement should be
met by use of suitable chemical admixtures (super plasticizers / water
reducing admixtures) to reduce water requirement in the concrete. The
design process takes care of various factors affecting strength, durability
and workability of concrete.
Thus, the minimum cement content specified ranges from 300 to 360 kg
per cum of concrete for various exposure conditions and for various grades
of concretes. It is the prerogative of the designer to find out the correct
quantity of cement over and above these minimum values in Table 5 of
IS456. The maximum cement content in concrete in normal course is also
limited to 450 kg. per cum of concrete as per Para 8.2.4.2 of IS 456. By
specifying cement content much higher than the stipulation in IS 456, the
entire purpose of designing is defeated. Very often designer may be easily
able to achieve required targeted mean strength in lesser cement than
specified in the agreement but he has to perforce specify the higher
quantity given in the agreement as intimated to him by client (project
authorities). Thus, the entire purpose of designing concrete in economical
and environmental friendly manner is lost. Unnecessary damage to the
environment is caused by emission of greenhouse gases in the
environment in the process of manufacturing of wasted cement. Besides
wastage / pilferage, this approach also gives rise to scope for wastage and
pilferage of cement in actual execution. .
d) High quantity of cement in Mix Design Report
It has also been noted that very high quantity of cement is sometimes
specified in the design mix by the laboratory. The design mix report issued
by the laboratory should not be directly used at site without approval of
project Engineer. The Engineer should be made responsible for approval of
design mix report. He should not instantly approve any design mix report
which shows abnormally high cement content. In such cases, he should
thoroughly check the design mix report particularly from economy angle. He should check if the use of water reducing admixtures or plasticizers can
help in reduction of water cement ratio and subsequent reduction in high
quantity of cement prescribed in the design mix report particularly for
higher grade of concrete where reduction in water cement ratio to value of
0.30 to 0.45 is needed. Thus, a design mix report showing very high
cement content should not be readily accepted at its face value but after
critical scrutiny only. The engineer should check the calculated design mix
proportion by means of trial batches as per para 5 of IS10262-2009.Grade of concrete Cement content in kg. per
cum of concrete
M 15 250 to 300
M 20 280 to 330
M 25 310 to 360
M 30 340 to 390
M 35 360 to 420
M 40 380 to 450
M 45 to M 55 400 to 450
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