need for ordinary portland cement€¦ · cement the fineness is to be limited to 225 m 2/kg as...
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Need for Ordinary Portland Cement
S.A.Reddi
Deputy Managing Director (Retd)
Gammon India Limited
Introduction
The requirements of strength, durability and economy are important factors in the design
and construction of concrete structures. A European Commission directive states that the
design life of structures must be defined in advance and that designs must be carried out
to suit the defined design life; construction should take requirements of durability into
account. The Indian standard codes have not a yet precisely defined design life of
structures. Normally, concrete structures have a life of more than 50 years, depending
upon various factors.
Some monumental structures are now designed for a minimum service life of 100 years.
Such structures are designed, constructed and operated to maintain their safety,
serviceability and appearance for a 100-year period under expected environmental
influences, without high costs for maintenance and repairs. In India, we have
monumental structures such as The Gateway of India (Fig.1), Mumbai, which is nearly
88 years old. The Great Belt bridge (Fig.2) in Denmark and the Channel Tunnel (Fig.3)
are outstanding international examples of structures with a design life of 100 years.
The Meerut Garages consist of
a series of shells 65 mm thick,
11 m span with prestressed
concrete edge beams spanning
about 40 m with floor area of
about 5000 sq.m. This was the
first prestressed concrete
structure in India, built in 1944.
The Coronation bridge (1939-1941) across the Teesta river in North Bengal is in
reinforced concrete with a central arch span of 80 m and a rise of approx. 40 m (fig.4).
Fig.1 Gateway of India
Fig.4 Coronation Bridge Fig.5 Pilot Bunder flats
Fig.2 Great Belt West Bridge Fig.3 Channel Tunnel
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The Pilot Bunder flats in Mumbai (fig.5) were built before 1948. All these structures
were built using OPC, still in service, indicating outstanding durability.
Durability provisions in IS 456-20001
The Indian Standard for Plain and Reinforced Concrete - Code of Practice, has dealt with
durability requirements extensively in a separate section. As per IS 456, “A durable
concrete is one that performs satisfactorily in the working environment during its
anticipated exposure conditions during service. The materials and mix proportions
specified and used should be such as to maintain its integrity and, if applicable, to
protect embedded metal from corrosion”.
Concrete should be impermeable to ingress of water, oxygen, carbon dioxide, chloride,
sulfate etc. Factors affecting durability are:
• environment
• cover to embedded steel
• type and quality of constituent materials.
• cement content, water/cementitious (w/c) materials ratio of concrete
• workmanship to obtain full compaction and efficient curing
• shape and size of the member
Exposure Min Cement (kg/m3) Min Concrete grade (Mpa) Max w/c
Mild 300 20 0.55
Moderate 300 25 0.50
Severe 320 30 0.45
Very Severe 340 35 0.45
Extreme 360 40 0.40
Minimum Cement Content
EN 206-1:20002 Concrete - Specifications, Performance, Production and Conformity, is
the Euro code to IS 456: 2000. The Euro code gives six exposure conditions for corrosion
alone depending on the degree of attack. On comparing the exposure conditions and the
minimum cement content as per the two codes it becomes evident that the values of
minimum cement are higher in IS 456 as compared to EN 206.
Exposure IS 456 EN 206
Mild 300 260
Moderate 300 280
Severe 320 300
Very Severe 340 300
Extreme 360 300
Fig.8 Exposure conditions and minimum cement content, minimum grade of
concrete and maximum w/c ratio as specified in is:456-2000
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It should be noted that the minimum cement content given above is specific to the
environmental condition and not to the grade of concrete. Thus for severe exposure
condition the minimum cement content is 320 kg/m3
and the minimum grade of concrete
is M30. If a higher grade of concrete is used for the same exposure condition, the
minimum cement content still remains 320 kg/m3. The maximum cement content for any
type of concrete and for any exposure condition is limited to 450 kg/m3. With proper mix
design and use of admixtures, it is possible to obtain the highest grade of concrete with
the minimum cement content specified in the code.
Effects of increased Cement Content
Cement is the most expensive component of concrete. Strict measures of waste control
should be implemented at site. Increased cement content in concrete leads to risk of
cracking due to drying shrinkage or early age thermal cracking and increased risk of
damage due to alkali silica reaction. Increased cement does not add value to concrete.
The belief of higher the cement content, higher is the strength is not true. In fact most of
the concrete structures constructed around the world use decreased quantity of cement by
replacing a part of cement with mineral admixtures to enhance the properties of concrete
such as reduction in the heat of hydration, increasing strength, durability etc.
Cement content alone has no significant influence on compressive strength3; w/c ratio
appears to be the main parameter for strength. A small reduction in strength is observed
with increasing cement content at equal w/c ratio. Cement content, at constant w/c ratio,
has no significant effect on carbonation/chloride ingress. W/c ratio for a given set of
conditions appears to be the dominant parameter affecting the durability of concrete.
It is possible and economical to design the mixes for even h9igh strength concrete with a
minimum specified cement content only if OPC is used
Effect of Increased Fineness of Cements4
Present day cements have higher fineness and higher lime content. As per IS Codes of
cement the fineness is to be limited to 225 m2/kg as verified by Blaines Test. Often
modern cements are ground to a fineness of 400 m2/kg or more. Due to the increase in
specific surface of finer cement, more water is required to obtain the desired workability
which decreases durability of the concrete. As rate of hydration depends on the fineness
of cement particles, higher fineness of cement leads to increase in the rate and
consequently increases the heat evolved. The expansion of concrete made with a given
alkali-sensitive aggregate is greater if the cement is finer. The finer the cement the higher
is the gypsum requirement.
Indian Cement Vs International Cement
Indian OPC 33 grade cement is inferior to the lowest grade of cement in Europe or USA.
It is not advisable to use 33-grade cement for any structural concrete. There is very little
difference in the quality of 43 and 53 grade cements. Indian 43 grade cement is
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equivalent to Euro 32.5 grade cement and Indian 53 grade cement is equivalent to Euro
42.5 grade. The difference is in specifications for testing methods in India and elsewhere.
As per the Comparison of BIS, ASTM & EN Cement Standards in Publication 5 of
Grasim Cement, “Although numerical values of strength specified may be similar, they
are not comparable, in view of the differences in test methods. As a very general
statement, it may be said that IS 43 grade may be comparable to 32.5 class of EN 197-1.
53 grade OPC (IS 12269) may satisfy 42.5 class of EN 197-1 & ASTM Type 1.”
Various types of cements and mineral admixtures and their uses under particular
conditions are now discussed:
Ordinary Portland Cement (OPC)
OPC is by far, the most commonly used cement in general concrete construction. OPC is,
in theory, available in three grades - 33, 43 & 53. The ground reality (April 2007) is that
very little of OPC is being produced in the country putting the users in difficulties. For
plain concrete (PCC) and reinforced concrete (RCC) of lower strengths, any grade of
cement can be used provided there is no exposure to sulphates.
For high strength concrete in RC structures as well as prestressed concrete structures, the
minimum grade of concrete is M40; often-higher grades up to M80 are used. Such
concretes require higher grades of OPC (43 / 53 grade). Portland pozzolana cement
cannot be used for such high grade concrete : slow strength development at early ages
and longer curing times and formwork stripping times are involved.
Blended Cements
Portland Pozzolana Cement (PPC) - IS 1489
PPC is produced either by grinding together clinker & pozzolana or by blending OPC and
pozzolana. PPC produces less heat of hydration, reducing the micro-cracks and offers
greater resistance to attack of aggressive waters than OPC. It is useful for special
structures such as marine, hydraulic construction and mass concrete structures. It has
been specified in IS 1489 that PPC can be used wherever 33 grade OPC is usable under
normal conditions. This specification has been prepared to enable manufacturers to
produce PPC equivalent to 33 grade OPC.
Thus, as per BIS, PPC is actually equivalent to OPC 33 Grade. The claims of
various manufacturers that PPC can replace OPC 43 and 53 Grades are not valid, but
incorrect and misleading. While PPC has advantages under certain situations, they cannot
replace OPC 43 and 53 Grades. The manufacturers’ claims violate BIS Specifications
and in fact mislead the end users into believing that they can replace OPC of higher
grades.
PPC can at best be used for low and medium grades concrete up to and including M-30;
can perhaps be used for Grades up to M-40, with PPC obtained from selected factories.
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This will be at extra cost to the end user, as more quantity of PPC is required to be used
to get the same strength. The formwork is required to be retained in place for a longer
period, the implication being the use of more sets of formwork to achieve the same time
cycle. The concrete is required to be cured for longer period, involving more
expenditure. For higher Grades of Concrete it is not normally possible to use PPC. The
only option is to use OPC 43 or 53 Grades.
The specific surface of PPC specified by the BIS Code is 300 m2/kg, thus it is finer than
OPC with specified specific surface of 225 m2/kg. The fineness of cement is a vital
property and has to be carefully controlled. The rate of hydration depends on the fineness
of cement particles; higher fineness may give more strength but is also detrimental to the
long-term performance of concrete. A higher early rate of hydration means a higher rate
of early heat evolution, leading to pre-mature of deterioration of concrete.
Majority of the structures constructed before 1970 had used OPC, with lower fineness;
they all are performing remarkably well. The current tendency on the part of cement
manufacturers towards finer grinding is leading to pre-mature deterioration of completed
structures. The erstwhile British Specification for Cement specifies upper limit for a
fineness of cement, purely from durability point of view.
Portland Slag Cement, (PSC) - IS 455
Portland Slag Cement is obtained by mixing Portland cement clinker, gypsum and
granulated blast furnace slag in suitable proportions and grinding the mixture to obtain a
homogenous mix. PSC has physical properties similar to OPC 33 Grade, but has low heat
of hydration and has relatively better resistance to chlorides, soils and water containing
excessive amount of sulphates or alkali metals, alumina, iron and acids.
The rate of hardening of PSC in mortar or concrete is slower than that of ordinary
Portland cement during the initial 28 days, but thereafter it increases. The heat of
hydration of Portland Slag cement is lower than that of OPC. Hence this cement can be
used in mass concrete structures effectively.
The compressive strength of Portland Slag cement, as per IS : 455 at 3, 7 and 28
days is the same as for OPC Grade 33. Any claim of manufacturers that the Portland
Slag Cement can achieve strengths equivalent to 43 or 53 Grades cement is thus not
valid, but again misleading the end user.
As per Forward to IS : 455, Indian Standards for Portland Slag Cement – Specification,
“The manufacture of Portland Slag Cement has been developed primarily to utilize blast
furnace slag, a waste product from blast furnaces. The development of manufacture of
this type of cement will considerably increase the total output of cement production in
the country and will, in addition, provide a profitable use for an otherwise waste
product”.
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Thus, today the intention of Bureau of Indian Standards in formulating Standards for
PPC and PSC has been totally twisted out of context. Instead of supplementing OPC
while utilizing the waste products, PPC and PSC are being erroneously projected as an
alternative to OPC. Apart from the additional cost to the end user, of using PPC and
PSC , these cements can never replace OPC for specific applications such as High
Strength Concrete.
The Need for OPC Grades 43 & 53
The Forward to IS: 12269, Specification for 53 Grade Ordinary Portland Cement sets
down the background for the need of 53 Grade OPC: “For certain specialized works, such
as Pre-stressed Concrete and certain items of Pre-cast Concrete requiring consistently
High Strength Concrete, the concrete industry quite often needs a special type of
Ordinary Portland Cement having the compressive strength much higher than the
minimum specified in IS: 269 and IS: 8112.
“The Cement and Concrete Sectional committee has therefore considered it necessary to
bring out a separate Specification for 53 Grade OPC, wherein the minimum 28 days
compressive strength requirements for Cement are specified keeping in view the needs of
the consumer for higher strength concrete and the manufacturing facilities available with
the manufacturers”.
The sentiments expressed in the Forward to IS Standards holds good even today. Its
importance is further highlighted by the development of high strength concrete industry
in India. At the time of formulation of Standards, 53 Grade cement, IS: 456 had allowed
for concrete strengths up to M-50 only. The revised IS: 456-2000, allows high strength
concrete up to Grade M-80.
A large number of structures are being realized with high strength concrete in India
during the last ten years. M-60 grade was first used for the Atomic Power project at
Kaiga, followed by Tarapur Atomic Power project. M-75 concrete was first used for the
JJ Hospital Flyover construction in Mumbai. A number of property developers and
builders are increasingly using high strength concrete. One of the tallest buildings under
construction in Mumbai will be using M-80 grade concrete for certain components.
These developments were made possible only by the use of OPC. Internationally, higher
grades of concrete up to M-120 have been realized by using OPC plus mineral
admixtures such as fly-ash, ground granulated blast furnace slag, etc. The formulations
are made by the end users and not necessarily the cement manufacturers.
Blended Cement Vs National Specifications
IS 456:2000 allows the use of all grades of OPC, Portland slag cement, Portland
pozzolana cement, sulphate resistant cement and low heat cement; type selected should
be appropriate for the intended use. Other combinations of Portland cement with mineral
admixtures may be used.
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MORTH specifications for Road and Bridge Works Cl.1006 permit only OPC 33, 43 and
53 grades and sulphate resisting Portland cement. IRC 21:2000 for Cement Concrete
(Plain and Reinforced), permits the unrestricted use of OPC, SRPC and slag cement for
all structures whereas Portland pozzolana cement is permitted only in plain concrete
members. Due to difficulties in obtaining OPC, a large number of National Highway
Projects are suffering inordinate delays
Difficulties in use of Blended Cements
o No third party certification for quality of pozzolana; source of fly ash not known
o No known suppliers of processed fly ash in India (except DIRK India in Nasik).
o No guarantee of whether fly ash is being processed before blending with cement.
o No information blending; whether inter-ground with clinker or mixed with cement.
o Lack of details of QS & QC for fly ash / slag.
50-70% slag is required for effective use. Most of the factories use lower percentage for
producing slag cement. Fly ash at Power stations is available for free; only transportation
cost has to be borne. Power consumption for production is lower than OPC; there is less
clinker to grind.Cost of blended cement should be lower than OPC; the cost benefits
should logically be shared with the buyers, but not done.No blended cement can satisfy
all specifications or uses. The manufacturers set proportions unless a client orders a
sufficient quantity of cement with alternative proportions. Due to the limited choice of
proportions their properties may not lead to optimum concrete properties for all purposes.
Blending of mineral admixtures at site
IS 456 allows addition of mineral admixtures as part replacement of cement, provided
uniform blending with cement is ensured. Thus the code allows two options
- by incorporation during the manufacture of cement or
- by incorporation during the manufacture of concrete
When the mineral admixture is incorporated in the cement factory, the end user has
difficulty in assessing the quantity incorporated; he also does not get the benefit of cost
advantage. In the absence of knowledge of percentage of fly ash / slag contained in the
blended cement, the mix design at the project site is also uncertain. Provided thorough
blending is ensured during the production of concrete, the end user is assured of both the
benefits. In fact in the developed world, blending at the project site is more common.
Such blending at project site requires batching plant with separate silos for cement and
mineral admixture. In the present scenario, this is no more a problem in India on major
project sites. There are more than 10.000 batching plants in operation. MORTH
specifications for Roads and Bridge works, clause 1707, specifies that an automatic
batching plant shall be used for all bridges with a length of 200 m or more. Site blending
of mineral admixtures was successfully employed for M40 concrete used at the Vizag
Sea Port. OPC 123 kg/cum and GGBS 287 kg/cum was used in the mix.
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The Bandra-Worli sea link project in Mumbai, presently under construction, is also using
OPC with mineral admixtures (fly-ash and micro silica) for M60 concrete. Some other
notable examples of site blending are listed below:
Project Concrete grade OPC % GGBS %
Construction of marine jetty for Chennai Port
Trust at Ennore
M45 30 70
Construction of 7 km long concrete road into
the sea for Chennai Port Trust at Ennore
M26 50 50
Hi-Tech city in Hyderabad M50 50 50
Construction of Bharat Forge factory in Pune M30 70 30
Vizag Port Trust M45 60 40
Some international examples include:
• Bahrain – Saudi Causeway
• Second Severn Crossing in the U.K. GGBS was used in all major elements of the
structure at level of 28% for deck units, 50% for piers and caisson units and 70%
for caisson infill concrete. The concrete grade was 40 MPa. Actual 28 days
strength achieved ranged between 70 and 80 MPa.
• Humber bridge, UK: GGBS at levels 30 – 60 %
Cement for Concrete in hot weather
Finer cements exhibit accelerated setting and increased heat of hydration. Such cements
should be used with caution in hot weather. Blended cements or OPC with site mixing of
GGBS or fly ash is recommended for low heat of hydration, improved cohesiveness and
better response to vibration. Cement should be stacked in advance to enable it to cool
down to the ambient temperature before it is batched into the concrete mixer.
Cement for High Performance Concrete (HPC)
OPC 53 grade from selected sources are used. HPC cannot be produced using lower
grades of cement or blended cement. To reduce the heat of hydration mineral admixtures
such as ground granulated blast furnace slag or fly ash are used. Silica fume is used in
HPC for strengths higher than M-75 to improve the properties of concrete. The silica
fume reacts with calcium hydroxide that forms during the hydration of cement, creating
an increased amount of CHS binder, thus providing increased strength and durability.
Water reducing admixtures are necessary in combination with silica fume to ensure good
workability. While using silica fume, the mixing time for concrete is increased. For a
given slump, the compaction of concrete with silica fume requires more efforts. Thus it is
necessary to increase the slump slightly in such concretes. Silica fume concrete suffers
less bleeding, thus improving the bond between the paste and the aggregates.
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Silica fume was first used for High Performance Concrete M60 in India in 1997 in the
Containment Dome of Nuclear Power Reactors (fig.6). Further trials by Nuclear Power
Corporation have confirmed that it is possible to produce M-60 grade of concrete with a
slump of more than 100mm at the pour point, with 5 to 6% silica fume.
Microsilica was also used in the construction of the JJ flyover (fig.7) in Mumbai in 2001.
M75 grade HPC was used for construction.
We can make durable concrete today, but need OPC
We have the knowledge and capability to make a good concrete, but we do not always do
so. Concrete should be fit for the purpose for which it was intended and for the expected
life during which it is to remain in service. Cement and cementious materials contribute
substantially towards durable concrete. Mineral admixtures form an integral component
of durable concrete and their use in preparation of concrete should be encouraged.
Along with cement, aggregates and water chemical admixtures are essential ingredients
of most concrete mixes, required to minimize the water content and thus increase
durability. Adequate cover to reinforcement is very important. In many of the structures
built in the past, the cover was inadequate, with resulting corrosion of the steel.
Adequate cover to reinforcement is also required for fire protection. The concrete in the
cover zone should be fully compacted. The quality of concrete in the cover zone is more
important than the quality of concrete anywhere else in the structure.
Let the consumer decide the type of cement he needs
Internationally, there are Specifications for various types / grades of Cement. Ordinary
Portland Cement (OPC) is most widely used. More than 50% of concrete produced in the
developed World uses Ordinary Portland Cement with or without mineral admixtures.
The decision regarding the types of cement to be used is that of the end user.
We are having strange situation in India where the manufacturers are dictating as to what
the consumer should use! Of-late, many of the cement majors are refusing to
manufacture / offer OPC, irrespective of consequences the end user. In consequence,
Fig.6 Kaiga Atomic Power plant
dome with M60 HPC
Fig.7 JJ flyover in Mumbai with M75 HPC
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many of the projects involving high strength concrete, particularly pre-stressed concrete
structures construction are inordinately delayed. Majority of the RMC Plants are unable
to supply High Strength concrete in the absence of OPC. Some are supplying using
exorbitant quantities of blended cements, increasing the cost to the end user.
The Financial Express, New Delhi, April 21, 2007 reports “Gujarat Ambuja net up 43%;
India Cements net zooms 10- fold “. Obviously, the extra ordinary level of profit is in
part due to reduced production of OPC and at the cost of the end user.
DNA, Mumbai Edition, April 14, 2007 headlines : “The road to cheaper cement from
Pakistan is, well, by road “. The Central Government has now withdrawn all import
duties. Quotations received from China and Pakistan indicate competitive landed prices
at Indian Ports / Border. However, there are bureaucratic obstacles. Bureau of Indian
Standards is required to certify each and every consignment of imported cement. While
the intention is honorable, there are practical difficulties. As on-date (April 2007), hardly
any international manufacturer of repute have applied to BIS for certification. Importers
who have offered cement from China / Pakistan are not able to get fast clearance from
BIS. Thus, in spite of cheaper availability, imports are not yet viable for immediate use.
The Way Forward
While the Government’s efforts to ensure prices at reasonable level commensurate with
manufacturing cost and international price levels are commendable, it is necessary to
ensure that the end user decides the type of cement he requires instead of allowing the
manufacturers to dictate. This was the broad consensus after deliberations on the issue in
a program organized by the Institution of Engineers, Maharashtra Centre in Mumbai on
6th
April 2007. Almost all the end user participated complained about non-availability of
OPC and consequent delays in construction of projects. Ordinary Portland cement of
various grades should be manufactured in large volumes to meet the demands of the end
users. While blended cement will continue to be used based on the requirements in
certain cases, attempts should not be made to insist that only blended cement should be
used. The manufacturers have an obligation to produce what the end user wants and not
the other way round.
The views expressed in the paper are those of the author only.