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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.