108 The Masterbuilder - July 2013 • www.masterbuilder.co.in

High Performance Concrete using Microsilica

Surendra SharmaGeneral Manager-Concrete. Elkem South Asia.

High Performance Concrete

Need for High Performance Concrete

Concrete structure gets deteriorated on account of chemical reactions such as corrosion of rebar or physical effects like porosity which permits easy access of harmful ions into the main body of concrete which makes concrete resistance weak there by shortening the life cycle of concrete. Some of the factors which affect durability of concrete are reinforcement corrosion, chloride diffusion, carbonation, sulphate attack, alkali silica reaction, frost action, leaching of concrete etc.

In practice, most of this deterioration of concrete cannot be seen as they developed as concrete grow older.

Fig.1 below shows some of the damages and therefore as a practicing engineer one need to design and place concrete which is durable and sustainable in the long run.

Therefore as a good practicing engineer, the best way of protecting these damages is to make concrete less permeable as high degree of water saturation is seen as one of the causes for durability problem. It becomes very essential to design concrete with less porosity to protect the concrete as water

Fig. 1

is only medium through which chlorides, sulphate and other harmful trace element can enter into concrete structure and cause damages to concrete from inside .i.e. corrosion.

All major infrastructure projects are designed for 100+ years and capital intensive. It becomes very essential to increase the life cycle and produce the concrete which can last for longer period and boost the nation’s economy. Replacing concrete or structure is a costly affair.

ACI defines High Performance Concrete as “concrete meeting special combinations of performance and uniformity require-ments that cannot always be achieved routinely using con-ventional constituents and normal mixing, placing and curing practices.

Microsilica: Elkem Microsilica® is one of the principle products supplied by Elkem Silicon Materials. It finds a wide range of applications in high-strength concrete, High Performance Concrete other building materials such as roof tiles and facade cladding, and fire proof products for heavy industry.

Microsilica is also used for sealing tunnels and drilling oil wells. Microsilica is proven mineral additive which has been in use since many years to provide high performance concrete solution to construction Industry. Worldwide 10 Million m3 or even more concrete is produced with Microsilica. The ACI defines Silica fumes as “very fine non crystalline silica produced in electric arc furnaces as a by- product of the production of elemental silicon or alloy containing silicon. Microsilica® is having a very fine particle. The average particle size of Microsilica is 0.15 µm.

The specific surface tested on BET method is minimum 15000 m2/Kg (See SEM Fig 2). Which provide better filler effects and make concrete less porous and robust and it helps in improving concrete resistance to chloride and sulphate attack.

Usually Microsilica available in condensed from. There are two type of Microsilica available in the market.

1) Undensified Microsilica

www.masterbuilder.co.in • The Masterbuilder - July 2013 109

2) Densified Microsilica

During Microsilica production, Undensified Microsilica is densified by use of hot air which helps in increasing its bulk density. It is essentially a non-hazardous material and fall into general category of nuisance dust, which is similar to cement and many other fine particles.

The relevant standard for Microsilica in India is IS 15388-2003. Please see below the other physical and chemical properties specification of Microsilica as outlined by different standard.

How does it work?

Microsilica has minimum 85%-99% of SiO2 which, make it very reactive pozzolanic material in concrete. As we all know, water is an essential element in the cement hydration process

and when one adds water to concrete; generally following chemical reaction takes place.

Cement hydration

Portland cement (C3S, C2S) + water = CSH + Ca (OH)2

Secondary Pozzolanic reaction with Microsilica

Pozzolana (SiO2) + Ca (OH)2 + water = CSH

Meaning you get more,

CSH-gel (binder)

Denser concrete/good interface

Higher compressive strength.

Also, above chemical equation shows that free lime, Ca(OH)2 which detrimental to concrete is turned to something good meaning more C-S-H (Calcium-Silicate-Hydrate) gel meaning additional binder for given concrete which provide improved strength and durability characteristics both at early age and later age besides improving plastic properties.

Other value attributes of Microsilica in Plastic stage of concrete

Generally the recommended dosage of Microsilica is 5 to 10%

Fig. 2

High Performance Concrete 0.45um

Bulk Density - Undensified Microsilica Densified Microsilica

130 to 480 Kg/m3

480 to 720 Kg/m3

Specific Gravity 2.2

Physical properties of Microsilica.

USA ASTM C

1240-2004

EUROPE CEN prEN

13263: 1999

INDIA IS 15388: 2003

CANADA CSAA23.5-98

NORWAY NS 3045

AUSTRALIAAS 3582

Sio2 %> 85.0 85.0 85.0 85.0 85.0 85.0

SO3%< 2 1 3

Cl %< 0.3 Report if > 0.10 Report

CaO %< 2

MgO

Si (free) %< 0.4

Total Alkalies 1.5

Free C

Moisture Content %< 3 3 2

LOI %< 6 4 4 6 5 6

Specific Surface m2/gm>

15 15-35 15 12

High Performance Concrete

110 The Masterbuilder - July 2013 • www.masterbuilder.co.in

by wt of cementitious content depending upon the specific characteristics that need to be improved in the concrete mix. Microsilica as stated earlier is a very fine material and it need proper dispersion well in concrete matrix and it occupies space between cement grains thereby improving permeability of concrete mass. Typically it is said that in one grain size of cement, 100 Microsilica grains can exits. It also helps in improving mobility of concrete when energy is applied to concrete.

Through the permeable structure of concrete the ingress of chloride and sulphate ions can damage the concrete as stated earlier and addition of Microsilica largely improved this and protects the concrete.

Fine particles also hold other constituent of concrete better and therefore other value addition of Microsilica is better cohesion. This property largely helps in short creating operation as well as helps in pumping the concrete. In addition it reduces bleeding and segregation and aids in efficient finishing and helps in slip form because of better finish.

These plastic properties have its own value and are of equal

important while placing and finishing concrete. The first and foremost principle a concrete mix designer should bear in mind is to make the concrete cohesive enough with limited stickness, good mobility and finishability at a given workability which enable concrete to hold its constituent together under pressure which will be excreted during mixing, transportation and placing of concrete. What is the use of a great mix design made in the laboratory which cannot be placed in field? And therefore these plastic properties becomes equally important, Once cohesion of concrete is satisfactory then mechanical properties such as compressive strength, E-Modulus etc and durability characteristics tested on durability standard in terms of permeability ,I-Sat, Rapid Chloride Penetration, Sorptivity, Water Penetration, chloride diffusivity etc are to be taken care of. These plastic, harden and durability characteristics have been seen as remarkable improved by addition of Microsilica. If someone is using higher content of cement, by adding Microsilica he/she can reduce cement content and make concrete less sticky too this would be a desirable largely in pumping operation. It also help us to reduce heat of hydration while strength is maintained which ultimately help to prevent early age cracking.

Other value attributes of Microsilica Hardened stage of concrete.

In fully hydrated Portland cement paste, approximately 1/4th of the hydration product by mass consists of oriented, heterogeneously distributed and weekly bonded layers of calcium hydroxide crystals; which de-bond easily under tensile stress thereby serving as potential sites for formation of micro cracks. Therefore, transformation of most of this calcium hydroxide into the calcium silicate hydrate paste (which is the predominant phase produced by hydration of Portland cement) would result in a much more homogeneous hydration product.

At hardened stages, it helps in,

- Improving Strengths,

Bulk Density Report Report

Pozzolanic Activity Index %>

105%-7d accel cur-ing, w/cm=variable

100%-28d Normal Curing w/p ration=0.5

85%-7d IS 1727 Factor N=1

85%-7d accel’dcuring

95%-28d Normal curing w/cm=0.5

Report

Retained on 45 micron sieve %<

10 10 10

Density, kg/M3

Autoclave Expansion %< 0.20%

Canadian Foaming Test No visible foam

NotesCharacteristic values. Not an official standerd in the approval process

Requirement of limiting alkali only in case of reactive aggregares

charateristic values

Fig. 3 Fig. 4

High Performance Concrete

112 The Masterbuilder - July 2013 • www.masterbuilder.co.in

Building Name Location App. Height (M) Date

Burj Khalifa Dubai 828 Tallest structure in the world, completed 2010

International Commerce Centre Hong Kong 484 Completed 2010

Petronas Twin Towers Kuala Lumpur 452 Tallest twin buildings in the world, completed 1998

Taipei 101 Taipei 508 Completed 2004, tallest LEED rating building

Guangzhou West Tower Guangzhou 432 Completed 2010

Trump Tower Chicago 423 Completed 2009

Two International Finance Centre Hong Kong 413 Completed 2003

Burj Al Arab Dubai 321 One of the world’s tallest hotels, opened 1999

311 South Wacker Drive Chicago 293 Completed 1999

Trianon Frankfurt 186 Completed 1993

World One Mumbai 442 On going

Polais Royal Worli Mumbai 320 On going

Oasis Towers Worli Mumbai 372 On going

Minerva Tower Mumbai 302 On going

Ireo Victory valley New Delhi 178 On going

- Reducing Permeability,

- Improving Acid Resistance,

- Improving Abrasion-erosion Resistance,

- Improving Durability

As you all, are aware addition of extra cement beyond certain mass depending upon cement type, per cu.m of cement does not increase your strength and highly uneconomical. And if one wants to design for higher strength and high performance concrete, addition of Micro silica helps a lot to achieve the desired plastic and hardened properties of concrete with robust and workable design. There are other mineral admixtures available based on Fly ash and GGBS, however they have their own limitation on SiO2 content and other and largely influence setting characteristics of concrete as they influence hydraulic properties of concrete and in today’s age everyone need high early strength to speed up the construction and high ultimate strength to go vertical in building high rise structure.

If you see, fig 3 and fig 4, fig 3 is a hardened concrete surface of concrete made with only cement and fig.4 is hardened concrete surface of concrete made with Microsilica and if you closely look at the interface of matrix or a cement glue (see letter A in the Fig 3 & 4), you will find that Microsilica concrete is having much less thickness of cement paste or glue or matrix and holding the aggregate better meaning economical

use of cementitious materials.

Besides this, there are enumerable advantages of Microsilica in the concrete and Microsilica is playing very important role to redefine High Performance concrete and allowing structural engineer to think of reduced size of structural elements and dream higher in the sky. And incorporation of Microsilica at designed stage of structure results in direct saving for the project. As a concrete mix designer we all should strike a balance between performance and economy and it is seen that reduced material cost per cu.m of concrete is taken only as means of economic mix design which is not true.

Therefore concrete cost must not be compared with material cost per cu.m but a comprehensive cost analysis is needed which includes all cost associated right from placing to maintenance of the structure till its design period is to be taken in to account which will the serve the purpose in longer run. Also there is need to go for performance based specification rather prescriptive based specification and Microsilica will ensure to meet the specifications.

Please see the summary of high rise executed in the recent past where Microsilica has been used in the project which one proves value additions of Microsilica.

References

- Silicafume user’s manual 2005- Internal references from Elkem ASA

High Performance Concrete