2nd Term New Material and Technology

S.R.P** NICMAR-2ND TERM- N.M.T Page 367

Admixtures Admixture is defined as a material, other than cement, water and aggregates, that is used as

an ingredient of concrete and is added to the batch immediately before or during mixing. Additive is a material which is added at the time of grinding cement clinker at the cement

factory. Ordinary concrete may fail to exhibit the required quality performance or durability in different

conditions. In such cases, admixture is used to modify the properties of ordinary concrete so as to make

it more suitable for any situation. Admixtures are no substitute for good concreting practices. Water Reducers or High Range Water Reducers, generally referred as plasticizers and

superplasticizers are developed in Japan and Germany around 1970. The use of plasticizers and superplasticizers have not become popular in India till recently

(1985). Use of RMC has promoted the use of admixtures in India, in recent times. It is difficult to predict the effect and the results of using admixtures because, many a time, the

change in the brand of cement, aggregate grading, mix proportions and richness of mix alter the properties of concrete.

Many admixtures affect more than one property of concrete. At times, they affect the desirable properties adversely. The effect of more than one admixture when used in the same mix is difficult to predict. One must be cautious in the selection of admixtures and in predicting the effect of the same in

concrete. ACI Committee 212, classified admixtures into 15 groups according to type of materials

constituting the admixtures, or characteristic affect of the use. Admixtures Plasticizers Superplasticizers Retarders and Retarding Plasticizers Accelerators and Accelerating Plasticizers Air-entraining Admixtures Pozzolanic or Mineral admixtures Damp-proofing and Waterproofing Admixtures Gas forming Admixtures Air-detraining Admixtures Alkali-aggregate Expansion Inhibiting Admixtures Workability Admixtures Grouting Admixtures Corrosion Inhibiting Admixtures Bonding Admixtures Fungicidal, Germicidal, Insecticidal Admixtures


Classification of Admixtures

Plasticizers A high degree of workability is required in situations like deep beams, thin walls of water

retaining structures with high percentage of steel reinforcement, column and beam junctions, tremie concreting, pumping of concrete, hot weather concreting, for concrete to be conveyed for considerable distance and in ready mixed concrete industries.

Conventional methods for obtaining high workability is by improving the gradation, or by the use of relatively higher percentage of fine aggregate or by increasing the cement content.

The addition of extra water to satisfy the need of workable concrete is amounting to sowing the seed of cancer in concrete.

The excess water will not improve the inherent good qualities such as homogeneity and cohesiveness of the mix whereas the plasticized concrete will improve the desirable qualities demanded of plastic concrete.

The practice all over the world now is to use plasticizer or superplasticizer for making concrete of higher workability.

S.R.P** NICMAR-2ND TERM- N.M.T 69

The organic substances or combinations of organic and inorganic substances, which allow a reduction in water content for the given workability, or give a higher workability at the same water content, are termed as plasticizing admixtures

The basic products constituting plasticizers are as follows: i) Anionic surfactants such as lignosulphonates and their modifications and derivatives, salts of sulphonates hydrocarbons ii) nonionic surfactants, such as polyglycol esters, acid of hydroxylated carboxylic acids and their modifications and derivatives

iii) Other products, such as carbohydrates etc. Calcium, sodium and ammonium lignosulphonates are the most used plasticizers. Lignosulphonic acid in the form of either its calcium

or sodium salt is a natural product derived from wood processing industries.

Plasticizers are used in the amount of 0.1 to 0.4 % by weight of cement.

Effect of surface-active agents on deflocculating of cement grains

Mechanisms Dispersion: Portland cement, being in fine state of division, will have a tendency to flocculate in

wet concrete. These flocculation entraps certain amount of water used in the mix and thereby all the water is not freely available to fluidify the mix.

When plasticizers are used, they get adsorbed on the cement particles. The result is that the cement particles are deflocculated and dispersed. When cement particles are deflocculated, the water trapped inside the flocs gets released and now available to fluidify the mix.

Retarding effect: The plasticizer gets adsorbed on the surface of cement particles and form a

thin sheath. This sheath inhibits the surface hydration as long as sufficient plasticizer molecules are available. The quantity of plasticizers will progressively decrease as the polymers become entrapped in hydration products.

All plasticizers are to some extent set retarders, depending upon the base of plasticizers, concentration and dosage used.

Superplasticizers (High Range Water Reducers) Relatively new category and improved version of plasticizer developed in Japan and Germany

during 1960 and 1970 respectively. Chemically different from normal plasticizers. Use of superplasticizers permit the reduction of water to the extent upto 30 % without reducing

workability in contrast to the possible reduction upto 15% in case of plasticizers. Used for production of flowing, self levelling, self compacting and for the production high

strength and high performance concrete. The mechanism of action is similar to ordinary plasticizer. Superplasticizers are more powerful as dispersing agents and they are high range water

reducers. Made it possible to use w/c ratio as low as 0.25 or even lower and yet to make flowing concrete

to obtain strength of the order 120 MPa or more. Made it possible to use fly ash, slag and particularly silica fume to make HPC.

Superplasticizers can produce: At the same w/c ratio much more workable concrete than the plain ones. For the same workability, it permits the use of lower w/c ratio. As a consequence of increased strength with lower w/c ratio, it also permits a reduction of cement

content.


Classification of Superplasticizers Following are a few polymers which are commonly used as base for superplasticizers

Sulphonated melamine-formaldehyde condensates (SMF) Sulphonated naphthalene-formaldehyde condensates (SNF) Modified lignosulphonates (MLS) Other types

India manufacture and use this four types] New generation Superplasticizers

Acrylic polymer based (AP) Copolymer of carboxylic acrylic acid with acrylic ester (CAE) Cross linked acrylic polymer (CLAP) Poly Carboxylate ester(PCE) Multicarboxylatethers(MCE) Combinations of above

Retarders Slows down the chemical process of hydration so that concrete remains plastic and

workable for a longer time. Used to overcome the accelerating effect of high temperature on setting properties of

concrete in hot weather concreting. Used in grouting oil wells : Cement grout is required to be in mobile condition for about 3 to 4

hours at high temp. (about 2000 C) without getting set. In RMC practices, setting of concrete will have to be retarded so that concrete when finally

placed and compacted is in perfect plastic state. Most commonly known retarder is calcium sulphate. It is interground to retard the setting of

cement. Addition of excess amount of gypsum may cause undesirable expansion and indefinite delay in the setting of concrete.

Other retarders are: starches, cellulose products, sugars( common sugar, skimmed milk powder), acids or salts of acids (Ligno sulphonic acids and their salts, hydroxylated carboxylic acids and their salts).

Retarding Plasticizers Plasticizers & Superplasticizers by themselves show certain extent of retardation. Retarders are mixed with Plasticizers & Superplasticizers at the time of commercial

production known as retarding plasticizers or superplasticizers . Used in RMC industry.

Accelerators Accelerating admixtures are added to concrete to increase the rate of early strength development in concrete to Permit earlier removal of formwork Reduce the required period of curing Advance the time that a structure can be placed in service Partially compensate for the retarding effect of low temperature during cold weather concreting In the emergency repair work In the past one of the commonly used accelerator was Calcium Chloride. Now a days it is not

used since recent studies have shown that it is harmful in RCC and prestressed concrete. Instead, some of the soluble carbonates, silicates, fluosilicates and some of the organic

compounds such as triethenolamine are used. Fluosilicates and triethenolamine are comparatively expensive.

Used in underwater concreting, repair work to the waterfront structures in the region of tidal variations, cold weather concreting etc.

Some of the modern commercial accelerating materials are Mc-Schnell OC, Mc-Schnell SDS, Mc-Torkrethilfe BE manufactured by Mc-Bauchemic (Ind) Pvt. Ltd.

.


Accelerating Plasticizers Accelerating materials like triethenolamine chlorides, calcium nutrite, nitrates and flousilicates

added to plasticizers or superplasticizers resulted in faster development of strength. List of some of the commercial plasticizers and superplasticizers manufactured in India Air-entraining Admixture 85% of concrete manufactured in America contains air entraining agent and it is considered a

necessary „fifth ingredient‟ in concrete making. Air entraining concrete is made by mixing a small quantity of air entraining agent or by using air

entraining cement. These air entraining agents incorporate air bubbles, which will act as flexible ball bearings and

will modify the properties of plastic concrete regarding workability, segregation, bleeding and finishing quality of concrete.

Air entraining agents Natural wood resins Animal and vegetable fats and oils, such as tallow, olive oil and their fatty acids such as stearic

and oleic acids Alkali salts or sulphated and sulphonated organic compounds Water soluble soaps of resin acids, and animal and vegetable fatty acids Miscellaneous materials such as the sodium salts of petroleum sulphonic acids, hydrogen

peroxide and aluminium powder etc. Vinsol resin, Darex, N Tair, Airalon, Orvus, Teepol, Petrosan and Cheecol are the common air

entraining agents and the first two enjoyed world-wide market Air entrained concrete has been used in the construction of Hirakud dam, Koyna dam, Rihand

dam etc. In these dams American air entraining agents Vinsol resin, Darex etc. were used Indigenous air entraining agents Aerosin-HRS., Rihand A.E.A., Koynaea, Ritha powder, Hico

etc. were developed in 1950s MC-Mischoel LP,MC-Michoel AEA, Complast AE 215, Roff AEA 330 are some of the

commercial brands available in India now Damp-proofing and Waterproofing Admixtures Waterproofing admixtures may be obtained in powder, paste or liquid form and may consist of

pore filling or water repellant materials The chief materials in the pore filling class are silicate of soda, aluminium and zinc sulphates

and aluminium and calcium chloride (chemically active pore fillers) Accelerate the setting time of concrete and thus render the concrete more impervious at early

stage Damp-proofing and Waterproofing Admixtures

The chemically inactive pore filling materials are chalk, fullers earth and talc and these are usually very finely ground.

These improve workability and facilitate the reduction of water for given workability and to make dense concrete which is basically impervious

Materials like soda, potash soaps, calcium soaps, resin, vegetable oils, fats, waxes and coal tar residues are added as water repelling materials

Inorganic salts of fatty acids, usually calcium or ammonium stearate or oleate is added along with lime and calcium chloride in some kind of waterproofing admixtures.

Calcium or ammonium stearate or oleate will mainly act as water repelling material, lime as pore filling material and calcium chloride accelerates the early strength development and helps in efficient curing of concrete all of which contribute towards making impervious concrete.

Butyl stearate, heavy mineral oil free from fatty or vegetable oil and asphalt cut-back oils are effective in rendering the concrete waterproof.

People are just about as happy as they make up their minds to be. -Abraham Lincoln


Gas Forming Agents

A gas forming agent is a chemical admixture such as aluminium powder. It reacts with hydroxide produced in the hydration of cement to produce minute bubbles of

hydrogen gas. The amount added is about one teaspoonful to a bag of cement (0.005 to 0.02% by weight of

cement). The action of Al powder causes a slight expansion in plastic concrete or mortar and helps to

improve the bond to reinforcing bars and effectiveness of grout in filling joints. Useful in grouting under machine bases Al powder is used as an admixture in the production of light weight concrete Zinc, magnesium powders and hydrogen peroxide are also used as gas forming agents

Air-detraining agents Admixture capable of dissipating the excess of air or other gas Tributyl phosphate is the most widely used air-detraining agent Water-insoluble alcohols and silicones have been proposed for this purpose Alkali-aggregate expansion inhibitors Alkali-aggregate reaction can be reduced by the use of pozzolanic admixture Air entraining admixture reduces the alkali-aggregate reaction Al powder and lithium salts are also used to reduce the alkali-aggregate reaction

Workability Agents The materials used as workability agents are:

Finely divided material ( bentonite clay, diatomaceous earth, fly ash, finely divided silica, hydrated lime and talc)

Plasticizers and superplasticizers

Air-entraining agents Grouting agents

Accelerators: used in situation where a plugging affect is desired - CaCl2, triethanolamine

Retarders and dispersing agents: to aid pumpability and to effect the penetration of grout into fine cracks or seams – mucic acid, gypsum and a commercial brand known as RDA (Ray Lig Blinder) etc.

Corrosion Inhibiting Agents Sodium benzoate is used as corrosion inhibiting admixture to protect the steel in RCC In this process 2% sodium benzoate is used in the mixing water or a 10% benzoate slurry is

used to paint the reinforcement or both Calcium lignosulphonate decreased the rate of corrosion of steel embedded in the concrete Sodium nitrate and calcium nitrate are efficient inhibitors of corrosion of steel Mc-Corrodur manufactured by Mc-Bauchimie (Ind) Pvt. Ltd. is the commercial admixture

available to inhibit corrosion Bonding Admixture

To increase the bond strength between the old and new concrete The commonly used bonding admixtures are made from natural rubber, synthetic rubber or

from any organic polymers. The polymers include polyvinyl chloride, polyvinyl acetate etc. Two types – re-emulsifiable types and non-re-emulsifiable types

Fungicidal, Germicidal and Insecticidal Admixtures

To impart fungicidal, germicidal and insecticidal properties to hardened cement pastes, mortars or concretes.

Polyhalogenated phenols, dieldren emulsion or copper compounds Coloring Agents.

Various metallic oxides and mineral pigments are used

RMC (India) Ltd. markets ready mixed colour concrete for decorative pavements


Miscellaneous Admixtures a) Damp proofers ( Accoproof, Natson‘s Cement Waterproofer, Trip-L-Seal, Cico, Feb-Mix-Admix,

Cemet) b) Surface hardeners ( Metal Crete, Ferrocrete No.1, Metal Crete Steel Patch, Arconate No.1)

Pozzolanic or Mineral Admixtures Pozzolanic materials are siliceous or siliceous and aluminous materials which in themselves

possess little or no cementitious value, but will, in finely divided form and in the presence of moisture, chemically react with calcium hydroxide liberated on hydration, at ordinary temperature, to form compounds, possessing cementitious properties.

Ancient Greeks and Romans used certain finely divided siliceous materials which when mixed with lime produced strong cementing material having hydraulic properties and such cementing materials were employed in the construction of aqueducts, bridges etc.

One such material was consolidated volcanic ash or tuff found near Pozzuoli (Italy) near Vesuvius. This came to be designated as Pozzuolana, a general term covering similar materials of volcanic origin.

Specimens of concrete made by lime and volcanic ash from Mount Vesuvious were used in the construction of Caligula Wharf built in the time of Julius Caesar nearly 2000 years ago is now existing in a fairly good condition.

Pozzolans in optimum proportions mixed with Portland cement improves qualities of concrete such as Lower the heat of hydration and thermal shrinkage Increase the watertightness Reduce the alkali-aggregate reaction Improve resistance to attack by sulphate soils and sea water Improve workability Lower costs Mineral admixtures are added to concrete in relatively large amounts, generally in the range 20 to

70 % by mass of the total cementitious material. Many industrial by-products have become the primary source of mineral admixtures in concrete

due to economic and environmental considerations. Power plants using coal as fuel, and metallurgical furnaces producing cast iron, silicon metal, and

ferrosilicon alloys are the major sources of by-products being produced at the rate of millions of tonnes every year in many countries.

Types 1. -Natural Pozzolans 2. -Artificial Pozzolans

1.Natural Pozzolans Clay and Shales Opaline Cherts Diatomaceous Earth Volcanic Tuffs and Pumicites

Natural Pozzolans Natural pozzolans such as diatomaceous earth, clay and shale, pumicites, opaline cherts etc.,

needs further grinding and sometimes needs calcining to activate them to show pozzolanic activities.


In Hirakud dam construction in Orissa, naturally occurring clay known as Talabara clay has been used as pozzolanic materials.

The natural pozzolans have lost their popularity in view of the availability of more active artificial pozzolans.

2.Artificial Pozzolans Fly ash Silica Fume Rice Husk Ash Surkhi Metakaoline Blast Furnace Slag

Fly Ash

Fly ash is finely divided residue resulting from the combustion of powdered coal and transported by the flue gases and collected by electrostatic precipitator

In U.K. it is referred as pulverized fuel ash (PFA)

Fly ash was first used in large scale in the construction of Hungry Horse dam in America in the approximate amount of 30% by weight of cement

In India, it was used in Rihand dam construction, Uttar Pradesh replacing cement up to about 15%

The use of fly ash as concrete admixture extends technical advantages to the properties of concrete and also contributes to the environmental pollution control.

75 million tonnes of fly ash is produced in India per year, the disposal of which has become a serious environmental problem.

Production of cement emits carbon dioxide and 7% of the world‘s CO2 emission is due to Portland cement industry.

One of the solutions to economise cement is to replace cement with supplementary cementitious materials like fly ash and slag.

There are two ways that the fly ash can be used

One way is to intergrind certain percentage of fly ash with cement clinker at the factory to produce Portland Pozzolana cement (PPC)

Second way is to use the fly ash as an admixture at the time of making concrete at the site of work.

The second method gives freedom and flexibility to the user regarding the percentage addition of fly ash.

ASTM broadly classify fly ash into two classes

Class F: Fly ash normally produced by burning anthracite or bituminous coal, usually has less than 5% CaO. Class F fly ash has pozzolanic properties only.

Class C : Fly ash normally produced by burning lignite or sub-bituminous coal. Some class C fly ash may have CaO content in excess of 10%. In addition to pozzolanic properties, class C fly ash also possess cementitious properties.


High Volume Fly Ash Concrete(HVFA) One of the practical methods for conserving and economizing cement and also to reduce the

disposal problem of fly ash is to popularize the high volume fly ash concrete system. HVFA is a concrete where in 50 to 60% fly ash is incorporated Developed for mass concrete application where low heat of hydration was of primary

consideration. HVFA showed excellent mechanical and durability properties required for structural applications

and pavement constructions. Silica Fume Silica fume, also referred to as microsilica or condensed silica fume is a product resulting from

reduction of high purity quartz with coal in an electric arc furnace in the manufacture of silicon or ferrosilicon alloy.

Condensed silica fume is essentially silicon dioxide (more than 90%) in noncrystalline form It is extremely fine with particle size less than 1 micron and with an average diameter of about 0.1

micron, about 100 times smaller than average cement particles. The use of silica fume in conjunction with superplasticizers has been the backbone of modern

HPC. The structures such as Key Tower in Cleaveland with a design strength of 85 Mpa, and Wacker

Tower in Chicago with specified concrete strength of 85 Mpa, and two Union Square in Seattle with concrete that achieved 130MPa strength - are testaments to the benefits of silica fume technology in concrete construction.

Silica fume by itself, do not contribute to the strength dramatically. It contributes to the strength property by being very fine pozzolanic material and also creating

dense packing and pore filler of cement paste. High strength of HPC containing silica fume are attributable, to a large degree, to the reduction in

water content which becomes possible in the presence of high dose of superplasticizer and dense packing of cement paste.

Packing of silica fume pastes Indian Scenario NPC was one of the first to use silica fume concrete in their Kaiga and Kota nuclear power projects. Used for one of the flyovers at Mumbai with 75 Mpa concrete. Bandra-Worli sea link project at Mumbai.

- Life is 10% what happens to us and 90% how we react to it.


Rice Husk Ash Obtained by burning rice husk in a controlled manner without causing environmental pollution. When properly burnt it has high SiO2 content and can be used as a concrete admixture. Rice husk ash exhibits high pozzolanic characteristics and contributes to high strength and high

impermeability of concrete. RHA consists of amorphous silica (90% SiO2), 5% carbon, and 2% K2O. The specific surface of RHA is between 40-100 m2/g. India produces about 122 million ton of paddy every year. Each ton of paddy produces about 40

kg of RHA and there is a good potential to make use of RHA as a pozzolanic material. RHA is patented under trade name Agrosilica in USA. When used in small quantity, 10% by weight of cement, Agrosilica enhances the workability and

impermeability of concrete. Surkhi

Commonest pozzolanic materials used in India. Surkhi was one of the main constituents in waterproofing treatments in conjunction with lime and

sometimes even with cement for extending valuable pozzolanic action to make the treatment impervious.

Surkhi is an artificial pozzolana made by powdering bricks or burnt clay balls. In some major works, for large scale production of surkhi, clay balls are specially burnt for this

purpose and then powdered. In the past , the term surkhi was used for a widely varying material with respect to composition,

temperature of burning, fineness of grinding etc. Now the terminology ―calcined clay Pozzolana‖ is used instead of the word surkhi, giving specific property and composition to this construction material.

Surkhi has been used as an admixture in the construction of Krishnaraja Sagar dam, Nizamsagar, Mettur, Lower Bhavani, Tungabhadra, Chambal, Kakrapara, Bhakra and in Rana Pratap Sagar dam.

In Bhakra Nangal dam scientifically made surkhi (burnt clay Pozzolana) was used about 100 tons per day at the rate of 20 % cement replacement.

In view of the large scale availability of fly ash and blended cement the old practice of using surkhi and the modern ―calcined clay pozzolana‖ has lost its importance.

Metakaolin Metakaolin are thermally activated ordinary clay and kaolinitic clay. Highly reactive metakaolin is made by water processing to remove unreactive impurities to make

100% reactive pozzolan. Such a product, white or cream in colour, purified, thermally activated is called High Reactive

Metakaolin (HRM). HRM shows high pozzolanic reactivity and having the potential to compete with silica fume. HRM is marketed in India by trade name ―Metacem‖.

Blast Furnace Slag Nonmetallic product consisting essentially of silicates and aluminates of calcium and other bases. The molten slag is rapidly chilled by quenching in water to form a glassy sand like granulated

material. The granulated material when further ground to less than 45 micron will have specific surface of

about 400 to 600 m2/ kg (Blaine). The chemical composition of Blast Furnace Slag (BFS) is similar to that of cement clinker.

Approximate Oxide Composition of Cement Clinker, BFS and Fly Ash Applications Workability improvement: With fresh concrete mixtures that show a tendency to bleed or

segregate, it is well known that incorporation of finely divided particles generally improves the workability by reducing the size and volume of voids.


Durability to thermal cracking: The total heat of hydration produced by the pozzolanic reactions involving mineral admixtures is considered to be half as much as the average heat produced by the hydration of

Portland cement. Durability to chemical attack: As the pozzolanic reaction involving mineral admixtures causes pore refinement that reduces the permeability of concrete, considerable improvement in the chemical durability of concrete containing mineral admixtures. Rihand Irrigation Project, Uttar Pradesh – 1962 ,Bhakra Nangal dam,Key Tower in Cleaveland Wacker Tower in Chicago ,Union Square in Seattle,Hungry Horse dam in America

Recent Developments in Concrete Ordinary concrete, made with natural aggregate, has a low strength – weight ratio compared to steel. This places concrete at an economic disadvantage when designing concrete members for tall buildings, long span bridges and floating structures There are three ways to address this problem First approach The density or the unit weight of concrete can be reduced by substituting lightweight aggregate in place of conventional aggregate. Lightweight aggregate made by calcination of clay or shale is commonly used to produce structural lightweight concrete that has about one-third less unit weight than conventional concrete. Second Approach Strength of concrete can be raised. High – strength concrete with compressive strengths ranging from 60 to 120 MPa is now available with the advent of superplasticizers or high-range water reducing admixtures. Third Approach Combination of first two approaches Use of high strength lightweight aggregate particles in superplasticized mixtures to produce high-strength, lightweight concrete. Recent Developments in Concrete Superplasticized concrete mixtures perform well on exposure to some aggressive environmental

conditions on account of their low water-cementitious ratio. Durable concrete are now being used in the construction of marine structures for a service life of

100 to 150 years, compared to 40 to 50 years with conventional concrete High-performance concrete, the family of special concrete types, which are superior to

conventional concrete in one or more properties such as workability, strength and durability Superplasticized concrete mixtures in combination with large proportion of fine mineral particles

and viscosity modifying chemical admixtures have been developed to produce self-consolidating concrete

Restrained shrinkage on drying is frequently the cause of concrete cracking as experienced in the construction of thin structural elements such as floor and pavement slabs. Shrinkage-compensating concrete containing expansive cements or cement additives are developed to counteract this problem

The concept of microlevel reinforcement is developed to counteract the deficiency of poor impact resistance of concrete. Fiber-reinforced concrete mixtures containing steel, glass, or polypropylene fibers are being employed to increase impact resistance.

Concrete mixtures containing polymers have shown higher imperviousness and excellent chemical resistance. Used to protect reinforcing steel from corrosion in industrial floors and bridge decks


Heavyweight concrete made with high density materials used for radiation shielding in nuclear power plants.

Pre-cooling of concrete materials eliminated the need for expensive post-cooling operations & made faster construction schedules possible.

Dams are now built with roller-compacted concrete at high speed and with less cost. Recent Developments in Concrete Structural Lightweight Concrete High-Strength Concrete Self-Consolidating Concrete High-Performance Concrete Shrinkage-Compensating Concrete

Fiber-Reinforced Concrete Concrete Containing Polymers Heavyweight Concrete Mass Concrete Roller-Compacted Concrete

High-Performance Concrete Generally the term high-performance concrete (HPC) is used for concrete mixtures that possess

the following three properties High-workability High-strength High-durability

ACI definition HPC is defined as a concrete meeting special combination of performance and uniformity requirements that cannot always be achieved routinely using conventional constituents and normal mixing, placing and curing practices. HPC is a concrete in which certain characteristics as given below are developed for a particular application and environment Ease of placement Compaction without segregation Early age strength Long-term strength and mechanical properties Impermeability Density Low heat of hydration Toughness Volume stability Long life in severe environments

U S Federal Highway Administration (FHWA) guideline for HPC HPC is a concrete that has been designed to be more durable and if necessary, stronger than conventional concrete. HPC mixtures essentially composed of the same materials as conventional concrete mixtures. But the proportions are designed or engineered to provide the strength and durability needed for the structural and environmental requirements of the project. Where is HPC required? Structures constructed in very severe environment like tunnels in sea beds, tunnels and pipes

carrying sewage, offshore piers and platforms, confinement structures for solid and liquid wastes containing toxic chemicals and radioactive elements, jetties and ports, sea link bridge piers and superstructures and high rise buildings, chimneys and towers, foundations and piles in aggressive environment.


Concrete has performed reasonably well in the past in favourable environment, if designed and constructed properly.

Unacceptable rates of deterioration in many recently constructed buildings, bridges and infrastructure projects exposed to hostile environments have caused great concern the world over.

Existing criteria were no longer valid for ensuring long term durability and concrete mixes have to be designed primarily for high performance.

Which are the specifications concrete should meet to classify as HPC ?

Impermeability

Penetration of moisture and harmful chemical ions can seriously affect performance of concrete.

Corrosion of steel or expansive reaction within concrete reduces durability of concrete within a few years of service life.

Since impermeability of concrete is the first line of defense it is extremely important for HPC to have a very low coefficient of permeability i.e. 1 x 10-14 m/sec.

Chloride-ion permeability test (AASHTO 277)

When concrete permeability coefficient is very low, a chloride-ion permeability test (AASHTO 277) is the most appropriate.

The rate of permeation of chloride ions is expressed in terms of Coulombs (C).

When the concrete mix shows 500 C or less current flow in a 6 hours chloride permeability test, it is considered to be virtually impermeable.

Which are the specifications concrete should meet to classify as HPC ?

Dimensional or Volume Stability Volume stability will depend on the following main characteristics of concrete: High elastic modulus Low thermal strain Low drying shrinkage Low creep

Conventional materials can produce concrete of high compressive strengths (above 60Mpa) but the increase in elastic modulus is not proportional. The increase in elastic modulus of concrete can only be achieved when suitable materials in correct proportions are incorporated in concrete mixes. Dimensional or Volume Stability Creep and drying shrinkage strains in normal concrete can be as high as 0.08% each. With proper

materials and mix proportions, it is possible to reduce the 90 days drying shrinkage strain to less than 0.04%.

Creep and drying shrinkage are highly dependant on the aggregate type and content. To achieve high dimensional stability it is desirable to reduce the magnitude of strains by limiting

the total volume of the cement paste in concrete and by using CA which has high strength and high elastic modulus.

With availability of good quality chemical and mineral admixtures it is possible to reduce the volume of the cement paste.

Drying shrinkage is more influenced by excessive mixing water than due to cement. Critical parameters involved in production of HPC Selection of appropriate materials Selection of mix proportions of various ingredients Sequence of mixing component materials


Simple model of concrete Concrete is a composite material consisting of two components-the aggregate skeleton and

cement paste. The permeability, strength, dimensional stability, workability and other properties of concrete

depend both on the aggregate to binder (cement + cementitious materials) ratio and the quality of each of the components which contribute to this ratio.

A dry mixture of well graded fine and coarse aggregates contains around 21-22% voids which needs to be filled up with the binder paste.

In actual practice, to produce workable concrete mixtures, at least 25% cementitious paste by volume is needed.

Influence of cement paste on quality & performance of concrete For low permeability and high strength of concrete it is desirable to reduce both water and

aggregate content. This has an undesirable effect as the stiff consistency of the mix results in difficulty in compaction. On the other hand, on increase of cement paste content in concrete, the strength and

impermeability are improved but dimensional stability is impaired. 35% of cement paste by volume, in HPC, represents an optimum solution in balancing the

conflicting requirements of strength, workability and most importantly dimensional stability. Other material selection factors, which have to be examined for volume stability Mismatch of elastic moduli or coefficient of thermal expansion between the cement paste and

aggregate will cause cracking when the structure is subjected to frequent cycles of temperature variations.

For a fixed cement paste to aggregate ratio, the use of aggregate with a very low elastic modulus results in higher creep and shrinkage of concrete.

Hence, such types of aggregate must not be used in HPC. From the point of long term dimensional stability, concrete mixtures containing CA derived from

limestone or basalt are generally known to perform very well in HPC. Choice of aggregate type FA with a fineness modulus between 2.5 to 3.0 are generally considered adequate. CA should have equidimensional particles (not flaky/elongated) obtained by crushing dense

basalt or limestone or an igneous rock. Single size aggregate is preferred to downgraded aggregate. Larger than 25 mm MSA generally impairs the strength and impermeability of concrete and

therefore are not recommended for use in HPC. Generally, 10-15 mm MSA is considered optimum for HPC. In HPC the aggregate-cement paste interfacial zone is strong, therefore the aggregate can be

the weak link as far as strength is considered. In HPC any internal defect present within the aggregate particles such as micro cracks, large

pores and inclusions of soft minerals, can influence the strength adversely. Reduction of MSA to 10-15 mm often eliminates such internal defects.

Type & quality of cement influence properties of HPC Commercial cements meeting the requirements of BIS code of practice for 53 grade OPCs (IS

12269) may vary considerably in chemical composition and fineness, both of which influence water requirement for normal consistency.

Physicochemical interactions between some cements and water reducing admixtures are known to cause rapid stiffening or slump loss.

Portland cements with higher C3A, high alkali and sulphate contents are more prone to slump loss problem.

Blended cement would perhaps be a better option to avoid loss of workability. Another alternative would be to use pozzolanic (fly ash) material or GGBS as mineral admixtures. Besides other advantages, mineral admixtures reduce heat of hydration and thereby reduce rapid

loss of slump.


Why are mineral admixtures in HPC indispensable ? Mineral admixtures are fine powders mainly composed of silicate glasses or non crystalline silica

which in the presence of moisture, calcium and hydroxyl ions, slowly hydrate to form cementing products.

Commonly used mineral admixtures: GGBS, flyash, CSF (micro silica). Important benefits of mineral admixtures Improve the rheological properties of concrete such as cohesiveness and stability – reduce

bleeding & segregation. Helps in increasing the time of set and reduces the rapid drop of slump thereby allowing adequate

concrete compaction. Improves impermeability of concrete and the strength development with age. Cement paste microstructure without mineral admixtures are found to be heterogeneous and

having local areas of high concentration of large pores. Factors on which performance of mineral admixtures would depend Fineness, particle size, pozzolanic and / or cementitious characteristics, degree of uniform

dispersion and curing conditions. A combination of 10% micro silica with 15% fly ash or slag by volume is optimum and gives

excellent particle packing in concrete. Role of chemical admixtures in HPC Dispersion of all fine particles in the mix, reduction of water content in HPC while maintaining the

desired workability Improving consistency, controlling the time of set and providing protection against deterioration by

freezing and thawing cycles. Conventional water reducing chemical admixtures (derivatives of sulphonated lignin) causes

excessive retardation. High range water reducing admixtures (Superplasticisers) provide high consistency at very

low water content without causing excessive set retardation. Superplasticisers are generally used in quantities ranging from 0.8 to 2 % by weight of

cementitious materials. Mix Proportioning of HPC The following considerations are generally required to develop the procedure for mix proportioning

of HPC: Cement paste to aggregate ratio Strength Water content Cement content Type and dosage of mineral admixture Type and dosage of chemical admixture Fine to coarse aggregate ratio

Cement paste to aggregate ratio By using a suitable CA of MSA of 10-15 mm, adequate stability of HPC concrete (elastic behaviour, creep and shrinkage) can be obtained at a fixed cement paste to aggregate ratio of 35-65 % by volume. Table No.1 : Relationship between average compressive strength and maximum water content per m3 of concrete


Volume of cement & cementitious material The cement paste volume is fixed at 35% for HPC. The cement paste contains anhydrous cement, mineral admixtures, water and entrapped air. The HPC mixes tend to trap around 2% air by volume when no air entraining admixtures are

used. Therefore if the volume of water (Table No.1) and air content (2%) are known the balance volume

of cement paste will be contributed by cement and mineral admixtures. Dosage of superplasticiser & FA/CA ratio Superplasticisers or high range water reducing chemicals can only be used in HPC. Superplasticisers are produced using either the derivatives of naphthalene or melamine. The selection of the derivative type would depend on the ambient temperature conditions.

(Naphthalene derivatives are more suitable in warm ambient temperature & melamines are preferred in cold ambient temp. environment)

For the first trial batch 1 % (solid content) superplasticiser by weight of cementitious material is recommended.

For the first trial batch, a 2:3 ratio between the FA and CA may be followed. Steps required to be followed for computing the proportion of the first batch of HPC mix

Sequence of batching various materials

Production of HPC involves the following 3 interrelated steps Selection of suitable ingredients for concrete having the desired rheological properties, strength

etc. Determination of relative quantities of the ingredients in order to produce durability Careful quality control of every phase of the concrete making process

Applications of HPC Long span bridges Offshore, oil drilling platforms High-rise buildings Tunnels Pavements

Nuclear structures Hydraulic structures Drainage Precast units


READY MIXED CONCRETE Introduction

If instead of being batched and mixed on the site, concrete is delivered ready for placing from a

central plant, it is referred to as RMC.

Central batching plant and mixing plant operates under very rigorous quality control systems.

Proper care during transportation ensured by use of agitator trucks [transit mixers].

Apart from being a material, RMC is also a service.

Ready Mixed Concrete

The various materials are mixed in correct proportion except may be the liquid part to give the end

user the desired results.

The end user does not have to bother about the various ingredients, wastage and storage of

materials and still gets the desired results.

Architect J.H.Magens from experience knew that concrete does not harden at frosty temperatures.

He first patented the idea of cooling concrete so that it can be transported over a longer distance.

RMC was patented in Germany way back on 10th January 1903.

In 1907, he discovered that the available time for transportation could be prolonged not only by

cooling fresh concrete but also by vibrating it during transportation. He obtained his second patent

for this.

First RMC plants were set up in Hamburg and Berlin in Germany.

One of the old RMC plants near Stuttgart, Germany, now refurbished and modernized

The first concrete mixed off site and delivered to a construction site was effectively done in

Baltimore, United States in 1913 just before the First world War.

The increasing availability of special transport vehicles, played a positive role in the development

of RMC industry.

The first concept of transit mixer was born in 1926 in the United States.

In 1939, the first RMC plant was installed in United Kingdom and first specifications on RMC were

published in UK.

Cement consumption by RMC plants in 1990

USA : 75%

Japan : 70%

U.K : 43%

Malaysia : 16%

RMC – INDIAN SCENARIO

In the 1950s RMC in India was initially used at large dam sites like Bhakra Nangal, Koyna wherein

large quantity of RMC was produced and transported either manually or mechanically using

ropeways and buckets or conveyor systems.

In 1979, RMC concept was introduced at the Tata Power Company‘s site utilising an imported 60

cum/hour computerised batching plant from Germany.

Concrete batching and mixing plant installed at Tata Power Company

RMC plant at 500MW Unit 6 Trombay Thermal Power Plant (1979-84)

The first attempt was made to commercially market RMC at Pune in the year 1991.

This attempt failed at the hands of a skeptical market.


Types of RMC

Transit mixed

Central mixed

Shrink mixed

Transit mixed

Materials are batched at central plant and loaded in trucks.

As the truck nears the site, water in measured quantity is added with ingredients to form the

concrete of required workability.

Transit mixing permits a longer haul and is less vulnerable in case of delay.

But the capacity of a truck used as a mixer is only about three quarters of the same truck used

solely to agitate premixed concrete.

Central mixed

Concrete is mixed in a central plant and transported in trucks to work site.

Truck is provided with an agitator which solely revolves to prevent undue stiffening of concrete

and prevent segregation.

Separate water tank is not required as in transit mixed and hence capacity is more.

But distance of supply to be limited.

Shrink mixed

Concrete is partially mixed in central plant and completely mixed in transporting- combination of

both central mixed and transit mixed.

Due to partial mixing in central plant, concrete capacity of truck can be increased by reducing

capacity of water tank.

Agitating speed is generally between 2 and 6 revolutions per minute while the mixing speed vary

between 4 and 16 revolutions per minute.

Speed of mixing affects the rate of stiffening while the total number of revolutions controls the

uniformity of mixing.


Why Use RMC ?

The quality of concrete will be superior than site mixed concrete. (depend on the controls &

checks exercised at site and at RMC producer‘s plant).

Prevention of wastage of materials on site due to poor storage conditions and repeated shifting of

the mixer location.

Using RMC can cause less congestion and better housekeeping on the site resulting in efficient

working environment

The modern RMC plants have an automatic arrangement to measure surface moisture on

aggregates.

This helps in controlling the w/c ratio which results in correct strength and durability.

RMC plants have proper facilities to store and accurately batch concrete admixtures (chemical &

mineral).

This accuracy is useful to improve properties of concrete both in plastic and hardened stage.

RMC plants have superior mixers than the rotating drum mixers generally used for mixing

concrete materials at site.

RMC plants have efficient batching and mixing facilities which improve both quality and speed of

concrete production.

Temperature control of concrete in extreme weather conditions can be exercised in a much better

manner than done at site.

Computerized batching in early 1980s at RMC plant

Modern computerized control room of RMC plant

Pan mixer at RMC plant

Ice flaking machine at RMC plant

RMC being pumped at site into the formwork

If we did all the things that we are capable of doing, we would literally astound ourselves.-S.R.Publications


Why Use RMC ?

RMC helps encourage mechanization and new technologies like pumped concrete, bulk

transportation of cement, production of SCC & HPC.

New materials like micro silica and fibres can be safely used in RMC which in conventional

concrete may pose problems.

Bulk cement being delivered and unloaded into RMC plant‘s silos

Introduction of RMC improves the rate of supply of concrete in the formwork and thereby

automatically improves quality of formwork, layout of rft. steel and its‘ detailing and safety/strength

of scaffolding and staging.

Adequately designed formwork is required to match the speed of RMC concrete supply

Proper layout of rft. is required to facilitate placing & compaction of RMC

Does RMC always assures quality of concrete ?

Not necessary - Even RMC needs to be supervised and adequate care has to be taken while

batching and mixing at RMC plant, transporting from the RMC plant to site and placing,

compacting, finishing and curing concrete at site.

Perfect understanding and co-operation b/w the RMC supplier, the agency receiving concrete at

site and the owner‘s supervising agency.

The requirements of concrete should be clearly specified and arrangements to receive RMC at

site should be properly organized.

Badly placed RMC results in segregation & large honeycombs and voids

Code of practice for RMC

IS 4926 : second revision in 2003 – adequate coverage to meet the needs of both the RMC

suppliers and consumers of RMC.

What is required to be specified for RMC?

Characteristic strength for grade (N/mm2).

Target workability or slump in mm at site.

Exposure conditions for durability requirements

Max. w/c ratio

Min. cement content

Type of cement

Mineral admixture and its proportion (kg/m3)

MAS

Rate of gain of strength (for formwork removal or prestressing etc.)

Max. temp. of concrete at the time of placing ( in extreme climatic conditions or in case of massive

concrete pours)

Method of placing

Rate of supply desired to match the placing and compaction speed planned at site.

Quantity of concrete required

Lift and lead of concrete transportation and placement at site

Information required to be supplied by the RMC supplier


What are the checks needed at site prior to receipt of RMC ?

Reinforcement layout for proper concrete placement without segregation.

Adequacy of formwork to take the hydrostatic pressure and adequacy of loading on propping

system to match the speed of placing.

Openings and chutes provided, at predetermined locations between reinforcement bars to lower

the placing hose (if pumped concrete is planned) to avoid segregation of concrete.

Incorrect planning & placing of rft. steel causes segregation of concrete

For pumping of concrete openings of adequate size are required to be left within the rft. cage

Incorrect placing of concrete causes segregation

Mortar from concrete stick to the rft. and CA fall below

Dropping concrete from a ht. more than 1m can cause segregation

Correct method of concrete placing without segregation

Adequacy of manpower and equipment for placing, compacting, finishing and curing of concrete.

Proper approach for transit mixers free from all encumbrances e.g. water logging, material

stacking etc.

Proper platform to receive concrete.

Proper precautions required to be taken to ensure that concrete from the transit mixer is unloaded

at the fastest possible speed and does not take more than 30 minutes.

If pumping is proposed, the location of the pump should be approachable.


Checks needed at site during concreting

Proper co-ordination b/w the RMC supply and placing & compacting gangs.

Proper signaling or communication at site is necessary.

Workability of concrete within accepted limits.

Adequacy of cohesiveness of concrete for pumpability.

Ensure that water or chemical admixtures are not added during transportation by RMC‘s

unauthorized persons and without the knowledge of the site in charge of the consumer.

Temp. of concrete at the time of receipt at site (if specified).

Continuous and steady supply at site and speedy unloading of the transit mixers.

Monitor speed and progress of placing to avoid formation cold joints.

Monitor proper placement without segregation.

Monitor placement of concrete at the closest possible point to its final location.

Arrange for curing as soon as finishing is completed. This is especially in case of slabs, pathways

and roads in hot/warm weather.

Formation of cold joints due to delayed placing of concrete in layers for a prestressed concrete

girder

Advantages of RMC

Good quality control hence better durability and strength

Speedy construction through mechanized operations

Lower labour and supervising costs

Disorderly storage of concrete ingredients at site can be avoided

Minimisation of material wastage

Clean environment

Shortage of water and interruptions with power supply at site are eliminated

Large volume production possible

Can be pumped horizontally and vertically

HPC ensuring durability & strength up to 60 MPa can be produced

RMC in India - Constraints

Mechanized approach, modifications in contract documents and specifications required by CPWD,

PWD which are for in-situ concrete.

High capital cost of batching plant.

Costlier than site mixed concrete by 10%.

Sales tax component 4-10% on sale of RMC by government.

Lack of bulk cement handling units- 50 kg sacks are to be replaced by jumbo bags.


REACTIVE POWDER CONCRETE

(RPC)

Introduction

Emerging technology that lends a new dimension to the term HPC.

Belongs to the class of chemically bonded ceramics.

Molecular consolidation and bonding takes place by chemical reaction at ambient or slightly

elevated temperatures unlike usual ceramics wherein solid fusion takes place at high

temperatures.

Ultra high strength and high ductility composite with advanced mechanical and physical

properties.

Developed by P. Richard and M. Cheyrazy at Bouygues Laboratory in France in early 1990s.

The world‘s first RPC structure, the Sherbrooke Bridge, Quebec, Canada, was constructed in July

1997. (33,000 psi ~230MPa)

A composite material that will allow the concrete industry to optimize material use, generate

economical benefits, and build structures that are strong, durable and sensitive to environment.

RPC

It consists of a special concrete where the microstructure is optimized by precise gradation of all

particles in the mix to yield maximum density.

It extensively uses the pozzolanic properties of highly refined silica fume and optimization of the

Portland cement chemistry to produce the highest strength hydrates.

HPC to RPC

HPC contains mineral components and chemical admixtures having a specific characteristic which

leads to achieve a maximum compressive strength of the order say 120-150MPa or so.

However, at such a level of strength, the coarse aggregate becomes the weakest link in concrete.

In order to increase the compressive strength of concrete even further, the only way is to remove

the coarse aggregate.

This philosophy has been employed in Reactive Powder concrete (RPC).

RPC

The ingredients of RPC are basically fine powders of cement, fine sand, quartz, very hard

particles of granite, silica fume (in optimal dosages), superplasticizers, steel fibres (optional).

Optimal quantity of superplasticizers is added to improve the workability of concrete, while W/C is

decreased.

By optimizing the granular packing of fine dry powders, an ultra-dense matrix is achieved

contributing to high compressive strength and ultra-high durability properties.

Extremely low porosity, low permeability, limited shrinkage and increased resistance to corrosion

contributes significantly to high durability characteristics.

Principle

Elimination of coarse aggregate for enhancement of homogeneity.

Utilization of pozzolanic properties of silica fume.

Optimization of the granular mixture for the enhancement of compacted density.

Optimal usage of superplasticizer to reduce w/c ratio and at the same time improve the

workability.


Application of pressure (before and during setting) to improve compaction.

Post-set heat-treatment for enhancement of the microstructure.

Addition of small sized steel fibres to improve ductility.(optional)

Composition

A very dense matrix is achieved by optimizing the granular packing of the dry fine powder. (Which

results to gain compressive strengths ranging from 200MPa to 800MPa).

Microstructure enhancement of RPC is done by heat curing.

Heat curing(normally at 900C ) is performed by heating the concrete at normal pressure after it

has set properly. This considerably accelerates the pozzolanic reaction, while modifying the

microstructure of the hydrates that have formed.

The high strength of RPC makes it highly brittle. So straight steel micro-fibres are generally added

to RPC to enhance its ductility.

o Straight steel fibres usually are about 30mm long

o Diameter of 0.15mm. (the fibres are introduced into mixture at the rate of 1.5-3% by

volume).

The major parameter that decides the quality of mix is its‘ water demand.

Grades of RPC

Comparison of HPC (80) Mpa and RPC 200

Mechanical Properties

Placing: The placement and curing of RPC (UHPC) can be performed using procedures similar to

those already established for use with HPCs.

Vibration: The fluid mix is virtually self-placing and requires no internal vibration. If required,

external form vibration causes the mix to smoothly flow into place.

Setting Time: An initial set of 24 hours, the curing process requires at least an additional 48

hours including a vapor bath at a constant 88 °C (190 °F).

Waterproofing: RPC have an ultra–dense microstructure giving good water proofing.

Ductility: As fracture toughness, which is a measure of energy absorbed per unit volume of

material to fracture, is higher for RPC, it exhibit high ductility.

Durability: RPC has ultra high durability characteristics resulting from its extremely low porosity,

low permeability, limited shrinkage and increased corrosion resistance.

The durability of RPC for different properties as compared to HPC

Compressive Strength

Ultra-high-performance concrete (UHPC) consists of a steel fiber-reinforced, reactive-powder

concrete that provides a compressive strength of 30000psi(1000psi ~ 7MPa), more than twice that

of any high-performance concrete.

The incorporation of fibres and use of heat curing was seen to enhance the compressive strength

of RPC by 30 – 50%.( the incorporation of fibres did not affect the compressive strength of HPC

significantly).

Flexural Strength

Plain RPC was found to possess marginally higher flexural strength than HPC. The increase of

flexural strength of RPC with the addition of fibres is higher than that of HPC.

(NC = Normal Curing, HWC = Hot Water Curing)

Flexural Strength(as per IS 516) at 28 days, MPa

Compressive Strengths of RPC and HPC

Water absorption of RPC and HPC age-wise


External water permeability of RPC and HPC

Rapid chloride permeability test(as per ASTM C 1202)

RPC mixture designs from literature

Mixture Proportions of RPC and HPC

\

Advantages

RPC is a better alternative to High Performance Concrete and has the potential to structurally

compete with steel.

Its superior strength combined with higher shear capacity results in significant dead load reduction

and limitless structural member shape.

With its ductile tension failure mechanism, RPC can be used to resist all but direct primary tensile

stresses. This eliminates the need for supplemental shear and other auxiliary reinforcing steel.

RPC provides improved seismic performance by reducing inertia loads with lighter members,

allowing larger deflections with reduced cross sections, and providing higher energy absorption.

Its low and non-interconnected porosity diminishes mass transfer making penetration of liquid/gas

or radioactive elements nearly non-existent.

Cesium diffusion is non-existent and Tritium diffusion is 45 times lower than conventional

containment materials.

Limitations

As the RPC mixture design, the least costly components of conventional concrete are basically

eliminated by more expensive elements.

In terms of size-scale, the fine sand used in RPC becomes equivalent to the coarse aggregate of

conventional concrete, the Portland cement plays the role of fine aggregate and the silica fume

that of cement.

RPC should be used in areas where substantial weight saving can be realized and where some of

the remarkable characteristics of the material can be fully utilized.

Remarks

By understanding the microstructure properties and parameters a wide variety of cement based

composite laminates may be provided.

Tensile strengths of the order of 50MPa and strain capacity on excess of 1% are possible.

Polymerize and synthetic fibres alter the energy absorption of the composites significantly.

Uniform fiber distribution at various size scales improves composite performance, as the specific

fiber spacing decreases, the strength of the brittle matrix is increased.

Small micro-fibres stabilize the micro-cracks and increase the strength.

Ultra-High-Performance Concrete extends precast strength.

Life is not lost by dying; life is lost minute by minute, day by dragging day, in all the thousand small uncaring

ways.


Self-Consolidating Concrete (SCC)

High-workability concrete mixtures that are commercially known by various names such as self-

consolidating concrete, self-compacting concrete, self-levelling concrete, or rheoplastic concrete

Self-Compacting Concrete (SCC) was first developed in Japan around the year 1980.

Professor H. Okamura from the University of Tokyo, Japan is mainly responsible for initiating the

development of such concrete.

It was developed to overcome deficiency of skilled manpower and problems of placing and

compacting congested civil engineering structures.

Necessity of SCC

Technology for SCC

SCC should be so designed that it would level by itself and further it would deform itself by self

weight

Major difficulty was on account of contradictory factors that the concrete should be fully flowable

but without bleeding or segregation

SCC should have higher viscosity to ensure flowability while maintaining no sedimentation of

bigger aggregates


Different concepts for production of SCC

There are three ways in which SCC can be made

1. Powder Type : SCC is made by increasing the powder content

2. VMA Type: Made by using viscosity modifying admixture

3. Combined Type : Made by increasing powder content and using VMA

o The above three methods are made depending upon the structural conditions,

constructional conditions, available material and restrictions in concrete production plant

etc.

How is SCC made?

High-range water reducing admixture [superplasticizer] is used to provide the high flowability of

the mix, much like a high slump concrete

Second, the aggregate content is proportioned. The size and shape of the coarse aggregate are

very important to the successful manufacture of SCC

The fluid properties are altered to provide a cohesive mix that will keep the aggregate and paste

together

As with any concrete mix, aggregate size must be limited to that which will pass through rebar

openings.

Rounded aggregate is desirable over angular aggregate because angular aggregate will have a

tendency to lock together. The coarse aggregate content will usually drop in an SCC mix, resulting

in a sand and aggregate ratio of 0.50 or greater.

Constituents of Normal Vs SCC (by Volume)

Materials for SCC

Cement: OPC 43 or 53 grade

Coarse aggregates: max.size 20mm. In case of congested rft. 10 to 12 mm desirable. Well graded

cubical or rounded aggregates desirable.

Fine aggregates: can be natural or manufactured

Water: Potable water to be used

Chemical admixtures:

o Superplasticizers are an essential component of SCC to provide necessary workability.

o The new generation superplasticizers termed poly-carboxylated ethers (PCE) is particularly

useful for SCC.

Mineral admixtures:

o Fly ash : to improve quality and durability

o Ground Granulated Blast Furnace Slag (GGBFS): to improve rheological properties

o Silica Fume: to improve mechanical properties


o Stone powder: Finely powdered lime stone, dolomite or granite may be added to increase

the powder content

o Fibres: enhance the properties of SCC in the same way as for normal concrete

Rheological properties of SCC

An SCC mix has the following characteristics:

Non-segregating: The aggregate will stay in suspension in the mix as it flows into the form

Non-bleeding: Water will not rise to the top of the mix or be observed on the outer edges of a flow

test

Vibration: No vibration is required during placement and SCC will flow around rebar

Flow spreads: Flow spreads of 18-inch diameter or greater are obtainable

Set time: The initial set time in many SCC mixes will increase upwards of 90 minutes, depending

on the admixtures used and water content of the mix.

Requirements for SCC

A concrete mix can only be classified as self-compacting if it has the following characteristics

o Filling ability

o Resistance to segregation

o Passing ability

Slump Flow Test & T50cm Test

Position the slump cone at the centre of the levelled flow table (Figure b).

Pour the concrete with a scoop from top

without tamping to fill the slump cone

completely. Strike off excess concrete.

Lift the cone vertically without any jerks

and allow the concrete to flow freely.

Note the time required for the concrete to

cover 50 cm diameter spread circle.

(T50cm time is the time required for the

concrete to cover 50 cm dia. spread circle

from the time the slump cone is lifted)

(Figure c).

Measure the average flow diameter (D1 +

D2)/2 of concrete after concrete stops

flowing (Figure d)

This value is known as slump flow value

(mm).

The permissible range of values for slump flow are 650-850mm and T50cm test time are 2-5

seconds.

It is to be noted that the material of the base plate can have some influence on these values.

In any case same base material or rather the same base plate is to be used in all tests, both in the

laboratory and on site.


V Funnel Test

L Box Test – Procedure

Blocking value for SCC should be in between 0.8 to 1.

If the blocking value is less than 0.8 it indicates viscosity is too high.

A ratio close to 1, indicates viscosity on the lower side but in an acceptable range.

If the ratio is more than 1, it indicates false results.


„U‟ Box Test – Procedure

Fill Box Test

This test is used to assess the filling ability of SCC.

The equipment consists of transparent rectangular box with a number of obstructions through

which concrete is made to flow.

Procedure

SCC vs. NC


Advantages of SCC over NC

Initial Mix Composition

Total powder content to be 160 to 240 litres(400-600 kg) per m3

The sand content may be more than 38% of the mortar volume

Coarse aggregate content should normally be 28 to 35% by volume of the mix

Water/cement ratio is selected based on strength. In any case water content should not exceed

200 litres/m3

Production and Placing

Aggregates: Aggregates should come from same source. There should not be much variation in

size, shape and moisture content

Mixing: Any suitable mixer could be used. Mixing time need to be longer than for conventional

concrete.

Placing: Formwork must be in good conditions to prevent leakage. The following rules are to be

followed to minimise the risk of segregation

-limit of vertical free fall distance to 5m

-limit the height of pour lifts (layers) to 500mm

-limit of permissible distance of horizontal flow from point of discharge to 10 m

Curing: SCC tends to dry faster and may cause more plastic shrinkage cracking. Initial curing

should be commenced as soon as practicable.

Mix Design

Procedure: The following sequence is followed

Determine the desired air content

Determine the coarse aggregate volume

Determine the sand content

Design the paste composition

Determine the optimum water to powder ratio and superplasticizer dosage in mortar

Finally the concrete properties are assessed by standard tests


Hardened properties of SCC

Hardened properties of SCC vary with the mix design

High-strength concretes are possible

Other hardened properties, such as modulus of elasticity, are similar to regular concrete mixes as

well

Durability and creep are unknown at present due to the lack of long-term performance studies

Testing to date shows that most SCC mixes exhibit similar shrinkage values to that of normal

concrete

Cost

The cost of an SCC mix will be greater than a standard concrete mix. The cost of an SCC mix, like

other concrete mixes, will be a function of strength, flow radius required and other admixtures that

have been added.

On an average, a general increase of 5 to 10 % can be expected.

Economics of SCC

Cost difference between normal concrete and SCC is not only the extra cost incurred for new

generation superplasticizers but also reduction in cost due to saving in formwork cost, purchase of

vibrating equipments as well as material saving for concrete cosmetics repair. These costs

coupled with durability of SCC, the gap between normal concrete and SCC is narrowed down

Applications

Concreting underwater piles and columns [many of the VMAs used by the ready-mixed industry

contain admixtures that are used to make anti-washout concretes and grouts]

Piles and columns with dense rebar cages [SCC was developed to alleviate the need to vibrate

concrete in structures containing dense rebar]

High-strength piles and columns [ The generally higher fines content of SCC mixes lends itself to

high strength concrete]

Limitations to use

The long-term behavior of SCC is yet to be tested

Long-term shrinkage, creep and durability testing data on SCC is not available domestically.

Application of SCC has not occurred in areas where these issues are important.

The use of SCC is only limited by the imagination of the industry. The more people know about

the characteristics of this material, the more they will be able to apply it to their own special

applications.

Examples

Akashi-Kaikyō Bridge

The Akashi-Kaikyō Bridge also known as Pearl Bridge in Japan was completed in 1998 and is the

world's longest suspension bridge (measure by the length of the centre span of

1,991 metres (6,532 ft)). It links the city of Kobe on the mainland of Honshū to Iwaya on Awaji

Island by crossing the busy Akashi Strait. It carries the part of the Honshū-Shikoku Highway.

The volume of the cast concrete in the two anchorages amounted to 290000m3. A new

construction system, which makes full use of the performance of self-compacting concrete, was

introduced for this. The concrete was mixed at the batching plant beside the site, and was

pumped out of the plant. It was transported 200 m through pipes to the casting site, where the

pipes were arranged in rows 3 to 5 m apart. The concrete was cast from gate valves located at 5

m intervals along the pipes. These valves were automatically controlled so that a surface level of

the cast concrete could be maintained. In the final analysis, the use of self-compacting concrete

shortened the anchorage construction period by 20%, from 2.5 to 2 years


Recent Developments in Reinforcing Steel

Introduction Reinforcing bars – Demands by Civil Engineers

• High tensile strength

• Corrosion resistance

• Better bond strength with concrete

• Easy weldability at necessary laps and locations

• Steel which could withstand close bends without exhibiting surface cracks

High strength steel and high strength concrete combination reduces the size of RCC members.

Development Scenario

Mild steel ribbed bars for better bond with concrete.

High strength deformed bars (H.S.D.Bars).

Cold twisted deformed bars (C.T.D Bars).

Thermo–processed rebars – commonly referred to as T.M.T. bars.

High-strength rebars using micro-alloys.

Ribbed bars with corrosion resistance through additions of Cr and Cu in the steel used for thermo-processed rebars

T.M.T.42 C.R.S bars of TISCO.

Mild steel ribbed bars

Mild steel ribbed bars were introduced around 1960.

For preventing ‗slip‘ and improving the mechanical bonding between steel rebars and cement

concrete.

Rolling mills in different countries followed different pattern of ribs.

All standards specified only the bond strength & testing procedures.

High Strength Deformed Bars

( H.S.D. BARS)

Introduction of H.S.D bars reduced quantity of steel used in RCC.

Steel bars having strength 400 – 500 N/mm2 with adequate ductility.

Cold Twisted Deformed Bars (C.T.D bars )

C.T.D. bars were introduced during 1970s.

Cold working increases steel strength – hence reduction of quantity of steel used in R.C.C.

structures.

Yield strength of around 400 N/mm2.

Higher strength C.T.D. bars did not gain acceptance since elongation values dropped to 12%

while the strength increased.

Other drawbacks – surface stress and visible cracks due to twisting which led to higher corrosion

rate and durability problems.

When cold twisting is done, the bar is subjected to very severe mechanical stress as the material

is deformed while in the plastic stage and any significant invisible defect would cause the bar to

fracture during the twisting itself.

This resulted in the protective scale layer to fall off during twisting.

Therefore, most of the European countries gave up the use of C.T.D bars within a few years.

However, in India C.T.D bars were commonly used upto 1992 under the brand name ―Torsteel‖.


T.M.T. Bars

( Online Quenching and Self-tempering)

Today‘s requirements – low cost deformed bars with min. guaranteed yield strength 500 N/mm2

and having adequate ductility.

Requirements of high seismic zones – 60% of land in India falls under high seismic zones (Zones

III, IV & V).

In the mid-eighties, rapid quenching technology known as ―Thermex‖ was developed & patented.

Tamm, an international authority on steel rft. was the inventor of the quenching system.

Thermex technology adopt online quenching and self-tempering of steel, with a guarantee for

elongation of above 16% in Fe500 & Fe600 grade.

The quenching and self-tempering process is known as ―Thermo Mechanical Treatment‖.

Thermex process

In this process the heat energy of the rolled bar, after the finishing stand of the rolling mills is

used.

In the normal process the heat of rolled bars (9500C - 10000C) is wasted and bars are allowed to

cool in the cooling bed to the ambient temperature.

The rolled bar leaving the rolling mill is guided through specially designed thermex pipes wherein

the surface temperature of 9500C - 10000C of the bar is brought down in a very short time,

approximately 1 sec, on account of drastic, intense and uniform cooling by high pressure water.

The system is installed between the last stand of the rolling mill and cooling bed and always

produced rebars with a concentric tempered ―martensite‖ periphery and ―pearlite ferrite core‖.

The thermex system permits production of rebars of different yield strengths by varying the

thickness of the hardened periphery by changing the quenching rate.

Basic Thermex Process

Objectives

• Low cost, high

strength rebars with high

ductility

• High yield strength of 500 N/mm2 or more.

• An elongation of around 20%.

• Excellent weldability

• Rebars that can be safely used in high earthquake-prone areas.

By this process, while the surface temperature of the steel bar is drastically brought down, the

temperature of the core is not affected to a large extent.

The pre-determined cooling of the bar by high pressure water transforms the peripheral structure

of the steel bar to ―martensite‖ and would need to be annealed to make the bars fit for use as

reinforcing bars.

As the rolled bars at around 9500C - 10000C passes at its normal speed through the Thermex

process/system, the periphery / surface is subjected to rapid cooling while the core remains

unaffected for a short time.


Cross section of thermo-processed rebar depicting the high strength periphery with a soft core

Micro-structure of thermo-processed bars showing martensite periphery and ferrite pearlite core

and rolled steel

Annealing is achieved through the heat available at the core.

The peripheral / surface part of the bar has been transformed into ―martensite‖ begins to gain

temperature from the core as soon as the bar leaves the Thermex system.

The thermal exchange in the steel bar is continued till it reaches the equalising temperature.

The difference in the peripheral and core temperature equalises finally at around 6000C.

During slow cooling on the cooling bed, the core transforms into ferrite and pearlite.

The result of this process is that the temporal martensite (25 – 35 % of area) is in the hardened

periphery and the soft core which is fine-grained for ferrite-pearlite (65 – 75%) is in the middle of

the steel bar.

The microstructure of thermo-processed bar, for martensite periphery and ferrite pearlite core is

different from the microstructure of rolled steel which has not undergone thermo processing.

Typical properties of thermex rebars


High-strength rebars using micro-alloys

High-strength rolled bars of high strength using micro alloys such as Niobium (Nb), Vanadium (V)

and Titanium (Ti) in various proportions were also tried out in advanced countries.

These rebars have limited usage due to the use of costly micro-alloys in their production.

Comparisons of different processes of making high-strength rebars

Thermo-mechanical Treatment

Many rolling mills adopt quenching and tempering technologies engineered by them and produce

steel which do not meet the requirements or which do not attain mechanical properties of

T.M.T.bars produced by Thermex process.

Since the method of steel bar production is not detailed out in I.S. 1786-1985, many rolling mills

produce bars under the T.M.T. brand name without any technology change in the mill set up and

also have no quenching system.

They take shelter under the pretext that hot rolling mills heat the billets and ingots and so thermal

work is involved.

They sell these bars as T.M.T.bars.

Failures of structures in Ahmedabad due to the Bhuj earthquake of Jan. 2001 is due to the use of

the T.M.T.bars produced by mills which do not adopt Thermex Technology.

Corrosion Resistance of T.M.T.Steel bars

Resistance to corrosion of rebars depends on the chemical composition and also on processing

parameters.

By reducing the carbon content in rebars, weldability improves, while strength is reduced.

To improve the strength of rebars with reduced carbon content, the manufacturing process has to

be modified as :-

• Cold twisting

• Addition of Micro-alloys in production process

• T.M.T. rebars ( online quenching and tempering)

In the first two processes, micro-cells are formed on the surface which may accelerate corrosion.


In the production of T.M.T.bars, strength of bars is carefully controlled by the water pressure for

specific diameter and length of quenching line.

In T.M.T. bars, the surface is tempered by the core heat of rebars giving a uniform stress-free

tempered martensite rim with very few micro-cells.

This provides an optimum combination of strength, ductility, toughness and corrosion resistance to

some extent.

Addition of some micro-alloying elements in the production of T.M.T.bars enhances the corrosion

resistance of the bars.

T.M.T. 42 C.R.S. bars

Tata Iron & Steel (TISCO) and Steel Authority of India (SAIL) are the two major producers of

T.M.T.bars.

Chemical composition of TISCON –

T.M.T. 42 C.R.S. bars %

Online control of T.M.T. bars

The control variables in the online process of quenching and tempering are

• Length of quenching line

• Cooling water flow rate

This can be easily adjusted during the rolling process and have a strong effect on yield strength of

the bars.

Effect of cooling water flow rate and quenching time on yield strength

Process for T.M.T.42 C.R.S.bars - Advantages

Costly addition of micro-alloying elements can be optimized.

High yield strength, good weldability, good cold forming properties and good bonding properties

can be achieved with lower carbon content.

Even a close bend will not cause surface cracking.

As the carbon content is less than 0.42% in T.M.T.42 C.R.S.bars, they can be easily welded.

Identification of quenched and tempered T.M.T. rebars

Field Test: Draw a random sample and file the surface of rebar. While filing the surface, check

whether the surface is hard or soft. Hard surface indicates T.M.T.

Take a cross section of the sample steel bar and smoothen to a fine polished state using grinder

or emery paper. Dip the smooth end in nitrol solution (5 to 10% nitric acid with balance ethyl

alcohol).

The result should show a uniform tempered martensite periphery with a soft core in case of good

quenched and tempered T.M.T bars.

Figure illustrates a good quenching & tempering ―Thermex‖ rebar.

The uniform and concentric hardened periphery and the softer

core are clearly visible.

Such bars will have desired tensile strengths coupled with high

elongation as required in high seismic zones.


Figure shows the uniformly tempered martensite periphery.

Depending on the size and grade, the hardened periphery

will be about 20 to 30% of the bar cross-sectional area for good

quenching and tempering rebars.

This is the ideal rebar for civil construction.


Figure shows a 25mm Thermex 500 rebar rolled by Metro Ispat,

Maharashtra.

The uniform tempered martensite periphery is visible.

Elongation measured was 19%.

This is a good rebar for civil construction.


Figure illustrates a highly over-quenched rebar.

The hardened periphery is about 60% of the total cross-sectional

area.

Produced by mill personnel who are not fully trained in quenching

and tempering technology, these bars will have high yield strength

and very poor ductility.

It is not advisable to use these in civil

construction.

Figure illustrates a bar produced by a bad quenching and tempering

system.

The quenching is not uniform.

Such bars should never be used in civil construction as their quality is

inconsistent and properties will vary from bar to bar.

Corrosion of steel in concrete & protective methods

Corrosion of reinforcement

Steel reinforcement plays an important role in concrete structures as concrete alone is not

capable of resisting tensile forces to which it is often subjected.

Good physical & chemical bond must exist b/w rft. and concrete surrounding it.

Due to inadequacy of structural design and / or construction, moisture and chemicals like

chlorides penetrate concrete and attack steel.

Steel oxides and rust is formed. This results in loss of bond b/w steel and concrete which

ultimately weakens the structure.

The chemical conversion of iron (Fe) to iron oxide (Fe2O3) and later to iron hydroxide (Fe(OH)2)

causes an increase in volume (expansion), over 6 times its original volume, within the concrete

mass and therefore concrete cracks under the tensile stresses and spalls.


Increase in volume of steel as corrosion process progresses

Cracks in concrete due to corrosion of steel

Corrosion of steel and spalling of concrete on a roof

slab

Corrosion of steel in concrete is an electrochemical

process.

In the steel, one part becomes anode and the other part

becomes cathode connected by electrolyte in the form of

pore water.

Simplified model representing corrosion mechanism

The positively charged ferrous ions Fe++ at the anode pass into solution while the negatively

charged free electrons e- pass through the steel into cathode where they are absorbed by the

constituents of the electrolyte and combine with water to form hydroxyl ions (OH)-.

These travel through the electrolyte and combine with the ferrous ions to form ferric hydroxide

which is converted by further oxidation to rust.

S.R.P** NICMAR-2ND TERM- N.M.T 6

Anodic & Cathodic reactions

Anodic reaction

Fe Fe ++ + 2e-

Fe ++ + 2 (OH)- Fe(OH)2 (Ferrous hydroxide)

4 Fe(OH)2 + 2H2O + O2 4Fe(OH)3 (Ferric oxide)

Cathodic reaction

4e- + O2 + H2O 4(OH)-

No corrosion takes place if the concrete is dry or probably below humidity of 60%.

Corrosion does not takes place if concrete is fully immersed in water because diffusion of oxygen

does not take place into the concrete.

Optimum relative humidity for corrosion is 70 to 80%.

The products of corrosion occupy a volume as many as six times the original volume of steel

depending upon the oxidation state.

The increased volume of rust exerts thrust on cover concrete resulting in cracks, spalling or

delamination of concrete.

Concrete loses its integrity in this kind of situations.

The cross section of reinforcement progressively reduces and the structure is sure to collapse.

40% of failure of structures is on account of corrosion of embedded steel reinforcement in

concrete.

Good quality concrete through good construction practices - choosing constituent material - good

rules to be followed during various stages of production - lowest possible w/c ratio - use

superplasticizers to cut down the w/c ratio to make dense concrete.

Corrosion control

Proper mix design

Use of right quality and quantity of cement for different exposure conditions.

Materials such as fly ash, blast furnace slag, silica fume etc. are required to be used as

admixtures or in the form of blended cement in addition to lowest w/c ratio to make concrete

dense.

The improvement in the microstructure of hydrated cement paste is ultimately responsible for

protecting the steel reinforcement from corrosion.


Why does raft. in concrete corrode ?

A properly designed and constructed concrete is initially water-tight and the rft. steel within it is

well protected by a physical barrier of concrete cover which has low permeability and high density.

Concrete also gives steel within it a chemical protection.

Steel will not corrode as long as concrete around it is impervious and does not allow moisture or

chlorides to penetrate within the cover area.

Steel corrosion will not occur as long as concrete surrounding it is alkaline in nature having a high

pH value.

Depending upon the quality of design and construction, there will be an initial period in which no

corrosion will occur as the external moisture or chloride is unable to reach the steel causing

corrosion.

This initial period will depend on the environment in which the structure is constructed.

Corrosion process

Accelerating factors

Wetting & drying cycles

Heating & cooling cycles

Loading & unloading cycles

Cyclic loading

Leaching of lime

Additions & alterations done on the structures

Isolated cracks

Voids

Entrapped air & large capillary pores get interconnected

External moisture & chlorides find their way to rft. steel and corrosion starts.

Corrosion process continues till such time large cracks develop and spalling of concrete occurs.

Carbonation of concrete

Concrete when produced is highly alkaline having a pH value b/w 12.5 and 13.5.

Alkaline environment around the steel passivates corrosion process.

Due to CO2 and humidity present in the environment the exposed surface of concrete loses its

alkalinity due to formation of carbonic acid.

This formation gradually penetrates into the concrete mass and is called carbonation of concrete.

When carbonation takes place beyond the concrete cover given to rft. steel, the environment

around the steel loses it alkalinity – dropped to less than pH value 9.

The mitigation of corrosion no longer takes place due to chemical protection.

Lower grades of concrete have shown much deeper carbonation than higher grades of concrete,

for a similar period of time.

Grades lower than M20 can carbonate beyond 25 – 35 mm within a matter of 20 years.

If quality of concrete in the cover region is poor, corrosion can take place much faster.

Slender sections like canopies, parapets, balcony slabs projecting on the building exterior show

greater evidence of deterioration than other structural members.


Corrosion of steel & spalling of concrete in a RCC canopy

Deterioration of balcony slabs and other structural members

Steps necessary for repairs

Visual examination of the cracked or spalled concrete surface.

To determine the extent of corrosion and depth of carbonation in the affected areas.

To clearly identify the areas which need repairs.

To remove the defective concrete and corrosion products from embedded steel.

To clean the concrete and steel surface completely free from rust and other materials.

To examine the reduction of steel rft. diameter and arrange for full or part replacement of corroded

steel.

To apply corrosion resistant barrier film or coating on the rft. to inhibit chances of future corrosion.

To apply bond coat on the old surface over which repairs have to be carried out.

To apply a strong, passive carbonation resistant concrete cover of proper generics and reinstate

the structural member to its original shape / form.

To apply a seal coat on the entire repaired surface to guard against future ingress of moisture and

other harmful chemicals.

To paint the repaired surface as an added precautionary measure and for aesthetics.

Corrosion resistant barrier film applied on steel rft.

Bond coat applied over the surface to be repaired

Application of cement concrete polymer cover over deteriorated concrete

Surface preparation for repairs

Rust compounds are like cancer cells in a human body.

They have to be removed completely so that the corrosion disease is removed and prevented

from spreading in future.

Rust on steel needs to be removed completely

Chipping tool used to remove deteriorated concrete

Protection of steel against future chances of corrosion

Application of thick cement slurry on steel bars just prior to placing of replacement material over it

- economical but the slurry film only containing cement may develop cracks.


Liquid epoxy application on bars

Stiffener & hardener have to be mixed properly using mechanical stirrers and the

replacement concrete has to be placed over the epoxy coated steel before the epoxy

hardens.

Very good barrier film but does not contribute in creating non-corrodible (alkaline)

conditions around the steel.

Slurry of water based polymer emulsion & cement prepared using mechanical stirrers is a better

option as it helps in maintaining alkaline environment and it also forms a strong barrier film around

the steel.

Additional (replacement) steel properly tied and fixed prior application of cementitious polymer

coating

Polymer cement mortar used to repair spalled concrete of RCC slab

Corrosion in poor and good concrete

Protective methods

Measures to control the corrosion of steel reinforcement

Metallurgical methods

Corrosion inhibitors

Coatings to reinforcement

Cathodic protection

Coatings to concrete

Design and detailing

Metallurgical methods

Mechanical properties and corrosion resistance property of steel can be improved by altering its

structure through metallurgical processes.

Rapid quenching of the hot bars by serious of water jets, keeping the hot steel bars for a short

time in a water bath etc.

Corrosion inhibitors

Using corrosion inhibiting chemicals such as nitrites, phosphates, benzoates etc.

Most widely used admixture is based on calcium nitrite - it is added to the concrete during mixing

of concrete - typical dosage is 10-30 litres per m3 of concrete.

Coatings to reinforcement

The object of coating to steel bar is to provide a durable barrier to aggressive materials.


Coating should be robust to withstand fabrication of reinforcement cage, and pouring of concrete

and compaction.

Simple cement slurry coating is a cheap method for temporary protection.

Fusion bonded epoxy coating

Effective method of coating rebars

Plants are designed to coat the straight bars in a continuous process.

Electrostatically charged epoxy powder particles are deposited evenly on the surface of the bar.

Costing thickness may vary from 130 to 300 microns, greenish in colour.

Reinforcement cage, partly coated with fusion bonded epoxy, for precast segment

Galvanised reinforcement

Dipping the steel bars in molten zinc

The zinc surface reacts with calcium hydroxide in the concrete to form a passive layer and

prevents corrosion.

Cathodic protection

One of the effective and extensively used methods for prevention of corrosion in concrete

structures in more advanced countries.

Due to high cost and long term monitoring required for this method, it is not very much used in

India.

Comprises of application of impressed current to an electrode laid on the concrete above steel

reinforcement. This electrode serves as anode and the steel reinforcement which is connected to

the negative terminal of a DC source acts as a cathode.

In this process the external anode is subjected to corrode and the cathodic reinforcement is

protected against corrosion and hence the name ―Cathodic protection‖.

Realkalisation & Desalination process

Realkalisation : This brings back the lost alkalinity of concrete to sufficiently high level to reform and

maintain the passive layer on the steel.

Desalination : The chloride ions are removed from concrete, particularly from the vicinity of the steel

reinforcement by certain electrical method to establish the passive layer on the steel.


In addition to the coating of reinforcement, a surface coating to the concrete member is given to

increase the durability.

Protective coatings to major concrete structures such as bridges, flyovers, industrial buildings and

chimneys have become a common specification in India as in other countries.

Biggest world map was drawn on cooling tower in Germany using Emce Colour-flex

Acrylic based protective cum decorative coating is given to J.J Flyover at Bombay


Epoxy coatings which does not allow the concrete to breathe should not be used for coating

concrete members.

Epoxy based coating material is not resistant to UV rays when exposed to sunlight.

Whereas the coating material based on acrylic polymer is resistant to UV rays and retains the

breathing property of concrete.‘

Design and detailing

The structural engineer should take all precautions in designing and detailing, with respect to

spacing between bars for the concrete to flow between reinforcements, to facilitate vibration of

concrete, to give proper cover to the steel reinforcements, to restrict the crack width etc.


Waterproofing Systems\/

- People only see what they are prepared to see.

Waterproofing

Maintains the appearance of the building.

Increases the life of the structure.

When the structure becomes older, it settles unevenly, forms cracks in the walls and then

leakage/seepage begins.

For better waterproofing work, the selection of quality material and workmanship is important.

Stress is laid on the procedures and stages of the work, along with the checking methods for

workmanship.

Waterproofing of roof slabs

The leakage and seepage of roof slabs of a structure are the most chronic problems that occur

from time to time after completion of the roof slabs or at a later stage.

The design and construction of the watertight roof has to take into consideration the effects and

impact of environmental conditions and construction practices in the area where the

structure is located.

Factors causing deterioration of roof slab

Intensity of rainfall, wind & Sun

Chemical reactions

Corrosion of embedded metal

Carbonation

Oxidation

Biological action in the form of moss and lichens, moulds, algae, fungi, etc.

Shapes of roof slabs

Normally governed by the climate in which they are constructed.

Pitched roofs in areas where rainfall is heavy and there is likelihood of snowfall - rainwater gets

drained from a series of points outside the perimeter walls into the RW disposal system besides

providing protection to the building below.

Flat roofs in hot and arid zones - Water drainage points should be located at slopes preferably in

the external perimeter of the wall for quick and uniform draining of RW.

Roofs slab construction material

Modern day roof slabs are generally of cement concrete.

◦ Concrete mix used should be durable, impervious and well compacted.

◦ Rft. should be protected from the ingress of moisture & chlorides.

Water seepage damages the concrete of the terrace slab causing corrosion of rebars and

spalling of concrete.

Concrete should act as a physical barrier and also preserve the alkaline environment around the

rebars to maintain its structural capacity and integrity.

Most vulnerable areas from the waterproofing angle are the roofs and sunken portions of

toilets and bathrooms.

The permeability in concrete is the function of porosity, pore size, pore distribution and continuity

of pores.


Concrete for the roof slab should be with low w/c ratio and minimum permeability since

reduction in permeability results in a good waterproof structure.

Sources of leakages in flat roof and leakage preventive methods

The rooftop to be leak proof, depends on:

◦ Structural design and detailing

◦ Quality & correct selection of construction materials

◦ Methodology & supervision of construction

◦ Type of usage and maintenance

◦ Adequate slopes of top surfaces towards the RW outlets

◦ Quality of exposed surface finish suitable for climate exposure

Leakage prevention system has to be planned at the design stage itself.

Overall design of structure with regard to waterproofing must consider:

◦ Good quality water barrier system

◦ Diversion and/or drainage of water

◦ Prevention of transmission of heat and cold insulation to the floor slabs below

◦ Prevention of penetration of UV rays

◦ Ability to withstand abrasive action due to movement of men and materials during usage

and maintenance

Defective structural design and improper slope of roof slab

The roof slab should be flat and have a slope of 1 in 60 to 1 in 80 towards RW outlets –

ponding of water due to incorrect design and poor workmanship results in cracking on account of

differential movement caused by wetting and drying of the surface which is subjected to

temperature changes.

Roof slabs subjected to increase in temperature not only spreads (expands) outwards but also

bows upwards from the center due to the higher temperature of the outer slab surface and cooler

temperature of the inner slab surface.

Leakage preventive methods

It would be advantageous if the WPS normally used to arrest seepage would provide

insulation(e.g. brickbat) which does not transmit the increase in external temperature to the

upper surface of the roof slab.

It is essential that materials for the construction of flat roofs should have either the same thermal

properties or be sufficiently protected so that the distortion, if any, is insignificant and does not

affect the WP works carried out.

Another alternative would be to use WP treatment with a flexible membrane which does not

crack due to expansion and contraction of the base slab.

Expansion joints/movement joints

Provided in roof slabs exceeding 45m in length at the junction of various arms of ―H‖, ―L‖, ―E‖, ―T‖,

and ―C‖ shaped buildings in the plan.

These joints should be made leak-proof with accurate detailing and sealing arrangements using

sealants.

The movement joints should also be provided in the parapet walls located above the movement

joint provided in the concrete slab.

It is not advisable to seal the joints directly at the floor level of the roof slab.

It is necessary to construct RCC inverted beams on either side of the joint filler and strengthen

the nosing and top surface.


The joints should be wide enough to allow easy installations and large enough to accommodate

the design movement without exceeding the sealant movement capacity.

Expanded polyethylene foams or shaltex board are used as joint filler material.

Thermocole should not be used as a filler material.

Characteristics of a good sealant

Easy installation

Good bond with faces

Low shrinkage

High ultimate strength

Impermeable

Resilient with a high degree of movement capacity

Good shelf and pot life

Capable of changing its shape without changing its volume after undergoing movement cycle.

Expansion joints/movement joints

Joint filler material should not be used for filling joints exceeding 50mm in width.

The sealing work of the joint should be carried out between two good and sound concrete nosing

instead of sealing the joint with the plastered nosing.

Nosing can also be strengthened with ferro cement lining or epoxy mortar.

Cross-sectional details for sealing 25mm wide horizontal expansion joint at terrace-level

Construction joints

Construction joints are unavoidable when roof slabs of large dimensions have to be constructed.

These joints are the weakest link and are prone to leakages.

Proper planning of joint details and detailed surface preparatory work before restarting the

concrete laying is essential to ensure water tightness of the joint.

The surface preparation at the joint is generally done either by water jet or by small chippers.

The loose particle on the surface and mortar coating of the already laid concrete are removed to

expose coarse aggregate surface.

Undulations up to 6mm on the surface of joint are advisable for good joint formation b/w the two

concrete pours.

Leakage through the parapet wall

Rain beating on the horizontal surface and on the inner as well as outer vertical faces of the

parapet wall is often heavy, accomplished by splashing due to rebound.

Water penetrates through cracks in the plaster, parapet wall coping and travels down through the

wall by capillary action to the RCC roof slab.

RW enters the roof through cracks or open junctions b/w wall and slab.


Further, the RW penetrates to the cracks in the top surface of the roof slab and dips below.

Ponding of rainwater may also occur b/w the slab and WPS on account of deflection in the slab

due to loading.

Incorrect and correct WP and construction of parapet wall

Ponding of rainwater

This may be due to RW outlets fixed at a higher level.

Choking of RW inlets on terraces of buildings also causes ponding of water and subsequent

leakages.

Incorrect & correct methods of fixing RW outlets on a flat terrace with masonry parapet wall

Leakage preventive measures through the parapet walls

Use dense bricks only for parapet wall construction.

Prevent plastic shrinkage cracks in plaster.

Avoid embedment of GI pipes in parapet walls as these pipes are likely to rust and leak through

the parapet wall.

Provide 150mm thick dense PCC coping with drip mould and inward slope to prevent leakage

through the top horizontal surface of the parapet wall. Alternatively provide 50mm thick stone slab

as coping.

Provide the drip mould at the junction of plaster at the inner surface of the parapet wall and WPS

below.

Leakage through RW inlet pipes

For smooth, quick and easy flow of RW, outlet pipes of minimum 100mm diameter should be

provided to cater to not more than 30m2 of roof area.

The mouth of the RW pipe should be covered with conventional cast iron jali and cleaned at

regular intervals specially before onset of the monsoons.

During the initial rains, leaves, solids etc. deposited on the roof by people and wind, seals the jali

obstructing easy flow of RW resulting in stagnation of water.

Choking of RW pipe inlet due to soil and vegetation deposits


Correct and incorrect method of fixing RW outlets

Many a times the horizontal pipe bends of the RW piping system are left inside the parapet wall

and fixed well within the inner face of parapet.

This causes seepage through the parapet wall and further into the building.

Leakage due to wrong usage of roof

The accessible roof slab is normally designed for movement of men and material for maintenance.

Quite often, garden and flowerbeds and even creepers are planted without any consideration.

The WPS may not be able to withstand such usages causing chemical attacks. Consequently the

WPS may develop cracks.

The roof slab may also deflect due to additional load resulting in ponding of water.

This ultimately results in leakage and seepage of RW.

Such situations should be avoided at the design stage itself if the utilization of the roof slab is

properly identified and proper consideration is given to the type of WP treatment.

Leakage due to poor workmanship

Poor workmanship can cause the following:

◦ The flat roof slab is not laid in proper line and level

◦ Selection of porous and inferior materials for construction

◦ Improper slope towards RW outlet. This will cause ponding of water on the roof surface.

◦ Terrace concrete slabs constructed without full bearing in load bearing structures.

◦ Inadequate bearing creates leakage sources at the joint b/w the vertical face at the end of

the roof slab and masonry.

◦ To avoid this leakage, the terrace slab with full bearing is required to be provided.

The cracks and voids (honeycombs) in the concrete roof slab are not grouted properly before

laying the WPS.

The peripheral junction of rainwater outlet in the parapet wall is not grouted resulting in leakages.

Application of membrane or chemical coating on an unsound concrete surface, cracked

plaster/concrete surface fails to perform in a satisfactory manner.

Incorrect application or choice of chemical binders results in loss of bond, thereby peeling the

membrane waterproofing layer.

Lack of thermal compatibility of membrane waterproofing with the concrete slab.

Incorrect and correct detailing and practice for construction and waterproofing of a verandah slab

Leakage due to poor workmanship

Improper construction of movement joint.


Casting of balcony slabs in a haphazard manner. Many times, concrete cantilever slabs are

provided with slopes towards the inner wall of the building and also vertical projection is

constructed on the top of this slab.

Due to this faulty construction, water stagnates and finds its way through slab and through

the internal wall junctions.

It is a good practice to provide a small inverted beam / band at the junction of the internal wall

and verandah slab to resist the ingress of water through the junction.

During concreting of cantilevered concrete slabs, the main reinforcement at the top gets

pressed down due to workers walking over it during concreting and also due to inadequate

supporting and fixing of the top steel.

Waterproofing Systems - Conventional methods

(a) Brickbat coba waterproofing

The popular conventional method for waterproofing flat roofs for several years has been brick jelly

concrete or brickbat coba with Indian Patent Stone (IPS) concrete finish or China Mosaic finish.

The precautions necessary to be taken for construction of brickbat coba waterproofing are as

under:

◦ Ensure that all materials used are of good quality

◦ Ensure that all brickbats are well soaked in water before use

◦ Provide CM 1:4 with 2% waterproofing compound as a screed

◦ On the edge, place brickbats of good quality with proper joints 20-25 mm thick to the

designed gradient

◦ The slope of the brickbat coba should be towards the RW outlet/gully traps

Grout the joints with CM 1:4 or 1:5 with 2% WP compound

Ensure proper and smooth finishing near traps and spouts

The junction between the parapet wall and slab should be first chased open and grouted with CM

1:4 with 2% WP compound. Coving should be 300 mm in height and 75 mm thick impregnated

with brick pieces placed and packed manually and covered with joint less plaster. The plaster

should end in recess over the parapet wall plaster where drip mould is provided. Ensure that this

watta is made immediately the next day and cured properly.

Cure the waterproofing for at least 7 days without interruption.

The brickbat coba WP increases dead weight on the roof slab, but helps in providing insulation

on the topmost floor and slope to the top horizontal surface of the terrace slab leading to the

drainage.

When rainwater enters the brickbat coba through leakage sources, it acts as a water reservoir

as brick has a tendency to absorb water. The trapped water finds its way below and seeps

through the weak points in the concrete slab.

China mosaic topping instead of IPS is provided for the quick and easy flow of rainwater due to its

smoothness and it reflects sunrays, thus improving the insulation over the concrete slab.

On account of poor workmanship, along with variations in day and night temperatures, the tile

pieces heave up ultimately leading to leakage of rainwater into the brickbat coba below and further

to the slab.


(b) Bitumen based products and coatings

It is normal practice to use bitumen impregnated felt as a waterproofing membrane for pitched

roofs.

Bitumen on attaining high temperature during the summer months, undergoes photo-oxidation

initiated by UV radiation.

This not only results in fast deterioration of bituminous felt (tar felt) but also formation of health

hazard compounds.

Pitched roof ending behind a parapet wall can cause problems of protection in heavy rainfall

areas.

In the case of bitumen impregnated felt water proofing system for pitched roof behind a parapet

wall, care has to be taken particularly at the junction of a slab and parapet by providing a proper

covering and tucking the bitumen impregnated felt in the chasing made in the parapet wall.

Tendency to increase the height of bitumen impregnated felt to the top of parapet for facilitating

avoidance of tucking by chasing a groove in the parapet should be discouraged.

In any case, tucking of bitumen-impregnated felt beyond a height of foot should be avoided as

it tends to slide down due to its own weight despite best adhesion on the wall surface.

Modern Techniques of Waterproofing

Most of the modern waterproofing systems cannot provide a slope to the top horizontal surface

of the roof slab towards the rainwater down-take outlets.

Therefore, when such modern techniques are used, it is essential that slopes towards RW

down-take pipes be made in the concrete slab itself.

The waterproofing materials used should have the following properties:

◦ The material should be truly compatible with the concrete base

◦ It must establish good bond with the existing surfaces without exhibiting shrinkage and

thermal expansion

◦ The material should not adversely react with the concrete or mortar surfaces

◦ The material used should be impermeable

◦ The material should not develop cracks due to thermal variations or varying conditions of

weather and loading

(a) Integral waterproofing admixtures

Chemical admixtures either in the form of powder or liquid are added to the concrete matrix

while mixing operations of concrete/mortar.

These admixtures are not substitutes for proper waterproofing systems and cannot ensure 100%

waterproofing of the structure.

Integral waterproofing admixtures provide beneficial effects and help in reducing the size of

capillary pores as fillers thereby reducing the chances of leakages.

Polymers can be incorporated in concrete slab itself to achieve the following:

◦ Minimizing shrinkage crack

◦ Decreasing permeability

◦ Improving resistance to ingress of water

(b) Waterproof protective coating

Most of the protective coatings used are either epoxy or polyurethane based.

In these surface coatings, the pores and capillaries are blocked hampering the breathing

capacity of concrete.


The impermeable surface coating allows water to build up behind the surface coating with the risk

of disruption to the coating and the possibility of damage.

The trapped water moves behind the surface of coating. As the protective coat is not flexible, on

finding weak spots, it would come out resulting in cracks and/or blisters on the surface.

Polymer solutions are also applied in the form of coatings.

These polymers are:

◦ Can be applied on any finished surface

◦ Should not be applied in a thick film

◦ Effective for short duration

◦ Generally not UV resistant

◦ Have Fire and health hazards

Due to the above criteria, solution coatings are used selectively.

(c) Mineral slurry with polymer compound

Polymer, in emulsion form is added to cement-sand slurry to make a paint-like material that can

be applied using either a brush or a spray gun.

The proportion of polymer in the slurry is about 40% expressed in terms of weight of cement.

Before applying slurry, the surface to be treated is to be cleaned, i.e. loose materials, dust,

fungus, oil and grease etc. need to be removed. Thereafter the paste is applied on the wet

surface.

The coating protects the concrete and rebar from deterioration.

The advantages are as follows:

◦ Coating surface can be 8-10 times thicker than pure polymer coating

◦ Adhesion is better

◦ UV ray resistant polymer can be used

◦ Retains breathing capacity of concrete

◦ Can be applied on damp surfaces.

In case of accessible terraces, this system of waterproofing has to be protected as it is unable to

resist aberration due to movement of men and materials.

(d) Elastomeric membrane forming products

The elastomers are a special class of membranes which are breathable and have high

elongation, weatherability and crack bridging abilities.

This system forms a seamless membrane and is UV ray resistant.

Since this system has low aberration resistance properties, it has to be protected for accessible

terraces.

(e) Impregnations

Solvent-based impregnants and solvent-free cement based impregnants are available in the

market.

Due to environmental problems, use of solvent-based impregnants is not encouraged.

Solvent-free cement-based impregnants penetrate into concrete through capillary action.

After penetration, the material undergoes chemical reaction. It blocks the capillaries and

prevents entry of water thus achieving effective waterproofing.

In this system, no extra load is added to the slab. The Petronas Towers in Kuala Lumpur,

Malaysia have used this system effectively.


Ferro cement technology

This technology is used by a few waterproofing specialists.

Cement mortar is spread over a skeleton of steel rod/wire mesh and after hardening and curing,

the composite becomes a versatile material.

It possesses certain advantages such as it is lightweight, resistant to cracking, is easy to

fabricate to any shape & has high impermeability and durability.

Variety of additives such as fly ash, granulated glass furnace slag, metakaolin, chemical

plasticizers/super plasticizers, silica fumes and polymers can be advantageously used to improve

the properties of the matrix such as workability, flowability, setting time, compressive strength,

bond strength, impermeability and durability.

The technique is labour-intensive. However, this type of waterproofing has not gained popularity.

General Preventive Measures

The roof should be inspected periodically, particularly before onset of the monsoons. All outlet

points must be cleaned and freed from loose debris and unwanted materials including plants, etc.

If the roof is not designed for live and material load, unnecessary movement of human beings

and materials on the roof should be avoided.

If punctures on the surface or loosening of tucking on parapet wall of bitumen impregnated felt or

cracks in the waterproofing surface and parapet are noticed, immediate patch repairs should be

carried out.

Waterproofing system should be replaced if found unserviceable.

Repeated replacement over the old unserviceable waterproofing should be avoided.

The treatment used earlier should be completely removed, taking care to see that the concrete

slab below is not damaged and then fresh treatment should be carried out.

CONCLUSION

Concrete if designed carefully and constructed properly is a leak-proof material.

Due to temperature variations, load variations, wetting and drying cycles, pores and cracks in

concrete get interconnected and cause seepage of water through the concrete slab.

This necessitates waterproofing treatment on otherwise dense, well compacted and well designed

concrete.

However, the best waterproofing treatment will not work if the concrete below is of poor quality.

It is therefore extremely important to produce good quality concrete or to have any type of

reliable roofing material which is installed in a correct and proper manner.

It's true that we don't know what we've got until we lose it, but it's also true that we don't know what we've been missing until it arrives.

-Shreedhar,,,


HIGH DENSITY CONCRETE

WHAT IS HIGH DENSITY CONCRETE?

High Density Concrete is defined as concrete with unit wt. ranging from 3360 kg/cu.m. to

3840kg/cu.m.

It is about 50% higher than unit wt. of Normal Concrete.

It can be produced up to a density of 5280 kg/cu.m. using iron as coarse and fine aggregate.

NECESSITY OF HIGH DENSITY CONCRETE

In Nuclear Power Project –

Large scale production of penetrating radiation & radioactive materials, particle accelerators,

industrial radiography & X – Ray, Gamma ray requires need of shielding material & protection of

operating personnel against biological hazards of such radiation.

HDC is more efficient for permanent shielding purposes.

PREPARATION OF HDC

COARSE & FINE AGGREGATES

Aggregates whose specific gravity is more than 3.5 is used.

Commercially employed aggregates are barite, magnetite, ilmenite, limonite, hematite etc.

Steel & Iron agg. In the form of shots, punching scrap also used as heavy wt. agg.

Heavy wt. agg. should be clean, strong, inert & relatively free from deleterious materials which might

reduce strength of concrete.

WATER CEMENT RATIO

• To produce HDC & High strength, it is necessary to control water cement ratio very strictly.

CEMENT CONTENT

• High strength cement is used.

OTHER FACTORS

• Desired increase in hydrogen content, required to slow down fast neutrons which can be

achieved by the use of hydrous ores.

• These materials contain high % of water of hydration.

• Lemonite & Goethite are reliable sources of hydrogen, as long as shield temperature doesn‘t

exceed 200 deg. C.

• Serpentine is good up to 400 deg C.

PROPERTIES

• High-density concrete is used extensively in nuclear power plants for radiation shielding against

biological hazards.

• Aggregates are the most important and critical component of the high-density concrete, properties

of the aggregates and the criteria for their use in concrete.

• Apart from the basic physical properties (i.e., compressive strength, density, and absorption),

knowledge of thermal properties of such concrete is required.

• Thermal properties, such as conductivity, diffusivity, specific heat, emissivity and coefficient of

thermal expansion are also important properties of HDC.


ADVANTAGES

SHIELDING PROPERTY – MOST IMPORTANT PROPERTY OF USING HDC.

Radiation shielding quality of concrete can be increased by increasing the density of concrete.

Another importance of shielding concrete its structural strength even at high temperature.

.

DISADVANTAGES

Due to high cement content, it may exhibit increased creep & shrinkage.

Tendency of segregation due to high density of aggregate.

With smooth cubical pieces of steel or iron used as coarse aggregates the compressive strength

doesn‘t exceed 21MPa.cm. regardless of water cement ratio.

Wear and tear of mixture drum may be high.

Formwork is made stronger to withstand higher loads.

APPLICATIONS

High-density concrete is used extensively in nuclear power plants for radiation shielding against

biological hazards.

It is widely used for radiation shielding in radiotherapy facilities and nuclear reactors.

For the prevention of radiation leakage from radioactive sources, as well.

It is also used in places where it is required to withstand high degree of temperature.

BACTERIAL CONCRETE

Concrete is the most used, strong & durable building material in the world.

The reinforcement takes over the load when concrete structure is subjected to tension and

prevents deformation, deterioration (i.e. crack development) and failure.

While concrete prevents corrosion of steel bars, protecting them from attacks of deteriorating

agents from the environment, like chlorides, sulphates, CO2.

These deteriorating agents are responsible for causing cracks, make concrete more permeable

and ultimately result in corrosion of reinforcement.

Thus, a reliable method to automatically repair cracks in concrete will increase and ensure

durability and functionality of concrete structures.

THUS BACTERIAL CONCRETE HELPS IN DOING THAT

Principle:

• The “Bacterial Concrete” can be made by embedding bacteria in the concrete that are able to

constantly precipitate calcite.

• Bacillus Pasteurii, a common soil bacterium, can continuously precipitate a new highly

impermeable calcite layer over the surface of an already existing concrete layer.

• The favorable conditions do not directly exist in a concrete but have to be created.

• An alkalophilic soil microorganism, B. pasteurii, produces urease that hydrolyzes urea to ammonia

and carbon dioxide.

• The ammonia increases the pH in surroundings, which in turn induces precipitation of CaCO3,

mainly as a form of calcite.


The bacterial cell surface with a variety of ions can non-specifically induce mineral deposition by

providing a nucleation site.

The high pH of these localized areas, without an initial increase in pH in the entire medium,

commences the growth of CaCO3 crystals around the cell.

APPLICATIONS

This technique is highly recommended for monumental & heritage structures.

The use of bacterial concrete lead to substantial savings, especially in steel reinforced concrete.

It will also mean durability issues can be tackled in a new and more economical way when

designing concrete structures.

Bacterial concrete is ideal for constructing underground retainers for hazardous waste

For residential buildings traditional repairing of cracks will remain the most economically attractive

solution for now.

Polymer Concrete • Despite being thought of as a modern material, concrete has been in use for hundreds of years.

• As much of the constituents of concrete come from stone, it is often thought that concrete has the

same qualities and will last forever.

• Due to the low tensile strength of concrete, when structural concrete elements deteriorate, are

subjected to extreme loadings, or react to corroded reinforcing steel, a portion of the concrete

separates from the component and results in a void that needs to be repaired.

Causes of concrete problems can be classified as:

• Defects: design, materials, construction

• Damage: overload, fire, impact, chemical spill

• Deterioration: metal corrosion, erosion, freeze/thaw, sulphate attacks

• And due to some Construction errors

• These problems can be solved by application of polymer in concrete construction.

What is a Polymer?

• A polymer is a large molecule containing hundreds or thousands of atoms formed by combining

one, two or occasionally more kinds of small molecule (monomers) into chain or network

structures.

• A typical polymer may include tens of thousands of monomers

• As polymers are relatively expensive, they are often added to the lower cost materials to form

composite material such as polymer concrete (PC), polymer cement concrete (PCC), and

polymer-impregnated concrete (PIC).


• The use of polymers in industry and in construction has increased phenomenally since 1950. This

increase is due to the many desirable properties that can be built into the polymer at a relatively

low overall cost.

The impregnation of monomer and subsequent polymerization is the latest technique adopted to

reduce the inherent porosity of the concrete, to improve the strength and other properties of

concrete.

The pioneering work for the development of polymer concrete was taken up by United States

Bureau of Reclamation (USBR).

The development of concrete-polymer is done by producing new material by combining the

ancient technology of cement concrete with the modern technology of polymer chemistry.

Types of polymers

Polymer Impregnated concrete

Monomers:-

1)Methyl methacrylate(MMA)

2)Styrene

3)Acrylonitrile

4)t -butyl styrene

5)vinyl acetate

Polymer cement concrete

Monomers:-

1)Polyster-styrene

2)Epoxy-styrene

3)Furans

4)Vinylidene Chloride

Polymer Impregnated concrete:-

It is produced by impregnation of pre-cast hardened Portland cement concrete with low viscosity

monomer that is subsequently converted to solid polymer under the influence of physical agents

(ultraviolet radiation or heat) or chemical agents (catalysts).

The concept underlying PIC is that if voids are responsible for low strength as well as poor

durability of concrete in severe environments, then eliminating them by filling with a polymer

should improve the characteristics of the material.

It is difficult for a liquid to penetrate it if the viscosity of the liquid is high and the voids in concrete

are not empty (they contain water and air). Therefore, for producing PIC, it is essential not only to

select a low-viscosity liquid for penetration but also to dry and evacuate the concrete before

subjecting it to the penetration process.

Polymer Cement Concrete:-

Polymer cement concrete is a modified concrete in which part (10 to 15% by weight) of the

cement binder is replaced by a synthetic organic polymer.

It is produced by incorporating a monomer, pre-polymer-monomer mixture, or a dispersed polymer

(latex) into a cement-concrete mix. To effect the polymerization of the monomer or pre-polymer-

monomer, a catalyst is added to the mixture.

The process technology used is very similar to that of conventional concrete. Therefore, polymer

cement concrete can be cast-in-place in field applications, whereas polymer impregnated concrete

has to be used as a pre-cast structure.

Generally, polymer cement concrete made with polymer latex exhibits excellent bonding to steel

reinforcement and to old concrete.

Polymer concrete (PC) is a composite material in which the binder consists entirely of a synthetic

organic polymer.

It is variously known as synthetic resin concrete, plastic resin concrete or simply resin concrete.

Polymer concrete consists of a mineral filler (for example, an aggregate) and a polymer binder

(which may be a thermoplastic, but more frequently, it is a thermosetting polymer.


GEOPOLYMER CONCRETE

Geopolymer Ingredients

Source Materials: Fly ash, Silica fume, Slag, Rice-husk ash, Red mud, etc.

Alkaline Liquids: Sodium hydroxide with sodium silicate, Potassium hydroxide with

potassiumsilicate.

Types of Geopolymers

Phosphate-based geopolymer

Fly ash based Geopolymers

Silico-phosphate geopolymers

Organic-mineral geopolymers

Silicone

Kerogen geopolymer

Production

• GPC are formed by reaction of an aluminosilicate powder with an alkaline silicate solution

at roughly ambient conditions.

• Metakaolin is a commonly used starting material for laboratory synthesis of geo-polymers,

and is generated by thermal activation of kaolinite clay.

• Geo-polymer cements can also be made from natural sources of pozzolanic materials,

such as lava or fly ash from coal.

Characteristics of Geopolymer concrete

Have high early strength and low shrinkage properties

Are resistant to freeze-thaw, acid, fire, sulphate and alkali aggregate reaction

Have high thermal resistance qualities useful for fire retardant walls, thermal banks and

insulation

Can be produced using batching processes similar to those used for Portland Cement

products

Can immobilize toxic and hazardous materials within the geopolymeric matrix, such as

heavy metals


Applications of GPC

• Mining: Paste back-fill, tailings dams, liners, capping media, shotcrete and acid resistant

concrete.

• Civil Construction: Stabilized fill, pavement materials, soil stabilization.

• Building Materials: Bricks, blocks, tiles, pavers, lightweight/fire retardant/acoustic panels, pipes,

precast and ready-mixed concrete products.

• Environment/Waste Management: Impermeable barriers and encapsulation of domestic,

hazardous, radioactive and contaminated materials in a very impervious, high-strength material;

liners and capping for landfills.

• Specialist Applications: Rapid-set binders, very high-strength binders, lightweight products,

super flat floors.

Glass-fibre-reinforced concrete (GRC) GRF COMPOSITION:

• Portland Cement

• Fine Aggregate

• Water

• Alkali-Resistant (AR) Glass Fiber

GRF ADVANTAGES Light weight (90% less than concrete)

• Limitless opportunities for architectural expression

• Weather Resistance

• Surface can be left uncoated

• Class A Fire Rating

GRF DISADVANTAGES:

• Used as NON-loadbearing only

• Requires separate anchorage system for installation

• Large panels must be reinforced

• Color additives may fade with sunlight

• May have different absorption rate than adjacent Historic material

Applications:

GRC is environmentally friendly

• GRC is durable against extreme weather conditions

• GRC products are lightweight and easy to handle

• GRC offers a wide variety of shapes and surface finishes

• GRC is impact resistant and non combustible

• unaffected by environmental conditions

• GRC can imitate traditional materials like stone, clay but unlike them neither heavy nor brittle

• Can be molded into different shapes like chimneys, ridges…

-Only by going too far can one possibly find out how far one can go


FIBRE REINFORCE CONCRETE ( FRC)

• Concrete is relatively brittle, and its tensile strength is typically only about one tenths of its

compressive strength.

• Regular concrete is therefore normally reinforced with steel reinforcing bars.

• For many applications, it is becoming increasingly popular to reinforce the concrete with small,

randomly distributed fibers.

• Their main purpose is to increase the energy absorption capacity and toughness of the material,

but also increase tensile and flexural strength of concrete.

COMPOSITION

• DEF: Concrete containing a hydraulic cement, water, fine or fine and coarse aggregate, and

discontinuous discrete fibers is called FIBER-REINFORCED CONCRETE (FRC).

• It may also contain pozzolans and other admixtures commonly used in conventional concrete.

• Fibers of various shapes and sizes produced from steel, plastic, GLASS, and natural materials are

being used; however, for most structural and nonstructural purposes, steel fiber is the most

commonly used of all the fibers.

• There is considerable improvement in the post-cracking behavior of concretes containing fibers.

Although in the fiber-reinforced concrete the ultimate tensile strengths do not increase

appreciably, the tensile strains at rupture do

CLASSIFICATION ACCORDING TO VOLUME FRACTION

LOW VOLUME FRACTION (<1%)

• The fibers are used to reduce shrinkage cracking. These fibers are used in slabs and pavements

that have large exposed surface leading to high shrinkage crack.

• Disperse fibers offer various advantages of steel bars and wiremesh to reduce shrinkage cracks:

(a) the fibers are uniformly distributed in three dimensions making an efficient load distribution

(b) the fibers are less sensitive to corrosion than the reinforcing steel bars,

(c) the fibers can reduce the labor cost of placing the bars and wiremesh.

MODERATE VOLUME FRACTION (BETWEEN 1 AND 2%)

• The presence of fibers at this volume fraction increase the modulus of rupture, fracture toughness,

and impact resistance.

• These composite are used in construction methods such as shotcrete and in structures that

require energy absorption capability, improved capacity against delamination, spalling, and

fatigue.


HIGH VOLUME FRACTION (GREATER THAN 2%)

• The fibers used at this level lead to strain hardening of the composites.

• Because of this improved behavior, these composites are often referred as High-Performance

Fiber-Reinforced Composites (HPFRC).

• In the last decade, even better composites were developed and are referred as Ultra-High-

Performance Fiber-Reinforced Concretes (UHPFRC).

AREAS OF APPLICATION

• Thin sheets

• Shingles

• Roof tiles

• Pipes

• Prefabricated shapes

• Panels

• Shotcrete

• Curtain walls

• Slabs on grade

• Precast elements

• Composite decks

• Vaults, safes

• Impact resisting structures

Vacuum concrete Introduction

• It has been amply brought out in the earlier developments that high w/c ratio is harmful to the

overall quality of concrete.

• Whereas low w/c ratio doesn't give enough workability for concrete to be compacted 100%.

• Due to these reasons, high workability and higher strength or very low workability and high

strength don't come hand in hand.

• Vacuum process of concreting enables to meet this conflicting demand to achieve high workable

concrete to get high strength.

• In this process excess water used for higher workability, is not required for hydration and it

adversely effects the strength of concrete.

• So, this excess water in concrete is withdrawn by means of vacuum pump, subsequent to the

placing of concrete.

• This process when properly applied, produces concrete of quality.

• It also permits removal of formwork at an early age to be used in other repetitive work.

General arrangement for vacuum concrete process


Process

• It essentially consists of a vacuum pump, water separator and filtering mat. The filtering consists

of a backing piece with a rubber seal all round the periphery.

• A sheet of expanded metal and then a sheet of wire gauge also forms part of the filtering mat.

• The top of the suction mat is connected to the vacuum pump.

• When the vacuum pump operates, suction is created within the boundary of the suction mat and

the excess of water is sucked from the concrete through the fine wire gauge.

• In this process at least one face should be opened to the atmosphere to create difference of

pressure.

• The size of the mat should not be less than 90cm x 60 cm.

Rate of extraction of water

• The rate of extraction of water is dependent upon the workability of mix, maximum size of

aggregate, proportion of fines and aggregate cement ratio.

• In general, the following tendencies are observed:

• The amount of water which may be withdrawn is governed by the initial workability or the amount

of free water. A greater reduction in w/c ratio can be obtained with higher initial w/c ratio.

• If the initial w/c ratio is kept the same the amount of water which can be extracted is increased by

increasing the maximum aggregate size or reducing the amount of fines in the mix.

• Although the depression of the w/c ratio is less, the lower the initial w/c ratio, the final w/c ratio is

also less, the lower the initial value.

• The greater the depth of concrete processed the smaller is the depression of the average w/c

ratio.

• Little advantage is gained by inc the period of treatment beyond 15 to 20 mins and max of 30

mins.

Advantages

High workability

• High strength

• Early removal of formwork

• Ease of operation and cost effective.

Disadvantage

• There is only one disadvantage ie uniform strength not possible.

• Reason is that there is a general tendency for the mix to be richer in cement near the processing,

due to fact that along with water cement gets sucked and deposited near the surface.

• The w/c ratio will be lower from 0.16 yo 0.30, than the original w/c ratio near the processed

surface.

Sometimes your joy is the source of your smile, but sometimes your smile can be the source of your joy.


Vacuum dewatered concrete • A high quality concrete should have high wear resistance, high compressive strength, reduced

shrinkage, minimum water permeability and should be free of pores.

• These properties can be achieved in vaccum dewatered concrete.

• For the last 10 years vacuum dewatered concrete is fairly widely used in construction of factory

floors.

• Vacuum dewatered concrete is equipment oriented. It requires formwork in the form of channels.

• Internal vibrators, double beam screed board vibrator for the full width, filter pads, vacuum pumps,

disc floater and power trowel are the based equipments required for dewatering process.

Application

• The major application of vacuum dewatering of concrete is applied for floors, concrete walls(ie

retaining walls), road pavements etc.

• This process is applied for the structural element where, properties like high workability, strength,

abrasion resistance, permeability, and hardness should be hand in hand.

HIGH VOLUME FLYASH CONCRETE

What is Fly Ash?

• A by-product of coal-fired power plants

• It is used as an admixture in high strength & high performance concrete.

• When used in concrete, it displaces more than 25% of the cement

• Creates a more stronger bond

• Reduces concrete‘s environmental impact

Fly Ash and Its Classification

• Fly ash is comprised of the non-combustible mineral portion of coal consumed in a coal fueled

power plant. Fly ash particles are glassy, spherical shaped having ―ball bearing‖ action — typically

finer than cement particles — that are collected from the combustion air-stream exiting the power

plant.

Fly ash improves the properties of concrete

• Fly ash as a water reducer :

The water demand and workability are influenced greatly by particle size distribution, particle

packing effect, and voids present in the solid system. Typical concrete mixtures do not have an

optimum particle size distribution, and this accounts for the undesirably high water requirement to

achieve certain workability.

Fly ash as a water reducer :

• To plasticize a cement paste for achieving a satisfactory consistency, much larger amounts of

water than necessary for the hydration of cement have to be used because Portland cement

particles, due to the presence of electric charge on the surface, tend to form flocs that trap

volumes of the mixing water.


How HVFA Contributes to Concrete Workability

• Fly ash produces more cementitious paste. It has a lower unit weight The ―ball-bearing‖ effect of

fly ash particles creates a lubricating action when concrete is in its plastic state. This creates

benefits in:

Workability: Concrete is easier to place with less effort, responding better to vibration to fill forms more

completely.

Ease of Pumping:

Pumping requires less energy and longer pumping distances are possible.

Improved Finishing

Sharp, clear architectural definition is easier to achieve, with less worry about in-place integrity.

Reduced Bleeding

As the water content is low in HVFA Concrete, the bleeding is very low or negligible.

Reduced Segregation

Improved cohesiveness of fly ash concrete reduces segregation that can lead to rock pockets and

blemishes.

Higher Strength: Fly ash continues to combine with free lime, increasing compressive strength over

time.

Decreased Permeability: Increased density and long term pozzolanic action of fly ash, which ties up

free lime, results in fewer bleed channels and decreases permeability.

Increased Durability

Dense fly ash concrete helps keep aggressive compounds on the surface, where destructive

action is lessened. Fly ash concrete is also more resistant to attack by sulphate, mild acid, soft

(lime hungry) water, and seawater.

Reduced Sulphate Attack: Fly ash ties up free lime that can combine with sulphates to create

destructive expansion.

Reduced Efflorescence: Fly ash chemically binds free lime and salts that can create efflorescence,

and dense concrete holds efflorescence producing compounds on the inside.

Reduced Shrinkage:The largest contributor to drying shrinkage is water content. The lubricating action

of fly ash reduces water content and drying shrinkage.

Reduced Heat of Hydration: The pozzolanic reaction between fly ash and lime generates less heat,

resulting in reduced thermal cracking when fly ash is used to reduce portland cement.

Reduced Alkali Silica Reactivity: Fly ash combines with alkalis from cement that might otherwise

combine with silica from aggregates, causing destructive expansion.

ADVANATAGES

• Improves workability and makes consolidation of concrete easier (No Honey-comb –No cold

joints)

• Lower Internal heat generation (No Thermal cracks)

• Decreases the potential for damage from alkali aggregate reactivity and sulphate attack.

• Decreases permeability

• Reduce Portland cement factor for better economy

• Better durability

2nd Term New Material and Technology

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Transcript of 2nd Term New Material and Technology