Scc Rmc Route Cidc 9.11

8/18/2019 Scc Rmc Route Cidc 9.11

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Special Concretes through the RMC route

Dr. Manamohan R Kalgal

Sr. Vice President

Head, Technical Services,

UltraTech Cement Ltd.INTRODUCTION

A significant proportion of concrete used in construction today is being

substantially modified to produce mixes which have properties (both in

fresh and/or hardened states) different from those of ordinary ones. It is

well known that the traditional concrete which was formed with the

cement aggregates and water is being modified by incorporating new

ingredients, either singly or in combination, to improve the quality of the

concrete and make it perform better in more demanding conditions of

service and also make it more suitable for handling by specialisedmethods of constructions.

Under service conditions, the finished product is expected to show

improved properties namely higher strength, toughness and durability.

The specialised construction methods demand mixes suitable for an easy

and reliable placing by pumping, spraying, underwater placing, extrusion

etc.

Such demands and the consequent developments produced a range of

concretes which can be broadly classified as the ‘ special ’ rather than the

‘ ordinary ’ ones which still prevail in the bulk of everyday construction.

Special Concretes are a fast growing area of concrete technology where

a stage has been reached in which the practical construction industry

has to adjust its site practice in order to maintain efficiency and high

quality of its products while using a wide range of mixes very different

from the traditional ones.

It is important to appreciate that there are no firm boundaries between

the special and the ordinary concrete mixes. In some situations, one and

the same concrete mix can show both an ordinary and a special

behaviour, depending on which properties are considered important or in

which type of practical application the concrete is used.

The experience suggests that a successful solution of the problem of

mixing some of the special concretes has to begin with the storage and

proportioning systems.


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This is not only because the storage and proportioning (batching) of the

special constituents of such mixes may be itself difficult but also because

the manner in which these constituents are added into the mixer may

have considerable influence on the mixing process as a whole. This can

influence significantly the necessary mixing time and the overall mixing

efficiency of the plant.

It is important to have good knowledge of the type of Special Concretes,

their properties, their production process from batching and mixing to

final placing, including appropriate and effective testing methods. These

issues are discussed briefly along with the status as of today in India.

HIGH PERFORMANCE CONCRETE

In recent years, the terminology "High-Performance Concrete" has been

introduced into the construction industry.

Definition

A high-performance concrete is something more than is achieved on a

routine basis and involves a specification that often requires the concrete

to meet several criteria. The American Concrete Institute (ACI) defines

high-performance concrete as concrete meeting special combinations of

performance and uniformity requirements that cannot always be

achieved routinely when using conventional constituents and normal

mixing, placing and curing practices. A commentary to the definition

states that a high-performance concrete is one in which certain

characteristics are developed for a particular application and

environment.

Properties

Examples of characteristics that may be considered critical for an

application are Ease of placement, Compaction without segregation,

Early age strength, Long-term mechanical properties, Permeability,

Density, Heat of hydration, Toughness, Volume stability, Long life in

severe environments etc.

Because many characteristics of high-performance concrete areinterrelated, a change in one usually results in changes in one or more of

the other characteristics. Consequently, if several characteristics have to

be taken into account in producing a concrete for the intended

application, each must be clearly specified in the contract documents.

A high-strength concrete is always a high-performance concrete, but a

high-performance concrete is not always a high-strength concrete. The


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compacted under its self weight without applying vibration.

Origin and growth

SCC was first developed in Japan in mid 1980. Since then, it has foundapplications in reinforced concrete sections containing congested

reinforcements. In 1988, Professor Okamura and his associates at theUniversity of Tokyo succeeded in developing SCC for commercial use.About 90% of the concrete used inPre-cast industry in Japan is saidto be SCC.

While most of the pioneeringresearch was going on in Japan,research also picked up in otherparts of the world. Europeancountries were interested inexploring the significance and

potential of SCC developed in Japan.

The European countries formed a large consortium in 1996 to promotethe development of SCC for practical applications in Europe. It hasreflected in the speed with which SCC is being adopted in manyEuropean countries in the construction of pre-cast and cast-in-placebridges and other structures. SCC is expected to replace conventionalconcrete to promote innovations, lessen environmental impact, improvedurability and reduce cost in almost all applications.

The use of SCC in the U.S. picked up a little later than in Japan and

Europe. The pre-cast concrete industry has been using SCC since 2000to produce pre-cast structural and non-structural elements, andarchitectural panels. The widespread acceptance of SCC in constructionhas been slow because of the limited experience in the workability,durability, constructability and the long- term properties of SCC.Constant efforts are on, in taking advantage of the characteristics of SCCin pre-cast and cast-in-place construction in the US and all over theworld.

Properties

Poor quality of vibration of concrete, in congested locations, has often

been a shortcoming of traditional concrete. In such situations, SCC,which flows under its self weight and does not require any externalvibration, has revolutionized the concrete placement. SCC allows easierpumping - even from bottom up, flows into complex shapes, transitionsand inaccessible spots and minimizes voids around embedded items toproduce a high degree of homogeneity and uniformity. Since SCC flowseasily, self-levels with minimal consolidation, placement is quick andeasy, saving placement time, vibration equipment and time, labour and


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equipment wear and tear. SCC's potential high early form strippingstrength and smooth finish mean faster turnaround and minimalcosmetic repairs and a positive impact on maintaining projects onschedule. By eliminating the need for consolidation, SCC results in fewersafety and noise concerns and costs.

Self-compacting concrete is gradually becoming the preferred formulationworldwide for many applications such as foundations, floors, walls andcomplex structures. It combines great strength and superb finishes withimproved productivity, providing outstanding construction solutions.

SCC through the RMC route

RMC plants where constant and superior quality control can bemaintained is the ideal source for SCC. In order for self compactingconcrete to be used as a standard concrete rather than a special one,new systems for its design, manufacturing and construction with SCCare being established. A system by which SCC can be supplied by RMC

manufacturers would involve testing of self compactability, mix-designmethod, acceptance testing method at job site, development/use of newtype of powder or admixture suitable for SCC. Okamura and Ozawa(1995) had proposed a simple mix proportioning system assuminggeneral supply from RMC plants. The coarse and fine aggregate contentsare fixed so that self-compactability can be achieved easily by adjustingthe water-powder ratio and super-plasticiser dosage only.

Several improvements and variants have been reported in SCC mixdesign since then. In mix proportioning of conventional concrete, thewater-cement ratio is fixed at first from the viewpoint of obtaining the

required strength. With SCC however, the water-powder ratio has to bedecided taking into account self-compactablity is very sensitive to thisratio. The mortar or paste in SCC requires high viscosity as well as highdeformability. This can be achieved by the employment of a superplasticizer with or without a viscosity modifying agent, resulting in a lowwater-powder ratio for highdeformability.

As regards a suitable acceptability testfor SCC, a major problem arises withregard to checking the whole concrete forself-compactablity since poorcompactablity cannot be compensated bythe construction work. Interval of takingsamples needs to be pre-fixed based onexperience. An interesting acceptancetest method was proposed by Ouchi etal.,(1999) wherein, a testing apparatus isinstalled between the agitator truck and the pump at the job site. The


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whole amount of concrete is poured into the apparatus. If the concreteflows though the apparatus, the concrete is said to be having sufficientself-compactability.

Indian Scenario

SCC was adopted in Indian research only in the 1990’s. It was used invarious projects in small scale without much of a success till 2000.Later, research and application stabilized with increasing awarenessabout the product. Designers have started to specify SCC mainly to beable to place concrete in congested reinforcement and in places whereaccess to vibration is not available. Pre-cast industry (which in itself issmall in comparisons to the potential) has not readily adopted SCC yet,but it is expected to pick up very soon. Indian standards - IS 456, hasalso adopted and introduced SCC in its latest amendment in 2007. Withincrease in labour costs and reduction in ready availability of skilled manpower to place and vibrate concrete, SCC has a great scope in Indian

concrete structures.Mix designs for various grades of SCC between M 25 and M 60(andrecently upto M80) have been developed and stabilised. Demonstrationson the performance of SCC have been carried out to various clients indifferent cities, in both pre-cast and cast in-situ applications.Considerable volumes of commercial supplies have been made from ourplants in Mumbai, Pune, Gurgaon, Noida, Chennai and Hyderabad.Recently, a successful landmark pour of 900 m3 of SCC of M 50 gradewas carried out for for M/S Indu Projects, Pune. SCC was poured into adensely reinforced Post Tensioned beam of dimensions said to be thelargest ever made in Asia.

FIBRE REINFORCED CONCRETE

Definition

Fibre-reinforced concrete is conventional concrete to whichdiscontinuous discrete fibres are added during mixing, so as to enhancethe mechanical properties of the concrete such as tensile and flexuralstrength, ductility, toughness and crack resistance.

The concept of fiber reinforcement is as old as the use of brittle materialslike clay, bricks or concrete. It was with the introduction of steelreinforcement bars however that ‘reinforced concrete’ came to be thedominating construction material during the industrial revolution. Themodern use of ‘fiber reinforced concrete’ started in the 1960s usingstraight, smooth discontinuous steel fibers. Thereafter, various sorts offiber materials have been investigated and are utilized for differentapplications. Steel fibers are the dominating fiber type, but there aremany others, such as polymeric fibers, mineral fibers and naturallyoccurring fibers.


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1.

The improvement in the properties of concrete when fibre is addedto it, depends on three factors:

2.

Physical properties of fibres and matrix

3. Uniform distribution of the discrete fibres throughout the matrix.

4.

Bond between fibres and matrix

Types of Fibres

Fibers have been produced from steel, carbon, glass, plastic,polypropylene, nylon, rayon, asbestos and also from natural materialssuch as cotton, coir, sisal and baggasse. For structural applicationsusing concrete, steel and glass fibers are generally used, since theypossess high modulus of elasticity and lead to strong and stiffcomposites.

Steel Fibers

Steel fibers are classifiedbased upon the nature of thesource steel, whether they arestraight or deformed, andtheir aspect ratio. Figureshows different shapescommonly adopted. The use ofsteel fibers in fiber-reinforcedconcrete results in increasedimpact resistance, toughness,and ductility. Failure of the

fibers in the concrete matrixusually occurs as a result offailure of the bond between the fiber and the concrete matrix, resulting inpull-out of the fibers.

Synthetic Fibers

Synthetic fibers, at current addition rates, are not designed asreplacement for structural (primary) reinforcement in concrete. However,synthetic fibers do offer numerous benefits to concrete in both the plasticand hardened states. Synthetic fibers are most commonly used to reduceplastic shrinkage cracks. Other benefits include a reduction in

permeability and an increase in impact and abrasion resistance andtoughness (ASTM C-1018 –89).

There are two general types of fiber currently available in the market. These are referred to as fibrillated and monofilament and are verydifferent in configuration and performance.


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The fibrillated type is easily recognized assmall bundles of interconnected fibersthat open up and disperse during themixing action to produce a threedimensional maze of fiber in the cement

matrix, resulting in excellent bondstrength and crack control of hardenedconcrete. Fiber lengths range from 6mm to50mm. Graded fibrillated fiber consistingof various lengths and fiber design forgreater ease of mixing and finishing areavailable.

Monofilament fibers do not offer the bonding characteristics of thefibrillated fiber; because of the shape the fiber acts more like a smoothtowel than a deformed bar. Monofilament fiber does not obtain themechanical bond achieved with the fibrillated fiber. Consequently,monofilaments do not impart the post crack holding strength provided byfibrillated fibers.

Recently CEMEX has developed AdvancedPolypropylene Fibre Concrete which is acombination of fine and coarsemonofilament polypropylene fibres whichtakes concrete fibre reinforcement to a newlevel of performance. The introduction ofdeformed coarse fibres into the concretemix allows much higher dosage rates,

resulting in increased toughness andductility of hardened concrete.

Synthetic Fiber Types

The number of fiber suppliers has grown inrecent years. The primary types of synthetic fibers commercially availablein the market are polypropylene, polyester and nylon. The fibers withineach type come in various lengths, thickness and geometries. Whateverthe fiber composition, they must show resistance to acid, alkali, mildew,salts, be non-corrosive and have low thermal conductivity; as tested inthe presence of moisture. (ASTM C- 1116-89).

POLYPROPYLENE: Of the synthetic fibers available, polypropylene is themost widely used in ready-mix concrete. Polypropylene is non-absorbentand has no effect on mixing water requirements. These fibers areproduced as either fibrillated or monofilaments. Polypropylene isresistant to alkali and all other chemicals normally present in concrete.


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POLYESTER: Though not as widely used as polypropylene fibers, a fewmanufacturers offer polyester fibers. Polyester fibers come only inmonofilament configuration, in lengths from 6mm to 50mm.

Like polypropylene, polyester is non-absorbent. Polyester, however, hasbeen shown to degrade in the alkaline environment of Portland cement

concrete. To retard this degradation, manufacturers of some polyestershave tried to coat the fibers to resist alkali attack. Long-termperformance of these fibers has yet to be determined.

NYLON: Like polyester fibers, nylon only comes in monofilament form.What primarily distinguishes them from polypropylene and polyesterfibers is their absorptive nature. They retain a natural balance of 4.5% ofwater.

Glass Fiber

Glass fiber reinforced concrete (GFRC), in simplest terms, is the

replacement of conventional large aggregate and steel rebar with ahomogenously dispersed network of tiny strands of glass in a slurry ofcement and sand. By using glass fibers as the matrix bound bycementitious adhesion, substantial increases in flexural, tensile andimpact strengths are achieved without losing the superb aging propertiesof concrete.

The combination of cement and glass fibers allows the homoqenouslyreinforced part (GFRC) to be made much thinner than one with onlyintermittent reinforcement. This is the essence of the commercialattractiveness of GFRC. Products made from GFRC generally weigh onlyten percent as much as conventional precast concrete products.

Carbon Fibres

This is a material with a lot of potential. Although carbon fibers are stillused in cement slurry and concrete, this area is no longer being pursuedas aggressively as it once was, primarily due to economics and codes thatdo not take into account the higher levels of performance.

Asbestos Fibres

Traditionally used as reinforcement in the chopped fiber form forapplications such as thin sheet-like materials or boards, structural andarchitectural panels that must withstand high loads and/or

deformations, and structural components where the fibers are added toobtain toughness and prevent cracking.

The overall use of asbestos prior to the determination of it as a healthhazard has been estimated to be as high as 2.5 to 3 million tons.


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Natural Fibres

There are many potential problems. Some fibers, like straw, have a thinwax-like coating, which must be removed by chemical treatment beforethe cement can adhere. Other fibers, like some species of wood fiber,contain soluble sugars and tannins which interfere with the cement

hydration process. Perhaps the most important problem is that manynatural fibers are not durable in the highly-alkaline matrix of cement. The alkalinity eventually embrittles the fibers, and they lose theirreinforcing ability. In addition, the fibers can still absorb some moisture,and will swell because of this (particularly those fibers nearest thesurface especially in warm humid environments).

Properties

Fiber reinforced concrete is a whole class of materials rather than asingle new type. Different combinations of fiber types, fiber contents andmatrix compositions mixed using various production methods yield a

vast range of material behaviour. With the change in fiber content alone,mechanical behaviour of the concrete may vary between being almost asbrittle as plain concrete to being close to elastic-plastic or evendeformation-hardening materials. Due to this feature of flexibility inchanging the mechanical behaviour for different uses, fiber reinforcedconcrete, the material design gets closely connected with the structuraldesign and vice versa. The improvement of ductility of concrete elementsand connections in structures in seismic areas has been a long feltdesire. Generally this is achieved by confining concrete but confinementreinforcements might result in congestion at joints create difficulties inconstruction. Several researchers and practitioners in the field havecome out with useful and innovative suggestions for using fibers inconcrete.

The most important property is the crack arrest and crack controlmechanism of the fibers. This in turn, improves all other propertieslinked with cracking, such as tensile strength, stiffness, ductility, energyabsorption and resistance to impact, fatigue and thermal loading.

Synthetic Fiber reinforcement combats cracking caused by severaldifferent types of stresses which cause Plastic settlement cracking,Plastic shrinkage cracking Drying shrinkage cracking

FRC through the RMC route The complexities of designing the appropriate dosage for the requirementand maintaining the uniform dispersion to obtain the desiredhomogenous concrete again point towards the supply of FRC from RMCplants.


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Batching and mixing

The fibres are normally supplied either pre-weighed in portions asspecified by the customer or as bulk goods weighed manually prior tobatching. Several factors can make the mixing of the fibres into theconcrete difficult, viz.:

1.

The tendency of the fibres towards entanglement (balling). This ismainly a function of the type/dimension of the fibres

2.

The difference between density of the fibres and the density of thefresh concrete. This makes it often necessary to choose the correcttime in the mixing cycle for the addition of the fibres. A typicalexample in is the addition of lightweight polymeric fibres. Theaddition before water is much better because the addition afterwater is into fresh liquid concrete and the buoyancy of the fibresmakes the mixing difficult.

3.

The changed character of the fresh mix following the addition offibres. There is little change in the mixing process when smallquantities of the fibres are added (e.g., 1 kg. of polymer fibres percum of concrete). However, trial mixes using the actually selectedmixer are necessary when large quantities of fibres are added.

Manual batching continues to be used in cases of fibres with whichit appears impossible to avoid entanglement. When usingmechanical systems, the entanglement can cause problems both atthe storage and at the batching stages of the production process.

The manual batching has a number of drawbacks

- An extended mixing time. This may be necessary to achieve ahomogenous mix.

- A possible need for an unnecessarily more expensive mixer inorder to achieve the desired homogeneity and the possibleunder-utilisation of the capacity of such plant.

- A substantial time consumption for the weighing andproportioning

- An insufficient control of the whole batching process

- A lack of documentation for the control of the quantity of fibres

actually added.

Mechanical batching plant which avoids the drawbacks mentioned above

has been developed for steel fibres and polymeric fibres

Placing:

Since synthetic fibers mix into the cement paste, they do not interfere

with structural reinforcement and they conform easily to unusually


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shaped forms, such as curved driveways, architectural precasts, etc.

Fibers increase the cohesiveness and "stackability" of the mix. Workers

may notice a slight change in the visual slump in the mix, but

workability will remain the same at any given water/cement ratio.

Finishing:Synthetic fibers are compatible with all concrete surface treatments and

finishes, such as stamped, exposed aggregate, broomed and trowelled

finish. Fibers are chemically inert and won’t stain, rust or discolor the

concrete. Since fiber-reinforced concrete bleeds more uniformly than

plain concrete, it is important not to begin finishing too early. Early

finishing may tend to expose fibers on the slab. Finishing at the proper

time, when the surface has begun to "tighten up" will ensure that the

fibers will not be a cosmetic problem. The fear of "hairy" concrete can be

easily overcome with proper finishing.Indian Scenario

Different grades of Fibre reinforced concrete are being tried to be

established by using polypropylene and polyester fibres for different

applications. Considerable quantities of FRC have been supplied from

our plants in Bangalore and small quantities have been supplied at

Mumbai and Gurgaon.

The work or bulk supply of FRC with glass, steel and other fibres, for

replacing reinforcement steel in RCC construction or for seismic

resistance is likely to pick up in the near future, once the consultants are

convinced of the benefits and specify FRC in large constructions.

HIGH VOLUME FLY ASH (HVFA) CONCRETE

In the modern construction practice 15%-20% of fly ash by mass of thecementitious material is now commonly used in developed countries.Higher amounts of fly ash on the order of 25%-30% are recommendedwhen there is a concern for thermal cracking, alkali-silica expansion, orsulfate attack. Such high proportions of fly ash are not readily acceptedby the construction industry due to a slower rate of strengthdevelopment at early age.

The high-volume fly ash concrete system overcomes the problems of lowearly strength to a great extent through a drastic reduction in the water-cementitious materials ratio by using a combination of methods, such astaking advantage of the super-plactisizing effect of fly ash when used in alarge volume, the use of a chemical super-plactisizer, and a judiciousaggregate grading. Consequently, properly cured high-volume concreteproducts are very homogenous in microstructure, virtually crack-free,


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and highly durable. Because there is a direct link between durability andresource productivity, the increasing use of high volume concrete willhelp to enhance the sustainability of the concrete industry.

Properties

Based on field experience and laboratory tests, the properties of HVFAconcrete, when compared to conventional Portland cement concrete, canbe summarized by P K Mehta as follows:

1.

Easier flowability, pumpability, and compactability.

2.

Better surface finish and quicker finishing time when power finishis not required.

3.

Slower setting time, which will have a corresponding effect on the joint cutting and lower power-finishing times for slabs.

4.

Early-strength up to 7 days, which can be accelerated withsuitable changes in the mix design when earlier removal offormwork or early structural loading is desired.

5.

Much later strength gain between 28 days and 90 days or more.(With HVFA concrete mixtures, the strength enhancement between7 and 90-day often exceeds 100%, therefore it is unnecessary toover-design them with respect to a given specified strength.)

6.

Superior dimensional stability and resistance to cracking fromthermal shrinkage, autogenous shrinkage, and drying shrinkage.In unprotected concrete, a higher tendency for plastic shrinkagecracking.

7.

After three to six months of curing, much higher electricalresistivity, and resistance to chloride ion penetration, according toASTM Method C1202.

8.

Very high durability to the reinforcement corrosion, alkali-silicaexpansion, and sulfate attack.

9. Better cost economy due to lower material cost and highlyfavorable lifecycle cost.

10. Superior environmental friendliness due to ecologicaldisposal of large quantities of fly ash, reduced carbon-dioxideemissions, and enhancement of resource productivity of theconcrete construction industry.

The high-volume concrete offers a holistic solution to the problem ofmeeting the increasing demands for concrete in the future in asustainable manner and at a reduced or no additional cost, and at thesame time reducing the environmental impact of two industries that arevital to economic development namely the cement industry and the coal-fired power industry. The technology of high-volume fly ash concrete is


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especially significant for countries like China and India, where, given thelimited amount of financial and natural resources, the huge demand forconcrete needed for infrastructure and housing can be easily met in acost-effective and ecological manner.

HVFA though the RMC route

Site-mixed concrete is still used on numerous medium and smallconcrete construction sites in India. It is for this category that the use ofHVFA concrete may pose a number of problems. They are as follows :

1.

In HVFA concrete, the proportion of fly ash in the totalcementitious materials is kept in the range of 50-60 percent.Mixing of two powdery materials – cement and fly ash – withalmost equal proportions in a low-efficiency mixer having acapacity of 200-l or 280-l is bound to pose difficulties. It is quitelikely that fly ash may not get dispersed evenly in the concrete mix.

2.

Further, HVFA concrete requires that the water-binder (w/b) ratiobe maintained between 0.32 to 0.38 and this is going to be thetoughest challenge in site-mixed concrete production. The watercontent in the HVFA concrete needs to be kept around 120-135l/m3 and if one is not able to control it then it is obvious that thedesired properties of strength and durability would not beachieved

3.

HVFA concrete necessarily includes a super plasticizer forincreasing the slump of this Concrete. We cannot expect themasons and supervisors handling site mixed concrete to beknowledgeable about the admixtures in general and super

plasticizers in particular. They are not capable of dealing with theproblem of cement-super plasticizer-fly ash compatibility. ? Alsothey cannot introduce an exact amount of super plasticizer whenproper dosing arrangements are just not available on a site-mixedconcrete work.

4.

The quality of fly ash is another big question. The IS 3812,specifies certain physical requirements like Blaine’s fineness, limereactivity, particles retained on 45 micron sieve, compressivestrength, soundness, etc. for ensuring that only the desired qualityof fly ash is used in concrete. The code has also introduced certain

uniformity requirements. However, the quality control exercised ona typical site-mixed concrete job site in India is just not equippedto exercise proper controls on the quality of fly ash that would beincorporated in concrete. Under such circumstances, there is apossibility that improper quality fly ash — even bottom ash — getintroduced in concrete.

5.

Proper supervision and quality control are generally lacking intypical site-mixed concrete jobs. There is a total dependence on the


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maistry and his team of labourers, who are not knowledgeableenough to understand the importance of keeping the w/b ratio atlower level, ensuring adequate mixing of fly ash and incorporatingadequate dosage of a good-quality superplasticiser in the mix.Further, HVFA concrete also needs sufficient curing, which is

usually an ignored area in site-mixed concrete. Of course, there arecertain enlightened contractors/builders and consultants in thecountry, who are fully aware of the implications of good qualityconcrete and are exercising good controls on many of theirconstruction sites using site-mixed concrete. Unfortunately, theirnumber is very low.

HVFA concrete should only be produced in an RMC facility or on projectshaving batching-mixing plant wherein sufficiently high level of qualitycontrol measures are exercised

FOAM CONCRETE

It is a lightweight, free-flowing material that is ideal for manyapplications, and can have a range of dry densities (typically 400-1600kg/m3) and strengths (1-15MPa). It can be easily placed, by pump ifnecessary, and does not require compacting, vibrating or levelling. Foamconcrete also has excellent resistance to water and frost, and provides ahigh level of sound and thermal insulation. It is very versatile, since itcan be tailored for optimum performance and minimum cost by choice ofa suitable mix design.

First used in mainland Europe during the 1920s, foam concrete hasproperties and applications that are different from conventional concrete,

where low compressive strengths are acceptable. The material is suitablefor a number of applications, including backfill and trenchreinstatement. One of the most cost-effective uses has been found insub-bases for roads.

Definition

The term 'foam concrete' is possibly more accurate than the morefrequently used `foamed concrete', as the product is not created byfoaming ordinary concrete but is a totally different material. In its mostfundamental form, foam concrete is composed of cement, water and airpores. The air pores are introduced by agitating air with a foaming agent

diluted with water, creating a mechanically manufactured foam. Thisfoam is then carefully blended with the cement slurry or base mix.

With ordinary concrete, there is a compact aggregate/sand skeleton, andcohesion is achieved by the cement matrix: compression loads arepredominantly transferred via this well-- stacked aggregate/sandskeleton. Foam concrete is composed without coarse aggregates, but witha substantial volume of foam bubbles. The bubbles are typically 0.3-0.4mm in diameter, surrounded by cement, the highest concentration


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being near the plateau border where there is the conjunction of threefoam bubbles. In foam concrete mortar, these bubbles provide thestability of the foam concrete. Once the material hydrates, load transfertakes place via the cement matrix around the bubbles.

Mix designs and properties

In addition to cement, water and foam, various fillers are used for foamconcrete. Popular fillers are pfa, sand, (lime) stone dust or chalk, andpolystyrene beads.

All materials that are compatible with cement and have fineness betweenthat of cement and sand can be used in foam concrete, provided thatthey have no adverse effect on the stability of the foam.

If mechanical properties, such as strength, are taken into account, it isbest to only use fine fillers, so sand with a maximum grain size of 1mmis more effective than sand with a maximum grain size of 4mm. The

minimum cement content depends on the overall filler fineness, but ingeneral the minimum can be set at 150kg/m^sup 3^. The water/cementratio also depends on the volume and fineness of the ingredients and canbe as high as 1.0.

The properties of foam concrete depend on only a few elements:

* volume

* cement content

* filler type (density of filler) material age.

The amount of cement, although important, does not determine the

eventual strength and that both density of foam concrete and porevolume are equally important. The presence of pfa gives the strength ofthe material an extra boost at greater age.

Applications

Road sub-bases

A recent development has been the use of foam concrete as a road sub-base. It is a highly effective way to improve unstable soil conditions or toreplace unsuitable soils. A typical application would be to raise theelevation of a roadway by using foam concrete as a subbase, especiallywhere soil is unstable. Foam concrete does not require compacting,imposes no lateral forces on adjacent structures, may be applied directlyto existing marginal ground such as peat concentrations or poor soils,and weighs 20-25% of the weight of standard soils. Foam concrete can beapplied on uneven ground as it does not need a completely flat surface,eliminating the requirement for surcharging with soil. Less consolidationis required for subsoils and it also achieves equilibrium with surroundingpressures.


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Trench Filling

Foam Concrete is a simple answerto trench reinstatement and void-fill. It reduces the time required toback-fill, thereby providing a major

saving to contractors by increasingproductivity. Since it is a freeflowing, self-levelling material, it willfill any voids and cavities in thetrench sides; eliminating settlementissues. There is no need to usecompactors, thus eliminatingvibration related illnesses amongoperatives.

Other Applications

Various other applications can be thought of like:

• As heat insulating materials for roofs of residential and IndustrialBuildings.

• As low temperature insulation material in cold storage plants.

• Pre-cast panels for non-load-bearing partition walls, to givelightweight and sound insulated construction.

•

Light weight-filling material for achieving slopes to facilitatedrainage.

•

To make complicated shapes for architectural facing work, becauseof high flowing and moulding ability without adding significantly tostructural dead weight.

Foam Concrete through the RMC route

It is important to make the slurryfirst, before making the foam.Ideally the foam should begenerated and delivered directlyinto the mixer of the ready mixtruck that contains the slurry.

The mixer should be rotated atapproximately 10 revolutions perminute. All of the foam should beallowed to blend into the slurry.A sample of the foamedconcrete should be tested for itswet density.

Foam concrete is manufactured


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plants around the world. They are available in powder, liquid andgranular forms, with no one form better then the other. To understandhow concrete gets colored, it should be understood that iron oxidepigment particles are ten times smaller in size then a particle of cement.When color is added to any cement based mix, the smaller pigment

particles cover the larger cement particle. This is why color is dosedbased on cement content (sack mix) and nothing else.

When it comes to major factors that affect colour, the most critical iswater to cement ratio. Controlling the amount of water added to theconcrete mix is critical to producing consistent colour. The addition ofwater permanently changes the concrete, typically lightening the finalcolour.

The second key factor to consider is the role that the gray cement playsin the final color. The color that is added to the mix has to overpower thegray base color of the concrete. These two colors come together to form

the final color we see. This is why colors (in gray cement) are all darkerearth tone shades. One can achieve lighter color shades in concrete, butthat requires the use of expensive white cement. Another importantconsideration regarding gray cement is that they are not all the sameshade of gray. This reinforces the practice of maintaining batch-to-batchconsistency. One should deal with reputable ready mix supplies thattake steps to control these variables, and never switch ready mixsuppliers in the middle of a colored concrete project! Make sure yourRMC supplier understands colored concrete and that they use cementfrom the same lot for the entire job. If a job starts with light cement andthen the RMC supplier tops off with another color of cement, color

differences are to be expected. Second only to water related issues,cement is a major culprit of color differences in large pours.

The remaining factors, while not as critical, are important and need to beconsidered and controlled: The proper curing of concrete is important toreduce surface shrinkage cracking and obtain the proper strength. It iseven more important in colored concrete because lack of curing producesinconsistent color. Slight color or shade differences in gray concrete areseldom noticed.

The last of the key factors that affect colour are sub base preparation,placement, finish and maintenance. As with most issues in architectural

concrete, color tends to magnify the above issues that are overlooked orunnoticed in gray concrete. When placing large areas of colored concreteover days, weeks or months, one should take into consideration theability to maintain color consistency.

Breaking up the large areas with bands of different color and or texturecan be considered. For large pours of one colour, it might be worthwhile


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concrete has the unique ability to allow storm water to pass through itsmass into the ground underneath. Pervious concrete offers significantenvironmental benefits as it reduces the requirement for drainagefacilities. Further it facilitates the recharge of ground water and thefiltration process purifies the water as it percolates below.

Building owners are realizing better land utilization and LEED creditswith pervious concrete parking lots. Pervious concrete applications canbe used as an alternative to complex drainage systems and waterretention areas reducing storm water runoff.

New possible applications for pervious concrete

1. Airport tarmac and terminal edge strips: So much rain water isconverted to storm water at airports because of the vast amount ofpaving for runways, ramps, tarmacs and gate areas. If some of thatcan be paved with pervious concrete, much rain water will reachthe aquifer.

2. Curbs, gutters and bicycle path strips: This is a way that coulddivert a tremendous amount of water from the storm water sewers.Consider: A 3' wide by 150' long strip of pervious concreteupstream from a catch basin could return over 25,000 gallons ofwater per hour to the aquifer, diverting it from surrounding bodiesof water. This estimate is based upon percolation rate of 1 gallonper minute per square foot.

3. Playground bases: A well constructed pervious concrete systemcovered with shredded tire mulch can provide a well drained easilymaintained playground surface.

4.

Drainage ditch linings.

5.

Sound absorption walls for highways.

6.

Well linings

7. Seawalls

8.

Sewage treatment plant - sludge beds etc.

Pervious Concrete through the RMC route

Pervious concrete is usually deliveredto the site in ready mix trucks andmixed at a specified mixing speed toresult in 75 to 100 rotations of themixing drum before discharge. Afterthe closely controlled amount of wateris added, one hour is usually themaximum time allowed for discharge. The batch must be placed, rolled,


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rapid return of traffic. Costs may be equal or less than the cost of usingstandard compacted backfill.

Flowable Fill through the RMC route

Ready Mixed Flowable Fill is a very specialized product that has a wide

variety of applications. It is an excellent choice for filling any subsurfacevoid when quality is critical or when inert filler is required. A ready mixconcrete producer can aid in developing a mix design for flowable fill.However, when ordering, consideration should be given to the followingproperties:

Strength: Applications that require removal of flowable fill at a later dateusually limit the maximum compressive strength to less than 1.4 MPa.

Setting and Early Strength: Hardening time can be as short as one hour,but can take up to eight hours depending on mix design and trenchconditions (e.g moisture, temperature).

Density in Place: The in-place density of normal flowable fill typicallyranges from 1400 to 2000 kg/m3.

Flowability: Flowability can be enhanced through the use of fly ash or airentraining admixtures.

Durability: Flowable fill is not designed to resist freezing and thawing,abrasive or erosive actions, or aggressive chemicals.

CONCLUDING REMARKS

There are a variety of “special” concretes being made available to theindustry today (and the list is growing), thanks to the dedicated efforts of

the RMC industry. Since these concretes require a number of ingredientsto be added very judiciously, elaborate storing and batching of rawmaterials is required. The modern Ready Mix Plants have thewherewithal to handle the complexities and deliver the right concrete atthe right time

References

1. Henry G. Russell, “What is high performance concrete?”, Jan1999, Concrete Products

2. H. Okamura and M Ouchi, “Self Compacting Concrete”, Journalof Advanced Concrete Technology, Vol. 1, No. 1, April 2003, JapanConcrete Institute.

3. Manamohan R Kalgal, “Fiber Reinforced Concrete For StructuresIn Seismic Areas”, Proceedings of the International Conference onIndustrial Structures – ISIS 2003, ACCE Coimbatore

4. Malhotra, V.M., and P.K. Mehta, “High-Performance, High-Volume Fly Ash Concrete”, Supplementary Cementing Materials forSustainable Development, Inc., Ottawa, Canada, 2002, 101 pp.


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Scc Rmc Route Cidc 9.11

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