SILICA FUME

The use of pozzolanic materials is an old as that of  the art of concrete construction. it was recognised long time age, that the suitable pozzolns used in appropriate amount, modify certain properties of fresh and hardened mortars and concretes.

It has been amply demonstrated that the best pozzolans in optimum proportions moxed with portland cement improves many 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 extensibility;

·                     Lower gusceptibility to dissolution and leaching;

·                     Improve workability;

·                     Lower costs.

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 presense of moisture, chemically react with calcium hydroxide liberated on hydration, at ordinary temperature, to formcompounds, possessing cementitious properties.

What is Silica fume?

Silica fume is a by-product from electric are furnaces used in the manufacture of silicon metal of silicon alloys. The material, which contains more than 80% silica in no crystalline state and in the form of  extremely fine particles (0.1 um average diameter), is highly pozzolanic. This product is excellent for use as a Portland cement supplement In addition to economic and energy saving potential, the use of pozzolanic admixtures in concrete leads to several technical advantages, such as reduction in thermal cracking caused by heat of cement hydration, improved durability to attack by sulphate and acidic waters, and high ultimate strength.

Unlike other by- product pozzolans such as fly ashes, a unique feature of CSF is that it has a better faster pozzolonic action.

How silica fume works...

CEMENTITIOUS COMPOUNDS

In cementitious compounds, silica fume works on two levels, the first one described here is a chemical reaction called the "pozzolanic" reaction. The hydration (mixing with water) of Portland cement produces many compounds, including calcium silicate hydrates (CSH) and calcium hydroxide (CH). The CSH gel is known to be the source of strength in concrete. When silica fume is added to fresh concrete it chemically reacts with the CH to produces additional CSH. The benefit of this reaction is twofold; increased compressive strength and chemical resistance. The bond between the concrete paste and the coarseaggregate, in the crucial interfacial zone, is greatly increased, resulting in compressive strengths that can exceed 15,000 psi. The additional CSH produced by silica fume is more resistant to attack from aggressive chemicals then the weaker CH.

Chemical composition of silica fume

Chemical composition of silica fume will mostly depend upon compositon of the principle product being made by furnace.additionally,the composition is also influnced by the furnace design; generally a furnace with a heat recovery system producessilica fume with lower ignition loss.the sio2 content of silica fume varies with the silica content of alloy being producer.unlike other by product pozzoloans such as fly ash,silica fume from a single source is little or no varition inchemical composition from one day to another,

Item	Requirement
Sio2	>90%
mgo	<1.5%
SO3	<1.1%
H2O	<0.4%
K2O	<2.25%
Na2O	<1.4%
CaO	<0.35%
Si	<0.5%
Cl	<0.06%
Fe2O3	<2.0%

Physical characteristics

The specific gravity of  CSF from silicon metal of ferrosilicon alloy industries is close to that of amorphous silica, that is approximately 2.2

The amorphous silica structure and the fine particle size re the principle reasons for the excellent pozzolanic activity of CSF, the surface area by Blaine air permeability method ranged from about 3.3 t 7.7 m2 / g The spherical shape of CSF can be confirmed by scanning electron microscopy of well- dispersed particles Figure 5.5 ) generally, the particles exist in the form of agglomerates.

Properties of fresh concrete using silica fume

Workability:-

Water demand increases in proportion to the amt of silica fume added. The increase in water demand of concrete containing silica fume is about 1% for every 1% cement substituted Workability is the ease with which a concrete mix can be handled. Water measures can be taken to avoid this increase by adjusting aggregate grading and use of super plasticizer. The addition of silica fume will lead to a cohesive mix due to more solid-to-solid contact and will have a lower slump. This reduces bleeding and segregation.

Bleeding and segregation:-

The effect of silica fume on the rheology of fresh concrete is considered a stablising affect in the sense that addition of fine particle to a concrete mix tends to reduce segregation and bleeding. When very fine particles of silica fume are added to concrete the size of flow channel is greatly reduced because these particle are able to find their way into empty spaces between 2 cement grains causing drastic segmentation of bleed water flow and reduce bleeding. Due to increase in the number of solid-to-solid contact, the cohesiveness of the concrete mix is greatly improved when silica fume is added. This makes the concrete highly attractive for pumping, shotcreting as it reduces segregation.

Time of set:-

 Silica fume in small amounts (10% by weight of cement) to ordinary concrete mixtures (250 to 300 kg/ m3 cement) either has no significant effect or alters the time of set of reference concrete only slightly. For example, the data reported by pistilli et al,27 showed that the presence of 24 kg/m3 CSF (FeSi - 75 type) in a concrete mixture containing 237 kg m3 Type 1 Portland cement increased the initial and the final times of set (ASTM Method (403) by 26 min and 29 min respectively.

Plastic Shrinkage:-

Freshly placed concrete mixtures that have not vet set (i.e., that are still in a plastic state ) are prone to surface cracking due to the phenomenon known as plastic shrinkage. Since concrete containing CSF shows little or no bleeding, there are several reports confirming the sensitivity of this concrete to plastic- shrinkage, cracking when exposed to drying conditions at early age. In order to overcome this problem, the surface of concrete must be protected against evaporation as soon as possible after placement. This precaution is a part of the standard concrete curing practice in hot weather.

Properties of hardened silica fume concrete

Drying shrinkage:-

The date from drying shrinkage tests by different researchers shows that the long- term shrinkage of concrete is not affected significantly by the addition of CSF, especially when the water content of the concrete mixture has not been changed.

Creep:-

Creep of concrete is inversely proportional to strength. Since CSF is a highly pozzolanic material, it is expected that the creep of concrete containing CSF will be lower than the corresponding Portland cement concrete a reference concrete made of a calcareous aggregate and normal Portland cement, and a concrete made of the same amount of the aggregate but with 25% (by weight) cement replaced by CSF. The ratio between water to cementations material was 0.435

Examination of the 1- year creep data showed that basic creep strains were similar in both the cases: however, the drying creep strain was about 370 X 10-6 for the reference concrete compared to 300 X 10-6 for the CSF concrete

Strength charecteristic:-.

If CSF is used as an addition, there is no deleterious effect on early strengths (i.e. I- day and 3 day strengths), and a noticeable strength increase is recorded during the 3 to 28 days moist- curing period 30 when most of the pozzolanic reaction takes place. Consequently, the relative strength increase during the 28 to 90 - day period is relatively low. From the results of the investigation reported the addition of 24 kg/m³ CSF to the concrete mixture containing 297 kg/m3 ASTM Type I Portland cement caused a strength increase of about 10% and 20% at 7 and 28 days, respectively; there were no differences in the strength of reference concrete and the CSF - concrete at the test ages I and 2 days.

Three series of both non air- entrained and air- entrained concrete mixtures were designed. The first contained 284kg / m³ of ASTM Type I Portland cement 0.6 ratio between water to cementations materials (i.e., Portland cement + CSF), and 0.5, 10, and 15 % cement replacement replacement by weight with CSF (94 % SiO2, 20 m2/g surface area) The second contained 340kg/m³ Portland cement 0.5 ratio between water to cementations materials, and similar levels of cement replacement with CSF as the first series. The third contained 431 kg /m³ Portland cement, 0.4 ratio between water to cementations materials.

The incorporation of CSF did not result in a significant change in compressive strength at 3 days with concretes of 0.6 and 0.5 ratio of water/ (cement+ CSF); however, the concrete with 0.4 ratio of water / (cement + CSF); showed increase in strength with increasing amount of the fume used in the test (i.e. 5 to 15 % cement replacement)

Regardless of water / (cement + CSF) ratio at 7 days and 28 days the compressive strength of concrete were increased.

All air- entrained concretes, both with and without: CSF, showed strength loss when compared with the corresponding non- air entrained concrete

DURABILITY ASPECTS

Permeability:-

Durability o a concrete to aggressive water is generally a direct function of its permeability. Manmohan and Mehta observed that highly reactive pozzolans, such as rice husk ash, are able to reduce the size of voids in hydrated cement pastes, thus making them almost impermeable even at an early age (7- 28 days) with as low as 10% additio of the pozzolan by weight of cement. a concrete moxture containing 100 kg / m3 portland cement 20% CSF, and a superplasticizer showed approximately the same permeability as a concrete containing 250 kg/m3 Portland cement but no CSF or plasticizer.

Abrasion resistance

Generally, in high- strength concrete there is a direct relationship between strength and abrasion resistance. It may be noted that exceptionally high strength in concrete cannot be achieved unless concrete is dense (low water cement ratio) and a strong aggregate has been used. The factors that have a beneficial effect on strength have also a beneficial effect on abrasion resistance. HENCE SILICA FUME CONCRETE HAS HIGH RESISTACE TO CONCRETE. A major reason for the improved resistance of concrete to acidic and sulphate waters is the reduction in the calcium hydroxide content of the cement paste which decreases linearly with the amount of CSF added. With 20% CSF by weight of cement, very little of the calcium hydroxide produced by the Portland cement hydration is left in a well hydrated cement paste.

Alkali- aggregate reaction

The amount of pozzolan needed for reducing the alkali - aggregate expansion depends on the reactivity of the pozzzolan. Whereas many researchers have reported that 30 to 40% replacement of a high- alkali Portland cement by a class F fly ash may be needed to control the alkali- aggregate expansion according to the ASTM Method C 227, even less than 10% CSF is found adequate for this purpose.

Resistance Corrosion of embedded steel

The service life of reinforced and prestressed concrete is often adversely affected by corrosion of the embedded steel. A strongly adherent iron - oxide film is usually present on the surface of steel.  and this film must get disrupted before the anodic and the cathodic processes associated with the corrosion phenomenon can make sufficent progress. In some situations, such as in the presence of chloride ions, it somehow gets disrupted: this explains why cholorides are known to accelerate the corrosion of steel in concrete.

The ability of concrete to protect embedded steel from corrosion also depends on electrical resistively.

Freeze – thaw resistance

Air void stability of concrete incorporating silica fume was studied. The test results indicated that the use of silica fume has no significant influence on the production and the stability of the air void system. However freeze-thaw testing on silica fume concrete showed acceptable results. The durability was considerable greater as compared to normal concrete

Industrial Use:

Case Study 1:

The construction of New Tjorn Bridge (Figure 5.15) in Sweden is believed to be the first industrial use of CSF for obtaining both a high strength and a lower heat of hydration than otherwise would have been possible According to Rockne and Svensson45 the bridge was designed to contain some very large structural members (Pylons) of a high - strength concrete (50MPa). The use of a high content of Portland cement in the concrete mixture would have caused excessive heat of hydration with risk of thermal cracking. The principal reason for using CSF was that this permitted a deduction of the cement content without loss of strength. The average strength of concrete was 62 MPa : reduction in the Portland cement content that was made possible by the use of CSF puls the post- cooling of concrete by circulation of cold air through imbedded pipes helped to lower the peak concrete temperature by 10 to 12 C. Case Study 2CSF was used to produce a very high- strength concrete foe a high - rise building in Montreal,canada. The average compressive strength of concrete cylinders was reported to be apa at 3 days. 75 MPa at 7 days, and 90 MPa at 28 days.

Case Study 2: -

In 1980.SKW Canada. Inc., xonstructed an experimental sidewalk in Becancour (Quebec) with a concrete containing CSF. Until the time of his report in1983, thesidewalkhassurvived three Canadian winters and numerous de-icing salt applications proving durability of silica fume concrete.

Cementitious application of silica fume

High Strength Concrete

High Performance Concrete (HPC) enhanced with silica fume is so strong it becomes an economical alternative to steel. Besides providing architects and engineers with greater design flexibility, there is more usable space, since smaller columns and beams can be used in high rise buildings and long span structural designs. Other advantages include:

• Tremendous compressive strengths up to 20,000 psi (140 MPa).
• High modulus of elasticity exceeding 6 million psi (40,000 MPa). High flexural strengths (2,000 psi @ 28 days) for airport pavements.
• Greatly reduced permeability to moisture, chlorides and chemical attack.
• Increased resistance to abrasion, erosion, corrosion.
• High early strengths for fast-track construction projects and precast applications.

 Shotcrete 

Greater economy, greater timesaving and more efficient use of sprayed concrete. Norchem silica fume produces superior shotcrete for use in rock stabilization; mine tunnel linings, and rehabilitation of deteriorating bridge and marine columns and piles. Greater bonding strength assures outstanding performance of both wet and dry process shotcreting, with greater impermeability and less rebound loss. Norchem silica fume provides improved cohesion that resists washout in tidal rehabilitation of piles and sea walls. In addition, thicker applications--up to 12 inches-- can be achieved with each pass of the nozzle. Superior performance characteristics are:

• High compressive and flexural strengths.
• Reduction of rebound loss up to 50%.
• Improved production time, with one-pass application thicknesses up to 12 inches           higher bonding strength.
• High electrical resistivity and low permeability

Oil Well Grouting

Whether used for primary (placement of grout as a hydraulic seal in the wellbore) or secondary applications (remedial operations including leak repairs, splits, closing of depleted zones), the addition of silica fume enables a well to achieve full production potential. Besides producing a blocking effect in the oil well grout which prevents gas migration, it provides these advantages:

• Improved flow, for easier, more effective application
• Dramatically increased impermeability, for better control of gas leakage
• Easier handling

• Better results

The use of CSF in concrete offers many advantages; however for several reasons the indiscriminate use of the material is not recommended. Incorporation of any additional ingredients into concrete requires extra handling and storage facilities Besides cost, special precautions will be needed for batching unusual materials such as CSF, and for curing concrete containing this admixture.

Cost. Although CSF is an industrial waste, due to its fine particle size and very low bulk density the cost of shipping and handling the material is rather high. The material that is being marketed in the form of a water suspension probably costs two to three times as much.

FORMWORK

What is Formwork 

Formwork is a die or a mould including all supporting structures, used to shape and support the concrete until it attains sufficient strength to carry its own weight. It should be capable of carrying all imposed dead and live loads apart from its own weight.
INTRODUCTION TO FORMWORK

•  Formwork has been in use since the beginning of concrete construction.
•  New materials such as steel, plastics and fiberglass are used in formwork.
•  greater attention is being given to the design, fabrication, erection and dismantling of formwork

DEFENITION:

•  As a structure,
•  Temporary which is designed to contain fresh fluid concrete.
•  Form it into the required shape and dimensions.
•  Support it until it cures sufficiently to become self supporting.

The term ‘formwork’ includes the actual material contact with the concrete, known asform face, and all the necessary associated supporting structure.

REQUIREMENTS OF A GOOD FORMWORK SYSTEM

§  How formwork can be erected and de-shuttered fast.

§  How good concrete quality and surface finish can be achieved.

§  What is the optimum stock of formwork required for the size of work force, the specified time schedule and flow of materials.

§  What is the overall cost savings that can be achieved using the right type of formwork.

§  How SAFETY can be improved for the site personnel.

In order to successfully carry out its function, formwork must achieve a balance of following requirements:

•  Containment
•  Strength
•  Resistance To Leakage
•  Accuracy
•  Ease Of Handling
•  Finish And Reuse Potential
•  Access For Concerted
•  Economy

Containment: formwork must be capable of shaping and supporting the fluid concrete until it cures.

Strength: formwork must be capable of safely withstanding without distortion or danger the dead weight of the fluid concrete is placed on it, labour weight, equipment weight and any environmental loadings.

Resistance to leakage: all joints in form work must be either close fitting of covered with form tape to make them grout tight. If grout leakage occurs the concrete Will leak at that point. Leakages cause honeycombing of the surface.

Accuracy: formwork must be accurately set out so that the resulting concrete product is in a right place and is of correct shape and dimensions.

Ease of handling: form panels and units should be designed so that their maximum size does not exceed that which can be easily handled by hand or mechanical means. In addition all formwork must also be designed and constructed to include facilities for adjustments, leveling, easing and striking without damage to the form work or concrete.

Finish and reuse potential: the form face material must be selected to be capable of consistently imparting the desired concrete finish (smooth, textured, featured or exposed aggregate etc.) At the same time it should also achieve the required number of reuse.

Access for concrete: any formwork arrangement must be provide access for placing of the concrete. The extent of this provision will be dependent on the ease of carrying out the concrete operations.

Economy: all the formwork is very expensive. On average about 35% of the total cost of any finished concrete unit or element can be attributed to its formwork; of this just over 40% can be taken for material for formwork and 60% for labour. The formwork designer must therefore not only consider the maximum number of times that any form can be reused, but also produce a design that will minimize the time taken for erection and striking.

FORMWORK BASED ON MATERIALS

MATERIALS FOR FORMWORK

                                                Formwork can be made out of a large variety of materials.

•  The material most commonly being used to date is timber. However, due to the depleting forest reserves and increasing cost of timber the use of alternate materials such as plywood and steel has become prominent.
•  More recently, materials such as plastics and fiberglass are also being used for pre-fabricating formwork.
•  The type of material to be used depends on the nature of construction as well as availability and cost of material.
•  The constraints on the project such as overall cost, time of completion also play a major role in the use of a particular material for formwork.

TIMBER FORMS

Timber is required for practically all jobs of formwork. The timber bring used for formwork must satisfy the following requirements:

1. It should be durable and treatable
2. It should have sufficient strength characteristics
3. It should be light weight and well seasoned without warping,
4. It should hold nails well.

Advantages of using timber forms:

1. It is economical for small construction jobs
2. It is design flexible and easy to erect
3. It has good thermal insulation which makes it useful to be used in colder

            Regions

Iv.      It can easily be made into any shape or size

Plywood forms (in combination with timber)

• Concrete shuttering plywood is bwp grade plywood, preservative treated and specially suited for use in concrete shuttering and formwork.
• The plywood is built up of odd number of layers with grain of adjacent layers perpendicular to each other.
• Plywood is used extensively for formwork for concrete, especially for sheathing, decking and form linings.
• There are two types of plywood - internal and exterior.
• The interior type is bonded with water resistant glue and exterior type is bonded with water proof glue.

Hardboard forms

•  Hardboard is a board material manufactured of wood fiber, which is then refined or partly refined to form a panel having a density range of approximately 50 to 80 pounds per cubic foot.
•  Hardboards are standard / non-tempered or tempered.
•  The tempered one being used for formwork. Tempered hardboard is solid or perforated hardboard panels impregnated with resin under high pressure to make them stronger and more resistant to moisture and abrasion.
•  The boards available in large sheets have a hard, smooth surface that produces a concrete whose surface is relatively free of blemishes and joint marks.
•  The thin sheets can be bent to small radii, which is an advantage when casting concrete members with curved surfaces.

ALUMINIUM FORMS

•  Forms made from aluminum are in many respects similar to those made of steel.
•  However, because of their lower density, aluminum forms are lighter than steel forms, and this is their primary advantage when compared to steel.
•  As the strength of aluminum in handling, tension and compression is less than the strength of steel, it is necessary to use large sections.
•  The formwork turns out to be economical if large numbers of reuses are made in construction.
•  The major disadvantage of aluminum forms is that no changes can be made once the formwork is fabricated.

PLASTICS

      These forms have become increasingly popular for casting unique shapes and patterns being designed in concrete because of the excellent finish obtained requiring minimum or no surface treatment and repairs.

            Different types of plastic forms are available like glass reinforced plastic, fiber reinforced plastic and thermoplastics etc.

   Fiberglass-reinforced plastic is the most common and has several advantages such as

1. The material allows greater freedom of design
2. Unusual textures and designs can be molded into the form
3. It   allows   the   contractor   to   pour   structural      and   finished   concrete

            Simultaneously

4. Because sections can be joined on the job site in such a way so as to eliminate joints, there is no size limitation

If carefully handled, a number of reuses are possible making it highly

            Economical

Vi.      It is lightweight and easily stripped

          The disadvantage of using plastic forms is that it does not lend itself to field fabrication  Hence, the design and planning of this form must be carefully carried out.Also care must take not to damage the plastic by the heat applied for accelerated curing of the concrete. Trough and waffle units in fiberglass are used in construction of large floor areas and multistoried office buildings.

STEEL FORMWORK:

            Mostly used in large construction projects or in situations where large numberof re-uses of the same shuttering is possible. Suitable for circular or curved shaped structures such as tanks, columns, chimneys. Etc. & for structures like sewer tunnel and retaining wall.

Advantages of steel formwork over timber form:

1.             strong, durable & have longer life
2.             Reuses can be assumed to vary from 100 to 120 wares timber varies from 10 to 12.
3.             Steel can be installed & dismantled with greater ease & speed resulting in saving in labour cost.
4.             Excellent quality of exposed concrete surface obtained. Thus saving in the cost of finishing        the  conc. surface.
5.             no danger of formwork absorbing water from the conc. & hence minimizing honeycombing

CONSTRUCTION OF FORMWORK:

•          propping and centering

•          shuttering

•          provision of camber

•          cleaning & surface treatment

Propping and centering:

                        The props used for centering may be of steel, timber post or ballies.pillars made up of brick masonry in mud mortar are also sometimes used as props.

Shuttering:

                        can be made up of timber planks or it may be in the form of panel unit made either by fixing ply wood to timber frames or by welding steel plates to angle framing.

Provision of camber

                        Certain amount of deflection in structure is unavoidable. It is therefore desirable to give an upward camber in the horizontal member of conc. Structure to counteract the effect of deflection.

Surface treatment

•  Before laying conc. The formwork should be cleaned of all rubbish particularly the sawdust savings & chippings etc.
•  Before laying conc. the face of formwork in contact with conc. shall be cleaned & treated with release agent like raw linseed oil or soft soap solution as to prevent the conc. getting struck to the formwork.

Order and method of removing formwork:

•  Shuttering forming vertical faces of walls, beams & column sides should be removed first. Shuttering forming sofit to slab should be removed next.
•  Shuttering forming soffit to beams, girders or other heavily loaded member should be removed in the end.

DURATION TAKEN FOR REMOVAL OF FORMWORK

1        WALLS COLUMNS & VERTICAL SIDES

            OF BEAMS                                                                            1-2 DAY

2        SLABS                                                                                   3 DAYS

3        BEAM SOFFIT                                                                      7 DAYS

4        REMOVAL OF PROPS TO SLABS

            A) SLAB SPANNINIG UPTO 4.5M                                    7 DAYS

            B) SLAB SPANNINIG OVER 4.5M                                   14 DAYS

5        REMOVAL OF PROPS TO BEAMS

            AND ARCHES

            A) SPANNING UPTO 6 MTS                                              14 DAYS

            B) SPANNING OVER 6 MTS                                              21 DAYS                  

TYPES OF FORMWORK

                        There are different types of formwork available for different purposes. Generally, the formworks for vertical concreting are called wall forms and those for horizontal concreting are called slab or floor forms. The various types of formwork available today in the market are discussed in detail.

TRADITIONAL FORMWORK

•  This usually consists of standard framed panels tied together over their backs with horizontal members called waling.
•  The waling is provided with the basic function of resisting the horizontal force of wet concrete.
•  One side of the wall formwork is first assembled ensuring that it is correctly aligned, plumbed and strutted.
•  The steel reinforcement cage is then placed and positioned before the other side of the formwork is erected and fixed.
• Plywood sheet in combination with timber is the most common material used for wall formwork.
•  The usual method is to make up wall forms as framed panels with the plywood facing sheet screwed on to studs on a timber frame. This allows for the plywood to be easily removed and reversed and used on both sides so as to increase the number of reuses.
•  The wall forms are susceptible to edge and corner damage and must be carefully handled.
•  Special attention must be given to comers and attached piers since the increased pressures applied by wet concrete could cause the abutments to open up, giving rise to unacceptable grout escape and a poor finish to the cast wall.

CLIMBING FORMWORK

•  Method of casting walls consists of a climbing formwork, the climbing of which may be manual or crane assisted.
•  It employs a common set of forms used in a repetitive manner for casting walls in set vertical lifts.
•  After each casting the forms are removed and raised to form the next lift until the required height has been reached.
•  These forms are widely used in the construction of industrial chimneys, silos, high rise towers & building cores, bridge piers & pylons, airport control towers, telecommunication, towers etc.

The climbing form has many advantages such as the following

•   Staged construction process allows balance of site resources.
•   Anchor accessories can be reused after each pour, reducing material costs on current and future construction programs.
•   In case of trolley mounted formwork, the panel retracts from the face, providing space for cleaning and fixing of concrete.
•  Formwork & access platforms lifted as one, minimizing crane support, reducing labour and material costs.
•   fine adjustments of the form face can be made during construction, providing accurate alignment of the form face vertically & laterally.

SLIDING FORMWORK OR SLIPFORMING

•  slip form means a continuously moving form, moving with such a speed that concrete when exposed has already achieved enough strength to support the vertical pressure from concrete still in the form as well as to withstand lateral pressure caused by wind etc.
•  Thus, the slip form concreting technique is a rapid and economical construction method that can be applied with great advantage to many types of construction projects such as chimneys, silos, water towers, bridge-columns, lift shaft cores and shaft lining etc.
• The technique is based on movable forms which are gradually lifted by hydraulic jacks.
•  It is a continuous process where wet concrete is added to wet concrete. Reinforcing steel and/or post tensioned cables are continuously fixed as the normal slipping speed is 3 to 6 meters per 24 hours. The slip form construction is designed for each project depending on the structure of the project. The advantages of slip forming are
• Minimum consumption of timber and steel plates.
• Total elimination of traditional scaffolding
• Minimum requirements of carpenters for assembling.
• It gives a monolithic structure.
• The concrete surfaces can be treated and finished while concrete is green,
• Depending on the weather conditions, it is possible to achieve a vertical rise to the tune of 4 to 5 m in summer and 2 to 3 m in winter.
• The procedure of continuous slipping is applied to making both inner and outer walls as well as columns of a building.
• Form climbs.

PERMANENT FORMWORK

• Permanent form or stay-in-place formwork is one in which the form is left as an integral part of the structure.
• Permanent formwork can also be utilized as the facing materials of in situ reinforced concrete. They can be of two types—participating and non-participating.
• The material used for these forms must be durable and of sufficient strength. Commonly used materials include polyvinyl chloride (pvc), galvanized coiled sheet steel, fabricated steel, carbon/epoxy thin shell.
•    The high initial cost of design and installation, lack of familiarity for installation and maintenance and more specified form design are some of the barriers to the use of this form.
•    However, there are various advantages like low cost of transportation and installation, precise form design, maximum flexibility, greater durability with reduced long term maintenance and versatility.

SPECIAL FORMS

            These are those forms that are specially designed and manufactured for a particular kind of construction. The need for a special formwork may arise due to several factors such as

•         when the contract demands the highest class of dimensional tolerance to be followed

•         Where the form work shape required becomes uneconomical or impracticable for site fabrication

•         Where the formwork is required to be self-contained i.e. self propelled,

•         Where rate of concreting, admixtures or types of concrete are such that concrete pressure developed within forms and stresses in the forms demand special attention where a substantial number of re-uses is envisaged

TABLE FORM

•  This is a special formwork designed for use in casting large repetitive floor slabs in medium to high-rise structures.
•  The main objective of reducing the time required re-erecting, striking and re-erecting slab formwork.
•  A system which can be put as an entire unit, removed, hoisted and repositioned without any dismantling.

GANGED PANEL FORM

•  The increasing pace in the construction of multi-storey and massive concrete structures, and the parallel progress in development of cranes and other mechanical methods of transporting forms have made the use of ganged prefabricated forms for the concreting of large sections of high walls very common.
• Ranging up to 30x50 ft, their size is limited only by the mechanics of handling. Large panels can be assembled flat on the ground where it is easier to work. Delay and lost motion are avoided in stripping because the gang forms are stripped as a unit.

TUNNEL FORM

            The tunnel formwork is a room sized structural steel fabricated form which is used to cast the rcc walls and floor slabs of a building as a monolithic structure in a continuous pour. The forms are then heated using hot air blowers for accelerated curing of the concrete. This system is most economical when the structure consists of large number of identical units. There exist two versions of this type of formwork. They are:

A.        The half tunnel formwork used to cast only one wall and slab simultaneously

B.        The full tunnel formwork used to cast two walls and a slab simultaneously

            The sequence of construction involves placing of reinforcement, electrical and sanitary conduits along with the tunnel forms. Concrete is then poured and the open side of the forms is covered and hot air blowers placed inside. The forms are removed the next day and placed on the next site using cranes. The optimum use of tunnel form is in multiunit shear wall structure with identical floor layout at each level.

DOKA FormWork System.

I. Doka System Components

            The various basic components that make up the various DOKA system are as follows:

1. Doka formwork beams
2. Doka formwork sheets
3. Dokadur panels
4. Doka floor props
5. Form ties and suspension cones
6. Multi-trip packaging

1. Doka formwork beams

           

            The core of the system lies in the usage of an Engineered timber component, the  H-Beam.

The H-beams are manufactured in a modern automated plant at Pondicherry under strict quality control the flanges are made of seasoned chemically treated timber. The web is made of boiling water proof ply wood and joined with the flange by the unique finger jointing method. The H-beams thus manufactured are  light, dimensionally stable and retains its structural properties over a period of time even after repeated usage. It is more predictable, easy to design and use, The number of reuses of H-Beams is more than 100 times (8 times that of conventional timber) and it consumes only 40% of timber volume required.

The H-beams are available in two size namely in H-16 - 16 cms  depth &  H-20 - 20 cms  depth,  length varying between 1m to 6m.

Salient Feature

•          Reduction in consumption of timber.

•          Making work at site minimized.

•          No. of reuses more than 8 times that of conventional timber.

•          Dimensionally stable, uniform in size and consistent in strength.

•          Cost ratio per use H-16 beam : conventional timber = 1:3.5

•          Economical and long - lasting.

•          Light weight @ 6kgs per RMT.

Max. Shear Force	Max. Bending Moment	EI
11 KN	4 KN/M	170 x 106 KG.CM2

Doka beam H20

Innovative end reinforcement .

• For less damage to the ends of the beams
• For outstanding durability

Outstanding production level.

• Ensures uniformly high quality and load-bearing strength for safe and dependable usage
• Is the basis for the reliability of the Doka beam formwork and Dokaflex floor formwork
• From mechanical strenght grading

Practical marks for all standard lengths.

§  As a grid for easy installation and checking of the Dokaflex 1-2-4 system

Doka beam H20 eco

Ends of beams bevelled for more strength but has no end reinforcement.

2. Doka formwork sheets

Doka has an extensive range of formwork sheets for the most varied areas of application. All sheets are made of glue-bonded layered wood and are extremely strong and dimensionally stable.

Formwork sheet 3-S Plus

Three-ply concrete-formwork sheet, made of European spruce (picea abies), designed specially for building. Produces a uniform concrete surface.

  Surface: Synthetic melamine resin glue with PU sealant and light corundum sanding on one side

  Bonding: Boilproof and weatherproof

  Edges: Impregnating emulsion,
Doka yellow

  Thicknesses: 21 and 27 mm

Formwork sheet 3-SO

Three-ply concrete-formwork sheet, made of European spruce. Produces a uniform concrete surface.

§  Surface: Synthetic melamine resin glue

§  Bonding: Boilproof and weatherproof

§  Edges: Impregnating emulsion,

§  Doka yellow

§  Thicknesses: 21 and 27 mm

Dokaplex Multi-ply sheets

High-grade multi-ply sheet made of Finnish birch hardwood for use again and again. Produces a high-quality, smooth concrete surface.

• Surface: Phenol-resin coating, 120 g/m²
• Bonding: Boilproof and weatherproof phenol-resin glue (BFU 100) to DIN 68705-T3
• Edges: Dispersion
• Thickness: 21 mm

3. Dokadur panels

Dokadur panels are the state of the art for floor-slab panels. All-round edge and surface sealing dependably protects the panel against the wear and tear of everyday construction work.

Maximised number of reuses and best-quality concrete surfaces.

• From special surface sealing by means of PUR varnish and melamine resin coating with precision-metered corundum sanding
• For improved safety at work, because risk of slipping is reduced
• From significantly reduced moisture absorption for much-reduced discolouration, structuring and cracking

Big savings on costs

• From easy cleaning of the surfaces, ready for the next use
• From all-round edge protection made of high-grade PU
• For exact edges with minimal cleaning
• For low costs on account of easy and fast reconditioning of the edge

4. Doka Floor Props.

Doka floor props are the right choice for every application. High load-bearing strength plus many practical details that help to make handling easier.

• The props are available in various sizes viz. CT-250,CT-300,CT-340 & CT-410. The number indicates the extended length of props in cms.

• Carrying capacity is rated from 20 kN to 30 kN.

• The tripods make the props self standing for  easier and faster erection of the shuttering system. The adjustments in height are obtained by operating the prop nut. The required dimension in plan is obtained by side-lapping of the H-Beams in the primary or secondary layer.

• A very accurate and convenient shutter is ready for tying of reinforcement and concreting.

• The system also facilitates re-propping. By adopting the method of repropping it is possible to reduce the total quantity of formwork materials significantly. The system is very well adapted for use alongwith the L&T-Doka Beam Forming Supporting system.

5. Form ties and suspension cones

Doka has a complete range of tried-and-tested formtie solutions and dependable suspension points for wall formwork, single-sided formwork and climbing formwork in uncompromising quality for maximum safety.

Doka tie rods and anchor accessories

§  Provide safety through superb manufacturing quality

§  Reduce labour costs for installing ties, because a hammer is all that is needed for easy installation

§  Are durable, robust and unaffected by dirt

Robustly dimensioned universal climbing cones

§  ensure firm connections between structure and formwork

§  for safety on high structures

§  for all kinds of climbing formwork

Safe suspension solutions for working and protection platforms

§  With different attachments to suit the application

§  Ideally matched to the carrying capacity of Doka working and protection platforms

§  Easy to install and reusable.

6. Multi-trip packaging

Multi-trip packaging such as containers, stacking pallets and skeleton transport boxes  keep everything in place on the site, minimise time wasted searching for parts, and streamline the storage and transport of system components, small items and accessories.

Savings on material overheads and labour costs.

• Through faster loading and unloading of system components, small items and accessories
• Through easy relocation to the next point where the parts are needed
• Through safe storage in stacks, particularly when space is at a premium

Stacking pallets 150 and 120 simplify the storage and transport of floor props, removable folding tripods, formwork beams and Dokadur panels. The clamp-on wheels make the stacking pallets mobile, so they can easily be steered through standard door-size openings in residential accommodation projects.

II. DOKA Floor System

No matter what the room height, the shape of the layout or the slab thickness, with Doka you always have exactly co-ordinated formwork in one single consistent system, comprising a conveniently small number of easy to manage system components.

Dokaflex 1-2-4

            Dokaflex 1-2-4 is the fast, versatile floor formwork for floorplans of any shape, for beams, slab overhangs and semifinished floor elements – and the ready reckoner is ideal for calculating the quantities of materials, so there's no need for formwork planning. The free choice of formwork sheets leaves nothing to be desired when it comes to the finished structure of the fair-face concrete.

• The L&T-Doka Fex system is suitable for RC-floors upto 4.40 m high.

• The plywood sheathing is supported by a layer of secondary H-Beams at the designed spacing. The primary layer of H-Beams  are supported with necessary accessories over the collapsible telescopic props fitted with tripods to ensure lateral stability.

Defined positioning grid with full flexibility in floorplan geometry

• For quick erection of the formwork, because the positioning points are clear
• Enables rapid adaptation to walls and columns by simply telescoping the transverse and longitudinal beams

Speedy progress and simple logistics

• Because there is only a small number of matched individual components
• With high-grade Eurex floor props with consecutively numbered pegging holes and low release forces
• With high-grade Eurex floor props with consecutively numbered pegging holes and low release forces
• Because the ready reckoner makes it easy to calculate the quantities needed
• Because dispensing with planning and preparatory operations cuts costs by a significant margin
• Because the maximum pitches for longitudinal and transverse beams and props are marked on the beams - for floor-slab thicknesses up to 30 cm

Safety and economy

• From durable and robust individual components
• Because the panels can be rented
• With high-grade polyurethane surround for first-class concrete surfaces and reduced investment costs
• With non-slip surfaces for significantly enhanced safety at work
• With high-grade Eurex floor props with consecutively numbered pegging holes and low release forces
• With the new, much-improved longitudinal and transverse beams for significantly reduced post-use costs

Every requirement for fair-faced concrete fulfilled

• By free choice of formwork sheets
• By the sealed surface of the rentable panels

Dokamatic table

The innovative design of the Dokamatic table makes for even faster formwork handling whenever large floor slabs have to be cast. Standard functional components can be installed for straightforward, speedy adaptation to changing requirements on the construction site.

Fast repositioning reduces labour costs

• Because fully assembled units are manoeuvred quickly into place - no laborious carrying of individual components from one location to the next
• Because practical shifting devices makes for virtually fatigue-free operations
• Because easier to handle and safer than hand-operated formwork, particularly as room heights increase

The Dokamatic table helps save on labour and on crane time: One man using the shifting trolley with attachable drive unit can move the tables to the next casting location on the same level. The system is optimised for minimal forming times on large-area projects and deals easily with varying requirements in terms of statics and geometry.

Dokamatic table sizes

• Comes in 4 rentable standard sizes with grid logic: 4.0 x 2.0 m, 4.0 x 2.5 m, 5.0 x 2.0 m and 5.0 x 2.5 m
• Special sizes for special applications can be supplied at any time
• Made up of high-grade system components such as the sturdy Dokamatic table waling 12 and Doka beams H20 top for outstanding durability and minimum post-use costs
• Fully assembled Dokamatic tables delivered to your site right on time

Load-bearing tower Staxo

Staxo is a high-strength load-bearing tower made of robust steel frames for high shoring and heavy loads. Integrated connectors for rapid assembly. This modern load-bearing tower system comprises only a few individual parts and is extremely versatile. A comprehensive range of safety accessories completes the system.

Highly stable and highly versatile

§  With 1.52 m wide frame with non-buckling vertical sections

§  With 50 cm grid for setting the frame spacing

§  With the tower unit's large footprint

§  Because horizontal loads are safely dissipated

§  Load-bearing capacity up to 70 kN/leg

Speedy assembly, even when the towers are high

§  Because there are only a few individual parts, they are light and easy to handle

§  Because the vertical adapters for the next lift are integrated, without loose parts or add-ons

§  With drop-in assembly battens and integrated climbing rungs

Less crane time needed

§  Because the towers can be pre-assembled on the flat, then hoisted into position

§  With shifting carriages for horizontal repositioning

Staxo frame

• Extremely strong, galvanised steel frame for straightforward height adjustment in a 30 cm grid; choice of three heights 0.90, 1.20 and 1.80 m
• Frame spacing with diagonal crosses from 1.00 to 2.50 m adjustable in 50 cm grid
• Reliably withstands horizontal forces such as wind loads
• Integrated next-lift frame adapters for ergonomic handling even high above the ground - no tools required
• Integrated climbing rungs and drop-in assembly battens support safe assembly and disassembly

Diagonal cross

• Timesaving integration of horizontal and diagonal braces in a single component
• Different lengths for variable frame spacing
• Colour clips and stamping for clear marking of the lengths
• Safe assembly with captive gravity catches

Setting the next lift is always quick and safe: no time is lost looking for parts, because the locking springs (1),

 (3) and connecting sleeves (2) are captive, integrated into the frames.

No additional parts or loose parts, so even high above the ground handling is still straightforward.

Height adjustment

Height adjustment accurate to the millimetre, even under load. By means of screw-jack U-spindle at the top, screw-jack foot or heavy-duty screw jack at the bottom. The heavy-duty screw jack 130 has an extension height of 130 cm and is available for jobs requiring maximum versatility.

           

Beam forming  support system

The L&T-Doka Beam Forming Support system is suitable for RC-Beams of depth between 30 cm to 120 cm.

Beam bottom

The plywood sheathing is supported by a layer of secondary H-20 Beams at the designed spacing to form the beam bottom. The primary H-Beams in turn support the secondary layer.

Beam sides

The plywood sheathing is supported by H-Beams at the designed spacing running along the length of the RC-beam  to form the beam sides. The H-beams are supported by the beam forming support which are clamped onto the H-20 beams provided for the Beam  bottom. The beam forming support ensures the right angle between the beam bottom and sides. The BFS extn. provides the necessary adjustment in depth.

The beam forming support with extension are available in three sizes viz.BFS with extn.600mm long, 900mm long & 1200mm long. 

III. Wall Formwork System

The L&T-Doka  Wall formwork system is suitable for casting of RC-Walls  including water tight structures.

The plywood sheathing is supported by H-Beams which are in turn supported by the steel walers. The wall formwork facilitates fixing of working platforms for access, checking of reinforcement, concreting etc., the panels also have provision for fixing for alignment system which ensure verticality. The  pressure due to concrete  are sustained by High strength  tie system.

The walers are available in sizes of 0.8m, 1.2m, 1.6m, 1.8m, 2m & 2.4m the inside corners are formed by “universal inside corner” and the outside by universal outside fixing or angle plates. The high strength tie system can be through tie system or lost anchor system depending on the structure. The H-Beams can be butt jointed to form larger size of panels.

The formwork panel along with the working platform and alignment systems can be lifted as a single unit using a crane thus the labour involved in each operation of erection and deshuttering is reduced to a minimum. The panels are formed in the carpentry workshop at site and the number of operations/assembly of components at each location is minimal and hence accuracy is maintained. Since large panels are handled as a single unit, the damage/loss of small components is eliminated  contributing to very high material productivity. In the absence of a crane the panels can be dismantled and handled separately. The wall formwork system can handle very large pressures generated due to pumping of concrete.

The planners place the Doka formwork beams H 20 (1) and the bracing, which consists of steel waling (2) to suit the anticipated load. The sheeting (3) is freely selectable – your choice of smooth fair-faced concrete, wood-textured surface, and so on.  

Doka framed formwork Frameco

The Frameco formwork system

• Forms wall heights up to 3.00 m without stacking
• Has sturdy, galvanised steel frames to produce smooth concrete surfaces
• Permissible fresh-concrete pressure 70 kN/m²

Only 2 anchors up to a height of 3.00 m

• Means fewer anchors have to be set, so forming work is faster
• Reduces labour costs for post-casting work on the anchorage holes

End-to-end 15 cm grid with only 5 panel widths

• Simplifies planning, forming and logistics on the construction site
• Reduces the number of cost-intensive closures
• Means matching panel formats for best possible utilisation of formwork
• Reduces quantities in stock and costs for rental, because of effortless adaptation to any floor plan

§  Makes for compact lifting units for fast positioning and short crane times
IV. Column Formwork System

The L&T-Doka  Column formwork system is suitable for casting of columns of minimum 15cm * 15cm .

In the Column formwork system the H-Beams along with steel walers and accessories makeup the assembly. The Column formwork facilitates fixing of working platforms for access, checking of reinforcement, concreting etc., The formwork panel along with the working platform and alignment systems can be lifted as a single unit using a crane thus the labour involved in each operation of erection and deshuttering is reduced to a minimum. The panels are formed in the carpentry workshop at site and the number of operations/assembly of components at each location is minimal and hence accuracy is maintained. Since large panels are handled as a single unit, the damage/loss of small components is eliminated  contributing to very high material productivity. In the absence of a crane the panels can be dismantled and handled separately. The column formwork system can handle very large pressures generated due to pumping of concrete.

Any column size, straight from the modular system ...

• For speedy forming of any column cross-section with standard parts
• For optimum adaptation to any special geometry up to 5 m in diameter
• For column cross-sections up to 120 x 120 cm with only one splice plate
• For first-class concrete surfaces with any sheeting
• Permissible fresh-concrete pressure 90 kN/m²

Some of the possible cross-sections of column

V. Climbing Formwork System

The L&T-Doka Climbing System are used for tall structures like cooling towers, etc where  it is very uneconomical to provide staging or scaffolding for supporting the  external or internal wall formwork.

In this system a bracket is hooked on to anchors called lost anchors provided in the already cast wall and supports/alignments are taken from these brackets. The brackets are suitably braced to prevent any sway and are provided with walkways/working platforms / handrails etc for safety and ease of working.

Varying degrees of sophistication are available in the

Climbing formwork systems, a few of which are as listed.

CB - 150 A - Simple Climbing, Crane handled - platform width 1.50 m.   The brackets & wall formwork are to be handled separately.

CB - 150 F- Traveling Climbing, Crane handled - platform width 1.50 m.  roll back arrangement  for deshuttering & cleaning of shutters. The wall formwork and climbing bracket are lifted as one unit.

Automatic climbing formwork, the wall formwork along with the climbing brackets slide along the wall using motors, thus eliminating the need for crane. This system is very often used for natural draught cooling towers.

MF - 240 - Simple  Climbing, Crane handled - platform width 2.40 m. these brackets can also be fitted with automatic climbers SKE-50 which is a hydraulic system with 5T carrying capacity or SKE-100 with 10T carrying capacity.

The MF 240 system

    * Travelling unit MF (1):

      Platform is 2.40 m wide for safe and convenient manipulation of the formwork. 75 cm retraction for easy cleaning of the formwork and for working on the reinforcement.

    * Climbing bracket MF (2):

      Combines with beam and frame formwork, high load-bearing capacity (50 kN per climbing bracket), angle of inclination ± 15° from vertical.

    * Working brackets (2):

      Modular principle for versatility: these brackets can be used as pouring, intermediate and suspension brackets.

The SKE system

The modular system for automatic climbing

    * Free positioning of brackets and automatic climbers as single-section climbing scaffold

    * Lift heights up to 5.50 m

    * Formwork systems:

      • wall formwork FF 20 and large-area formwork Top 50

      • framed formwork Framax and Alu-Framax

    * Climbing speed 5 min/m

SKE 50

    * Load-carrying capacity 5 metric tons per bracket

    * Ideal solution for a huge number of climbing tasks

    * Up to 40 automatic climbers per hydraulic unit

                       

SKE 100

    * Load-carrying capacity 10 metric tons per bracket

    * Platform system for simultaneous work at different levels

    * Extremely strong brackets permit wide, variable spacing