Formwork: The term given to the whole setup of temporary moulds into which concrete is poured. Along with formwork, a system is provided to support the moulds and the concrete till the concrete is set and has gained enough strength this is known as the Staging. It is also the term given to either temporary or permanent moulds into which concrete or similar materials are poured. The formwork constitutes 30% of the project cost and 60% of the project construction time. A temporary framework is used to support the walkway platform, the access ways and that supports the labours who are laying the skin and placing the reinforcement is know as Scaffolding. The material which is used to form the mould into which the concrete is pored is called as Skin. The skin can be made up of ply, steel or aluminium. The process of placing the skin is known as the Shuttering. Requirements of a good formwork system: The essential requirements of formwork or shuttering are:
The material for form should be cheap and it should be suitable for reuse for several times/
It should be practically waterproof so that it does not absorb water from concrete. Also its shrinkage and swelling should be minimum.
It should be as light as possible.
It should be strong enough to take the dead and live loads during the construction
The joints in the formwork should be rigid so that the bulging, twisting or sagging due to dead and live load is as small as possible. Excessive deformation may disfigure the surface of concrete
The construction lines in the formwork should be true and the surface plane so that the cost of the finishing the surface of concrete on removal of shuttering is least.
A formwork should be such that it can be easily erected.
A formwork should be easily removable without any damages to itself so that it can be repeatedly used.
Factors to be considered for selection of system:
The number of repeat uses
The desired finish
The amount of labour to install each use
Availability of experienced personnels to install it
Handling equipment available to move large sections around
The amount of wet concrete deliverable per cast
The amount of capital available
The speed of project
Versatility
Pour to strip times
The acceptability of cold joints
Materials The various materials used as forms are: Timber: It is the most common material for shuttering and is almost available everywhere. It is comparatively cheaper and could be easily shaped. Timber deteriorates under the stress of heat and contact with water. It is also difficult to evaluate the proper strength of timber. Thus, a scientific approach of design becomes difficult. Plywood: It has got a better surface and gives a smoother finish. These are useful and economical for larger panels with repetitive uses. Plywood is available is different varieties such as resin , plastic coated plywood, texturised surfaces for exposed concrete surface. Hardwood: they are manufactured from wood fibre under controlled combination of pressure, heat and moisture. These are tempered with the impregnation of drying oils which are stabilized by heating. They have improved strength, low water absorption, and better abrasion resistance and are used as formwork lining. Fibre forms: They are used as lost forms for concrete. They are left in place on the exposed face of the concrete where it gives architectural look and also improves acoustical and insulation properties Gypsum boards: They are generally used to provide artistic design or ornamental pattern for the exposed concrete face. It is reinforced with an organic fibre or coir to make it tougher. Concrete quality may improve due to absorption of extra water present in concrete by these boards. Plastic forms: PVC, neoprene and polyester strengthened with glass fibre are some of the plastic forms in use. They are manufactured as per required shapes, do not rust and are easy to clean. Lost forms: Forms, which are not removed after casting, form a part and parcel of the structure that behaves as composites. These are known as lost forms or permanent formwork.
Steel form: Steel forms consists of angles, tubes, joists, flat plates, are very much in use as shores, bracings, runners, slabs, shutters etc. Steel forms are also in use in combination with timber. These are very strong and can be used repetitively without much damage done to these forms. The design calculations can be easily done since the characteristics of steel are known. The advantages and disadvantages of the various forms are listed below. Untreated wood o Advantages Low initial cost Low experience cost Low weight Versatility usually contains sugars that acts as a retardant and prevents sticking o
Disadvantages Poor finish, patching almost demanded where the finished concrete will be seen Low per unit use (4-7) High labour as each set must be rebuilt Must wait until concrete is well set (24 hours) to strip Leaves a dusty finish as there are sugars in the sap of soft wood
Untreated Plywood o Advantages Same as above but gives a smooth finish o Disadvantages Same as above but greater cost and less availability in some countries Coated Plywood Densified shuttering: It is manufactured from selected veneers peeled from hardwood plantation and treated with specially manufactured phenolic resin for longer durability and better performance. The surface of shuttering ply is overlaid with 10GSM phenol impregnated film. The sides are resealed by acrylic paint to preserve moisture absorption and swelling o
Advantages
Will increase durability
Many plywood systems are available:
Oil saturated paper
Vinyl and water resistant glue surface
Simple oil based paint
Increases the number of uses by double as long as edges are not exposed to the setting of cement
o
Epoxy paint coatings will extend the use of plywood to over 100 uses
Disadvantages
Increase initial cost
Increase weight
Release agents is a must
If uncoated surfaces are exposed, the number of uses will be the same as untreated plywood
Steel form o
o
Advantages
Has very high reuse rates
Very smooth surfaces are possible
Strong and can be stripped late
Fast to install in simple walls and like
Disadvantages
Costs are 6-10 times ply
Thicker the surface greater the weight
Steel dents easily
Low versatility
Release agents are demanded
Aluminium Form o
o
Advantages
Not as easy to damage as steel
Assembles fast
Disadvantages
More work to clean before each use
Can be very costly to buy
Glass Reinforced Plastic formwork o
Advantages
Very useful for complex shape and special features
Easy to disassemble
o
Light (not heavy)
Damages on the formwork can be easily be repaired
Disadvantages
Expensive at first
Classification of formwork
According to size: There are two sizes of formwork – small sized and large sized. Any size which is designed for operations by workers manually is small sized Timber and aluminium forms are usually in the form of small sized panels. In case of large sized formwork, the size of formwork can be designed as large as practicable to reduce the amount of jointing and to minimize the amount of lift. The stiffness required for the same can be dealt with the introduction of more stiffeners such as studs and soldiers
According to material: Materials used are traditionally quite limited due to finding the difficult balance between cost and performance. Timber in general is still the most popular formwork material for its relatively low initial cost and adaptability. Steel and aluminium forms are becoming more commonly used now-a-days because of their higher number of repetitions but the cost and labour requirements is high for the same.
Design criteria:
For Slab o
Thickness
o
Load calculation
o
Pouring method
o
Type of form
o
Spacing of props/ vertical shore
o
Capacity of spanning member
o
Floor Height
For walls/columns o
Rate of rise
o
Temperature at site
o
Pressure diagram
o
Material selection
o
Height of form
o
Handling facilities of forms
o
Selection of forms
o
Section of columns and wall
Formwork design loads: 1. Vertical loads: a) Weight of wet concrete b) Self weight c) Live load during construction d) Uplift due to uneven placing of concrete 2. Lateral loads a. Density b. Rate of placing c. Vibration d. Concrete temperature e. Slump f.
Admixture
g. Thickness of concrete element 3. Horizontal loads a. Mainly on alignment/ supporting props b. Due to wind c. Due to dumping of concrete d. Due to equipment Types of formwork systems 1. Al- Titan system The skin is backed up by aluminium secondary beams (1.7m, 2.3m, 3.3m length) also known as aluminium doka. These beams have strip of wood running along their length on the top to facilitate fixing the plywood with nails. This fits into the groove of primary beam and forms a decking to enable the placing of plywood and also replaces the 4”x3” runner which is generally used as beam bottom. The aluminium secondary beam (2.3m or 3.3m) is used in wall or column shuttering. The holes are provided to fix the steel walers which generally take care of concrete pressure. Aluminium primary usually 1.2m or 1.8m long is used to support the slab along with secondary beam. The groove on either side receives the secondary beam. The end collars will sit in the fly plate of drop head. The drop heads are supported by the staging. The main advantages are:
The component are light weight hence easy to handle
The drop heads facilitate in achieving faster cycle time as the skin and the beams can be
removed soon with only supporting system in place.
Deshuttering is easy
Large number of repetitions can be obtained.
Primarybeam
Secondary beam
Drop head 2. L&T Doka System The skin is stiffened by timber beams (I- sections). These are designated as H16 (160mm deep) as secondary beams and H20 (200mm deep) as primary beams. The skin is nailed to the flanges of secondary timber beams which rest on primary timber beam. These primary beams are supported over four way heads which are attached to a supporting system. For vertical members, the beams used are of H16.
Primary beam
four way head
3. Conventional system It is the commonly used system mainly because of its ease and availability of the components. The skin is stiffened by wooden runners of dimensions 2”x3” or 3”x4”. In some cases LVR’s are also used. These stiffeners rest on wooden rubber wood, placed 300 to350mm c/c, or LVR’s, placed 400mm c/c, this in turn rests on ISMC channels placed on stirrups heads that is supported by the scaffolding system. Types of scaffolding systems: a. Cuplock system This system takes both vertical and inclined loads. Its components are: Cuplock verticals: made from 40NB tubes. The bottom cups are welded every .5m and the top cups are movable. They are available in sizes .5m, 1m, 1.5m, 2m, 2.5m and 3m. the maximum safe loading capacity is 2T when the horizontals are placed at 2m spacing Cuplock horizontals: made from 40NB tubes with blades welded at ends. These are used to make cuplock staging grids. These are also called as bracings. These are spaced as per design requirements along with cuplock vertical. The various sizes are 2.5m, 2m, 1.8m, 1.5m, 1.2m, 1m and .86m.
Spigot: when the room height is great, a need for using one cuplock vertical on top of another is required. In such a case a spigot is used. These have holes on either side to accommodate the
spigot pin. At site instead of spigot pins, 8mm diameter bolts is used. These spigots have diameter less than that of cuplock vertical.
Base jack: They are used at the bottom of verticals. A screw jack arrangement is used to adjust the height. They are threaded to a length of 600mm out of which 500mm can be safely extruded. Its safe load carrying capacity is 6T
Stirrup heads: They are used at the top of verticals. It also has screw jack arrangement to adjust the height. They are threaded to a length of 650mm out of which 500mm can be safely extruded.
b. Flex systems This system is ideally suited for different types of floors with varying thickness and shapes. It is simple and quick to use. This system mainly consists of floor props and tripods. They can be used for beam and slab formwork, shoring, aligning formwork for walls and columns. The tripod provided which vertical support is given ensures stability against inclined loads. The props are telescopic and adjustments can be made in height. The lightweight H-16 beams and the four way heads combine to give a versatile formwork system. The four way heads hold the light weight beams whether they are kept continuously joined or overlapping. This can be adopted for varying room dimensions without the need to cut the H-16 beams. Such beams can be supported at any point along the length, with no restriction of spacing. Hence it can be easily adopted for varying floor thickness and changing room dimensions.
12 MM PLY H-16 BEAMS (SEC)
AS SHEATHING
FOURWAY HEAD
H-16 BEAMS H-16 BEAMS
(PRIMARY BEAM FORMING HEAD )
(PRIMARY) CT PROPS
TRIPOD
Arrangement of doka flex system
c. HD tower systems It is ideal in places where the scaffold has to carry heavy loads or the scaffold height required is high. The vertical members are made from 50NB medium class pipes and horizontal members are made out of 40NB pipes. The horizontal members are welded into vertical members thus forming the basic frame. The safe load carrying capacity is 6T per leg. A foot plate is provided at the bottom to transmit the load uniformly to the ground from the HD tower. A U-head is provided at the top to receive and transmit the load to HD tower.
H-16 BEAMS
STEEL WALER
U-HEAD
SHORT PROPS
H.D.TOWER FOOT PLATE
SYSTEM TOWER SPINDLE
Arrangement of Doka HDT system
MISC Components: Steel walers: it is made up of two ISMC 100 laid back to back with a spacer plate. It is fixed around the periphery of columns, walls or beam sides to withstand the concrete pressure and prevent bulging of concrete. They are available in .8m, 1.2m, 1.6m, 2m and 2.4m lengths. At site, cuplock horizontals tied together with binding wire is also being used.
Steel waler Tie rods and wing nuts: They are steel rods which are threaded so that nuts called wing nuts can be attached to them. They are used to fix the steel walers around the columns, walls or beam sides. In case of beams and walls, PVC pipes are placed through the section at required intervals and the tie rod passes through these PVC pipes. The tie rod also passes through the space created by the spacer plates in the walers. The wing nut is then fixed and tightened to secure them. When concrete is poured into moulds, the tie rods come into tension. The maximum allowable tension force is 50kN.
Mild Steel panels: They are used in place of plywood as skin. They are of two types: wall form panels and floor form panels. These panels are stiffened by a steel strip which has grooves in it. A single clip or a double clip can be fixed into these groves to attach a pipe along the length. This is done to ensure that the panels are in line. Coupler: they are used to connect two pipes together. They can be either right angle coupler or swivel type coupler, where the angle between the two pipes can be changed. Mivan system: It is the most advanced formwork systems. It is fast, simple and adaptable. It produces total quality work which requires minimum maintenance and when durability is the prime consideration. It
is a totally pre-engineered system where in the complete methodology is planned to the finest details. In this system the walls, columns and slab are casted in one continuous pour on concrete. Early removal of forms can be achieved by the air curing/ curing compounds. These forms are made strong and sturdy, fabricated with accuracy and easy to handle. The components are made out of aluminium and hence are very light weight. They afford large number of repetitions (around 250). The repropping is simple hence short cycle time can be achieved. Construction with Mivan A) PRE – CONCRETE ACTIVITIES a) Receipt of Equipment on Site – The equipments is received in the site as ordered. b) Level Surveys – Level checking are made to maintain horizontal level check. c) Setting Out – The setting out of the formwork is done. d) Control / Correction of Deviation – Deviation or any correction are carried out. e) Erect Formwork – The formwork is erected on site. f) Erect Deck Formwork – Deck is erected for labours to work. g) Setting Kickers – kickers are provided over the beam. After the above activities have been completed it is necessary to check the following. 1. All formwork should be cleaned and coated with approved realize agent. 2. Ensure wall formwork is erected to the setting out lines. 3. Check all openings are of correct dimensions, not twist. 4. Check all horizontal formwork (deck soffit, and beam soffit etc.) in level. 5. Ensure deck and beam props are vertical and there is vertical movement in the prop lengths. 6. Check wall ties, pins and wedges are all in position and secure. 7. Any surplus material or items to be cleared from the area to be cast. 8. Ensure working platform brackets are securely fastened to the concrete. B) ON CONCRETE ACTIVITIES At least two operatives should be on stand by during concreting for checking pins, wedges and wall ties as the pour is in progress. Pins, wedges or wall ties missing could lead to a movement of the formwork and possibility of the formwork being damaged. This – effected area will then required remedial work after striking of the formwork.
Things to look for during concreting: i. Dislodging of pins / wedges due to vibration. ii. Beam / deck props adjacent to drop areas slipping due to vibration. iii. Ensure all bracing at special areas slipping due to vibration. iv. Overspill of concrete at window opening etc. C) POST – CONCRETE ACTIVITIES i) Strike Wall Form- It is required to strike down the wall form. ii) Strike Deck Form- The deck form is then removed. iii) Clean, Transport and stack formwork iv) Strike kicker Formwork – The kicker are removed. v) Strike wall – Mounted on a Working Platform the wall are fitted on next floor. vi) Erect Wall – Mount Working Platform and the wall is erected. Normally all formwork can be struck after 12 hours.
The post-concreting activities include: CLEANING All components should be cleaned with scrapers and wire brushes as soon as they are struck. Wire brush is to be used on side rails only. The longer cleaning is delayed, the more difficult the task will be. It is usually best to clean panels in the area where they are struck. TRANSPORTING There are basic three methods recommended when transporting to the next floor:
The heaviest and the longest, which is a full height of wall panel, can be carried up the nearest stairway.
Passes through void areas.
Rose through slots specially formed in the floor slab for this purpose. Once they have served their purpose they are closed by casting in concrete filter.
STRIKING Once cleaned and transported to the next point of erection, panels should be stacked at right place and in right order. Proper stacking is a clean sign of a wall – managed operation greatly aids the next sequence of erection as well as prevents clutters and impend other activities
Components of formwork The basic element of the formwork is the panel, which is an extruded aluminium rail section, welded to an aluminium sheet. This produces a lightweight panel with an excellent stiffness to weight ratio, yielding minimal deflection under concrete loading. Panels are manufactured in the size and shape to suit the requirements of specific projects. The panels are made from high strength aluminium alloy with a 4 mm thick skin plate and 6mm thick ribbing behind to stiffen the panels. The panels are manufactured in MIVAN’S dedicated factories in Europe and South East Asia. Once they are assembled they are subjected to a trial erection in order to eliminate any dimensional or on site problems.
All the formwork components are received at the site whining three months after they are ordered. Following are the components that are regularly used in the construction. Beam components 1) Beam Side Panel: - It forms the side of the beams. It is a rectangular structure and is cut according to the size of the beam
BEAM SIDE PANEL
2) Prop Head for Soffit Beam: - It forms the soffit beam. It is a V-shaped head for easy dislodging of the formwork.
PROP HEAD FOR SOFFIT BEAM. 3) Beam Soffit Panel: - It supports the soffit beam. It is a plain rectangular structure of aluminium.
BEAM SOFFIT-PANEL 4) Beam Soffit Bulkhead: - It is the bulkhead for beam. It carries most of the bulk load.
BEAM SOFFIT BULKHEAD
Deck Components 1) Deck Panel: - It forms the horizontal surface for casting of slabs. It is built for proper safety of workers.
DECK PANEL
2) Deck Prop: - It forms a V-shaped prop head. It supports the deck and bears the load coming on the deck panel.
DECK PROP
3) Prop Length: - It is the length of the prop. It depends upon the length of the slab.
DECK PROP LENGTH 4) Deck mid – Beam: - It supports the middle portion of the beam. It holds the concrete.
DECK MID-BEAM
5) Soffit Length: - It provides support to the edge of the deck panels at their perimeter of the room.
SOFFIT LENGTH 6) Deck Beam Bar: - It is the deck for the beam. This component supports the deck and beam.
DECK BEAM BAR Other Components 1) Internal Soffit Corner: - It forms the vertical internal corner between the walls and the beams, slabs, and the horizontal internal cornice between the walls and the beam slabs and the beam soffit.
INTERNAL SOFFIT CORNER
2) External Soffit Corner: - It forms the external corner between the components
EXTERNAL SOFFIT CORNER 3) External Corner: - It forms the external corner of the formwork system.
EXTENAL CORNER
4) Internal Corner: - It connects two pieces of vertical formwork pieces at their exterior intersections.
INTERNAL CORNERS Wall Components a) Wall Panel: - It forms the face of the wall. It is an Aluminium sheet properly cut to fit the exact size of the wall
WALL PANEL
b) Rocker: - It is a supporting component of wall. It is L-shaped panel having allotment holes for stub pin.
ROCKER
c)
Kicker: - It forms the wall face at the top of the panels and acts as a ledge to support
KICKER d) Stub Pin: - It helps in joining two wall panels. It helps in joining two joints
STUB PIN
Work cycle MIVAN is a system for scheduling & controlling the work of other connected construction trades such as steel reinforcement, concrete placements & electrical inserts. The work at site hence follows a particular sequence. The work cycle begins with the deshuttering of the panels. It takes about 12-15hrs. It is followed by positioning of the brackets & platforms on the level. It takes about 10-15hrs simultaneously. The deshuttered panels are lifted & fixed on the floor .The activity requires 7-10 hours. Kicker & External shutters are fixed in 7 hrs. The wall shutters are erected in 6-8 hrs One of the major activity reinforcement requires 10-12 hrs. The fixing of the electrical conduits takes about 10 hrs and finally pouring of concrete takes place in these. This is a well synchronized work cycle for a period of 7 days. A period of 10-12 hrs is left after concreting for the concrete to gain strength before the beginning of the next cycle. This work schedule has been planned for 1010-1080 sq m of formwork with 72-25cu m of concreting & approximate reinforcement. The formwork assembling at the site is a quick & easy process. On leaving the MIVAN factory all panels are clearly labeled to ensure that they are easily identifiable on site and can be smoothly fitted together using formwork modulation drawings. All formwork begins from corners and proceeds from there.
The system usually follows a four day cycle: Day 1: -The first activity consists of erection of vertical reinforcement bars and one side of the vertical formwork for the entire floor or a part of one floor. Day 2: -The second activity involves erection of the second side of the vertical formwork and formwork for the floor Day 3: - Fixing reinforcement bars for floor slabs and casting of walls and slabs. Day 4: -Removal of vertical form work panels after 24hours, leaving the props in place for 7 days and floor slab formwork in place for 2.5 days. Advantages of mivan:
High quality formwork ensures consistence of dimensions
On removal of mould a high quality concrete finish is produced to accurate tolerances and verticality
Total system forms the complete concrete structures
Custom designed to suit project requirements
Unsurpassed construction speed
Panels can be reused up to 250 times
Can be erected using unskilled labour
Limitations of mivan:
Because of small sizes finishing lines are seen on the concrete surfaces.
Concealed services become difficult due to small thickness of components.
It requires uniform planning as well as uniform elevations to be cost effective.
Modifications are not possible as all members are caste in RCC.
Large volume of work is necessary to be cost effective i.e. at least 200 repetitions of the forms should be possible at work.
The formwork requires number of spacer, wall ties etc. which are placed @ 2 feet c/c; these create problems such as seepage, leakages during monsoon.
Due to box-type construction shrinkage cracks are likely to appear.
Heat of Hydration is high due to shear walls.
Remedial measures
Ties used in shutter connection should be carefully grouted.
Shrinkage cracks likely to occur around door and window openings in the wall can be minimized by providing control strips in the structure which could be concreted after a delay of about 3 to 7 days after major concreting
Heat of hydration can be reduced by the use of fly ash.
Conventional Vs Mivan Advantages of Mivan formwork over conventional construction 1. More seismic resistance: - The box type construction provides more seismic resistance to the structure. 2. Increased durability: - The durability of a complete concrete structure is more than conventional brick bat masonry. 3.
Lesser number of joints thereby reducing the leakages and enhancing the durability.
4. Higher carpet area- Due to shear walls the walls are thin thus increasing area. 5. Integral and smooth finishing of wall and slab- Smooth finish of aluminium can be seen vividly on walls. 6. Uniform quality of construction – Uniform grade of concrete is used. 7. Negligible maintenance – Strong built up of concrete needs no maintenance. 8. Faster completion – Unsurpassed construction speed can be achieved due to light
weight of forms
9. Lesser manual labour- Less labour is required for carrying formworks. 10. Simplified foundation design due to consistent load distribution. 11. The natural density of concrete wall result in better sound transmission coefficient. Special types of formwork: Climbing Platform for Lift Shafts This is a system developed particularly for constructing lift shafts (Lift Core wall), this system eliminates the need for providing scaffolding up to the working stage from the floor, and hence it is most suitable for high rise construction. This consists of a platform with climbing pawls attached at each corner. These pawls are made to fit and rest in pockets left out during the concreting of the wall. Once the next stage is ready, this platform can be pulled up by a crane, during which the pawls drop down due to their own
weight and roll up to the next pocket. Once the pocket of the next stage is reached, the crane releases the platform and the pawls fit into the pockets, hence providing a working platform for the next stage. Types
Climbing formwork (crane-climbing) - in this type of climbing formwork, the formwork around the structure is displaced upwards with the help of one or more cranes once the hardening of the concrete has proceeded far enough. This may entail lifting the whole section, or be achieved segmentally.
Climbing formwork (self-climbing) - In this type of formwork, the structure elevates itself with the help of mechanic leverage equipment (usually hydraulic). To do this, it fixes itself to sacrificial cones or rails emplaced in the previously cast concrete.
Lift shaft platform resting in pockets
Lift shaft platform being lifted to next level
Jump forms These are forms that are able to be moved in leaps either horizontally or vertically. They are simple reusable panels which can be fixed to a previous rise of wall. They may be used when identical units are being constructed. The panels must be stronger than single use panels for the same section of concrete because they must withstand movement. The extra costs of building stronger panels are recovered when they are re-used. The total building time is less and the overall use of material is less.
Slip forms A slip form is made so that it can move slowly whilst being continually kept full of concrete. The form is not deep and it moves so that the concrete is not in the form for long time. The concrete is left behind by the form when it is just strong enough to support itself. Typically, the concrete stays in the vertical slip form for 1.5 to 6 hours. Because the form is continually filled it produces joint less concrete. That’s useful for construction of containers, such as water tanks, cooling towers etc. where breaks in concrete should be avoided.
Pipe racks
Core walls
Permanent formwork Permanent formwork is a structural element that is used to contain the placed concrete, mould it to the required dimensions and remain in place for the life of the structure. It can be of two types: Participating permanent formwork or non participating permanent formwork. Participating permanent formwork makes some predetermined contribution to the strength of the structure. Non-participating permanent formwork makes no strength contribution but may provide additional benefits such as improved durability, finish or insulation properties. The correct use of permanent formwork in construction will reduce costs and save time by:
eliminating or reducing the need for false work
reducing the skill level needed on site
increasing the potential for standardisation and repetition
permitting off-site fabrication in factory conditions followed by scheduled and appropriate
deliveries
speeding up erection times, particularly in building works
eliminating the need to strike formwork and false work
allowing early access for following or concurrent operations
eliminating the programme limitations of re-use of formwork
The correct use of permanent formwork in construction will reduce hazards by: eliminating or reducing the need to erect formwork and false work in difficult locations, for example over rail tracks and water, particularly in bridge works providing a safe working platform early in the erection process Eliminating the need to strike formwork in difficult locations and confined spaces. The correct use of permanent formwork in construction will reduce maintenance costs by:
improving curing of concrete and reducing shrinkage cracking
ensuring adequate cover to the reinforcement and providing associated benefits such as increased resistance to chloride ingress and carbonation, where appropriate
in many instances improving the durability of the structure
providing the decorative finish required
Insulated Concrete Formwork The ICF consists of twin-walled expanded polystyrene (EPS) panels or blocks that are quickly built up to create formwork for the walls of a building. This formwork is then filled with factory produced, quality assured, ready-mixed concrete to create a robust structure. The forms are thermally efficient with the EPS remaining in place to provide both complete thermal insulation and a uniform surface ready for the direct application of external finishes or proprietary cladding systems. ICF construction sandwiches a heavy high strength material between layers of light, highly insulated one (EPS or foam). The combination creates a wall with many desirable properties such as air tightness, strength, sound attenuation, insulation and mass. Buildings constructed with ICF walls have a more even temperature throughout the day and night.
Insulated concrete formwork The Table or Flying Form System Another type of formwork is table or flying form systems. These consist of slab formwork tables which are reusable. These tables do not have to be dismantled and can be used in high buildings where cranes or elevators are used to lift the tables. Once the table is positioned, the space between the wall and table is filled. Tables vary in size from eight square meters to 150 square meters. This type of formwork is a huge saver of both labour and time and is a favourite of construction engineers and architects. However, table formwork is best used in the construction of large, but simple structures. Formwork should be placed at the correct height so that there is sufficient space to remove them once the concrete has set or cured. Due to this reason, the support systems of table formwork need to be height adjustable. Adjustable metal props can be used to support the systems. Some use steel or aluminium to insert stringers and supports into the systems, while others use metal frame shoring towers to attach the decks to. Others attach the decks to walls or columns that have been pre-cast which means that contractors do not need to use vertical props, simply support shoes bolted through holes.
Table formwork system Tunnel form system Tunnel form is a formwork system where slabs & walls of a building is cast in a continuous pour using room sized structural steel mould. The components of this system are made of steel. The system creates an efficient load-bearing structure for use in a wide variety of applications. It is particularly effective in projects suited to repetitive cellular construction such as residential blocks, hotels, student accommodation, barracks and prisons. The solid, strong monolithic structure can be 40 or more storeys in height and the accuracy of the system suits the installation of prefabricated elements such as cladding panels and bathroom pods. It has been suggested that tunnel form can reduce the cost of the frame of a typical hotel or similar building by about 15% when compared with traditional construction, chiefly due to the increased speed of construction. Savings in construction time of up to 25% over traditional frame construction methods can be achieved; for low-rise construction, which would traditionally be in brick or block, savings of 45% on the construction time have been achieved
Fabric Formwork Fabric formwork uses a flexible textile membrane in place of the rigid formwork panels usually used in concrete construction. Rigid materials such as dimensional lumber, plywood, or aluminum are conventionally used to restrain the fluid forces of the concrete. In order to restrain these forces, the form will have tension forces on one surface, compressive forces on the other surface, and a neutral plane between the two surfaces. Fabric, however, can only operate in tension, and as a result can be an extremely efficient method of forming concrete. When wet concrete is contained by a tensile membrane, the fabric deflects into precise tension geometries. This produces efficient structural curves and extraordinary surface finishes. Fabric weighs approximately 1/300th that of dimensional lumber. But the pressure that concrete exerts on the fibre results in bulging or deflection of the structure. This means that fabric can only be used where the bulging is not detrimental to the finished concrete. Fabric forms can be used to form columns, walls, beams, slabs and panels in both pre-cast and in-situ construction. From a structural/architectural perspective, fabric formwork awakens concrete to its fluid origins, introducing new horizons for architectural form and structural expression.
Fabric formwork being placed at site Advantages of Fabric Forms
Material Reduction: Fabric forms use hundreds of times less material than conventional rigid formwork. Concrete and steel can also be conserved by forming more efficient curved geometries.
Cost Savings: Fabric forms cost far less than rigid forms due to the efficiency of the tensile-only membrane. In addition, certain fabrics can be reused many times over.
Improved Concrete Quality: Permeable fabrics improve surface finish, compression, strength and impermeability by filtering air bubbles and excess mix water from the wet concrete.
Waterproof Concrete: Inexpensive plastic-coated fabric forms provide a permanent waterproof shield when left on a concrete cast - useful, for example, in damp-proofing foundation footings.
Beauty: Fabric-cast concrete is distinguished by its soft curves and immaculately detailed surfaces, offering unique forms of architectural beauty to concrete products and construction.
Formwork Failures The most common reason due to which failure occurs when concrete is fresh is due to the formwork failure. The main reasons for which Formwork failure can occur are:
Improper stripping and shore removal Premature stripping of forms, premature removal of shores, and careless practices in reshoring can produce catastrophic results.
Inadequate bracing This is one of the more frequent causes of formwork failure, however, other effects are that induce lateral force components or induce displacement of supporting members. Inadequate cross bracing and horizontal bracing of shores is one of the factors most frequently involved in formwork accidents. High shoring with heavy load at the top is vulnerable to eccentric or lateral loading. Diagonal bracing improves the stability of a structure as struts to solid ground.
Vibration Forms sometimes collapse when their supporting shores or jacks are displaced by vibration caused by: * Passing traffic * Movement of workers and equipment on the formwork * The effect of vibrating concrete to consolidate it. Diagonal bracing can help prevent failure due to vibration.
Unstable soil under mudsills Formwork should be safe if it is adequately braced and constructed so all loads are carried to solid ground through vertical members. Shores must be set plumb and the ground must be able to carry the load without settling. Shores and mudsills must not rest on frozen ground; moisture and heat from the concreting operations, or changing air temperatures, may thaw the soil and allow settlement that overloads or shifts the
formwork. Site drainage must be adequate to prevent a washout of soil supporting the mudsills.
Inadequate control of concrete placement The temperature and rate of vertical placement of concrete are factors influencing the development of lateral pressures that act on the forms. If temperature drops during construction operations, rate of concreting often has to be slowed down to prevent a built up of lateral pressure overloading the forms. If this is not done, formwork failure may result. Failure to regulate properly the rate and order of placing concrete on horizontal surfaces or curved roofs may produce unbalanced loadings and consequent failures of formwork.
Lack of attention to formwork details Even when the basic formwork design is soundly conceived, small differences in assembly details may cause local weakness or overstress loading to form failure. This may be as simple as insufficient nailing, or failure to tighten the locking devices on metal shoring.
Placing of formwork At site, the cuplock system with wooden runners is being used.
Columns: First the construction of the kicker is done. The ply of dimension is 1.2mx2.4m is cut into the required shape. This is stiffened with the help of wooden runners placed at 200300mmc/c. a thin coat of shuttering oil is applied on the surface which acts as the release agent. With reference to column starter these boards are put into place and nailed. Props are provided to support them; these props rest on rigid surface and are used to either pull or push the boards. The inner dimensions of the formwork are checked. Line dori suspended alongside the boards is used to check its plumb. Distances of the line dori from the surface of the ply measured at salient points and adjustments are made using the props.
Footing: After the reinforcement is tied, the cover blocks are placed and the ply is placed. These ply boards are held in position with the help of props. The verticality is checked with the help of plumb and also the inner dimensions are checked. The level of concrete is marked on the ply using a nail.
Beams and slabs: A detailed plan of scaffolding layout is made, based on the shuttering layout drawing, and the scaffolding is erected accordingly. The scaffolding is approximately drawn up to the required level. The ISMC channels, which act as the primary beams are put in the stirrup heads and on top of it wooden runners or rubber wood whichever is used is placed. The beam bottom skin is then nailed to these runners which act as secondary beams. Beam side skin is then nailed and fixed. The slab bottom is fixed after this. All this is done with reference to the column
alignment. The dimensions of beams are checked and all required rectifications done. The corrections are made by adjusting the scaffolding heights.
Cycle time Cycle time is the time period for which the formwork is utilized for a particular stage. It starts from the time the material is made available for that particular stage, includes the time for erection, time for which it remains in place and the time taken for de-shuttering. After deshuttering the material can be utilized for the next stage De-shuttering time It is the time for which the form work remains in place after the concrete has been poured. This depends on several factors like the type of member (Slab, Beam bottom, beam sides, column, etc…).
Span of the member (in case of beams and slabs).
Depth of the member.
Type of formwork used (If there is a provision for Props being left under, the Deshuttering time can be improved).
Use of admixtures in concrete. Etc…
At site: Description
Deshuttering time
Wall, column and vertical beam sides
24-48hrs
Slabs ( props left under)
3 days
Beam soffits (props left under)
7 days
Removal of props to slab Spanning up to 4.5m
7 days
Spanning greater than 4.5m
14 days
Removal of props to beams and arches Spanning up to 6m
14 days
Spanning greater than 6m
21 days
Usage ratio This is the ratio of the total area of shuttering done (measured in m2) and the total amount of shuttering material available on site (measured in m2), in a given time period. This value gives an idea about the utility of the material. Higher the value greater is the utility.
Economy in formwork The following steps or measures can be taken to reduce the cost of formwork and hence achieve economy:
The use of irregular shapes for forms should be avoided as far as possible.
The structural components should be so designed as to permit the use of commercially available forms.
The working drawings of the formwork should be prepared and checked before fabricating it.
The forms should be designed to provide adequate but not excessive strength and rigidity.
The use of construction joints should be made to improve the quality of forms and to make re-use of forms
Misc learnings:
Staging should be placed such that it should be at a distance of 300mm from column face. If the distance becomes more, then extra supports should be provided
When ply is being cut for beam side, the width is equal to beam depth- slab depth + runner width
1 lt of shuttering oil can cover 35sqft of film coated plywood
One board of ply requires about 40g of nails to prepare shuttering for column, beam and slab
For the UG tank, the water stop tie rods are to be used thereby preventing leakage. After the concreting.
For pile caps, about 120gms nails is required for making and fixing.
For circular column, steel form with angle sections is used to make the work easier compared to the usage of wooden form
Good practices: The formwork after being used should be cleaned properly and stacked separately for easy use. Adequate release agents should be applied to the board such that it can be easy deshutterd but care should be taken that concrete won’t get stained The shuttering ply should be planned and cut in such a way that minimum wastages occur. The shuttering usage ratio should be kept high thereby increasing the utility of the ply The column box shuttering can be reused for beam bottom after 12-15 repetitions. The amount of staging procured can be more than the amount of ply procured. There by the staging can be laid before providing greater work front also the placing of ply becomes easier. Exercising proper care and maintenance of the shuttering material increases their durability and more number of repetitions; thereby making the investment worthy.
