Shuttering also called as Forms or moulds are the set of receptacles in which concrete is poured, so that it will have desired shape and outline when set on the construction site. Once adequate strength is developed by concrete it supports its own weight post which the shuttering can be taken out.
Pre-requisites of a good formwork / Shuttering
- A good shuttering should possess enough strength to take the live load and dead load during the progress of construction activity.
- Workmanship of shuttering work should also ensure that joints in the shuttering should be rigid enough to prevent bulging, twisting, or sagging due to the dead load and live load of construction activity. Permissible limits are issued to the workmanship in case of bulging, twisting and sagging which should be strictly followed. Excessive deformation of shuttering beyond permissible limits may disfigure the surface of concrete and reduce its strength.
- The construction line and level in the shuttering work should be true, this gives a smooth and desired finish to the concrete surface on removal of shuttering.
- Shuttering surface should be smooth and it should facilitate as easy to remove without damaging itself and ensure higher number of repetitions for the construction project.
- From economic point of view the cost of shuttering should be relatively less and it should provide higher number of repetitions.
- Shuttering should be water resistant and it should not absorb moisture from concrete.
- Shuttering shrinkage and swelling should be minimum.
- Joints of shuttering should be leak proof.
Factors affecting selection of shuttering systems
- Building Design
- Slab type: Depending on the type of slab to be executed on site various shuttering methods are adopted. Various slab types are one way slab supported beams or bearing walls, two-way flat slab, two-way slab supported by beam, waffle slab, one way slab, beams and girder, two-way flat plate, one way joist slab.
- Lateral loads supporting system (floor height): Depending on vertical supports like in rigid frame system, shear walls, framed shear wall, framed tube, tube in tube design also the shuttering methodology is applied.
- Building shape: Based on the shape of building i.e., if the structure is typical or non-typical the shuttering system is adopted for execution of construction activity on site.
- Job specifications
- Required concrete finish: Before starting the construction activity, required specification of concrete finish is decided. Various types of concrete finish are as-cast concrete finish, exposed concrete, architectural concrete finish. Basis these aspects shuttering type is adopted for execution.
- Construction speed: Depending on the required floor cycle, rate of placement and construction sequence also shuttering design is adopted.
- Local conditions
- As per the prevailing conditions of the project location in terms of skill of labors and the quality of workmanship shuttering methodology should be adopted.
- Weather conditions also act as a determining factor for shuttering methodology.
- Site characteristics like if it’s a sub-urban open site, urban restricted site or scale of site also determines the guiding factor for shuttering to be adopted for construction site.
- Organization support
- Based on the available capital and various other aspects of shuttering like its initial cost, potential reuse of shuttering material and stripping of the shuttering, decisions on the type of shuttering usage is made.Hoisting equipment provided by construction owner on site and its characteristics like carrying capacity, maximum and minimum radius of operation, the crane involvement.Office support in terms of quality management and safety management.Yard facility provided by client and its distance from site.
Classification of shuttering
Various categories in which shuttering can be classified are:
- According to size
- According to location of use
- According to materials of construction
- According to nature of operation
- According to product make
Classification according to the size of shuttering:
As per size, the shuttering can be classified into two types i.e., small size and large size. Small size shuttering work are operated by workers manually. Very often these small size shuttering is handled by single worked to avoid possible waiting time of erection. Usage of small size shuttering have limitations since it cannot be used for large scale projects where otherwise large size shuttering are used which are heavy. Handling of large-scale shuttering requires mechanical hoisting equipment. In the current market practice the most common types of shuttering used are of ply and aluminum formwork. These are designed in such a way that they can be handled manually as well as mechanically.
Classification according to the location of use:
Depending on the location i.e., as per building elements like slab and beam, column, shear wall, staircase shuttering and related support system is designed. In case of staircases there are not much effective shuttering as this element cannot be standardized. Staircases are basically three dimensional in nature which involves suspended panels and riser boards in shuttering along with the need to compliance with different spatial and dimensional variances as per respective design considerations.
Classification according to material of construction:
In case of shuttering material in construction industry we come across limitations as we do not have much options due to cost and feasibility aspects. Available options for shuttering are timber shuttering, plywood shuttering, steel or metallic shuttering, aluminum shuttering, plastic shuttering and coffer shuttering.
Timber shuttering material is one of the early adopted shuttering materials and it is still used in the construction industry. Timber shuttering has a drawback, it might get infected with termites hence it is important to make sure before its use that it is termite free. Before using timber shuttering, one should consider the fact that the life span of timber shuttering is short and it consumes a lot of time while execution on site. One should take care while using timber shuttering, that it should be cut in large pieces and the same should have the condition to be reused again. Timber shuttering is usually used when the labor cost for the same is less and when multiple elements which are to be casted require flexible formwork. Advantages of this shuttering material is that it is easy to erect and remove, they prove to be economical in small projects, they are highly workable and light weight.
Plywood shuttering is manufactured artificially, it is manufactured in different sizes and thickness used as formwork for casting of concrete member. Ply material is lightweight, durable and strong. For shuttering works plywood is one of the most widely used materials for decking, sheathing and linings in shuttering. Plywood shuttering possess similar properties as that of timber shuttering.
Steel shuttering consist of various panels of sizes which is fabricated from these plates fixed along the edges by steel angles. The panels are erected and held together using suitable clamps or nuts and bolts. Steel shuttering is mostly used for large scale infrastructure projects like bridges, flyovers, etc. Due to long life and higher repetitions of steel shuttering, it has proven to be one of the most popular shuttering. In spite of being one of the costly options in shuttering, its usage in large scale projects and structures results in being feasible option. Concrete surface obtained from steel shuttering usage comes out as a levelled and smooth surface. Steel shuttering is also the most adopted system in case of curved or circular structures like columns, tanks, sewers, chimneys, retaining walls and tunnel. Various advantages of steel shuttering over conventional shuttering are, it is efficient, long lasting and durable, it minimizes honey combing and is waterproof, it gives a smooth as well as levelled finish to the concrete surface and it can be reused upto at least 100 times.
Aluminum shuttering has become very popular in past few years. It is quite similar to that of steel shuttering along with an added advantage of being light weight due to its low density. However, the strength of aluminum shuttering is lower than that of steel shuttering. If large scale repetitions are achieved then the usage of aluminum shuttering becomes more economical.
Plastic shuttering is another interesting material of shuttering which is erected on site by the mechanism of interlocking panels or what we call modular systems. It is made up of light weight and robust plastic. Its application is done for the construction of simple concrete structures i.e., in small projects wherein there are repetitive tasks. These shuttering sets are washable and are light weight which can be further used multiple times. Plastic shuttering is best suitable in case of mass housing schemes and construction of similar type of structures. Due to its light weight property its handling and de-shuttering cost is very minimum. Its production is also economical.
Coffer shuttering can also be termed as stay in place shuttering system. Coffer shuttering comprises of two-layer filtering grids which is further reinforced with horizontal and vertical stiffeners and are linked by articulate connectors. Coffer shuttering can also be folded for a convenient transit on site. Methodology in this case is coffer shuttering is erected on site and is kept in place after concrete is poured on site, coffer shuttering in this case acts as reinforcement. Coffer shuttering is produced in the prefab factory and then transported on site for placement. Coffer shuttering can be used for any type of shuttering.
Classification according to the nature of operation:
Shuttering can be operated by power lifted methods or manually. There are systems which are equipped with mobility at a certain degree to ease the erection and to allow horizontal moment using rollers, tracks or rails.
Classification according to the product make:
Shuttering of various makes are available in the market. Various companies used various materials like steel, aluminum etc. and various design approach to devise best possible shuttering set.
IS standard code for Shuttering
IS 456-964 is used as a standard code of practice for shuttering works in India.
General: The shuttering shall conform to the lines, shapes and dimensions as per the reference plans. While placing and compacting of concrete shuttering should be rigid. Shuttering should also be water tight to prevent water loss from concrete.
Cleaning and treatment of concrete: All saw dust, rubbish, chippings should be removed from the interior of the shuttering before placing of the concrete. Interior surface of the shuttering should be oiled properly so that concrete does not get stuck to the shuttering.
Stripping time: Until good strength for concrete is achieved shuttering should not be removed. For vertical members like column, shear walls, non-structural walls stripping of shuttering should be done after completion of 16-24 hrs. The slab soffit can be stripped off the shuttering after at least 3 days wherein the props should be refixed immediately. The beam soffit can be stripped off the shuttering after at least 7 days wherein the props should be refixed immediately. For slab span less than 4.5 m, props should be fixed in place for a minimum period of 7 days. For slab span more than 4.5 m, props should be fixed in place for a minimum period of 14 days. For beam span less than 6 m, props should be fixed in place for a minimum period of 14 days. For beam span more than 6 m, props should be fixed in place for a minimum period of 21 days.
Removal procedure for shuttering: All shuttering should be removed without any vibrations and shocks as it might cause damage to the concrete. One should check and ensure that concrete is sufficiently hardened before the soffit and struts are removed from the position. Adequate precautions should be taken for one to allow the decrease in the rate of hardening in cold water conditions.
Camber in shuttering: To ensure that beam do not sag when they would have taken up their deflection, it is desirable to give shuttering an upward camber / slope. The same should be done only as per the design calculations of the beam.
Tolerances: Shuttering set should be erected in such a way that the internal dimensions are within the permissible tolerance as prescribed by the designer. Usually, a tolerance of up to 5 mm is given on site during execution.
Various loads on formwork: The shuttering erected on site has to support loads like live load due to labor movements, wet concrete weight, hydrostatic pressure of the wet concrete which acts against the vertical faces or inclined faces of the shuttering, impact of concrete which is poured in the shuttering. A live load of 370 kg/m2 is considered against the movement of labor and equipment including concrete impact during placing. The hydrostatic pressure is at its peak during poring activity and the pressure decreases when concrete is hardened. Setting time of concrete can be considered as 45 mins to 1 hour. The height of concrete is considered for calculation of hydrostatic pressure on shuttering. For a height of 1.5 m of concrete, equivalent weight of 2300 kg/m3 for concrete can be considered for calculations. For height greater than 1.5m the equivalent weight of concrete decreases. An equivalent weight of fluid concrete of 1200 kg/m3 to be considered for 6 m height of concrete in single pour. For height between 1.5 m to 6 m within the setting time of 45 min, a linear interpolation can be used for unit weight between 1200 to 2300 kg/m3 respectively.
Assembly of shuttering
- Preparing straight and right-angle shutter panels as per the approved shuttering design.
- Before using the shuttering material on site, it is important to check the material visually and confirm its sturdiness.
- The erection of shuttering should be carried out in such a way that, even if extreme combination of loads act on the shuttering it should support safely within prescribed tolerance limit and prevent bulging and deflection of concrete.
- Utmost care should be taken to ensure that, while erecting shuttering for slab its staging should rest on solid hard ground or ISMB sections should be used which should be properly anchored to concrete structure. Solid blocks or bricks should not be used below the base plate of staging as they are more prone to crushing when wet concreting is in progress. PCC floor is recommended for staging below the slab on site.
- Say if a plywood material is used on site for shuttering, one should ensure that it is cut accurate to line and length, its width and depths should be cut precise so that we get a desired finish of concrete surface. One should prefer waterproof plywood boards as they give higher repetitions.
- Staging done for shuttering should be sturdy, strutted, propped and thoroughly braced.
Application of oil and grease on shuttering
The prime objective of application of oil on shuttering surfaces is to facilitate easy removal of shuttering during de-shuttering activity. Shuttering oil creates a separate medium between shuttering and concrete. This in turn helps keep shuttering surface clean and increases life of shuttering and its number of repetitions. The shuttering oil should be applied on the horizontal surfaces like slab and beams before 48 hrs. of concreting activity and for vertical surfaces like column and shear wall oil is applied 2-3 hrs. in advance of concreting activity. The oil is applied by foam on surfaces which is gently smeared in oil. On site during execution, it is usually recommended to use shuttering oil of reputed makes or a mixture of grease and diesel is also recommended for use. Usage of burnt machine oil or reusing of oil should be strictly prohibited for site execution, otherwise it may affect the concrete surface also against plastering activity.
Process of column shuttering
- Erect the shuttering panel which could be supported by battens and braces or square tubes or steel walers on all four sides for columns or walls and check the level and line along the length and depth.
- Start fixing the panels in place with level and alignment, check from one side till the end on the same side of the panel.
- Once the shutter for one side is placed then start placing the cover blocks from one side at 600 center to center.
- Likewise other side of shutters are fixed in the sequential manner and simultaneously its line and level should be checked.
- Adequate props should be placed in place to maintain the alignment of the vertical assembly.
- Spacers are fixed in regular interval of 1 m from top to bottom between the shutters to maintain the uniformity of the vertical structure.
- To maintain the alignment of the shutters on both sides and for checking and rectification purpose a line thread on top of plyboards can be fixed.
- For checking of the line and level of the corners of shuttering, interior surface corners can be referred for right angle checking, accordingly the supports can be tightened to match the required lengths.
- Separate staging should be done for labors while concreting activity. Moreover, separate staging for concrete pump line should be done and in no case support of shutters to be taken.
- While concreting, wet concrete should be placed in a layer of 300 mm height and up to 900 mm height per hour so that it restricts the designed concrete pressure.
Reasons for failure of shuttering
- Improper load calculations for shuttering and not designed according to the intended purpose.
- In case horizontal pressure is not taken into consideration for design.
- Noncompliance in the de shuttering process like early removal of shuttering, not following proper sequence of de shuttering and negligence in the shore removal.
- Improper packing at the bottom of vertical props and inadequate diagonal bracing could cause failure in shuttering assembly.
- Improper compaction and vibration of concrete.
- If the soil under the support vertical props and base plate is unstable.
- If plumb of shoring and proper slant supports is not maintained.
- Due to negligent workmanship during concrete placement by stacking excess concrete load in a particular area.
- Due to alteration or mixing of vendor supplied material while execution.
- If the assembly consist of usage of damaged elements like loose weld joints of spans, slippery wedges, broken clamps etc.
Shuttering methodology used in monolithic construction
Monolithic construction can be defined as a methodology of construction wherein walls and slabs are casted together. This method is well known to the construction industry. This method is aggressively used in construction of high-rise structure. Its initial application was in the seismic zone as it can withstand high loads and pressures.
Various formwork companies are researching to develop more advanced systems which would produce faster and more efficient output.
Tunnel Formwork: Tunnel formwork is a precise example of monolithic construction. This technology exists since past 50 years, but in recent years it has started gaining popularity in India. This system is entirely made up of steel and is used for casting of RCC walls and slabs. While executing the works the reinforcement, plumbing offsets, sleeves, electrical conduits and the tunnel formwork are all placed as per the specified procedure. After the concreting activity is completed, the formwork are de-shuttered and mechanical systems like cranes are used to lift the formwork system. This type of formwork is best suited where the repetitions of the formwork are higher i.e., a typical floor plate is repeated multiple times in the construction. Tunnel formwork gives higher accuracy, speed and quality along with becoming one of the feasible options if number of repetitions achieved are higher. Tunnel formwork permits lengthy lifespan and several reuses. Tunnel formwork can achieve almost 500 repetitions for a typical slab cycle. Moreover, we get an added advantage of reduced slab cycle, it can give a slab cycle of 2-3 days. This form work methodology can give a cost reduction of up to 35% for projects of large scale. This methodology is best suited for mass housing and projects of large scale. This methodology has very less to negligible maintenance.
Few challenges to adopt the tunnel formwork technology are as follows. The design of the structure should be compliant to that of tunnel formwork which restricts the construction owner to explore more in the design. The customization and manufacturing of the formwork set takes time hence the vendor should be on boarded well in advance so that the design is finalized soon and production of the formwork is started in time. The initial investment of the tunnel formwork is higher and at times construction owners may not be in a comfortable position to make upfront investment. The concrete required for casting in tunnel formwork is a free flow concrete, it should allow casting in a single pour which should have appropriate workability and strength development at early stage. The required resources and labor work force should be skilled and at times this may create a possibility of shortage of appropriate manpower for this construction methodology.