FORMWORK, FALSE WORKS & SCAFFOLDING

FORM WORKS

Formwork is the process of making a temporary mould into which concrete is poured and formed. Formwork plays a key role in concrete construction. They mould the concrete to the desired size & shape, control its alignment and position. 

Requirements of a Good Formwork

Formwork also carries the weight of freshly placed concrete and itself besides live load due to materials, equipment and workmen. Therefore Formwork should be strong enough to withstand all types of dead and live loads. . The design of formwork should be easy to handle , quick erection and removal. joints between formwork must have tight enough to prevent leakage of grout.

Formwork should be rigidly constructed and efficiently propped and braced both horizontally and vertically, so as to retain its shape. Construction of formwork should permit removal of various parts in desired sequences without damage to the Concrete.

The formwork should be accurately to the desired line and levels should have plane surface. The material of the formwork should not warp or get distorted when exposed to the elements.

Types of Formwork 

Timber Formwork

The timber used for formwork should be well seasoned and light in weight. It should have smooth and even surface on all the sides which comes in direct contact with concrete. The timber should be free from loose knots and easily workable with nails without splitting.

Plywood Formwork

The Plywood should be boiling waterproof grade. Plywood shuttering is durable under alternate wetting and drying conditions. Plywood has a hard surface and it possesses adequate strength to withstand a load of concrete and the forces caused by pouring of concrete and vibrations.

Plywood formwork is very economical as it can be use repeatedly, depending upon the care taken during erection and dismantling.

Steel Formwork

Steel formwork formed by steel panels fabricated with angles and thin steel plates. Steel panels can e fabricated in any modular shape or size, mostly circular and curved.

Steel panels are stronger, durable and have longer life span. These can be insttalled and dismantled easily with great speed. They do not shrink or warp. 

Compared to Plywoog, steel is costly. Initial investment will be high, but easy to maintain. Steel panels are  heavy in weight and needs lifting equipment.

Aluminium Formwork

Formed with Aluminium panels, this formwork increases the speed of construction and saves time and labour. Aluminium Formwork can be reused up to 250 times. Though initial cost is high, cost effective in building large number of symmetrical structures. Skilled labours are required for erection, alignment and maintenance.

Holes caused during formwork by wall tie should be grouted properly, or else there will be a leakage problem in the future.

Mivan Company Ltd. from Malaysia is manufacturing aluminium formworks. Mivan formworks are widely used in mass residential units. It is economical and useful for fast construction.

Plastic Formwork

Plastic formwork is assembled either from interlocking panels or from a modular system and is used for relatively simple concrete structures. It is not as versatile as timber formwork due to the prefabrication requirements. It is used for concrete columns and piers and stays in place, acting as permanent axial and shear reinforcement for the structural member. It also provides resistance to environmental damage for both the concrete and reinforcing bars.

Column Formwork

Column box or shuttering for columns is made of Plywood sheets or Steel sheets fabricated with adequate stiffeners. A thin films of oil or grease should be applied to inner surface of the shuttering to enable easy removal of the column after the concrete hardens. Plastic Panels also can be used for Column shuttering.

Column Formwork is to be designed to be able to accommodate relatively high fresh concrete pressures as comparatively small cross-sections are concreted quickly.

Beam Formwork

Formwork for beams takes the form of a three-sided box which is supported and propped in the correct position and to the desired level. Beam formwork is generally formed with either timber or metal panels. It should be built up tightly and sustained competently and braced both horizontally and vertically with the intension of preserving its shape.

The joints in the formwork should be firm against seepage of cement grout. The formwork should be arranged correctly to the preferred line and levels with plane surface. It should be supported with firm base.

Slab Formwork

Components of slab formwork are formwork panels, joists, Props, bracing and other supporting materials that enables the concrete to be poured and set above the ground. Slab formwork essentially supports the weight of the concrete during the curing process and when the concrete slab is positioned on permanent supports.

There are a variety of materials used in slab formwork, including plywood, timber, metal, aluminium, and sometimes even plastic components that are used to shape and give strength to the concrete.

Wall Formwork

Formwork for a concrete wall is normally built up on both sides of the wall. Reinforcement bars are laid on wires before the spreaders are placed and the wall is tied. The studs are approximately 600 mm apart. All studs are braced.

Formworks for wall are subjected to relatively lower lateral prssure than Column forms due to their large cross sectional area. Wall Formwork panels should be checked for adequate tying and bracing. Form panels should be checked for adequate tying and bracing.

Centring and Shuttering

Centring - formwork which supports the horizontal surface such as Beam, Slab bottom is known as Centring.

Shuttering - Formwork which supports the vertical surface such as Columns, Shear walls is known as Shuttering.

But Centring and Shuttering have no significant difference between them.

Major Companies manufacturing Form works are MIVAN, PERI, DOKA, COFFOR etc.

Stripping : Operation of removing formwork is known as Stripping or De-shuttering. 

Order and method of removing Formwork

• Shuttering forming vertical faces of walls, beams and Column sides should be removed first. Shuttering forming soffit to slab should be removed next
• Shuttering forming soffit to beams, girders or heavily loaded members should be removed at the end.
• Time of removal of formwork depends on type of Cement, ratio of Concrete mix and weather condition.
• Over loaded, misaligned formwork, Inadequate bracing, improper stripping are some of the reasons for formwork failures.

Stripping Time as per IS 456:2000 (For OPC Cement)

 FALSE WORK

A transitory Support system for the permanent structure until it can withstand its own weight is called as False work. Falsework may be required to support steel and timber frameworks and masonry arches as well as in situ and precast concrete construction like Flyover, box girder bridge, culverts, etc. False works have Props or Scaffolding only.

SHORING

Shoring is the process of temporarily supporting a building, vessel, structure, retaining wall or trench with shores (props) when in danger of collapse or during repairs or alterations. Shoring comes from shore, a timber or metal prop.

Professionals use shoring to fix unstable walls, demolish structures, change existing walls, construct new walls and repair cracked or broken walls or foundations. When deciding which type of shoring to use, professionals take several variables into consideration, like the soil located on the site, the proximity of the site to other structures and the environment of their worksite, like if it's wet, dry or near a body of water.

Shoring is often used to provide lateral support:

• To walls undergoing repair or reinforcement.
• During excavations.
• To prevent walls bulging out.
• When an adjacent structure is to be pulled down.
• When openings in a wall are made or enlarged.
• Temporary vertical supports to Bridges also.

Types of Shoring

• Horizontal shoring or flying shoring
•  Vertical shoring or dead shoring
•  Inclined Shoring or raking shoring

STAGING

Staging is a temporary member which is used to support formwork (either it may be for centering or shuttering). It is done by props, jacks, H frames, cup lock system, wooden ballies, etc. Staging should be firm enough so that during concreting it should properly hold the concrete & shuttering so that shuttering does not bulge of settle.

SCAFFOLDING

Scaffolding is a fixed / movable platform used to lift, support, and supply materials during a construction, repair or cleaning of a structure. In Construction, they are usually used for activities such as Plastering, Painting, Brick work at heights, etc.

 In addition, it also provides some degree of support for a standing structure during the construction phase.

Types of Scaffolding

• Single Scaffolding.
• Double Scaffolding.
• Cantilever Scaffolding.
• Suspended Scaffolding.
• Trestle Scaffolding.
• Steel Scaffolding.
• Patented Scaffolding.
• Wooden Gantries Scaffolding.

Single Scaffolding
Single Scaffolding is generally used for brick masonry Construction, It is made of only one row. It comprises of putlogs, standards, ledgers, etc. that are corresponded to the wall within a distance of 1.2m. The standards are placed at a distance of 2–2.5m. Ledgers connect the standards at vertical interval of 1.2 to 1.5 m. Putlogs are taken out from the hole left in the wall to one end of the ledgers. Putlogs are placed at an interval of 1.2 to 1.5 m.

Double Scaffolding

Normally used for the construction work of stone masonry, plastering, the double scaffolding is also called “mason’s scaffolding”. This scaffolding is made by two rows to support. The first row is 20–30cms away from the wall and the 2nd row is put almost 1m away from the row so that they can form a stiff support for the construction work.

Cantilever Scaffolding

This a type of scaffolding in which the standards are supported on series of needles and these needles are taken out through holes in the wall. This is called single frame type scaffolding. In the other type, needles are strutted inside the floors through the openings and this is called independent or double frame type scaffolding. Care should be taken while construction of cantilever scaffolding.

Generally cantilever scaffoldings are used under conditions such as

• When the ground does not having the capacity to support standards,
• When the Ground near the wall is to be free from traffic,
• When upper part of the wall is under construction.

Suspended Scaffolding

This one is used for the painting, cleaning and repair purposes for the exteriors of the house. The working platform of the scaffolding is suspended from the roofs through wires or chains and it can be pulled up or down depending on the area of the work carried out in the construction sites.

Trestle Scaffolding

This is mostly used for interiors work like paintings, cleaning, repairs, etc. This type of scaffolding is made in a way that it is supported on movable tripods or ladders and can be used up to 5 meters in height.

Steel Scaffolding

The steel scaffoldings are made of steel tubes and are joined by couplers or fittings that are also of steel materials. It is very easy to work on such type of scaffoldings as they have better robust, superior and durable characteristics. Also, it is easy to dismantle or erect such scaffoldings. But it is a bit costly as compared to the other types of scaffoldings.

Patented Scaffolding

Patented scaffoldings are readymade scaffoldings which are available in the market.  These are made up of steel but these are equipped with special couplings and frames etc., The working platform of the patented scaffolding is set in the brackets in a way that it is adjustable to required extents

Wooden Scaffolding

Wooden scaffolding can be used in the construction or repair of both large and small buildings. These scaffoldings are not as stable as scaffolds made from other materials. Bamboo scaffolding is widely used in Hong Kong and Macau, with nylon straps tied into knots as couplers. In India, bamboo or other wooden scaffolding is also mostly used, with poles being lashed together using ropes made from coconut hair (coir).

SLIP FORM

Slip form is a method of construction in which concrete is poured into the top of a continuously moving formwork. As the concrete is poured, the formwork is raised vertically at a speed which allows the concrete to harden before it is free from the formwork at the bottom. Slip form is most economical for structures over 7 storeys high such as bridges and towers, as it is the fastest method of construction for vertical reinforced concrete structures, but it can also be used for horizontal structures such as roadways.

As long as there is a regular shape or core, formwork can be used for a smooth, continuous pour that requires no joints. The formwork supports itself on the core rather than relying on other parts of the building or permanent works, and rises at a rate of about 300 mm per hour.

The concrete to be used needs to be workable enough to be placed into the form and consolidated by vibration, yet quick-setting enough to emerge from the form with strength. The concrete must be constantly monitored for its setting rate to ensure the forms are not being raised too fast.

Usually, the formwork has three platforms:

• Upper platform: This acts as a storage and distribution area.
• Middle platform: This is the main working platform and sits at the top of the poured concrete level.
• Lower platform: This provides access for concrete finishing.

It is essential that the middle supporting platform is rigid so that all parts of the structure move simultaneously. The shuttering can drag if there is inconsistency in the rate at which the platform is moving which can cause difficulties.

The types of structure that are most commonly constructed used slip forming techniques include:

• Service cores for commercial buildings
• Lift and stair shafts
• Silos
• Chimneys
• Concrete gravity structures such as oil platforms
• Bridge pylons and piers
• Mine headgear towers
• Shaft linings
• Surge shafts
• Liquid containment vessels

Advantages of slip forming

• Slip forming can achieve high production rates, however, once continuous concreting has begun there is little flexibility for change and so very careful planning is required.
• Crane use is minimised.
• Only minimal scaffolding and temporary works are required allowing the construction site to be less congested, and so safer.
• The exposed concrete can be finished at the bottom of the rising formwork.
• Slip form systems require a small but skilled workforce on site.
• There is flexibility in that tapering structures with wall reductions (either gradual, over a short distance, or stepped) can be achieved

Cost of Formworks

• For normal works cost of formwork is about 30%-40% of the concrete cost.
• For special works cost of formwork is about 50%-60% of the concrete cost.

 Formwork cost is controlled by the following factors

•  Formwork Material cost
•  Formwork erecting cost
•  Formwork removal cost
•  Formwork jointing cost (Nails and Cables)
• Labor charges.