Formwork is the term given to either temporary or permanent molds into which concrete or similar materials are poured. In the context of concrete construction, the falsework supports the shuttering moulds.
The construction of formwork takes time and involves expenditure upto 20 to 25% of the cost of the structure or even more. Design of these temporary structures are made to economic expenditure. The operation of removing the formwork is known as stripping. Stripped formwork can be reused. Reusable forms are known as panel forms and non-usable are called stationary forms.
Timber is the most common material used for formwork. The disadvantage with timber formwork is that it will warp, swell and shrink. Application of water impermeable cost to the surface of wood mitigates these defects. Hence PERI uses formwork girders like VT20K and GT24 in place of timber which are very nailable and have very high life.
A good formwork should satisfy the following requirements:
Economy in Formwork
The following points are to be kept in view to effect economy in the cost of formwork:
Formwork can be made out of timber, plywood, steel, precast concrete or fibre glass used separately or in combination. Steel forms are used in situation where large numbers of re-use of the same forms are necessary. For small works, timber formwork proves useful. Fibre glass made of pre-cast concrete and aluminium are used in cast-in-situ construction such as slabs or members involving curved surfaces.
When the material for formwork has been chosen, and the anticipated loading estimated, a form should be designed strong enough to carry the anticipated loads safely, and stiff enough to hold its shape under full load. At the same time the builder or contractor wants to keep costs down by not overbuilding the form.
Loads imposed by fresh concrete against wall or column forms differ from the gravity load on a horizontal slab form. The freshly placed concrete behaves temporarily like a fluid, producing a hydrostatic pressure that acts laterally on the vertical forms. This lateral pressure is comparable to full liquid head when concrete is placed full height within the period required for its initial set.
With slower rate of placing, concrete at the bottom of the form begins to harden and lateral pressure is reduced to less than full fluid pressure by the time concreting is completed in the upper parts of the form. The effective lateral pressure −a modified hydrostatic pressure− has been found to be influenced by the weight, rate of placement, temperature of concrete mix, use of retardant admixtures, and vibration.
Factors affecting lateral pressure on forms are:
Gravity loads are from all live and dead loads applied to and supported by the formwork. These include the weight of the concrete and reinforcing steel, the weight of the forms, and any construction loads from workers, equipment, or stored materials. Loads from upper floors may also be transferred to lowerlevel forms in multistory construction. The largest loads are generally due to the weight of the concrete being formed and to the construction live load from workers and equipment. it is common to use a value for design of 25KN per cubic meter , which includes an allowance for the reinforcing steel.
Where lightweight or heavyweight concrete is used, the density of that particular mix should be used in calculating formwork loads. Trying to predict what value should be used for construction loads to account for the weight of workers and equipment is difficult. The weights of the workers and equipment would have to be estimated for each situation and their locations taken into account when trying to determine worst-case loadings. As a guide to the designer in ordinary conditions,
ACI Committee 347 recommends using a minimum construction live load of 2 KN/sqm of horizontal projection when no motorized buggies are used for placing concrete. When motorized buggies are used, a minimum construction live load of 6 KN/sqm should be used. The dead load of the concrete and forms should be added to the value of the live load.
In addition to fluid pressures, formwork must also resist lateral loads caused by wind, guy cable tensions, starting and stopping of buggies, bumping by equipment, and uneven dumping of concrete. Because many formwork collapses can be attributed to inadequate bracing for handling lateral loads, it is important that these loads be properly resisted by an adequate bracing system
• The dead load of the concrete and reinforcing, and the dead load of the forms when significant
• The construction live loads (workers, equipment, storage of materials)
• Design strength of concrete specified
• Cycle time for placing of floors of building
• Structural design load for the floor element supporting construction loads (slab, beam, girder, etc.) — include all loads the engineer designed the slab to carry
• The rate at which the concrete will gain strength under job conditions — use to find the strength of the concrete when loads are applied to it
• The way the loads applied at the different levels are distributed to the floors at the different levels by the elements of the form system
1. Wall Formworks
2. Slab Formworks
3. Monolithic Formworks
4. Climbing Formworks
The girder wall formwork consists of timber and/or steel girders connected to each other that form the support surface for the formlining. The optimization of individual elements and the development of a wide range of accessories for connections and attachments still makes girder wall formwork cost-effective in spite of the relatively high number of components. A major advantage is the flexibility in order to be able to adapt to complex shapes and high loads. In particular, architectural concrete structures with special requirements for surfaces and anchor patterns can often only be efficiently realized with girder wall formwork.
The versatile PERI VARIO girder wall formwork system, customized and assembled specially for the project: Form-lining shall be 18mm densified film faced plywood. Specially designed large elements, makes shutter handling efficient in the Climbing System.
Slab formwork is designed as per the depth of concrete slab over, filling the slab portion between beams having large surface area.The loads on plywood are usually considered as being uniformly distributed over the entire surface of the plywood. The concentrated load “d” can by transformed in a uniform distributed load, by dividing it to the span “l”
With the PERI UNO formwork system, walls, columns, slabs, beams and stairways are realized using the monolithic construction method. The system offers a suitable and fast solution for forming frequently repetitive floor plans - horizontally or vertically.
The UNO system is characterized through easy and safe handling whereby the low weight is an important factor. The individual panels can be easily moved and transported by hand. Through the use of special wedge couplers, the wall and slab panels are assembled in a very short time.
The individual panels are produced according to project specifications as well as taking into consideration the high PERI quality standards. As a result, all structural dimensions and building shapes are possible.
With PERI UNO, the usual number of re-use for monolithic formwork systems is achieved.
Climbing formwork is a special type formwork for vertical concrete structures that rises with the building process. While relatively complicated and costly, it can be an effective solution for buildings that are either very repetitive in form (such as towers or skyscrapers) or that require a seamless wall structure .
Various types of climbing formwork exist, which are either relocated from time to time, or can even move on their own (usually on hydraulic jacks, required for self-climbing and gliding formworks).
PERI ACS and RCS variants for facades, building cores, shaft structures and inclined bridge pylons make the construction of high concrete structures extremely economical, fast and safe. The efficient sequence of operations from floor to floor facilitate allow a very high level of productivity and exceptionally short cycle times.