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Modern Concrete Formwork Systems: Types, Reuse, Reliability & Risk Assessment

Formwork has been used widely in construction practice since the discovery and establishment of Portland cement concrete as a favored building material. Formwork is a

temporary structure that can be incorporated into the permanent structure or removed after the concrete has reached design strength. Formwork costs can constitute from 35 to 60 percent of the concrete cost on projects involving large quantities of concrete work. Formwork is significantly important activity for concreting. Good quality of formwork can contribute a great to good quality of concrete. It not only holds the concrete during its wet stage but has many other important functions in this ac-tivity of concreting. Bad formwork has often yielded failures of minor as well as major magnitude. It is also fairly popular as shuttering. Its functional as well as financial share in the entire concreting activity can’t be ignored.

Various Available Formwork Materials:

Formwork can be made out of timber, plywood, steel, pre-cast 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 anticipated. For small works, timber form-work proves useful. Fibre glass made of precast concrete and aluminum are used in cast-in-situ construction such as slabs or members involving curved surfaces.

Timber Formwork: It should be (i) well seasoned, (ii) light in weight, (iii) easily workable with nails without splitting and (iv) free from loose knots. Refer Figure 1.

Plywood Formwork

Resin bonded plywood sheets are attached to timber frames

Sonjoy Deb, B.Tech, Civil Associate Editor

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to make up panels of required sizes. The cost of plywood form-work compares favorably with that of timber shuttering and it may even prove cheaper in certain cases. Refer Figure 2.

Plastic Formwork

These interlocking and modular systems are used to build widely variable, but relatively simple, concrete structures. The panels are lightweight and very robust. They are especially suited for low-cost, mass housing schemes. Refer Figure 4.

Figure 1: Typical Timber Formwork

Figure 4: Typical Plastic modular formwork

Figure 2: Typical Plywood Formwork

Figure 5: Typical Aluminium Formwork System

Figure3 : Typical Steel Formwork

Steel Formwork

This consists of panels fabricated out of thin steel plates stiffened along the edges by small steel angles. The panel units can be held together through the use of suitable clamps or bolts and nuts. The panels can be fabricated in large number in any desired modular shape or size. Steel forms are largely used in large projects or in situation where large number reuses of the shuttering is possible. This type of shuttering is considered most suitable for circular or curved structures. Refer Figure 3.

Aluminium Formwork

The aluminum formworks can be recycled after smelting. Aluminum formworks are light in weight and highly precise in assembly. The installation process is easier and the installa-tion time can be reduced by 30% compared with the wooden formworks. A area of 2,000 m2 aluminum formworks can be completed in about 35 hours. It is unique because it forms all of the concrete in a building including walls, floor slabs, columns, beams, stairs, window hoods, balconies and various decorative features in exact accordance with the architects’ design. The dimensional accuracy of the concreted work also results in consistent fittings of doors and windows. The smooth–off form finish of the concrete eliminates the need for costly plastering. Aluminium Formwork System provides Aluminium Formwork for RCC load bearing or RCC framed multi-storied buildings and enables the walls and slabs to be poured in the same operation. Refer Figure 5.

Fiberglass formwork

The use of fiberglass as a material to make forms for con-crete work has increased rapidly within the past two years. Among the reasons are:

- Fiberglass forms provide a means of producing a concrete surface that is architecturally acceptable without rubbing and grinding.

- Special patterns and designs can be readily molded into the material.

- Large areas or sections may be made without joints or seams.

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- When repeated usage is possible, fiberglass is often the most economical form material. Perhaps most important one is fiberglass forms allow the

architect complete freedom of design. Refer Figure 6.

reuse. Formwork systems based on reusability criteria can be grouped under two heads as mentioned below.

Low-reuse systems: These are loose form systems based on common or special materials. Assembled from stock com-ponents, they typically have low initial cost, are easy to modify, and are very adaptable. They are available for purchase only, and may require slow rates of concrete placement. Their pri-mary use is in small quantity applications, with few repetitions and many penetrations.

High-reuse systems: These include hand-set panel sys-tems, equipment set gang forms, and plate-girder forms. Some experts recommend building form modules or gangs when more than 10 reuses are needed. Heavy-duty forms are well-suited for building massive girders and pier caps. Having larger sys-tem components, these systems can require heavy lifting capa-bilities and advance fabrication, and are generally not suited for modifications or many penetrations.

The reusability of formwork depends on the type of formwork materials and nature of the construction. The more symmet-rical and repetitive the structural element layout is, the more favorable formwork reuse is. Complicated structural elements such as curved concrete beams or any irregularly shaped con-crete structure may require custom made formwork, which does not favor formwork reuse. Refer Figure below for Steel plate-girder forms are being used to form raker beams for a Chicago sports facility. Offering a high number of reuses, these systems are well-suited for projects with repetitive structural features.

Caring for Formwork for Reuse

Formwork materials must be handled carefully and acci-dental dropping must be avoided as possible. Used formwork panels should be inspected for damage before they are used again. Formwork highly effects the final appearance of the finished structure, so any defect must be checked first and re-

Figure 6:Typical Fibre Glass formwork systems

Figure 8: Reuse of Plastic Modular Formwork

Figure 7: Corrugated & Flat Permanent GRC Formwork

Figure 9: Steel plate-girder forms being used in Chicago sports facility Yield-ing Maximum Repetitions

Corrugated & Flat Permanent Formwork

Glassfibre Reinforced Concrete (GRC) has been extensively used as permanent formwork for the past twenty years, cor-rugated or flat to suit all supporting beam design. Formwork manufactured from GRC is capable of supporting various slab thicknesses over a variety of spans between main bridge beams. In addition, it’s characteristics allow it to behave as a composite part of the in-situ concrete under normal in-service dynamic loading. Refer Figure 7.

The Aspect of Formwork Reuse

Costs of forming account for about half the value of typical cast-in-place concrete construction. Planning for maximum form reuse can reduce forming costs and provide an oppor-tunity to win bids in competitive markets. The ability to reuse formwork depends on the nature of the construction. Regular, repetitive structural features favor reuse. Complicated layouts may require many custom revisions. Site layout, phasing of the work, ability to fabricate formwork components, and availability of cranes or other lifting equipment also are important factors.

Opportunities for form reuse are found both on small and large jobs. On small jobs, plywood reuse may be the most im-portant consideration. On larger projects with complex require-ments, the choice of forming system and form ties can affect form

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paired or removed; otherwise the formwork’s defect will reflect on the finished surface of the concrete. After the formwork is repaired, any excessive concrete on the formwork surface must be removed also. Aluminum formwork should be oiled to pre-vent it from getting rusty. After the used formwork is repaired, excessive concrete removed and oiled, they should be stock-piled and covered with suitable fabric like canvas.

Reusing Formwork Saves in Construction Costs

Formwork is a large proportion of the total cost of a building structure, approximately 10 percent of total cost of the struc-ture. Reusing formwork for concrete is one of the ways to re-duce construction costs. The initial cost of formwork materials will drop to 40 percent after five reuses. The economical value or reusing formwork lies in the ease with which it can be stripped from concrete. With that in mind, formwork designers must kept abreast in advancing technology of formwork manufacturing so that quality can be maintained without ignoring its economical value in facing new challenges in reusing concrete formwork.

Proven Formwork Innovations :

The features that manufacturers of various forming sys-tems have developed result in more efficient concrete con-struction. Below listed are a few of the important formwork innovations:

Self-aligning connections—Channelized panel-locking mechanisms mean there is no need to make form adjustments after moving assemblies to align panels. Channel-locking mechanisms connect and align two panels with a single hammer blow at any position. Connections for crane lift hooks permit fast positioning, and conveniently located tie-off bars on forms ensure worker use and safety. Refer Figure 10.

prop spacing and formwork loads. Fast and early reshoring of elevated slabs is possible with new decking systems. Workers lower the quick-release dropheads on shores as rapidly as specifications allow (often striking 1 to 2 days after a pour), strip the slab panels, and reshore with few component parts and no loose hardware. Most notable on projects using these new slab-forming systems is the open, uncluttered maneuvering room on the reshore level, with up to 50% fewer props required.

Fill-in minimized—With traditional plywood forms, carpen-ters can spend more than half of their time fabricating and in-stalling fill-in formwork for irregular openings around columns, corners, and other penetrations. This fill-in can consume more than 50% of the forming labor. The high versatility of the new systems with integrated parts significantly reduces the neces-sity of job built formwork.

High-pressure forms— High pressure forms meet the de-mands of today’s new concrete mixes. High pressure forms has a higher pour rating than any system on the market plus the ability to handle external vibration. Due to an increased use of reinforcing steel, many contractors expect growth in the use of high-slump self-consolidating concrete (SCC), which means faster pour rates and higher lifts, requiring increased formwork pressure ratings. Typical formwork is designed for pressure ratings around 1500 psf, but there are stronger systems on the market with a rating of 2000 psf. Refer Figure 12.

Reliability & Risk Assessment of Formwork Reusability:

Formwork is generally designed according to guidelines set by various associations or publications such as the American Ply-wood Association (APA), National Design Specification for Wood Construction (AWC, 2005), and ASCE Design Loads on Struc-tures During Construction manual. Perusal of the more com-monly available guidelines indicate that re-use of formwork is generally not formally factored into the design of formwork. Formwork is subjected to a wide variety of loads and exposures when in use, and it stands to reason that there would be a re-duction in the strength or structural capacity of the formwork as it undergoes multiple uses. Reliability and risk related to formwork activities are also topics that are underexplored. Use of formwork often involves working at heights, and on tempo-rary platforms, which are factors that affect the efficiency and safety of construction workers. In addition, activities such as strip-

Figure 11: The main working platform is completely enclosed inside trapezoi-dal metal sheeting 2m high

Figure 10: Formwork with self aligning connections

Off-the-truck utility—Components can be erected right off the truck, with concrete placement the same day. Once the integrated forms are assembled into gang panels, the larger assembly is then lifted into place by crane for the next forming sequence. No adjustment or realigning of panels is required. Refer Figure 11.

Nonwood panel— Several Formwork Systems manufac-turer’s have invested in a nonwood panel. The nonwood material’s are water-repellent, does not shrink, has unrestricted nailabili-ty, and provides triple the working life of plywood. Nailing a non-wood panel does not violate the integrity of the panel—unlike the nailing of a wood panel, which introduces moisture into the plywood and leads to delamination.

Engineered shoring removes guesswork—The new alumi-num slab-formwork system provides a systematic erection se-quence that eliminates errors and guesswork in determining

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ping of formwork from concrete and assembling forms on site create a certain amount of risk to the workers. The location of the activity and the activity itself affects the productivity of the worker as well as the safety of the worker. These effects need consideration during design and by regulatory agencies such as OSHA. Most safety issues such as fall protection, scaffolding, and use of power tools, have been addressed in the OSHA stan-dards for construction (29 CFR 1926) by themselves. However, these issues need further investigation from the perspective of formwork use. OSHA Fatality and Catastrophe Investigation Summaries give an idea of the various types of incidents associ-ated with concrete formwork, as well as the causes of incidents associated with formwork use. There are no mandatory rules regarding the use and re-use of formwork, but just guidelines for use.

Conclusion

Formwork development has paralleled the growth of con-

crete construction throughout the 20th century. Formwork is a classic temporary structure in the sense that it is erect-ed quickly, highly loaded for a few hours during the concrete placement, and within a few days disassembled for future re-use. Also classic in their temporary nature are the connections, braces, tie anchorages, and adjustment devices which forms need. The form design should be such that it can be erected and disassembled efficiently in order to maximize productivi-ty. The disassembly or stripping of forms depends on factors such as the bond between concrete and the form, rigidity and shrinkage of concrete. The ability to reuse formwork depends on the nature of the construction. Regular, repetitive structural features favor reuse. Complicated layouts may require many custom revisions. Site layout, phasing of the work, ability to fab-ricate formwork components, and availability of cranes or other lifting equipment also are important factors.

Reference

1. http://www.sciencedirect.com/science/article/pii/S03601323990000862. http://www.geoplast.it/sites/default/files/catalogue_geotub_panel_

en_0.pdf3. http://denia.lt/wp-content/uploads/geopanel_star_eng.pdf4. http://www.program.com.hk/upload/catalog/en.pdf5. http://www.concreteconstruction.net/images/Form%20Reuse_tcm45-

342553.pdf6. http://files.hostgator.co.in/hostgator181958/file/aluminiumform-

worksystemintro.pdf7. http://www.djc.com/stories/images/20101028/CostEffective_flying-

form_big.jpg8. http://www.worldhighways.com/categories/road-highway-structures/

features/modern-formwork-systems-fast-flexible-safe/9. http://www.peri.com/files/jpg1/vario-gt-24-01.jpg w

Figure 12: High Pressure Formwork Systems

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