Formwork
Permanent FormworksPermanent formworks are part of the permanent structure of the building. The main advantages are that they save time and labour as there is no need to dismantle the formwork. They reduce costs and save time by (source: CIRIA C558):
·
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 falsework.
·
Allowing early access for following or concurrent operations.
·
Eliminating the programme limitations of reuse of formwork.
The use of permanent formwork can reduce construction and maintenance costs, shorten construction time, and improve safety by reducing hazards during construction. It also reduces construction waste generation during construction.
Maintenance costs are reduced by:
· Improving curing of concrete and reducing shrinkage craking.
·
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.
DefinitionsPermanent 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.
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.
Potential applications and benefits of permanent formwork (source: CIRIA C558):
Checklist of recommended applications of materials for permanent formwork
Description of some permanent formwork.
METTALLIC
Profiled steel
The major use is in the high rise steel frame building sector where its speed of erection proved to be a great asset.
Common usage:
Composite beam and slab type concrete floors with the formwork spanning between either steel or concrete beams.
Ribberd floors within the depth of the permanent works supporting members.
Span Range:
Typical maximum span indicated is based on a150 mm slab.
Trapezoidal sections permitting a simple or multiple span of 1.5 to 3.8m for 1.2mm sheet thickness.
Trapezoidal section designed for use in slim floor systems permit spans of up to 6.5m unpropped and 9m propped.
Re-entrant sections permitting a simple or multiple span of 2.0 to 5.5m for 1.2mm sheet thickness
A
dvantages:
Economies of cost and speed over traditional methods.
Improves safety in difficult situations by providing a safe working platform and cover for other trades.
Large span range.
Light and easy handle.
Stiffens supporting steel frame.
Easily cut and fitted to awkward shapes.
Sealing between panels can be provided by lapping edges and ends.
Disadvantages:
Access or ribbed profiles difficult when in position.
Can compromises reinforcement detailing if used in bridges works.
The exposed steel sheet providing the tensile reinforcement is susceptible to fire damage and additional reinforcement may be needed in the slab to provide adequate fire resistance.
Coated mesh
A variation on a small open mesh is the use of larger, structural steel mesh with an additional coating to retain the concrete.
Common usage:
For below ground structures such as ground beams, column bases and pile caps.
External support provided by backfill to structure.
Can be used in above ground applications if external support is provided by either temporary falsework or standard lattice type walings and through ties.
A
dvantages:
Lower cost and higher speed over traditional methods.
Light and easy to handle.
Easily cut and fitted to awkward shapes by unskilled labour.
Reduces overdig, consequent backfilling and spoil removal.
Disadvantages:
Requires phased reinforcement fixing and backfilling.
Care needed during backfilling to prevent distortion and consequent loss of cover.
Large footprint spacer blocks required to prevent puncture of the polyethylene coating.
Links must be detailed and fixed to take backfill loads.
Examples in Hong Kong
Concrete Permanent Formwork Semi-Precast slabsThe Cambridge House, Quarry Bay.
The Rosedale on the Park Hotel, Causeway Bay.
Metal Permanent Formwork StaircasesThe Charter House, Central.
Steel mesh
Steel mesh can be used as permanent formwork, generally in vertical applications, with either a small opening, similar to largest aggregates size, to retain the placed concrete directly or as a larger size mesh with additional coating.
Common Usage as permanent formwork:
Construction joints in slabs.
Deep construction joints.
Vertical construction joints to walls.
Wall and column formwork, for further finishing or rendering.
Soffit formwork.
Curved wall formwork.
Arch formwork.
Span Range:
At the base of thick walls and deep stop ends supports at 225 to 325mm centers, depending on thickness.
Shallow stop ends 675 to 725mm for 500mm slab depending upon thickness.
For soffit of 300mm slab supports at 425 to 575mm depending upon thickness.
A
dvantages:
Economies of cost and speed over traditional methods.
Light and easy to handle.
Easily cut and fitted to awkward shapes by unskilled labour.
Benefits can be taken of reduced formwork pressures when using expanded mesh.
Pour can be monitored from partially exposed face.
Continuity of reinforcement through stop ends can readily be accommodated.
Disadvantages:
Produces an unformed face which is acceptable for stop ends to receive further concrete but must be rendered/faced for a fair finish.
CEMENTITIOUS Concrete Lattice productsPrecast concrete panels up to 2400mm wide reinforced by welded lattices which project into, and provide a mechanical linkage with, the in-situ concrete slab.
Common usage :Composite concrete floors spanning between either steel or concrete beams.
Composite bridge decks spanning between either steel or concrete beams.
Span Range :Up to approximately 3.8m for self-supporting units.
Larger spans, up to 10m, will require propping at construction stage.
Longer panels are available for particular applications.
A
dvantages:
Lower cost and higher speed over traditional methods.
Large areas quickly placed with mechanical handling.
Large span range.
Compatible material.
Can simplify reinforcement detailing, particularly when used in building works.
Disadvantages:
Relatively heavy, will require some mechanical handling.
Access difficult when in position, will require the provision of access routes.
Can compromises reinforcement detailing, particularly when used as planks in bridgeworks.
Tolerances of panels and permanent works, creep during positioning.
AttentionTo minimize demolition waste and provide maximum recycling opportunities, the use of permanent formwork should be carefully considered. Permanent formwork using different material types may be difficult to recycle.
Large Panel Formwork
Large panel formwork consists mainly of large pieces of metal formwork. It is most suitable for construction activities where formwork is highly repetitive as it can be reused many times. Therefore, its design is specified for the construction of the load bearing walls of typical floors in high rise tower blocks.
Compared with traditional timber formwork, metal panel formwork has several advantages. The use of large panel formwork can save time and labour in erecting, striking and re-erecting the formwork as the panel is handled as one unit. It can also produce a concrete surface, which is neater than is produced normally by conventional timber formwork, and the surface essentially needs no additional applied finishes for levelling. On site waste audit records also show large panel formwork systems are effective in reducing concrete waste generated by losses due to damaged formwork, which usually accounts for 30% of the total concrete waste. However, because of the weight of large panel formwork, tower cranes should be available for its handling.
Large panel formwork
Removal of formwork
Different large panel formwork systems are widely used today by contractors for constructing standardised housing blocks, and are broadly classified as wall forms, table forms and tunnel forms.
Wall FormsThe wall forms are combined with the slab form so that the wall and slabs can be formed monolithically in one casting operation, and the number of joints between panels is minimised. There is less chance of grout leakage or an uneven surface finish at the joint, which is a common fault with in traditional timber formwork.
Table FormsStandard modules of housing blocks are relatively large in span and large table forms are widely used for assembly time reduction, fewer joints and better surface finishes. The table method uses separate vertical forms for walls and horizontal table forms for floor slabs. The work is done in two stages. First, the walls are cast, and forms are stripped, the tables are then positioned, and the horizontal slabs are cast.
Tunnel FormsThe half tunnel is composed of vertical and horizontal panels set at right angles and supported by struts and props. The walls and slabs are cast in a single operation. Like the wall-forms and table forms, this reduces not only the number of joints, but also the assembly time. Therefore, the casting of walls and slabs can be completed in the one day.
Steel FormsBecause steel forms are very durable, one set of steel forms can be used to complete a project, and then reused for another project or scrapped for recycling. Steel forms can produce a better quality concrete finish when compared with timber forms. Although their initial cost is higher, they can be cheaper in the long run, particularly with their recycling potential.
Composite Steel DeckingSteel decking, becoming a permanent part of a composite slab, serves as both the working platform and formwork for supporting the in-situ slab concrete in the construction stage. The steel of the decking can also utilised to provide some of the required permanent reinforcement for the slab. In other words, the decking can replace both timber formwork and bottom reinforcement, reducing the amount of temporary works and formwork required.
Aluminum FormsAluminum formwork consists of small aluminum framed panels which are easy for handling. It is fast to erect and strike and the floor cycle can be 4 days. It is durable and can be reused over 100 times. Although it is more expensive than steel and timber, its merit is its lightweight and recycling potential.
Plastic FormsPVC and polyurethane forms can be used to replace timber forms in either rigid or flexible formats. Special floor forms made of fibreglass are occasionally used (6%). They can be reused over 50 times and produce a concrete surface finish similar in appearance to fine snake skin. If a smooth finish is desired, a paste wax or water-based release agent can be applied to the form.
Rib Loc Circular ColumnThis UPVC form is usually used to cast circular columns of diameter ranging from 150 to 3000mm. It is made by spirally winding a ribbed plastic profile into a tube. This enables column forms to be constructed which are strong, lightweight, easy to strip and which provide an excellent surface finish. The surface also incorporates the inherent “waxiness” of extruded plastic and no form oil is required. Square stabilisation timbers are fixed at the foot of the column formwork to stop the formwork moving out of position. Vertical bracing timbers are placed against possible movement while concreting. The process is cost efficient with recycling potential.
UPVC column forms
Erection of column forms
PecaformThis is made by laminating a layer of polyethylene to each side of a high tensile steel wire mesh. This combination creates a material that is both light and structurally strong, making it very easy to handle. It can be used for constructing ground beams, pile caps, footings, curved structures, ribbed and waffle slabs. The formwork is cut-to-size and bent to shape at factory and arrives at site ready for installation. There is no need to strip formwork after the concrete has cured. No waste is produced. A clean and neat site can be obtained in the foundation stage with Pecaform.
Construction of pile caps using Pecaform
Backfill around pile caps about 150mm from the top level of Pecaform
High Density Overlaid (HDO) PlywoodThe coating of conventional plywood with a thick layer of polymer resin may improve the quality and durability of the formwork. For example, applying a medium density overlay or a high density overlay on plywood can increase the number of reuses to 20. The form can be easily dismantled and handled by small cranes and can be adjusted to suit architectural requirements.
Example Supplier for Aluminium Formwork VSL Hong Kong Limited 3/F Stelux House, 698 Prince Edward Road East, San Po Kong, Kowloon,Permanent Formwork: What’s All the Fuss About?
There are two main types of formwork in the building and construction industry: temporary formwork and permanent formwork.
But, what are they? And what’s the fuss about permanent formwork?
Here’s a guide to help you out.
Temporary formwork has long been a mainstay of the industry. As the name suggests, it is a temporary but reusable formwork that is removed from around the concrete once it is laid and completely cured.
Permanent formwork is a relative newcomer to the world of the construction industry. However, its effectiveness has seen it adopted by Solutions Built – we specialise in installing AFS Rediwall, Logicwall, Dincel or Hebel formwork solution – and the rest of the world with lightning speed.
Unlike temporary formwork, permanent formwork stays in place, acting as an additional stabiliser, making it an appealing alternative to precast concrete and temporary building methods. In a nutshell, the process uses hollow panels in place of traditional block and brick work. These hollow panels are then filled with concrete, remaining in place forming a permanent structure.
In addition to providing extra stability, permanent formwork protects concrete from any deterioration that might be caused by surrounding soil it comes into contact with.
Formwork has largely been used in the laying of horizontal slabs. But now, with new procedural and technological advancements it’s possible to pour vertically and use the permanent formwork to create walls.
It also reduces the time and processes required for both commercial and residential builds. Safer sites for workers, quicker project turnarounds and reduced costs; these are areas of paramount importance to any reputable construction company, and to the customer. What’s not to love?
Well, it’s suitable for just about any construction project.
Small or large builds, residential or commercial.
And it’s a welcome process that’s been widely heralded as revolutionary within the industry. Notably, many architects have also praised and adopted the practice.
Which tends to suggest that permanent formwork doesn’t look to be going anywhere, anytime soon.
With our focus on creating durable buildings on budget and on schedule, we recommend that all our our clients consider permanent formwork. If you would like to find out more about how we can help with your upcoming project, please Get in touch.