Table of Contents
Building Construction - Formwork
Formwork is a temporary arrangement used in building construction to mold, hold and support concrete or other materials until they set and gain sufficient strength. It is cured enough to support itself.
General
- Construction:
Construct formwork and falsework such that it is strong to support the load imposed on them by fresh concrete and stresses imposed by vibrating equipment and traffic. Prevent settlement of support. All joints should be tight against the escape of cement and fines. Make due allowance (including camber) for settlement and movement of forms under fresh concrete. If metal ties are used in conjunction with bolts to pass through the concrete, do not leave the metal closer than 50 mm (2”) from the face of the concrete.
- Usage:
Do not use damaged formwork. Provide shear bars as per the Engineer in Charge’s instructions at all construction joints. Maintain formwork fabrication tolerances and surface irregularities as follows:
- Variations from plumb in lines and surfaces of piers, walls, and rises: 6mm (¼”) per 3M (10 ft) but not more than 25mm (1”).
- For exposed corners, columns, central joints, grooves, and other construction lines: 6mm (¼”) in any bay or 6M (20 ft).
- Variation from the position of linear building lines and related columns, walls, and partitions: 13mm (½”) in any bay or 6M (20 ft) max and 25mm (1”) in 12M (40 ft) or more.
- Variation in the size & location of sleeves, floors & wall openings: 6mm (¼”).
- Variation in cross-sectional thickness of slab & walls: between minus 6mm (-¼”) and plus 12mm (+½”).
- Variation in footing plan dimensions: between (-) 12mm (-½”) and (+) 50mm (+2”). Layout displacement not more than 12mm (½”) and verticality deviation not more than 1.0% of the footing width, in the direction of displacement, but not more than 50mm (2”) on top.
Suitable openings as required shall be provided in the formwork for the passage of piping ducts, channels, etc. All forms shall be so constructed that they can be removed without damaging the concrete. All exposed joints, edges, and external corners shall be chamfered. 37.5 mm (1½”) at 45°, except as otherwise shown. Internal corners are filleted where indicated or required. Forms that are to be used more than once shall be maintained in serviceable condition and shall be thoroughly cleaned before reuse.
Design of Formwork
- Scaffolding and Shuttering:
All scaffolding and shuttering shall be of steel unless specifically allowed by the Engineer-in-Charge.
- Conformance:
Forms shall result in a final structure that conforms to shapes, lines, and dimensions of the members as per the design, drawing, and specifications.
- Substantial and Tight Forms:
Forms shall be substantial and sufficiently tight to prevent leakage of mortar.
- Bracing:
Forms shall be properly braced or tied together to maintain position and shape.
- Protection of Structure:
Forms and their support shall be designed so as not to damage previously placed structures.
- Design Considerations:
Design of formwork shall include consideration of the following factors:
- Rate and method of placing concrete.
- Construction loads including vertical, horizontal, and impact loads.
- Special form requirements for construction of shells, folded plates, domes, architectural concrete, or similar types of elements.
- Removal of Forms:
Forms shall be removed in a manner that does not impair the safety and serviceability of the structure. Concrete to be exposed by form removal shall have enough strength not to be damaged by removal operations.
Coating / Oiling
Shortly before concrete is placed, forms for exposed surfaces shall be coated with approved non-staining form oil, which shall not interfere with the setting of the concrete nor be otherwise deleterious. After oiling, surplus oil on the form surfaces and any oil on the reinforcing steel or other surfaces requiring bond with the concrete shall be removed. Forms for unexposed surfaces may be thoroughly wetted in lieu of oiling, immediately before placing the concrete.
Removal of Formwork
Formwork should be removed after completing the required sufficient period (sufficient time has elapsed for the concrete to attain sufficient strength before removal of forms). Forms shall be removed with care so as to avoid injury to the concrete. Forms shall be removed as soon as practicable, keeping in view the minimum concrete setting time requirements, to avoid delay in water curing, and to enable the earliest practicable repair of surface imperfections. In order to avoid excessive stresses in the concrete that might result from swelling of the forms, wooden forms for wall openings shall be loosened as soon as this can be accomplished without damage to the concrete. Forms for the openings shall be constructed in such a manner as to be removed until the strength of the concrete is such that form removal will not result in perceptible cracking, spalling, breaking of edges of surfaces, or other damage to the concrete. In general, the approximate elapsed time before removal of forms is as follows:
