Formwork and shuttering in construction: types and striking times

Formwork and shuttering: what they are

Formwork in construction is everything that shapes and supports concrete until it hardens: the face material in contact with the concrete, the frame behind it and the props or ties that carry the load. Shuttering in construction usually means the face panels themselves, often plywood or steel sheets; in building construction formwork the two words are used almost interchangeably.

Formwork is temporary, but it decides the size, line and surface of the finished member. India’s code for plain and reinforced concrete, IS 456:2000, states the two basic duties in clause 11.1: stay rigid while the concrete is placed and compacted, and prevent loss of slurry. Tanzanian requirements were not checked for this page; IS 456 is quoted here as a published example of a concrete code, not as the local requirement.

Types of formwork by material

Types of formwork by material

Material	What it is	Typical use	What to ask the supplier
Sawn timber	boards and battens cut and nailed on site	small jobs, odd shapes, edges	timber species and moisture content
Film-faced plywood	birch or other plywood with a phenolic film on both faces	wall and slab panels, columns	thickness, film weight, expected uses (figures below)
Steel formwork	steel plate on steel frames	repeated columns, walls, precast moulds	panel weight, permitted concrete pressure
Aluminium	aluminium frames with plywood or aluminium faces	system panels for repeated floors	panel weight, compatible props and ties
Plastic	moulded panels or disposable moulds	small repeated elements	sizes available, number of uses

A further option is formwork that stays in place: the GRCA design guide lists permanent formwork among the products made from sprayed glass reinforced concrete. Manufacturer figures are given below for film-faced plywood only; for the other materials, the supplier’s data sheet is the place to check weight, permitted pressure and number of uses.

Formwork for columns, beams, slabs and stairs

Column formwork. Four sides boxed and clamped, with the clamps closer together near the bottom, where the pressure of wet concrete is highest. Formwork to columns is vertical formwork, which IS 456 lets you strike first.

Beam formwork. A soffit board with two side panels, carried on props. The sides can come off early; the soffit and its props stay longer, as the striking table shows. Where a pour has to stop, the stop end forms a construction joint, one of the joint types set out on the expansion joint page.

Slab formwork. Floor slab formwork is a deck of panels on joists and bearers, held up by props. Shuttering for a slab must also carry workers and the reinforcement before the pour, which is why the slab spacers and the deck have to be level and firm (see rebar spacers and cover blocks).

Stair formwork. A sloping soffit deck like a small inclined slab, with riser boards fixed across it to form each step. Formwork for steps is closed at the sides by stringer boards, and the risers are braced so they do not bow when concrete is placed.

Film-faced plywood: thickness, weight and reuse

Plywood makers publish the figures a buyer needs. The Metsä Wood FormPLUS data sheet gives these values for its birch panels, built from 1.4 mm veneers with a 220 g/m² phenolic film:

Film-faced plywood: thickness, weight and reuse

Thickness	Plies	Weight
12 mm	9	8.2 kg/m²
15 mm	11	10.2 kg/m²
18 mm	13	12.2 kg/m²
21 mm	15	14.3 kg/m²

The same sheet puts the number of uses at 30 to 100, depending on the film, site practice, the concrete finish required, the release agent and how panels are stored and handled. The Doka data sheet for Dokaplex gives a guide value of up to 50 uses for its 18 mm sheet, which has 220 g/m² of film per side against 120 g/m² on its 4 mm and 9 mm sheets.

Thickness also sets how far the deck can span between joists. The Doka sheet, quoting the Handbook of Finnish Plywood, gives a bending stiffness (EI) of 4.56 kNm²/m along the grain for the 18 mm sheet and 6.97 kNm²/m for the 21 mm sheet, about 53% more (our calculation). Across the grain the figures fall to 3.38 and 5.40, and on that sheet the face grain runs across the length of the panel, so the way a panel is laid on the joists changes how much it deflects. Doka lists the 18 mm sheet at 12.2 kg/m², the same as Metsä; a 150 × 250 cm panel of that weight comes to about 46 kg (our calculation).

Before the pour: what the formwork must do

IS 456 asks for three things before concrete goes in:

1. The forms are rigid enough not to move or bulge during placing and compaction (clause 11.1).
2. The joints are tight enough to stop cement slurry escaping (clause 11.1).
3. Rubbish, chippings, shavings and sawdust are removed from inside the forms (clause 11.2).

Both plywood data sheets add a fourth: a release agent suited to the panel film, applied before each use. Inside the forms the bars rest on spacers so the concrete cover holds during the pour; Kitovu stocks Plastic Rebar Chairs with Base (Rebar Spacers) in Zanzibar.

When formwork can be struck

IS 456 clause 11.3 sets the rule: forms stay until the concrete has reached at least twice the stress it will carry when they are removed. For normal conditions — ambient temperature not below 15°C, ordinary Portland cement and adequate curing — clause 11.3.1 gives these minimum periods:

When formwork can be struck

Formwork	Minimum period before striking
Vertical formwork to columns, walls, beams	16–24 hours
Soffit formwork to slabs (props refixed at once)	3 days
Soffit formwork to beams (props refixed at once)	7 days
Props to slabs spanning up to 4.5 m	7 days
Props to slabs spanning over 4.5 m	14 days
Props to beams and arches spanning up to 6 m	14 days
Props to beams and arches spanning over 6 m	21 days

Source: IS 456:2000, clause 11.3.1. The code says the periods may be changed for other cements and lower temperatures, and admixtures that slow setting change them too (see concrete admixtures).

Two further rules follow. The props left under a slab or beam must carry its full dead load plus any load from work above while it cures (clause 11.3.2). Where formwork has re-entrant angles, it should come off as soon as possible after the concrete has set, so the form does not restrain the concrete and cause shrinkage cracks (clause 11.3.3).

Looking after shuttering between pours

Both data sheets tie the number of uses to handling. The Doka sheet asks for concrete residue to be removed immediately after stripping, and warns against pointed or sharp tools, wire brushes and high-pressure cleaners on the film. Metsä asks for panels to be cleaned after each use, scratches sealed with waterproof paint, holes filled with a water-resistant filler and a fresh coat of release agent before the next pour. Its film is not UV resistant, and the colour may change after long exposure to sunlight.

On shuttering work in construction that repeats floor after floor, this care decides whether a panel gets near the top or stays at the bottom of Metsä’s range of 30 to 100 uses.