BUILD RIGHT FORMWORK Many builders have had a concrete and formwork failure that they would rather forget. It’s an area of construction where listening to the wisdom of experienced tradespeople is vital, but the following pointers may also help. By Des Molloy, Freelance Technical Writer, Golden Bay
he construction of formwork is taken the person doing the formwork is equally vital in Building formwork is something that is best for granted by all except those doing it. producing a quality result. learned by listening and working alongside more It is difficult to find information about experienced tradespeople. Learning only by T it – there is no mention in the Building Need to be able to build and unbuild experience can result in failures along the way. Code, no New Zealand standard to follow The design and build of formwork is usually (Australia has AS 3610), no ‘how to’ pamphlets done by site staff. Unless a proprietary system Huge forces to consider in the local merchants’ shelves and of course is being used, it is up to the carpenters on site Wet concrete exerts huge forces against there is usually no lasting monument to well to plan how to build and support the formwork, the forms. The concrete and reinforcing together constructed examples. and also how to unbuild it. Being able to strip weigh approximately 2.4 tonnes/m3. Lateral Finishing carpenters often take the glory the formwork effectively is an important part of forces are generated in walls creating horizontal when a job is being reviewed, yet the work of the task. bursting pressures, which are increased by
Note: Double waling and she-bolts may be used to tie the top of the wall together.
spacer used to keep cleat packed off form to keep place outside formwork; excavate forms apart until formwork apart, support trench; place steel – erect inside concrete placed and to reinforcing and holding down formwork tie forms together bolts and allow trowelling verticals at 600 mm pre-made plywood max. centres for 25 mm shutters fixed securely thick boxing pre-built shutter packing (dotted) to form together rebate for concrete slab floors brace at 1200 mm support cleat to top 100 x 50 mm braces at double brace – base of centres waling 1200 mm centres boxing is too close to trench edge to peg peg loose spreader – remove as concreting pull nail to remove proceeds spreader 200 x 50 mm plate, 100 x 50 mm double 150 x 50 mm set to line before formwork ties waling/strong backs at plate excavation fitted through 700 centres conduits pegs to hold base boxing constructed of of shutter in place brace to foot of wall 150 x 25 mm boards double waling closely butted required when bottom she-bolts at form ventilation using she-bolts 900 mm centres openings in walls that trench outline have a suspended 150 x 50 mm starter bars from footing floor over bottom plate pour footing with nib to locate boxing
Note: 1. Formwork may be either plywood or timber. Plywood is more suitable where a Note: 1. Formwork may be either plywood or timber. Plywood is more suitable where a fair-face finish is required. fair-face finish is required. 2. Pegs and plate beside trench should be in place before digging the trench. (If driven 2. Pegs and plate beside trench should be in place before digging the trench. (If driven after trenching, pegs may cause the side of the trench to collapse.) after trenching, pegs may cause the side of the trench to collapse.)
Figure 1: Foundation wall formwork – low walls. Figure 2: Foundation wall formwork – walls up to 2 m.
22 BUILD 120 October/November 2010 Note: Place sole plate before excavating trench. distance piece removed 25 mm thick timber rough-sawn 18 mm plywood when beam ¾ full boards closely butted
top plate 150 x 50 mm joists
raking cut to peg ease striking
ledger 100 x 50 mm 18 mm plywood support
head tree 100 mm
brace (typically 25 mm 150 x 75 mm thick at 600 mm centres) ledgers framing wedged up to level fork heads 150 x 50 mm sole plate steel props at 900–1,200 centres
side braces left out for clarity
Figure 3: Formwork for thickened edge slab up to 600 mm high. Figure 4: Beam box less than 600 mm.
internal vibrators during compacting. (This exerts being forced out of gaps and open footings when Make sure it’s very strong similar pressure to a hydraulic jack.) there is a head of pressure above. Care has to be It’s unlikely that anyone has been sacked, Formwork needs to resist lifting and bending in taken in making sure forms will keep the concrete injured or killed by making formwork using every direction and be able to also support the live where it is meant to be until it hardens. too many props, strong-backs or holding down loads of people and equipment during the pour. When designing the formwork, think about: fixings… but they have from not using enough. Once things start to shift, you’ll never get it back. ❚❚ access for placing the concrete, vibrating it When the formwork is completed, subject it and stripping the forms to a strenuous test of pushing and pulling in all Planning is vital ❚❚ how to clean out before the concrete pour directions. It has to be extremely strong. The formwork may not have to look good (except – specific ports may be needed in high wall for the face that is imparting the surface finish) forms And now the pour but it has to be strong and yet still able to be ❚❚ how you will screed the top of the concrete The most forces on wall forms are applied at the stripped. When deciding on what to use for accurately – the top of the forms or time of placing. There will be fewer forces if the formwork, consider that: sometimes a line of nails can be used pour is done in layers (approximately 1–1.5 m), ❚❚ proprietary systems will have all the ❚❚ how to tie walls together – the use of but it is important that there is no more than engineering calculations done and have all conduits and through-bolts may be 30 minutes gap between these otherwise a the necessary components for the job precluded, necessitating snap-ties or she- construction joint will form. ❚❚ using higher grade timbers is often quicker bolts, which remain in the concrete. Keeping formwork in place aids the curing than using ‘boxing’ grade, due to its When building the formwork, remember: process, so don’t rush to strip it just because likelihood of being truer ❚❚ any deflection of formwork should be no less the concrete is hard. ❚❚ absorbent form surfaces can draw moisture than 3 mm or 1⁄₂₇₀ of the span Finally, remember the easiest time to clean from the concrete ❚❚ don’t cut timber to length unless necessary the forms is straight after stripping. ❚❚ plywood sheathing of forms should have the ❚❚ make sure all formwork running joints are CCANZ have some useful information outer ply face grain running parallel with the tightly butted together to minimise grinding, booklets – IB 29 Formwork for concrete, IB span of the sheet – it is stronger this way stoning or ‘bagging’ work after stripping 41 Formwork detailing and IB 64 Surface ❚❚ a 90 mm nail will take a lateral load of ❚❚ sealing cut edges will stop the drawing out of retarders. See www.ccanz.org.nz, then 360 N in radiata pine. water from the concrete mix Publications. ❚❚ never hammer nails fully home on formwork. Designing and building Leaving the heads proud aids the taking Wet concrete doesn’t quite act as a fluid, but it apart of the forms. Consider using hex-head does have many of the same attributes, such as drive screws.
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