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
T
he construction of formwork is taken
for granted by all except those doing it.
It is difficult to find information about
it – there is no mention in the Building
Code, no New Zealand standard to follow
(Australia has AS 3610), no ‘how to’ pamphlets
in the local merchants’ shelves and of course
there is usually no lasting monument to well
constructed examples.
Finishing carpenters often take the glory
when a job is being reviewed, yet the work of
the person doing the formwork is equally vital in
producing a quality result.
Need to be able to build and unbuild
The design and build of formwork is usually
done by site staff. Unless a proprietary system
is being used, it is up to the carpenters on site
to plan how to build and support the formwork,
and also how to unbuild it. Being able to strip
the formwork effectively is an important part of
the task.
Building formwork is something that is best
learned by listening and working alongside more
experienced tradespeople. Learning only by
experience can result in failures along the way.
Huge forces to consider
Wet concrete exerts huge forces against
the forms. The concrete and reinforcing together
weigh approximately 2.4 tonnes/m3. Lateral
forces are generated in walls creating horizontal
bursting pressures, which are increased by
Note: Double waling and she-bolts may be used to tie the top of the wall together.
cleat packed off form to keep
formwork apart, support
reinforcing and holding down
bolts and allow trowelling
packing (dotted) to form
rebate for concrete slab floors
double brace – base of
boxing is too close to trench
edge to peg
pre-made plywood
shutters fixed securely
together
brace at 1200 mm
centres
peg
150 x 50 mm
plate
boxing constructed of
150 x 25 mm boards
closely butted
trench outline
spacer used to keep
forms apart until
concrete placed and to
tie forms together
place outside formwork; excavate
trench; place steel – erect inside
formwork
200 x 50 mm plate,
set to line before
excavation
pegs to hold base
of shutter in place
form ventilation
openings in walls that
have a suspended
floor over
verticals at 600 mm
max. centres for 25 mm
thick boxing
pre-built shutter
support cleat to top
waling
100 x 50 mm braces at
1200 mm centres
loose spreader
– remove as
concreting
proceeds
pull nail to remove
spreader
formwork ties
fitted through
conduits
double waling
required when
using she-bolts
150 x 50 mm
bottom plate
100 x 50 mm double
waling/strong backs at
700 centres
brace to foot of wall
bottom she-bolts at
900 mm centres
starter bars from footing
pour footing with nib
to locate boxing
Note: 1.Formwork may be either plywood or timber. Plywood is more suitable where a
fair-face finish is required.
Note: 1.Formwork may be either plywood or timber. Plywood is more suitable where a
fair-face finish is required.
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.)
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.)
Figure 1: Foundation wall formwork –­low walls.
22 BUILD 120 October/November 2010
Figure 2: Foundation wall formwork –­walls up to 2 m.
Note: Place sole plate before excavating trench.
25 mm thick timber rough-sawn
boards closely butted
18 mm plywood
top plate
150 x 50 mm joists
distance piece removed
when beam ¾ full
raking cut to
ease striking
peg
ledger
100 x 50 mm
support
18 mm plywood
head tree 100 mm
brace (typically 25 mm
thick at 600 mm centres)
150 x 75 mm
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.
internal vibrators during compacting. (This exerts
similar pressure to a hydraulic jack.)
Formwork needs to resist lifting and bending in
every direction and be able to also support the live
loads of people and equipment during the pour.
Once things start to shift, you’ll never get it back.
Planning is vital
The formwork may not have to look good (except
for the face that is imparting the surface finish)
but it has to be strong and yet still able to be
stripped. When deciding on what to use for
formwork, consider that:
❚❚ proprietary systems will have all the
engineering calculations done and have all
the necessary components for the job
❚❚ using higher grade timbers is often quicker
than using ‘boxing’ grade, due to its
likelihood of being truer
❚❚ absorbent form surfaces can draw moisture
from the concrete
❚❚ plywood sheathing of forms should have the
outer ply face grain running parallel with the
span of the sheet – it is stronger this way
❚❚ a 90 mm nail will take a lateral load of
360 N in radiata pine.
Designing and building
Wet concrete doesn’t quite act as a fluid, but it
does have many of the same attributes, such as
Figure 4: Beam box less than 600 mm.
being forced out of gaps and open footings when
there is a head of pressure above. Care has to be
taken in making sure forms will keep the concrete
where it is meant to be until it hardens.
When designing the formwork, think about:
❚❚ access for placing the concrete, vibrating it
and stripping the forms
❚❚ how to clean out before the concrete pour
– specific ports may be needed in high wall
forms
❚❚ how you will screed the top of the concrete
accurately – the top of the forms or
sometimes a line of nails can be used
❚❚ how to tie walls together – the use of
conduits and through-bolts may be
precluded, necessitating snap-ties or shebolts, which remain in the concrete.
When building the formwork, remember:
❚❚ any deflection of formwork should be no less
than 3 mm or 1⁄₂₇₀ of the span
❚❚ don’t cut timber to length unless necessary
❚❚ make sure all formwork running joints are
tightly butted together to minimise grinding,
stoning or ‘bagging’ work after stripping
❚❚ sealing cut edges will stop the drawing out of
water from the concrete mix
❚❚ never hammer nails fully home on formwork.
Leaving the heads proud aids the taking
apart of the forms. Consider using hex-head
drive screws.
Make sure it’s very strong
It’s unlikely that anyone has been sacked,
injured or killed by making formwork using
too many props, strong-backs or holding down
fixings… but they have from not using enough.
When the formwork is completed, subject it
to a strenuous test of pushing and pulling in all
directions. It has to be extremely strong.
And now the pour
The most forces on wall forms are applied at the
time of placing. There will be fewer forces if the
pour is done in layers (approximately 1–1.5 m),
but it is important that there is no more than
30 minutes gap between these otherwise a
construction joint will form.
Keeping formwork in place aids the curing
process, so don’t rush to strip it just because
the concrete is hard.
Finally, remember the easiest time to clean
the forms is straight after stripping.
CCANZ have some useful information
booklets – IB 29 Formwork for concrete, IB
41 Formwork detailing and IB 64 Surface
retarders. See www.ccanz.org.nz, then
Publications.
BUILD 120 October/November 2010 23