Formwork, an introduction and some basic points.

What I am going to be talking about in this section is concrete formwork or shuttering. It is sometimes incorrectly called falsework. It is the temporary, usually timber and plywood mould, that holds the wet concrete in the correct position until the concrete has set and cured.

formwork in Thailand
Concrete shuttering for a new river boat jetty Bangkok 2005. I don't want to comment on the quality control of these guys, except to say that when I saw the completed building about a year later there was a dirty bog crack through the polished terrazzo floor. I am using it as an example of how simple formwork can be and still do the job.

Don't think I have put this shot of a concrete pour in Bangkok here as a bit of a joke. I get visitors from all over the world, and I hope to provide something for everybody. I will be showing modern Australian methods and materials, but I will also try to show people how to do it with only the basics of timber, ply and tie wire or the odd bolt or two.

I have worked on very similar jobs myself. We used what we called "bush toms" . They were sapling trees cut down for props, 100mm to say 150mm dia. Shuttering can be done with the simplest of materials. Bear this in mind if you only have a small one off job to do. You don't need to buy expensive formply that is designed for strength and many repetitive uses. Search around for something cheaper. I have used downgraded or cover sheets of MDF for a few odd forming jobs.

formwork made up of sawn boards
Underneath of a suspended slab. WWII era. In case you are wondering what the bits sticking out of the top of the columns are, they are galvanised ant caps to stop termites getting into the building. The ant caps would have been cut over the projecting column rebar and soldered before the slab was formed and poured.

Here's a shot of the underneath of of a building built in 1941 using mainly boards of about 150 wide to make up the shutters or forms. They would have used a lot of what we would call makeshift materials to do this, but they achieved an excellent job. The fact that the floor is still in fairly good condition is a testament to the quality of the mixed on site concrete.

A lot of people tend to think that formwork is a semi skilled occupation. To be fair there are a lot of guys that started off as labourers and finish up as formwork carpenters without any formal training. Also, to be sure there is a fair bit of hard manual labour involved, but it is a very tricky job and it takes just as much know how to do it properly as any other jobs in the building trade.

Formwork on suspended slab
A suspended slab on a multi storey project.
More on suspended slabs

The guys on this job have to get it right. They started off with a drawing, a couple of walls and a few columns to work off. There is going to be 32 tonne of reo steel up here shortly, and maybe 300 to 400 tonnes of wet concrete after that. There has already been a significant amount of money spent to get to this stage, imagine finding out that they had something wrong just before the pour

So you trim carpenters who think you are above the humble form workers, think again!

Here is a quote from a Thomas S... a reader who lives in Butuan in the Philippines.

"I REALLY loved the remark about the high noses of trim carpenters and how useless they can be building formwork. it should be coupled with a remark about the best form carpenters are ones who plan to strip before they plan to build."

I'll repeat Tom's last point, it can't be overstated.

the best form carpenters are ones who plan to strip before they plan to build.

Thanks Tommy, never a truer word said.

a few general points applicable to all formwork jobs

  • Once again I will mention that there are many different standards, or codes of practice governing the construction of concrete formwork.
    • Check out your local conditions, at the least think about the following points:-
    • The need for edge protection.
    • Access, and platforms for workers.
    • Protection from falling objects.
    • Wind bracing for walls. The final bracing of walls, due to be poured, is (or should be) well inside the requirements for wind loadings, but think about keeping temporary braces on anything high enough to cause hurt to a worker if it falls down due to a gust of wind.

In my sketches and photographs on this page and others, I am only giving pointers on how to go about doing any particular job.

Just because I show formwork using 100 x 75 and 150 x 100 softwood in the photos, that just means that the guys on that job had that size of timber available.

You may well be using salvaged timber of lighter sizes. No problem, just close up your spacings.

I have had RHS steel on a job that was later used in the roof, but as strong straight support for formwork it was excellent. I just made sure it was cleaned as soon as possible.

  • Don't cut timber to exact size unless it is necessary.
  • Formwork by it's nature is temporary. The finished job is the concrete. That has to look straight and true.
  • The look of the formwork is not critical. If you can let timbers lap, or fly over at the ends and not cut them to length, do it.
  • I repeat, don't cut timber unless it is necessary. You might need that extra length next time.
formwork props
Formwork intro - A propping system for a suspended slab.

"Better ten props too many than one too short"
The old guy who said that to me when I was doing my first formwork job meant of course, not only props, but any temporary bracing, struts, tie bolts etc. etc.

To the best of my knowledge nobody has ever been:-
  • Killed
  • Injured
  • Sacked
  • Created a "right pigs ear of a job".
as a direct result of putting in too many props etc.

But each of the above has happened many times by people cutting corners and not using enough props.

Hydrostatic pressure

column formwork
Formwork intro - Column formwork closer spacing of the column clamps at the bottom.
More on column formwork

Wet concrete delivers hydrostatic pressure. Simply put, when concrete is wet, or as the engineer's say in a fluid state, it acts like any other liquid in spite of all the extra stuff in it. So that the deeper the concrete pour, the greater the pressure at the bottom of the forms.

  • Forms that are holding the edge of a pavement slab 100mm thick have hardly any sideways push against them.
  • If that same slab was 1M thick it would be a different story.
  • So pay particular attention to holding the bottom of the formwork. As a general rule, if the bottom holds, so will the rest. Blowouts invariably happen at the bottom of a deep pour.
  • The photo on the right shows a column form that will be poured about 3m deep.
  • Notice that the column clamps are close spaced at the bottom and wider at the top.
  • Also because the column is 700 wide or so, they have taken the precaution of putting in through bolts to stop the bottom column clamps bending.

Forming up concrete above the ground, is not a trivial matter.
A concrete beam over a 4m wide wall opening is a potentially dangerous situation. Do it well and carefully and make sure of your supports.
A collapse could bring down a few tonnes of material with disastrous results.

  • Nail everything. Don't just wedge it in tight and say it's OK.
  • When the concrete is being poured there is a heck of a lot of vibration going on and things that are not fixed in position can and do shake loose.
  • Say you get a large load of concrete dropped on a suspended slab. The formwork in the immediate area bows downwards and the bit in the next section can lift up a touch. If the props are not fixed at the top or otherwise braced, they can fall out and leave a section of the slab unsupported.

This actually happened to a contractor I used to know, many tears ago. Twenty or thirty tonnes of concrete, steel, men and machinery fell in a tangled mess.

Incredibly nobody was seriously hurt. So, take heed. Fix your props firm.

For this reason I like to see a guy watching the formwork during the pour. Checking props and braces etc. Just to catch any possible movement before it gets too bad.
I have done it enough times myself, and If nothing else, it is good to have someone not physically involved in the rest of the work, just watching out for the odd little things that happen.

  • It is always good practice to have a removable panel in the lowest part of the formwork somewhere.
  • This is to facilitate the removal of rubbish. The panel gets lifted out just before the pour and the formwork can be hosed or blown out, shifting all the bits of sawdust, nails and tie wire without any trouble.
  • Don't forget to replace the panel before pouring:-)
  • Keep the formwork neat and with tight joints. Gaps as small as 3mm will let out the fines and cement juice, leaving ugly and weak patches in the concrete finish.
  • When formwork under the pressure of wet concrete moves, it is most times impossible to push it back.
  • An extra small timber brace may have stopped the form from moving in the first place, but once the form has pushed out of line, no amount of pushing with steel props will get it back, short of emptying the form of concrete. Which is usually not possible because of the reo steel.
  • So,make sure it doesn't move in the first place. Make it stronger than you need.
  • As soon as the formwork is stripped, clean It.
  • It seems to get tens times harder the longer you leave it.
  • Clean it and oil the ply with proper recommended form oil. I have not heard of any home brewed oil mixtures that aid in the stripping process, and yet don't stain the finished concrete. Get the proper stuff.

Shutters, or forms are the terms used for made up sections that actually touch the concrete. They are often made up of ply nailed to timber. Also they can be steel, or a combination of both.

The surface finish of the form is reflected on the surface of the finished concrete. So if the form is rusty, you might get rust on the surface of your concrete. If you have holes in your ply, plug them up, or else accept ugly lumps and loss of fines on your finished concrete surface.

Apart from the nails fixing ply to make up forms, the rest of the nails on a formwork job are never driven home fully. They are left, or bent over so that they can be pulled out easily with a claw hammer or pinch bar when it comes time to strip the forms.

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I did the foundations for a large electrical substation a few years ago and next to our job there was a control building being built by a very large construction company.
They had a large (say 200 cu.M)suspended slab to pour, using Bondek, (permanent steel slab formwork that gets left keyed into the slab).
The young structural engineer that was supervisor for the job came to me one day and asked me what I thought about the formwork next door.

I don't like to rubbish somebody else's job, that's their problem, but the young guy was obviously concerned and I had my doubts myself, safety issues involved as well.
I told him that if it was my job I'd double up on the number of props supporting the slab and tie them all together with scaffold tubes.
The following day they did just that, and had a successful pour the day after. The original layout probably was OK, but for a very little extra cost, why take the chance?



Please Note! The information on this site is offered as a guide only!  When we are talking about areas where building regulations or safety regulations could exist,the information here could be wrong for your area.  It could be out of date!  Regulations breed faster than rabbits!
You must check your own local conditions.
Copyright © Bill Bradley 2007-2012. All rights reserved.
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