Now that you’ve learnt how to mix concrete and work out the different mixes to use for different jobs, you’ll need to know how to keep the runny stuff where you want it, right?
Simple plank shuttering
If you’re doing a simple path or hardstanding, perhaps a slab for a shed or summerhouse, then it’s pretty simple and obvious, just border the area with planks on edge, using pegs or stakes to keep them upright. It’s not quite that simple, you could do it that way but the results wouldn’t be that great. The general idea is right, it just needs a bit more care for a good looking job.
Firstly the planks should be trimmed so that they are level with the top surface of the slab. This will allow you to smooth the concrete up to and just over the top of the planks when finishing off, leaving a tidy finish that it’s less likely to crack and split with cold and frosty weather.
If the shuttering is too low the concrete will spill over. Too high and you won’t be able to flatten the surface off, it will rise up at the edges and look stupid when you take the planks away. Not to mention breaking off when water gets in and freezes; chunks will fall off and make it look scrappy and generally rubbish.
More rigid shuttering for a better finish
It’s also worth considering new, clean, planed all-round (PAR) wooden planks for the job. Although it seems like a waste of good wood this will prevent the cement mix leaching into the grain of the wood, making it harder to take it off and more likely to damage the face and corners of the concrete in the process. Alternatively cut sheets of plywood (try to get weather and boil-proof, or WBP) to tack onto your planks. Exterior-grade MDF or hardboard will do just as well.
The supporting pegs should be driven a good six inches into the ground and the planks nailed to the pegs to prevent them falling over while the slab cures. Cut the tops of the pegs flush with the top edger of the planks, again to make it easier to screed (the technical term for the smoothing over of the top layer of the slab).
Once the slab is well on the way to being properly cured you can take the shuttering away. With a slab that’s not too thick you can probably get away with knocking the planks away with a rubber mallet then lifting the planks and pegs out of the ground.
There is a risk that some concrete will come away and weaken the edge of the slab, or at least make it look bad. Whether this is a problem or not depends on what you’re doing. If it’s a base for a shed it might not matter to you, if it’s a path or patio you probably want it to look better.
You might be better off coating the wood with a special release agent as this will ensue the shuttering or formwork will come away easily and without damage. This way you can also leave the shuttering in place until the concrete has cured completely, safe in the knowledge that it will come away easily.
For larger areas of concrete, particularly thicker slabs of concrete, you’ll need higher and sturdier shuttering which is usually referred to as formwork. But although you could get thicker and wider planks, they’ll be too heavy to manipulate easily. This is particularly the case if you are building something like steps in a garden, where you’ll have broader vertical faces to support while the concrete cures.
Although the terms aren’t hard and fast, ‘formwork’ is usually used to refer to shuttering that has been created by building a framework out of 2-by-2. Make top and bottom rails then join them with noggins (cross pieces between the rails) at both ends and at regular intervals along the length of the formwork.
Formwork will need better bracing. Use 2-by-2 to make horizontal braces at the back, at least as long as the formwork is high, then join the ends of those to just below the top of the top rails with diagonal braces. This creates strong triangular supports at the back of the formwork. Peg the horizontal braces at their rear ends, again at least six inches deep.
Face the inside (the side the concrete will butt up against) with plywood, hardwood or MDF as before. You should definitely use release agent (sometimes known as “mould oil”) with formwork of this kind as the area of concrete that will touch it will be much larger. Any portion that sticks and comes away with the formwork will ruin those faces.
Learn as you go
There’s a lot more to concrete than we’ve covered on this website but being able to use it gives you a lot of control over the DIY you are capable of doing, both inside and outside the home.
Take your time and be prepared to break up and discard work if you’re not happy with it. Learn from your mistakes, mix up another batch of concrete and try again, practice really does make perfect.