Here it is.
Casting paving slabs
Materials
Release agent/oil
10mil Gravel
River sand/sharp sand
Cement
Pigment/dye (optional)
Most of these can be found at places like B&Q or you may find them cheaper at your local builders merchants. I have release agent and wide range of moulds and pigment dye for sale on Ebay.
Preparation
First it’s important to make sure your moulds are clean, as any dust, debris or dye left from previous moulds will show on your next cast. The best practice is to clean your moulds after use as this will lengthen the life of your moulds.
Greasing
The next step is to grease your moulds with a release agent. An easy way to do this is put the release oil into a spray bottle and spray the inside of the mould, then spread the oil around with an old cloth paying particular attention to the sides and corners. Just a thin layer of oil will do the job.
Mixing
Now you need to mix your concrete. The best way to do this is use a container/bucket to measure your materials this is important especially when adding dye, as just measuring by shovel load will be hard to get the same colour time and time again.
A good ratio for a strong mix is 3 parts gravel 1½ parts river sand and 1 part cement, using a standard builders bucket this should be enough concrete to make just short of 2 square meters.
You want your mix to be not too stiff and also not too runny. This is hard to explain on paper but if you try to pour the mix out of a bucket and it doesn’t pour out it’s too stiff and if you pour it and it runs out fast it’s too runny somewhere in between is just right. This is all trial and error and if you don’t get it right the first time you’ll know to either add more or less water the next time.
To change the colour of the mix you need to add pigment. You need to add about 5% to whatever cement you are adding to your mix. Once again it’s important to use the same measure of pigment to keep the colour consistent when mixing multiple mixes.
Pouring
Now it’s time to pour your mix into your moulds.A vibrating table is best for this but are expencive. It’s best to have your moulds sat on a level surface as the mix is going to set level, it doesn’t have to be perfectly level but something like.
Pour your mix into the centre of the mould. Gently tap the sides and shake the mould until the concrete has spread and filled the mould. Try not to over fill the mould as the job will get messy.
Once the mould is filled you need to get the trapped air out. The best way to do this is with a vibrating table but these can be expensive. A simple way to do this is tap the sides of the mould gently with something like a hammer or a piece of wood, also pick the mould up and gently bang it on the surface it’s sat on. You should start to see bubbles surfacing at the top of your mix keep doing this process until the bubbles stop surfacing. The longer you do this the better the result.
You then need to let the concrete set in the mould, in good temperatures concrete will set within 24 to 36 hours.
Demoulding & Curing
Once your concrete is set it’s time to demould a good idea is to get two pieces of wood a bit longer than your mould to demould onto thus enabling you to get your hands under the paving slab to move it.
Holding one hand on the top of the slab and the other on the bottom of the mould carefully turn the mould over and lay on the pieces of wood. Gently lift the mould and usually the slab will drop out, if not a few gentle persuasive taps should release it.
You have to be very careful from this point onwards as the slab is not cured fully, slight little knock can break corners off. Curing normally takes up to Three weeks and it’s best to have them stored leant up against each other with spacers it between them as this will help with the curing process.
