The first thing we really should understand about your concrete mix is that it is a product that shows no mercy, it is starting to set the moment water is added to the mix in the truck. It is a chemical reaction and there is nothing you can do to stop it.
So now I’ve scared you into never wanting to try it, well not really I hope, I just want you to know as I always advise people when they want to learn how to concrete, that it is best to start off small than to take on too much and find yourself in trouble with your concrete mix.
Laying concrete is, as with most things, successful when you have the right conditions, but if you understand its characteristics then you can adjust your procedure to cope with what the weather is doing, as well as all the other elements that what seems to be, working against you.
So we will go through the character of concrete, as well as the terms that are used in the trade. You will need to familiarise yourself with these for when you are purchasing your concrete mix.
Concrete is the mix made up from a ratio of Sand, gravel and cement. Although when you purchase pre-mixed concrete there are basically two types of sand and two sizes of gravel in the mix, as well as a few other chemicals, which we don’t really need to go into.
The basic ingredients to make up 1.0m3 of concrete mix is as follows:
- River Sand (course) 450 kg
- Concrete sand (fine) 250 kg
- 20mm Gravel 750 kg
- 10mm Gravel 300 kg
- Cement Dust 250 kg
100lts of water is added to make the mix 80 Slump, only when the above materials are what we call saturated surface dry, which means they have a moisture content of no more than 8%. If it has been raining and the materials are wet then the batcher at the concrete plant has to make allowances for this during the pre-mixing.
Keep in mind you don’t really want too much water in the concrete mix. This will bring the cement dust to the surface and can weaken the concrete slab. 80 to 100 slump is the range but the less the better. We have often poured in winter especially, concrete at 65 slump.
This will make up 1.0m3 of concrete at 20MPa and so to increase the strength to 30Mpa we use 300kg of cement dust. Technically it takes 28 days for concrete to reach its maximum strength and hardness, conditionally after pouring concrete it should be kept moist for that time and have no load or stress for it to set properly.
Allowing conditions to remain constantly moist during this time is the best thing for your concrete mix as it slows down the setting process.
HERE ARE SOME CONCRETING TERMS.
1.0m3 :- Is one cubic meter on concrete, when you purchase pre-mixed concrete it’s sold in multiples of 0.2 m3 lots.
Slump :- Is the measurement given to the amount of water in the mix. It is measured by concrete being placed inside a cone and the cone then placed on the ground with the point up, the cone is then removed and the amount that the concrete sinks is the slump. In this case it sinks 80 mm. making 80 slump. This is the most common pre-mix made, if you were not to specify a particular slump then it is made at 80.
Mpa. :- is the measurement of strength or hardness of the concrete in Mega Pascals. This measurement will vary according to the use of the concrete you are laying, for example if the concrete has to be very hard to endure trucks turning on the surface for example, then the Mpa may be increased to 30. 32Mpa at 150mm (6”) thick to give you an idea is actually water proof.
Pre-mixed Concrete is usually sold in lots of 0.2m3 so when ordering you will have to round up your measurement to the nearest in 0.2 m3 lots.
Going off :- Is a term we use that just simply means the concrete is setting, going hard etc.
Concrete is green :- The concrete has not fully set yet,
Slurry :- Is the part of the mix that has no stones, generally you work the slurry up on the surface to get a nice finish.
Formwork :- Is the material usually timber used to contain the concrete until it is set enough to remove.
MIXING YOUR OWN CONCRETE.
If you are mixing the concrete yourself then the ratios that you will be working with to make a 20Mpa mix would be approx.
4 gravel – 3 sand – 1 cement.
Whether it be shovels, a bucket or whatever, if you stick to this ratio you won’t go far wrong.
The reinforcing for your concrete. (often referred to as mesh)
F52 :- Bar is 5mm diameter and should be used for Pedestrian use ONLY
F62 :- Bar is 6.3mm diameter and used for pedestrian and vehicles I recommend up to 6 ton.
F72 :- Bar is 7.1mm diameter and can be used for vehicles weighing from 6 to 12 ton.
Generally the sheets of steel are 6.0m x 2.4m and the spacing for these bars is 200mm x 200mm. that is what the 2 stands for in F72, you can get reinforcing mesh with 100mm (4”) spacing but for around the home these are the most common sizes.
POURING CONCRETE AROUND THE HOUSE.
Paths, Courtyards and general Walkways.
These have to be a minimum of 75mm (3”) thick, a minimum of 20Mpa and 80 slump with a minimum of F52 mesh.
Driveways and any surface that will have vehicle traffic load.
These concrete slabs have to be a minimum of 100mm (4”) thick, a minimum of 20Mpa, 80 Slump with a minimum of F62 mesh.