Concrete Mixing for the Handyperson
How do I make concrete?
Firstly, remember that concrete is a mixture of cement, fine aggregate (sand), coarse aggregate (stone or metal usually 20 mm or 14 mm in size) and water.
The proportions or parts of each material in the mixture affects the properties of the final hardened concrete.
To measure a part, a suitable container such as a small bucket or ice-cream container must be used. Do not rely on measuring by the shovelful.
How do I make Grade 20 (20-MPa) concrete; and, how do I estimate materials quantities to purchase?
Table 2 indicates the quantity of each material required to make up to one cubic metre of Grade 20 or Grade 25 concrete and Table 3 the typical
applications of each grade for most household projects.
Cement should be Type GP (General Purpose Portland Cement) or Type GB (General Purpose Blended Cement).
Sand should be concreting sand - sometimes referred to as sharp sand NOT brickies' sand or plasterers' sand.
Coarse aggregate is stone or metal usually 20 mm or 14 mm maximum size. All-In materials means sand and coarse aggregate already mixed together.
How do I mix concrete?
Concrete must be mixed so that the cement, sand, coarse aggregate and water blend into an even, consistent mix.
If too much water is used, the strength and durability of the hardened concrete is decreased.
Too much sand gives a sticky mix, too much coarse aggregate gives a harsh bony mix, both are difficult to work and finish.
To measure a part, use a suitable container such as a small bucket or an ice-cream container - not a shovel.
The following advice is for a standard 1:2:4 mix - see Mix A in Table 2.
For convenience and practicality, concrete would normally be mixed in a mechanical mixer. Turn the mixer on. Begin by loading a measured amount (4 parts) of coarse aggregate. Next add the sand (2 parts) by measured amount. Then add the cement (1 part) by measured amount. Blend materials together until there is no visible sand in the mix.
Now begin adding water. Use only enough water to get an even, workable mix. The amount of water used might typically be approximately 75% (or 3/4) of the volume of cement.
INSERT Table 2 Concrete mixing, Guide for Quantities of Materials
INSERT Table 3 applications of Concrete Grades
This is only a guide. Less water may be required due to inherent dampness of materials - particularly sand. Care in adding water is the key to a successful mix.
Never add just water to the concrete to make it more workable, always add cement also.
When should I use reinforcement?
Concrete is strong in compression and weak in tension. The introduction of reinforcement in the top half of a slab improves the strength of concrete in tension, helping to reduce cracking and to increase the distance between control joints in the concrete.
Generally, it is best to avoid the use of reinforcement in domestic paths and driveways by ensuring a firm sub-base and using frequent full-depth control joints. (Slabs should be preferably divided into squares by joints.) If soil conditions are known to be poor, eg expansive clay soils, consult a professional engineer.
If reinforcement is used, it should be placed at 1/3 of the slab depth from the top surface and supported on bar chairs. F42 or F52 reinforcing fabric is usually recommended for paths, or F62 for driveways. However, the first two are so light that it is difficult to ensure that they remain in the correct position in the slab.
Reinforcement should always have sufficient cover of concrete to prevent water penetration causing rusting. Generally, a minimum cover of 25 mm is recommended, but 40 mm should be used if the slab is near the sea.
Garden paths are generally 75 mm thick and, if reinforced, a layer of F42 mesh is placed 25 mm below the surface. Driveways, suitable for cars but not trucks, are generally 100 mm thick and a layer of F62 mesh would be placed 30 mm below the surface.
Bar chairs supporting the reinforcement
should be spaced at approximately 600-mm
centres. They either have a special flat base
or are stood on tin lids. Reinforcement
must not be lifted up to correct its
position during concrete placement.
Reinforcement is generally
discontinued at joints to permit crack control.
The Building Department of the local council will provide advice on local soil types, special ordinances, etc regarding driveways and paths.
Ensure adequate concrete cover to mesh both at the surface and at the edges of the concrete (minimum 50 mm).
Concrete footings to support a structure -
You must seek advice from a professional engineer to ensure that the concrete footing is adequately designed to support the structure. Reinforcement (usually trench mesh) must be placed in the footings at the locations shown on the plan.