But how do I know what cement to use?
Let’s start at the beginning. A respectable concrete mixture comprises a combination of Portland cement, sand, stone, and water. The strength of a concrete mixture lies in the quantity of each ingredient. Concrete should be allowed to dry over an extended period – if it dries too quickly it won’t develop its full strength.
ALL cement that is sold in South Africa must meet the requirements of SANS 50197 (for common cement), as well as those of the National Regulator for Compulsory Standards (NRCS), as outlined in NRCS VC9085. Kwikbuild Cement is no exception.
The strength grade of the concrete should be indicated on every bag, as well as a note that specifies the details of the composition, and a Letter of Authority (LOA) which is issued for each type of cement from its source. It is important to remember that the use of masonry cement in concrete is prohibited, and LOA numbers can be verified by the NRCS directly, either online or by phone.
To determine how much concrete you need, Cemcon.sa provides the following formulas to calculate the volume of concrete required:
- For rectangular shapes: volume = length x width x thickness
- For circular shapes: diameter x diameter x 0,08
What does a good mixture look like?
First things first – a good quality sand is of utmost importance. In other words, the sand and stone should not contain any kind of debris, such as leaves, grass, compost, clay lumps, etc. Regarding water content, any water that is good enough to drink should be used in the mixture.
Now let’s tackle the mixture. The method that follows is to assess the water requirement and workability of your cement mixture. You will need to measure out the ingredients for your mixture as follows:
- 5kg cement
- 25kg dry sand
- 5 litres of water = (Good Quality Sand)
- Add 1 litre of water = (Average Quality Sand)
- Add 1,5 litres of water = (Poor Quality Sand)
- Note: these quantities can be scaled down accordingly, to suit your needs.
Please note that this mix above is a test to determine the sand quality on site, it is not a typical mixture used for building practice.
Please stick to 1:6 ratio (Wheelbarrows) for Mortar and Plaster
Begin by mixing the solid ingredients (the sand and cement) on a non-porous surface.First you’ll spread out the sand (about 100mm thick) and then you will spread out the cement on top of it and mix thoroughly. Then heap up the dry ingredients and make a hollow in the middle. (Just use a Wheelbarrow for this) Next you will add each amount of water consecutively, mixing until your mixture reaches a suitable consistency (the ideal consistency is similar to that of thin porridge). If your desired consistency is reached using only the five litres of water, then the quality of the sand is high(Good Quality). If both the five litre and one litre of water were required, then the quality of the sand is average. If all three quantities of water are required, then the quality of the sand is considered poor.(Stop, If sand is poor it should not be used) And if additional water is required, then the quality of the sand is considered very poor.
It follows then, that only good quality sand should be used in all mortar and plaster work, while average quality sand is preferred for mortar, for interior plaster and screed. Try to avoid using poor or very poor quality sand at all costs.
Next you will add the correct quantity of stone to the mixture, mixing thoroughly until each stone is covered in the mortar. Ig the mixture is too stiff, add more water. However, if you add too much water the mixture will have a slushy consistency, which means the concrete will be weak.
How do I use the concrete?
Place the concrete down as close as possible to its final position. If it is placed on the ground, then the soil should be damp (but not wet) when the concrete is placed down. The concrete should be well-compacted and worked along the edges and in the corners with a spade.
Concrete slabs can be compacted by using a wooden beam that is about as wide as the slab itself. This can be done first by making a chopping movement and then by making a sawing movement. Then, you will would-float the surface so that it will become even but still rough, as smooth surfaces outdoor are slippery, which is dangerous.
Finally, the concrete can be cured by keeping it damp.(7 Days at least) Once it stiffens, cover it with either a plastic sheeting held firmly along the edges, or with hessian or sacking that will be kept wet. The appropriate period for curing concrete is seven days. This will ensure the quality of the concrete and limit cracking once the concrete has dried.
