Let’s face it – a lot of people use the words “concrete” and “cement” interchangeably mistakenly thinking these refer to the same thing.
This couldn’t be more incorrect. Cement refers to a broad variety of fine-ground powders that harden when mixed with water.
On the other hand, concrete is a hardened construction material made by combining a chemically inert mineral aggregate – usually sand, crushed stone or gravel, or a mixture of two or all three, cement as the binder, chemical additives (optional) and water.
Cement is therefore is only one of the many components of modern concrete.
The cement and water that make up the paste coats the surface of the aggregate, both the fine and the coarse ones. Through hydration, a chemical reaction, the paste hardens as it dries to form the rock-like mass which is concrete.
When concrete is dry, it naturally acquires a hardness and consistency that can be likened to stone. This makes it deal for road construction, building of bridges, factories, railroads, sewerage and water supply systems, airports, mass transportation systems, waterways and other durable structures.
Concrete may differ in strength and density, depending on the proportions of its three key ingredients: cement, water and aggregate.
Quality Depends on Proportion
Depending on the kind of structure that would be made with the concrete, you will prepare proportions of water, cement and aggregate. These proportions and how you mix these elements will determine the quality of the concrete and the final structure.
Thus, if the mixture does not have enough paste to fill the gaps between the aggregates properly, it becomes difficult to manipulate the concrete and your mixture may become porous and rough on the surface.
On the other hand, if the mixture contains excess cement paste, it will be easy to apply, and the final concrete will be smooth on the surface but can easily crack.
Given this, we see that the strength of the concrete depends on the ratio of water to cement.
This ratio refers to the weight of the water used to mix the concrete divided by the weight of the cement used.
To produce high-quality concrete, lower the ratio of water to cement as much as possible. Ensure that the fresh concrete can be properly placed and consolidated.
Setting the proportion of concrete can be done in two ways:
By Volume
This is usually used for small jobs. In doing this, you use gauge boxes to measure the fine and coarse aggregate.
Each gauge box has a volume equal to one bag of cement. This method is less accurate but takes less time.
By Weight
This method is way more accurate and makes for a more uniform process. It doesn’t have the uncertainties of bulking, hence is generally preferred over volume batching.
In either case, the proportions of cement, water, and aggregate should produce concrete that has the following properties:
- Maximum density – the concrete should be as strongest and water-tight as it can be.
- Can be easily manipulated while it’s fresh. it should be easy to economically place in the formwork.
With these, you are ready to make your first concrete. But, how do you go about it?
Step 1: Gather or Buy the Raw Materials
You may have to buy cement, but can gather the two other raw materials: water and builder’s sand/gravel or crushed limestone.
Determine the volume of concrete that you’ll need. You can do this by multiplying the thickness of the target area to its square footage.
Step 2: Find a Suitable Container for Mixing
You may not always have a mixer unless you are in the construction business. But you can always borrow or rent a mechanical mixer. Don’t worry too much on the cost as this will considerably reduce the labour required.
For a small project, improvise a manual mixing box by building one from wood planks. You can also use a strong wheelbarrow to mix the materials.
Step 3: Mix The Cement and Sand
Start off by preparing your dry mix. You do this by mixing the cement and sand. You can choose from these two concrete preparation methods:
- Method 1 – make a basic mixture for mortar using the ratio of 1 to 2 to 3 for water to cement to sand by volume.
- Method 2 – create a general purpose concrete using the ratio of 1 to 2 to 3 for cement to sand to gravel by volume.
Step 4: Add the Crushed Stone or Gravel
Add the gravel or crushed stone at a ratio of up to 5 portions of gravel for every 1 portion of cement and sand mixture.
Don’t worry. The high ratio of gravel has no adverse effect on the tensile strength of the concrete unless it’s too much. Just be sure to leave enough cement paste to fill the spaces between the gravel. If this doesn’t happen then you know you’ve added too much of the gravel.
Note that concrete with too much gravel will be difficult to level and it will be hard to achieve a smooth finished surface where it’s curved.
Step 5: Add Water
Water determines the quality of your concrete so it’s best to add water to the mixture slowly. Mix out the concrete continuously every time you add water until the concrete becomes plastic enough to place in your form.
Step 6: Mix the concrete thoroughly
Once the desired volume of water has been added, continue mixing until the concrete achieves a uniform consistency.
The final wet concrete should be even all over, and have no packets of dry material. About two or three extra minutes of mixing after the desired consistency has been achieved will give you the perfect concrete.
With your concrete ready, you can then pour it and float it to create the structure you intended.