What are the correct concrete mixing ratios?
Concrete was probably the most used building material of the twentieth century. Walking around any city, you’ll see huge skyscrapers built from concrete. Such is concrete’s versatility; it’s also suitable for smaller projects. If you’re undertaking a small project at home, you may be tempted to mix your own concrete. If you are, you’ll need to know the correct ratio for mixing concrete. The key point here is that there is not one magical ratio for all types of concrete; different types of concrete require different ratios.
This guide is here to help you understand how and why you need to get the ratio right. It’ll explain what the ingredients are, what each ingredient’s role is, and how you can combine them in the correct way depending on what your project is. Whatever you’re building, you’ll find the right concrete ratio below.
WHAT ARE THE INGREDIENTS OF CONCRETE?
One of the most remarkable things about concrete is that it is a relatively simple concoction. You only actually need four ingredients to make concrete. Even at the most advanced level of engineering, with the latest technology and materials available, concrete is still made up of the same four ingredients. These are cement, aggregate, sand, and water. Each of these ingredients plays a crucial role in making up a strong and durable material.
Knowing what each ingredient does individually will help you when it comes to making up the ratio. Like making a cocktail, the more you emphasize one ingredient, the more that ingredient’s strengths will come to the fore. Below are outlines of each of the four ingredients of concrete:
CEMENT
Cement functions as an adhesive within concrete. Its key role is to hold the aggregate and sand together once the cement has been set. The primary substance within cement is calcium oxide, which is produced when limestone is superheated. Different types of cement will contain different secondary materials, but these usually include silicon, aluminum, and iron.
When you think of cement, you probably think of an extremely hard, dense material. In reality, however, cement can be the weak link within concrete. Cement is responsible for providing the tensile strength to concrete, a job it only does to a limited extent. Concrete that faces torque (i.e., twisting forces) usually requires rebar because cement does not possess a great deal of tensile strength. The more cement you add to your concrete, the better the sand and aggregate will stick together.
AGGREGATE
Aggregate is made up of small pieces of rock that have been washed and crushed. This is the material that does much of the work when it comes to the compressive strength of concrete. To put it another way, aggregate is the reason why you can use concrete as the foundation of your home.
SAND
Sand also provides a great deal of compressive strength (almost as much as aggregate). However, its primary role within concrete is to fill in the small gaps between the particles of aggregate. This eliminates air pockets and other spaces, which would undermine the compressive strength of the overall concrete.
WATER
Water’s role in the whole process is as a chemical agent. By adding water to the mix, it creates a chemical reaction to the quicklime within the cement. This means that it sticks to the aggregate and the sand, thereby sticking them to one another. In addition, adding the water to the mixture makes it pourable so that you can add it to a mold or use it to create a slab. The water then evaporates off over the course of days or weeks – a process known as curing.
The core ingredients are always present in cement. However, mixing them in slightly different ratios will give you a different set of results. As you add proportionally more of one ingredient, its strengths come to the fore, so depending on precisely what task you are undertaking, the ratio required will shift. The below section will explore this in more detail.
WHAT ARE YOU USING THE CONCRETE FOR?
One of the most important things to consider when determining what ratio concrete mix you need is exactly what your project is. There is no single correct ratio in the same way that there’s not a single correct type of concrete. The first thing you’ll need to consider is precisely what you’re using the concrete for. The below table will give you a means of converting the project you’re working on to a required strength in psi.
Concrete Use | Required Strength |
|---|---|
Small slabs, posts for fences | 2500 psi |
Slabs, flooring, walls, footings | 3000 psi |
Outdoor patios, walkways, slabs | 3500 psi |
Driveways and large exterior slabs | 4000 psi |
Commercial or industrial flooring, very large slabs | 4500 psi |
As you can see, the larger the weight-bearing requirements of the concrete, the higher the psi must be. This naturally has ramifications when it comes to the ratios involved. If you’re comparing the psi required with that of different grades of concrete, you will get an idea of what sort of pressure you’ll need. The table below gives an overview:
Concrete Grade | Strength |
|---|---|
M5 | 725 psi |
M10 | 1450 psi |
M20 | 2900 psi |
M30 | 4350 psi |
M40 | 5800 psi |
M50 | 7250 psi |
M60 | 8700 psi |
M70 | 10150 psi |
What the above table shows is that you’re unlikely to want to or need to mix anything above an M30 grade of concrete. Anything this level and above, you’re far better using professionally-mixed concrete (and usually installed by a professional). For everything below that level, you can usually mix it yourself. The next section will connect the project with the ratios.
WHAT ARE THE MIXING RATIOS?
If you’ve decided what your project is and got a rough sense of what strength or grade of concrete you’ll need, you can now work out which ratio is best for you. The below table will give you all the information you need, giving you the ratio of Cement to Sand to Aggregate.
Concrete Grade | Type of Project | Ratio (Cement : Sand : Aggregate) |
|---|---|---|
M5 | Supportive foundations or slab coverings in a single room used for domestic animals | 1 : 5 : 10 |
M7.5 | To create a level surface to place under a footing or to prevent a foundation from touching the floor | 1 : 4 : 8 |
M10 | Floors, garden walls, drainage works, foundations for steps, roads | 1 : 3 : 6 |
M15 | Patio slabs, paths, and non-structural or load-bearing work | 1 : 2 : 4 |
M20 | Foundations for sheds or homes, internal flooring, garages, walls, patio paving | 1 : 1.5 : 3 |
M25 | Footings, foundations, landfill, groundwork, flooring, kerbing | 1 : 1 : 2 |
Effectively, the two rules of thumb to follow – you should always use twice as much aggregate as sand, and the stronger you want the overall concrete to be, the less cement you add to the mixture.
HOW MUCH WATER SHOULD YOU USE?
You’ll notice that water is not included in the ratios above, and this is because the addition of water is one of the trickiest aspects of mixing concrete. There are a number of different variables involved in how much water to add. These include:
- The air temperature
- The humidity levels
- The amount of sunlight the concrete will be curing in
As a general rule, you’ll want to add the least amount of water possible while still allowing the concrete to be workable and adhesive. The best way to achieve this is to add the water in small amounts to the completed mix and stir thoroughly. As you do this, you’ll notice the changes in the makeup of the concrete. Once it’s at the right level, don’t add any more.
HOW DO YOU CURE THE CONCRETE?
The final step in the process is to cure the concrete – this means allowing the water to evaporate off so that it is fully set. This can take as long as 28 days, depending on the factors listed above. The best thing about it is that curing is a passive process, meaning you don’t have to do anything but wait. You should take care not to place heavy weights on the concrete before it is fully cured. Even if it feels dry to the touch, there may still be water inside.
As mentioned throughout this guide, concrete does not need to be a complicated or daunting material to work with. In fact, it’s one of the simplest materials you can work with. Simply use a concrete mixer or a wheelbarrow to mix the concrete up (using the ratios above), and then pour it into the required container. That’s all you have to do.
Where there is some complexity is in getting the ratios right. You may need to do some math, particularly because you’re working with some solids (aggregate and sand), some liquids (water), and some materials that seem to fall between the two (cement).
However, you’re not the first person to need to do this math, and if you make a mistake, you can always add a little more of the other ingredients, so the ratios work out okay. Pick your project, pick your ratio, and get mixing – it’s well worth it.