Concrete is arguably the most important construction material in use today, with an estimated 7.5 billion cubic meters of it being produced every year – translating into one cubic meter for every man, woman, and child alive. Concrete is used for a variety of projects, from massive skyscrapers to pavement to little DIY projects in your backyard.
Concrete is not only widely available in most countries, but it’s also extremely affordable, due to the natural abundance of its constituent materials. You can either order it in bulk from your local concrete plant or you can make it yourself with store-bought materials – if you only need a small amount. For many DIY projects, making the concrete yourself is a very viable option.
However, you should know that although sand and aggregate lasts indefinitely, the cement itself won’t. In other words, you need to plan your project and calculate the amount of concrete you’ll need so you won’t be left with a large amount of cement after the project is finished. This is when the question of how one would go about calculating all this comes up.
Calculating the Required Concrete for a Given Project
The first step is to measure and determine the volume of the concrete specimen you’re trying to make. This could be a flat slab, a square stump, or a long rectangular garden divider. Depending on the project, it may be simple to calculate, or it may be a little more difficult. But in any case, it’s always smart to add a margin of error in case your calculations aren’t perfect.
In many cases it’s often enough to just make an educated estimate and round it to an appropriate higher number. Let’s take the example of a short, rectangular wall that divides two sections of your garden. The dimensions are 0.2 meters thick, 0.2 meters tall, and 2 meters long – a standard, cute, stubby little dividing wall. So we multiply these numbers together to get the volume – 0.08m^3.
Now we need to find out the density of concrete in order to calculate the necessary concrete to fill this volume. Concrete is a composite material made of several different constituent parts, and as such, its density depends entirely on how it’s made. However, for smaller volumes, a general number can be used safely. A fairly standard concrete density value is 2400kg/m^3, or 150lbs/ft^3.
Using this number, we multiply by the volume, 0.08m^3, resulting in 192 kilograms. This means that it will take about 192 kgs of concrete to make our little wall. Let’s round this number up to 200 kgs to account for error. Here we should remember that “concrete” is the resultant material of all the constituent ingredients, including water, which means we now need to subdivide these constituent parts to determine how much of each we’ll need.
The ratio of cement:sand:gravel varies depending on overall volume, with larger applications typically having a significantly higher gravel content than smaller applications. The reason for this is because you usually don’t want larger aggregate showing on the outside face of the finished concrete, especially if it causes a rough uneven surface. As the overall volume of concrete increases the amount of larger aggregate can increase without causing this problem.
A typical cement:sand:gravel ratio for smaller projects is 1:3:1, with the water to cement ratio about 0.5:1, by weight. Depending on the project, you could add 2 or even 3 parts of gravel, but like I said above, you don’t want to add to the point of making an uneven surface. A 1:2 ratio of cement to sand can be used for projects requiring higher compressive strength, although this is probably the limit.
With the 1:3:1 ratio in mind, you can calculate how much you’ll need of each component. If the total weight is 200 kilograms, and we have 5 parts, each part will weigh 40 kgs. Using this calculation, we’ll be needing 40 kgs of cement, 120 kgs of sand, and 40 kgs of gravel. And using the 0.5:1 ratio of water to cement, we’ll need 20 liters of water.
The suggested mixing procedure is to mix the aggregates and sand together first, the cement next, and the water last – half at a time. You’ll most likely need to break it down to smaller batches. You should note that after a certain threshold, the less water a mixture has the stronger it will turn out. Obviously, it needs a certain amount for hydration to take place, but after that, any additional water will only weaken it.
Also, the hydration process the water triggers only affects the cement itself, which means any additional sand or gravel will have little to no effect on the amount of water a mixture needs. This is why the ratio of water to cement doesn’t include sand or gravel, as cement is really the only component that matters in this respect. So adding more sand in runny concrete won’t do anything – you must add cement.
This is why first of all, you should calculate the various components of your concrete. But if mistakes are made and too much water was mixed in, be sure to have a little extra cement on hand so you can rectify the problem. Note that too much water in the mixture is the main cause of poor-quality concrete, and is responsible for premature cracks, excessive shrinkage, and low compressive strength, to name a few symptoms.
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