What Makes Up A Cubic Metre of Concrete?
Cement isn’t always the easiest thing to budget for, given that it is mixed with other materials to make concrete, and that concrete is poured, sometimes into unconventional shapes. Still, it helps to know roughly how many bags of cement you need for a given area. As a rough guide, assuming 25kg bags of cement, you would be looking at around fourteen bags of cement per cubic metre of concrete. Of course, this a general guideline amount for something that can vary a lot.
Bag Size Matters
This should really go without saying, but it’s worth noting that not all bags of cement are created equally. Our above estimate is based on 25kg bags of cement, which is a size commonly found in DIY stores, and the most likely size a regular consumer who is not working in construction would buy.
That being said, you can also get concrete in other sizes of bag, such as 50kg. And, if you are working in the trade and have access to trade sales, you could potentially buy cement in even larger bags. A trade-sized bag of cement can be up around 800kg in weight! Based on our above guideline amount, that would be enough for nearly three cubic metres from just one bag.
What Goes into a Cubic Metre of Concrete?
So, if we can’t use “bag” as a reliable metric due to the differing sizes, it may help to talk about the actual amounts that are going into this cubic metre. As a rough guide, you are looking at between one and a half to two tonnes per cubic meter of concrete; however, that is not all cement. These figures assume a ratio of one to four in favour of sand. So for every one bag of cement, you would be mixing in four bags of sand—though it should be noted that sand often comes in different sizes, such as 20kg rather than 25kg.
A significant portion of this weight will be added by water, which is essential for concrete. The amount of cement factored in is also exaggerated slightly to account for wastage, since it is basically impossible to mix concrete without wasting some, no matter how good you are.
In most cases, you will be using an aggregate in your concrete mix (not mortar, however), and that will change the proportions of your mix. You will need to be careful to ensure the amount of cement in your overall mix is sufficient since the cement is responsible for binding the sand and aggregate together.
Cement is essentially a glue within the concrete, and the sand and aggregate is the substance that brings strength to the final product. Aggregate provides a way of getting more mileage out of your cement by increasing the volume without needing to increase the cement by a proportional amount. Though, as mentioned above, there still needs to be enough cement in the mix to give the concrete enough strength to do its job.
The size of the aggregate has a direct effect on how smooth the final product can be. For example, if you use gravel that is on average 10-20mm across, you are going to get a very rough finish compared to concrete that is just mixed with sand and cement.
Why is This Information Important?
Planning out any construction project is a difficult task, and even though the difficulty scales with the size of the project, it doesn’t make smaller projects any easier. If you are embarking on a project—perhaps something of a DIY nature—one of the most important things to get right is the budget. If you go into any kind of construction project blind on the financial front, you are going to end up with a nasty shock when the bills start rolling in.
For something like bricks or wood, it is relatively straightforward to calculate how much of those raw materials you need. For cement, however, there are many factors that complicate the issue. For example; what are you using the cement for? Our above guideline amount of fourteen bags assumes you are making regular concrete that will contain sand and an aggregate of some kind, but perhaps you are making concrete without the aggregate, or maybe you are mixing mortar, which also requires cement.