Mixing concrete is something of an art form in that there is not one single ratio that is suitable for every situation. Some mixes are more watertight; others offer more strength. When it comes to mixing the strongest possible concrete, the best concrete mix ratio is typically;
- One part cement
- Two parts sand
- Three parts gravel
- Half a part of water
Each part refers to an arbitrary unit of measurement. You can measure your ingredients in any unit you like, just so long as the other ingredients are measured in the same units. For example, one part cement and two parts sand means that there should be twice as much sand as there is cement, regardless of how much cement that might be.
It is also worth noting that many factors go into what makes the best possible mix of cement. For example, it may be possible to get a stronger mix of concrete by adjusting the ratios, but that adjustment may make the concrete not feasible due to the added costs it would incur. An example of this would be increasing the amount of cement in the mix, which is a far more costly material than sand or gravel.
Similarly, there are factors outside of the concrete to consider. For example, the strongest possible concrete mix may not be necessary if the structure that is being poured is going to be reinforced. Another factor is the kind of stresses the structure will undergo. Concrete excels in some areas but is less impressive in others, and if you are pouring a concrete structure that will be placed under the kinds of stresses that concrete is not typically good with, you may face a situation where it is far more practical to get the additional strength needed from other places—such as the reinforcement we mentioned above—rather than trying to make the concrete as strong as possible.
What Goes Into Concrete?
It is all well and good talking about the ratios you should mix concrete at, but what are those ingredients, and what do they do in the overall concrete mix?
Cement is the crucial ingredient in concrete, so much so that the name “cement” is often used interchangeably with concrete. It is the cement that reacts with the water in the mixture, binding the aggregate materials together and creating the solid, durable material we know of as concrete.
Cement has some unique properties that are not always intuitive to people who are not familiar with construction. For example, it is commonly thought that concrete hardens as it “dries” out. This is almost the opposite of what happens since the hardening is a result of the cement reacting with water, and that water then becomes part of the concrete structure in the form of hydrates.
Evidence of these reactions can be seen in the heat that cement generates when it is curing, which is the phenomenon that is responsible for concrete burns, and why you should avoid getting concrete on your skin as much as possible.
Speaking of water, the water used in the mix must be neutral, even pure, so that any contaminants in there don’t interfere with the reactions between the water and the cement. If the water were contaminated with such things, it would cause the concrete to be weaker than it should be since not all of the cement would have been able to react with the water.
It is also worth noting that cement—that is, Portland Cement, the most commonly used kind today—is hydraulic. This means it can cure underwater since it is not relying on the “drying” of the mixture to harden. In fact, in cases where large areas are being concreted, it is not uncommon for the concrete to be hydrated further after the initial pouring. This may be a process as simple as spraying more water on the curing concrete, though special hydrating solutions can also be used.
We mentioned above that cement works to bind aggregates together, and sand is part of that. It can help to think of cement as more of a glue than some kind of stone-like material. Sure, you could make a strong structure from just that glue, but the cement’s real strength lies in binding other, stronger objects together. Those objects come in the form of aggregates and sand. Sand makes up the fine structure of the concrete, and it is strong because sand is largely made up of small pieces of stone. Using sand allows for strength to be maintained in the spaces between the larger aggregates—which we’ll get into in the next section.
Like water, it cannot just be any old sand that is used in a concrete mix. The sand grains need to have a particular type of shape, angular edged. This allows them to interlock with each other, forming a much stronger structure than they would have done with more rounded grains of sand. The best source of this ideal sand for concrete mixing is found in nature in riverbeds and beaches, though it is also possible to manufacture sand.
Another reason that sand is so crucial to the mix—beyond the strength it brings—is the economic factor. Sand is much cheaper to source naturally, or manufacture than cement is to make, so adding sand to the mix greatly reduces the cost of concrete.
The final main ingredient in concrete is the aggregates. Aggregates can be things like gravel or crushed stone and, generally speaking, they serve the same purpose as sand, but on a larger scale.
The bulk of the cured concrete’s strength comes from the aggregate since the cement is effectively just there to hold the aggregate together. Therefore, the stronger the aggregate, the stronger the concrete. This is because any force attempting to crush the concrete is basically attempting to crush hundreds and thousands of little stones.
As with the sand, aggregate also reduces the cost of concrete significantly since the aggregate takes up a lot of volume, reducing the amount of volume that needs to be filled with sand or cement.
It is important to remember that concrete has a great deal of compressible strength but a relatively small amount of tensile strength. This means that if you are creating a structure that will be facing more tensile stress than your concrete can handle, you are going to have to reinforce the concrete to bring that weakness up to scratch.
The most common way of doing this is through the use of steel bars that are arranged in a kind of framework throughout the concrete structure. When the concrete is poured and has cured, it binds the steel bars just as easily as it binds the sand and aggregate.
This is relevant here because the strongest concrete mix may not be necessary. If you find yourself needing to reinforce your structure regardless, then making the concrete stronger may be a waste of resources and money.
Properties of Different Concrete Mixes
Concrete is porous by nature, which is something that can be mitigated if you need a more watertight finish by skipping the aggregate. You could even reduce the ratio of sand, though you will need to ensure your new, weaker concrete mix is up to the task. Increasing the amount of proportion of cement in your concrete mix will create a more watertight finish, but the trade-off is expense. If you are increasing the ratio of cement in your mix, you are going to be spending more money on the overall project, especially if it is a large project.
With a larger aggregate, you can reduce the amount of money you need to spend on ingredients for your mix. That being said, larger aggregates are not ideal for all situations. For instance, if you are trying to create a smooth surface, you would stay away from large gravel due to the fact that the pieces of stone would force a roughness onto the surface of the cement that would not be ideal.
For the most part, there is a choice to make between highly structural, durable concrete, and extremely smooth concrete. It is possible to polish certain cured concrete mixes to a glass-like polish, but that would not be possible with a bumpy, large-gravel-filled concrete mix. Similarly, it would not be possible to achieve the same degree of strength without a strong aggregate.
There are a lot of factors that go into even deciding what metric you are judging “strong” to be, which can make settling on the strongest concrete mix ratios difficult. We mentioned that the strongest mix consists of one part cement, two parts sand, three parts gravel, and half a part water. This is indeed a recipe for a strong concrete mix, but, depending on your circumstances, you may not need the strongest.
It’s also important to note that the ingredients you use make a difference. Things like contaminated water, weak aggregate, subpar cement—all of these things—and plenty more—can cause problems for your concrete mix. If you’re something of a novice, always be sure to follow the instructions that come with your cement.