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If you have not worked with concrete that often you may be unsure of the exact materials needed to make concrete. I often hear others even use the terms cement and concrete interchangeably. Some DIYers unfamiliar with concrete wonder if you can make concrete without the use of gravel.
No, you cannot make concrete without gravel. The ingredient mix of 1 part cement, 2 parts sand, and 3 parts gravel (stone) is what makes up concrete. Without gravel, concrete would not be strong or be able to be used as a foundation or base that could withstand heavy loads.
This answer is a little simplistic, and there is a lot more to concrete than the gravel and sand that goes into it. Here, we’ll get into what makes up concrete, how to combine the perfect concrete mix, and what substitutes you can use for gravel.
The Contributing Components of Concrete
There are many types of concrete, but essentially concrete is a mix of cement and rocks.
The fine, powdery cement is called Portland cement, and it is mixed with water to create a paste. Manufacturers then mix this paste with sand and rocks, often called aggregates, and this creates concrete (source).
What Does Sand Add to Concrete?
Aggregates make up 60 to 75% of concrete. Therefore, the type of aggregate used plays a vital role in the consistency of concrete (source).
Sand is a vital part of most concrete mixtures, due to its relative affordability and natural availability in specific environments. The addition of sand and other aggregates not only strengthens the concrete mix but also makes it bulkier and increases grip.
Sand is a fine material consisting of small grains, and these tiny particles fill spaces found in between coarse aggregate material. Therefore, without sand, or any fine material, the concrete requires extra paste, or it would become porous and hard to work with (source).
When we use concrete, hydration occurs. While we usually associate hydration with drinking eight glasses of water a day, hydration is a different process in cement. Hydration is when compounds in cement bond with the molecules in water. This also starts the hardening process of concrete.
When you add sand to cement with gravel, it ensures that the hydration process is smoother. If there were only cement and gravel in concrete, the mix would significantly shrink and weaken the structure. Sand limits the shrinking.
To find out more about the qualities of sand and how it contributes to concrete, take a look at “Can I Make Concrete with Just Sand and Cement?”
What Does Water Add to Concrete?
Water activates the binding agent in concrete. It gives concrete the flexibility to pour, but it is also part of the hardening process. Water is a necessary part of the hydration chemical reaction as it reacts to compounds in the cement.
What Does Gravel Add to Concrete?
Both sand and gravel play an essential role in determining the texture of the concrete and the amount of water necessary for the mixture. The strength of concrete comes from the gravel added to it.
However, gravel also comes in a range of sizes. Small, finer pieces of gravel create a “fine” grade of concrete, which is appropriate for external, smooth surfaces. Larger pieces of gravel create a “course” concrete that is extremely strong but somewhat unwieldy.
Gravel adds a lot of strength to concrete but, if you use too much, it has the opposite effect and damages the integrity of the concrete mix. Therefore, it is important to know what type of concrete you need for a particular job.
There are a few types of aggregate that can be used instead of gravel, and they have their distinct advantages and disadvantages.
Two gravel substitutes are perlite and vermiculite. These are both hard, porous materials that help create a lighter concrete that has excellent insulating properties (source).
Expanded Polystyrene (EPS) is an inexpensive substitute for gravel. It has the advantage of providing aeration in the concrete and can be used to create lightweight concrete.
However, the PSI strength of this concrete is significantly lower at three PSI, making it more appropriate as an insulator or as a plaster. We’ll explain PSI a little later.
Glasscrete is an environmentally friendly replacement for fine gravel. It uses recycled glass and has shown to be a favorable replacement as there is almost no strength loss, and it has improved maneuverability (source).
Another environmentally friendly substitute is plastic. An unorthodox choice, plastic can be a cheaper alternative to concrete. However, it does not have the strength of gravel and will usually require steel fiber or some other reinforcement to make it viable in the long term (source).
Types of Concrete
When concrete is mixed, it receives an “M” grade to conform to standard mixes. Standard mixes also have a PSI (pound per square inch) measurement, which indicates strength. The basic terminology for concrete is either Normal, Standard, or High-Strength (source).
There are approximately 23 different types of concrete, but we’ll examine the most commonly used ones here (source). Normal strength concrete is for smaller constructions that don’t require a lot of strength, such as pathways, garages, and internal floor slabs.
Plain concrete is used in construction, but it does not have any reinforcements in it. It is used in footpaths and in the construction of smaller buildings that do not require a lot of tensile strength.
In comparison, reinforced concrete is a mix of plain concrete, but it is poured into spaces that have been reinforced with metals bars or rods. Another durable concrete is prestressed concrete that is used in heavy-load projects, like bridges and overpasses.
Mortar is a mix of cement, water, and sand only. This mix is used for construction, especially building with bricks.
The Perfect Concrete Mix Ratio
When it comes to mixing concrete, the proportions are vital. A concrete mix that is too thin and runny will not be able to hold anything. One that is to dry won’t settle properly, while one with too much gravel will be stiff and unwieldy. Therefore, it’s best to get it right the first time.
However, the perfect mix of concrete is dependent on the job that you are completing. Ratios may differ, especially when using gravel substitutes. Certain materials require different ratios.
When it comes to concrete, both water and the aggregates in the mixture play a role in strength and longevity. Water is not something to be picky about; as long as it is clean and drinkable, you can use it for a concrete mix.
Specific aggregates are required in concrete, however. The size and ratio of the aggregate make a difference in the final product and its potential applications. Aggregates make up a large portion of the concrete mixture, so they should be correctly used.
Concrete mix ratios are usually expressed in 3 numbers — X:Y:Z. The first number refers to the amount of cement used, the second to sand, and the third to gravel. The amount of water used in each mix is also different.
M5, 7.5, 10, 15, and 20 all fall under the “Normal Grade of Concrete.” These mixes are generally more granular and used in spaces that you wouldn’t see after construction.
M10 is a very granular mix, composed of a 1:3:6 ratio. Due to its porous nature, it is useful for putting down patio slabs and pathways.
The classic use for concrete is in building, specifically with brick. M25 is part of the standard concrete grade and is the most commonly used concrete mix. Due to its multi-purpose nature, you can use it for both the foundation and the building itself.
M25 is made of a 1:1:2 mix and has a psi of 3625. This concrete is strong, but it is a little more expensive to make due to the lower ratio of sand and gravel, which means you use more cement.
Testing Your Concrete Mix
A good way to test whether or not a concrete mix is workable is to use what is called the “slump test.” You conduct the test using a bottomless container. Even a styrofoam cup with the bottom removed will be adequate.
The cup or cone should be packed as tightly as possible with concrete. Place the cone face down on a surface and remove it, leaving the concrete behind.
If the concrete is close to the original size of the cone, or if it slumps down to less than 50% of the size of the cone, then the mixture is too hard or soft will be practically unworkable.
If the concrete stays between 50% to 75% of the original cone or cup, that indicates a good texture, and the concrete should be usable (source).
Technically, you cannot call it concrete without gravel, but technology and innovation have shown that there are several substitutes that you can use to create concrete without traditional gravel.
Your chosen substitute depends on what you’re trying to build, but there are plenty of viable options available for your next project.