What is concrete made of? In its simplest form, it’s made of three basic ingredients. The mixture includes water, aggregate, and cement.
The ingredients sound simple, which they are. But, there’s a huge amount of complexity in the mix design. You need to choose the exact quantities and characteristics of your ingredients.
These ingredients then create the most versatile material on Earth. So much so, that concrete has become the most widely used construction material on Earth.
In fact, our entire global infrastructure relies on this magical material.
To better appreciate the ingredients, I’m going to go over the basics of concrete and how it’s made.
What is concrete?
It starts out as a liquid. Then when it dries, it can become stone hard. Hence the nickname, “liquid rock.”
To point out right off the bat, cement is not concrete. Cement is rather one ingredient in concrete.
Also, concrete refers to many different materials in the construction industry. It’s not a single material as many would think.
The following are some mainstream concrete formulas you may have heard of:
- Reinforced concrete
- Rapid-set concrete
- Asphalt concrete
- Portland Cement Concrete (PCC)
Each of these formulas has a different application. This versatility makes this material the first choice of engineers for all structures. This includes roads, skyscrapers, dams, bridges, and all other mega structures.
Plus, concrete has all the following powerful characteristics:
- Molds into any shape when first poured
- Strong, durable, resilient, with a long life
- Remains durable in any environment
- Doesn’t burn, rust, or rot
- 100% recyclable
- Low maintenance
- Vibration and sound insulating
- Doesn’t emit any gas or toxic compounds when cured
- Easy to use
- Produced locally
How is concrete made?
Many just think of concrete as a mixture of paste and aggregates. Where the paste is cement and water, and the aggregate is sand and rock.
But, looking deeper into how it’s made, you can quickly see the complexities. Before we scratch the surface of the complexities, let’s go over the ingredients once more. The ingredients include the following:
- Fine aggregate (any natural sand particles)
- Coarse aggregate (gravel, crushed stone, recycled concrete)
- Optional: admixtures – chemicals used to change the properties of a mix
Material ingredients don’t get any more basic than this.
Just think about it. Water falls from the sky and the other ingredients you find in the ground.
Now sure, cement, the heart of the mix, you need to make. So this part is more involved.
The point is though, the overall mixture is cheap for how incredibly awesome it is. Plus, we won’t ever have a shortage of these ingredients like other natural resources.
Important Note: Portland cement is the most common type of cement used in concrete. It’s a fine gray powder.
It’s made by heating limestone and clay minerals in a kiln. This forms clinker. Then you grind the clinker and add 2% to 3% of gypsum.
Next, how do all these ingredients bind together? Cement!
But, the key is water.
When cement reacts to water, it goes through a chemical reaction called hydration. During this reaction, nodes form on the surface of each of the cement particles.
The nodes then expand and link up with other nodes. The nodes stick to the crushed rock and sand too and bind these ingredients. This all is a part of the hardening process.
At the same time, a great amount of heat increase occurs. Because hydration is an exothermic process.
What’s cool is, you can measure your concrete’s temperature. You’ll then know how far along the hydration process is. In return, you can estimate the concrete’s compressive strength. This is also called measuring concrete maturity.
In short, when cement mixes with water it forms a paste. This paste then coats the fine and coarse aggregate materials acting like glue. This glue then hardens creating the hard material you see on roads and buildings.
Important Note: aggregates either have a smooth surface or a jagged surface. The smoother the surface, the more workable concrete becomes. While rougher surfaces are less workable but stronger.
Generally, the inclusion of stones and sand increases the strength of concrete. Because for example, the cement bonds to the jagged edges of the stone. This then forms stronger bonds and thus increasing the end material strength.
The proportion of water and cement
The chemical reaction of cement with water brings out cement’s amazing qualities.
As we discussed, the combination of water and cement forms a paste. To create the perfect paste though, you need to properly measure the ratio of cement to water. This is the Water/Cement (W/C) ratio.
The W/C ratio is the weight of water to the weight of cement in a mixture. Depending on the application of concrete, the W/C will vary.
For high-strength concrete, the W/C ratio needs to be as low as possible. But, you still need enough water for the concrete to remain workable. Otherwise, you can’t pour and mold it.
If you add too much water though, you’ll create a cement soup. In other words, you’ll dilute the cement paste too much. You’ll then end up creating a weak material.
Without a doubt, the W/C ratio plays an important role in every mix design. Because varying the water content even a little will have a major impact on your end product. This includes the following changes:
- Finished appearance
The proportion of aggregates and paste
The key to a perfect mixture is the proper measurement of ingredients.
If you use more aggregate than paste, the gaps of your mixture won’t fill. Thus, you’ll end up with porous concrete with rough surfaces.
Now, what if you do the complete opposite and add excess paste? Your concrete will increase in cost, and it’ll more easily crack.
Because aggregates reduce the amount of shrinkage as concrete cures. But with more paste, cracks will naturally become more common.
All in all, aggregates and paste need to be in the right proportion. This will then improve the structural properties of your mixture.
Important Note: the type of aggregate used depends on your application of concrete. And it’s an important decision, as aggregates can make up to 60% to 80% of a mix design’s volume.
One application is a lean concrete mix where fine aggregates are only used. This mix produces a cheap material that’s easy to remove.
You can’t just use any water to create your paste.
When water has excess impurities, you can cause the following concrete problems:
- Increase in setting time
- Decrease in strength and durability
- Corrosion of embedded materials
- Volume instability
In short, you want to use water with limited chlorides, sulfates, alkalis, and solids. This means almost all drinkable water is usable.
If you’re uncertain about the quality of your water though, perform an impurity test.
Admixtures for concrete
You can add a wide range of chemicals to cement. Each chemical has a different use case you need to consider for your project.
It’s always important to first check if an additive is permissible for your project.
The following are the five most common admixtures you can add to your mix:
#1 Air entraining agents: improve workability. This is especially important when you work in freezing weather conditions. Also, when concrete becomes exposed to thawing, sulfates, and seawater.
#2 Accelerators: help accelerate the hardening. Often used in cold weather conditions. The agent used is calcium chloride and non-calcium chloride.
BUT, don’t use calcium chloride accelerators for concrete exposed to seawater. Also, don’t use in reinforced concrete, or concrete that’s in contact with aluminum or any other non-ferrous materials.
#3 Pozzolans: replace or improve the cement in mixes. This way you can use less cement to achieve the specified required concrete strength.
#4 Retarders: slow the hardening. Often used in hot weather conditions to delay setting time.
#5 Water reducers: improve concrete quality. Thus, you can get the desired material strength at lower water to cement ratios. This helps with the workability of your mix.
What are the grades of concrete?
Concrete has different grades, that define its strength and composition.
Typically, you’ll find concrete grades denoted as “M5” for example. The “M” stands for mix, and the 5 denotes the strength of concrete. This is the strength concrete should have 28 days after you pour it.
The strength is a measurement in MPa (megapascals) in the metric system. Also, psi (pounds per square inch) in the imperial system. These are both units of pressure.
Next, your ingredient mix ratio depends on the type of structure you’re building. This mix ratio includes the following ingredients:
- Fine aggregate
- Coarse aggregate
You’ll find the mix ratios typically shown in the following format:
(1 : 5 : 10) is (1 part cement : 5 part fine aggregate : 10 part coarse aggregate)
The following is a table listing the major grades of concrete.
|Grade of Concrete||Mix Ratio||Strength (MPa)||Strength (PSI)|
|Normal Grade of Concrete|
|M5||1 : 5 : 10||5 MPa||725 psi|
|M10||1 : 3 : 6||10 MPa||1087 psi|
|M15||1 : 2 : 4||15 MPa||1450 psi|
|M20||1 : 1.5 : 3||20 MPa||2900 psi|
|Standard Grade of Concrete|
|M25||1 : 1 : 2||25 MPa||3625 psi|
|M30||Design Mix||30 MPa||4350 psi|
|M35||Design Mix||35 MPa||5075 psi|
|M40||Design Mix||40 MPa||5800 psi|
|M45||Design Mix||45 MPa||6525 psi|
|High Strength Concrete Grades|
|M50||Design Mix||50 MPa||7250 psi|
|M55||Design Mix||55 MPa||7975 psi|
|M60||Design Mix||60 MPa||8700 psi|
|M65||Design Mix||65 MPa||9425 psi|
|M70||Design Mix||70 Mpa||10150|
To reemphasize, each grade has a separate construction application. The more familiar you become with construction, the better you can choose the proper grade to use.
Concrete is one of the most versatile materials known to humankind. There’s a reason why it’s the building block to our modern world.
What’s more amazing is how simple the ingredients are. BUT, at the same time, concrete is insanely complex and the mixtures require deep science.
Not surprisingly, this science has given us amazing mega structures spread across the globe.
Look no further than the amazing Roman Empire structures that still stand tall. Many structures are intact, after nearly 2,000 years!
While today, the Three Gorges Dam in China holds back 10.4 trillion gallons of water. Equally impressive, the Burj Khalifa in Dubai stands 2,717 feet tall.
Each of these projects used a different application of concrete, with a unique mix design.
In short, without concrete, life, as we know it today, wouldn’t exist.
What are your thoughts on this amazing material and its applications? Do you think there’s a more versatile material?
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Koosha started Engineer Calcs in 2020 to help people better understand the engineering and construction industry, and to discuss various science and engineering-related topics to make people think. He has been working in the engineering and tech industry in California for over 15 years now and is a licensed professional electrical engineer, and also has various entrepreneurial pursuits.
Koosha has an extensive background in the design and specification of electrical systems with areas of expertise including power generation, transmission, distribution, instrumentation and controls, and water distribution and pumping as well as alternative energy (wind, solar, geothermal, and storage).
Koosha is most interested in engineering innovations, the cosmos, our history and future, sports, and fitness.