Concrete can be used to make driveways, slabs and structures. It’s a very popular building material that has been in use for thousands of years, although the recipe may vary. The material is widely used, but a lot of people still confuse this material with cement.
To better understand the difference between the two, it’s important to know how concrete is made and how it works.
How is Concrete Made?
Concrete is less complicated than many people assume. In fact, concrete ingredients consist of a few materials:
- Cement
- Air
- Water
- Sand
- Gravel
Each of the ingredients plays an important role in the composition of the concrete that is used in the building process. The concrete mix ratio can vary depending on the properties that you want from the cement.
A traditional recipe often includes toe 10-20-30-40 rule, which stands for:
- 10% cement
- 20% air/water
- 30% sand
- 40% gravel
Concrete mix ratios depend on the role that the concrete will play in the building process. Understanding the key ingredients in the mixture is just as important as the chosen ratio:
Air and Water
Air and water are very important parts of the concrete mixture, especially water. In climates where freezing occurs, the cement may have small air bubbles that allow the concrete to expand when thawing occurs.
If the air bubbles are too big, they can cause cracking, so there’s a very delicate process behind choosing the right air and water ratios.
The inclusion of water often has the largest impact on the cement and can cause:
- Cracking
- Shrinkage
- Loss of strength
If too much water is used in the mixture, it will lead to the material cracking and shrinking. Higher ratios of water lead to a weaker material. Builders and concrete companies can perform a slump test, which is used to determine whether the cement mixture contains too much water to be viable.
Water-reducing admixtures may be used in the concrete to lower the amount of water required while improving the strength. While these admixtures may be a good option for industrial use, it’s rare that they’re used in residential applications because they can be very expensive.
Cement
Concrete contains cement, but it’s just 10% of most mixtures. The cement is used to bind all of the materials together and act as a sort of glue. When the concrete is placed, the cement is what causes the hardening process to occur.
While many people consider cement to be a single product, there are multiple types of cement that can be added to the concrete mixture to provide the desired results:
- Residential
- Sulfate conditions
- Climates where freezing is a concern
- Industrial or special placements
- Extreme sulfate conditions
In the United States, the first two types of cement are mostly used due to the moderate climate. The properties of the concrete will depend on each project’s needs and demands. Hardened concrete can have low permeability, require additional strength or be resistant to freezing.
When concrete is used in building, the climate and location will play a significant role in whether or not certain cement ratios are used or not.
Gravel and Sand (Aggregates)
Aggregates are added to the recipe and account for a large percentage of the mixture, 70% or higher in most cases. The reason that sand and gravel are added is because they’re far more affordable than concrete.
They’re also stronger materials than cement.
Large gravel and fine sand are used in most mixes to provide the strongest concrete possible. Sand is used to fill in the gaps and spots in the mixture where air bubbles can form. Again, small air pockets are great for freezing climates, but if the air pockets are too large, they’ll cause the concrete to crack and weaken.
Gravel fills in a larger space than sand and has a high strength.
How Does Concrete Work?
Hydration plays a major role in the concrete material that you’re used to seeing in the building process. Concrete must harden, and the hardening process occurs rapidly as the water, aggregates and cement combine.
Hydration, a chemical process of water, occurs, leading to the nodes of the concrete forming. As the hardening process begins, the nodes of the cement begin to form on all of the cement particles.
Nodes linking eventually occurs as the nodes grow and expand.
Eventually, the nodes will adhere to the aggregates. It’s important that the concrete mixture be placed as quickly as possible because the mixture will start to stiffen rapidly. If the mixture becomes too stiff, it won’t be possible to place the concrete properly.
Curing and Concrete
Different applications will require different curing processes. Special techniques can be used to protect concrete in:
- Extreme hot temperatures
- Extreme cold temperatures
If the concrete is allowed to remain moist, it will be more durable. As concrete ages, it continues to get stronger. Why? Hydration. Most of the hydration process occurs in the first month after the concrete is poured, but the hydration will continue for years, making the concrete stronger in the process.
Water fog can be sprinkled on the exposed surface or by using other materials. The process allows the concrete to remain moist for longer.
Placing Problem Correction
Concrete material may have placing problems that must be corrected. When these placing problems occur, a few methods of correction can include:
- Adding cement, ash, sand or entrained air can be used when a mixture has too much bleedwater.
- Aggregate materials may need to be adjusted when pumping concrete because larger aggregates may not pass through the pump.
- Retarders can be used when more time is needed for the setup. Setting time slowing is common and may require the addition of water and ice, liquid nitrogen injections or by sprinkling water on the mixture.
- Accelerators can also be used if the mix needs to be set quicker. Heating the water and aggregates is the most common method of acceleration.
Concrete is a hard, durable and strong material that strengthens over time. When used for the right application, the material offers long-lasting and affordability.
