What Is Concrete?
The terms concrete and cement are often used interchangeably. However, the two aren’t the same product. Cement is the main ingredient in concrete and acts as a binder. Portland cement is the most common cement used in concrete. Along with cement, concrete also contains water, a coarse aggregate, and a fine aggregate like gravel or sand. The strength and durability of concrete is determined mainly by the proportions of the ingredients.
Concrete strength is measured in pounds per square inch (PSI) in the imperial system and megapascals (MPa) in the metric system. PSI is important because it determines if the concrete mix meets the needs of the project. The greater the PSI, the greater the concrete strength and durability. PSI ranges from 2500 to 5500 for normal concrete, while high-strength concrete is 6000 PSI or higher.
What’s the Best Concrete for My Project?
There are many uses for concrete, including walkways, patios, deck footings and more. Before you jump into a project, you’ll need the right concrete.
When mixing concrete, the water ratio is important. Bagged mixes have the perfect ratio of ingredients. However, too much water will cause an uneven distribution of sand and gravel and leave voids, or little holes, as a result of trapped air in the concrete. Over time, voids can produce cracks and pits, which are a hazard. To prevent voids, make sure you have enough water in the mix. Start with the minimum amount of water and gradually add more if needed.
After the concrete is poured, it needs to cure before you can put weight on it. For residential uses such as a driveway or sidewalk, the curing process can take 24 to 48 hours after the initial set of the concrete before it’s safe to walk on the surface.
Different Types of Concrete
Concrete is available in a variety of types. Not all concretes are created equal. Depending on the project and your needs, one type of concrete may work best for an application, whereas a different type of concrete is more suited for other applications.
Concrete mix types vary, as well as setting and curing times. Be sure to follow the manufacturer’s instructions.
When mixing or working with concrete, always wear proper protective gear, including safety glasses, a respirator and gloves.
Normal Concrete Mix
Normal concrete is also known as regular concrete, and it’s most commonly used in residential projects. Normal concrete has many advantages.
- It’s good for large or small projects like sidewalks, steps and curbs.
- It has favorable workability because the ingredients are correctly proportioned.
- It has an average setting period between 30 to 90 minutes.
- It’s used for applications with a 2-inch minimum thickness.
When achieving early strength is a concern, high-strength concrete is a good choice. It’s a commercial-grade mix with a high cement content, sand and a high-strength aggregate. High-strength concrete has some advantages.
- It’s more durable than normal-strength concrete.
- It’s used in high-rise buildings, parking structures and occasionally in bridges, etc.
- It’s great for load-bearing columns.
- It requires reduced maintenance and repair.
- It has a compressive strength of more than 6000 PSI.
Crack-Resistant Concrete Mix
Crack-resistant concrete mix is a commercial high-strength concrete pre-blended with synthetic fibers along with the cement, water and gravel. The fibers act as reinforcement, which helps to increase tensile strength and reduce surface shrinkage cracking. Crack-resistant concrete has many advantages.
- It provides greater impact resistance.
- It eliminates the need to use wire mesh.
- It’s a great option for high-impact applications like sidewalks, patios and floors.
- It’s used for applications with a 2-inch minimum thickness.
- It achieves a compressive strength of 4000 PSI.
Fast-setting concrete, as the name implies, sets quickly. It’s a special blend of cement, gravel and sand. There are multiple advantages to fast-setting concrete.
- It sets in 20 to 40 minutes.
- It’s a good choice for setting anchors and posts, like mailbox, lamp or fence posts.
- There’s no mixing involved. Simply pour the dry mix into the hole and add water.
- It can achieve a compressive strength up to 3000 PSI in three hours.
For more on working with fast-setting concrete to set posts, read Setting a Post With Concrete.
Sand/topping mix concrete differs from other concrete in that the pre-blended mix only contains cement and sand.
- It’s an ideal choice for patching damaged concrete, leveling surfaces like steps or floors, and repairing cracks.
- It's used for applications with a 2-inch maximum thickness.
- It’s a high-strength mix that achieves a compressive strength of 5000 PSI.
Ready-mix concrete is a mix of cement and water along with a coarse material like crushed stone, sand or gravel. It’s pre-mixed in a specific quantity for the job according to your requirements and delivered to you in a concrete truck, which is a huge timesaver, or you can pre-mix it yourself by adding water. There are a few downsides to pre-mix concrete: It adds to the cost of your project. It’s not suitable for small jobs. Also, because it’s mixed off-site, it needs to be transported from the mix site.
How Much Concrete Do I Need?
No matter the size of your project, it helps to know beforehand the amount of concrete you’ll need for the job. You need to calculate the volume of concrete by determining the cubic yards.
Using measurements in feet, multiply the length by the width to get the square footage. Convert the depth from inches to feet by dividing the number of inches by 12. The depth represents how deep you want your slab. Next, multiply the depth by the square footage to determine the cubic feet. Finally, divide the cubic feet by 27, the number of cubic feet in a cubic yard.
For a 12-foot-by-12-foot slab, the square footage is 144. The desired depth of the slab is 6 inches, which is 0.5 feet. Multiply 144 by 0.5 to get 72 cubic feet. Divide 72 by 27 to get 2.67 cubic yards. Add an extra 10% to allow for spillage. The total amount of concrete you’ll need is 2.94 cubic yards.
Now that you know the amount of concrete mix you need, it’s time to figure out how many bags you’ll use. To do this, divide the cubic yards by the bag yield.
For example, using the cubic yards above (2.94), if an 80-pound bag yields 0.022 cubic yards, divide 2.94 by 0.022 for a total of 134 (rounded up) bags.
If math isn’t your strong suit, use our Concrete Slabs Calculator to do the figuring for you.
What Tools Do I Need?
Completing a concrete project takes thought and planning. There are various tools you’ll need during the process.
As mentioned earlier, you should always wear the appropriate gear when working with concrete, including safety glasses, a respirator, rubber gloves and rubber boots. In addition, when working with power tools, always wear hearing protection.
If you’re mixing concrete by hand, you’ll need a bucket of water, a mortar hoe or shovel, and a mixing trough or wheelbarrow. For larger jobs when a lot of bags of concrete are needed, a portable cement mixer is appropriate.
When cutting wood forms or tube concrete forms, use a circular saw or miter saw. When cutting rebar or remesh, an angle grinder or a reciprocating saw fitted with metal cutting blades will get the job done.
If you're installing flooring over the concrete or if you’re in a high-humidity area, you’ll need to install a vapor barrier to prevent damage.
Once the concrete is poured and spread, use a screed to level the surface. To ensure straight lines, snap a chalk line across the wet concrete as a guide. If working across a large or extended surface, like a sidewalk or driveway, you’ll need to use a groover to create grooves in the concrete to help control cracking. When you’re finished grooving, use a float to remove any marks left by the groover.
When finishing a concrete application, using a hand trowel with a sweeping motion smooths the surface. If there’s too much aggregate on top of the concrete, using a concrete tamper while the concrete is still wet helps to push aggregate below the surface for a smoother finish. For large jobs, a plate compactor is more efficient.
For more on sealing concrete, read How to Seal Concrete.