Understanding Concrete Compressive Strength (What is PSI?)

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Concrete is known to be strong and long-lasting, but just how strong it needs to be to do its job varies, depending on the application.

The compressive strength of concrete refers to how many pounds per square inch (psi) it can handle when force is applied. It is a reliable indicator of how it will perform under heavy loads or internal pressures caused by freeze-thaw cycles. Understanding what the different psi ratings mean is essential for specifying the right type of concrete for your project.

How do you test the compressive strength of concrete?

Testing is the only way to determine whether a concrete mix has enough compressive strength, or load-bearing capacity, for the intended use. These tests are conducted on cylindrical concrete specimens (per ASTM C39) using a machine that compresses the cylinders until they crack or break completely (see Concrete Testing Procedures). Generally the higher the psi rating, or the weight under which a square inch of concrete surface area will fail, the stronger and more durable the concrete will be.

Note that concrete cylinders are usually tested 28 days after casting as a quality-control check to determine if compressive strength levels are acceptable. Although concrete will continue to gain strength after 28 days, enough hydration has taken place during this time frame to provide a good estimate of the final strength.

What is the best psi for driveways and other residential concrete?

The minimum compressive strength requirements for residential concrete are often set by local building codes. For concrete slabs that will bear lighter loads, such as sidewalks, patios, steps, and interior flatwork, a minimum of 2500 psi may be sufficient. For a standard residential concrete driveway or garage floor slab, a range of between 3000 and 4000 psi is often required to provide the necessary load-bearing capacity to support vehicle traffic (see table).

When choosing the best psi rating for a driveway, it’s important to consider the weight of the vehicles that will be using it as well as climate conditions. If the driveway will support heavy vehicle traffic (such as an RV or truck) or be exposed to frequent freeze-thaw cycles, a 4000 psi rating or even higher may be recommended for maximum durability.

Is higher psi concrete always better?

There is little benefit to using concrete with a higher psi rating than recommended for the intended use. Higher psi concrete generally has a lower water-cement ratio, which makes it stronger but also more difficult to work with. What’s more, a concrete mix with a higher psi is often more expensive and may not be worth the additional cost, especially for surfaces that aren’t exposed to heavy traffic or harsh weather.

Keep in mind that while the compressive strength of concrete is important, a high psi rating will not boost concrete tensile strength, or its ability withstand pulling-apart forces caused by subgrade settlement, heavy loads, drying shrinkage, and thermal expansion and contraction. To improve concrete tensile strength, some type of steel reinforcement may be required (see A Guide to Reinforcing Concrete Slabs).

How do I order concrete with the right psi rating?

To get concrete with the right strength capacity for your project, tell your ready-mix supplier the psi you're looking for and they will then proportion the concrete to achieve the desired performance. If you're unsure of what psi you need for a certain project, tell the supplier the anticipated exposure and service conditions for the concrete and they can design a mix suitable for the purpose by adjusting the ratio of portland cement, aggregate, and water.

What factors affect concrete compressive strength?

Several factors affect the compressive strength of concrete, including:

• The quality of the raw materials in the mix
• The water-cement ratio
• The type and grade of aggregate used

Generally, the more portland cement that is added to the mix, the higher the psi rating of the concrete. For example, a traditional 3000 psi mix contains 5 sacks of cement per cubic yard of concrete. To achieve 4000 psi, you’ll need 6 sacks of cement (which is why higher psi mixes are often more expensive).

Curing conditions also play a crucial role in the strength development of concrete. When properly cured, concrete has an adequate amount of moisture for continued hydration, which improves its durability and wear resistance (see Guide to Concrete Curing Time and Methods).

What happens if the concrete compressive strength is too low?

If your concrete fails to achieve the designated compressive strength, as determined by 28-day concrete cylinder tests, it can be due to several factors:

• Improper cylinder handling, curing, and testing (the most common reason for low strength results).
• An incorrectly proportioned concrete mix.
• The addition of water to the fresh concrete on the jobsite in efforts to improve workability.

If the process for making the test samples was right, the quality of the concrete mix is the most likely cause of the low strength results. Before tearing out the concrete or taking other remedial measures, consider how the concrete is being used. It still may be strong enough to serve its purpose, especially if the concrete will not be bearing heavy loads. Additional core testing of the hardened concrete may be needed to determine if the concrete is of sufficient strength and durability.

CONCRETE COMPRESSIVE STRENGTH REQUIREMENTS