# What Is the Compressive Strength Of Concrete?

The compressive strength of concrete is a measure of how well it can withstand being compressed or subjected to pressure.

It is determined by testing cylindrical samples of the concrete in a machine that applies pressure until the concrete crushes or fails. The compressive strength is calculated by dividing the maximum load the concrete can withstand by the cross-sectional area of the sample.

In the United States, the strength is typically reported in pounds per square inch (psi), while in the International System of Units (SI), it is measured in mega pascals (MPa).

The required compressive strength of concrete can vary based on the intended use of the concrete, with residential structures typically requiring a strength of 2500-4000 psi (17-28 MPa) and higher strengths (up to 10,000 psi or 70 MPa) being necessary for some commercial and specialized applications.

Concrete’s compressive strength is typically evaluated by comparing the results of testing samples from a batch of concrete to the strength specified in the job contract documents, known as the design strength.

The average of at least two tests is generally used to determine the concrete’s strength, and in most cases, the strength is evaluated at 28 days after the concrete is poured.

If the average strength of the tests falls below the specified design strength, it is important to consider the possibility that the testing procedure may have contributed to the failure, rather than assuming that the concrete itself did not meet the required specifications.

## How Do You Calculate The Compressive Strength Of Concrete?

Compressive strength is a measure of how much load a material can bear before it fails. It is determined by applying a force to compress the material, and measuring the force required to cause it to deform permanently.

To perform this test, a cubic or cylindrical specimen of the material is used because it ensures that there are flat, parallel surfaces at the top and bottom of the specimen.

The test is conducted under controlled conditions, including the application of equal and opposite forces to compress the material from both top and bottom.

The compressive strength is calculated using the formula CS = F/A, where CS is the compressive strength, F is the force at the point of failure, and A is the initial cross-sectional surface area of the specimen.

To calculate the compressive strength of a concrete cylinder, for example, the diameter of the cylinder’s cross-sectional faces is used to calculate the surface area, which is then plugged into the formula along with the force at which the cylinder failed. The resulting compressive strength is typically measured in pounds per square inch (psi).

Example: You’ve been asked to calculate the compressive strength of a concrete cylinder. The cross-sectional faces of the cylinder each measure 6 inches across, and the cylinder failed at 71,000 pounds of force. What is the compressive strength of that sample of concrete?

To find the compressive strength of a concrete cylinder, you need to first calculate the surface area of one of the cylinder’s cross-sectional faces.

The surface area can be found using the formula for the area of a circle, which is pi multiplied by the radius squared. The radius of the circle is equal to half the diameter of the face, which in this case is 6 inches.

Once you have calculated the surface area, you can then use the formula for compressive strength, which is the force at which the cylinder failed divided by the surface area.

In this example, the force was 71,000 pounds and the surface area was 28.26 square inches, resulting in a compressive strength of 2,512 psi.

This value is consistent with the standard compressive strength of concrete used in residential construction.

## What Is The Compressive Strength Of Concrete At 28 Days?

Strength of concrete is generally tested after 28 days as concrete cube strength or concrete cylinder strength. The reason for testing concrete strength after 28 days is discussed.

## Why Do We Test Concrete Compressive Strength After 28 Days?

Concrete becomes stronger over time after it is poured and set. It takes an unknown amount of time for concrete to reach its full strength.

The rate at which concrete gains strength is higher in the first 28 days of being poured, and then the rate slows down.

The table below shows the percentage of strength that concrete gains at different ages, based on the grade of the concrete used:

Age: Strength (percent)

1 day: 16%

3 days: 40%

7 days: 65%

14 days: 90%

28 days: 99%

From the table, we can see that concrete gains 16% of its strength in one day, 40% in three days, 65% in seven days, 90% in 14 days, and 99% in 28 days.

This shows that concrete gains strength quickly in the first few days after being poured, with 90% of its strength achieved in just 14 days.

After 28 days, concrete continues to gain strength, but at a much slower rate compared to the first 28 days.

It is generally assumed that concrete reaches its final strength after about one year, although the exact time it takes is unknown.

In construction, the compressive strength of concrete is typically tested after 28 days, as this is considered to be close to the final strength of the material.

Rapid methods for testing the strength of concrete are also available, which can be used when time is limited and the strength of a structural element needs to be determined for further construction work.

## What Is The Normal Range Of Compressive Strength For Concrete?

Concrete is often used in construction projects, and its compressive strength typically ranges from 2500 to 4000 pounds per square inch (psi), or 17 to 28 megapascals (MPa).

In some cases, such as in residential and commercial buildings, the strength may be even higher. There are also certain applications that require concrete with a strength of more than 10,000 psi (70 MPa).

## Why Do We Need To Test The Concrete Compressive Strength At 7 14 And 28 Days?

Concrete typically reaches its maximum strength at 28 days, but due to the high cost and risk involved in construction, the strength of concrete can be tested at 7 and 14 days to predict its target strength at 28 days.

The strength of concrete is important in construction as it determines whether or not the concrete meets the necessary specifications and codes for different structures.

For example, the strength of first-floor columns in high-rise buildings is more critical than that of non-bearing walls.

If the required strength is not achieved, it can result in costly, dangerous repairs or even failure of the structure.

It is essential to consider the strength requirements when designing the mix for a construction project, as the proportions of the ingredients depend on the target strength.

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