Concrete’s compressive strength is one of the key characteristics that makes it such a versatile material.
This strength is measured in psi, or pounds per square inch, and it’s an indication of how much weight a given amount of concrete can support before it starts to crack or break.
The higher the compressive strength, the more load-bearing capacity the concrete has. In simple terms, it’s a measure of how hard the concrete is.
Concrete compressive strength for general construction ranges from 22 N/mm2 to 25 N/mm2 or 15 MPa to 30 MPa or 2200 psi to 4400 psi, where it goes higher in industrial and commercial structures.
The compressive strength of concrete is determined by testing cylindrical specimens in a laboratory environment.
The cylinders are filled with freshly-mixed concrete and then pressed from both ends with an increasing force until they fracture. This high pressure causes stress on the material, which lets us calculate its compressive strength.
The most common type of concrete is referred to as “normal weight” concrete, which has a compressive strength in the range of 2,500-5,000 psi.
Other types can have higher or lower strengths depending on the mix design and other factors. The type of aggregate used during mixing also affects the compressive strength of the concrete, with higher-strength aggregates creating a stronger material.
The Role of Understanding Concrete’s Compressive Strength
Concrete’s compressive strength plays an important role in construction projects, and it can be used to guide decisions about the type of concrete that should be used for particular applications.
For example, a driveway or sidewalk might require one type of concrete with a lower compressive strength, while the columns of a building might require concrete with a much higher compressive strength.
Understanding what the compressive strength of concrete is can also help architects and engineers plan for potential problems such as cracking or inadequate load-bearing capacity.
By choosing the right type and mix design for each application, they can ensure that the concrete will meet their project’s demands.
In general, the compressive strength of concrete is defined as the resistance to failure under the decided force. It is the maximum load applied at any cross-sectional area. You can calculate the compressive strength of concrete by using the formula below:
F = P / A
Where,
F is the compressive strength
P is the maximum load applied in Newton
A is the cross-sectional area
Generally, the concrete mix may be designed to get the required durability and engineering properties. Therefore, testing the compressive strength after 28 days of pouring concrete is pretty necessary.
Some of the other engineering properties of hardened concrete involve density, tensile strength, creep coefficients, elastic modulus, coefficients of thermal expansion, etc.
What Is The Reason For Testing Concrete Compressive Strength After 28 Days?
Concrete gains strength with time as it matures. The concrete will gain strength for 28 days, which is considered the reference age of concrete. Since concrete’s strength doesn’t stay constant, it is tested at certain ages to determine its strength. For most concrete structures, the 28-day compressive strength is used as an indication of when a building can be put into use.
The compressive strength of the concrete also affects how well it will resist other environmental forces like temperature changes and water damage. Testing for this value helps ensure that concrete will hold up well in different conditions.
The table below lists the compressive strength of concrete per time period.
| Days After Casting | Compressive Strength Gained |
|---|---|
| One day | 16% |
| Three Days | 40% |
| Seven Days | 65% |
| 14 Days | 90% |
| 28 Days | 99% |
Compressive Strength Gained After Casting
As per the table, you can observe that concrete gains around 16 percent strength in one day of pouring, 40 percent in three days, 65 percent in seven days, 90 percent in 14 days, and about 99 percent in 28 days.
Hence it is clear that concrete strength gains faster before 28 days, and the strength will gradually increase between 28 to 90 days.
This is one of the major reasons why we use the 28-day benchmark when testing for the compressive strength of concrete.
It is also important to note that the 28-day compressive strength test should only be used as a reference and not as a general indication of when a structure can be put into use. Proper engineering calculations should also be done before using any concrete for construction purposes.
Calculation Of Concrete Compressive Strength
The basic formula used to evaluate the compressive strength of concrete is:
Compressive strength F = load at failure P / Cross-sectional area A
The test is conducted using a compression testing machine, and the failure load is recorded as the compressive strength of concrete. If you are looking to measure the compressive strength of concrete in everyday life, you can make use of a rebound hammer or ultrasound pulse velocity (UPV) meter.
These tools are relatively inexpensive but provide accurate results. The rebound hammer test gives an estimate of the compressive strength by measuring the surface hardness of concrete.
The UPV method measures the rate at which ultrasonic pulses pass through a concrete sample and can be used to estimate compressive strength with accuracy.
What Are The Factors Affecting Compressive Strength Gain?
Below is a list of some critical factors that impact the rate of compressive strength gain:
Size Of Aggregates
Concrete is made by combining cement, aggregates, water, sand, and other ingredients. But even when properly mixed, there are some instances that micro cracks can happen.
The size of the aggregates must be taken into consideration to reduce this risk. These cracks develop because of the thermal and mechanical properties of cement and aggregate mix, leading to concrete failure.
Concrete experts recommend that the maximum aggregate size should be kept to one-fourth of the minimum plan dimension of the member. This reduces fracturing and increases compressive strength gain.
Mix Design
The mix design plays a vital role in determining the strength of concrete. A proper combination of cement, sand, water and other additives needs to be taken into account for maximum compressive strength development.
Air Entrainment
Concrete air entrainment was one of the concepts developed by cold nations to prevent damage because of thawing and freezing. Moreover, experiments show the multidimensional benefits of air entrainment with the improved workability of concrete at a low ratio of water to cement.
The needed workability at low water contents helped acquire the high strength of concrete, which leads to light concrete with great compressive strength.
Water To Cement Ratio
Everybody might know about the danger of excess water to concrete strength. Cement plays a significant role as a binding material in concrete that requires water for the hydration procedure but is limited to around 0.20% to 0.25% of cement content.
Excess water turns out to be beneficial in contributing to the workability and finishing of concrete.
Excess water is considered dangerous because the drying of water in a concrete matrix leaves large spaces between cement and aggregate. This interstitial space becomes primary cracks during the test of concrete strength.
You can check the compressive strength of concrete as per their grades in 3, 7, 14, and 28 days respectively in the table below:
| Concrete Grades | 3 days strength | 7 days strength | 14 days strength | 28 days strength |
|---|---|---|---|---|
| M 10 | 4 | 6.5 | 9 | 10 |
| M 15 | 6 | 9.75 | 13.5 | 15 |
| M 20 | 8 | 13 | 18 | 20 |
| M 25 | 10 | 16.25 | 22.5 | 25 |
| M 30 | 12 | 19.5 | 27 | 30 |
| M 35 | 14 | 22.75 | 31.5 | 35 |
| M 40 | 16 | 26 | 36 | 40 |
| M 45 | 18 | 29.25 | 40.5 | 45 |
| M 50 | 20 | 32.5 | 45 | 50 |
Concrete Compressive Strength: Final Thoughts
In the construction industry, the compressive strength of concrete is one of the most important parameters used to evaluate performance and quality. It is also a key factor in determining the structural integrity of any structure.
When it comes to designing concrete structures, engineers have to consider various factors that can impact compressive strength, such as mix design, water-cement ratio, air entrainment, and size of aggregates.
Using the tables and data provided above, you will be able to evaluate the compressive strength of your concrete samples with accuracy and determine whether they meet industry standards.
The compressive strength test is a reliable and cost-effective way to ensure that your structure is strong and durable. Use it to achieve optimal results in all your building projects.
