Before starting any type of concrete construction, you must ensure the capacity of the concrete to withstand different types of loads. Compressive strength is one of the important factors influencing the structural strength and load-bearing capacity of concrete in construction.
In today’s blog on Brick & Bolt, you will understand the different methods used and factors influencing the compressive strength of concrete.
What is the Compressive Strength of Concrete?
The compressive strength of concrete indicates the maximum load a concrete sample can endure without failing. It is generally measured in megapascals (MPa). Before designing any concrete structure, you must consider the compressive strength of the concrete. It depends upon the concrete’s mix proportions, curing practices, and the properties of its ingredient materials. Higher compressive strength indicates greater durability and resistance to different types of stresses. Commonly, the compressive strength of concrete after 28 days is considered standard. This time is essential for the concrete to achieve its essential strength.
While high strength is crucial, it’s important to understand the difference between high-strength concrete and high-performance concrete to choose the right type for your project.
Testing Methods for Concrete Compressive Strength
Every country has its own set of regulations that must be adhered to, and concrete testing techniques vary from place to place. You can test the compressive strength of concrete samples with destructive or non-destructive methods.
The following are the destructive compressive strength testing methods:
- Cube Test
- Cylinder Test
- Concrete core testing
1. Cube Test
Concrete cube testing is one of the most common concrete strength test methodologies for measuring compressive strength. Concrete samples are prepared by pouring the concrete mix into cubes, normally with dimensions of 150 mm x 150 mm x 150 mm.
The concrete will gain complete strength after 28 days of pouring. The test will be conducted for 7, 14, and 28 days. After 7 days, it should have gained 65% of its strength and 90% after 14 days. After the standard curing period of 28 days, these cubes are subjected to axial loading in a compression testing machine until failure takes place. The force at which the cube fails, divided by its cross-sectional area, gives the compressive strength. While compressive strength is crucial, it’s not the only measure of concrete’s performance. Another important property is flexural strength of concrete, which measures its ability to resist bending under load. Understanding both compressive and flexural strength is essential for comprehensive concrete design and quality control.
2. Cylinder Test
Similar to the cube test, the cylinder test also involves the casting process of concrete into cylindrical molds, usually 150 mm in diameter and 300 mm in height. Cylinders should have a length-to-diameter ratio of 2:1 and a diameter three times the coarse aggregate’s maximum nominal size. After curing, the cylinders are tested for compressive strength. This concrete strength test is commonly preferred for its statistical reliability and also for selected methods for longer and more slender structures, offering an accurate representation of the concrete’s properties when placed in situ.
3. Concrete Core Testing
Core testing involves drilling sites in existing concrete structures, taking multiple cylindrical samples of the concrete that are then examined and tested in a laboratory. Testing the concrete for strength in this way is mandatory to verify the integrity of the concrete, particularly if earlier laboratory tests indicated doubts about its compressive strength or overall quality. Even if core testing provides reliable and accurate data, it is also invasive and will need careful handling to prevent related damages.
Non-Destructive Testing Methods
Non-destructive testing (NDT) methods allow engineers to test the strength of concrete without damaging structures, effectively identifying potential quality issues.
Penetration Method
The penetration method employs a device like the Windsor probe, which drives a hardened steel probe into the concrete using a powder-actuated charge. The depth of penetration correlates inversely with the concrete’s compressive strength; thus, deeper penetrations indicate lower strength. This method is comparatively quick and requires minimal setup.
Rebound Hammer Test
The rebound hammer is also known as the Schmidt hammer. It measures the surface hardness of concrete by using a spring-driven mass to strike the concrete and then record the rebound energy. The rebound value shows a good correlation with compressive strength, which makes this method very beneficial for large-scale evaluations of concrete surfaces. Although the rebound hammer test is simple, it requires careful correlation with cylinder test results for accurate strength estimations.
Pull Out Test Method
The pull-out test involves inserting a metal disk into the surface of the concrete and applying a tensile load until the disk is extracted. The force needed to extract the disk gives an estimate of the concrete’s compressive strength. This technique is helpful for evaluating early-age concrete strength and can be performed in situ without large-scale sampling.
Ultrasonic Pulse Velocity Method
Ultrasonic pulse velocity (UPV) testing is a versatile test. It uses ultrasonic waves to distinguish internal defects and assess the quality of concrete. Engineers use this method to measure how long the sound waves take to travel through a patch of concrete as an indicator, not just of its density but also – indirectly — of its compressive strength. The detection of defects such as voids and different types of cracks in UPV is more useful for maintaining quality.
Radioactive Methods
It is one of the major non-destructive tests. Tests with radioactivity encourage the identification of an in-situ density test to determine a successful compressive strength. This test is useful for detecting the position of reinforcement, measuring density, and detecting honeycombing in concrete components. It uses very simple equipment, and, except for the cost of purchase, it can be relatively cheap to run. You can easily test the concrete samples of up to 18 inches. This method is best for high-risk, difficult-to-reach locations, but regulatory and safety restrictions might limit the usage.
Factors Affecting Compressive Strength of Concrete
Several factors significantly impact the compressive strength of concrete, emphasising the importance of careful material selection and curing practices.
Water-Cement Ratio
Compressive strength is most sensitive to changes in the water-cement ratio. Concrete will become hard when the water is less, and if it is more, porosity will increase, reducing compactness and hence reducing density, ultimately decreasing strength. Typically, the target ratio for optimal concrete strength is between 0.4 and 0.6.
Aggregate Quality and Size
Aggregates should be of good quality and size to help in creating a proper mix of concrete that will lead to higher compressive strength. Good high-quality aggregates, appropriate grading, and perfect cohesion give higher capability for load distribution, resulting in improved strength. Proper compaction of concrete is also crucial to ensure optimal aggregate distribution and minimisation of voids. On the other hand, low grades of aggregates or inadequate compaction will result in considerable deficiencies in the concrete matrix.
Curing Conditions
Good concrete curing is important to make concrete gain its maximum strength. Curing conditions such as proper moisture and temperature ensure proper hydration, which in turn results in the strength of concrete. Insufficient curing allows the surface of cured material to be dry and cracked, which affects its long-term service performance.
Age of Concrete
Concrete strength matures with age, and test samples may produce extremely misleading results if not tested at the appropriate time. Although concrete typically gains most of its strength within 28 days, it continues to get stronger for months or years under the proper conditions.
Admixtures and Supplementary Cementitious Materials
Using admixtures in concrete, such as superplasticisers or fly ash, allows for the improvement of concrete properties, such as compressive strength. They can improve workability, permeability reduction, and proper hydration, consequently producing better structural performance.
Generally, understanding and evaluating the compressive strength of concrete includes thoughtful consideration of different testing methods and factors. The careful selection of appropriate tests—whether traditional or non-destructive—for a thorough understanding of critical variables such as water-cement ratio, aggregate properties, and curing conditions ensures the durability and safety of concrete structures. By prioritising the factors that affect compressive strength, engineers and construction professionals can effectively mitigate risks and enhance the longevity of their projects. For construction services in Hyderabad, consider Brick & Bolt, a reputable firm with a track record of delivering high-quality concrete structures.