The compressive strength of concrete is an essential factor in the safety, durability, and performance of concrete structures. Proper testing and understanding of this property help engineers design more reliable and long-lasting buildings, roads, and infrastructure. By ensuring that the compressive strength of concrete meets the required standards, we can confidently build structures that will stand the test of time.
Compressive Strength of Concrete: Definition, Importance, and Testing Procedure
Concrete is one of the most widely used construction materials worldwide, and its compressive strength is a key factor in determining the safety and durability of structures. This property refers to the ability of concrete to withstand compressive forces—an essential aspect for ensuring that concrete structures can bear loads effectively without failure.
In this guide, we will explore what compressive strength is, why it matters, and the testing procedures used to measure it. Also, we will discuss the factors that affect the compressive strength of concrete and the standards that guide testing practices.
How is Compressive Strength Measured?
To determine the compressive strength of concrete, standardized test specimens are used, usually in the form of cubes or cylinders. These specimens are prepared in molds and cured under controlled conditions. Once the curing process is complete, the specimen is placed in a compression testing machine, which applies an increasing load until the specimen fails.
What is Compressive Strength of Concrete?
Compressive strength of concrete refers to its ability to resist compressive loads. Essentially, it determines how much pressure the material can endure before failing. This property is crucial for assessing the load-bearing capacity of concrete, which in turn influences the design and safety of concrete structures, such as buildings, bridges, and pavements.
Compressive strength is typically measured in pounds per square inch (psi), Newton per mm² (N/mm²), or megapascals (MPa), depending on the system of units used.
The compressive strength is calculated by dividing the maximum load (at failure) by the cross-sectional area of the specimen.
Why is Compressive Strength Important?
The compressive strength of concrete is a critical factor in the design of any concrete structure. By testing the specimens only we can get many detailed information about the concrete used in the structures at very low cost. Since, the structures are designed to carry specific loads, and concrete must have the strength to bear these without cracking or collapsing. Compressive strength not only ensures that a structure is safe but also influences its durability.
For concrete exposed to harsh environmental conditions—like seawater, freeze-thaw cycles, or deicing salts—the higher the compressive strength, the better the structure will resist degradation over time.
Test Procedure for Compressive Strength of Concrete
Here is a basic overview of the testing procedure used to measure concrete’s compressive strength:
Casting the Specimens:
Concrete specimens are typically cast in casting moulds in the form of cubes (150 mm x 150 mm x 150 mm) or cylinders (150 mm in diameter and 300 mm in height) moulds. After 24 hrs the specimens are removed from casting moulds.
Curing the Specimens:
Curing is essential to allow the concrete to achieve its desired strength. Specimens are typically cured for 7 or 28 days in a laboratory setting under controlled temperature and moisture conditions.
Testing the Specimens:
After curing, surface moisture is removed and each specimen is placed in a compression-testing machine, where the load is applied until failure occurs. The maximum load the specimen can withstand is recorded.
Calculating Compressive Strength:
Factors Influencing Compressive Strength of Concrete
Several factors influence the compressive strength of concrete, including:
Water-to-Cement Ratio: A lower water-to-cement ratio generally results in stronger concrete.
Type of Cement: Different types of cement can have varying compressive strengths.
Aggregate Type: Strong, well-graded aggregates contribute to higher compressive strength.
Curing Conditions: Proper curing in moist conditions is essential for maximizing compressive strength. Insufficient curing may result in weak concrete.
Compressive Strength of Hardened Concrete as Per IS:516 (Part-1)
Test Specimens
- The test specimen shall be a cube or a cylinder, meeting the requirements of IS: 1199 (Part-5) and IS:516 (Part-4) for concrete cores. Cylindrical specimens cast in the lab or cores extracted from the structures, shall be capped as per 6.6 of IS 1199 (Part-5).
- Damaged specimens shall not be tested. Specimens that are badly honeycombed shall not be regarded as being representative of the quality of concrete supplied.In general, standard cube and cylinder specimens shall not be tested if they are badly honeycombed as this is an indication of poor specimen making. When such specimens are tested, the test report shall include the fact that the specimen was honeycombed.
Apparatus for Test
- The test shall be carried out using a compression testing machine conforming to IS 14858. The test machine shall be in calibration at the time of test. The calibration shall be carried out at least once per year.
Age at Test
- Tests shall be made at recognized ages of the test specimens, the most usual being 7 and 28 days. Ages of 56 days, 90 days and one year are recommended if tests at greater ages are required. Where it may be necessary to obtain the early strengths, tests may be made at the ages of 24 h ± 30 min and 72 h ± 2 h. The ages shall be calculated from the time of the addition of water to the dry ingredients.
Number of Specimens
- At least three specimens shall be tested at each selected age.
Test Procedure
- Preparation and Positioning of Specimens For specimens stored in water, excess moisture shall be wiped from the surface of the specimen before placing them in the testing machine. The dimensions of the specimens to the nearest 0.2 mm and their weight shall be noted before testing.The time between the extraction of the specimen from the curing tank, and the testing, shall be as short as possible or not more than 2 hours. During the time the specimen is outside the curing tank, it shall be protected from drying, may be by covering with wet cloth. All testing machine bearing surfaces shall be wiped clean and any loose grit or other extraneous material removed from the surfaces of the specimen that will be in contact with the platens.
- Do not use packing, other than auxiliary platens or spacing blocks, between the specimen and the platens of the testing machine. Cube specimens shall be compressed perpendicularly to the direction of casting.The specimen shall be centered on the lower platen to an accuracy of 1 percent of the designated size of cubic,or diameter of cylindrical specimens. Where physical means of ensuring centering are provided on the testing machine and they are in calibration, these shall be deemed to satisfy the requirements for accuracy of centering. If auxiliary platens are used, the top auxiliary platen shall be aligned with the top of the specimen. With two-column testing machines, cube specimens shall be placed with the troweled surface facing a column.
Loading
- The load shall be applied without shock and shall be increased continuously at a constant rate of 14 N/mm²/min until no greater load can be sustained.The maximum load indicated shall be recorded.
Assessment of Type of Failure
- For cube specimens, if the failure is satisfactory (see Fig. 1), this fact shall be recorded. If the failure pattern is unsatisfactory, this fact shall be recorded andthe type of failure recorded using the pattern number inFig. 2 closest to that observed. For cylindrical specimens, if the failure is satisfactory( see Fig. 3), this fact shall be recorded. If the failure pattern is unsatisfactory, this fact shall be recorded and the type of failure recorded using the pattern letter in Fig. 4 closest to that observed.
Test Results of the compressive strength is given by the equation
fc= F / Ac
Where,
fc = Compressive strength, in MPa;
F= Maximum load, in N; and
Ac = Cross-sectional area, in mm², of the specimen on which the compressive force acts.
Testing and Reporting Compressive Strength of Concrete Cube
Average of three values shall be taken as the representative of the batch provided the individual variation is not more than ± 15 percent of the average. Otherwise repeat test shall be made, however if there is no further sample, then the average of two closest values may be taken as the average result. The actual dimensions of test specimens shall conform to IS 10086. If the actual dimensions are within the tolerance limits as mentioned in IS 10086, the strength may be calculated on the basis of designated size. If the actual dimensions are outside this tolerance, the strength calculation shall be based on the actual dimensions of the test specimen, however, perpendicularity of the surface of specimens should be maintained as per IS 10086.The compressive strength shall be expressed to the nearest 0.5 MPa.
The following information shall be included in the report:
a) Details of the concrete like grade, mix details, etc, in case of cast specimens; and details of the structure,like structure type, origin member/structure, in case of cored specimen;
b) Type of specimen: cast (cube/cylinder) or drilled core;
c) Size of the specimen, and capping details, if applicable;
d) Identification mark;
e) Age of specimen;
f) Date of test;
g) Curing conditions;
h) Weight of specimen;
j) Dimension of specimen;
k) Maximum load;
m) Details of the machine used for testing(manual/automated, loading range, date of calibration, etc);
n) Compressive strength of specimen (to the nearest 0.5 MPa); and
p) Type of failure (satisfactory or unsatisfactory and,if unsatisfactory, the nearest type)
FAQs on Compressive Strength of Concrete
1. What is compressive strength in concrete?
Compressive strength refers to the capacity of concrete to withstand axial loads that push it together, measured by the maximum force it can resist per unit area before failing. It’s a crucial factor in determining the strength and durability of concrete structures such as buildings, bridges, and roads.
2. How is compressive strength of concrete measured?
The compressive strength of concrete is measured using a compression testing machine. Concrete specimens, typically cubes or cylinders, are subjected to increasing compressive loads until failure occurs. The maximum load sustained by the specimen is recorded and used to calculate the compressive strength using the formula:
Compressive Strength = Load (N)/Cross Sectional Area (mm²)
3. Why is compressive strength important in construction?
Compressive strength is vital because it ensures concrete structures can safely withstand the loads they will bear over time. Whether it's a sidewalk or a high-rise building, the compressive strength directly influences the load-bearing capacity and durability of the structure. High-strength concrete is essential for heavy-load applications, such as bridges, skyscrapers, and foundations.
4. What factors affect the compressive strength of concrete?
Several factors influence the compressive strength of concrete:
- Water-to-Cement Ratio: A lower ratio results in stronger concrete.
- Type of Cement: The choice of cement, such as Portland cement, impacts strength.
- Aggregate Quality: Well-graded, strong aggregates contribute to higher strength.
- Curing Conditions: Proper curing in moist conditions enhances strength.
- Mix Proportions: The exact ratio of ingredients (cement, water, aggregates) is crucial.
5. What is the ideal curing time for concrete to achieve maximum compressive strength?
Concrete generally achieves most of its strength within the first 28 days of curing. Standard testing for compressive strength is done at 7 and 28 days to gauge early and full strength development. However, some concrete can continue to gain strength for up to 90 days or even longer with extended curing.
6. How can the compressive strength of concrete be improved?
To enhance concrete’s compressive strength:
Optimize the mix design: Adjust the water-to-cement ratio for a denser mix.
Use high-quality aggregates: Ensure aggregates are clean and well-graded.
Proper curing: Maintain consistent moisture levels during curing to prevent premature drying.
Incorporate admixtures: Use chemical additives such as superplasticizers or silica fume for stronger concrete.
7. What is the difference between compressive strength and tensile strength of concrete?
Compressive strength measures a material's ability to withstand compression (being crushed), while tensile strength refers to the material’s ability to resist tension (being stretched or pulled). Concrete generally has high compressive strength but low tensile strength, which is why reinforcement with steel (rebar) is used to improve its overall strength.
8. What are the common testing methods for concrete compressive strength?
The most common method for testing compressive strength involves the cube test or cylinder test. Concrete specimens are cured, and then a load is applied to the specimen until failure. The compression-testing machine measures the maximum load sustained, which is then used to calculate the strength. The tests are usually conducted at 7, 28, or even 56 days.
9. How is the compressive strength test result interpreted?
The test result, typically reported in megapascals (MPa) or pounds per square inch (psi), indicates the maximum load per unit area the specimen can endure. To ensure consistency and reliability, the average compressive strength from multiple specimens (usually three) is considered. The result should be within ±15% variation for accuracy.
10. Can the compressive strength of concrete be affected by temperature?
Yes, the temperature during the curing process can significantly impact the compressive strength of concrete. High temperatures accelerate hydration and may lead to quicker strength development, but they can also cause cracking if not managed. Conversely, low temperatures can slow down hydration, leading to a longer curing period and potentially lower strength.
11. What is Compressive Strength in Concrete as per IS Code?
Compressive strength of concrete is a key property that defines the load-bearing capacity of the material under compression. According to the IS 456: 2000 code, compressive strength is one of the primary parameters used to assess the quality and performance of concrete in structural applications. The IS code outlines guidelines for testing concrete specimens and establishing appropriate strength values based on age and curing conditions.
12. What is the Compressive Strength of Concrete at 7 Days, 14 Days, and 28 Days?
The compressive strength of concrete increases over time as it undergoes hydration. The standard testing days are:
7 days: Provides an early indicator of strength development.
14 days: Used to estimate the strength progress before full curing.
28 days: This is the standard age for assessing concrete's full compressive strength, commonly used for reporting and comparison in structural design.
At 28 days, concrete typically achieves about 70-80% of its final strength, with the remaining strength developing over time.
13. What is the Compressive Strength of Concrete in N/mm²?
The compressive strength of concrete is typically measured in Newtons per millimeter squared (N/mm²) or megapascals (MPa). For example:
M20 grade concrete typically has a compressive strength of 20 N/mm² (MPa).
M25 grade concrete typically has a compressive strength of 25 N/mm² (MPa).
These values may vary based on mix design, curing conditions, and other factors.
14. Where Can I Find Compressive Strength of Concrete in PDF Format?
The compressive strength of concrete and related testing standards can be found in official IS code documents. For a detailed breakdown of the compressive strength values, test methods, and guidelines, you can refer to PDFs of IS 456: 2000 (for general concrete design) and IS 516 (for concrete testing methods). These documents can often be accessed from official government or engineering sites like the Bureau of Indian Standards (BIS).
15. What is the Compressive Strength of Concrete at 7 Days, 14 Days, and 28 Days PDF?
A PDF version of the compressive strength of concrete at different ages can be found in various concrete testing guides. Generally, a 7-day strength test gives an early assessment, while the 14-day and 28-day tests are more reliable. Specific standard values are outlined in IS 456 and IS 516.
For example, IS 456 specifies that concrete should achieve at least 60% of its 28-day strength at 7 days and about 80% at 14 days for typical mix designs.
16. What is the Compressive Strength of Concrete After 7 Days as per IS 456?
IS 456:2000 recommends that 7-day strength should be at least 60% of the characteristic compressive strength achieved at 28 days. For instance, if the characteristic strength of concrete is 25 MPa (M25 grade), the concrete should have a compressive strength of approximately 15 MPa at 7 days.
17. What is the Compressive Strength of Brick?
While concrete is typically used for structural applications, bricks also need to have certain strength properties. The compressive strength of bricks varies depending on the type of brick:
Clay bricks usually have a compressive strength of 3.5 to 12.5 MPa (507.63 psi to 1812.98 psi).
Concrete bricks can range from 10 MPa to 40 MPa, depending on their intended use.
Bricks are generally tested in a similar manner to concrete, using a compression testing machine, but they are assessed for their ability to withstand localized pressure, especially in masonry applications.
