Compressive Strength of Concrete | Cube Test, Procedure, Apparatus & Results
Compressive Strength of Concrete?
The compressive strength of concrete refers to its capacity to withstand applied loads without experiencing cracking or deformation. When a material is subjected to compressive forces, it tends to decrease in size, whereas tensile forces cause elongation or expansion.
The compressive strength of concrete can be determined by dividing the maximum load applied to a concrete cube (with dimensions of 15x15x15 cm) at the point of failure by the cross-sectional area of the cube.
The compressive strength of concrete used in normal construction work typically ranges from 15 MPa (2200 psi) to 30 MPa (4400 psi) or even higher in commercial and industrial structures.
The strength of concrete is influenced by various factors including the water-cement ratio, the type and strength of cement used, the quality of concrete materials, and the implementation of quality control measures during the concrete production process. These factors play a crucial role in determining the final strength and durability of the concrete.
Cube Test of Concrete
The size of the aggregate used for making concrete can have an impact on its strength. For most concrete works, cubic molds with dimensions of 15cm x 15cm x 15cm are commonly used to test the compressive strength of concrete samples. The size and shape of the molds provide a standardized testing method for evaluating the strength characteristics of the concrete.
This concrete is filled in the mould and tempered properly so as to minimize any air voids available in concrete.
After 24 hours, the molds are opened, and the test specimens are carefully removed. These specimens are then placed in water for a specified period of time for curing. Curing is an essential process in which the concrete is kept in a moist environment to promote hydration and ensure proper strength development. By soaking the test specimens in water, the curing process is facilitated, allowing the concrete to gain strength and durability over time.
The upper surface of these specimens should be made even and smooth. To achieve this, a layer of cement paste is spread uniformly across the entire surface of the specimen. This ensures that the top surface is level and provides a consistent and smooth finish. The cement paste helps in creating a strong bond between the specimen and any subsequent testing apparatus.
These cubes are tested by a compression testing machine after 7 days of curing or 28 days of curing.
Why do we test concrete for 7 days, 14 days & 28 days?:
Concrete reaches its maximum strength after 28 days of curing. However, in the construction sector, where a significant amount of investment is involved, it is common practice to check the concrete strength at 7 and 14 days to predict the target strength of the construction work. By testing the concrete strength at these earlier time periods, engineers and contractors can assess the progress of the concrete’s strength development and make necessary adjustments to the construction schedule and design. This allows for better control and management of the construction project.
From the table below, it is evident that concrete gains approximately 16% of its target strength within 24 hours after casting. Furthermore, the concrete reaches about 65% of its target strength by the 7th day of curing.
Concrete continues to gain strength after 7 days of curing, reaching approximately 90% of its target strength by the 14th day. However, the rate of strength gain slows down significantly after this point. It takes a total of 28 days for the concrete to achieve approximately 99% of its ultimate strength. This is why the 28-day curing period is often used as a standard for assessing the strength of concrete in construction projects. It is important to note that the actual strength gain may vary depending on various factors such as mix design, curing conditions, and the specific type of concrete used.
Age in Days Percentage of Strength
1 Days 16%
3 Days 40%
7 Days 65%
14 Days 90%
21 Days 94%
28 Days 99%
Compressive Strength of Different Grades of Concrete at 7, 14, 21 and 28 Days:
|Concrete Grade||Compressive strength in N/mm² at 3 days||Compressive strength in N/mm² at 7 days||Compressive strength in N/mm² at 14 days||Compressive strength in N/mm² at 28 days|
Apparatus for Concrete Cube Test
Standard Compression testing machine
Cube mould of standard size 15 cm x 15 cm x 15 cm
Calculations of Compressive Strength
To check the compressive strength of concrete, follow these steps:
Prepare a concrete cube sample with dimensions of 15 cm x 15 cm x 15 cm.
Calculate the cross-sectional area of the cube: Area = length x width = 15 cm x 15 cm = 225 cm^2 (or 22500 mm^2).
Apply a gradually increasing load to the cube until it fails or fractures. Record the maximum load applied, which in this case is 400 kN (400,000 N).
Calculate the compressive strength using the formula: Compressive strength = Maximum load / Cross-sectional area.
Compressive strength = 400,000 N / 22,500 mm^2 = 17.77 N/mm^2.
This calculation provides the compressive strength of the concrete cube, which in this case is 17.77 N/mm^2. Remember to repeat the testing process for multiple cubes to ensure accuracy and reliability of the results.