Concrete compression testing is one of the most important tests in civil engineering because it tells us how strong concrete really is. This test checks whether concrete can safely carry loads in buildings, bridges, and other structures. The test is carried out using a Compression Testing Machine (CTM) in accordance with Standard IS: 14858(2000), the Indian Standard for concrete testing.
This article explains the complete process of concrete compression testing in a simple, step-by-step manner — starting from sample preparation and ending with the final test result.
A cube has flat square faces, which allows uniform load application. A cylinder has a circular cross-section and taller height, which causes stress to distribute differently. Because of this shape difference, cylinders usually give slightly lower compressive strength values than cubes, even when made from the same concrete.
The materials are mixed manually in a mixing tray or using a mixer. Proper mixing ensures that cement paste coats all aggregates evenly, which directly affects concrete strength.
A tamping rod works by manually compacting concrete through repeated strokes, while a vibrating table uses mechanical vibrations to settle concrete evenly. Proper compaction prevents honeycombing and weak zones inside the specimen.
As per IS standards, specimens are usually cured for 28 days. Proper curing helps concrete achieve its designed strength. Poor curing can significantly reduce compressive strength, even if the mix design is correct.
The specimen is then wiped clean and prepared for placement in the testing machine.
As load increases, the built-in load indicator continuously displays the applied force. This indicator may be digital or analog, but its function is to show the exact load acting on the specimen at any moment.
Compressive Strength = Failure Load / Cross-sectional Area
Area = 150 × 150 = 22,500 mm²
Compressive strength = 600,000 / 22,500 = 26.67 N/mm²
This result shows that the concrete strength is higher than M20 and close to M25 grade.
This article explains the complete process of concrete compression testing in a simple, step-by-step manner — starting from sample preparation and ending with the final test result.
Why Concrete Compression Testing Is Done ?
Concrete is strong in compression, but its strength depends on materials, mixing, compaction, and curing. Compression testing helps engineers verify that the concrete used at a construction site meets the required strength. Without Concrete Compression test, there is no reliable way to confirm concrete quality.Selection of Test Specimens (Cube and Cylinder)
Concrete strength is tested using standard specimens. In India, the most commonly used specimen is the 150 mm concrete cube, prepared using steel cube moulds. In some cases, concrete cylinders are also used, especially for comparison with international practices.A cube has flat square faces, which allows uniform load application. A cylinder has a circular cross-section and taller height, which causes stress to distribute differently. Because of this shape difference, cylinders usually give slightly lower compressive strength values than cubes, even when made from the same concrete.
Preparation of Concrete Samples
Concrete of two different grades, such as M20 and M30, is prepared to compare strength performance. The raw materials — cement, sand, aggregate, and water — are first measured using a weighing balance, which works by accurately measuring mass to ensure correct mix proportions.The materials are mixed manually in a mixing tray or using a mixer. Proper mixing ensures that cement paste coats all aggregates evenly, which directly affects concrete strength.
Filling and Compaction of Moulds
The fresh concrete is poured into cube or cylinder moulds in layers. These moulds are made of steel and maintain exact specimen dimensions. To remove trapped air and improve density, compaction is done using a tamping rod or a vibrating table.A tamping rod works by manually compacting concrete through repeated strokes, while a vibrating table uses mechanical vibrations to settle concrete evenly. Proper compaction prevents honeycombing and weak zones inside the specimen.
Initial Setting and Demoulding
After filling, the moulds are kept undisturbed for about 24 hours. During this time, the concrete begins to harden due to hydration of cement. Once initial strength is gained, the specimens are removed from the moulds using basic hand tools. This process is known as demoulding.
Curing of Concrete Specimens
Curing is one of the most critical steps in concrete testing. The demoulded specimens are placed in a curing tank filled with clean water. The curing tank maintains moisture and temperature, allowing cement hydration to continue.As per IS standards, specimens are usually cured for 28 days. Proper curing helps concrete achieve its designed strength. Poor curing can significantly reduce compressive strength, even if the mix design is correct.
Preparing for Compression Testing
Before testing, the dimensions of the cube or cylinder are measured using a vernier caliper or steel scale. These instruments work by providing precise measurements of length and area, which are needed for accurate strength calculation.The specimen is then wiped clean and prepared for placement in the testing machine.
Testing Using Compression Testing Machine (CTM)
The core of the process is the Compression Testing Machine (CTM). The concrete specimen is placed centrally between the CTM platens. The CTM works by applying a gradually increasing compressive load through a hydraulic system.As load increases, the built-in load indicator continuously displays the applied force. This indicator may be digital or analog, but its function is to show the exact load acting on the specimen at any moment.
Failure of Specimen and Load Recording
The load is applied until the concrete cube or cylinder cracks and fails. The maximum load shown on the load indicator at the moment of failure is recorded. This value represents the failure load, which is the key input for calculating compressive strength.Calculation of Compressive Strength
The compressive strength of concrete is calculated using a simple formula:Compressive Strength = Failure Load / Cross-sectional Area
Sample calculation:
If a 150 mm cube fails at 600 kN,Area = 150 × 150 = 22,500 mm²
Compressive strength = 600,000 / 22,500 = 26.67 N/mm²
This result shows that the concrete strength is higher than M20 and close to M25 grade.
