The compressive strength of concrete is a fundamental property that plays a crucial role in determining the structural integrity and durability of concrete structures. It is a measure of the concrete's ability to withstand axial loads or forces that tend to squeeze or crush the material. Compressive strength is one of the key factors considered in the design and assessment of concrete structures, and it is vital in ensuring that concrete can support the applied loads without failing.

This piece of writing will cover all about the Compressive Strength of concrete. This topic is important for all the upcoming civil engineering examinations, including SSC JE CE and RRB JE Civil.

What is Compressive Strength?

Compressive strength is the ability of concrete to resist crushing or compression under a load. It is one of the most important mechanical properties of concrete, as it helps determine the load-carrying capacity of structures such as columns, beams, slabs, and foundations. The strength is measured in terms of the maximum stress the concrete specimen can withstand before failure. Concrete with higher compressive strength is more durable and suitable for heavy structures like bridges, dams, and multi-storey buildings.

How is Compressive Strength Measured?

The compressive strength of concrete is determined using a cube or cylinder that is tested in a compression testing machine (CTM). A standard-sized specimen is cast, cured under specific conditions, and then crushed in the CTM after a set period (usually 7 or 28 days). The load at failure is recorded and divided by the cross-sectional area to calculate the compressive strength. The standard IS code for this test is IS 516:1959.

To accurately test the compressive strength of concrete, standardized specimens are required. The shape and size of these specimens affect the result, as different geometries distribute stress differently during testing. In India, cubes are widely used, whereas cylinders are more common in countries like the USA. The choice depends on national standards and testing facilities. The table below shows the commonly used specimen types along with their standard dimensions and application areas:

Compressive Strength Test Procedure

Conducting a compressive strength test requires careful attention to the preparation and handling of the concrete specimen. Even minor errors like air voids, improper curing, or misalignment in the testing machine can significantly affect the results. That's why following the proper procedure as per IS 516:1959 is crucial.

Here’s a step-by-step explanation of how the test is conducted:

1. Sample Preparation: The fresh concrete is poured into moulds in layers and compacted properly to eliminate air pockets.
2. Curing: After 24 hours, the specimen is demoulded and submerged in water at 27°C ± 2°C to allow proper strength development.
3. Surface Cleaning: Before testing, the cube is removed from the curing tank and dried to remove excess surface moisture.
4. Testing: The cube is placed centrally in the compression testing machine. Load is applied at a controlled rate until the specimen fails.
5. Calculation: The maximum load at failure is divided by the cross-sectional area of the cube to get the compressive strength in MPa.

Formula to Calculate Compressive Strength

To calculate the compressive strength of concrete, you can use this basic formula:

Compressive Strength (in MPa) = Load at Failure (in Newton) ÷ Area of the Cube Face (in mm²)

For a standard cube of size 150 mm × 150 mm:

• The area = 150 × 150 = 22,500 mm²

Now, suppose the cube failed under a load of 500 kN (which is equal to 500,000 Newtons). Then:

Compressive Strength = 500,000 ÷ 22,500 = 22.22 MPa

This value shows how much compressive load the concrete can bear per square millimetre.

Factors Affecting Compressive Strength

Compressive strength depends on several interrelated factors. Even small changes in water content, curing time, or material properties can influence the result. For consistent quality and durability, all these factors must be well-controlled during construction. Below are the major factors that impact compressive strength:

• Water-Cement Ratio: A lower ratio usually results in stronger, denser concrete.
• Cement Quality: Higher-grade cement increases strength potential.
  Aggregate Properties: Clean, well-graded, and angular aggregates contribute to better bonding.
• Mix Design: Proper proportioning of ingredients ensures balanced strength and workability
• Curing Conditions: Poor curing reduces strength gain and surface hardness.
• Compaction: Inadequate vibration can leave voids, reducing overall strength.

Standard Curing Periods

Concrete doesn't achieve its full strength instantly. Strength gain is a gradual process, which is why specimens are tested at different curing durations. These periods help us understand both early-age and long-term performance of concrete under real conditions.

Here’s a table showing standard curing periods and their significance:

Compressive Strength Formula

Compressive strength formula for any material is the load applied at the point of failure to the cross-section area of the face on which the load was applied.

Compressive Strength = Load / Cross-sectional Area

Apparatus Required

Fig 1: Apparatus Required

How is Compressive Strength of Concrete Cubes Determined?

In the cube test, two types of specimens are employed: cubes measuring either 15cm x 15cm x 15cm or 10cm x 10cm x 10cm, depending on the aggregate size. Typically, 15cm x 15cm x 15cm cubical moulds are preferred for most applications.

The concrete is poured into the mould and thoroughly compacted to eliminate voids. After 24 hours, the moulds are removed, and the test specimens are placed in water for curing. The top surface of these specimens is levelled and smoothed by applying a layer of cement paste.

The compressive strength of hardened concrete is determined by using a compression testing machine after either seven days or 28 days of curing. The load is applied gradually at a rate of 140 kg/cm2 per minute until the specimens fail. The compressive strength of the concrete is determined by dividing the load at failure by the specimen's area.

Apparatus for Concrete Cube Test

Compression Testing Machine

A compression testing machine, often referred to as a compressive strength testing machine, is an essential apparatus used to determine the compressive strength of materials, particularly concrete and other construction materials. This machine applies axial loads to test specimens in a controlled and gradual manner to assess their ability to withstand compression forces without failing or deforming. It plays a crucial role in quality control and design in construction and material industries, ensuring that structures and materials meet the required strength standards.

Fig 2: Compression Testing Machine

Preparation of Concrete Cube Specimen

The concrete test specimens are made from the same concrete used in the construction field. For the cube test, six cubes of 15 cm size are prepared using a mix of M15 or above.

Mixing of Concrete for Cube Test

• Mix the cement and fine aggregate on a watertight, non-absorbent platform until the mixture is uniformly blended.
• Add the coarse aggregate and blend it with the cement and fine aggregate until it is evenly distributed.
• Add water and mix until the concrete appears homogeneous and reaches the desired consistency.

Sampling of Cubes for Test

• Clean the moulds and apply oil.
• Fill the moulds with concrete in layers approximately 5 cm thick.
• Compact each layer with a tamping rod, using not less than 35 strokes per layer.
• Level the top surface and smooth it with a trowel.

Curing of Cubes

The test specimens are stored in a moist environment for 24 hours, after which they are marked, removed from the moulds, and submerged in clear freshwater until shortly before the test.

Precautions for Tests

The curing water should be tested every 7 days, and its temperature must be maintained at 27±2°C.

Procedure for Concrete Cube Test

• Remove the specimen from the water after the specified curing time and wipe off excess water from the surface.
• Measure the dimensions of the specimen to the nearest 0.2 mm.
• Clean the bearing surface of the testing machine.
• Place the specimen in the machine so that the load will be applied to opposite sides of the cube.
• Align the specimen centrally on the base plate of the machine.
• Gently rotate the movable portion by hand until it touches the top surface of the specimen.
• Gradually apply the load without shock at a continuous rate of 140 \(kg/cm^2/minute\) until the specimen fails.
• Record the maximum load and note any unusual features in the type of failure.
• Note: A minimum of three specimens should be tested at each selected age. If the strength of any specimen varies by more than 15% from the average strength, the results of such specimens should be rejected. The average strength of the three specimens determines the crushing strength of the concrete. The concrete must meet the specified strength requirements.

Cube vs Cylinder Strength Tests

Concrete compressive strength can be measured using cube or cylinder specimens, depending on regional practices. While IS standards prescribe 150 mm cubes, many international codes like ASTM use 150 mm diameter × 300 mm height cylinders. The key difference lies in how failure occurs:

• Cube strength tends to be about 15% higher than cylinder strength due to the smaller height-to-width ratio, resulting in more confinement.
• Cylinder tests provide better insights into the material’s true compressive behavior under axial load.

Understanding this distinction is crucial in interpreting design values, especially when converting strengths across codes or comparing results globally.

Strength Development Over Time

The compressive strength of concrete does not develop instantly after casting. It increases gradually over time, influenced by the hydration of cement and curing conditions. Understanding this strength gain pattern is essential for determining safe periods for formwork removal, post-tensioning operations, and structural loading. Concrete typically gains about:

• 16% of its 28-day strength in 1 day
• 40% in 3 days
• 65% in 7 days
• 90% by 14 days
• 100% by 28 days (standard reference)

In many IS codes, the 28-day strength is taken as the benchmark. However, high early strength may be achieved through the use of admixtures or rapid-hardening cement, especially in precast or time-sensitive construction.

Factors Affecting the Compressive Strength of Concrete

• Water-Cement Ratio:  A lower water-cement ratio increases strength but reduces workability. Maintaining an optimal range (generally 0.4 to 0.6) ensures better bonding and durability.
• Quality of Cement: The grade, fineness, and freshness of cement directly impact the final strength. Higher-grade cement (like OPC 43/53) generally leads to higher compressive strength.
• Aggregate Properties: The size, shape, texture, and gradation of aggregates affect compaction and load distribution. Well-graded and clean aggregates improve strength.
• Mix Proportioning: The correct ratio of cement, fine aggregate, and coarse aggregate is essential. A well-balanced mix ensures density and reduces the risk of segregation.
• Compaction of Concrete: Proper compaction eliminates air voids and honeycombs, which enhances the overall strength. Mechanical vibrators are often used for this purpose.
• Curing Conditions: Adequate curing (usually 7 to 28 days) helps in proper hydration of cement. Lack of curing can lead to reduced strength and surface cracks.
  Use of Admixtures: Plasticizers, superplasticizers, and other chemical admixtures enhance workability and allow strength gains without increasing water content.
• Type of Cementitious Materials: Additions like fly ash, slag, or silica fume can improve long-term strength and durability when used in proper proportions.
• Temperature & Weather: High temperatures accelerate setting but may lead to early cracking. Cold weather slows hydration. Both extremes affect strength if not controlled.

Compressive Strength of Concrete Calculation

• Size of the cube =15cmx15cmx15cm
• Area of the specimen (calculated from the mean size of the specimen )=225 \(cm^2\)
• Characteristic compressive strength(f ck)at 7 days =
• Expected maximum load =fck x area x f.s
• Range to be selected is …………………..
• Similar calculation should be done for 28-day compressive strength
• Maximum load applied =……….tones = ………….N
• Compressive strength = (Load in N/ Area in \(mm^2\))=……………\(N/mm^2\)
• =……………………….\(N/mm^2\)

Compressive Strength of Concrete at Various Ages

Compressive Strength of Different Grades of Concrete at 7 and 28 Days

This blog enumerated all about Compressive strength of concrete and the procedure to determine it. This topic is important for your upcoming exams. If you are preparing for State and Central level Civil examinations and other diploma-level exams, get enrolled in AE/JE Civil Coaching on the Testbook App. Also, attempt a wide range of SSC JE Civil Mock Tests and SSC JE Civil Previous Year Papers available on the Testbook App itself.