Understanding Concrete Strength: Cube Testing, Procedure, and Results

Concrete serves as the backbone of construction projects, and its quality is pivotal. The compressive strength test on concrete cubes provides a comprehensive evaluation, ensuring that the concreting process meets the necessary standards. This test gauges the concrete’s ability to withstand loads without cracking or deflecting.

Factors Influencing Compressive Strength

Several factors influence the compressive strength of concrete, including the water-cement ratio, cement strength, quality of concrete materials, and the overall quality control during production.

Test Methods: Cube vs. Cylinder

The compressive strength test can be conducted on either a cube or a cylinder. Standard codes, such as ASTM C39/C39M by the American Society for Testing Materials, recommend cylindrical specimens for this evaluation.

What is Compressive Strength?

Compressive strength is the material or structure’s ability to withstand surface loads without cracking or deflecting. When subjected to compression, a material tends to reduce in size.

Compressive Strength Formula

The formula to calculate compressive strength is straightforward:

Compressive Strength=LoadCross-sectional AreaCompressive Strength=Cross-sectional AreaLoad​

Procedure for Concrete Cube Test

The concrete cube test involves specific steps to ensure accurate results. Here’s a detailed procedure:

Apparatus for Concrete Cube Test

• Compression testing machine

Preparation of Concrete Cube Specimen

• Use the same concrete proportion and materials as those used in the actual construction.
• Create six cubes of 15 cm size (Mix: M15 or above).

Mixing of Concrete for Cube Test

• Mix the cement, fine aggregate, and coarse aggregate until a uniform blend is achieved.
• Add water gradually to attain the desired consistency.

Sampling of Cubes for Test

• Clean molds and apply oil.
• Fill molds in layers, compacting each layer adequately.
• Level the top surface with a trowel.

Curing of Cubes

• Store specimens in moist air for 24 hours, followed by submersion in freshwater until the test.

Procedure for Concrete Cube Test

1. Remove the specimen from water after the curing period.
2. Measure the specimen’s dimensions.
3. Clean the testing machine’s bearing surface.
4. Place the specimen centrally on the base plate.
5. Apply the load gradually until failure.
6. Record the maximum load and any unusual features in the failure.

Calculations of Compressive Strength

• Size of the cube: $15\text{cm}\times 15\text{cm}\times 15\text{cm}$
• Area of the specimen: 225 cm2225cm2
• Characteristic compressive strength (fckfck​) at 7 days.

Similar calculations for 28-day compressive strength.

Reports of Cube Test

• Identification mark
• Date of test
• Age of specimen
• Curing conditions
• Appearance of fractured faces

Results of Concrete Cube Test

• Average compressive strength at 7 days and 28 days.

Compressive Strength at Various Ages

The strength of concrete increases over time. Here’s a comparative table:

Age	Strength Percent
1 day	16%
3 days	40%
7 days	65%
14 days	90%
28 days	99%

Compressive Strength of Different Concrete Grades

Grade	Minimum Compressive Strength (N/mm² at 7 days)	Specified Characteristic Compressive Strength (N/mm² at 28 days)
M15	10	15
M20	13.5	20
M25	17	25
M30	20	30
M35	23.5	35
M40	27	40
M45	30	45

In summary, understanding the compressive strength of concrete through cube testing is essential for ensuring the durability and reliability of construction projects. Regular testing, adherence to standards, and meticulous reporting contribute to the overall quality control in the construction industry.