Compressive Strength Test of Concrete

Engineers commonly perform this test on concrete in construction because it gives a general idea about the characteristics of concrete. Based on this test and the applicable acceptance criteria, one can accept the concrete or decide that further evaluation is required.

Also, compressive strength as a concrete property depends on several factors related to the quality of the materials, mix design, sampling, curing, specimen preparation, and quality control during concrete production.

Compressive Strength Test Formula

Compressive strength = Maximum failure load / Loaded cross-sectional area
f_c = P / A

Concrete cube under compressive load showing the strength formula load divided by area — Compressive strength is the maximum failure load divided by the loaded area

When the load is measured in newtons and the area is measured in square millimetres, the result is N/mm², which is equal to MPa.

Cube Test or Cylinder Test?

Depending on the applied code, the test specimen may be a cylinder [150 mm diameter x 300 mm high is common] or a cube [150 mm x 150 mm x 150 mm is common]. For example, ASTM C39/C39M covers cylindrical concrete specimens. Cube testing follows the standard specified for the project.

Do not directly compare cube and cylinder results as if they are identical. Specimen shape, dimensions, end preparation, curing, and loading procedure affect the measured strength.

Relation Between Cube and Cylinder Strength

For normal-strength concrete, the cylinder result is commonly lower than the cube result. The shorter cube receives more restraint from the machine platens, while a cylinder has a larger unrestrained zone. However, there is no one conversion factor that is accurate for every concrete. The ratio changes with the concrete strength, specimen size, aggregate, end preparation, curing, and test procedure.

As a quick approximation, engineers sometimes take the cylinder strength as about 80% of the cube strength. Use this only as a rough comparison. For design or acceptance, use the strength class and specimen shape stated in the applicable code and project specification.

Concrete class	Characteristic cylinder strength	Characteristic cube strength
C20/25	20 MPa	25 MPa
C25/30	25 MPa	30 MPa
C30/37	30 MPa	37 MPa
C35/45	35 MPa	45 MPa
C40/50	40 MPa	50 MPa

For example, C25/30 means a characteristic cylinder strength of 25 MPa and a characteristic cube strength of 30 MPa. These are paired strength-class values. They are not a rule for converting one individual test result into the other.

Compressive Strength Test Idea

First, technicians sample the concrete, place it in the mould, and consolidate it according to the applicable standard.
Next, technicians remove the moulds after the initial curing period and store the specimens under the specified curing conditions.
Then, at the specified test age, the technician checks the dimensions and condition of each specimen.
After that, the technician centres the specimen in the compression testing machine and applies the load continuously at the rate required by the applicable test standard.
Finally, record the maximum load. Then, divide the failure load by the loaded cross-sectional area.

Apparatus

Compression testing machine with suitable capacity and current verification
Cube or cylinder moulds that meet the specified dimensions and tolerances
Sampling and mixing equipment, when specimens are prepared in a laboratory
Compaction equipment required by the applicable standard
Trowel, scoop, measuring tools, identification labels, and curing facilities

Sample Preparation

Technicians take samples from the concrete used on site or prepare them from a laboratory batch. The sampling method is important because the specimen must represent the concrete being evaluated.

Common specimen sizes: cylinders 150 mm x 300 mm or cubes 150 mm x 150 mm x 150 mm. However, the standard may permit other sizes according to the nominal maximum aggregate size.

The applicable standard and project specification determine the number of specimens, the ages of testing, and how individual results form one strength test. Therefore, do not assume that three specimens are required in every code.

Mixing Concrete

You may mix concrete in a laboratory batch mixer. When the standard permits hand mixing, first mix the cement and fine aggregate until the mixture is thoroughly blended. Then, add the coarse aggregate and mix until it is uniformly distributed. Finally, add water and mix until the concrete appears homogeneous and has the required consistency.

Placing Concrete in Moulds

Clean the moulds and apply the permitted release agent without leaving excess oil.
Fill and consolidate the concrete in the number of layers and by the method required by the standard. Do not use a fixed number of strokes from a different test method.
Finish the top surface and identify the specimen without damaging it.

Curing

Store the moulded specimens under the required initial-curing conditions. After demoulding, continue standard curing until the test age. Record any departure from the specified temperature, moisture condition, or timing because poor curing can reduce the measured strength.

Compressive Strength Test Procedure

First, remove the specimen from curing and wipe off surface water.
Next, confirm the specimen identification, age, dimensions, and end condition.
Then, clean the bearing faces of the specimen and testing machine.
After that, place and centre the specimen correctly. For a cube, apply load to the faces specified by the applicable standard. For a cylinder, use the required capped, ground, or unbonded end treatment.
Next, apply the load continuously and without shock at the test standard’s specified rate.
Finally, record the maximum load and note the failure pattern or any unusual condition.
Then, calculate and report the compressive strength using the measured loaded area.

Cube Compressive Strength Example

Assume a 150 mm concrete cube failed at a maximum load of 720 kN.

Item	Calculation	Result
Loaded area	150 x 150	22,500 mm²
Failure load	720 kN x 1,000	720,000 N
Compressive strength	720,000 / 22,500	32.0 N/mm² = 32.0 MPa

Cylinder Compressive Strength Example

Assume a concrete cylinder with an average measured diameter of 150 mm failed at a maximum load of 600 kN.

Item	Calculation	Result
Loaded area	π x 150² / 4	17,671 mm²
Failure load	600 kN x 1,000	600,000 N
Compressive strength	600,000 / 17,671	34.0 N/mm² = 34.0 MPa

The cylinder area is calculated from the measured average diameter, not from the nominal diameter when the test standard requires measured dimensions.

What to Record in the Test Report

Project, concrete mixture, sample, and specimn identification
Sampling, casting, and testing dates and the specimen age
Specimen type, nominal dimensions, and measured loaded area
Curing and moisture condition
End-preparation method for cylinders
Maximum load and calculated compressive strength
Failure pattern, defects, damage, or other unusual observations
Test method and any deviation from it

Sample Compressive Strength Test Report

The following is a simple example of how the results can be arranged. Keep cube and cylinder results in separate groups, and do not average the two specimen shapes together.

Project and Sample Information
Project	Office Building	Concrete location	First-floor columns
Mix identification	M-30-01	Specified age	28 days
Sampling date	4 May 2026	Testing date	1 June 2026

Specimen ID	Shape and size	Area	Failure load	Strength	Observation
CY-01	Cylinder, 150 x 300 mm	17,671 mm²	600 kN	34.0 MPa	No visible defect before testing
CY-02	Cylinder, 150 x 300 mm	17,671 mm²	618 kN	35.0 MPa	Fracture type recorded separately
CU-01	Cube, 150 x 150 x 150 mm	22,500 mm²	720 kN	32.0 MPa	No visible defect before testing
CU-02	Cube, 150 x 150 x 150 mm	22,500 mm²	742.5 kN	33.0 MPa	Failure pattern recorded separately

Report note: the numbers above show the calculation and report format only. A real laboratory report must include the test method, curing condition, machine identification, end preparation, measured dimensions, failure pattern, defects, deviations, and the acceptance criteria required by the project.

Common Errors That Can Reduce the Result

Poor or non-representative sampling
Insufficient or excessive consolidation
Loss of moisture or temperature outside the required curing range
Damaged specimens or uneven bearing surfaces
Incorrect centring or loading rate
Using the wrong specimen area or incorrect unit conversion

How to Interpret the Result

A single specimen result is not automatically the acceptance decision for the concrete. The specified code defines how specimens form one strength test and how to evaluate groups of tests. If a result is low, check the sampling, curing, specimen condition, testing records, and acceptance provisions before deciding what action is required.

Also, 7-day results are useful for monitoring early strength development, but they are not automatically a fixed percentage of the 28-day strength. Cement type, supplementary cementitious materials, water-to-cement ratio, temperature, and curing can change the strength-development rate.

References

Characteristic Compressive Strength vs Target Mean Strength