Cylinder/Cube Strength Ratio: In-depth Data

This article gives in-depth information on the concrete cylinder/cube strength ratio. Concrete cube and cylinder specimens’ compressive strengths have a complex connection. They do not normally give the same strength, even when they are made from the same concrete.

There are multiple factors that affect this relationship between the concrete cube and cylinder strengths. The main factors are:

1. The method for casting, curing, and testing
2. Geometry of the specimen
3. Characteristic strength
4. Loading direction and machine properties
5. Aggregate grading

Let us talk about each one of these. First, it is important to define the ratio used in the comparison.

The ratio used in this article is:

K = cylinder compressive strength / cube compressive strength

A value of 0.80 is often used for a quick comparison, but it is not a universal conversion factor. The ratio generally changes with concrete strength and test conditions. A project-specific correlation is preferable when measured cube results must be compared with cylinder results.

Quick Cylinder/Cube Strength Comparison

Cylinder/Cube Strength Ratio Calculator

Use the calculator for a preliminary comparison. Select a user-entered ratio when a project or specification gives a value. The strength-dependent equation is included as a literature estimate, not as a replacement for a specified or experimentally established correlation.

Concrete Strength Class Reference Data

In a European concrete strength class written as C30/37, the first value is the characteristic cylinder strength and the second value is the characteristic cube strength. Therefore, the class itself indicates a ratio of 30/37 = 0.81. These values are class definitions; they should not be used to convert an individual measured result.

Factors Affecting the Cylinder/Cube Strength Ratio

The main factors affecting the cylinder/cube strength ratio are:

1. The method for casting, curing, and testing
2. Geometry and size of the specimen
3. Concrete compressive strength
4. Loading direction and testing-machine properties
5. Aggregate grading and maximum aggregate size

Let us talk about each one of these.

The Method for Casting, Curing, and Testing

The type of mould used to cast cylinder specimens can affect the cylinder/cube strength ratio. Cylinders can be cast in rigid or non-rigid moulds, whereas cubes are commonly produced in rigid steel or cast-iron moulds.

Cylinders made with some non-rigid moulds have been reported to give slightly lower strengths than cylinders made with rigid moulds. The capping or end-preparation method and the planeness of the bearing surfaces can also affect the cylinder result.

Care must be taken to ensure that mould and specimen tolerances are met. Otherwise, the measured cube strength, cylinder strength, and resulting ratio may be misleading. Specimen age, curing temperature, moisture condition, loading rate, and testing-machine alignment should also be controlled before two sets of results are compared.

Geometry of the Specimen

Several parts of specimen geometry affect a compressive-strength result:

1. The height-to-maximum-lateral-dimension ratio (h/d)
2. The lateral dimension (d)
3. The overall specimen volume
4. The cross-sectional shape

Height-to-Dimension Ratio (h/d)

A perfectly uniform uniaxial stress condition is not achieved in a normal compression test. Friction between the machine platens and the specimen restrains lateral expansion near the loaded ends. This creates a multiaxial state of stress that makes the restrained zones appear stronger.

For a standard cylinder with a height-to-diameter ratio of 2, the middle portion is less affected by platen restraint. In a cube, the restrained regions influence a greater portion of the specimen. For the same concrete, this is one reason why the cube result is commonly higher.

The following figure shows the general relationship between h/d and measured strength.

The strength ratio is more sensitive to h/d below about 1.5 than above it. Cubes can therefore be more sensitive than standard cylinders to small differences in specimen dimensions and end restraint.

Lateral Dimension and Specimen Volume

Concrete strength is not simply inversely proportional to d². Specimen size has a statistical and physical effect: a larger specimen has a greater probability of containing a weak region, while smaller specimens can show more scatter and a stronger influence from the mould wall and aggregate size.

The lateral dimension of a 150 × 300 mm cylinder and a 150 mm cube is the same, which helps when comparing these standard specimens. A 100 mm cube, however, is not geometrically identical to a 150 mm cube, and maximum aggregate size becomes more important.

A 150 × 300 mm cylinder has approximately 1.57 times the volume of a 150 mm cube. The specimen volume, end restraint, aspect ratio, and surface-to-volume ratio act together, so no single size correction should be applied without reference to the governing test standard.

Concrete Compressive Strength

Concrete strength itself affects the cylinder/cube strength ratio. For normal-strength concrete, the cylinder strength is often treated as approximately 0.80 of the cube strength. However, the ratio generally increases as strength increases and can approach 1.0 for very-high-strength concrete.

This is why one conversion factor cannot accurately cover every strength level. Neville reported the following strength-dependent expression, also discussed in an ACI Concrete International comparison:

K = 0.76 + 0.21 log₁₀(f_cyl / 20)

where K is the cylinder/cube ratio and f_cyl is the cylinder strength in MPa. It is an estimate based on published relationships. Experimental scatter can be considerable, so the equation should not replace project test data.

Direction of Loading and Machine Characteristics

Compression loads are normally applied in the direction of casting for cylinders and perpendicular to the direction of casting for cubes. Because both specimens are placed and consolidated in layers, the loading direction is another factor in their relationship.

Testing-machine stiffness, platen alignment, bearing-surface preparation, and load rate can change the measured result. When two laboratories or specimen types are compared, these testing details should not be ignored.

Aggregate Grading and the Wall Effect

Aggregate grading affects concrete strength in a structure and in a test specimen. In a small specimen, the ratio between the specimen dimension and nominal maximum aggregate size becomes especially important. Test standards therefore limit the maximum aggregate size permitted for a specimen.

Aggregate grading also affects the specimen through the wall effect, as shown below.

More space exists between aggregate particles and the mould wall than between particles within the concrete. This produces a mortar-rich zone near the wall. Because it is a surface effect, its influence becomes greater as the specimen surface-to-volume ratio increases.

The approximate surface-to-volume ratio is 0.033/mm for a 150 × 300 mm cylinder and 0.040/mm for a 150 mm cube. This suggests that the cube can be more sensitive to changes in aggregate grading and the wall effect.

Worked Conversion Examples

Example 1: Cube to Cylinder Using K = 0.80

For a measured or specified cube strength of 40 MPa:

Cylinder strength = 0.80 × 40 = 32 MPa

Example 2: Cylinder to Cube Using K = 0.80

For a cylinder strength of 30 MPa:

Cube strength = 30 / 0.80 = 37.5 MPa

These are arithmetic examples. Whether the conversion is allowed for design, compliance, or acceptance depends on the governing document and the basis of the specified strength.

When Not to Convert the Results

• Do not convert a failed acceptance result only to make it pass.
• Do not use a strength-class pair as a universal conversion for individual test results.
• Do not compare specimens of different age, curing condition, moisture condition, or loading rate as if geometry were the only variable.
• Do not apply the 0.80 rule automatically to high-strength concrete.
• Do not mix results from different specimen types in one statistical strength record unless the permitted correlation is established.

For one project, it is best to specify one specimen type and use it consistently. Where both types are unavoidable, cast paired specimens from the same batches and establish the relationship over the required strength range.

References

• Neville, A. M., Properties of Concrete, 5th Edition, sections on the influence of specimen type and size.
• ACI Concrete International, January 2024, discussion of cylinder-to-cube compressive-strength conversion.

Read Also:

Compressive Strength of Concrete Cube vs Cylinder

Compressive Strength Test of Concrete

Calculation of Concrete Target Mean Strength