Two main types of concrete specimens are used for compression testing. Cylinder specimens are used in the United States, Australia, Canada, France, and New Zealand. Cube specimens are used for compression tests throughout much of Europe, including Great Britain and Germany. The significant differences between the two procedures are twofold. First, cylinders may be cast in single-use or rigid metal molds, but cubes must be cast in rigid metal molds. Second, cylinder specimens must be capped to distribute the applied load evenly over the ends, but cubes need not be capped and are flipped on their sides for loading.
Numerous factors influence the cylinder/cube strength ratio, these five being the most important:
1. Testing procedure - type of cylinder mold and capping system, and planeness of cube surfaces.
2. Specimen geometry - ratio of height-to-maximum lateral dimension (h/d) is recognized as the most important geometrical factor, and is inversely related to specimen strength. Other factors are d and volume V.
3. Strength level- this is positively correlated with the cylinder/cube strength ratio.
4. Pattern of load distribution - fixity of the loading platens and loading direction for cubes perpendicular to the direction of casting are significant, for segregating concretes.
5. Aggregate Grading - changing aggregate grading affects cube strength more than cylinder strength, and increasing aggregate coarseness decreases the cylinder/cube strength ratio.
Strength test results have not been accurately converted between specimen types. Substantial variation exists among the numerous conversion factors and equations developed for this purpose. Although results vary among investigations, the cylinder/cube strength ratio is between 0.65 and 0.90 for 150 x 300 mm (6 x 12 in.) cylinders and 150 mm (6-in.) cubes.
In 1955, R. L'Hermite proposed a simple equation for the cylinder/cube strength ratio as a function of the cube strength:
Where fcu is cube strength in psi. Some strength ratios based on L'Hermite's equation are given in the following table: