Cubes vs. Cylinders: Mold Materials and Shapes

ASTM specifies non-shrink mortars be tested in a 2"x 2" metal cube ( ASTM C-109) instead of a 4"x 8" cylinder mold (ASTM C-39). Both test method specifications are testing compressive strength, but how is it decided whether to cast a cube specimen or a cylinder specimen?

Hydraulic mortars (including non-shrink grouts) are designed for precision load transfer applications, so they are designed with expansive properties. Non-shrink grouts are typically placed in confined forms where the expansive properties are limited to an upward direction. This results in maximum effective bearing area support and stronger physical properties. As a result of the non-shrink grout being restricted from expanding freely, it has now developed into a stronger and denser material.

Testing conditions that represent the application should produce representative test data. For example, if a non-shrink grout freely expanded in a non-restrictive mold material (such as plastic), the compressive strength test data would be low and would produce a less dense finished product. This could be a factor if "value-engineering" were used.

Traditional concrete is not designed with expansive properties. Therefore, a resistive mold material is not necessary. In fact, concrete will naturally undergo some shrinkage (generally about 1%) within the first 24 hours. Consequently, plastic or disposable cylinder molds (with dimensions of the hardened specimen that comply with ASTM C-39) are the standard mold material for concrete test specimens in use today.

In his book Properties of Concrete, Adam Neville says:

It is difficult to say which type of specimen, cylinder or cube is "better", but even in countries where cubes are the standard specimen, there seems to be a tendency, at least for research purposes to use cylinders rather than cubes. Cylinders are believed to give a greater uniformity of results for nominally similar specimens because their failure is less affected by the end restraint of the specimen; their strength is less influenced by the properties of the coarse aggregate used in the mix; and the stress distribution on horizontal planes in a cylinder is more uniform than on a specimen of square cross section. (594)

He goes on to say that cylinders are cast and tested in the same direction whereas for cubes the test is transverse.