I've heard that the two-cone fracture pattern is typical when compressive-strength cylinders are tested. Does that mean any other fracture pattern indicates a testing or concrete problem?
Figure 1, from ASTM C 39, "Standard Test Method for Compressive Strength of Cylindrical Concrete Specimens," shows some of the other possible fracture types. ASTM's "Manual of Aggregate and Concrete Testing," included in Vol. 04.02 of Annual Book of ASTM Standards, states that the conical fracture is typical. In cases where the compressive strength is lower than anticipated, the manual suggests that fracture type may help in determining the cause. The manual cites a fracture pattern similar to that shown in Figure 2 as the cause of a 40% reduction in compressive strength that was a result of nonstandard testing procedures. In our experience, the cone, cone-and-split, and cone-shear are fairly typical fracture types for normal concretes. Concretes with high sand contents may fail in the shear mode. In normal-strength concretes, columnar splitting often indicates a testing problem. The splitting may be caused by the presence of a lubricant on the cylinder cap, which reduces friction between the testing machine platen and the ends. This reduces the lateral confining pressure that's usually present, and thus reduces the apparent strength. Columnar splitting and a lower apparent strength can also be caused by a cap that's convex rather than plane.