The size and shape of concrete specimens influence their compressive strength due to their effect on the development of fracture processes before the maximum load. Small specimens yield higher strengths than larger ones, so the size effect becomes an issue…
The size and shape of concrete specimens influence their compressive strength due to their effect on the development of fracture processes before the maximum load. Small specimens yield higher strengths than larger ones, so the size effect becomes an issue since material properties in structural elements may be smaller than those measured in the laboratory. Analogously, cubes resist more compression than cylinders (of slenderness 2) due to the constraints exerted by the load platens, which may also affect fatigue results. Regarding fatigue, there are very few studies on the effect of the size and shape of the specimen in the resisted number of cycles, especially in fiber concrete, which justifies our work on this topic. We tested cubes of 40 mm, 80 mm, and 150 mm in edge length and cylinders of 75 mm (S), 100 mm (M), and 150 mm (L) in diameter (all of slenderness 2). All the specimens were of the same self-compacting steel-fiber reinforced concrete and were loaded from 20% to 85% of their corresponding quasi-static strength. L and M cylinders endure a smaller number of cycles than S cylinders on average. Likewise, 150 mm and 80 mm cubes live shorter lives than 40 mm cubes (many of them are runouts). Cubes endure longer fatigue lives than cylinders of compara- ble size. Some of the reasons explaining these effects are: L and M cylinders have a larger maximum pore size due to the pouring process in the molds; cylinders show wall-effect (fiber alignment and porosity decrease close to the walls); load platens constraint is weaker in cylinders; and this material shows an autogenous self-healing due to the cyclic loads, which is especially noticeable in the small cubes. These results indicate that careful selection of the correct testing specimen is crucial when determining the constitutive laws for the fatigue of steel-fiber concrete.