Failure of slender and stocky reinforced concrete columns: tests of size effect

The paper reports the results of a series of tests of geometrically similar pin-ended tied reinforced concrete columns of different sizes, with ratio 1:2:4, and slendernesses of 19.2, 35.8 and 52.5. The model columns had square cross-sections of sides 0.5, 1 and 2 in. (12.7, 25.4 and 50.8 mm), reduced-size steel bars with steel ratio 4.91%, and reduced-size aggregate with maximum size 1/8 in. (3.2 mm). The axial load was eccentric, with end eccentricities 0.25 of the cross-section side. It is found that for all slendernesses the failure loads exhibit a strong size effect in which the nominal stress at maximum load (load divided by cross-sectional area) decreases as the size is increased. This contradicts the current design codes, which exhibit no size effect, and indicates that the failure is governed by fracture mechanics. The results are in good agreement with the size-effect law previously proposed by Bažant. For a higher slenderness, the size effect increases and the brittleness of failure, characterizing the proximity to the behaviour described by linear elastic fracture mechanics, increases. The role of bond slip in the observed size effect remains to be clarified.