Experimental investigation on the triaxial fatigue behaviour of plain concrete

An experimental research programme was carried out with the aim of investigating the effects of cyclic triaxial loading upon the mechanical behaviour of concrete. A triaxial press was used, capable of applying on cylindrical specimens an axial load and a confining pressure cyclically varying in time according to independent programs. Cycles (at 1 Hz) were characterized by different (a) mean stress, (b) amplitude, (c) ratio r of the mean confining pressure to the mean axial stress and (d) phase angle between loads. Some uniaxial cyclic tests were also performed Those specimens that did not fail within about 400 000 cycles were monotonically tested to failure in uniaxial compression. Despite their scatter; the results obtained qualitatively show that: (a) other test parameters being constant, fatigue life is usually shorter in tests in phase opposition rather than in phase coincidence; (b) as the cycle amplitude increases, fatigue life decreases (mainly in tests in phase opposition); (c) fatigue life increases as the mean confinement increases; (d) a strong correlation exists between 'secondary creep rate' and fatigue life, irrespective of the ratio between mean stresses, (e) all specimens submitted to post-cyclic failure tests exhibited a decrease in elastic modulus in proportion to the increase in r during cycles; the specimens submitted to uniaxial cycles experienced an increase in static strength in comparison with the virgin ones.