  We study the creep behavior of a disordered brittle material (concrete) under
successive loading steps, using acoustic emission and ultrasonic sensing to
track internal damage. The primary creep rate is observed to follow a
(Omori-type) power-law decay in the strain rate, the number of acoustic
emission events as well as the amplitudes of the ultrasonic beams, supporting a
brittle-creep mechanism. The main outcome is however the discovery of
unexpected history effects that make the material less prone to creep when it
has been previously deformed and damaged under primary creep at a lower applied
stress. With the help of a progressive damage model implementing thermal
activation, we interpret this as an aging-under-stress phenomenon: during an
initial creep step at relatively low applied stress, the easy-to-damage sites
are exhausted first, depleting the excitation spectrum at low stress gap
values. Consequently, upon reloading under a larger applied stress, although
previously damaged, the material creeps (and damages) less than it would under
the same stress but without pre-creeping. Besides shedding a new light on the
fundamental physics of creep of disordered brittle materials, this has
important practical consequences in the interpretation of some experimental
procedures, such as stress-stepping experiments.
