  Acoustic emission (AE) measurements performed during the compressive loading
of concrete samples with three different microstructures (aggregate sizes and
porosity) and four sample sizes revealed that failure is preceded by an
acceleration of the rate of fracturing events, power law distributions of AE
energies and durations near failure, and a divergence of the fracturing
correlation length and time towards failure. This argues for an interpretation
of compressive failure of disordered materials as a critical transition between
an intact and a failed state. The associated critical exponents were found to
be independent of sample size and microstructural disorder and close to
mean-field depinning values. Although compressive failure differs from
classical depinning in several respects, including the nature of the elastic
redistribution kernel, an analogy between the two processes allows deriving
(finite)-sizing effects on strength that match our extensive dataset. This
critical interpretation of failure may have also important consequences in
terms of natural hazards forecasting, such as volcanic eruptions, landslides,
or cliff collapses.
