text: As hard rocks, concrete can withstand high compressive stress but not tensile stress. As a consequence, concrete is easily damaged when expansion phases are formed in its mass. 
  The most ubiquitous, and best known, expansive phases are probably the iron oxides produced by the oxidation of the carbon steel reinforcement bars embedded into concrete. Corrosion products are formed around rebar located in carbonated concrete (and thus no longer passivated against corrosion), or directly exposed to the atmospheric oxygen once cracks have started to form. The damages produced by rebar corrosion are clearly visible with bare eyes and their diagnostic is easy. 
  Other deleterious expansive chemical reactions more difficult to characterize and to identify can occur in concrete. They may be first distinguished according to the location where they occur in concrete: inside the aggregates or in the hardened cement paste.