Concrete is a poor conductor of heat, but can suffer considerable damage when exposed to fire. Unraveling the heating history of concrete is important to forensic research or to determine whether a fire-exposed concrete structure and its components are still structurally sound. Assessment of fire-damage concrete structures usually starts with visual observation of color change, cracking and spalling. On heating, a change in color from normal to pink is often observed and this is useful since it coincides with the onset of significant loss of concrete strength. This paper presents results of cores strength, as well as, optical microscopy investigations of fire-damaged concrete. Samples were taken from concrete that had been exposed to fire. Optical microscopy has focused on microstructure of cement paste, aggregates, microvoids and cracks, as well as, on quantification the crack patterns found in heated concrete samples. The physical condition of concrete sample in combination with the microscopic examination, enable a petrographer to make a reasonable estimation of the minimum exposure temperature and its relative impact to the depth of damage in concrete.