Petrographic testing is the use of microscopes to examine samples of rock or concrete to determine their mineralogical and chemical characteristics. Samples for petrographic examination can be taken from lump samples or cores. They are impregnated with resin. Suitable surfaces (either polished or thin sections) are prepared by sawing, lapping and polishing. The samples are examined through a petrological (geological polarising) microscope, using either reflected or transmitted light. The lighting may be normal, ultra-violet or polarised.
Petrographic examination can determine a range of parameters, as follows:
Type, proportions, grading, shape and condition of the aggregates
Nature of the cement paste, including mineral additions such as fly ash/pfa, the degree of hydration
Presence of deleterious material, cement paste and air voids
Depth of carbonation
Bond between the aggregate and the paste
Evidence of sulfate attack, frost damage, some forms of chemical attack and alkali-silica reactions
Depth of fire damage and temperature of the fire
Measurement of air entrainment.
Detailed information on petrographic testing is given in Concrete Society Technical Report 71, Concrete petrography: An introductory guide for the non-specialist. General information on the use of petrography, and the use of other testing techniques, is given in Technical Report 54, Diagnosis of deterioration in concrete structures.