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Thin Section Petrographic Examination

Introduction


Petrography involves the identification, systematic description and geological classification of rocks. The three principal rock groups, i.e. igneous, sedimentary and metamorphic, may all be examined and classified by petrological techniques. These techniques can be similarly used to provide valuable information concerning the characteristic minerals and structures occurring within the concrete. Microscopic analysis permits observations on:

Aggregate
Cement type
Cement content:

low <12%
medium 14-16%
high >20%

Water/Cement ratio
Carbonation
Cracking
Porosity and permeability

Concrete can suffer from a variety of problems for which petrography can be usefully employed in diagnosis e.g.

Alkali-Aggregate reactivity
Shrinkable aggregates
Frost susceptible aggregates
Sulphate attack
Reinforcement corrosion
Drying shrinkage
Leaching

Presence of deleterious materials:

Dolomite
Salts i.e. gypsum, halite etc.
Mica and clay minerals
Organic matter
Iron pyrites
Too high a proportion of fine dust

Test Method and Equipment


An area measuring approximately 24.5cm2 is selected and cut from the core, then dried at 30oC for several hours, before being vacuum impregnated with coloured epoxy resin. The concrete slice is then glued to the etched surface of a glass slide and reduced to a thickness of 30 microns by a series of grinding and polishing processes, and finally sealed with a cover slip. The thin-sections are subsequently examined under a combination of plane and cross-polarised light using a petrographic microscope.

The majority of rocks are so finely crystalline that a microscope is necessary to allow precise identification of the constituent minerals. The extreme thinness of the section enables the various minerals to be distinguished according to their behaviour in transmitted light. Polarising filters within the petrological microscope produce crossed polarised light in which the crystals affect the light path to produce characteristic interference colours. This feature together with other properties such as refractive index, crystal shape and texture enable almost all minerals and rock types to be identified.

References


ASTM, C856-95 Standard Recommended Practice for the Petrographic Examination of Hardened Concrete. American Society for Testing & Material, Philadelphia, USA.

Building Research Establishment. 1999. Alkali-Silica Reaction in Concrete. BRE Digest 330, Watford, UK.

Kerr, P.F., 1977. Optical Mineralogy. New York, McGraw Hill Books.

Lin, W.M., Lin, T.D. & Powers-Couche, L.J. Microstructures of Fire-Damaged Concrete. American Concrete Institute Materials Journal. Vol 93, No.3, May - June 1996.

Smart, S.A.S. Concrete - The Burning Issue. Concrete. July/August 1999. pp. 30-34.

St. John, D.A., Poole, A.W., & Sims, I., 1998. Concrete Petrography. Arnold.

Power, T.O. and Hammersley, G.P. "Practical Concrete Petrography". Concrete, August 1978.

French, W. J. "Concrete Petrography: A Review". Quarterly Journal of Engineering Geology, 24, 17-48

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