Question

Explain the importance of petrographic examination and discuss the mechanism of concrete distress that suitable to be analysed using this method

Answer 1

Petrography is a branch of petrology that focuses on detailed descriptions of rocks. Someone who studies petrography is called a petrographer. The mineral content and the textural relationships within the rock are described in detail. The classification of rocks is based on the information acquired during the petrorgaphy analysis.

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:

(1). Type, proportions, grading, shape and condition of the aggregates

(2.) Nature of the cement paste, including mineral additions such as fly ash/pfa, the degree of hydration

(3.) Presence of deleterious material, cement paste and air voids

(4.) Depth of carbonation

(5.) Bond between the aggregate and the paste

(6.) Evidence of sulfate attack, frost damage, some forms of chemical attack and alkali-silica reactions

(7.) Depth of fire damage and temperature of the fire
Measurement of air entrainment.

Mechanism of Concrete

Petrography is the most powerful method of diagnosing the causes and extent of varioustypes of concrete surface distress (scaling, aggregate popout, mortar lift-off, spalling,dusting, discoloration, staining, efflorescence, blistering, delamination, joint or crackedge chipping, surface abrasion) in pavements, sidewalks, driveways, gutters, bridge andparking garage decks, and industrial floors that call for repair.

petrography determines:

(a) whether or not a distress is due to improper materials,construction procedures, installation practices, or environmental impacts;

(b) the extentand severity of the distress; and

(c) the amount of concrete needed to be replaced (i.e.,

full or partial-depth repair). The common repair strategies are surface preparation, partialor full-depth removal of the distressed concrete, and placement of a repair material or anew concrete

A microscopical examination performed by an experienced concrete petrographer can provide information into whether a defect is a sign of serious problems, and how future defects can be avoided. Such examinations include observations of the overall condition of the concrete, composition and condition of the aggregate particles, occurrence and extent of cement paste-aggregate reactions, characteristics of cement paste, strength of the paste-aggregate bond, and parameters of the air-void system. A petrographic examination can tell you if the concrete was adequately proportioned, adequately mixed, suitably compacted, and adequately finished and cured. Air-void parameters can be quantitatively determined. Microscopical observations can determine if the cement is properly hydrated, estimate the water-cement ratio, detect the presence of absence of fly ash or slag, and detect the presence or absence of reactions between aggregates and cement paste. When submitting samples to a testing lab for petrographic examination, use samples that represent the problem. An ideal sample is a full-thickness core with at least a 2-inch diameter. If possible, also provide some samples from areas that do not show the problem.