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What is Petrography?

  • Petrography is the 150+ year-old science of geology that deals with the textural, compositional, and mineralogical description and classification of terrestrial (igneous, sedimentary, and metamorphic) and extraterrestrial (lunar and meteoritic) rocks. 

  • Classical petrography encompasses detailed macroscopical and microscopical examinations and supplementary chemical analysis needed for detailed description and classification of natural rocks. 

  • Since clinker, cement, pozzolan, slag, aggregate, concrete, masonry units, mortar, grout, plaster, building stone, tile, terrazzo, and various other construction materials are essentially made using or processing various natural rocks, petrography has significant applications in the characterization and analyses of these materials. 

  • Since the mid-nineteenth century, petrography has been used for the analysis of Portland cement – one of the most widely used construction materials in modern civilization. 

  • The application of petrography in the examination of concrete dates back to the early twentieth century. 

  • Despite its early heritage, it is has been only during the last 3 to 4 decades that the construction industry has started to accept this century-old science in the routine quality evaluation and particularly in failure investigation of various construction materials. Concrete petrography is now a well-recognized discipline amongst engineers and architects.  

Applications of Petrography in Construction Industry

  • Materials Characterization – Compositional, textural, and microstructural analysis of construction materials to determine and evaluate their bulk composition and ingredients.

  • Quality Evaluation – Evaluation of the overall quality of a material; its integrity; assessment of durability and short and long-term performances; conformance to project specifications; evaluation of the effects of manufacturing and construction procedures on various properties of a construction material; and evaluation of the original material in a restoration project to determine a suitable repair material.

  • Failure Investigation – Diagnosing various distresses in a material in service due to improper or inferior quality of the material; improper construction practices; inadequate design; and severe exposure conditions causing various physical and/or chemical attacks in the material during its service life.

  • Depth and Degree of Deterioration – Determination of the depth or extent of deterioration in a structure and its severity prior to the assessment of repair and rehabilitation schemes.

  • Evaluating Materials, Methods, and Efficiency of Repair and Rehabilitation – Evaluation of the performance of a repair material, suitability of a repair material and method in a project, extent of repair needed, condition evaluation before and after repair, and cause(s) of repair failures

Construction Materials Examined by Petrography

  • Cementitious materials including Portland cement clinker, Portland cement (ASTM C 150), blended hydraulic cement (ASTM C 595), fly ash (ASTM C 618, C 593), ground granulated blast furnace slag (ASTM C 989), silica fume or microsilica (ASTM C 1240), volcanic ash, metakaolin and other pozzolanic materials, masonry cement (ASTM C 91), stucco cement (ASTM C 1328), plaster cement (ASTM C 926), mortar cement (ASTM C 1329), hydraulic cement (ASTM C 1157), expansive hydraulic cement (ASTM C 845), etc.

  • Aggregates used in concrete, mortar, grout, plaster, concrete masonry units, etc. that are classified as normal weight aggregate (gravel, crushed stone, and natural or manufactured sand; ASTM C 33, C 294), lightweight aggregates (air cooled slag, blast-furnace slag, volcanic cinder, scoria, and breccias), expanded clay, shale and slate, etc.; (ASTM C 330, C 331, and C 332), heavyweight aggregates (e.g., iron oxides), slag aggregates, and recycled concrete aggregates.

  • Portland cement concrete and its various modifications, e.g., ready-mixed concrete (ASTM C 94); blended cement concrete; precast-prestressed-post tensioned concrete; fiber-reinforced concrete (ASTM C 1116); polymer-modified concrete (ASTM C 1439); high strength concrete; and high-performance concrete.  Portland cement concrete is by far the most widely examined construction material in petrography.

  • Miscellaneous Portland cement-based products such as Portland cement plaster, stucco, (ASTM C 926), shotcrete (ASTM C 1141, 1436, and 1480), patching and anchoring grout, metallic and mineral surface hardener (dry shake), Portland cement overlay.

  • Pipes - Prestressed concrete cylinder pipes, reinforced concrete pies, sewer pipes, and vitrified clay pipes.

  • Clay Masonry Units – Facing bricks (ASTM C 216), glazed bricks and tiles (ASTM C 126), building bricks (ASTM C 62), structural clay facing tiles (ASTM C 212), structural clay load-bearing and non-load-bearing tiles (ASTM C 34, 56), hollow bricks (ASTM C 652), thin veneer brick units, fire clay and high alumina refractory bricks (ASTM C 27), sewer and manhole bricks (ASTM C 32), insulating fire bricks (ASTM C 155), light and heavy vehicular traffic paving bricks (ASTM C 902, 1272), bricks for fireplaces (ASTM C 1261), chemical resistant bricks (ASTM C 279), silica refractory brick (ASTM C 416), and industrial floor bricks (ASTM C 410).

  • Concrete Masonry Units – Concrete building bricks (ASTM C 55), sand-lime brick (ASTM C 73), load bearing concrete masonry units (ASTM C 90), non-load-bearing concrete masonry units (ASTM C 129), prefaced concrete masonry units (ASTM C 744), solid interlocking concrete paving units (ASTM C 936), concrete grid paving units (ASTM C 1319).

  • Stone Masonry Units – Building Stones - Marble (ASTM C 503), limestone (ASTM C 568), granite (ASTM C 615), sandstone (quartz-based stone, ASTM C 616), and slate (ASTM C 629), dimension (ashlar) stone and rubble stone masonry units.

  • Mortars and Grouts for Unit Masonry – Lime mortars, Portland cement-lime mortars (ASTM C 270), masonry cement mortars (ASTM C 270), and grouts; evaluation of aggregates for masonry mortars (ASTM C 144) and grouts ASTM C 404); and estimation of proportions of cement, lime, and sand in masonry mortars (ASTM C 270).

  • Stone Products – Building stones, stone claddings and veneers, stone pavers, natural stones, dimension stones, and various other decorative or architectural stone products not related to stone masonry.

  • Lime and Gypsum-based products – Quicklime (ASTM C 5), lime and limestone (ASTM C 50, 51), hydrated lime (ASTM C 207), hydraulic hydrated lime (ASTM C 141), gypsum (ASTM C 22), anchoring grouts, gypsum plasters (ASTM C 28), gypsum casting and molding plaster (ASTM C 59), gypsum Keene’s cement (ASTM C 61), gypsum wallboard (ASTM C 36), dry wall products, gypsum concrete (ASTM C 317), gypsum veneer plaster (ASTM C 587), etc.

  • Asbestos – Detection of various forms of asbestos (serpentine and amphibole minerals, e.g., tremolite, antigorite, amosite, etc.) in building materials.

  • Tile, Terrazzo, and Other Floor Coverings – Ceramic, vinyl, and quarry tiles; Portland cement based and epoxy-based terrazzo; and other resilient floor coverings.

  • Architectural cast stones (ASTM C 1384) from historic and architectural constructions.

  • Many proprietary, shrinkage compensating, fast setting, high early strength mortars and grouts used for patching, repair, and anchorage in concrete (ASTM C 928, 1107).

  • Soil and backfill materials – mineralogy and composition of surface clays and their potential for expansion.

Tools Used in Petrographic Examinations

  • Stereomicroscope (reflected-light microscope), where white light from an illuminator is reflected from an "as received," saw-cut, fractured, lapped, polished, or thin section of material and examined through objective and eyepiece lenses at magnifications up to 100 times.

  • Petrographic microscope, where a “polarized” light is transmitted through a thin section of a sample and through objective and eyepiece lenses and examined at magnifications up to 1000 times. Both crystalline and glassy materials can be identified; specific minerals are identified by their characteristic optical properties.

  • Fluorescence-light microscope, where a petrographic microscope is modified by: (a) inserting an excitation filter between the sample stage and polarizer to generate a short-wavelength, ultraviolet light from a polarized-light, which creates fluorescence in the sample embedded in a fluorescent dye, and (b) inserting another barrier filter to block the long wavelengths. Fluorescence highlights the open spaces, cracks, and voids in a material; and based on the variation in degree of fluorescence, the spatial variations in the porosity of a material can be assessed.

  • Metallurgical microscope (combined reflected and transmitted light modes), where polarized light is reflected from a polished section of a sample and examined at magnifications up to 1000 times. A petrographic microscope can be modified to provide transmitted polarized light, reflected light, and fluorescent light (in both transmitted or reflected modes) observations.

  • Scanning electron microscope (SEM) equipped with a secondary electron detector, a backscatter electron detector, and an energy-dispersive X-ray spectrometer (EDS), where accelerated electron beams are focused to a small area on a saw-cut, fractured, lapped, polished, or thin-section of material and examined at magnifications in excess of 100,000 times.  A secondary electron detector provides the detailed, three-dimensional morphology of material at a high resolution from low energy secondary electrons generated near the surface region of the material after the electron bombardment.  Backscatter electrons reflect back from the sample surface after the bombardment of the primary electron and provide a detailed microstructure and differentiate phases of compositions from their average atomic numbers.  An EDS detector captures characteristic X-rays of elements generated from the material, which helps determine the elemental or oxide compositions of a material at a small scale, at a point, in an area, along a line, or of an area as different elemental maps. 

  • X-ray diffractometer, which determines the presence and abundance of various crystalline components (especially the fine-grained phases) in a material.

Preparing Samples for Petrographic Examinations

  • Samples selected for petrographic examination must be representative of the larger sample or structure of the concern. The sampling strategy is more crucial in failure investigation, where samples from distressed and relatively less distressed or sound areas are usually selected for determining the cause and different degree and extent of deterioration.  Following a detailed field investigation, samples should be collected from the distressed and relatively sound or less distressed areas in pristine conditions for laboratory analysis.  The number of samples should be adequate to cover the area of interest.  In most cases, drilled cores, saw-cut sections, or freshly broken pieces are collected.  Concrete sampling can be done following the recommendations of ASTM C 823 "Standard Practice for Examination and Sampling of Hardened Concrete in Construction".  Materials received for petrographic examinations are first examined visually in an "as received" condition, and then microscopically on freshly fractured sections, saw-cut sections, lapped sections, thin sections (0.2 to 0.3 mm thick, capable of transmitting light), polished sections, and oil immersion mounts using the microscopes described earlier.  Materials should be adequately photographed to record the pristine condition prior to any destructive sample preparation procedures described below.

  • Depending on the nature of the material, various methods of sample preparation are followed in microscopical examinations to obtain as much compositional and microstructural information as possible. 

  • Sectioning for reducing a sample to a manageable size by using diamond saws. 

  • Lapping or grinding to achieve a smooth, flat surface using lapidary (e.g., cast iron) wheels and abrasives (e.g., SiC, alumina powder, diamond). 

  • Thin sectioning to observe the mineralogy and microstructure of a material by successive precision sectioning and controlled, fine grinding by hand or more efficiently by using thin-sectioning equipment.

  • Polishing to achieve a shiny surface is done on polishing wheels with cloths and diamond or other fine abrasive pastes.

  • Etching or staining a polished section with a chemical reagent highlights a particular component by absorbing into that component and removing a surface layer in the solution, or, in the latter, by reacting with the component of interest. 

  • Powder mount or oil-immersion mount preparation of material is used to examine the composition of a specific area of interest in a material.  A needle is used to pick up a small portion of the sample, and its constituents are examined in a petrographic microscope by immersing it in an oil of a known refractive index.  Minerals are identified by their characteristic refractive indices and other optical properties.

  • The method and extent of sample preparation should be based upon the kind of information needed for the purpose of the examination.  Freshly fractured sections are helpful in studying the composition, properties, and conditions of a material in a stereomicroscope that have not been weathered or altered by the environment or other means.  Lapped cross sections are useful for examinations of the overall composition and microstructure of a material at low (10X) to moderate (100X) magnifications by using a stereomicroscope.  Thin sections are useful for examining finer details in composition, mineralogy, and microstructure in a petrographic microscope at magnifications of up to 1000X or in an SEM at much higher magnifications.  Polished sections are useful for examination in metallurgical microscopes, staining/etching, and in SEM.  Powder mounts or oil immersion mounts are useful in examining the composition of any small area of interest (usually in a freshly fractured section) from any location in the sample using a petrographic microscope or SEM.  Saw-cut sections are sometimes used for either light or electron microscopes.  Treating samples with a fluorescent or colored dye-mixed epoxy or alcohol can highlight cracks, voids, and porous areas.  Epoxy impregnation improves the overall integrity of broken or fragile samples.  Appropriate preparation of a sample is a crucial step in the examination, which, if not done properly, can destroy or prevent the gathering of useful information from a material. 

Industry Standards and Methodologies on Petrography

  • Industry standards from the American Society for Testing and Materials (ASTM) on petrographic examinations of aggregate, concrete, and mortar:

  • ASTM C 295: "Standard Guide for Petrographic Examination of Aggregates for Concrete;"

  • ASTM C 457: "Standard Test Method for Microscopical Determination of Parameters of the Air-Void System in Hardened Concrete;"

  • ASTM C 856:  "Standard Practice for Petrographic Examination of Hardened Concrete;"

  • ASTM C 1324: "Standard Test Method for Examination and Analysis of Hardened Masonry Mortar;"

  • ASTM C 1721: “Standard Guide for Petrographic Examination of Dimension Stone;” and,

  • ASTM C 1723: “Standard Guide for Examination of Hardened Concrete Using Scanning Electron Microscopy.”

  • The European standards on petrography include:

  • BSI 812, Part 104 "Method for qualitative and quantitative petrographic examination of aggregates" and

  • BSI 1881, Part 124: "Methods for analysis of hardened concrete". 

Petrography Applied to Concrete and Aggregate

  • Determination of the overall quality of concrete in an existing structure for assessment of its future short and long-term performance, and the impact of the existing condition in the future.

  • Evaluation of aggregates to be used in concrete or mortar for potential alkali-aggregate reactivity, soundness, potential oxidation of iron sulfide minerals, and frost resistance.

  • Evaluation of concrete mixture proportioning, degree of mixing, retempering, consolidation, finishing improprieties (premature and/or prolonged finishing, adding water during finishing), inadequate curing, degree of cement hydration especially at the surface region and during placement in cold weather condition, and their impact on the properties and performance of freshly placed concrete.

  • Determination of the overall quality of concrete in an existing structure (parking garage, bridge deck, condominium complex, outdoor slab-on-grade, precast elements, etc.) for assessment of overall concrete durability, i.e., its resistance to the affects of various deleterious agents in the environment.

  • Determination of air content and other air-void parameters of concrete for assessment of freeze-thaw durability and scaling resistance.

  • Estimation of the amount of total cementitious materials used, proportions of various pozzolans, and water-cementitious materials ratio for assessment of strength of concrete in a new construction.

  • Evaluating the reasons for low strength of laboratory cured concrete cylinders compared to the anticipated designed strength.  Petrography is the most powerful method to determine the cause of low strength, which could be from high water content, high air content, low cementitious materials content, low degree of cement hydration, early freezing of concrete, or other reasons.

Diagnosing Concrete Deteriorations By Concrete Petrography

  • Concrete surface distress such as scaling, spalling, aggregate popout, mortar lift-off, blistering, delamination, dusting, efflorescence, discoloration, and cracking.

  • Chemical attacks in concrete by acid, alkali, sulfate, carbonate, and chloride solutions.

  • Cracking and spalling due to alkali-aggregate reaction.

  • Corrosion of reinforcing steel in concrete.

  • Cracking, spalling, and loss of strength due to frost attacks in plastic and hardened concretes.

  • Cracking, spalling, discoloration, and severity of distress from fire attacks  

  • Loss of mass and strength in concrete exposed to a marine environment by the ingress of seawater containing dissolved magnesium sulfate, chloride, and carbon dioxide, corrosion of reinforcing steel in concrete by the dissolved chloride of seawater.

  • Cracking due to delayed hydration of free lime and magnesia in the paste due to prolonged exposure of concrete to moisture.

  • Cracking, spalling and exfoliation due to salt weathering and salt hydration distress.

  • Cracking due to concrete shrinkage at the plastic or semi-plastic state, drying shrinkage, and plastic settlement.

  • Concrete deterioration due to various external and internal sulfate attacks.

  • Curling and cracking of a concrete slab-on-grade due to the thermal and/or moisture differential between the slab top and bottom.

  • Low strength gain due to abnormal setting behavior (quick or delayed setting) of concrete.

  • De-bonding of surface sealer, protective coating, paint, and overlays.

  • Weathering, acid attacks, frost attacks, chloride and carbonation induced corrosion of wires, and other environmental attacks on underground prestressed concrete cylinder pipes and reinforced concrete pipes; microbial-induced sulfuric acid attacks in concrete sewer pipes.

  • Various deteriorations in Portland cement plasters (stuccos) such as cracking, discoloration, de-bonding of stucco from the substrate, corrosion of wire mesh backing, etc.

  • Each mechanism of deterioration leaves a series of characteristic compositional, macrostructural, and microstructural evidence that is commonly searched for during petrographic examinations of concrete.  Identification of this evidence helps determine the cause and extent of deterioration.

Durability Evaluation By Concrete Petrography

  • Assessment of the resistance of the concrete to external aggressive agents from the overall quality of concrete including the estimated water-cementitious materials ratio, degree of cement hydration, degree of consolidation, finishing and curing practices employed, extent of cracking, and extent of chemical alterations of concrete.

  • Assessment of chloride-induced and carbonation-induced corrosion of reinforcing steel in concrete from the chloride profile and depth of carbonation of the concrete in relation to the depth of the corroded steel, respectively.  Assessment of the materials, thickness, and quality of the concrete cover over the reinforcing steel for preventing corrosion.

  • Assessment of the freeze-thaw durability of concrete from air entrainment; the amount, size, fineness, distribution, and spacing factor of entrained air voids; frost resistance of aggregates (evaluation of aggregates for potential frost damage in concrete such as D-cracking in pavement); density/porosity of the paste; volumetric proportion of the paste; evaluation of the results of laboratory cyclic freezing and thawing and deicing salt scaling resistance tests. 

  • Assessment of resistance of concrete to various forms of sulfate attacks in concrete (e.g., external sulfate attack, internal sulfate attack, chemical sulfate attack, and physical sulfate attack) from various chemical, physical, and microstructural evidence such as: (a) the bulk sulfate content of concrete and sulfate profiles from the exposed surface inward; (b) the depth and degree of decomposition of the paste; (c) the presence of calcium sulfate dihydrate (gypsum); calcium sulfoaluminate hydrates (ettringite, monosulfate); sulfate salts (sodium, calcium sulfate hydrates) and other products of sulfate attacks in the concrete, (d) extent of microcracking; and (e) the overall loss of mass and strength of the concrete. 

  • Assessment of the extent and damaging action of alkali-aggregate reactions in concrete from: (a) the type, composition, location, and abundance of alkali-silica gel in the paste, cracks, and voids in the concrete; (b) reaction rims in potentially alkali-silica or alkali-carbonate reactive aggregate particles; and (c) microcracking in concrete extending from reactive aggregate particles into the paste. The presence of potential alkali-silica reactive particles (chert, flint, chalcedony, tridymite, cristobalite, volcanic rocks, strained quartz and quartzite, greywacke, and glass), alkali-carbonate reactive particles (calcitic dolomite and dolomitic limestone containing large euhedral crystals of dolomite in a fine-grained matrix of calcite, dolomite, quartz, and clay), and the evidence and products of their reactions with cement alkalis in pore solutions of concrete can be diagnosed only by petrography.  

  • Assessment of acid-induced corrosion of concrete from the depth of alteration of the cement paste; leaching, decomposition, and loss of the calcium hydroxide component of cement hydration from the paste; and the degree of dissolution of calcareous aggregates by acidic solutions.

  • Assessment of abrasion resistance of concrete from hardness, density, degree of curing, water-cementitious materials ratio, type of aggregates, and the presence of hardener at the wearing surface (and in near-surface region) of the concrete.

Role of Concrete Petrography in Repair & Rehabilitation

  • Diagnosing the causes of concrete deterioration prior to the repair.

  • Determining the extent and severity of the deterioration and the amount of deteriorated concrete to be repaired.

  • Evaluating a suitable repair material for a particular deterioration.

  • Evaluating the preparation of the surface of the deteriorated concrete substrate prior to the placement of the repair material.

  • Evaluating the bond between a repair material and the original concrete.

  • Determining the degree of infiltration of a repair material in the cracks.

  • Evaluating the effectiveness of a proposed repair scheme in a small test area.

  • Evaluating the improvement of the overall condition of a structure by a repair material and method by comparing the conditions before and after the repair.

  • Investigating the causes of a repair failure.

Petrography Applied to Masonry Units and Mortars

  • Evaluation of overall quality, condition, composition, and volumetric proportions of ingredients in historic mortar containing one or a combinations of various clay, gypsum, lime, or cement-based cementitious binders, e.g., non-hydraulic lime, feebly hydraulic lime, hydraulic lime, calcitic lime, dolomitic lime, calcined clay, brick dust and other pozzolans, gypsum binders, natural cements, and early-age Portland cements to prepare a suitable matching mortar.

  • Evaluation of overall quality, condition, composition, and volumetric proportions of ingredients in modern masrony mortars containing Portland cement–lime, Portland-Pozzolan cement-lime, or masonry cement to prepare a suitable matching pointing mortar.

  • Testing of historic and modern masonry mortars by following various methods outlined in ASTM C 1324 and RILEM methods. 

  • Evaluation of clay, stone, and concrete masonry units for their quality, composition, condition, suitability and anticipated performances in a project, short and long-term durability, performance, and behavior in an existing structure.

Diagnosing Various Masonry Problems By Petrography

  • Water leakage through masonry.

  • Cracking and spalling of jointing mortars and masonry.

  • Reasons for an inadequate bond between masonry units and jointing mortars.

  • Efflorescence, staining, and other discoloration on masonry walls.

  • Disintegration of jointing mortars, grouts, and sealants.

  • Distress in a masonry wall and foundation due to cyclic freezing and thawing.

  • Evaluating the effectiveness of a surface sealer or coating or the reasons for coating failure on a masonry wall.

  • Distress in dimension stone cladding such as volume instability (bowing or dishing), cracking, discoloration, de-bonding, moisture and/or thermal hysteresis, etc.

  • Deterioration of clay, stone, and concrete pavers from cyclic freezing and thawing, wetting and drying, heating and cooling, abrasion, impact, disintegration, heaving, and efflorescence.

Petrography Applied to Stone, Tile, and Various Floor Coverings

  • Evaluation of building/dimension stones - their overall quality, composition, and suitability in a particular project and environment; their freeze-thaw durability, and their resistance to other chemical and physical agents. 

  • Evaluation of overall quality of terrazzo, ceramic, porcelain, stone, quarry tile, and other floor covering products

Diagnosing Various Distress in Tiles and Other Floor Coverings 
By Petrography

  • Distress in ceramic tile floors such as cracking, de-bonding from the setting bed mortar, popping, discoloration, disintegration of the setting bed mortar and jointing grouts, and moisture penetration related distress.

  • Distress in vinyl tile floors such as discoloration, bumps, blisters, and de-bonding from the substrate due to moisture entrapment, or adhesive failure.

  • Distress in terrazzo floors due to cracking, de-bonding of terrazzo from setting bed mortar, discoloration, marble chipping, and other deteriorations.

  • De-bonding of resilient floor coverings due to high moisture emission through the concrete substrate, alkali-aggregate reaction in the concrete substrate (which is often concentrated at the surface region by moisture upwelling and moisture-induced alkali enrichment at the surface region), adhesive failure, or other reasons.

  • Blistering, cracking, crazing, and de-bonding of paint from various substrates ranging from concrete floors to drywall.

Diagnosing Miscellaneous Deteriorations 
By Petrography

  • Examination of soil and other subbase materials for the presence of potentially expansive materials in the presence of moisture (e.g., certain clays, metallic materials, iron sulfide).

  • Following the above line of examination, investigation of possible reasons for heaving of concrete or asphaltic pavements (either frost-induced or moisture related expansion of subbase materials including oxidation of iron sulfide minerals).

  • Comparing an unknown cementitious material from a sewage pipe with a known material to investigate the alleged blockage of the sewage line by the unknown material.

  • Examinations of various slag products (steel slag, blast-furnace slag, air-cooled slag, granulated slag) for their potential use in concrete and in other building materials and their potential unsoundness.

  • Examination of fireproofing materials; asbestiform minerals for possible health hazards; galvanized metals for possible corrosion-induced deteriorations, etc.

  • Investigation of plaster deteriorations in many in-ground swimming pools, such as spot etching, cracking, staining, discoloration, spalling, blistering, etc.

Things Petrographic Examinations Cannot Do

  • Petrography provides a semi-quantitative estimation of the proportion of cementitious materials and the water-cementitious materials ratio of hardened concrete.  The accuracy of this determination depends upon the number of samples examined, the construction practices, the type and number of reference materials used for comparison of the water-cementitious materials ratio, the degree of cement hydration, spatial variations of these properties in a structure, experience, and other factors.  The strength of a material cannot be assessed without direct or in situ strength testing.  Petrography, however, can determine whether or not the strength of a material is higher or lower than the strength of another similar material and the reasons for strength variations.

  • The types of chemical admixtures and various property-enhancing organic chemicals added in a construction material cannot be assessed without doing the appropriate supplementary chemical analysis.  However, the effects of such chemicals on the microstructure and properties of materials can be assessed.

  • The absolute age of a material cannot be assessed.  The relative age of cracks in a structure in the same concrete and exposure conditions can be assessed from determining the depths of carbonation.

  • The arial (lateral) extent of deterioration cannot be assessed without a field and nondestructive survey or adequate sampling of the entire area.  The extent of deterioration at a particular location, however, can be assessed from a core.

  • Depending upon the project and the purpose of the examinations, the extent of information obtainable from petrographic examination may depend upon the sampling strategy (e.g., number, location, and extent of sampling); therefore, caution is needed to extend the conclusions from one or a couple of samples to the broader scale in the structure.

Two Comprehensive Guides on Petrography Applied to Concrete and Other Building Materials

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