Petrography 101: Everything you wanted to know about concrete, but were afraid to ask.

Guest Author: Jeff Varga, Certified Professional Geologist, President/Petrographer at The Rock Doctor, Inc.

Petrography: A branch of Geology that uses methods and techniques to determine the optical properties of minerals and thin sections of rocks to identify mineralogy and conditions of rock formation.

Concrete is mostly comprised of aggregates. The applicable petrographic methods and techniques are utilized for examination of concrete, concrete raw materials, and other construction products. The scientific instruments of choice commonly used by Concrete Petrographers are microscopes: stereomicroscopes, transmitted light petrographic microscopes and/or scanning electron microscopes, or SEM, to identify the basic components in concrete. Concrete Petrography is mainly utilized as a troubleshooting tool. It can help in providing answers “after the fact” when deficiencies arise. It can evaluate the material in question as to “Why did it occur?” or “How bad is it?” There are limiting factors to the scope of petrographic examinations, the “Can Do's” and “Cannot Do's” (see Right Column). Constraints also include adequate sampling and adequate field information.

Routine examinations of hardened samples and raw materials commonly follow the guidelines of ASTM C 457-12 “Standard Test Method for Microscopical Determination of Parameters of the Air-Void System in Hardened Concrete” for air-void content and/or air-void system parameter determinations and ASTM C 856-11 “Standard Practice for Petrographic Examination of Hardened Concrete.”

Investigative choices are based on experience and competence of Petrographer and can provide definitive answers in a limited time. Concrete Petrographers are usually “looking through the keyhole,” trying to see what's in the room—the big, jobsite picture. Pertinent information from the field is key to better answers and more useful reports as the quality of a report reflects the quality of information provided.

So, what information is contained in a Petrographic Examination?

The sections typically encountered are as follows:

  1. INTRODUCTION
    Sample information, location, statement of reported problem, sample markings, reported information, etc.
  2. METHODS
    ASTM Test Methods, Practices and Standards and other industry accepted test methods utilized in the examination.
  3. GENERAL CONDITION
    Includes descriptions of both macro-features and micro-features present in the sample. These include the type of finish, evaluation of curing and carbonation (if present), distribution of components, and any workmanship effects. This is a cross-sectional or “Top-to-Bottom” examination of the sample.
  4. CEMENTITIOUS MATRIX
    Includes descriptions of paste color, water absorptivity, amount of unhydrated cement particles, hardness, and any pozzolanic materials, if present. These features are used to evaluate the condition of the paste and are directly related to the water:cement(itious) or w:c(m). There is no industry accepted standard or test method for determining w:c(m) for field concrete.
  5. AIR-VOID CONTENT
    Includes the volumetric amount of air-voids present and/or Air-Void System parameters, size, shape, distribution of voids, and identification of secondary deposits present in voids.
  6. COARSE AGGREGATE
    Includes descriptions of observed topsize, condition, shape, distribution, rock types, and any potential or occurring deleterious reactions, such as alkali-silica reaction or ASR.
  7. FINE AGGREGATE
    Includes descriptions of condition, shape, distribution, rock types, and any potential or occurring deleterious reactions.
In some cases, experienced Petrographers do not need microscopes to determine concrete problems

The sections of a Petrographic report (as shown on left) are objective. The observed features are evaluated by the Petrographer and related to the reported problem in a subjective Discussion section of a report. The evaluation of features present is dependent on the ability, experience, and skill of the Petrographer. The report results are summarized and the most probable cause(s) of the reported problem are in a Conclusion section.

The most common problems encountered are low compressive strength, loss of surface or surface distress, improper mixing and/or retempering, cracking, concrete deterioration related to durability, such as freeze-thaw damage, alkali-aggregate reactions, chemical attack, and corrosion. Concrete Petrographers can also assist in understanding issues such as floor covering failures and other applied repair product deficiencies or failures.

The following quotation from K. Mather (1966), serves as a mission statement for concrete petrographers:
“The best petrographic examination is the one that finds the right questions and answers them with maximum economy in minimum time, with a demonstration clear to all concerned that the right questions were answered with all necessary and no superfluous detail. In practice, the approach to the ideal varies depending on the problem, the skill with which the questions are asked, and the skill of the petrographer. One measure of the petrographer's skill is knowing when to stop, either because the problem is adequately solved, or, in some cases, because it has been shown to be insoluble under the circumstances.”

Petrography: Can vs. Cannot

Petrography Can:

  • Evaluate Proportioning of Mix Design – including proportions of aggregate, paste and air-voids and determine whether the Air-Void system parameters are suitable for freeze-thaw durability.
  • Evaluate Mixing - including aggregate, paste and air-void distributions
  • Assess degree of cement hydration
  • Assess degree of compaction - including any workmanship effects.
  • Segregation – distributions of aggregate, paste and air-voids
  • Over-vibration and placement issues
  • Verify proper curing and determine carbonation zone detection
  • Describe w:c or w:cm from paste characteristics
  • Detect Retempering
  • Detect freezing of plastic concrete
  • Freeze-thaw distress of paste and/or aggregates
  • Identify aggregates
  • Soundness and deleterious reactions of aggregates
  • Identify Contaminants
  • Assess bonding
  • Evaluate discoloration and staining
  • Evidence of external or internal attack
  • Pre-job evaluations – examinations prior to any repair procedures
  • Potential problems / Troubleshooting

Petrography Cannot:

  • Determine TYPE of cement
  • Determine Quantitative amounts of cement, water, fly ash, GGBF slag, other pozzolans – cementitious materials observed are residual components and the ORIGINAL amounts prior to chemical reaction or hydration cannot be determined.
  • Use of chemical admixtures – but some effects can be observed.
  • “Who” cut corners
  • Water:cement ratio – Again, there is no industry accepted standard or test method for determining w:c(m) for field concrete.
  • Why air-void content of concrete at plant differs from job-site air content?
  • Determine type of AEA used
  • Determine setting characteristics

Concrete Petrography: Case Study 1

Material in Question: Less than One-Year Old Hardened concrete driveway

Customer: Homeowner

Reason for Request: Weak, wearing, and dusting finished driveway surfaces.

Symptoms: Early-age distress in service life with wearing of exposed, fine broom finished concrete driveways.

Diagnosis: The exposed top surface exhibited a carbonation zone from 1/16 of an inch thick up to 3/16 of an inch thick. The paste of this zone was softer as compared to the bulk paste. This suggests that the top surface had dried and carbonated at an early age. No evidence of a curing compound was found.

All Portland cement based concretes will carbonate over time. When carbonation occurs early in the life of the concrete, the strength development of the affected area can be compromised. Abrasion and/or wear in service of the affected zone should be expected. Proper curing, that is, the timely application of a suitable membrane curing compound at the manufacturer's suggested rate, can prevent early carbonation from occurring. Other industry accepted methods, such as water curing, fogging/misting with water, etc. that prevent early evaporation and loss of moisture at the exposed surfaces of freshly placed concrete can also be used. The essential factor is to prevent water loss at the exposed surface and allow the concrete to properly hydrate. The observed condition of the concrete surface was likely the result of early carbonation.

Result: Based upon our report, the most probable cause for the reported surface distress was early carbonation of the concrete due to lack of adequate curing. The concrete driveway was removed and replaced at no expense to the homeowner.

Concrete Petrography: Case Study 2

Material in Question: 30 year-old concrete deck

Customer: Parking Structure Owner

Reason for Request: Widespread cracking and suspected loss of durability.

Symptoms: Cracking at exposed concrete surface and white, gelatinous exudations.

Diagnosis: Reactive chert particles present with widespread cracking within paste and aggregate particles. Alkali-Silica Reaction (ASR) takes place between certain reactive, poorly crystalline or metastable silica minerals, volcanic or artificial glasses, and other siliceous-bearing aggregates (e.g., opal, chalcedony, cherts, rhyolites, dacites, etc.) and the alkalis from Portland cement paste or external sources. A reaction product gel forms that, in the presence of water, expands and may cause cracking and/or expansion of mortar and concrete.

Three conditions, (1) sufficient moisture, (2) alkalis, and (3) reactive forms of silica or aggregate(s), must be present for ASR to occur. If one of these components is not present, the reaction will not occur. Numerous fine cracks and microcracking were observed with associated gel exudations in cracks and aggregate particles.

Result: Based upon our report, the most probable cause for the reported cracking was active and ongoing ASR. Potential repairs of the structure were postponed and mitigation/service life studies are being conducted.

About the author:

Jeff Varga, CPG/AIPG is President/Petrographer with The Rock Doctor, Inc. based in Hudson, Ohio. Jeff is an active member of ACI and ASTM including ASTM Subcommittee C9.65, Petrography. Jeff has 25 years of experience as Concrete Petrographer and received his BS in Geology from The University of Akron.

www.rock-doctor.com
Email: jeff@rock-doctor.com


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