Simplified Guide to Petrographic Examination of Aggregates (IS 2386 Part VIII - 1963)

 IS.2386.8.1963

Simplified Guide to Petrographic Examination of Aggregates (IS 2386 Part VIII - 1963)

The standard describes two methods for petrographic examination:

  1. Method I: A routine visual and basic microscopic examination for quick assessment.
  2. Method II: A detailed petrographic analysis for in-depth investigation, serving as the reference method.

These methods help identify the suitability of aggregates for concrete by evaluating their mineral composition, physical properties, and potential for deleterious reactions, such as alkali-aggregate reactivity.

Why Perform Petrographic Examination?

Petrographic examination identifies the mineralogical and physical characteristics of aggregates, which affect the strength, durability, and stability of concrete. It detects potentially deleterious constituents (e.g., opal, chert, or clay minerals) that could cause issues like alkali-silica reaction (ASR) or poor mechanical performance in concrete structures.

1. Method I: Routine Petrographic Examination

Purpose

To perform a quick visual and basic microscopic examination to segregate and identify aggregate constituents based on petrographic, physical, and chemical differences.

Apparatus

  • Screens: IS sieves (150 µm, 300 µm, 600 µm, 1.00 mm, 2.5 mm, 4.75 mm, 10 mm, 12.5 mm, 20 mm, 25 mm, 40 mm, 50 mm, 80 mm).
  • Balance: 2 kg capacity, sensitive to 0.1 g.
  • Anvil and Hammer: For breaking pebbles, with a tray to minimize loss of rock chips.
  • Hand Lens and Microscopes: Stereoscopic and petrographic microscopes for visual and mineralogical identification.
  • Auxiliary Equipment: For chemical tests (e.g., acid tests) and sample preparation.

Procedure

  1. Sample Preparation (Clause 2.3):
    • Screen aggregates to obtain specified quantities for each size fraction:
      • 80 mm: 4000 g
      • 40 mm: 1000 g
      • 20 mm: 200 g
      • 10 mm: 100 g
      • 4.75 mm and smaller: 25–50 g
    • Dry samples in an oven at 105–110°C to remove moisture.
  2. Examination (Clauses 2.3–2.4):
    • Examine each size fraction separately using a hand lens or stereoscopic microscope.
    • Check for coatings (e.g., opal, calcium carbonate, gypsum, clay, or organic matter) and assess their bonding strength and potential impact on concrete durability.
    • Break coarse aggregate particles using an anvil and hammer to expose fresh surfaces.
    • Identify particles by:
      • Petrographic Identity: Rock type (e.g., granite, limestone, sandstone).
      • Physical Condition: Good, satisfactory, fair, or poor (based on strength, durability, and abrasion resistance).
      • Chemical Stability: Non-reactive or deleterious (e.g., opal, chalcedony, tridymite, cristobalite) in concrete.
    • Use chemical tests (e.g., acid tests) or immersion oils under a petrographic microscope to confirm mineral identity if needed.
  3. Reporting (Clause 2.4):
    • Summarize findings in a report detailing:
      • Petrographic identity, proportion, and characteristics of each constituent.
      • Physical and chemical quality ratings (see Clause 2.4 for quality scale).
      • Suitability for concrete under anticipated service conditions.
      • Recommendations for additional tests (e.g., alkali-aggregate reactivity tests per IS 2386 Part VII) if deleterious substances are detected.

Example

A coarse aggregate sample is examined:

  • Sample: River gravel, sieved to 20 mm fraction (200 g).
  • Examination: 60% granite (good physical quality, non-reactive), 30% limestone (satisfactory, non-reactive), 10% chert (fair, potentially reactive due to opal content).
  • Coatings: Some limestone particles have clay coatings, removable by washing.
  • Report: Granite and limestone are suitable; chert requires further testing (e.g., mortar-bar test) due to potential alkali-silica reactivity.

2. Method II: Detailed Petrographic Examination

Purpose

To conduct a comprehensive petrographic analysis for precise identification of aggregate constituents, their physical and chemical properties, and their suitability for concrete.

Apparatus and Supplies

  • Sample Preparation (Clause 3.1.1):
    • Rock-cutting saw (350 mm diamond blade).
    • Horizontal grinding wheel (400 mm) and polishing wheel (200–300 mm).
    • Abrasives (silicon carbide grit, emery).
    • Microscope slides (25 x 45 mm), Canada balsam, xylene, mounting medium.
    • Laboratory oven, Jones riffle sampler, Plattner mortar.
  • Examination (Clause 3.2.2):
    • Polarizing microscope with mechanical stage, low/medium/high-power objectives, compensators, and immersion media (refractive index 1.400–1.785).
    • Stereoscopic microscope (6X–60X magnification).
    • Magnet, needle holder, dropping bottle, Petri dishes, lens paper.
    • Photomicrographic camera for documentation.

Procedure

  1. Sampling (Clause 3.3):
    • Quarries: Collect ≥2.5 kg from each stratum, with no piece <0.5 kg, or use drilled cores.
    • Sand and Gravel Deposits: Sample via test pits to anticipated production depth, with quantities based on sieve size.
    • Ensure samples are representative of the deposit.
  2. Examination of Natural Gravel (Clause 3.4.2):
    • Sort pebbles by rock type using visual inspection, stereoscopic microscope, or acid/scratch tests.
    • Check for deleterious coatings (e.g., opal, gypsum, soluble salts).
    • Assess physical condition (e.g., fracturing, porosity) and chemical stability (e.g., presence of reactive silica minerals).
    • Examine at least 300 particles per sieve fraction for reliable statistical results.
  3. Examination of Natural Sand (Clause 3.4.3):
    • For sizes >600 µm, reduce samples using a Jones riffle sampler to obtain ≥300 particles per fraction.
    • Examine under a stereoscopic microscope, submerging grains in water to reduce reflection and aid identification.
    • Use petrographic microscope for fine-grained or suspect particles (e.g., those containing opal or chalcedony).
    • Record particle shape, surface texture, grain size, mineral composition, and presence of deleterious substances.
  4. Examination of Crushed Stone (Clause 3.5.3):
    • Follow similar procedures as for gravel, focusing on particle counts per sieve fraction.
    • Assess fracturing caused by crushing, which may affect durability.
  5. Additional Test for Undulatory Extinction (UE) (Clause 3.7, Amendment No. 1, June 1999):
    • For aggregates with >30% quartz, measure the undulatory extinction angle using a polarizing microscope in transmitted light.
    • Record the angle between the first and second extinction positions of quartz grains.
    • Average UE angles across grains in a thin section.
    • Determine the percentage of quartz grains showing UE via point-counting.
    • Note: UE indicates crystal lattice distortion, which may correlate with alkali reactivity (e.g., strained quartz).
  6. Calculations and Reporting (Clause 3.6):
    • Calculate the weighted percentage of each constituent in each sieve fraction (Clause 3.6.1.3).
    • Sum weighted percentages to obtain the overall composition (Clause 3.6.1.4).
    • Construct tables (e.g., Table III and IV) showing composition and condition by sieve fraction and for the whole sample.
    • Report trace constituents (≤0.5%) separately.
    • Prepare a summary report (brief, for engineers) and a detailed report (including test procedures, data, tables, and photographs).
    • Describe deleterious properties quantitatively and their potential effects on concrete (e.g., cracking due to reactive minerals).

Example

A crushed stone sample is analyzed:

  • Sample: Quarry aggregate, sieved to 10 mm fraction (100 g).
  • Examination: 50% quartzite (UE angle 20°, 40% quartz with UE), 30% granite, 15% sandstone, 5% chert (potentially reactive).
  • Calculations: Weighted composition shows 2% chert in the whole sample.
  • Report: Quartzite’s high UE suggests potential reactivity; recommend alkali-aggregate reactivity testing (IS 2386 Part VII). Granite and sandstone are suitable.

Key Tips for Students and Engineers

  • Method I:
    • Ideal for quick assessments when time or resources are limited.
    • Focus on identifying coatings and reactive minerals (e.g., opal, chert).
    • Use simple tools (hand lens, stereoscopic microscope) for cost-effective analysis.
  • Method II:
    • Requires specialized equipment (e.g., polarizing microscope, thin-section preparation tools).
    • Essential for critical projects (e.g., dams, bridges) where aggregate quality is paramount.
    • Ensure precise calibration of immersion media refractive indices for accurate mineral identification.
  • General:
    • Examine at least 300 particles per sieve fraction for statistical reliability.
    • Cross-reference findings with IS 2386 Part VII (Alkali Aggregate Reactivity) for suspect aggregates.
    • Document observations thoroughly, as reports guide engineering decisions.

Common Mistakes to Avoid

  • Method I:
    • Overlooking minor coatings that may affect concrete bonding.
    • Inadequate particle breaking, missing internal defects.
  • Method II:
    • Insufficient particle counts (<300), leading to unreliable composition estimates.
    • Poor thin-section preparation, affecting microscopic analysis.
    • Neglecting temperature control for immersion media, causing inaccurate refractive index measurements.
  • Document Issues:
    • Typographical errors: “2366”, “2866”, “2966” instead of “2386”; “AGREGATES” instead of “AGGREGATES”; “Petorgraphic” instead of “Petrographic”.
    • Truncated text limits procedural details; consult the full standard for clarity.
    • Amendment No. 1 (1999) adds UE testing but lacks detailed procedural steps in the provided excerpt.

Notes on Document Issues

  • Typographical Errors: References to “2366”, “2866”, “2966”, or “2.86” are typos for “2386”. “AGREGATES” is a typo for “AGGREGATES” (Pages 5, 7, 13, 26). “Petorgraphic” is a typo for “Petrographic” (Page 12).
  • Truncation: Large sections are missing, limiting details on procedures, calculations, and table formats (e.g., Tables I–IV).
  • Amendment: Amendment No. 1 (June 1999) adds undulatory extinction testing for quartz-rich aggregates (>30% quartz) to assess alkali reactivity potential.
  • Reference Standards:
    • IS 2:1960 (Rules for Rounding Off Numerical Values).
    • IS 2386 Part I (Particle Size and Shape).
  • Year: Adopted in 1963, reaffirmed in 2002, with Amendment No. 1 in 1999.

For complete details, refer to the full IS 2386 (Part VIII) - 1963 document, including Amendment No. 1.

 

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