Methods of Test for Aggregates for Concrete, Part VIII: Petrographic Examination
1963 Edition

According to the standard, sample preparation begins with collection under the supervision of a geologist knowledgeable in concrete aggregates, following IS 2430 sampling guidelines. Samples of gravel and natural sand are sieved as per IS 2386 Part 1, with special attention to material passing the 75-micron sieve without washing. Each sieve fraction is examined individually starting from the largest size. Fine fractions below 600 microns are reduced to approximately 4 to 5 grams using a Jones riffle or coning and quartering, then mounted in immersion oil on glass slides with cover glass. These slides are examined under petrographic microscopes with a mechanical stage to identify and count grains systematically, ensuring representative samples for accurate petrographic evaluation.

IS 2386 Part 8 specifies the use of several key instruments: a hand lens for preliminary examination; a stereoscopic microscope for detailed observation of coarser grains (over 600 microns), useful for wet and dry identification; and a petrographic microscope for detailed mineralogical analysis, particularly for grains not identified stereoscopically. Immersion media with refractive indices ranging from 1.410 to 1.785 in 0.005 increments are used to study optical properties. An Abbe refractometer is necessary for precise calibration of immersion media indices. Additionally, a photomicrographic camera with appropriate lighting, such as a zirconium arc lamp, can document microscopic features. This comprehensive equipment setup ensures thorough petrographic examination of concrete aggregates.

The standard employs petrographic examination to detect deleterious materials in aggregates through two approaches: a routine petrographic method (Method I) and a detailed reference method (Method II). Coarse aggregate particles are broken and inspected individually using hand lenses or microscopes. Each particle is classified based on petrographic identity, physical condition, and anticipated chemical stability in concrete. Physical quality is rated from good to poor, indicating the particle’s influence on concrete strength and durability. Chemical assessment distinguishes innocuous from harmful particles, especially regarding alkali reactions. This systematic segregation allows estimation of deleterious content, ensuring only suitable aggregates are selected for concrete production.

The standard assesses aggregates using three primary criteria. First, petrographic identity involves microscopic mineral identification and segregation by rock types such as granite or sandstone. Second, physical quality is categorized into four grades: good (high strength and durability), satisfactory (moderate to high durability), fair (moderate durability under ideal conditions), and poor (low durability causing concrete deterioration). Third, chemical quality focuses on the potential for alkali-aggregate reactions, classifying materials as innocuous if they do not provoke harmful chemical effects. Particles are examined individually after breaking, with coatings like calcium carbonate or clay removed prior to evaluation. These criteria, supported by petrographic tables, ensure aggregate suitability for concrete applications.

Reporting involves preparing both a concise summary and a detailed document. The summary identifies the sample source and intended use and provides an accessible description of composition and key properties for engineers. The detailed report includes descriptions of test methods, comprehensive data on constituents, and features of each significant component, supplemented by tables, photographs, and visuals. Interpretation entails summarizing observations regarding aggregate suitability for service conditions, noting any limitations or recommending further testing if necessary. The objective is to identify physical and chemical characteristics that influence concrete quality. Clear, structured reporting supports informed decisions on aggregate acceptance or rejection, thereby assuring concrete durability and performance.