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4.5 QUALITY CONTROL OF AGGREGATES (FINE AND COARSE)

Aggregates form about 60–75 % of the total volume of concrete; their quality directly affects the workability, strength, and durability of concrete and mortar. Quality control ensures uniformity, compliance with standards, and durability of the finished structure.

4.5.1 Objectives of Quality Control for Aggregates

• To verify compliance with grading, strength, and cleanliness specifications.
• To minimize variations in aggregate properties between different sources and batches.
• To ensure aggregate performance under expected service and environmental conditions.
• To control deleterious materials (clay, silt, organic impurities).

4.5.2 Relevant Standards and Codes

Type of Aggregate	Applicable Indian Standards
Fine Aggregates (Sand)	IS 383, IS 2386 (Part I to VIII), IS 1542 (for sand for plaster), IS 2116 (for masonry mortar)
Coarse Aggregates	IS 383, IS 2386 (Part I to VIII)
Sampling & Acceptance	IS 2430 – Methods of Sampling of Aggregates
Moisture Content	IS 2386 (Part III)
Alkali Reactivity	IS 2386 (Part VII)
Deleterious Material	IS 2386 (Part II)

4.5.3 Sampling and Acceptance Criteria

• Sampling: As per IS 2430, at least 3 samples are taken from different portions of the lot; combined to form a representative sample.
• Acceptance: Aggregate is accepted when physical and grading requirements conform to IS 383 (2016 or latest revision).
• Frequency: Minimum once per 100 m³ of concrete or as directed by the specification.

4.5.4 General Requirements for Good Aggregates

• Hard, durable, clean, and free from organic impurities, clay, and deleterious materials.
• Properly graded to achieve dense packing and low void content.
• Sound and chemically inert under exposure to moisture and alkalis.

4.5.5 Laboratory Quality Tests (for Fine and Coarse Aggregates)

Test	Description / Purpose	Typical Requirement	Standard Reference
Sieve Analysis / Grading	Determines particle-size distribution to ensure proper workability and strength.	Fine Aggregate: Zone I – IV (As per IS 383) ; Coarse Aggregate: Passing 20 mm and retained on 4.75 mm sieve (typical)	IS 2386 (Part I)
Specific Gravity and Water Absorption	Indicates density and porosity; affects mix design and durability.	Fine Agg: 2.6–2.9; Coarse Agg: 2.6–2.9; Water Absorption ≤ 2 %	IS 2386 (Part III)
Bulk Density and Voids	Determines packing efficiency and void ratio.	Bulk Density (Loose): 1300–1600 kg/m³; Voids < 45 %	IS 2386 (Part III)
Moisture Content	Essential for adjusting water–cement ratio during mixing.	Generally 0.2 – 2 % for coarse; 0.5 – 5 % for fine aggregates	IS 2386 (Part III)
Silt Content / Material Finer than 75 µm	Checks for dust or clay; excess reduces bond strength.	≤ 3 % (by weight) for natural sand ; ≤ 15 % for manufactured sand	IS 2386 (Part II)
Organic Impurities Test	Detects decaying vegetation and organic matter that retard setting of cement.	Colour of solution lighter than standard solution	IS 2386 (Part II)
Crushing Value Test	Measures resistance to crushing under gradually applied load.	≤ 30 % for concrete other than wearing surfaces; ≤ 45 % for base courses	IS 2386 (Part IV)
Impact Value Test	Determines toughness of aggregates against sudden impact.	≤ 30 % for concrete ; ≤ 35 % for other works	IS 2386 (Part IV)
Abrasion Test (Los Angeles / Deval)	Measures hardness and resistance to wear.	≤ 30 % for surface courses ; ≤ 50 % for other works	IS 2386 (Part IV)
Flakiness and Elongation Index	Determines shape characteristics of coarse aggregates.	Combined Index ≤ 30 %	IS 2386 (Part I)
Alkali–Silica Reactivity (ASR)	Checks potential for deleterious expansion with cement alkalis.	Expansion ≤ 0.10 % (after 14 days in NaOH solution)	IS 2386 (Part VII)
Soundness Test (Sodium / Magnesium Sulfate)	Assesses resistance to disintegration due to weathering.	Weight loss ≤ 12 % (Na₂SO₄); ≤ 18 % (MgSO₄)	IS 2386 (Part V)
Deleterious Materials (Clay Lumps, Coal, Shells)	Ensures aggregates are free of harmful impurities.	≤ 5 % (by mass)	IS 2386 (Part II)

4.5.6 Interpretation and Acceptance

Parameter	Criteria for Acceptance
Grading	Must conform to IS 383 limits for respective zones or nominal sizes.
Strength Tests	Aggregate crushing value, impact value, and abrasion value within permissible limits.
Durability	Soundness and ASR tests within specified limits.
Cleanliness	Silt content and deleterious material within allowable limits.

4.5.7 Common Causes of Rejection

• Excessive dust or silt content.
• Poor grading leading to segregation.
• Presence of clay lumps or organic matter.
• Unsound aggregate showing high loss in sulphate test.
• Failure in impact or crushing value criteria.

4.5.8 Field Checks and Practical Control

• Visual inspection for cleanliness and grading.
• Hand sieving for approximate particle-size distribution.
• Periodic sampling during storage and mixing.
• Protection of aggregates from dust and moisture variations by covering stockpiles.

4.5.9 Key Takeaways

• Consistent aggregate quality is crucial for durable concrete.
• Test results must be regularly monitored and plotted for trend analysis.
• Maintain records of source approval, sampling frequency, and test results for audit and traceability.