Which of the following is/are deleterious material in aggregate?
Correct Answer: D. All option are correct
📚 Detailed Explanation: Deleterious Materials in Aggregate
Deleterious (harmful) substances are materials found mixed with aggregate that adversely affect the strength, durability, or workability of concrete. IS 383 and IS 2386 (Part II) specify the identification and permissible limits of these substances. Coal, clay lumps, and soft fragments are three of the most commonly cited deleterious materials.
Why D (All option are correct) is correct: Coal particles can expand on wetting and cause pop-outs and surface staining; they also weaken the concrete matrix. Clay lumps coat aggregate surfaces, preventing proper bonding with cement paste, and increase water demand. Soft fragments (shale, chalk) have low crushing strength and fail before the cement matrix, creating weak spots in the hardened concrete. All three are therefore classified as deleterious.
Deleterious Materials and Their Permissible Limits (IS 383)
| Deleterious Material | Permissible Limit (by weight) | Main Harm |
|---|---|---|
| Coal & lignite | ≤1% (FA), ≤1% (CA) | Pop-outs, staining, volume change |
| Clay lumps | ≤1% (FA), ≤1% (CA) | Weak bond, increased water demand |
| Soft & friable particles | ≤3% (CA) | Low strength, fracture under load |
Key Concepts for Students
- IS 2386 (Part II) provides the test methods for identifying deleterious materials, including the organic matter colour test and clay lump percentage.
- The total deleterious material in fine aggregate must not exceed 5% by weight as per IS 383.
- Sea sand is also deleterious due to chloride content — it must be washed before use to prevent reinforcement corrosion.