Which of the following factors influence the durability of concrete?
Correct Answer: D. All options are correct
📚 Detailed Explanation: Three Factors Governing Concrete Durability
Why D (All options) is correct: IS 456:2000 Clause 8 defines durability as the ability of concrete to resist weathering, chemical attack, abrasion and other deterioration processes throughout its design life. All three listed factors are explicitly recognised in this definition and in the exposure classification and design provisions of IS 456.
Factor A: Cover to Embedded Steel
| Cover Depth | Effect on Durability |
|---|---|
| Adequate cover (≥ IS 456 minimum) | Protects steel from CO2 (carbonation) and Cl¹¯ (chloride) ingress; maintains alkaline environment (pH >11.5) that passivates steel |
| Inadequate cover | Faster carbonation front reaches steel; chloride threshold exceeded; corrosion starts; expansive rust causes spalling |
Factor B: Cement Content and Water-Cement Ratio
| Parameter | High Value → Lower Durability | Low Value → Higher Durability |
|---|---|---|
| w/c ratio | High w/c → porous paste → easy ingress of water, salts, gases | Low w/c → dense paste → impermeable; durable |
| Cement content | Too low → insufficient paste to fill voids → porous mix | Adequate content + correct type = dense, durable matrix |
Factor C: Shape and Size of Member
| Aspect | Durability Impact |
|---|---|
| Surface/volume ratio | High S/V (thin slabs) → fast drying, more shrinkage cracking, more surface attack |
| Corners and re-entrant angles | Stress concentrations → earlier cracking → easier ingress of aggressive agents |
| Large sections | Thermal gradients → differential expansion → internal cracks (mass concrete) |
IS 456:2000 Minimum Requirements for Durability
| Exposure Class | Max w/c | Min Cement (kg/m³) | Min Grade | Min Cover (mm) |
|---|---|---|---|---|
| Mild | 0.55 | 300 | M20 | 20 |
| Moderate | 0.50 | 300 | M25 | 30 |
| Severe | 0.45 | 320 | M30 | 45 |
| Very Severe | 0.45 | 340 | M35 | 50 |
| Extreme | 0.40 | 360 | M40 | 75 |
- Durability is governed by all three: cover depth, cement content/w-c ratio, and member shape/size.
- Defective cement and weak aggregates produce low-grade, porous concrete with poor durability.
- The ingress of water, salts, and gases through pores is the primary mechanism of concrete deterioration.