Shrinkage of concrete:
Correct Answer: A. Increases bond strength
📚 Detailed Explanation: Shrinkage Increases Bond Strength in Reinforced Concrete
Why A (Increases bond strength) is correct: When concrete undergoes drying shrinkage, it contracts volumetrically. If steel reinforcement bars are embedded within the concrete, this contraction causes the concrete to grip the steel more tightly — like a vice closing around the bar. This friction/mechanical grip increases the bond strength between concrete and steel, benefiting the composite action of reinforced concrete.
Mechanism of Shrinkage-Induced Bond Enhancement
| Stage | What Happens | Effect on Bond |
|---|---|---|
| Concrete placement | Steel bar surrounded by fresh mix; no significant bond yet | Bond = 0 (wet state) |
| Initial setting | Cement paste begins to stiffen around bar | Chemical adhesion starts |
| Drying shrinkage | Concrete volume contracts; concrete grips steel bar | Friction bond increases significantly |
| Long-term | Continuous shrinkage maintains lateral pressure on bar | Bond strength sustained; increases with curing time |
Bond Strength Components in Reinforced Concrete
| Component | Mechanism | Affected by Shrinkage? |
|---|---|---|
| Chemical adhesion | Gel bond between cement paste and steel oxide layer | Indirectly (paste quality) |
| Friction | Normal force × coefficient of friction at interface | YES — shrinkage increases normal force |
| Mechanical interlock | Concrete flowing into ribs/deformations of bar | YES — shrinkage pushes concrete into deformations |
⚠ Caveat — Excessive Shrinkage: While moderate shrinkage grips the steel, excessive differential shrinkage can cause cracking parallel to reinforcement (splitting cracks), which severely reduces bond. The beneficial effect holds within normal design limits.
- Shrinkage increases bond strength because concrete contracts and grips steel bars more tightly.
- This effect is one reason why a higher aggregate content (which restrains shrinkage) is balanced by bond requirements in mix design.
- Deformed (ribbed) bars benefit from both mechanical interlock and shrinkage-induced grip.