Which of the following methods explains the process of underwater concreting? (i) Pumping technique (ii) Hydro valve method (iii) Toggle bags method (iv) Bagged concrete method
Correct Answer: C. (i), (ii), (iii), and (iv)
📚 Detailed Explanation: All Four Methods Are Used for Underwater Concreting
Why C [(i), (ii), (iii), and (iv)] is correct: Underwater concreting is a specialised operation required for bridge piers, marine foundations, dams, and retaining walls in water. Multiple methods have been developed. All four listed methods — pumping, hydro valve, toggle bags, and bagged concrete — are recognised techniques for placing concrete underwater. The tremie method is also widely used (most common) but was not listed as one of the four options.
Underwater Concreting Methods
| Method | Principle | Application |
|---|---|---|
| (i) Pumping technique | Concrete pumped through a pipe (bottom discharge) from surface; pipe submerged with outlet kept below concrete surface to prevent water mixing; combined with tremie principle for anti-washout | Large marine foundations; bridge caissons; modern preferred method |
| (ii) Hydro valve method | Specially designed valve at bottom of concrete container allows concrete to be released underwater in a controlled manner, preventing water intrusion by hydrostatic pressure differential | Shaft foundations; enclosed cofferdam pours |
| (iii) Toggle bags method | Canvas or jute bags (toggle bags) filled with semi-dry concrete are lowered on a rope and placed manually by divers; bags are slit or opened at the placement location; concrete flows out and knits together | Small repairs; inaccessible underwater locations; cofferdam sealing |
| (iv) Bagged concrete method | Permeable fabric bags filled with dry or semi-dry concrete are stacked or placed by divers/crane; seawater saturates the mix and initiates hydration; bags degrade leaving a consolidated mass | Scour protection; gabion-like revetment; emergency repairs |
| Tremie method (most common) | Steel tube (tremie pipe, φ200–300mm) lowered to base; concrete fed by gravity; pipe kept submerged in fresh concrete; raised as pour proceeds; never withdrawn from concrete surface | Bridge piers; caissons; bored pile foundations; most widely used underwater concreting method |
Requirements for Concrete Mix Used Underwater
| Property | Requirement | Reason |
|---|---|---|
| w/c ratio | 0.4–0.55 | Flowable but not segregating; anti-washout |
| Slump | 150–200 mm (very high) | Self-levelling without vibration (vibration causes washout) |
| Cement content | Minimum 400 kg/m³ | Rich mix for cohesion and impermeability |
| Anti-washout admixture | Often used in modern practice | Prevents cement and fines from washing away in water |
| Vibration | None (or minimal) | Vibration disperses concrete and causes washout |
- All four methods — pumping, hydro valve, toggle bags, and bagged concrete — are valid underwater concreting techniques.
- The Tremie method is the most common and preferred method for large underwater pours.
- Key principle in all methods: concrete must not fall freely through water — it must be placed in one continuous mass to prevent washout and segregation.