Placing in Bags and Underwater Concreting

Placing in Bags and Underwater Concreting

Key Concepts and Points:

1. Placing in Bags:

   • Definition: A method used to place concrete in underwater or hard-to-reach areas by filling and then depositing bags filled with concrete.
   • Materials: Typically uses burlap, jute, or synthetic bags for strength and permeability.
   • Process:
     • Fill bags with a predetermined amount of concrete.
     • Seal bags to prevent leakage.
     • Lower bags into position using cranes or divers.
     • Carefully arrange to ensure structural integrity.
   • Application: Used in repairing underwater foundations, constructing breakwaters, and forming underwater walls.

2. Underwater Concreting:

   • Definition: The process of placing concrete in an underwater environment, requiring specialized techniques to ensure quality and durability.

   • Methods:

     • Tremie Method: Utilizing a tremie pipe to place concrete underwater by gravity flow.
     • Pump Method: Employs concrete pumps to deliver concrete through a delivery pipe submerged in water.
     • Bagwork Method: Concrete is placed in pre-filled bags which are then positioned underwater.
     • Pre-packed Aggregate Concrete (PAC): Pre-placed aggregates are first positioned, and then grout is pumped into the voids.
     • Sprayed Concrete: Concrete is pneumatically projected using high-velocity air.

3. Key Considerations:

   • Mix Design: Importance of designing a concrete mix that has suitable workability, cohesion, and minimal segregation.
   • Anti-washout Admixtures: Added to concrete mix to prevent cement particles from washing out in the presence of water.
   • Pumpability: Concrete must be pumpable without segregation, maintaining uniformity and consistency.

4. Execution:

   • Site Preparation: Ensures a clean underwater site free of debris.
   • Formwork: Forms are usually used to contain the concrete in the desired shape.
   • Placement Speed: Must be consistent to avoid the formation of cold joints.
   • Inspection: Use of divers or cameras to monitor and inspect the placement.

5. Challenges and Solutions:

   • Hydration and Setting: Ensuring concrete sets properly in an underwater environment.
   • Contamination: Managing the cleanliness of water to avoid contamination of concrete.
   • Hydrostatic Pressure: Countering the pressure of water at depth to maintain structural integrity.
   • Visibility and Monitoring: Overcoming poor visibility underwater; employing technologies for precise placement and monitoring.

6. Quality Control:

   • Testing: Routine testing of concrete samples for strength, permeability, and consistency.
   • Monitoring Admixtures: Regular assessment of admixture performance specific to underwater usage.
   • Diver Inspections: Regular inspection by certified divers for structural assessments and quality evaluations.

By focusing on these aspects, one can gain a comprehensive understanding of placing in bags and underwater concrete techniques, their importance, execution challenges, and quality control measures.