VIBRATION
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Overview of Vibrating Concrete
Vibrating concrete is a crucial process in construction to ensure the strength, durability, and uniformity of concrete structures. It involves the use of mechanical devices to consolidate freshly poured concrete by removing entrapped air and helping the mix settle properly around reinforcement and into formwork.
1. Need for Vibrating Concrete
Freshly poured concrete traps air, which can make up 5-20% of its volume if not properly compacted. This trapped air:
- Weakens structural integrity
- Reduces durability and impermeability
- Causes honeycombing and surface blemishes
- Inhibits bonding with reinforcement
Concrete vibration helps:
- Achieve maximum density
- Improve bonding between concrete and rebar
- Increase compressive strength
- Enhance surface finish
2. Timing of Vibration
Timing is critical to ensure proper consolidation without damaging the structure:
- Immediately after placement: Begin vibration as soon as the concrete is placed in layers.
- Layer by layer: Each layer (typically 300-500 mm thick) should be vibrated before placing the next.
- Do not delay: Delayed vibration may lead to cold joints and inadequate consolidation.
- Avoid over-vibration: This can cause segregation of aggregates and bleeding (excess water rising to the surface).
3. Protocols and Best Practices
a. Insertion Technique
- Insert the vibrator vertically and quickly.
- Allow it to sink under its own weight.
- Keep it in place for about 5–15 seconds (until air bubbles stop surfacing and the concrete settles).
- Withdraw slowly to avoid voids.
b. Vibration Pattern
- Vibrate at regular intervals (typically every 30–45 cm).
- Overlap zones of vibration to ensure full coverage.
- Avoid touching reinforcement directly.
c. Layering and Re-Vibration
- Each new layer should penetrate into the previous one for proper bonding.
- Re-vibration may be done if needed within 15–30 minutes of placement, provided the concrete is still workable.
d. Safety & Equipment Care
- Operators should wear PPE (gloves, ear protection, boots).
- Inspect equipment for damage or wear before use.
- Avoid operating in waterlogged or unstable positions.
4. Equipment Used for Vibrating Concrete
a. Internal (Poker) Vibrators
- Most common for cast-in-place concrete
- Inserted directly into concrete
- Comes in various diameters (25–100 mm)
- Electric or petrol-powered
b. Form (External) Vibrators
- Attached to formwork
- Suitable for precast elements and thin walls
- Helps achieve surface finish without internal intrusion
c. Surface Vibrators
- Used for slabs and pavements
- Examples: Screed vibrators, pan vibrators
- Vibrate from the top surface
d. Table Vibrators
- Used in precast concrete manufacturing
- A vibrating table shakes the entire mold
