What is compaction and how is it achieved? | Q & A

Compaction is the process of densifying concrete or soil by removing entrapped air or voids, so that the material achieves maximum strength, stability, and durability.

In RCC work, compaction mainly refers to compacting fresh concrete.

Why Compaction Is Important

• Increases strength of concrete

• Prevents honeycombing and voids

• Improves bond between concrete and reinforcement

• Reduces permeability and improves durability

How Compaction Is Achieved (Concrete) 1. Manual Compaction

Methods:

• Rodding

• Tamping

Used for:

• Small works

• Thin sections

Limitations:

• Less effective

• Not suitable for RCC

2. Mechanical Compaction (Most Common)

Achieved using vibrators:

a) Needle (Internal) Vibrator

• Inserted into concrete

• Most effective for beams, columns, slabs

b) Surface Vibrator

• Used for slabs and pavements

c) Shutter / External Vibrator

• Used where needle cannot reach

How to Do Proper Compaction (Site Practice)

• Place concrete in layers

• Insert vibrator vertically

• Vibrate for 5–15 seconds per point

• Ensure vibrator reaches previous layer

• Stop when:

  • Air bubbles stop

  • Surface becomes glossy

Compaction in Soil (Short Note – Interview Extra)

• Achieved by:

  • Rolling

  • Ramming

  • Vibratory rollers

• Purpose: increase bearing capacity and reduce settlement

One-Line Interview Answer (Perfect)

Compaction is the process of removing air voids from concrete or soil to increase density, strength, and durability, achieved mainly by mechanical vibration in concrete works.

Common Interview Follow-Up

Q: What happens if compaction is inadequate?
A: Honeycombing, low strength, and poor durability.

Q: What happens if over-compaction is done?
A: Segregation of concrete.