Types of vibrators and their uses: 1. Needle vibrator : It is used when concrete is placed in layers not more than 45- 60cm high (or 8 to 10 times the diameter of needle) 2. Surface or Screed vibrator : For compacting plain concrete road surface of thickness less than 20 cm. 3. External vibrators : External vibrators are more often used for pre-casting of thin in-situ sections of such shape and thickness as cannot be compacted by internal vibrators. 4. Vibrating tables : It is used for compacting stiff and harsh mixes and used in precast structural members in factories and laboratory specimens.