Vibrator (Concrete Vibrator)

A concrete vibrator — commonly a handheld immersion ("stinger") vibrator — is used to consolidate wet concrete as it's placed, working out trapped air voids and helping the mix flow tightly around rebar and into the corners of formwork.

Under-vibration leaves honeycombing (visible voids) at the surface; over-vibration can cause aggregate to segregate from the paste. Getting it right is one of the more skill-dependent steps in a pour.