Why honey combing occur in hardened concrete?

 

Honeycombing is the term used to describe areas of the concrete surface that are coarse, harsh and stony. It may be caused by insufficient fine material in the mix, perhaps due to incorrect vibartion, aggregate grading or poor mixing.

dense of reinforcement and no space between steel bars.. bad using of vibrator ... slump of concrete less than 160 mm 

Honeycombs 

These are hollows and cavities in concrete mass on surface or inside the concrete mass.

Honeycombs are visible to naked eyes and can be detected by ultrasonic testing.

Reasons are as ,

Less vibration during concrete,

Less cover to reinforcement bars, 

very stiff concrete.

mostly on junction of beam to beam to column

Improper gradation of aggregate in concrete.

Segregation in concrete.

Etc

It may be caused by insufficient fine material in the mix perhaps due to incorrect aggregate grading or poor mixing. Alternatively, honeycombing may be caused by leakage of grout or mortar fraction from the concrete at formwork joints.