How violent are surface honeycombs on a concrete column?

 

Honeycombs in concrete casting may occur due to

1. Improper batching of the required materials. Coarse aggregate might have been added more than necessary.
2. Defective formwork. When formwork is not reasonably water tight, the finer material is pumped out leaving coarser material inside
3. Segregation of concrete. This may be due to improper placing. Eg. If concrete is dropped from a heights more than 1.50 meter, it may lead to segregation. Further excess compaction of concrete with needle vibrator may lead to segregation.
4. If the honeycomb is within the surface area it poses less problem. It can be rectified by applying rich mortar. But if the honeycomb is deeper it's difficult to repair. The strength and durability of the member is affected drastically.

It's always better to avoid formation of honeycombs so as to achieve the best results. 

Depending on the type and amount of honeycombing, it may be very dangerous and the strength of column may be reduced by 30%. And if it happens, the further construction on/above the column may be unsafe.

Before going in details, I would like to defined honeycombing in a simple manner.

Honeycombing means vacant space between the aggregates on the concrete surface. After removal of formwork, some holes which are porous are observed on the surface of concrete, that porous face or surface is known as honey combing. (as you see in the figure) 

Now, coming to the question -How violent are surface honeycombs on a concrete column?

Honey combing leads to ununiform cross-sectional area of column which disturbs the mechanism of load transfer (or load distribution or bending moment) from top to bottom. Thus, column may be subjected to ununiform load distribution throughout the cross-section which may cause deflection in the column.

Due to honeycombing, the reinforcement bar may be subjected to moisture and open atmosphere or it can be protected by very thin layer of mortar from atmosphere. It leads the corrosion in reinforcement and ultimately reduces the strength of column.

These two are the most adverse effects of honey combing. One other bad effect of it is bad appearance which is not dangerous.