Honeycombs are hollow spaces and cavities left in concrete mass on the surface or inside the mass where concrete could not reach. These look like honey bees nest. Improper vibration and workability of concrete are main causes of honeycombs in concrete.
Honeycombs which are on sides are visible to naked eyes and can be detected easily as soon shuttering is removed. Honeycombs which are inside mass of concrete can only be detected by advanced techniques like ultrasonic testing etc.
Causes of Honeycombs in Concrete
Honeycomb is due to non-reaching of concrete to all places due to which cavities and hollow pockets are created, main reasons are:
- Improper vibration during concrete.
- Less cover to reinforcement bars
- Use of very stiff concrete (this can be avoided by controlling water as per slump test).
- Places like junction of beam to beam to column and to one or more beams are the typical spots where honeycombs are observed. This is due to jumbling of reinforcement of beams and column rods at one place; special attention is required at such place during concreting and vibrating.
- Presence of more percentage of bigger size of aggregate in concrete also prevents concrete to fill narrow spaces between the reinforcement rods.
Remedies for Honeycombs in Concrete
Strictly speaking wherever honeycombs are observed in concrete, the affected area should be broken and the portion should be re-concreted after applying grouting chemical to the old surface.
Honeycombs as a defect not only reduces the load bearing capacity but water finds an easy way to reinforcement rods and rusting and corrosion starts.
Corrosion is a process which continues through reinforcement rods even in good concrete, this result in losing grip between rods and concrete, which is very dangerous to safety and life of concrete structures.
Reinforced concrete structures have failed within 20 or 30 years of their construction due to honeycombs, which is less than half their projected life. Especially no risk should be taken in case of columns, machine foundations, rafts, beams etc., where breaking and recasting is the only best way.
In case of honeycombs on surface pressure grouting with cement based chemicals which are non-shrinkable can be adopted after taking opinion of the designer and acting as per his advice.
It will not be out of context to point out that contractors and their supervisors are in the habit of hiding honeycombs by applying super facially cement plaster on the honeycombs, hence site engineer must be very cautious.
At places of junction of columns and beams concrete with strictly 20mm and down aggregates should be used with slightly more water and cement to avoid honeycombs.
Taping with wooden hammer the sides of shuttering from outside during concreting and vibrating will help minimizing honeycombs to a great extent in case of columns and beams.
Use of thinner needle say 25mm or less with vibrator at intricate places of concreting will also help in reducing honeycombs.