What is Honey comb in concrete?
Honeycombs are the hollow spaces and cavities left in concrete mass on surface or inside the concrete mass which is caused by the mortar not filling the spaces between the coarse aggregate particles
Honeycombs which are on sides and bottoms are visible to naked eyes and can be detected when shuttering is removed. Honeycombs which are inside mass of concrete can only be detected by advanced techniques like ultrasonic testing etc.
Causes of honeycomb in Concrete
- Addition of more water than designed at site for achieving workability.
- When number of cover blocks to rebar is less or the size of the cover blocks is not sufficient, honeycombs might occur.
- Poor grading of aggregate & using of more coarse aggrgegates than designed mix proportions and over sized aggregates.
- Improper vibration of concrete in formwork.
- Over vibration would make the concrete segregated while under vibrated concrete would have honeycombs in it.
- Concrete is poured from more than allowable height.
- When joints of formwork are loose there can be leakage of grout through these joints Honeycombs might occur.
- Use of stiff concrete mix or the concrete is already set before placing.
Effects of Honeycombs
- Reduces the load bearing capacity of structure.
- Water and air enters inside the concrete.
- Reduce durability of concrete.
- Rusting and corrosion of reinforcement will occurs.
- It causes less structural strength.
Remedies of Honeycomb in concrete
- sufficient cover blocks should be provided.
- Use proper methods of compacting and ensure concrete is fully compacted. To prevent concrete from honey comb, Vibrator should run in concrete in proper manner during filling of concrete. The vibrator should run with specific time. The duration of running vibrator should not exceed more than 30 seconds, otherwise it will start separating water from concrete, which will let the concrete to lose its workability and strength.
- Use a mix with appropriate workability for the situation in which it is to be placed.
- Proper cover block thickness should be maintained for particular concrete structures.(Eg:- walls, columns, staircases – 25mm, slabs- 40mm, beams- 50mm, pile caps- 75mm) .
- Insert vertically Vibrate by layers Vibrate the whole depth of layer Insert at intervals. In some cases even after running the vibrator honey comb will appear and in such cases add the admixture to cement mortar and paste it in voids and concrete will regain its strength.
- Ensure the mix has sufficient fines to fill the voids between the coarse aggregate.
- Use proper methods of placing concrete.
- Ensure the reinforcement layout and the section shape will permit the concrete to flow around the reinforcement and completely fill the forms.
- Rebar congestion can be minimized by having larger (that is, lesser) rebar or by increasing the sectional dimensions of elements where possible.