Honeycomb in concrete appears when mortar does not fill the space between the coarse aggregates completely. Honeycombing in concrete is caused due to the absence of cement mortar. The coarse aggregates are bounded with very loose bonds in the absence of fines.
It is called honeycomb because its structure looks like the shape of honey bee nests. The main reason for the appearance of honeycomb is the improper consolidation of concrete.
Honeycombs can be visually seen on the surface. In such a case, the area affected by the honeycomb is chipped out and repaired. Honeycomb inside the structural component could be estimated by ultrasonic pulse velocity test and x-ray methods
What Causes Honeycombing in Concrete?
Honeycomb in concrete is caused by improper vibration. Vibrators are used systematically at all the places in concrete to ensure good compaction. If it is inserted at a haphazard angle or randomly at different places in concrete, proper compaction may not happen.
Generally, Coarse aggregates settle down at the bottom, and cement, sand slurry remains at the top. The cement mortar could not reach the bottom thus honeycomb appears. Following are causes of honeycomb in concrete due to improper vibration-
- A small vibrator is used than required.
- The frequency of the vibrator is too low.
- Amplitude too small.
- Immersion time is shorter than required.
- Large spacing between insertion points of the vibrator.
- Inadequate penetration of vibrator.
Less quantity of sand
If the quantity of sand used in concrete is less than required the pore space between the coarse aggregates remains empty which gives rise to honeycomb.
Poor Aggregate Grading
Concrete to be perfectly compacted should consist of proper grading of aggregates. If the concrete has all sizes of aggregates from small to big sizes, the small ones will fill the space between the bigger aggregates and sand will fill the gap between the small aggregates.
But in the case of all size aggregates having all the aggregates of the same size the empty spaces between them may not get filled. Thus concrete should have proper aggregate grading.
If concrete is very stiff i.e. WC ratio is lower than required then the concrete may not flow properly and empty spaces between the coarse aggregate may not get filled. To avoid this Good workable concrete should be used.
Less Spacing between the reinforcing bar
If the reinforcing bars are too dense or do not have enough spacing between them in that case coarse aggregates are filtered out and cement sand slurry flows inside the reinforcing mesh. This is also a reason for the appearance of the honeycomb-like structure.
The spacing between the reinforcing bars should be set properly so that concrete can flow without obstruction.
If formwork is not proper or has open spaces then the cement, sand paste could flow out of the component where concreting is done. In that case, the structure is left only with only the coarse aggregate having very little to no fines. Leaking of joints could be a reason for honeycomb in concrete.
Junction of the beam to beam and beam to column may be subjected to the honeycomb because of the heavy placement of reinforcement. The stiff concrete may not get filled in these places even after vibration. These congested sections should be properly designed.
- highly congested placement of reinforcement.
- Narrow sections used
- Reinforcement splices increase the denseness of the section.
- High temperature could also lead to a honeycomb in concrete. The workability of the fresh concrete reduces due to the temperature thus the mix becomes stiff.
- If excessive freefall of concrete also leads to honeycombing. Coarser aggregates will be separated with the fines and honeycombing appears.
- If the concrete has to travel more distance in forms then the possibility of honeycombing cant be ruled out. Coarser particles will not reach the far end.
- The improper concrete mix could lead to segregation.
How to Avoid Honeycomb in Concrete
- The concrete should be compacted properly. Faulty Vibrators should not be used. Excessive vibration should not be given. Proper care should be taken during the compaction process.
- Spacing of insertion of vibrator should not be too large and adequate size vibrators should be used.
- The quantity of fines to coarse aggregate should be properly designed. Fine aggregate should be sufficient enough to fill the spaces between the coarse aggregate.
- Grading of coarse aggregate should be properly done. Concrete should consist of all sizes of aggregates preferably to avoid honeycomb in concrete.
- The concrete mix should not be very stiff. Workable concrete should be used.
- Spacing of reinforcing bars should be properly designed too closely spaced bars should not be used. If say reinforcement is too close higher diameter bars should be used.
- Narrow sections should not be used. To avoid congestion couplers etc. should be used in place of mechanical splicing.
- Concrete should not be allowed to travel more distance in the forms.
- Faulty formwork should not be used. Fines should not flow out of the formwork in any case. Joints should be perfectly sealed.
- Concrete should not be poured from a greater height. Height more than 1m should be avoided.
- Concreting on a too-hot day should be avoided.
How to treat Honeycomb in concrete
Repair with Cement Mortar
If the honeycomb appears at the surface of the beam or column or any other concrete component and the defect is minor can be repaired by applying fresh cement mortar onto the surface if the concrete is fresh or if the formwork is removed not later than 24 hours.
If the defect is extensive or concrete formwork is removed more than 1 day ago, in that case following repair methods should be done.
Repair with dry pack
A dry pack is a mixture of Portland cement and sand having just enough water to hydrate the cement.
The surface should be properly cleaned and roughened. The bounding grout of cement to the sand ratio of 1:1 in a fluid consistency. All surfaces are thoroughly brushed with the grout and before bonding grout dries up dry packing is done.
Another way is the portion to be received treatment is presoaked overnight with wet rags. The next day this wet surface is lightly brushed with dry cement and packing work is started.
It is the most common concrete repair material. In this method, new concrete is applied to the defective are without the use of a bonding agent or Portland cement grout. The surface is cleaned roughened and kept in saturated surface dry condition.
Repair material should have the same water-cement ratio as that used in old concrete but should not exceed 0.47 by weight. The lowest practical slump should be used. Repair material is then applied over the defective area.
Vibrators like form vibrators are used for removing entrapped air. The lowermost portion of the lift is filled with concrete having a good slump however as the height of the lift increases top portions can be drier and drier.
Epoxy Bonded Replacement Concrete
This method of repair consists of Portland cement and an epoxy resin bonding agent. The component to receive treatment is cleaned and roughened first. An epoxy resin bonding agent is then applied to the surface.
When this epoxy resin bonding agent is still fluid, epoxy bonded concrete should be applied. This should be compacted thoroughly with vibrators or temping.
Effects of Honeycomb in concrete
- The load-carrying capacity of the member reduces considerably.
- Due to the pores water, air and unwanted material find its way inside the structure.
- Reinforcement gets corroded which leads to the poor bond between the reinforcement and concrete. Eventually, the strength of the RCC structure reduces.
- The durability of the structure is considerably reduced.
- IF not treated well in advance this could lead to the loss of property and lives.
Is Honeycombing in concrete bad?
Honeycombing reduces the durability of the structure. The strength of concrete reduces considerably. Reinforcement gets corroded which further reduces the load-carrying capacity of the structure.
What causes concrete Honeycombing?
Improper vibration, less quantity of sand, poor Aggregate Grading, low workability, Less spacing between the reinforcing bar, leakage from the joints, heavy and narrows sections are the reason for honeycombing.
What is honeycomb in concrete, how is it formed, how do you eradicate honey comb in concrete?
Honeycomb in concrete appears when mortar does not fill the space between the coarse aggregate completely. It is formed due to Improper vibration, less quantity of sand, poor aggregate grading, low workability, less Spacing between the reinforcing bar, leakage from the joints, heavy and narrows Sections. Honeycombing can be repaired by the application of cement mortar, dry pack, replacement concrete, and epoxy-bonded replacement concrete.
How can Honeycombing be avoided?
The concrete should be compacted properly. Proper use of a vibrator, sufficient quantity of fines, proper grading of coarse aggregate, use of workable concrete, sufficient freefall, and properly designed section helps in avoiding honeycomb
What is the effect of honeycomb in concrete?
Loss of strength of Concrete, corrosion in the reinforcement, reduced durability of concrete are caused by the honeycombing.