Honeycombs are hollow spaces in concrete caused when mortar fails to fill spaces between coarse aggregate. There are three types: small (<25mm deep), moderate (25-50mm deep), and large (>50mm deep exposing rebar). Honeycombs reduce durability and strength. They are caused by poor workability, improper compaction, rebar congestion, leaky formwork, and poor aggregate grading. To prevent honeycombs, proper mix design, placement, compaction, formwork, and cover are needed. Small and moderate honeycombs can be repaired by removing loose material, cleaning, applying grout, and curing. Large honeycombs require removing concrete to sound material and applying mort
3. What are honeycombs?
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.
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4. Types of Honeycombs
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Small Size Honeycombs
Depth is less than 25mm
Moderate Size Honeycombs
Deeper than 25mm but steel bars have not
exposed
Larger Size Honeycombs
Deeper than 25mm & bars have come out
5. Where can we find Honeycombs?
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.
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6. Effects of Honeycombs
Water and air enters inside
Rusting and corrosion of reinforcement
Reduce durability
Less structural strength
Reduces the load bearing capacity
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8. Poor Workability8
Slump Test can be used to find out the workability of concrete. If the slump is
less workability is less.
Workability might be less because
Low water content
Finer cement particles
High temperature
Ratio of fine-to-coarse aggregate being low
9. Improper compaction9
The purpose of compacting concrete by vibration is
the elimination of entrapped air and forcing the
particles in to a closer configuration that will
definitely build the more strength.
Over vibration would make the concrete
segregated while under vibrated concrete would
have honeycombs in it.
Poor concrete compaction occurs due to ineffective
vibration or rebar congestion.
Over vibration
Under vibration
Inserting at angles
Touching reinforcement
Touching formwork
10. Improper cover & placement of rebar
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Rebar congestion - If rebar is placed too close
together or too close to formwork it will trap the
larger pieces of aggregate while the mortar in
the mixture may not pass through & interfere
with concrete flow and vibration.
Insufficient cover – When number of cover
blocks to rebar is less or the size of the cover
blocks is not sufficient, honeycombs might
occur.
11. Movement of Formwork & Grout leak
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When joints of formwork are loose there can be leakage of grout through these
joints. Honeycombs occur.
Increased water content also results in bleeding, hence, increased water
content can also mean that cement slurry (grout) will escape through the joints
of the formwork.
12. Presence of more percentage of bigger size of aggregate in
concrete also prevents concrete to fill narrow spaces between the
reinforcement rods.
Voids often occur because aggregate is too large to pass through
a condensed area of reinforcement.
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13. Practices to minimize Honeycombing in
concrete
Use a mix with appropriate workability for the situation in which it is to be
placed.
Ensure the mix has sufficient fines to fill the voids between the coarse
aggregate.
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14. Insert vertically
Vibrate by layers
Vibrate the whole depth of layer
Insert at intervals
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Use proper methods of compacting and ensure concrete is fully compacted
Use proper methods of placing concrete
Avoid segregation
Avoid delays
Pour by layers
15. Proper cover block thickness should be
maintained for particular concrete
structures.(Eg : walls, columns, staircases –
25mm, slabs- 40mm, beams- 50mm, pile
caps- 75mm)
Sufficient cover blocks should be
provided
Rebar congestion can be minimized by
having larger (that is, lesser) rebar or by
increasing the sectional dimensions of
elements where possible.
Ensure the reinforcement layout and the
section shape will permit the concrete to
flow around the reinforcement and
completely fill the forms.
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16. Practices to minimize Honeycombing in
concrete
Check that the formwork is rigid and well
braced, the joints are watertight and any
penetrations through the formwork. e.g.:
form ties are properly sealed.
Provide enough supports to formwork
Defective formworks and accessories
should be repaired or replaced to prevent
grout leakage during concreting.
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17. Repairing Honeycombs
For small & moderate honeycombs
Hack and remove the loose particles in order to
appear sound surface.
Surface is cleaned by water jet to obtain dust
free surface.
Construction grout is applied.
Texture and color should then be matched for
aesthetic finish.
Curing is done.
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18. For large Honeycombs
Hack & remove the weak concrete to expose
sound concrete surface.
To keep clearance around reinforcement 12mm
thick concrete layer is removed.
By using water jet surface is cleaned
Barra emulsion is used as a bonding agent.
Over the wet surface barra emulsion is applied
and before drying, 1:2 cement mortar is applied
After mortar has set sufficiently, final surface is
prepared by trowel.
Texture and colour is matched for aesthetic finish.
Curing is done.
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Before
After
