In this article, we will shall discuss on 15 unseen errors lead to concrete honeycombing at site, the ways of identifying these errors and most importantly the ways of avoiding them.
One of the most frequent areas of construction site defect that is most misunderstood is concrete honeycombing. Most site engineers have considered it a minor surface problem, however, in reality, honeycombing could significantly lower structural strength, service life, and durability of RCC members.
The problem with honeycombing is that it is not very often noticed on the surface but contains severe internal cavities. Honeycombing in the majority of cases is not caused by a single major error, but rather by a set of small mistakes that are hidden in the course of concreting.
What Is Concrete Honeycombing?
Concrete honeycombing is a phenomenon in which openings, holes or bare coarse aggregate are present on the surface or within hardened concrete. These holes appear like a honeycomb formation, thus the name.
Given below are the reasons Why Honeycombing Is Dangerous.
- Decreases compressive strength.
- Allows the entry of water and air.
- Speedens up reinforcement corrosion.
- Concessions unload carrying capacity.
- Causes high cost of repairs or retrofit.
15 Unseen Honeycombers That Cause Concrete Honeycombing.
1.Incorrect Concrete Mix Design.
An imbalanced mix design is one of the major causes of honeycombing, which are hidden.
Common site issues:
- Excess coarse aggregate
- Lack of fineness of aggregate.
- Low cement content
- Unsuitable ratio of water-cement.
- Uncivil, non-adhesive amalgam can not run smoothly around reinforcement and create its gaps.
Prevention:
- Use approved mix design
- Guess mixes RCC site-made should be avoided.
- Make sure that aggregates are well graded.
2. Too much Reinforcement Congestion
Highly congested reinforced flow slows down the flow of concrete, in particular around beam-column joints and footing necks.
Hidden problem:
Concrete does not seem solid when it is poured over and it has holes beneath bars.
Prevention:
- Keep the bar spacing to minimum as required by IS codes.
- Smaller aggregate size where there is no escape of congestion.
- Pour concrete in layers.
Also Read:
- Top 15 Most Common Building Material Failures
- 15 popular cement test mistakes at site
- Quality Tests on Cement, Sand and Aggregates
3. Poor Compaction (Most Imageable Error)
The most frequent reason of honeycombing on Indian construction sites is improper vibration.
Typical site errors:
- No vibrator used
- Vibrator lifted off too fast.
- Vibrating only top surface
- It is a pouring tool that uses needle vibrator.
Prevention:
- Mechanical needle vibrators should be used.
- Fit vibrator in a vertical position at periodic intervals.
- Vibrate until no more bubbles of air come to the surface.
4. Over-Vibration of Concrete
Yes–excessive vibration also is bad.
What happens:
- Segregation of aggregates
- Cement slurry rises
- Coarse aggregates settle
- Voids form at the bottom
Prevention:
- Only insertion intervals of 5-15 seconds of vibrating.
- Stop when concrete is glossy.
5. Improper Coarse Aggregate Size.
Silent honeycombing activators are the use of large-sized aggregates in thin RCC members.
Common site mistake:
Beam or column using congested columns, slabs or beams with 20 mm aggregate
Prevention:
- Aggregate 10 mm where there is a congestion.
- Adhere to specifications and structural drawings.
6. Low Workability of Concrete.
Low slump concrete is incapable of flowing and filling up form work corners, particularly in columns and shear walls.
Why this happens:
- Low water content
- No admixtures used
- Dry aggregates
Prevention:
- Maintain specified slump
- Use plasticizers or super plastics.
- Do slump tests regularly.
7. Incorrect installation of Concrete.
The pouring of concrete at an excessive height or at a single point only leads to segregation.
Hidden site issue:
On the surface, concrete appears fine appearing but the coarse aggregates roll away, leaving empty spaces.
Prevention:
- Limit free fall to 1.5 m
- Use chutes or tremie pipes
- Lay concrete near the final location.
8. Leaky or Badly nailed Formwork.
Holes in formwork will enable the leakage of cement slurry and concrete will be left hollow.
Signs:
- Outside shuttering slurry stains.
- after de-shuttering rough concrete surface.
Prevention:
- Use foam, tape or putty to seal joints.
- Make sure that the shuttering is tight.
- Oil formwork properly.
9. Improper Formwork Design
Weak or anisotropic formwork will make it move when concreting and disrupt compaction.
Hidden danger:
Bulls in the formwork → shift of the concrete creates air bubbles.
Prevention:
- Apply stiff and sound formwork.
- Before the concreting, check verticality and alignment.
10. Lag Time between Mixing and Placing.
Concrete which begins to set prior to placement loses its workability.
Common causes:
- Traffic delays
- Poor site coordination
- Re-tempering using water (extremely risky)
Prevention:
- Install concrete in initial setting time.
- Avoid adding water at site
- Plan pour sequence properly
11. Inappropriate Layering in the Process of Concreting.
When the thick layers are poured without sufficient vibration, the layers get air trapped in between.
Site mistake:
Whole column height casting without layered compaction.
Prevention:
- Pour in 300–450 mm layers
- Vibrate each layer properly
- Make sure that there is overlap of vibrator between layers.
12. Neglecting Corner and Edge Zones.
Honeycombing is most likely to occur at corners and edges and cover zones.
Why it happens:
Vibrator is not inserted close to the edges of the shutter because of the potential damage.
Prevention:
- Vibrate near the edges of the formwork.
- Tight areas should have smaller diameter vibrators.
13. Low Quality Control and Supervision.
The Honeycombing multiplies tremendously in the absence of technical control during the concreting.
Hidden truth:
Majority of the honeycombing failures occur after working or night concreting.
Prevention:
- Make sure that there is an engineer on-site during pouring.
- Use concreting checklists
- Train labor properly.
14. Wet-End Concrete Re-Tempering
The addition of water to stiff will temporarily increase the workability but will ruin the quality of concrete.
Result:
- Increased bleeding
- Loss of cohesion
- After setting, severe honeycombing occurred.
Prevention:
- Reject stiff concrete
- Admixtures should be approved.
15. Missing Mock Trials and Pour Planning.
This is because rushing into concreting without trial pours or planning causes some defects which can be avoided.
Common oversight:
None of the planning of the number of vibrators, labor, or pour order.
Prevention:
- Conduct trial mixes
- Plan manpower and equipment
- Prepare contingency plans
How to Spot Honeycombing in the Early Stage.
Given below are the observations on how to spot honeycombing in early stage.
- Obvious gaps following de-shuttering.
- Exposed aggregates
- Hammer tapping, hollow-sound.
- Low rebound hammer values
The above observations allows avoiding serious structural damage in the future through early detection.
Reconstruction of Honeycombed Concrete
The following are the reconstruction methods of spotted honeycombed concrete areas.
- Depending on severity: Polymer-modified mortar
- Moderate: micro-concrete repair.
- Severe: Chipping and pressure grouting/jacketing.
Always seek the advice of a structural engineer prior to repair.
Final Thoughts:
Honeycombing of concrete is not a possibility, it is an operation failure. Majority of the cases are due to some hidden site-level errors that are easily avoided by proper planning, supervision and implementation.
It is cheap to avoid honeycombing but it is expensive and dangerous to repair once it has begun. As an engineer of a site, a contractor or a homeowner, keep in mind:
Good concrete does not mean pouring faster, it is pouring it right.