Honeycombing refers to voids in concrete caused by the mortar not filling the spaces between the coarse aggregate particles. It usually becomes apparent when the form work is stripped, revealing a rough and ‘stony’ concrete surface with air voids between the coarse aggregate. Sometimes, however, a surface skin of mortar masks the extent of the defect. Honeycombing may extend some depth into the member. Honeycombing is always an aesthetic problem, and depending on the depth and extent may reduce both the durability performance and the structural strength of the member.
Honeycombing is caused either by the compaction not having been adequate to cause the mortar to fill the voids between the coarse aggregate, or by holes and gaps in the form work allowing some of the mortar to drain out of the concrete. In some cases, the member shape and detailing/placement of the reinforcement compounds the effect of inadequate compaction.
PRACTICES TO MINIMIZE THE OCCURRENCE OF HONEYCOMBED CONCRETE
To minimize the incidence of honeycombed concrete:
- Ensure the mix has sufficient fines to fill the voids between the coarse aggregate.
- Use a mix with appropriate work ability for the situation in which it is to be placed.
- Ensure the concrete is fully compacted and the placing methods minimize the risk of segregation.
- Ensure the reinforcement layout and the section shape will permit the concrete to flow around the reinforcement and completely fill the forms.
- Check that the form work is rigid and well braced, the joints are watertight and any penetrations through the form work, eg form ties, are properly sealed.
Fixing of honeycomb in concrete
It is always better to avoid imperfections such as honeycombing in concrete rather than have to repair them. However, if honeycombing does occur then it can be repaired using the following techniques. The extent and depth of the honeycombed area first needs to be defined. This can be done by chiselling out the affected area to expose sound concrete or by using non-destructive testing techniques such as impact-echo. If the honeycombed area is small in extent and depth does not significantly jeopardise the quality of the cover concrete protecting the reinforcement then, it can be repaired by patching with mortar of a similar colour to the base concrete.
Any lightly attached stones should be removed before the mortar is worked into the spaces between the aggregate ensuring that it completely fills the honeycombed area. The area should be slightly over filled and screeded off to give a similar texture to the surrounding surface. The patch should then be cured. Consideration needs to be given to the appearance of the repaired surface relative to adjacent untreated surfaces. As a general rule, mortar used for patching should be made from the same materials as the original concrete except that a proportion of off-white cement should be mixed with the original cement to lighten the colour and thus better match the existing surface.
If the honeycombing is extensive and penetrates down to the reinforcement or even deeper than it is necessary to cut out the defective concrete and replace it with sound concrete. It is essential that the reinforcement be surrounded by sound concrete. The advice of a suitably qualified engineer should be obtained to check that the load-carrying capacity of the member, as repaired, will be satisfactory.