Honeycombing in concrete
Figure 1: Honeycombing on a Concrete Element
Honeycombing is a term used to describe the hollow spaces and cavities; or the rough, pitted appearance on the concrete surface or inside the concrete mass, resulting from the mortar not adequately filling the spaces between the coarse aggregates. The honeycombing usually becomes evident once the formwork has been stripped off the concrete to reveal a rough and stony surface which has air voids in between the coarse aggregates. Sometimes, the honeycombing might not be immediately visible after formwork stripping because a layer of mortar on the concrete surface may mask the extent of the damage. It is possible for honeycombing to extend a significant depth into the concrete member.
Honeycombing is unsightly and thus has a negative impact on the aesthetics of a concrete structure. It can also have a big impact on the durability performance and structural integrity of a concrete member, depending on the severity of the damage. The voids in the concrete reduce the amount of concrete cover to the reinforcement and create paths for water and harmful agents to enter the concrete, resulting in deterioration processes such as corrosion. The effects of honeycombing are therefore an even bigger concern in coastal regions where water and air contain deleterious agents, and in water retaining structures where the concrete is exposed to water the majority of their operating life.
Causes
The possible causes of honeycombing include:
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Poor workability of concrete, where the mix is too stiff, making it difficult to place and compact it;
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Insufficient compaction, which may leave entrapped air bubbles in the concrete and which may result in concrete not being spread out evenly throughout the formwork;
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Improper rebar placement interferes with the flow and compaction of the concrete, particularly when rebars are congested;
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Formwork movement and unsealed formwork joints result in the leakage of mortar through the joints, leaving coarse aggregates exposed;
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Poor aggregate grading may result in a big number of larger sized aggregates which may not be able to fit in small gaps such as the spacing between rebars, resulting in voids;
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Placing concrete from heights causes segregation of the coarse aggregates and the mortar;
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High water cement ratio results in a high water content in the concrete mix and causes segregation of the coarse aggregates and the mortar.
Preventative Measures
The following measures can be taken to prevent or minimise the occurrence of honeycombing in concrete:
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Ensure that the concrete mix contains a sufficient amount of fines that will be able to fill voids between the coarse aggregates;
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The workability of the concrete mix should be suitable for the application and placement- a lightly reinforced element may require a different concrete slump from a heavily reinforced concrete element;
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Concrete should be compacted sufficiently so that the risk of segregation is minimised. If necessary, different poker vibrator sizes should be used at different locations in a concrete element to ensure that all the areas of the concrete element are reached and compacted;
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The reinforcement layout and the shape of the concrete element should allow the concrete to flow around and encapsulate all rebars, as well as fill all parts of the formwork;
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The formwork should be rigid and adequately braced and the joints should be properly sealed and watertight so that no mortar can leak through it;
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Concrete fall should be kept to a minimum in order to minimise the risk of material segregation. A flexible concrete drop chute/pipe should be used to reduce the concrete free fall height;
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The means of providing concrete cover to the reinforcement should be such that it facilitates proper compaction. The spacer blocks should not hinder vibrating activities;
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The concrete placing job should be properly planned and organized to ensure that there is sufficient time to batch, mix, transport, place and compact the concrete adequately before it sets;
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Site supervision is critical to ensure that placing and compaction activities are carried out in the most efficient and effective manner
Remedies
It is best to prevent the occurrence of honeycombing, however in the event that it is observed, some remedial techniques should be implemented. A condition assessment of the concrete structure/element should be carried out to evaluate the extent and depth of the honeycombing. Once the damage has been evaluated, a repair strategy should then be formulated. The most important and usually the first assessment method employed is visual inspection, where one physically looks at the structure to determine the severity of the damage, as far as practically possible. The visual inspection will then inform the next assessment techniques that should be employed. Acoustic methods such as ultrasonic pulse velocity and impact echo tests can also be used to detect any flaws in the concrete compaction. Cover surveys can also be carried out to determine the depth of concrete cover to reinforcement, which can be affected if honeycombing is present within the concrete.
For small damages, honeycombing can be repaired by patching the damaged area with repair mortar. Loose particles should be removed, the surface should be cleaned and the mortar should be applied in a manner where it is slightly overfilled to ensure that all the voids are completely filled. The area should then be screeded off so that the texture is similar to that of the surrounding concrete. Curing of the patched area should be carried out afterwards. For aesthetics, the colour of the repair material should match that of the surrounding concrete as closely as possible.
For larger damages, the damaged area should be broken out until sound concrete is reached. Rebars need to be surrounded by good quality concrete. Once the defective concrete has been removed, the repair may be carried out similarly to that of small damages, using a suitable mortar. A bonding agent may also be required to ensure a good adhesion between the substrate and the repair material. For large damages, it is advisable to seek the advice of a qualified structural engineer to assess if the damage has had any impact on the load carrying capacity of the concrete member and to assist in the formulation of a suitable and comprehensive repair strategy.
Conclusion
Honeycombing is a defect/deterioration mechanism which can cause problems to the aesthetics and the durability of a concrete member/structure. There are various possible causes, all of which result in voids forming in the concrete once it has hardened. It is advisable to avoid or minimise honeycombing as far as possible. Depending on the extent and depth of the honeycombing, suitable repair methods may be employed to remedy the damage. The repair method implemented should ensure that voids are either completely covered up or damaged concrete is removed and replaced with good quality material. For extensive damages, the advice of a structural engineer and/or a concrete repair specialist should be sought for condition assessment, structural integrity analysis and repair strategy formulation.
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