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Surface voids in concrete

Surface voids in concrete

Surface voids are the cavities or little holes that appear on the surface of concrete castings. Surface voids (commonly known as pitting) are referred to as “bug holes” or “fish eyes”. These voids may produce an unacceptable appearance on the surface of the finished casting. Understanding the causes of surface voids and what can be done to minimize them is the first step to a more efficient and productive casting operation.

Surface voids are generally attributed to the following three factors:

  • Release agent

  • Water

  • Air

– sometimes a combination of the three

Factors causing surface voids

Releasing Agent

Release agents act as a “lubricant” between the mold and the concrete. The proper application of a release agent will yield castings without surface voids. When a release agent is over applied, it may “pool” or “puddle” on the lower extremities of the mold and when concrete is poured these pools prevent the concrete from filling in 100%.


Similar to release agents, water is also trapped against the mold’s working surface resulting in voids. As the concrete cures and the residual water evaporates, a cavity is left behind on the surface of the casting. However most voids caused by water are a result of a high water to cement ratio.


In most circumstances, air voids have an irregular shape and tend to be much larger (1/2″ or 1.27 cm.) than those caused by water or release agents. The air voids are caused by air trapped between the mold surface and the concrete. They generally appear in low slump concrete and can be found underneath irregular (non-spherical) shaped pieces of crushed aggregate.
This is a result of having too little mortar to fill the spaces around the aggregate.

How To Eliminate Voids In A Concrete Casting:

While many variables must be considered in the elimination of surface voids in concrete castings, there are a number of precautions that can remedy this unsightly problem.

  1. Make sure that releasing agents are applied in thin films.

  2. Techniques used during compaction can eliminate most surface voids.

    i) Suitable size of needle virator should be used

    ii) Concrete should be compacted layer by layer

    iii) Needle should not touch reinforcement and formwork

  3. Tapping the mold with rubber hammer / wooden hammer can eliminate any residual voids.

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