Blowholes are individual rounded or irregular cavities that are formed against the formwork and become visible when it is stripped. Small blowholes tend to be approximately hemispherical while larger ones are irregular and often expose coarse aggregate particles. Generally, they are regarded as an appearance problem though a concentration of large blowholes may lead to loss of durability. Under AS 3610 Formwork for Concrete, the size/extent of blowholes is therefore one of the criteria by which an off-form surface finishes can be evaluated. Blowholes are caused by the entrapment of air against the inside face of the formwork.

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