Blowholes are small surface voids or cavities in concrete formed by trapped air or water bubbles that rise and become lodged against formwork during placement.
Explanation
Blowholes typically appear as round or elongated holes on formed concrete surfaces once the formwork is removed. They are caused by air or bleed water becoming trapped between the concrete and the form face, particularly where consolidation is inadequate.
Factors contributing to blowhole formation include poor vibration, overly stiff mixes, high formwork absorption, excessive release agents, and complex reinforcement arrangements that restrict concrete flow.
While usually shallow and non-structural, blowholes can vary in size and distribution. In architectural or exposed concrete, they are often considered a defect due to their impact on surface finish. In structural applications, their significance depends on depth, frequency, and exposure conditions.
Why It Matters
Although generally cosmetic, blowholes can affect the durability of concrete if they create pathways for moisture and contaminants. In aggressive environments, this may increase the risk of reinforcement corrosion over time.
In repair and construction projects, controlling blowholes is important for both aesthetic quality and long-term performance, particularly where protective coatings or waterproofing systems are applied.