CEMENT & CONCRETE
Minor concrete defects and their remedies Bryan Perrie, managing director, TCI
The durability of concrete is exceptional and well recognised but, sometimes, operational errors or unforeseen circumstances can cause unsightly, super ficial sur face damage, says Br yan Perrie, managing director of The Concrete Institute (TCI).
T
he quality of any cast concrete product depends on material selection, equipment used and workmanship. Sur faces of acceptable quality and accuracy are difficult to achieve and, depending on functional and aesthetic requirements, some repairs may be necessar y. Repairs may also be necessar y where sur faces are damaged in ser vice. Minor defects include: • Small cavities: such as those formed by tie-bolts and impact damage to the sur face. • Honeycombing: concrete in which mortar is partially or completely absent, caused by mortar loss during placement, or segregation of the stone and mortar. • Misplaced surfaces: usually the result of incorrectly aligning or positioning
Blowholes are relatively small voids of roughly spherical surface shape formed against formwork or moulds, usually caused by inadequate compaction of the thin layer of concrete in contact with formwork, or the incorrect mould release agent for the formwork type
formwork, or movement of formwork during concrete placement. Under- or over filling of formwork may also be the cause. • Blowholes: relatively small voids of roughly spherical sur face shape formed against formwork or moulds, usually caused by inadequate compaction of the thin layer of concrete in contact with formwork, or the incorrect mould release agent for the formwork employed.
Basic types of repair materials Repair materials are formulated to reduce the likelihood and severity of cracking. This is done by reducing potential dr ying shrinkage, which will reduce the tendency to crack. Two approaches are possible. The first approach is the use of ‘semi-dr y’ mixtures of cement, water and aggregate, compacted by being rammed into position. Semi-dr y mixtures have low water content and therefore ver y low potential dr ying shrinkage – and a consequent resistance to cracking. But water content is critical: if too low, thorough compaction is impossible; if too high, the repair may slump or crack. These mixtures can be used only in confined spaces such as
The cause of honeycombing: concrete in which mortar is partially or completely absent, caused by mortar loss during placement, or segregation of the stone and mortar
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IMIESA January 2021
