Factors determining the durability of reinforcement

Environment

The external environment of the concrete provides the agents which commonly cause corrosion in reinforcement: oxygen, water, carbon dioxide and chloride ions.  Marine structures and structures close to coastal waters are particularly at risk from corrosion of reinforcement due to the ingress of chloride ions from sea spray and salt-laden air.

Away from the sea coast most corrosion of reinforcement in concrete is due to the process of carbonation, which reduces the alkalinity of the surrounding concrete.  This process can occur at any geographic location. The rate of carbonation is at a maximum when the relative humidity is about 50 per cent, and increases with increasing temperature.

Surveys have shown that the corrosion problem in relatively new buildings is worst in coastal areas.

Chloride tolerance. Though zinc can be depassivated and attacked in the presence of chloride ions, the tolerance of galvanized reinforcement to chloride depassivation is substantially higher than that of black steel.  In a survey of a number of long-serving marine structures* galvanized bars were shown to have been exposed to chloride contents as high as 2.2% (by approximate weight of cement) over periods of 10-20 years, with less than 10% loss of original coating thickness and no record of failure.  This should be compared to chloride levels in the range of 0.2-0.3% by weight of cement leading to severe corrosion of black steel in similar circumstances.

*Tonini, DE and Cook, AR (1978) ‘The performance of galvanised reinforcement in high chloride environments – field study reports.’ International Corrosion Forum, NACE, Houston.

 

Quality of concrete

In preventing corrosion of reinforcement, the most critical property of concrete is permeability. The degree of permeability determines the extent and rate of the diffusion of chloride ions and carbon dioxide through the concrete. Permeability is a function of mix design, compaction and curing.

 

Depth of Cover

Lack of concrete cover for reinforcement has been identified as a major problem associated with ‘failures’ in high rise buildings.* In a survey of 95 Sydney buildings ranging in height from 5 to 36 storeys and aged between 2 and 17 years, the average depth of concrete cover at sites where spalling occurred was 5.45mm. The maximum depth of cover at any failure point was 18 mm compared with recommended covers to AS 3600 ‘Concrete structures’ in the range 25 – 30 mm, depending on the type of member.

* Marosszeky, M and Sade, D (1986). “Concrete durability – the problem of reinforcement corrosion and improving workmanship”. Building Research Centre, University of NSW.

 

Cracks in concrete

The type and size of cracks have an important influence on durability of concrete. Cracks caused by shrinkage or thermal stresses may contribute significantly to reinforcement corrosion, particularly when they run parallel to reinforcing bars and are close to the concrete surface.

Crack widths of less than 0.1 mm are generally regarded as not causing significant corrosion risk, provided cover is adequate and the structure is not exposed to highly corrosive environments. Flexural cracks are not generally a problem as they decrease in width from a maximum at the surface and become narrower at the level of the reinforcing steel.

 

Surface treatment of concrete

In the production of architectural finishes the concrete surface is sometimes washed or treated to expose the aggregate. These practices are not recommended if there is any possibility of aggressive chemicals such as acids or salts being left behind to permeate the concrete.

Etching, washing and mechanical concrete surface finishing may also result in loss of the valuable cement-rich paste which forms the surface layer of the concrete, reducing carbonation resistance and depth of cover.

 

Reaction between galvanized coatings and concrete

During initial contact of galvanized reinforcement with wet concrete, the outer zinc layers of the galvanized coating react to form stable insoluble zinc salts. Attack ceases as the concrete hardens and the galvanized coating remains intact.

 

Corrosion Protection Provided by Galvanizing

In areas where the reinforcement may be exposed accidentally due to thin or porous concrete, cracking, or damage to the concrete, the galvanized coating provides extended protection. Since the corrosion product of zinc occupies a smaller volume than the corrosion products of iron, any small degree of corrosion which may occur to the galvanized coating causes little or no disruption to the surrounding concrete mass.  Studies were made at the Structural Engineering Materials Laboratory, University of California, Berkeley California, of the effects of corrosion on reinforced concrete test prisms.

Galvanized Reinforcement for Concrete

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