CROSS AUS: Reinforcement corrosion and its effect on structural performance

Collaborative reporting for safer structures. Report 1286: Reinforcement corrosion and its effect on structural performance.

The reporter is concerned about the degradation of many older concrete-framed buildings

The reporter is concerned that many older, reinforced-concrete-framed buildings are showing signs of reinforcement corrosion, which may adversely affect their structural performance. They observed evidence of this appearing as cracking and spalling of cover concrete. In several cases, they drew this to the attention of the building owners, but no action was taken to address the concerns.

In some instances, the visual damage appeared to be relatively minor. However, it is the reporter’s opinion that this may be because, in some older, more porous concretes, the products of corrosion of reinforcement can be absorbed into the concrete matrix. The reporter’s concern in such cases is that the partial loss of the bond between reinforcing bars and the surrounding concrete may lead to a significant reduction, or complete loss, of structural performance.

The reporter is particularly concerned about this in framed structures at beam-to-column junctions subject to combined axial and bending actions. The loss of bond could result in a significant reduction in bending capacity, and at these locations there may be little visible evidence of movement or distress. Therefore, a failure may be sudden and brittle, leading to collapse of the structure.

The reporter adds that confinement reinforcement and shear reinforcement, in the form of ligatures around the primary reinforcing bars, is the closest reinforcing steel to the concrete surface and often the first reinforcement to suffer degradation – with obvious consequences for robustness of reinforced concrete structures.

In recent years, CROSS has published several Safety Reports related to the corrosion of reinforcement in older concrete buildings. The reporter adds that while it is widely recognised that reinforced concrete buildings can have a long useful life, the difficulty is determining exactly how long that useful life is and when such buildings become unsafe, unusable or uninsurable.

In the reporter’s opinion, increasing deterioration of exposed reinforced concrete with time is almost inevitable, along with the corresponding reduction in structural capacity. They would like to see a more proactive approach to inspections, investigations and repairs to extend the life of these older concrete structures. They believe the wide publication of information obtained from such inspections would help in assessing the magnitude and urgency of dealing with the issue.

Comments

The reporter has drawn attention to the problem of potential reinforcement corrosion in older reinforced concrete-framed buildings including:

• the apparent lack of routine building mainframe maintenance inspections in building management practice; and
• the disconcerting lack of response from owners or their agents when alerted to the observed deterioration.

As intimated by the reporter, the loss of structural integrity in concrete buildings is typically in the form of corrosion of embedded reinforcement, evidence of which appears as cracking and spalling of concrete cover and rust staining. Such corrosion generally develops by one of three mechanisms:

• carbonation – where the pH of the concrete cover has changed. Once this process extends to the depth of the reinforcement, corrosion can occur (most common)
• chloride attack – which can be either from within the concrete matrix for older structures, or from the external environment, particularly in coastal regions. Some floor levelling compounds have also been known to cause corrosion because of high chloride contents; and
• stray electrical currents – which are not that common.

The reporter has raised the possibility there could be reinforcement corrosion progressing internally, without any obvious external signs, that results in a loss of bond. This would not appear to be the general experience with normal reinforced concrete, although it has been reported with more porous concretes such as reinforced autoclaved aerated concrete in the UK.

Research tends to show that, as the corrosion product is many times larger than the steel it came from, bond for plain bars increases until the cover cracks. With deformed bars, since the main component of bond is in the bearing on the ribs, any reduction in bond is likely to be minimal prior to corrosion initiated cracking. As the reporter notes, wider publication of the results from inspections of older buildings would be most valuable, and CROSS would like to hear from anyone who has experience of this potential loss of bond mechanism.

Further reading

• CROSS Safety Report 201: Large Panel System (LPS) building structural refurbishment issues b.link/CROSS_201
• CROSS Safety Report 265: Multi-storey car parks – structural condition concerns b.link/CROSS_265
• CROSS Safety Report 267: Multi-storey car parks demolished due to structural defects b.link/CROSS_267
• CROSS Safety Report 995: Defects found in precast (prefabricated) concrete façades b.link/CROSS_995
• Concrete Institute of Australia’s recommended practices b.link/CIAU_practice
• ACRA’s guide to concrete repair and protection b.link/ACRA_guide
• IStructE’s appraisal of existing structures (third edition) b.link/IStructE_appraisal

Where signs of corrosion occur, investigations should be carried out by a competent party to determine the extent of corrosion and to recommend appropriate repairs or maintenance. Concrete repair methods have improved dramatically over recent years and there are many solutions that have performed satisfactorily. These include the removal of concrete surrounding the affected reinforcement and the checking and treating of the reinforcement as required. Following this, the concrete is reinstated with a suitable concrete repair material.

Active and passive methods to reduce the rate of reinforcement corrosion, such as cathodic protection, are also available. Care is suggested where confinement reinforcement is affected by corrosion. An important aspect of any structural investigation is to assess the quality of the original design, provided the relevant information is available.

Owners and managers of concrete buildings should take action before the reinforcement inside the concrete starts to corrode. Early action is likely to result in a reduction in both the cost of any remediation and the risk of the building becoming structurally unsafe. Owners should consider testing their buildings for concrete integrity (carbonation front, chloride content) 40 years after construction. Routine inspections (such as those for bridges) by suitably qualified professionals allow early warning of issues and ‘no-surprises’ planning of maintenance and repairs.

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