Concrete Cancer: Identification, Assessment, and Remediation of Spalling Concrete

Concrete spalling, commonly referred to as "concrete cancer," is one of the most serious and widespread building defects affecting structures across Australia. This progressive deterioration of concrete not only compromises the structural integrity of buildings but can also pose significant safety risks to occupants and the public. As forensic structural engineers, we regularly investigate concrete spalling failures and have developed comprehensive approaches to assessment and remediation.

Understanding Concrete Spalling

Concrete spalling occurs when the concrete surface breaks away in flakes or chunks, typically exposing the underlying steel reinforcement. This process is often initiated by the corrosion of steel reinforcement within the concrete, which expands as it rusts, creating internal pressure that eventually causes the concrete cover to crack and fall away.

The term "concrete cancer" is particularly apt because, like cancer, the deterioration spreads progressively if left untreated. What begins as minor surface cracking can eventually compromise the entire structural element, requiring extensive and costly remediation.

Primary Causes of Concrete Spalling

Chloride Attack

The most common cause of concrete spalling in Australia is chloride-induced corrosion, particularly in coastal areas. Chloride ions from salt air penetrate the concrete and reach the steel reinforcement, initiating corrosion even in alkaline conditions. This is especially problematic in:

• Buildings within 1km of the coastline
• Structures exposed to de-icing salts
• Swimming pool areas and water treatment facilities
• Underground structures exposed to groundwater

Carbonation

Carbonation occurs when carbon dioxide from the atmosphere reacts with the alkaline compounds in concrete, gradually reducing the pH. When the pH drops below about 9, the protective alkaline environment around the steel reinforcement is lost, allowing corrosion to begin. This process is accelerated by:

• Poor quality concrete with high permeability
• Insufficient concrete cover over reinforcement
• Exposure to high CO2 concentrations
• Wet-dry cycling that accelerates carbonation

Poor Construction Practices

Many spalling problems can be traced back to construction defects:

• Insufficient concrete cover over reinforcement
• Poor concrete compaction leaving voids and honeycombing
• Inadequate curing allowing rapid moisture loss
• Use of contaminated aggregates or mixing water
• Incorrect concrete mix design for the exposure conditions

Identification and Assessment

Visual Indicators

Early identification of concrete spalling is crucial for effective remediation. Key visual indicators include:

• Rust staining on concrete surfaces
• Fine cracks running parallel to reinforcement
• Loose or hollow-sounding concrete when tapped
• White efflorescence deposits on surfaces
• Exposed or corroded reinforcement
• Concrete pieces falling from the structure

Professional Assessment Techniques

Comprehensive assessment requires specialized techniques:

Half-Cell Potential Testing

This electrochemical test measures the corrosion potential of embedded steel reinforcement, helping identify areas of active corrosion before visible spalling occurs.

Concrete Core Testing

Core samples allow laboratory analysis of concrete strength, permeability, chloride content, and carbonation depth, providing crucial data for remediation design.

Cover Meter Surveys

These surveys map the location and depth of reinforcement, identifying areas with insufficient cover that are prone to spalling.

Delamination Surveys

Chain dragging or hammer tapping identifies areas where the concrete has delaminated from the reinforcement but hasn't yet spalled.

Case Study: Apartment Building Balcony Failure

We recently investigated a 25-year-old apartment building where concrete spalling on balconies had progressed to the point where falling concrete posed a safety hazard to pedestrians below. The building was located 800m from the coast, and residents had noticed rust staining and minor cracking for several years before seeking professional assessment.

Our investigation revealed:

• Chloride levels in the concrete exceeded the threshold for corrosion initiation
• Concrete cover over reinforcement was only 15mm instead of the required 40mm for the exposure classification
• The concrete had carbonated to the depth of the reinforcement in several areas
• Poor drainage details allowed water to pond on balcony surfaces

The remediation required complete reconstruction of the balcony slabs at a cost of $1.2 million. Early intervention when rust staining first appeared could have limited the damage to localized repairs costing approximately $200,000.

Remediation Strategies

Patch Repairs

For localized spalling, patch repairs may be appropriate:

• Remove all loose and contaminated concrete
• Clean reinforcement and apply corrosion inhibitor
• Apply bonding agent and repair mortar
• Apply protective coating system

However, patch repairs often fail if the underlying causes aren't addressed, leading to further deterioration around the repair areas.

Cathodic Protection

For widespread corrosion, cathodic protection systems can halt the corrosion process by applying a small electrical current to the reinforcement. This is particularly effective for:

• Large structures where replacement is impractical
• Heritage buildings where maintaining original fabric is important
• Structures with widespread but not severe deterioration

Concrete Replacement

For severe spalling, complete replacement of affected elements may be necessary:

• Remove all contaminated concrete
• Replace corroded reinforcement
• Use appropriate concrete mix for exposure conditions
• Ensure adequate cover and proper curing
• Apply protective systems as required

Prevention Strategies

Design Phase

• Specify appropriate concrete grades and cover depths for exposure conditions
• Design effective drainage and waterproofing systems
• Consider protective coatings for high-risk areas
• Specify corrosion-resistant reinforcement where appropriate

Construction Phase

• Ensure proper concrete placement and compaction
• Maintain adequate cover to reinforcement
• Implement proper curing procedures
• Use quality materials appropriate for the exposure

Maintenance Phase

• Conduct regular inspections for early signs of deterioration
• Maintain drainage systems and waterproofing
• Address minor defects before they become major problems
• Apply protective coatings as part of planned maintenance

The Economics of Early Intervention

The cost of addressing concrete spalling increases exponentially with time. Consider these typical scenarios:

• Early intervention (rust staining): $50-100 per m² for protective coatings
• Minor spalling: $200-400 per m² for patch repairs
• Moderate spalling: $500-800 per m² for extensive repairs
• Severe spalling: $1000-2000 per m² for replacement

These figures don't include the indirect costs of scaffolding, temporary works, disruption to occupants, and potential safety risks.

Conclusion

Concrete spalling is a serious but manageable building defect when addressed promptly and professionally. The key to successful management is early identification, proper assessment, and appropriate remediation based on the specific causes and extent of deterioration.

At AZTA Engineering, our forensic investigations of concrete spalling failures inform our approach to both assessment and remediation. We understand how concrete deteriorates and use this knowledge to develop cost-effective solutions that address both immediate safety concerns and long-term durability.

Remember: concrete cancer doesn't heal itself. Early diagnosis and treatment are essential to prevent minor problems from becoming major structural failures.