7. Case Studies on Durability Failures

Case Study 1: Bridge Deck Corrosion due to Chloride Attack

Location: Coastal highway bridge, Gujarat
Issue: Severe corrosion observed within 10 years of construction.
Root Cause:

• High chloride ingress due to poor waterproofing and inadequate concrete cover.
• Water-cement ratio was > 0.55; concrete had high permeability.
  Outcome:
• Corroded rebars led to delamination of cover concrete.
• Bridge required partial demolition and retrofitting using corrosion-resistant rebars.

This case study highlights a specific incident where a coastal highway bridge in Gujarat experienced severe corrosion. Corrosion is a significant durability failure in concrete structures, especially when exposed to harsh aquatic environments. The issue arose just 10 years after construction, indicating a lack of sufficient protective measures. The major contributing factors included: 1) Poor waterproofing, which allowed chlorides from marine environments to penetrate the concrete. 2) An inadequate concrete cover, which failed to protect rebar from exposure to aggressive agents. 3) A high water-cement ratio exceeding 0.55, which increased the concrete's permeability, allowing water and chlorides to infiltrate and contribute to corrosion. The outcome was quite severe, necessitating partial demolition of the bridge and costly retrofitting with corrosion-resistant materials.

To relate this to a more everyday situation, think of a car that is constantly exposed to saltwater due to proximity to the ocean. If the car isn’t washed regularly, the salt can corrode the metal parts, leading to rust and structural issues. Just like the bridge, the car needed regular maintenance and protective coatings to prevent deterioration.

Case Study 2: Carbonation in High-Rise Residential Building

Location: Delhi NCR
Issue: Carbonation depth reached 30 mm within 7 years.
Cause:

• Low cement content, inadequate curing.
• High relative humidity supported fast carbonation.
  Detection:
• Phenolphthalein test showed no pink coloration in outer layers.
  Action Taken:
• Application of anti-carbonation coatings.
• Surface densifiers used and cover concrete replaced where pH < 9.

This second case study discusses a high-rise residential building in Delhi NCR, where carbonation was identified as a significant problem. Carbonation is a chemical reaction that occurs when carbon dioxide from the atmosphere reacts with calcium hydroxide in the concrete, leading to a lower pH and compromising the material's durability. In this case, the carbonation depth reached 30 mm within only 7 years, primarily because of low cement content and poor curing practices. The high relative humidity in the area further expedited the carbonation process. The detection involved using a phenolphthalein test, which determined that the outer concrete had carbonated and no longer showed the necessary protective alkaline properties (indicated by color change). The response included applying anti-carbonation coatings and replacing cover concrete that had become too acidic (with pH lower than 9).

Imagine baking bread. If you don’t use enough yeast (analogous to cement) and don’t let it rise properly (like curing), the bread won’t have the right structure or texture. Similarly, inadequate cement and curing in concrete led to a weak structure that was easily carbonated. Preventative measures, like coating the bread with oil to prevent it from hardening on the outside, are like the anti-carbonation treatments applied to the building.

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