Corrosion and Concrete: Causes, Prevention, and Repair
Chloride-Induced Corrosion in Concrete
Q1. A) Explain chloride-induced corrosion.
When chlorides are present, water carrying chloride ions is deposited on the concrete surface. This allows the chloride ions to permeate the concrete cover through a combination of capillary action, evaporation, deposition, and diffusion. When chloride ions reach the steel surface, they combine with hydrogen ions in the pore water, creating acids. In sufficient concentrations, these acids neutralize the alkalinity of the concrete and facilitate the breakdown of the passive layer, allowing localized corrosion to establish. As a result of the localized nature of chloride attack, corrosion tends to proceed in a very specific form known as “pitting.” As the name suggests, this type of corrosion forms “pits,” which are typically characterized by holes in the steel surface that tend to be deeper than they are wide. Pitting is possibly the most insidious form of corrosion, as relatively little weight loss can result in a significant reduction in section size, and its localized attack can be difficult to detect prior to severe deterioration.
Different Types of Rebars
Q1. B) What are the different types of rebars?
Types of Rebar
- European Rebar: The main advantage of European rebar is its cost-effectiveness.
- Carbon Steel Rebar: The most common rebar, known as “black” bar, is used on every type and scale of project, with few exceptions.
- Epoxy-Coated Rebar
- Galvanized Rebar
- Glass-Fiber-Reinforced-Polymer (GFRP)
- Stainless Steel Rebar
Sulfate Attack on Cementitious Systems
Q2. A) Explain internal and external sulfate attacks on cementitious systems.
- External sulfate attack occurs when sulfates enter the concrete from the surrounding environment.
- Internal sulfate attack occurs when there is an excess of sulfate in the original mixture. Sulfate attack on concrete is a key durability problem. Internal sulfate attack (ISA) on concrete is a reaction between sulfates present in the original mix of concrete and the calcium aluminate in cement and water, which can show signs of deterioration.
Protecting Post-Tensioned Members from Corrosion
Q2. B) What are the precautionary measures to save post-tensioned members against corrosion?
Monitoring the electrical resistance of the tendons over time allows for the detection of the penetration of (chloride-containing) water at defects into the ducts, long before corrosion damage occurs. For the first time, a simple, cost-effective early warning system for post-tensioned tendons is available.
Surface Preparation for Concrete Repair
Q3. A) How to prepare the surface for the repair of concrete structures?
Surface preparation includes the removal of damaged and/or deteriorated parts of the substrate concrete and previously applied coatings, whereas cleaning commonly refers to the removal of loose particles and contaminants on the surface.
Electrochemical Chloride Extraction Technique
Q3. B) Explain the Electrochemical Chloride Extraction Technique.
Electrochemical chloride extraction (ECE) is a process that extracts or removes chloride ions from chloride-contaminated reinforced concrete structures. An electrical current is applied between the embedded steel and an external anode as a rehabilitation option to mitigate the ongoing corrosion of the embedded steel.
Rapid Chloride Penetration Test
Q4. A) What is the rapid chloride penetration test used for?
The rapid chloride permeability test (RCPT-ASTM C 1202) is commonly used to evaluate the resistance of concrete to chloride ion ingress, owing to its simplicity and rapidity. RCPT is an electrical indication of concrete’s ability to resist chloride ion penetration. In one reservoir is a 3.0% NaCl solution, and in the other reservoir is a 0.3 M NaOH solution. The total charge passed is determined and used to rate the concrete according to the criteria included.
Causes of Cracks in Concrete
Q4. Write the causes of cracks in concrete.
- Excess water in the mix
- Concrete drying too fast
- Improper strength of concrete poured on the job
- Lack of control joints (in concrete slab)
- Foundations poured in the winter