Concrete structures form the backbone of modern infrastructure, serving as the foundation for buildings, bridges, and various civil engineering projects. However, the longevity of these structures is often compromised by the corrosion of embedded steel reinforcement bars (rebars). This corrosion leads to crack formation and structural deterioration, primarily due to the ingress of oxygen and moisture through the concrete matrix. In regions where groundwater contains high levels of chlorides, the problem is exacerbated by carbonation reactions that reduce the concrete’s alkalinity, accelerating the corrosion process. To mitigate these issues, incorporating inhibitors into the cement mix has emerged as an effective strategy to enhance the durability and extend the lifespan of concrete structures.
Causes of Corrosion in Concrete
High Initial Alkalinity of Concrete
When freshly produced, concrete exhibits a high alkalinity with a pH range of 12–13. This environment fosters the formation of a passive oxide layer on the surface of steel rebars, providing protection against corrosion. The passive layer acts as a barrier, preventing the electrochemical reactions that lead to rust formation. However, certain environmental exposures can break down this protective layer, rendering the steel vulnerable to corrosion.
Carbonation of Concrete
One of the primary causes of the loss of passivating alkalinity is carbonation. This chemical process involves the reaction of atmospheric carbon dioxide (CO₂) and sulfur dioxide (SO₂) with the soluble calcium hydroxide (Ca(OH)₂) present in the concrete. The reactions can be represented as:
- Carbonation Reaction:
CO₂ + Ca(OH)₂ → CaCO₃ + H₂O - Sulphation Reaction:
SO₂ + Ca(OH)₂ → CaSO₄ + H₂O
These reactions convert calcium hydroxide into calcium carbonate (CaCO₃) and calcium sulfate (CaSO₄), which are less alkaline. As a result, the pH of the concrete decreases, diminishing its ability to maintain the passive oxide layer on the steel surface. The carbonation front progresses inward from the surface, and once it reaches the steel reinforcement, the protective environment is lost, making the steel susceptible to corrosion in the presence of moisture and oxygen.
Chlorides in Concrete
Chloride ions, originating from de-icing salts, seawater, or contaminated aggregates, pose a significant threat to the integrity of concrete structures. The concentration of chlorides required to initiate corrosion depends on the pH of the concrete. In highly alkaline concrete, higher chloride concentrations are necessary to disrupt the passive layer. However, when the alkalinity is reduced due to carbonation, even small amounts of chlorides can penetrate the passive layer and promote corrosion. The critical chloride threshold decreases, accelerating the onset of corrosion in steel rebars.
Process of Corrosion in Concrete Structures
Volume Expansion and Cracking
The corrosion of steel in concrete is an electrochemical process that leads to the formation of iron oxides and hydroxides (rust). These corrosion products occupy a volume that is four to twelve times greater than the original steel. This significant volumetric expansion exerts tensile stresses within the concrete, leading to cracking, spalling, and delamination. These defects not only compromise the structural integrity but also provide pathways for aggressive agents to penetrate deeper, exacerbating the corrosion process.
Electrochemical Nature of Corrosion
Corrosion in concrete is facilitated by the presence of an electrolyte, which, in this case, is the moisture contained within the concrete’s pore structure. The steel reinforcement acts as both an anode and a cathode at different locations along its length:
- Anodic Reaction (Oxidation):
Fe → Fe²⁺ + 2e⁻ - Cathodic Reaction (Reduction):
O₂ + 2H₂O + 4e⁻ → 4OH⁻
The flow of electrons from the anodic to the cathodic areas generates an electrical current, driving the corrosion process. The formation of hydroxide ions (OH⁻) further contributes to the alkalinity but is insufficient to halt the ongoing corrosion once the passive layer is compromised.
Role of Inhibitors
Inhibitors introduced into the concrete mix can significantly disrupt this electrochemical process. Upon reaching the steel surface, inhibitors adsorb onto the metal, forming a protective film that impedes both anodic and cathodic reactions. This film acts as a barrier, reducing the permeability of aggressive species like chlorides and limiting the availability of oxygen and moisture at the steel surface.
Corrosion Rate of Steel: Comparative Analysis
A study comparing the corrosion rates of steel in standard cement and inhibitor-admixed cement revealed substantial differences:
- Regular Cement: Corrosion rate of 0.0191 millimeters per year (mmpy).
- Cement with Inhibitors: Corrosion rate reduced to 0.0029 mmpy.
This significant reduction indicates that the use of inhibitors can decrease the corrosion rate by approximately 85%. Such a decrease translates to a considerable extension of the service life of reinforced concrete structures, particularly in aggressive environments.
Corrosion Prevention with Inhibitors
Mechanism of Action
When inhibitors are incorporated into the concrete mix, they function by:
- Adsorption onto Steel Surface: Forming a molecular layer that blocks active corrosion sites.
- Passivation Enhancement: Reinforcing the natural passive oxide layer or facilitating its reformation.
- Complexation with Aggressive Ions: Binding with chloride ions to form stable complexes, reducing their availability to participate in corrosion reactions.
Longevity and Effectiveness
The protective effects of inhibitors are long-lasting due to their chemical stability within the concrete matrix. Weight loss experiments on steel rebars have demonstrated that the use of inhibitors can increase their lifespan by up to five times. This enhancement is particularly beneficial in structures exposed to harsh conditions, such as marine environments or areas with high industrial pollution.
Impact on Concrete Properties
Importantly, the addition of inhibitors does not adversely affect the fundamental properties of concrete:
- Strength: Compressive and tensile strengths remain comparable or slightly improved.
- Setting Time: Initial and final setting times are minimally impacted, ensuring workability.
- Durability: Improved resistance to cracking and spalling due to reduced internal stresses from corrosion.
Comparison of Physical Characteristics
A comprehensive evaluation of the physical properties of cement and inhibitor-admixed cement yielded the following results:
Compressive Strength (N/mm²)
- Mortar Cubes (as per IS 1489 Part I:1991):
- 3 Days:
- Cement: 16.000
- Cement + Inhibitors: 16.500
- 7 Days:
- Cement: 17.750
- Cement + Inhibitors: 19.830
- Concrete Cubes (28 Days):
- Both samples achieved a compressive strength of 38.70 N/mm².
Tensile Strength (N/mm²)
- Cement: 1.240
- Cement + Inhibitors: 1.260
Consistency and Setting Time
- Consistency for 33 mm Penetration:
- Cement: 0.328
- Cement + Inhibitors: 0.290
- Setting Times (minutes, as per IS 1489 Part I:1991):
- Initial Setting Time:
- Cement: 149
- Cement + Inhibitors: 135
- Final Setting Time:
- Cement: 345
- Cement + Inhibitors: 330
These results indicate that the inclusion of inhibitors has a negligible or slightly positive effect on the mechanical properties of the concrete. The minor reductions in setting times can be advantageous in certain construction scenarios where faster setting is desirable.
Corrosion-Resistant Properties of Concrete
Long-Term Performance (After 3 Years)
An evaluation of the corrosion-resistant properties over a three-year period showed that inhibitor-admixed concrete significantly outperformed regular concrete:
- Impedance Testing:
- Charge Transfer Resistance (Rct):
- Cement with 10,000 ppm added chloride: 5.2 K-ohm
- Cement + Inhibitor with 10,000 ppm added chloride: 37 K-ohm
Interpretation of Results
- Higher Rct Values: Indicate greater resistance to corrosion, as the charge transfer process at the steel-concrete interface is impeded.
- Durability Factor: The inhibitor-admixed concrete exhibited a durability factor seven times greater than that of regular cement under identical conditions.
Graphical Representation
A graphical comparison (not included here) would typically illustrate the stark contrast in corrosion rates and durability factors, reinforcing the quantitative data with visual evidence of the inhibitors’ effectiveness.
Conclusion
The use of inhibitor-admixed cement represents a significant advancement in the construction of durable concrete structures. By addressing the primary causes of steel rebar corrosion—carbonation and chloride ingress—inhibitors enhance the longevity and integrity of concrete. The protective film formed on the steel surface acts as a formidable barrier against aggressive agents, effectively reducing corrosion rates and preventing crack formation. The maintenance of key physical properties such as compressive strength and setting times ensures that the incorporation of inhibitors does not compromise the material’s performance. Given these substantial benefits, the adoption of inhibitors in cement is strongly recommended, especially in environments prone to carbonation and chloride exposure. This proactive approach to corrosion prevention is essential for extending the service life of concrete structures and reducing long-term maintenance costs.