Reinforced concrete elements can experience serviceability issues and even structural failure due to the corrosion of steel bars. However, it is possible to take steps to prevent this problem during construction.
One key approach is to use concrete that has low permeability and high quality. Although conventional concrete is not completely impermeable, it is possible to achieve the desired level of impermeability by paying close attention to various aspects of construction. This includes factors such as workmanship, the composition of the concrete mixture, and the curing process.
By taking these practical measures, designers and site engineers can help to prevent the corrosion of steel reinforcement in concrete structures. This is an important consideration for ensuring the longevity and safety of these structures, and should be a priority for anyone involved in their design or construction.
How to Prevent Reinforcement Corrosion on Site?
1. Water-Cement Ratio (w/c ratio)
Low-permeability concrete is achievable by utilizing a low water-to-cement (w/c) ratio. This not only enhances the concrete’s durability but also enhances the reinforcement protection. In accordance with the ACI 318M-11 Building Code requirements for structural concrete, the maximum w/c ratio recommended for concrete exposed to moisture and an external source of chlorides from deicing chemicals, salt, brackish water, seawater, or spray from these sources is 0.40. Additionally, a minimum concrete strength of 35 MPa is also advised. In situations where the concrete surface is expected to degrade severely, ACI 357R-84 recommends using a similar water-to-cement ratio as provided in Table-1. Therefore, to ensure better protection of the concrete, it is recommended to utilize a concrete strength of 42 MPa.
Table-1: Water/ Cement Ratios And Concrete Compressive Strength for Three Weather Conditions
| Zone | Maximum w/c ratio | Concrete compressive strength (fc’) MPa |
| Submerged | 0.45 | 35 |
| Splash | 0.40 | 35 |
| Atmospheric | 0.40 | 35 |
2. Content
Increasing the amount of cement in a material leads to an increase in its binding capacity with carbon dioxide (CO2) and chloride (CL). However, it is important to be mindful of excessive cement usage as this can negatively impact other important factors such as water/cement ratio, curing, and compaction quality. In fact, these factors can have a greater influence on the penetration of chloride and carbonation than the amount of cement used.
To ensure the durability of materials in corrosive environments, it is recommended by the American Concrete Institute (ACI) 357R-84 that a minimum cement content of 356 Kg/m3 be used. This guideline takes into account the various factors that can affect the performance of the material and suggests a minimum cement content that can provide adequate protection against corrosion.
3. Cement Type
The durability of concrete is greatly affected by the composition of cement used. One specific factor that has a significant impact is the tricalcium aluminate (C3A) content in Portland cement. When the C3A content is increased, the resistance to corrosion is improved as a result of the reaction between chloride ions and the hydrated tricalcium sulfoaluminate, which creates an insoluble compound called Friedel salt in the hardened cement paste. This reaction is illustrated in Figure-1, which demonstrates the effect of C3A on the initiation of reinforcement corrosion.
However, the effectiveness of C3A is limited when the amount of chloride content is higher, as the reaction can only occur with a specific amount of chloride. Additionally, the concrete’s resistance to sulfate attack decreases as the C3A content increases. As a result, the ACI 357R-84 recommends the use of ASTM I, II, and III (Canadian Standard Association (CSA) 10, 20, and 30) cement types with a C3A content ranging between 4-10% to optimize both corrosion resistance and sulfate resistance.
Overall, the composition of cement plays a crucial role in determining the durability of concrete, with the C3A content being a significant factor. While increasing C3A content can improve resistance to corrosion, there are limitations to its effectiveness and implications for sulfate resistance. As a result, selecting the appropriate cement type with a suitable C3A content is crucial for optimizing concrete durability.
Figure-2: Effect Of C3A Content on Time to Initiate Reinforcement
4. Pozzolans
Pozzolanic materials such as silica fume, blast-furnace slag, and fly ash are commonly used in the production of concrete to increase its resistance against chloride and sulfate attacks. These materials, when combined with water and calcium hydroxide, can produce concrete that is both low permeable and high strength.
The use of pozzolans in concrete production is particularly effective in resisting sulfate attacks. In fact, the ACI 318M-11 allows for the use of Type V (or 50 according to CSA) cement with pozzolans specifically for this purpose.
Overall, the incorporation of pozzolans into concrete mixes can greatly enhance its durability and resistance to harmful chemical attacks, ultimately resulting in a stronger and longer-lasting concrete structure.
5. Admixtures
Admixtures are chemical materials that are utilized to safeguard steel reinforcement against corrosion. These materials can be used in combination with a low water-cement ratio by employing water-reducing admixtures and superplasticizers, which provide appropriate workability and ultimately lead to better impermeability. However, it is crucial to avoid admixtures that contain calcium chloride since they can result in steel corrosion. To ensure the best outcome, time setting modification and water reduction admixtures should be utilized according to the guidelines outlined in ASTM C494M.
6. Aggregates
The presence of aggregates in concrete has a significant impact on its permeability since they make up approximately 70% of the mix’s volume. As the size of the coarse aggregates increases, so does the permeability of the resulting concrete. In fact, the permeability of most mineral aggregates is 10 to 1000 times higher than that of the concrete paste itself. Consequently, it is crucial to take into account the moisture content of the aggregates when calculating the water-to-cement ratio, and they must be washed beforehand to ensure accurate measurements.
7. Permissible Chloride Content
The Building Code known as ACI 318-11 sets out guidelines for construction. One of the provisions in this code pertains to the acceptable levels of water-soluble chloride ion content in concrete. To provide specific details on this requirement, the code includes a table, referred to as Table-2, which outlines the maximum allowable concentration of these ions. This information is important for builders and contractors to ensure that their concrete structures meet the safety standards set forth by the code.
Table-2: Maximum Water-soluble Chloride Ion Concrete in Concrete, % Weight of Cement
| Exposure conditions | Maximum water-soluble chloride ion (Cl–) content in concrete, percent by weight of cement* | |
| Reinforced concrete | Precast concrete | |
| Concrete exposed to moisture and an external source of chlorides | 0.15 | 0.06 |
| Concrete exposed to moisture but not to external sources of chlorides | 0.30 | 0.06 |
| Concrete dry or protected from moisture | 1.0 | 0.06 |
| *Water-soluble chloride ion content contributed from the ingredients, including water, aggregates, cementitious materials, and admixtures, shall be determined in the concrete mixture by ASTM C1218M at an age between 28 and 42 days. |
8. Concrete Cover Thickness
The corrosion of reinforcement in concrete is greatly affected by the depth of concrete cover, which is considered the most significant factor. Applying additional concrete cover can help delay moisture penetration and chloride ingression, which are known to accelerate corrosion. The thickness of concrete cover is influenced by several parameters, and these parameters have a direct impact on reinforcement corrosion. Understanding and managing these parameters is crucial in determining the appropriate concrete cover thickness to prevent corrosion and ensure the durability of reinforced concrete structures.
The time to corrosion of reinforcements embedded in concrete, denoted as Rt, is influenced by several factors including the depth of concrete cover (Si), chloride ion concentration (K), and water to cement ratio (w/c), especially when the concrete is exposed to saline water continuously. ACI 318M-11 provides recommendations for minimum concrete cover depths to protect against corrosion. For conventional concrete, a minimum cover depth of 65 mm is recommended, while for precast concrete, a minimum cover depth of 50 mm is recommended. Additionally, ACI 357R-84 specifies concrete cover requirements for different exposure conditions, as detailed in Table-3.
Table-3: Recommended Concrete Cover Over Reinforced Steel
| Zone | Cover over reinforcing steel | Cover over post-tensioning ducts |
| Atmospheric zone not subject to salt spray | 50 mm | 75 |
| Splash and atmospheric zone subject to salt spray | 65 mm | 90 |
| Submerged | 50 mm | 75 |
| Cover of stirrups | 13 mm less than those mentioned above |
9. Compaction
Concrete elements are susceptible to reinforcement corrosion, which is greatly influenced by the degree of compaction during pouring. Insufficient compaction can accelerate the corrosion of concrete elements. For example, a decrease in compaction by 10% can result in a 100% increase in permeability and a 50% reduction in concrete strength. Hence, the adequacy of compaction plays a crucial role in preventing corrosion.
10. Curing
Proper curing and control of temperature and moisture can effectively reduce the permeability of concrete. In fact, if adequate curing is not employed, the permeability of the concrete surface layer can increase by 5-10 times. It is important to note that a short curing period can result in chloride ions penetrating the concrete before a passive protective film is formed. In order to provide guidance on the process of concrete curing, the ACI committee 308 has developed recommendations for industry professionals to follow.
11. Permissible Crack Width
Concrete cracks can have a significant impact on the corrosion of reinforcement within a structure. To mitigate this issue, the American Concrete Institute (ACI) has recommended that a maximum crack width of 0.15 mm is acceptable at the tension side of an element that is exposed to wetting and drying.
It has been observed that longitudinal cracks along steel reinforcement are more damaging than cracks that are transverse to the longitudinal reinforcement. This is due to the fact that transverse cracks only allow for a small area of ingression, whereas longitudinal cracks can cause the concrete cover to spall off.
To provide guidance on permissible cracks under different exposure conditions, Table-4 has been developed. This table outlines the maximum allowable cracks for various exposure scenarios. By following these guidelines, it is possible to mitigate the detrimental effects of concrete cracks on reinforcement corrosion, ensuring the structural integrity and longevity of the building.
Table-4: Guide To Recommended Crack Width for Reinforced Concrete Under Service Loads
| Exposure conditions | Crack width (mm) |
| Dry air or protective membrane | 0.41 |
| Humidity, moist air, soil | 0.30 |
| Deicing chemicals | 0.18 |
| Seawater and seawater spray, wetting and drying | 0.15 |
| Water-retaining structures† | 0.10 |
| †Excluding non-pressure pipes. |
12. Protective Coatings
There are various methods available to prevent corrosion, including reinforcement bar coating and cathodic protection. However, these methods are more expensive compared to the use of low permeable concrete protection.
Among the methods of protection by coating, anodic and barrier coatings are the most significant. Anodic coatings work by increasing the potential of the reinforcement to prevent corrosion, while barrier coatings prevent the contact of corrosive agents with the reinforcement.
On the other hand, cathodic protection involves changing the concrete environment by using volunteering anodes or directing ion flow away from the reinforcement. This method is also effective in preventing corrosion, but its implementation can be more costly than other protection methods.
FAQs
What is the effect of corrosion on reinforced concrete elements?
Steel bars are an essential component of reinforced concrete structures. However, the corrosion of these bars can have severe consequences for the integrity of the entire structure. This is because the corrosion of steel bars can negatively impact the serviceability of reinforced concrete elements and may even lead to structural failure. Therefore, it is crucial to address the issue of steel bar corrosion to maintain the safety and reliability of reinforced concrete structures.
What are the main causes of reinforcement corrosion?
Reinforcement corrosion can be attributed to two primary causes: carbonation and penetration of chloride ions. When concrete structures are exposed to carbon dioxide from the atmosphere or other sources, carbonation occurs. This process lowers the pH of the concrete, which reduces the passivity of the reinforcing steel and makes it more susceptible to corrosion.
On the other hand, chloride ions can penetrate the concrete and reach the reinforcing steel through various means, such as through exposure to seawater or de-icing salts. Once the chloride ions reach the steel, they can break down the protective layer on the surface of the steel, leading to corrosion.
Both carbonation and chloride ion penetration can have detrimental effects on the structural integrity of concrete. It is essential to take measures to prevent or mitigate the effects of these causes to ensure the longevity and safety of concrete structures.
How to prevent reinforcement corrosion?
One crucial measure to prevent corrosion of steel bars on construction sites is to use concrete that is of high quality and has low permeability. This is essential in controlling various mechanisms that can cause corrosion. Concrete with these characteristics acts as a barrier that restricts the entry of harmful substances, such as moisture and chloride ions, which can accelerate the corrosion process. By ensuring that the concrete used in construction is of high quality and has low permeability, the risk of corrosion of steel bars can be significantly reduced. This is particularly important in environments where steel bars are exposed to harsh conditions, such as in coastal areas where saltwater can easily penetrate the concrete and initiate corrosion. Taking precautions to ensure that the concrete used in construction has these desirable properties can help prolong the lifespan of steel bars and enhance the durability of the overall structure.