Reinforced concrete elements are vulnerable to a significant problem – the corrosion of steel bars. This issue can lead to a loss of structural integrity and even cause the failure of the entire structure. However, it is possible to prevent the corrosion of steel bars by taking various precautions during construction.
Protect Reinforcement from Corrosion at Site
One key factor in preventing corrosion is the quality of the concrete used in the construction process. Concrete that is highly impermeable and of good quality is critical to controlling different corrosion mechanisms. Although conventional concrete may not be entirely impermeable, it is possible to produce high-quality, low-permeability concrete by paying careful attention to various aspects of the construction process.
Site engineers and designers play a crucial role in ensuring that reinforcement corrosion does not occur. By focusing on details such as workmanship, concrete mixtures, and curing, they can produce concrete that is highly resistant to corrosion. These practical measures are essential to protect the steel bars in reinforced concrete elements, ensuring their long-term durability and safety.
How to Prevent Reinforcement Corrosion on Site?
1. Water-Cement Ratio (w/c ratio)
Low-permeability concrete is a type of concrete that can offer better reinforcement protection by applying a low water-to-cement ratio. This approach can prevent the penetration of harmful substances such as chlorides from external sources like deicing chemicals, salt, brackish water, seawater, or spray. In fact, the ACI 318M-11 Building Code requires a maximum water-to-cement ratio of 0.40 and a minimum concrete strength of 35 MPa for concrete that is exposed to moisture and external sources of chlorides.
To ensure that concrete surfaces do not degrade severely, it is recommended to use a concrete strength of 42 MPa while maintaining the specified water-to-cement ratio. This recommendation is provided by the ACI 357R- 84 guidelines, which align with the requirements outlined in the ACI 318M-11 Building Code. By following these guidelines, the resulting low-permeability concrete can offer improved durability and longevity in structures that are exposed to moisture and external sources of chlorides.
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. Cement Content
The amount of cement used in concrete has an impact on its ability to bind with carbon dioxide (CO2) and chloride (CL). As the cement content increases, the binding capacity for these substances also increases. However, it is important to note that using excessive amounts of cement can lead to other issues.
Factors such as water/cement ratio, curing, and compaction quality can have a greater influence on the penetration of chloride and carbonation than the cement content alone. Therefore, it is recommended by the American Concrete Institute’s (ACI) publication 357R-84 that a minimum cement content of 356 kg/m3 is sufficient for use in corrosive environments.
By adhering to this minimum cement content, the concrete’s ability to bind with CO2 and CL can be maintained while also ensuring that other important factors are not compromised. This balance can help to prevent issues related to corrosion and deterioration of the concrete over time.
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3. Cement Type
The durability of concrete is significantly influenced by the composition of cement used in its production. An increase in the tricalcium aluminate (C3A) content of Portland cement can greatly enhance its resistance to corrosion. This is due to the reaction between chloride ions and hydrated tricalcium sulfoaluminate, which creates an insoluble compound called Friedel salt in the hardened cement paste.
Figure-1 demonstrates the impact of C3A on the initiation of reinforcement corrosion. However, the effectiveness of C3A diminishes when the amount of chloride content exceeds a certain level, as C3A can only react with a specific amount of chloride. In addition, increasing the C3A content in cement can result in a decrease in concrete’s resistance to sulfate attack. As a result, 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%.
4. Pozzolans
Pozzolanic materials are increasingly being used in the production of concrete that can resist chloride and sulfate attacks. Examples of such materials include silica fume, blast-furnace slag, and fly ash. The combination of water and calcium hydroxide with pozzolans produces concrete that is low permeable and high strength.
The use of pozzolans in concrete production is so effective in reducing permeability and increasing strength that the American Concrete Institute (ACI) 318M-11 allows for the use of type V (50 according to CSA) cement with pozzolans to resist sulfate attacks. This is a significant development in the use of pozzolans in concrete production and highlights their effectiveness in enhancing the durability and strength of concrete structures.
By utilizing pozzolanic materials in concrete production, engineers and builders can create structures that are resistant to harmful substances like chloride and sulfate, which can cause structural damage over time. This can lead to longer-lasting structures that require less maintenance and repairs, ultimately saving time and money. The use of pozzolans in concrete production is an important advancement in construction technology and offers promising benefits for the future of infrastructure.
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5. Admixtures
Admixtures are chemicals that are added to concrete to enhance its properties. One of the primary benefits of using admixtures is their ability to protect steel reinforcement from corrosion. By using water-reducing admixtures and superplasticizers, it is possible to reduce the amount of water required for concrete without sacrificing workability. This results in better impermeability and durability of the concrete structure.
However, it is essential to avoid using admixtures that contain calcium chloride as they can lead to steel corrosion. This is a significant concern since steel reinforcement is critical to the structural integrity of concrete.
The American Society for Testing and Materials (ASTM) has established standards for the use of admixtures in concrete. In particular, ASTM C494M outlines the use of time setting modification and water reduction admixtures. These admixtures can be used to adjust the setting time of concrete and reduce the amount of water needed, respectively. Following ASTM guidelines can help ensure that concrete structures are durable and long-lasting.
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6. Aggregates
The volume of aggregates in a concrete mix is significant as it constitutes around 70% of the total volume. As a result, the permeability of concrete is significantly influenced by aggregates. When the size of coarse aggregates increases, it leads to a corresponding increase in the permeability of the concrete. Mineral aggregates, in particular, have a higher permeability rate by 10-1000 times compared to the concrete paste. Hence, it is crucial to factor in the moisture content of the aggregates in the calculation of water-cement (w/c) ratio, and they should be washed before use to mitigate their impact on concrete permeability.
7. Permissible Chloride Content
The ACI 318-11 Building Code includes regulations regarding the acceptable levels of water-soluble chloride ion content in concrete. These regulations can be found in Table-2 of the code. The purpose of these regulations is to ensure that concrete structures maintain their integrity and durability over time. High levels of chloride ions in concrete can cause corrosion of reinforcing steel, which weakens the structure and can lead to costly repairs or even collapse. Therefore, it is important to adhere to the maximum allowable limits of chloride ion content in concrete as specified by the ACI 318-11 Building 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 |
| The water-soluble chloride ion content in the concrete mixture, including water, aggregates, cementitious materials, and admixtures, should be measured using ASTM C1218M. This test is conducted between 28 and 42 days of age. |
8. Concrete Cover Thickness
The durability of reinforced concrete structures is strongly influenced by the depth of concrete cover provided to the steel reinforcement. This is because a sufficient amount of concrete cover can delay the penetration of moisture and ingress of chlorides, which are key factors that contribute to the corrosion of the reinforcement.
In order to ensure the longevity of a concrete structure, it is important to consider several parameters that can affect the thickness of the concrete cover and subsequently the corrosion of the reinforcement. These parameters may include the exposure conditions, such as the level of humidity and temperature, the quality of the concrete mix used, and the type and condition of the reinforcing steel.
The relationship between these parameters and the thickness of the concrete cover can be described by an equation, which provides a means to predict the amount of concrete cover required to mitigate the risk of reinforcement corrosion. By taking into account these various factors and providing adequate concrete cover, the durability and service life of a reinforced concrete structure can be significantly improved.
Where:
The time it takes for reinforcements embedded in concrete to corrode when continuously exposed to saline water can be predicted based on several factors. These include the depth of the concrete cover, measured in centimeters, the concentration of chloride ions, measured in parts per million (ppm), and the water to cement ratio (w/c).
To prevent corrosion, the American Concrete Institute (ACI) recommends a minimum concrete cover of 65 mm for conventional concrete and 50 mm for precast concrete. ACI also provides specific guidelines for different exposure conditions in their document ACI 357R-84, which is summarized in Table-3.
By following these recommendations and guidelines, it is possible to estimate the time it will take for reinforcements embedded in concrete to corrode when exposed to saline water. This information can be invaluable for ensuring the long-term durability and safety of concrete structures.
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. Curing
Proper curing and control of temperature and moisture are effective methods for reducing the permeability of concrete. The surface layer of concrete becomes significantly more permeable if adequate curing is not employed, with an increase in permeability of 5-10 times. It is essential to ensure that the curing period is sufficient to prevent the ingress of chloride ions into the concrete before the formation of a passive protective film.
To provide guidance on concrete curing, the American Concrete Institute (ACI) committee 308 has established a standard practice. The recommendations provided by this committee can assist in achieving optimal curing outcomes for concrete structures. By following these guidelines, it is possible to minimize the risk of chloride ion ingress and ensure that the concrete remains durable and long-lasting.
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10. Compaction
A crucial factor that directly impacts reinforcement corrosion in concrete elements is the degree of compaction during pouring. Inadequate compaction can hasten the corrosion of such elements. For instance, if the degree of compaction is reduced by 10%, the permeability of the concrete will increase by 100%, and the concrete’s strength will decrease by 50%. This highlights the significance of adequate compaction for preventing corrosion.
11. Permissible Crack Width
According to ACI 224-01, the presence of cracks in concrete has a significant impact on reinforcement corrosion. To mitigate this problem, the guideline recommends a maximum permissible crack width of 0.15 mm on the tension side of the concrete element that is exposed to wetting and drying.
It is worth noting that longitudinal cracks along the steel reinforcement are more harmful than transverse cracks to the longitudinal reinforcement. This is because transverse cracks allow ingress for a smaller area, while longitudinal cracks have the potential to spall off the concrete cover. Therefore, it is important to take extra precautions when dealing with longitudinal cracks.
Table-4 in the guideline provides the maximum permissible crack widths for various exposure conditions. By following these guidelines, it is possible to minimize the effects of cracks on reinforcement corrosion and ensure the longevity and safety of concrete structures.
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 ways to prevent corrosion in reinforced concrete structures, including the application of reinforcement bar coating and cathodic protection. However, these methods are generally more expensive compared to using low permeable concrete protection.
Coating methods of protection include both anodic and barrier coatings, which are considered to be the most significant methods. Anodic coatings work by preventing the flow of electrons from the anode to the cathode, while barrier coatings create a physical barrier that prevents corrosive agents from coming into contact with the reinforcing steel.
On the other hand, the cathodic protection technique involves changing the concrete environment by using volunteering anodes or directing the flow of ions away from the reinforcement. This method works by applying an electrical potential to the steel reinforcement, which effectively stops the corrosion process from occurring.
FAQs about Protection of Reinforcement from Corrosion
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u003cstrongu003eWhat is the effect of corrosion on reinforced concrete elements?u003c/strongu003e
Steel bars play a crucial role in reinforced concrete elements by providing tensile strength to the structure. However, the corrosion of these steel bars can have severe consequences, ultimately leading to the failure of the structure. Corrosion occurs when the steel bars are exposed to environmental factors such as moisture, oxygen, and chloride ions, causing them to rust and weaken over time.u003cbru003eu003cbru003eWhen steel bars corrode, they undergo a process called oxidation, which causes the metal to expand and create pressure on the surrounding concrete. This pressure can result in cracks, spalling, and delamination of the concrete, which can lead to a loss of strength and stiffness in the structure. As the corrosion progresses, the steel bars can lose their load-bearing capacity, compromising the safety and serviceability of the structure.u003cbru003eu003cbru003eThe consequences of steel bar corrosion can be severe and far-reaching, making it a critical concern for the durability and longevity of reinforced concrete structures. Therefore, it is crucial to take preventative measures to mitigate the risks of corrosion, such as proper concrete cover and protective coatings on the steel bars. Regular inspection and maintenance of the structure can also help to identify and address any signs of corrosion early on, minimizing the potential for further damage and ensuring the safety of the structure.
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u003cstrongu003eWhat are the main causes of reinforcement corrosion?u003c/strongu003e
The two main culprits responsible for reinforcement corrosion are carbonation and chloride ion penetration. When concrete is exposed to carbon dioxide from the air, it undergoes a process known as carbonation. During this process, the alkaline environment surrounding the reinforcing steel is gradually neutralized. As a result, the steel becomes vulnerable to corrosion.u003cbru003eu003cbru003eChloride ions are another key factor in the corrosion of reinforcement. Chlorides penetrate concrete through a variety of means, including salt-laden sea spray, deicing salts used on roads, and even contaminated water used in mixing concrete. Once inside the concrete, chloride ions break down the passive layer of protection on the surface of the steel, allowing it to corrode.u003cbru003eu003cbru003eBoth carbonation and chloride ion penetration can significantly reduce the lifespan of reinforced concrete structures. Regular maintenance and protective measures can help prevent or mitigate the effects of these damaging processes.
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u003cstrongu003eHow to prevent reinforcement corrosion?u003c/strongu003e
To prevent the corrosion of steel bars on construction sites, several precautions can be taken. One of the most important factors is to use high-quality and low-permeability concrete. This can help in controlling various mechanisms that cause corrosion.u003cbru003eu003cbru003eCorrosion of steel bars is a common problem on construction sites, and it can lead to significant structural damage over time. To avoid this, it is essential to ensure that the concrete used is of high quality. High-quality concrete has lower permeability, which means that it is less likely to allow moisture and other corrosive substances to penetrate the surface.u003cbru003eu003cbru003eIn addition to using high-quality concrete, other precautions can also be taken to prevent corrosion. For example, coatings and sealants can be applied to steel bars to protect them from moisture and other corrosive substances. Proper ventilation and drainage systems can also help to reduce the buildup of moisture, which is a common cause of corrosion.