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Corrosion of steel bars is highly detrimental for reinforced concrete elements and might undermine their serviceability and even cause structural failure. However, various precautions can be taken to prevent corrosion of steel bars on construction sites.
Concrete with high quality and low permeability is essential for controlling various corrosion mechanisms. Therefore, although conventional concrete is not entirely impermeable, paying proper attention to different aspects of construction such as workmanship, concrete mixtures, and curing can ensure the production of low-permeability concrete with excellent quality.
These practical measures are crucial for designers and site engineers to prevent reinforcement corrosion.
How to Prevent Reinforcement Corrosion on Site?
1. Water-Cement Ratio (w/c ratio)
Low-permeability concrete can be produced by applying a low w/c ratio which, in return, can provide better reinforcement protection. ACI 318M-11 Building Code requirements for structural concrete recommends a maximum w/c ratio of 0.40 and minimum concrete strength of 35 MPa for concrete exposed to moisture and an external source of chlorides from deicing chemicals, salt, brackish water, seawater, or spray from these sources.
Furthermore, ACI 357R- 84 provides a similar water-to-cement ratio, as shown in Table-1. Therefore, it is recommended to use a concrete strength of 42 MPa as long as the concrete surface is expected to degrade severely.
Table-1: Water/ Cement Ratios And Concrete Compressive Strength for Three Weather Conditions
|Zone||Maximum w/c ratio||Concrete compressive strength (fc’) MPa|
The binding capacity of CO2 and CL is increased as cement content is increased. However, if the amount of cement is raised without measure, water/cement ratio, curing, and compaction quality will have a greater effect on chloride and carbonation penetration compared with cement content. Therefore, it is recommended by ACI 357R-84 that minimum cement content of 356 Kg/m3 could be used for a corrosive environment.
3. Cement Type
Cement composition affects concrete durability considerably. For instance, as the content of tricalcium aluminate (C3A) in Portland cement increases, corrosion resistance is improved significantly. This is because chloride ions' reaction with the hydrated tricalcium sulfoaluminate creates insoluble Friedel salt in the hardened cement paste. Figure-1 illustrates the effect of C3A on reinforcement corrosion initiation.
However, the effectiveness of C3A decreases when the amount of chloride content is more as C3A reacts only with a specific amount of chloride. Moreover, concrete resistance to sulfate attack is declined by increasing C3A content. That is why ACI 357R-84 recommends employing ASTM I, II, and III (Canadian Standard Association (CSA) 10, 20, and 30) cement type but with C3A content ranging between 4-10 %.
Pozzolanic materials such as silica fume, blast-furnace slag, and fly ash are utilized for concrete production that resists chloride and sulfate attacks. So significant is the combination of both water and calcium hydroxide with pozzolans that produce low permeable and high strength concrete. ACI 318M-11 permits type V (50 according to CSA) cement with pozzolans for resisting sulfate attacks.
Admixtures are chemical materials that are used to help protect steel reinforcement from corrosion. It is possible to use a low w/c ratio by employing water-reducing admixtures and superplasticizers, which provide proper workability, leading to better impermeability. Admixtures that possess calcium chloride should be avoided as it leads to steel corrosion. Time setting modification and water reduction admixtures should be used according to ASTM C494M.
Aggregates affect concrete permeability considerably as it occupies around 70 % of the volume of concrete mix. Concrete permeability increases as the size of coarse aggregates increases. The permeability of most mineral aggregates is higher by 10-1000 times compared to concrete paste. That is why it is vital to include aggregate moisture content in w/c ratio calculations, and they should be washed.
7. Permissible Chloride Content
ACI 318-11 Building Code specifies the maximum water-soluble chloride ion content in concrete (see Table-2).
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 depth of concrete cover is considered the most significant factor affecting corrosion of reinforcement. Moisture penetration and chloride ingression can be delayed by applying additional concrete cover. Several parameters influence concrete cover thickness, and as a result, reinforcement corrosion. The following equation explains those parameters:
Rt: Time to corrosion of reinforcements embedded in concrete which is exposed to saline water continuously, years
Si: Depth of concrete cover, cm
K: Chloride ion concentration, ppm
w/c: Water to cement ratio
ACI 318M-11 recommends a minimum concrete cover for corrosion protection of 65 mm for conventional concrete and a minimum cover depth of 50 mm for precast concrete. Furthermore, ACI 357R-84 specifies concrete cover for different exposure conditions, as shown 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|
|Cover of stirrups||13 mm less than those mentioned above|
Reinforcement corrosion is directly affected by the degree of concrete compaction. Therefore, if adequate compaction is not provided during concrete pouring, it will lead to corrosion of concrete elements more quickly. For instance, due to decreasing degree of compaction by 10%, permeability will increase by 100%, and concrete strength will be reduced by 50%. From this, it is clear that compaction adequacy is very significant for the prevention of corrosion.
Concrete permeability can be reduced by proper curing and control of both temperature and moisture. The permeability of the concrete surface layer is increased by 5-10 times if adequate curing is not employed. If the curing period is too short, chloride ions ingress the concrete before forming a passive protective film. ACI committee 308 (Standard Practice of Curing) provides recommendations on concrete curing.
11. Permissible Crack Width
The presence of concrete cracks affects reinforcement corrosion significantly. Therefore, the ACI 224-01 recommends that a maximum crack width of 0.15 mm is permissible at the tension side of the element exposed to wetting and drying.
In addition, it is found that longitudinal crack along steel reinforcement is far more detrimental compared with cracks transverse to longitudinal reinforcement. This is because the latter allows ingression for a small area while the former could spall off the concrete cover. Table-4 provides maximum permissible cracks for different exposure conditions.
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|
|Seawater and seawater spray, wetting and drying||0.15|
|†Excluding non-pressure pipes.|
12. Protective Coatings
The application of reinforcement bar coating and cathodic protection is another way to prevent corrosion. However, they are more expensive compared with low permeable concrete protection. Both anodic and barrier coatings are the most significant methods of protection by coating. In the cathodic technique, the concrete environment is changed by employing volunteering anodes or directing ion flow away from the reinforcement.
Corrosion of steel bars is highly detrimental for reinforced concrete elements and might undermine their serviceability, and even cause structural failure.
Carbonation and penetration of chloride ions are the primary causes of reinforcement corrosion.
Various precautions can be taken to prevent corrosion of steel bars on construction sites. Concrete with high quality and low permeability is essential for controlling various corrosion mechanisms.
How to measure reinforcement corrosion in concrete structures?