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In this article, we will discuss how to prevent reinforcement corrosion on site.
| Summary: Corrosion of steel bars is highly detrimental to reinforced concrete elements and might undermine their serviceability and even cause structural failure. However, various precautions can be taken to prevent the corrosion of steel bars on construction sites. Concrete having high quality and low permeability is very essential for controlling different corrosion mechanisms. Therefore, although conventional concrete is not fully impermeable, paying proper attention to various aspects of construction such as workmanship, concrete mixtures, and curing can make sure the production of low-permeability concrete with excellent quality. These practical measures are important for designers and site engineers to protect against reinforcement corrosion. |
A. How to Prevent Reinforcement Corrosion on Site?
1. Water-Cement Ratio (w/c ratio)
Low-permeability concrete can be made by involving a low w/c ratio which, in return, can deliver better reinforcement protection. ACI 318M-11 Building Code provisions for structural concrete suggest a maximum w/c ratio of 0.40 and minimum concrete strength of 35 MPa for concrete uncovered to moisture and an external source of chlorides from deicing chemicals, salt, brackish water, seawater, or spray from these sources.
Furthermore, ACI 357R- 84 delivers a similar water-to-cement ratio, as shown in Table 1. Therefore, it is suggested to employ a concrete strength of 42 MPa as long as the concrete surface is expected to degrade severely.
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
The binding capacity of CO2 and CL is raised as cement content is improved. However, if the portion of cement is raised without analysis, the water/cement ratio, curing, and compaction quality will have a more effective effect on chloride and carbonation penetration compared with cement content. Therefore, it is suggested by ACI 357R-84 that minimum cement content of 356 Kg/m3 could be utilized for a corrosive environment.
3. Cement Type
Cement composition involves concrete durability greatly. For instance, as the content of tricalcium aluminate (C3A) in Portland cement increases, corrosion resistance is enhanced greatly. This is because chloride ions’ reaction with the hydrated tricalcium sulfoaluminate makes insoluble Friedel salt in the hardened cement paste.
However, the effectiveness of C3A drops when the quantity of chloride content is more as C3A reacts only with a specific amount of chloride. Moreover, concrete resistance to sulfate attack is reduced by improving C3A content. That is why ACI 357R-84 suggests employing ASTM I, II, and III (Canadian Standard Association (CSA) 10, 20, and 30) cement type but with C3A content varying between 4-10 %.
4. Pozzolans
Pozzolanic materials such as silica fume, blast-furnace slag, and fly ash are used for concrete production that opposes chloride and sulfate attacks. So important is the combination of both water and calcium hydroxide with pozzolans that have low permeable and high strength concrete. ACI 318M-11 permits type V (50 according to CSA) cement with pozzolans for opposing sulfate attacks.
5. Admixtures
Admixtures are chemical materials that are employed to help cover steel reinforcement from corrosion. It is possible to use a low w/c ratio by using water-reducing admixtures and superplasticizers, which deliver proper workability, directing to better impermeability. Admixtures that contain calcium chloride should be bypassed as it guides to steel corrosion. Time-setting modification and water reduction admixtures should be operated according to ASTM C494M.
6. Aggregates
Aggregates affect concrete permeability considerably as it settles around 70 % of the volume of concrete mix. Concrete permeability raises as the size of coarse aggregates raised. The permeability of most mineral aggregates is increased by 10-1000 times compared to concrete paste. That is why it is important to have aggregate moisture content in w/c ratio calculations, and they should be cleaned.
7. Permissible Chloride Content
ACI 318-11 Building Code defines the maximum water-soluble chloride ion content in concrete (see Table 2).
Optimum Water-soluble Chloride Ion Concrete in Concrete, % Weight of Cement
| Exposure conditions | Optimum water-soluble chloride ion (Cl–) content in concrete, percent by weight of cement* | |
| Reinforced concrete | Precast concrete | |
| Concrete open to moisture and an external source of chlorides | 0.15 | 0.06 |
| Concrete is open 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, containing water, aggregates, cementitious materials, and admixtures, shall be calculated in the concrete mixture by ASTM C1218M at an age between 28 and 42 days. |
8. Concrete Cover Thickness
The depth of the concrete cover is thought the most significant factor affecting the corrosion of reinforcement. Moisture penetration and chloride ingression can be hindered by applying more concrete cover. Several parameters control concrete cover thickness, and as an outcome, reinforcement corrosion. The following equation describes those parameters:
Where:
Rt: Time to corrosion of reinforcements embedded in concrete which is open to saline water continually, years
Si: Depth of concrete cover, cm
K: Chloride ion concentration, ppm
w/c: Water to cement ratio
ACI 318M-11 suggests the lowest 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 defines concrete cover for various exposure conditions, as shown in Table 3.
Recommended Concrete Cover Over Reinforced Steel
| Zone | Cover over reinforcing steel | Cover over post-tensioning ducts |
| Atmospheric zone not applied to salt spray | 50 mm | 75 |
| Splash and atmospheric zone applied to salt spray | 65 mm | 90 |
| Submerged | 50 mm | 75 |
| Cover of stirrups | 13 mm less than those noted above |
9. Compaction
Reinforcement corrosion is instantly affected by the degree of concrete compaction. Therefore, if satisfactory compaction is not provided during concrete pouring, it will lead to the corrosion of concrete elements more quickly. For instance, due to decreasing the degree of compaction by 10%, permeability will rise by 100%, and concrete strength will be decreased by 50%. From this, it is obvious that compaction satisfactoriness is very important for the prevention of corrosion.
10. Curing
Concrete permeability can be decreased by proper curing and control of both temperature and moisture. The permeability of the concrete surface layer is improved by 5-10 times if sufficient curing is not used. If the curing period is too quick, chloride ions ingress the concrete before forming a passive protective film. ACI committee 308 (Standard Practice of Curing) delivers recommendations on concrete curing.
11. Permissible Crack Width
The existence of concrete cracks affects reinforcement corrosion significantly. Therefore, ACI 224-01 suggests that a maximum crack width of 0.15 mm is permissible at the tension side of the element uncovered for wetting and drying.
In addition, it is found that longitudinal crack along steel reinforcement is far more dangerous compared with cracks transverse to longitudinal reinforcement. This is because the latter permits ingression for a small area while the former could spall off the concrete cover. Table 4 provides the maximum permissible cracks for various exposure conditions.
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
The use of reinforcement bar coating and cathodic protection is another way to control corrosion. However, they are more costly 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 modified by using volunteering anodes or directing ion flow away from the reinforcement.
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