How to Repair of Concrete Affected by Steel Corrosion - homeslibro | lceted

Repair of Concrete Affected by Steel Corrosion

In this article, we have explained how to repair the corrosion of steel in reinforced concrete members. As we know, steel corrosion not only corrodes steel but also affects the concrete around the steel in the structure and causes its cracking. This is due to the carbonation of concrete. We proceed as follows for repairs:

1.   First, we make a visual observation of the problem.

2.   Then, we determine the nature of corrosion whether it is carbonation or chloride induced.

The nature of corrosion, as we can see with our eyes, also tells us how corrosion has taken place, Therefore, it clarifies whether it is due to carbonation or chloride effect. Now, the treatment of the corroded structure is explained here depending on its nature.

 

Procedure for Repair of Carbonation-Induced Corrosion

If the cracking of concrete is due to carbonation-induced corrosion of steel, we can have two distinct cases of the state of corrosion, namely,

1.   Where the carbonation has not reached the level of steel

2.   Where it has reached the level of steel

Test for the extent of carbonation can be carried out by spraying 0.2% solution of phenolphthalein indicator on freshly chipped concrete which is in contact with steel and also on the concrete near the reinforcement. The change in colour of the concrete indicates that the concrete is in good condition. When there is no change in colour, it shows that it is carbonation-affected concrete. The pH value can also be found by analysing the samples of mortar collected by drilling, dissolving them in distilled water and conducting titration. There can be two cases of corrosion which are to be treated as follows:

 

Case 1: (where the carbonation depth has not reached the reinforcement level)

In this case, we are concerned with carbonation for a small depth only. The crack can be sealed by widening it and placing a polymer-modified cement slurry. In important cases, we may use injection grouting with grouting nipples and slurry pumps.

Case 2: (where the carbonation depth has reached reinforcement level)

This is a bad case. In this case, we proceed along the following steps after supporting the member with external supports to carry the load on the member, if necessary.

Step 1: Remove all the defective concrete that has been carbonised and expose the reinforcement bars. If the bars are rusted very much, then it is good to remove the concrete so as to give a clear air gap all around the steel of about 15 mm in slabs and up to 25 mm in beams and columns. (This is a must in chloride corrosion.) Clean the steel thoroughly with a wire brush and manual labour or by shot blasting or other means. We may use a water jet to clean the surface. Rust remover chemicals are also available for easy removal of rust. Additional steel with necessary supports may have to be provided if needed. This addition is compulsory if the rusted steel is around or over 15%. It can be tied or welded to old steel.

 

Step 2: Apply a corrosion-inhibiting slurry coat to steel using acrylic polymer and cement (with 43-grade cement) in a ratio of 1:5. Alternately, use an epoxy bonding coat with no cement. These chemicals are available in the market. (In the former days, we used to paint the steel with red oxide chromium paint. Two coats one after the other at intervals of not less than four hours were specified.

 

Step 3: Make the exposed concrete damp (but not saturated) with water. Apply a bond coat of latex or acrylic-modified cement slurry (using commercially available chemicals) over the concrete surface. This coat is applied to bond the old concrete with the new concrete or mortar (as described in step 4). If this bond coat is not applied, the old concrete and new concrete will not stick together. (These bond coats are conventional cement slurry to which we add chemicals like synthetic latex.)

 

Step 4: Replace the concrete that we have removed with latex-modified concrete (with small aggregates) or with latex-modified mortar. This work should be carried out before the bonding coat dries up. Compact this concrete wall.

 

Step 5: Apply to cure paint on the finished surface for curing without watering or use the conventional method of curing the concrete (or mortar) with wet gunny bags for 7 days.

 

Step 6: Apply decorative paint after curing to match the surrounding surfaces.

 

Note: There is also ready-to-use mineral-based one-component material which acts as a combined corrosion inhibitor and bond coat. One such chemical is Zentrifix KMH of MC Bauchemie available in the market. A 100 parts by weight of this material is slowly added to 20–25 parts by weight of water and mixed thoroughly. This is applied to the prepared steel and prewetted concrete as corrosion inhibitors and bonding coats.

 

Repair of Chloride Induced Corrosion

There can be two cases under this head that are as follows:

Case 1: Corrosion due to chlorides in the concrete used for construction

This is the worst case. The sand, water or aggregate used to make the concrete may have contained chlorides. shows the formation of corrosion by voltaic action. This case is very difficult to cure, as the cause is internal. It is better to rebuild the structure. For temporary relief, we may proceed as follows: In this case, for repair, we have to completely remove the concrete around the steel, leaving a clear gap. We then carry out the operations of cleaning both steel as well as concrete, applying bonding coats and filling the space with very good concrete and small aggregates mortar. As corrosion can occur at any place of the structure, it is better to rebuild the member rather than make frequent repairs.

 

Figure Repair of corrosion.

 

Case 2: Corrosion due to chloride from the surrounding atmosphere in which the structure stands

This occurs in structures like water tanks, and bridges exposed to air near the sea, even if there is no chloride inside the structure. This case is not as bad as the above-described case because this can be stopped if we take proper precautions, as explained below.

 

Figure: Surface coating of RC structures situated in open marine areas.

 

In this case, the cause is external, not internal. For repair, we first repair the corroded concrete and steel, as indicated above. Then, we paint the structure exposed to the marine air with special paint available for this purpose to eliminate the future intrusion of chloride. These paints are classified as anti-carbonation and chloride protective coatings. The figure above shows a bridge near the seacoast painted with protective paint. Water tanks exposed to sea air (built near the seacoast) should be also painted with this type of paint.

For undersea reinforced concrete structures, special precautions are to be taken for choosing the materials and method of construction.

 

How to Repair Concrete Broken by Sulphate Action

We must first make a chemical test of the concrete to ascertain whether the water-soluble sulphate ions are present in the set concrete. It is to be expressed as a percentage of the weight of cement. Check whether it exceeds the permissible value of 4%. If it does, then it is sure that the sulphate action is present. For repair, we have to remove the broken parts and reconstruct them with sulphate-resisting cement. Such cases require frequent inspection and repair.

 

How to avoid Corrosion of Reinforcement in Contact with Brickwork?

Another common corrosion of steel occurs when steel is in contact with brickwork. When we strengthen brickwork with steel (for example, when we build a half brickwork partition wall), the steel must be fully embedded in good concrete to avoid corrosion of steel.

 

How to Visually Distinguish between Carbonation Induced and Chloride Induced Corrosion

For an experienced person, it is easy to distinguish between the two types of corrosion, Carbonation induced corrosion is the general corrosion along the reinforcement in wet situations. This corrosion also occurs much more rapidly after construction than chloride-induced corrosion. In chloride-induced corrosion, we can notice distinct anode and cathode regions, as it is an electrochemical process.

 

Conculsion

During the construction of reinforced concrete structures, we should take proper care of all the steel reinforcement. We should also take steps to protect steel from corrosion by adopting a low water-cement ratio (not more than 0.5) by using plasticisers and compacting the concrete well. The object is to reduce the number of pores in the concrete and thus, reduce the carbonation of steel.

We should also take care of using good water and fine aggregates free from chlorides to eliminate chloride corrosion. Structures like bridges, water tanks etc. which are near the sea coast and open to the atmosphere, should be painted with special paints to reduce the effect of chlorides and carbon present in the air and also to reduce the penetration of moisture into the structures. This reduces both chloride corrosion and carbonation of steel.

When we deal with repairing steel corrosion and the breaking up of concrete structures due to corrosion of steel, we must first diagnose the cause of corrosion whether it is carbonation or chloride effect or sulphate effect Then, we must plan the repair accordingly, as described. The repaired structure should be also monitored periodically to check whether the repair that has been made is working well.