How to Reduce Corrosion of steel in concrete during construction? | Types of Corrosion and defects breaks concrete

Steel corrosion in Reinforced concrete is a very important topic that every civil engineer should know, and in this article, we will first discuss two corrosion mechanisms of steel in reinforced concrete construction. Because the amount of rust produced by the corrosion of steel exceeds the amount of original steel, corrosion of steel causes cracking of the concrete cover in reinforced concrete structures. The following topics are also considered in this article:

 

Corrosion of Steel and Breaking up of Concrete

Basically, there are two types of corrosion that result in the cracking of concrete.

1. Carbonation-induced corrosion

2. Chloride-induced corrosion

The two types of defects that can also break up concrete are as follows:

1. Sulphate attack and break up of concrete

2. Alkali aggregate reaction and break up of concrete

 

In reinforced concrete elements, the first to crack is the concrete cover. It is always understood that the steel floor in an RC structure should be in accordance with the structural exposure of the elements. (These depend on the distance of the structure to the sea). When steel corrodes excessively, we can only see both broken concrete and corroded steel.

 

It is also interesting that cast iron pipes do not deteriorate when buried in corrosive soil. The corroded area remains intact and acts as a protection against further corrosion. Several cast iron water pipes buried underground remained in the field for a long time. Steel pipes do not last long. In industrial cities, CO2 in the air and additionally, industrial pollutant gases such as SO2 and H25 also affect concrete. Rainwater in these places becomes acidic due to its reaction with these gases and this acidic water can lead to the rapid corrosion of steel. Details of the four types of concrete cracks (listed above) are given here.

 

Cracks Formed by Carbonation Induced Corrosion (Ordinary Corrosion)

The carbonation-induced corrosion takes place in steel in RC members as follows:

The water-cement ratio required for complete hydration of most of the cement is only about 0.23. Only this water is required for chemical action when we make concrete. But for easy placement of concrete, we use a water-cement ratio of 0.5 or more. The final evaporation of the excess water that we have used (in excess requirement of hydration of cement) increases the permeability (ease of air and water to go through) of the resulting concrete in the long run. Even though longer curing may improve gel formation and reduce permeability by a little more, this long-time curing is not generally carried out in the field.

 

The important factor of why it takes time to rust is also discussed here. The hydration of Portland cement releases alkalies with a high pH value of the order of 12.6 to 13.5. In this condition, oxygen cannot react with steel and produce rust, even if the oxygen can reach steel through a small crack or the pores present

 

in the concrete. However, as the concrete dries up and the pores allow free access to the atmospheric air, the carbon dioxide from the atmosphere reacts with alkalinity and reduces the pH value. If the pH value falls lower than 11 to 11.5 (i.e., when the passive nature around steel is destroyed by carbon dioxide), the oxygen of the air in the presence of moisture reacts with steel and produces rust or corrosion. This mechanism of rusting of steel is called carbonation-induced corrosion Figure below because of the role of carbon dioxide. This basic action should be borne in mind while dealing with the ordinary corrosion of steel.

 

Figure: Corrosion of steel (a) carbonate corrosion and (b) chloride corrosion.

 

Cracks Formed by Chloride-Induced Corrosion

The second type of steel corrosion and breaking of concrete is chloride-induced corrosion. This type of corrosion is different from corrosion due to carbonation. It is the corrosion that takes place in the reinforced concrete structures under the following situations where chlorides are present:

1. Near the seacoast

2. In the sea

3. In the concrete in which the sand used for making concrete contains chlorides, as in sea sand

4. In the salty water which we use.

With this type of corrosion, the steel corrodes very fast. In this case, an electrochemical reaction (like the flow of current in a battery cell) takes place figure above the steel generally gets pitted and rusted. The shape of corrosion on steel due to the chloride effect is, hence, different from that due to carbonation and the two can easily be identified by looking at the corroded. steel. Laboratory tests are also available to distinguish these two types of corrosion described above.

 

Cracking of Concrete by Sulphate Attack and Alkali

Aggregate (Silica) Reaction

These are two other types of reactions whereby concrete (in mass concrete and in reinforced concrete also) can get broken up. These are explained below:

1. Sulphate attack: Concrete, even without reinforcement (such as in concrete pipes), carrying sewage can cause expansion of plain concrete which can lead to cracking. This is due to a chemical reaction between hardened concrete and sulphate ions.

Sewers made of plain concrete (need not be reinforced) carry sewerage. The reducing bacteria present in sewage can produce sulphide gas which produces sulphuric acid. This can damage the sewers and pipes carrying sewerage. Hence, for such works, we should use special sulphate-resisting cement for making concrete. Otherwise, the concrete breaks up.

There is a record in Chennai where the top large reinforced concrete cover of a sewage tank gave way when a few people started playing cards sitting on the top of the cover. The bottom of the cover which was not visible was attacked by the sulphate from the sewage water below. Hence, when loaded from the top, it gave way.

2. Cracking of concrete by alkali aggregate reaction: This is a different reaction. Aggregates containing certain reactive constituents (like reactive silica) can react with the alkali and hydroxyl ions released during the hydration of concrete and cause the disintegration of concrete. This is called an alkali-aggregate reaction. It is reported that this effect was seen in the use of river shingles containing reactive silica for making concrete for the spillway of the Hirakud dam in Odisha. The spillway had to be repaired after the cracking developed

 

Steps to be Taken During Construction to Reduce Corrosion of Steel

From the above discussion, we can infer the steps to be taken to eliminate both types of corrosion of steel—carbonation-induced corrosion and chloride-induced corrosion (even with good cover) in reinforced concrete during the construction stage. They can be summarised as follows:

Methods of reduction of carbonation: Carbonation-induced corrosion of steel can be minimised if we make fully compacted concrete with minimum pores. This can be achieved to some extent by reducing the water-cement ratio while laying concrete. It should not be more than 0.5. For good compaction of concrete, we need workability, and this can be achieved by the use of plasticisers. Then, the porosity can be brought to a minimum and the accessibility of air to reach steel can be reduced.

As a second means, we can paint the surfaces of the structure after it is built with special paints. There are also a number of chemicals like plasticisers which can reduce the amount of water needed, and polymers that can fill the voids and prevent the entry of air. These work internally.

There are also many types of special external coatings that are available in the market for the prevention of corrosion. Paintwork is done especially if the structure is fully exposed to the air and rain, as in the case of bridges, water tanks etc. Most of the modern exposed bridges and water tanks are nowadays painted with these special paints, especially if they are near the seacoast.

 

Elimination of chloride corrosion: For eliminating this type of corrosion, the constituents (sand, water, aggregates we use for making concrete) should be free from chlorides. Special care should be taken during the construction stage itself for testing the water and the sand that we use for making concrete for chlorides.

 

Providing specified cover: In all cases, enough cover for reinforcement should be given in the construction. There are two separate specifications for the cover, as shown in the Table below

Case 1: for regions beyond 15 km from the sea and 

Case 2: for coastal regions at a distance of 15 km from the sea.

 

For all structural elements below ground level, the minimum cover should be 75 mm. (This may be reduced a little if the highest water level in the rainy season is 1.5 m below the bottom level of the foundation.)