Water Ingress - A Cause of Reinforcement Cancer - A Cure

Dr. Surendra P. Bhatnagar, Chairman & Managing Director, Tech-Dry (India) Pvt. Ltd., Bangalore.

We would begin with the fact that concrete is known since ages but it is very weak in tension in comparison to its compressive strength. It is because of the low tensile strength of the concrete that reinforcing bars are used. The combination of concrete and steel offers us reinforced concrete, which is used in the construction of most of the concrete structures including bridges and roadways. It was a great discovery and was a revolution in the history of concrete technology. However, in late 60s prematured concrete delamination and concrete stress was seen mostly in coastal areas because of chloride ions.

In 1970s deicing using common salt also added to further deterioration of reinforced concrete and report of 581, 862 bridges in and off the federal aid system, about 101, 518 bridges were rated as structurally deficient. The estimated cost to eliminate all backlog bridge deficiencies including structurally and functionally is approximately $78 billion and it could increase to as much as $112 billion.

The cancer in reinforced concrete is a serious concern because the structure would become undurable. In concrete, the presence of abundant amount of calcium hydroxide and relatively small amounts of alkali elements, such as sodium and potassium, gives concrete a very high alkalinity with pH of 12 to 13. Reinforcing steel in concrete normally does not corrode because of the formation of a passive oxide film on the surface of the steel due to the initial corrosion reaction. The process of hydration of cement in freshly placed concrete develops a high alkalinity, which in the presence of oxygen stabilises the film on the surface of embedded steel, ensuring continued protection while the alkalinity is retained.

Once corrosion sets in on the reinforcing steel bars, it proceeds in electrochemical cells formed on the surface of the metal and the electrolyte or solution surrounding the metal.

The combination of concrete and steel offers us reinforced concrete
Corrosion can also occur even in the absence of chloride ions. For example, when the concrete comes into contact with carbonic acid resulting from carbon dioxide in the atmosphere, the ensuing carbonation of the calcium hydroxide in the hydrated cement paste leads to reduction of the alkalinity, to pH as low as 8.5, thereby permitting corrosion of the embedded steel:

CO2 + H2O H2CO3

H2CO3 + Ca(OH)2 CaCO3 +2H2O

The rate of carbonation in concrete is directly dependent on the water/cement ratio (w/c) of the concrete, i.e. the higher the ratio the greater is the depth of carbonation in the concrete. In concrete of reasonable quality, that is properly consolidated and has no cracking, the expected rate of carbonation is very low.

For example, in concrete with w/c of 0.45 and concrete cover 25 mm (1 in.), it will require more than 100 years for carbonation to reach the concrete immediately surrounding the steel. Carbonation of concrete or mortar is more of an issue in Europe—thereby prompting the application of electrochemical realkalinization of concrete there—than in the United States. In India even RMC are using w/c ratio of 0.45 which is quite high.

The emissions of air pollutants such as sulfur dioxide (SO2) and nitrous oxides (NOx), resulting from burning of fossil fuels are high in percentage because of environmental pollution and these gases convert into acides –in the form of acid rain which is more dangerous than carbon dioxide and chloride.

The rapid prematured deterioration of many concrete bridges, roof and other structures has raised concern. The only solution is to get the structure completely waterproofed and use minimum w/c ratio.

The combination of concrete and steel offers us reinforced concrete
The application of prestressed concrete technology in building bridges is relatively recent. Therefore, the existing prestressed concrete members in bridges are still relatively young, and the corrosion and the concrete deterioration problems associated with this type of concrete members have only become evident in the early 1980s. Although prestressed concrete members were generally manufactured with concrete of relatively higher strength, time has shown that they are subject to the same adverse effects of reinforcement corrosion as reinforced concrete members are. Documented cases of prestressed strands breaking as a result of corrosion make this a most pressing problem. Since PS/C members rely on thetensile strength of the strands to resist loads, loss of even a few strands per member could prove catastrophic. In addition, because of the high stress the strands are subjected to, corrosion effects are accelerated. Even small corrosion pits could cause fracture of a strand, as compared to non prestressed reinforcing steel that will literally rust away before breaking. Of the 581,862 bridges in the National Bridge Inventory, slightly more than 10 percent have prestressed concrete superstructures, all of which will eventually need some protective measures applied to them. In addition, many of the other structures not in the inventory have prestressed substructure members that will likewise need some degree of corrosion protection.

The common man thinks that water intrusion in the building causes only cosmetic changes and utmost inconvenience and it does spoil paints and interiors. As this paper explains it is more important to safe guard reinforcement, otherwise our buildings will not be safe and durable. Recently, number of products have been developed worldwide and by our own research group which can safe guard reinforcement. This paper is important because we are spending millions of rupees in infrastructure and we should learn to protect them otherwise we will be spending equal amount on repair and rehabilitation.

The human tragedy, property loss repetition of 1999 Turkey earthquake and the similar type of earthquake in Gujarat and Uttarkashi, where corrosion was found as the main cause for disaster could have been avoided if the consultants and the people in this profession learn to accept changes.

All our consultants and scientists working in this field have knowledge of these inherent problems with the building. As Swami Vivekananda said “Knowledge is inherent in man. No knowledge comes from outside; it is all inside. What we say a man ‘knows,” should in strict psychological language, be what he “discovers” or unveils”; what a man “learns” is really what he “discovers,” by taking the cover off his own soul, which is a mine of infinite knowledge.” He further said, “We say Newton discovered gravitation. Was it sitting anywhere in the corner waiting for him? It was in his own mind; the time came and he found it out. All knowledge that the world has ever received comes from the mind.” All we need is change and discover the knowledge, which we have within ourselves.
NBM&CW April 2006
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