Page 189 - CW E-Magazine (27-2-2024)

Special Report

Prediction of corrosion in concrete structures

INTRODUCTION material that will cause the calcium DR. S.K. CHAKRAVORTY
corroded concrete structure oxide or hydroxide to be removed, lower- Consultant (Plant Engineering)
(whether a bridge, jetty, build- ing the pH of the cement mix, will cause
A ing, or a simple ramp) is unsafe. instability. The most common cause for Email: chakravorty4410@gmail.com
This makes corrosion prediction/inves- deterioration of concrete is contact with concrete structures start showing signs
tigation of critical structures a impor- inorganic and organic acids. Those that of distress within a short period. Usu-
tant task in asset management. Figure-1 form soluble salts with calcium oxide ally the condition of the structures is
shows the pillars of a flyover damaged or calcium hydrate are most aggres- monitored by visual inspection and
due to concrete corrosion. sive. Typical compounds that can cause remedial measures are resorted to
problems include sour milk, industrial only when the condition becomes seri-
wastes, fruit juices, sonic ultrapure ous due to heavy rusting of steel rein-
waters, and organic materials that ferment forcements, followed by cracking and
and produce organic acids (Table 1). spalling on concrete. It is desirable to
monitor the condition of such strategic
Many reinforced and pre-stressed structures right from the construction

Table 1: Effects of various chemicals on concrete
Fig. 1: Pillars of flyover damaged due to Chemicals present Effect on Concrete
concrete corrosion Chemical plants
Corrosion monitoring of reinforced
concrete structures can be quite chal- Acid waters, pH 6.5 or Disintegrates slowly
less
lenging. It is hard to detect corrosion
at the early stages; on the other hand, Ammonium nitrate Disintegrates
it is almost too late when we can see Benzene Liquid loss by penetration
corrosion signs on the surface. Detect- Sodium hypochlorite Disintegrates slowly
ing corrosion in concrete structures Ethylene Disintegrates slowly
requires appropriate knowledge, proper
inspection tools and experience. Many Phosphoric acid Disintegrates slowly
structural engineers will like to know Sodium hydroxide, Disintegrates slowly
best practice(s) in corrosion investiga- 20% & above
tion. Some want to know where to start; Electricity generating utilities
others are concerned about accuracy & Ammonium salts Disintegrates
precision of test methods; and some
will like to know if the results of such Coal Sulphides leaching from damp coal may oxidise to
investigation(s) are of any use from a sulphurous/sulphuric acid; causes disintegration
structural engineering perspective. Hydrogen sulphide Dry, no effect; in moist oxidising environments
converts to sulphurous acid; disintegrates slowly
When water is added to Portland Sulphuric acid (10-80%) Disintegrates rapidly
cement, a chemical reaction takes place Sulphur dioxide With moisture forms sulphurous acid; disintegrates
during hardening. This reaction pro- rapidly.
duces calcium hydroxide and trical-
cium silicate hydrate. The alkalinity of Pulp & paper mills
concrete is provided by the presence of Chlorine gas Slowly disintegrates moist concrete
calcium oxide from the cement. Con- Sodium hydroxide Disintegrates
sequently, concrete attack can be due Sodium hypochlorite Disintegrates slowly
to chemicals that react with Portland
cement binder and form conditions that Sodium sulphide Disintegrates slowly
physically deteriorate the material. Any Sodium sulphate Disintegrates concrete of inadequate sulphate resistance.

Chemical Weekly February 27, 2024 189

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