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Deleterious effects of Aggressive Chemicals

Er. Parbhakar Dwivedi
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1. Sulphate Attack

Soluble sulphates are present in

1. Ground water

2. Soil

3. Clay bricks

Soluble sulphate react with Tricalcium Aluminate of cement in the presence of moisture and form products which occupy much bigger volume then that of original constituents and so expansive reaction results causing disintegration and weakening of concrete, masonry, plaster and formation of cracks. The reaction is very slow and cracks start coming to notice after 2-3 years. This reaction to take place, the presence of following three things

Severity of sulphate attack depends upon:-

1. Amount of soluble sulphates

2. Permeability and porosity (voids) in concrete & mortar

3. Proportion of C3, A in cement

4. Presence of moisture/dampness/leakage of water in the particular building component.

2. Carbonation

When concrete hardens due to hydration of cement, some calcium hydroxide is liberated which sets up a protective alkaline medium inhibiting galvanic cell action and preventing corrosion of steel. In course of time, free hydroxide in concrete reacts with atmospheric carbondioxide, forming, calcium carbonate. This reaction known as Carbonation lowers alkalinity of concrete and reduces its effectiveness as a protective medium. In good quality dense concrete, carbonation is confined mainly to surface layers of concrete and depth of carbonation may not exceed 20mm in 50 years.

3. Chloride Attack

Chlorides may be present in the water used for mixing or may be introduced into concrete by coming in contact with environments containing chlorides, such as sea water, de-icing salts (over road surface). Penetration of the chlorides starts at the surface, then moves inward. Penetration takes time depending upon:

(a) Amount of chlorides coming into contact with the concrete

(b) Permeability of concrete and

(c) Amount of moisture content.

Eventually the concentration of chlorides in contact with reinforcing bars will cause corrosion when moisture and oxygen are present

As the rust layer builds up, tensile forces generated by the expanded volume of the rusted iron cause the concrete to crack and delaminate. When cracking and delaminating progress, accelerated corrosion takes place because of easy access of corrosive salts, oxygen and moisture. Corrosion then begins to affect reinf. Bars buried further within the concrete.

The concentration of chlorides necessary to promote corrosion, among other factors, is greatly affected by the pH of concrete. Studies have revealed that a threshold level of 8000 ppm of chloride ion was required to initiate corrosion with only 71 ppm of chloride ions. Chlorides are not consumed in the corrosion process, but instead act as catalysts to the process and remain in concrete. Cracks and construction joints in concrete permit corrosive chemicals to enter the concrete and access embedded reinforcing steel. Reinforcement detailing rules given in the codes generally provide that the crack widths are less than 0.3 mm for soil exposure conditions, humidity, and moist air.

4. Alkali Aggregate Reaction

In O.P.C., Alkalis namely sodium oxide (Na2,O) and Potassium Oxide (K2,O) are present to some extent. These alkalis chemically react with certain siliceous mineral (constituents of some aggregates) and cause expansion, cracking and disintegration of concrete. Because of lowering of alkalify, rusting of reinf. Is also promoted in presence of moisture. Like sulphate attack. This reaction is also very slow and takes no. of years for developing cracks. Cracks are of map pattern. Mineral admixtures such as pozzolana, flyash, ground blast furnace slag and silicafume when used as part replacement in concrete, can be effective in suppressing the Alkali- aggregate reaction.

5. Freeze-Thaw Disintegration

It occurs on concrete surfaces when freezing water contained in the pore structure expands as it is converted into ice. The expansion causes localized tension forces that facture the surrounding concrete matrix. The fracturing occurs in small pieces, working from the outer surfaces inward. The rate of freeze-thaw deterioration is a function of porosity, moisture saturation, and number of freeze-thaw cycles; horizontal surfaces that trap standing water aid the process.

6. Abrasion

Abrasion is wearing away of the surface by rubbing and friction. Generally, the surface is uniformly worn away, including the cement matrix and aggregate Factors affecting abrasion include, compressive strength of concrete, aggregate properties, finishing methods, use of toppings and curing and the type of loading, movement on the slab.

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