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TYPES OF CHEMICAL ATTACKS ON CONCRETE STRUCTURES

Chemical attacks on concrete structures causes deterioration of structure and its durability is affected. The life of structure reduces and it can lead to failure of structures. The various types of chemical attacks and their effects on concrete structures are discussed below:

(a) Chlorides attacks on concrete structures:

High concentrations of chloride ions cause corrosion of reinforcement and the products of corrosion can disrupt the concrete. Chlorides can be introduced  into the concrete either during or after construction as follows.

(i) Before construction Chlorides can be admitted in admixtures containing calcium chloride, through using mixing water contaminated with  salt water or improperly washed marine aggregates.

(ii) After construction Chlorides in salt or sea water, in airborne sea spray and from de-icing salts can attack permeable concrete causing corrosion of reinforcement.

Chlorides are discussed in BS8110: Part 1, clause 6.2.5.2. The clause limits the total chloride content expressed as a percentage of chloride ion by mass of cement to 0.4% for concrete containing embedded metal. It was common practice in the past to add calcium chloride to concrete to increase the rate of hardening. The code now recommends that calcium chloride and chloride-based admixtures should not be added
to reinforced concrete containing embedded metals.

(b) Sulphates attacks on concrete structures:

Sulphates are present in most cements and some aggregates. Sulphates may also be present in soils, groundwater and sea water, industrial wastes and acid rain. The products of sulphate attack on concrete occupy a larger space than the original material and this causes the concrete to disintegrate and permits corrosion of steel to begin. BS8110: Part 1, clause 6.2.5.3, states that the total water-soluble sulphate
content of the concrete mix expressed as SO3should not exceed 4% by mass of cement in the mix. Sulphate-resisting Portland cement should be used where sulphates are present in the soil, water or atmosphere and come into contact with the concrete.

Super-sulphated cement, made from blast furnace slag,can also be used although it is not widely available. This cement can resist the highest concentrations of sulphates.

(c) Carbonation of concrete structures:

Carbonation is the process by which carbon dioxide from the atmosphere slowly transforms calcium hydroxide into calcium carbonate in concrete. The concrete itself is not harmed and increases in strength, but the reinforcement can be seriously affected by corrosion as a result of this process.

Normally the high pH value of the concrete prevents corrosion of the reinforcing bars by keeping them in a highly alkaline environment due to the release of calcium hydroxide by the cement during its hydration. Carbonated concrete has a pH value of 8.3 while the passivation of steel starts at a pH value of 9.5. The depth of Carbonation in good dense concrete is about 3 mm at an early stage and may increase to 6–10 mm
after 30–40 years. Poor concrete may have a depth of Carbonation of 50 mm after say 6–8 years. The rate of Carbonation depends on time, cover, concrete density, cement content, water-to-cement ratio and the presence of cracks.

(d) Alkali—silica reaction in concrete structures:

A chemical reaction can take place between alkali in cement  and certain forms of silica in aggregate. The reaction produces a gel which absorbs water and expands in volume, resulting in cracking and disintegration of the concrete.

BS8110: Part 2, clause 6.2.5.4, states that the reaction only occurs when the following are present together:

  1. a high moisture level in the concrete

  2. cement with a high alkali content or some other source of alkali

  3. aggregate containing an alkali-reactive constituent

The code recommends that the following precautions be taken if uncertainty exists:

  1. Reduce the saturation of the concrete;

  2. Use low alkali Portland cement and limit the alkali content of the mix to a low level;

  3. Use replacement cementitious materials such as blast furnace slag or pulverized fuel ash. Most normal aggregates behave satisfactorily.

(e) Acids attacks on concrete structures:

Portland cement is not acid resistant and acid attack may remove part of the set cement. Acids are formed by the dissolution in water of carbon dioxide or sulphur dioxide from the atmosphere. Acids can also come from industrial wastes. Good dense concrete with adequate cover is required and sulphate-resistant cements should be used if necessary.

 

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