Causes

Defects/deterioration in any structure is dependent on the type of structure, how it has been designed and detailed, how constructed, how maintained and its environment. The common causes are similar regardless of these factors which primarily influence the degree and extent of deterioration. Causes can be classified on the basis on material of construction. The causes of cement concrete deterioration include alkali aggregate reactions, unsound cement, contmated water and aggregates, sulphate attack, corrasion, freezing and thawing etc. 

Alkali-Aggregate Reaction 

Reaction in concrete between alkalis from the cement in the pore water of the concrete and reactive components in the aggregate is termed alkali aggregate reaction. These are believed to be three types of alkali-aggregate reactional alkali-silica, alkali-silicate, and alkali-carbonate. They can cause cracking and, occasionally, significant weakening of the concrete. The alkali-silica reaction is the, most common. Alkaliqilica reaction can cause substantial reductions in the engineering properties of concrete. At 0.1% expansion, compressive strength decreases about 1276, loss of flexural strength can be as much as 50%, and the elastic modulus is reduced approximately 20%. 

Unsound Cement 

Portland cement as a hydrated paste is the binder of concrete. This hinder governs most of the properties of concrete.  Cement is available in several types, with and without air-entraining agents, to meet various construction requirements (e.g. early high strength) and for compatibility with certain types of aggregates.  Because of differences in raw materials, cements ground to the same fineness can have differences in strength, heat of hydration, reduction of laitance, bleeding tendency, and durability.  As mentioned previously, the alkalis in the cement can react with minerals in the aggregates resulting in cracking and expansion of the concretes.  Sulphates in ground water and in soil against which concrete is placed, can also react with chemicals in the cement resulting in disintegration of the concrete. 

Contaminated Water and Aggregates 

The usual rule regarding mixing water is that; if it meets the requirements for drinking, it is satisfactory for making concrete. Mixing water can be contaminated by impurities such as sodium and potassium carbonates and bicarbonates; sodium chloride; sodium sulphate; calcium and magnesium bicarbonates; calcium chloride; iron salts; sodium iodate; phosphate arsenate; borate; sodium sulphide; hydrochloric and sulphuric acids, sodium hydroxide; and other salts and suspended particles. It can also be contaminated by algae and other organic materials. Minute concentrations of contaminants are usually tolerable and numerical limits on concentration of different impurities have been suggested. 

Reactive. Aggregates 

Aggregates for concrete are generally inert. However, aggregates can contain minerals which react unfavorably with the chemicals in the cement paste. These reactive components of aggregates include calcium, magnesium, silica and iron oxides; ferrous sulphides; calcium sulphate; zeolites; clay minerals, and dolomitic limestone. 

Sulphate Attack 

Sulphates in ground water (or mixing water), or in soils against which concrete is in contact, can react with the cement paste in concrete and result in.disintegration of the concrete.  The problem is fairly common. Sulphates of sodium, magnesium, and calcium can react with the cementing constituents of the concrete.  Depending upon the composition of the cement, these chemical reactions may be accompanied by large volume changes in the concrete. 

Corrosion 

Concrete can corrode when it comes into contact with both organic and inorganic acids. Organic materials include acetic acid; oxalic and dry carbonic acids, carbonic acid in water: lactic-and tannic acid; and vegetable oils. Inorganic acids include the sulphates already mentioned and chlorides of magnesium and calcium.  Corrosion can also be the result of environmental action by wind and rain. 

Freezing and Thawing 

Disintegration of concrete is also caused by the disruptive action of freezing and thawing. Dry concrete, with or without entrained air, is usually not damaged by Fielding acid thawing. Frost also has a destructive effect on a weak and porous concrete. The severest conditions for frost action arise when concrete has more than one face exposed to the weather and is in such a position that it remains wet for long periods.  Alternate wetting and drying has also an adverse effect on the concrete. 

Fatigue damage of reinforced concrete is dependent upon both the fatigue life of the reinforcing steel and of the concrete.  As for all fatigue damage, the number of cycles of stress and the stress range are the most important parameters affecting fatigue damage. Failure of concrete under repeated loading results in progressive micro-cracking of the Concrete. Because there are no plastic deformations to blunt micro-cracks, concrete does have an endurance limit analogous to that for steel. Research on concrete deteriorationindicates that fatigue damage of concrete may be more prevalent than realized.