Deterioration of concrete structures happen mainly because of the following reasons:-
1. Poor quality of concrete leading to inadequate strength and large porosity, permeability and honeycombs in the structure due to which water and various aggressive chemicals such as CO2, Oxygen, Chlorides and sulphates find easy entry into the interior of concrete.
2. Effect of weathering and aggressive environment around the concrete. Aggressive compounds may be present either in subsoil or ground water or external environment or aggressive liquid stored in the structure or inside the concrete itself.
3. In adequate cover to the reinforcement steel due to which corrosion inducing agents find easier and quicker contact with reinf. Steel
4. Poor treatment of construction joints, expansion joints, control joints due to which water and other chemicals find an easy access.
It should be noted that if the concrete is of good quality and highly impermeable and water tight, then the effect of other factors in deteriorating the concrete reduces substantially. How voids and pores in the porous and permeable concrete lead to harmful effects on concrete is explained below.
Concrete has a porous structure due to evaporation of water present in the concrete mix in excess of that required for cement hydration. As the excess volume of water leaves the fresh concrete, it leaves behind capillaries in the hardened concrete, which form passage for the ingress of external/ environmental chemicals into concrete. This porosity is termed as capillary porosity and obviously increases with an increase in water cement ratio. The capillary pore structure allows water under pressure to pass slowly through the material. The permeability rate through good quality dense concrete is extremely slow.
Though non-structural cracks do not endanger safety of a building but may look unsightly or may create an impression of faulty work or may give a feeling of instability. In some situations, due to penetration of moisture through non-structural cracks, they may spoil the internal finishes thus
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