i) Concrete degradation- Concrete can be damaged by fire, aggregate expansion, sea water effects, bacterial corrosion, calcium leaching, physical damage and chemical damage (from carbonatation, chlorides, sulfates and distilled water). This process adversely affects concrete exposed to these damaging stimuli.

Degradation occurs due to-

1) Aggregate expansion.

2) Corrosion of reinforcement bars.

3) Chemical damage.

4) Bacterial corrosion.

5) Physical damage.

6) Thermal damage.

7) Radiation damages.

ii) Setting and hardening of cement-

The term ‘Setting‘ is used to describe the stiffening of the cement paste. Setting of cement refers to changes of cement paste from a fluid to rigid state. Setting differs from Hardening of cement. The setting characteristics of Portland cement paste are defined by initial set and final set. Initial set indicates the approximate time at which the paste begins to stiffen considerably. Final set roughly indicates the time at which the cement paste has hardened and can support some load.

Initial setting time indicates the beginning of the setting process when the cement paste starts losing its plasticity. Final setting time is the time elapsed between the moment water is added to the cement and the time when the cement completely lost its plasticity and can resist certain definite pressure.

These times of set are tested according to standardized procedures and have no special relationship to concrete setting behavior. Setting types are affected by minor constituents in the cement such as alkalis and Sulfates, by fineness, water-cement ratio, ambient temperature and inclusion of mineral and chemical admixtures. Concrete generally sets more slowly than cement paste because of the higher water-cement ratios.

The term ‘Hardening’ refers to the gain of strength of a set cement paste, although during setting the cement paste acquires some strength.

Heat speeds up the setting and hardening of cement, and cold slows it down and can even completely stop the processes.

In order to crystallize or hydrate) cement requires a quantity of water equal to 25% of its weight. But in order for it to be laid and remain sufficiently workable, twice this amount is usually required. However, too much water can reduce the strength and durability of the concrete. The paste (i.e. the mixture of cement and water) acts both as a lubricant and an adhesive.

Hardening does not start as soon as cement and water are mixed, but after a certain length of time known as the "initial set time" during which the concrete can be mixed, transported, laid and vibrated.