What are the reasons behind in UAE overhead water storage tank are not RCC, why use Fiber tank ?

as we are in humidity climate, the introduction of reinforcing steel into concrete has created a new problem that affected the durability of concrete tanks. When embedded reinforcing steel corrodes, it can cause concrete to crack and break.

Lime, which is found in concrete, can also leak into the water. 

in term of cost the fiber glass is cheaper the RCC tank.

Fiberglass is less in weight

Easy to install

No reinforcement required

More Economical overall in  cost

most appropriate answer is given by Mr. Abdelaziz Al khawajah cost is secondary in skyscraper, it is the quality of water that matters. 

the cost is most important now in gulf so , they are searching for good quality with less cost 

Agreed with Fazlur answer. I like to add that could be better for health than RCC

Using Fibre Glass has the following advantages.

• Fiberglass is lightweight, prefabricated  and installation at higher elevation is easier than conventional construction RCC overhead tank
• The Fibre glass is heat resistant,  so water will remain comparatively cooler in summer month. The RCC tank absorbs  heat and so  water will become very hot in summer  and RCC tank also  takes long time to cool down
• In term of cost, Fibreglass construction is cheaper than RCC tank