Hey Mariwan Abdullah,

The hair line cracks which are visible to you are on the top surface of RCC slab. These cracks occur due to shrinkage in cement concrete.  

When we mix more quantity of water than that of actually required quantity according to water cement ratio, the upper layer of cement concrete dries at faster rate and bottom layer of cement concrete which has more quantity of water does not dry accordingly.

Due to dryness in upper layer cement concrete shrinkage at the top and hair line cracks occur on upper surface of the RCC slab. This also happens during hot weather when there is lot of difference in temperature of cement concrete mix and outside surrounding temperature of cement concrete rate in RCC slab. As there is very hot weather and temperature is more than 40 degree Celsius, you must keep RCC slab wet 24 hours before implementing repair process. For this process the water should be filled on RCC slab before 24 hours.

After 24 hours, remove the water on RCC slab and grout the cracks with cement mortar 1:1 in the cracks two or three times because the mortar in cracks will settle then fill it second time or third time. After one hour place wet gunny bags on cracks which are filled with mortar. Keep gunny bags wet by sprinkling water on them. Next day the water should be spread over RCC slab at least for three days. It would be better if you implement the process of repair in evening.

thanks