A little minutes after placing the concrete of slab , cracks began show in many places. How to treat these cracks in this slab?

Depends on the type of cracks :

1.If the cracks are surface cracks caused by heat or low W/C ratio it is advisable to proceed with moderate curing.

2.If the cracks are structural it might be due to non-workable concrete or concrete with initial setting time lapsed or due to improper design mix. In such a situation proper compressive strenth tests should be carried out to determine the suitability of the slab to be used or demolished or provided with Treatment/Support.

Mix some more water and cement

 smoothing surface by using cement and fan ,

Also try to reduce using of chemicals admixtures  .

 

It depends with the mixtures of that concrete slab ,,it wise to remix it with the recommended ratios

It depends, on the mixture of the concrete...if the cement was not enough u have to do it all over again..so That it would be solid, patching it wont help

just by cementing the slab or replace the slab

I am not an expert on this issue ......

I would say the cement is to try or is trying to fast ...my grandfather was a brick mason and would have added  water ... or pouring and additional slap on top to seal the cracks .... and hopefully the bandage will hold.

 

Cracks are signs the foundation below the slap is not even/not properly done ...

 

As many noted, it all depends on the mixture/recipe ...., it also indicate to me that perhaps the dirt below is settling which may causes more problems in the long run.... and hopefully the cracks were not caused by an earth quake ... so there are many things to consider ... 

  Because of bleeding, its Happen after placing the concrete

the way to treatment this cracks its to use curing material.

 

for treating this cracks re vibrate the concrete again and better to use mechanical smoothing tool like helicopter this will give you dense and smooth concrete without cracks.  

use "DMAIC" process to rectify gaps and issues

The cracks can be due to various reasons and in order to fix this you have to investigate. The solution dependents upon the cause. If it is due to settlement of the form-work or due to settlement of surface below it then that area have to be fixed