Dear sir, madam
we have poured the slab it was 35 centigrade degree. After pouring I face with cracks although I use burlaps to cover it (cracks nearly 1/8 inches). How can I avoid these cracks and after pouring how frequently do I have to water it?  Can I water it in the afternoon? (here traditionally they say to water only in the morning and in the evening. Not in hot afternoon.  Also there are no cracks within beams and columns - only on the slab and there is leakage through the sub floor and salt color tracks of the leakage. Although this problems I face with, I can't even hammer a steel nail on the slab it is so hard. Does it show that no structural problem?

Well it was surprise for me to find something like that web site.  It is amazing. I would like to express my gratitude to you.

This is a very common problem, even though yours sounds a little on the severe side.

In general, these are shrinkage cracks, caused by rapid loss of water from the fresh concrete while it hydrates and dries.  Higher quality (stronger) concrete uses less water and will experience less shrinkage.  Reinforced concrete will experience the shrinkage, but the steel will hold the cracks together and you will not have those wide cracks.

As to your specific situation.

1.  Pouring concrete at high temperatures requires special care.  35 degrees Celsius (95 deg F) is very hot and will cause the concrete to dry faster with larger cracks.  To minimize that problem you should use stronger concrete (less water) and start misting the surface as soon as it appears dry.  Do it very carefully since you do not want to disturb the "skin" before it hardened, or add water to the fresh concrete.  At these temperatures, normal concrete will probably be hard enough after 3-4 hours, but that depends on admixtures used in your area.

2.  Covering with burlap is good practice if you keep it moist at all times, and apply it before the concrete lost too much water.

3.  The reason for watering in early morning and evening is to save water.  When you water around noon time, the heat will cause the water to evaporate almost instantly instead of penetrating and soaking the concrete.  However, if you water the burlap and cover with plastic sheets you may overcome that problem.

4.  I assume that your slab is placed directly on the ground without any steel.  These are not structural slabs and will not "collapse."  They will, however, remain cracked and cause many problems inside the house.  If you plan to use floor tiles, the cracks might show through the tiles and cause them to crack too.  The salt deposits you observed (efflorescence) are an indication that moisture is coming through.  These will not stop and might get worse.

5.  One thing you can do to minimize the cracks is to reinforce the slab.  In the USA we use welded wire-mesh (steel wires welded to form a "net" with square holes) to arrest the cracks.  In more severe situations we will also use steel rebars centered in the slab.

6.  The most basic method of dealing with this problem is to provide "joints" at regular intervals.  These joints are pre-cut "cracks" that force the cracking to happen where you want it and in straight lines.  The joints can be cut by a concrete saw as soon as the concrete is strong enough to allow the equipment on top (but not later because it will start cracking...), or placed in the concrete during pouring by scoring the fresh concrete to about 1/3 of depth.  Joints should be placed every 10-15 feet (3-4.5 meters) and divide the slab into square sections.

Hope that helps.  If you have access to publications of the American Concrete Institute (ACI) you can find many sources that will help you further.