They do make a chemical that slows down the curing process of the crete mix but I don't know if they do have or not in Thailand. It's called retarter. I said that because one mix will not cover the entire slab. It will take you many wheel barrels of mixing if you are doing that way. By the time you have filled the entire slab, the concrete most likely will set up on you. You may try to keep it wet but the more water you add to it the weaker the concrete will get. You can pour the slab into 2 sections but you must keep in mind that you must use rebar or wire sections to keep the 2 slabs intact for the next pour. The concrete expands during the curing process. Any concrete projects will have cracks in them; that's why you put expansion joints in them so the crack will crack below the surface.
If you would like to have better quality; I recommend you have the truck delivered the mix. I would extend you pad a bit more if they are going to charge you extra for small load. I don't know but in America, they required a minimum of 3 cubic yards for delivery. Make sure you have couple of friends to help you put it down. You also want to put light broom finished on it before it begins to harden so you will have a non-skid surface.
I know a little bit about the concrete coz I have been doing this for over 20 yrs in the military. I did a lot of runway repairs after we bomb the **** out of Iraq. It don't make sense...we go bomb it...and we go fix it. lol
Best of luck.
