Is there enough exposed pipe to get the solder to melt? it could be the mass of the concrete is absorbing the heat and the pipe isn't getting hot enough.
Copper isn't supposed to have joints when buried in concrete. If the pipe is of an age where its starting to get pin holes, its time to replace the entire pipe.
Houstonremodeler is correct: 1. you can not have a copper connection under the slab. 2. even if you successfully repair this leak, other leaks, in other areas, will follow.
You have to repipe the entire house (through the attic).
The price of copper is going up. The price of water is going up. So repipe ASAP.
A previous poster mentioned using bread to plug the pipe so that you won't get water seeping in to where you're trying to solder.
You should use only pure white bread ("balloon bread"), and not the crust. Ball it up into a nice dense plug and shove it in the pipe. Use a stick or dowel to push it in several inches, one bread plug on each side of the repair.
After the repair, remove the aerators from your faucets and run the faucets. The bread will dissolve and flush out of the pipes.
If you use anything but pure white bread, or if you use the crust, it might plug up a valve or something somewhere along the way.
Don't overheat the pipe. No amount of flux will allow you to solder on burned copper.
The "Senior Member" designation under my name doesn't mean I know a lot, it just means I talk a lot.I've been a DIYer since I was 12 (thanks, Dad!). I have read several books on various home improvement topics. I do not have any current code books I can refer to. I was an apprentice plumber for two years.
If you go with the *old* bread trick and you have faucets with cartridges -- don't be surprised if they plug up from chunks of that bread when it breaks apart.
"" an ounce of perception -- a pound of obscure "
I would try to avoid using bread. Like mentioned before, it can clog other components and cause additional work and expense.
Instead, I would let the water out the lowest point in the house. Sometimes it could be at the water meter.
You could use a sharkbite as a last resort. Something to rember is that the old line probably has a lot of calciam in it so it will take much more heat.
When I was working with an old time plumber, he would "tin" the copper pipe. When I asked him why, he told me in his day , some copper pipe they used was bad and would not take solder. I am wondering if this is the problem?
copper under concrete should be brazed using k copper