How can I remove large nails from cement?
If you have acess to Dremel or other rotary tool you can use a cutoff /abrasive wheel attachment to cut them off. They may have been inserted by a power tool that fires them in with a gunpowder charge. If this is the case prying them out will be difficult if not impossible. You could try a Sawzall but these type of fasteners are very hard and you will go through a lot of blades. Some type of cut off wheel or grinder is the way to go. You may forgo the expense of buying one by renting one. If you opt for the grinder you will be able to grind away a little of the concrete so the nail will be below grade. You can then patch the low spots to level the finished surface. Be aware that any type of cutting wheel you use will throw sparks and if you grind the concrete it will generate losts of dust so use appropriate safety precautions.