I just wanted to give my opinion on deck piers being place under an existing patio or driveway. You should not just assume that there are piers under your driveway or patio even if there is a deck there already. You should have it checked by your contractor or a secondary agency to verify things are done correctly from before. In most municipalities in the U.S. you are now required to prove there are existing piers in place and pass code. The municipality should have a record of this if your house is newer than 10 years old, but would not hurt to ask if they have record before you start spending money or muscle power to find out. If there is no record and can not verify piers exist just install new piers.
It is a pretty simple process really. Don’t let the idea that you have to go through concrete even be a concern for you. If you have a concrete coring company come out or rent the equipment to core holes for the piers through the existing patio or driveway it is fairly simple. You want to remember to core the concrete the next available size up form the desire pier hole size. This is so you can fit a gas auger bit in the hole to auger your pier in the dirt below. Note: double and triple check your pier placement because this is not a movable item if you are wrong.
After you have the concrete cored and you have auger the dirt out of the holes the pier hole should be roughly 3 times the size of the post supporting your deck and 3 feet deep, but consult the local municipality for codes and ordinances. Do not forget must municipalities require a inspection before you pour concrete in the pier to verify they are correct.
Now that you have done everything to this point you are ready to pour concrete in your piers. Pour the concrete in the pier hole to be approximately 3/8 of an inch lower the the bottom of the existing concrete slab. After you have the holes filled to the desired level take a flat strap or piece of rope and lay on the new concrete leaving a foot out of each side of the pier hole. You will need to drill a few holes in the bottom of the core you cut out of your existing driveway or patio and put in some Tapcom screws in the core leaving them sticking out about a inch or so. These screw will go into the new wet concrete and when concrete the concrete cures it will dry around the screws hold the cap down during the freeze thaw cycles. Then lay the concrete core out of that you cut out earlier back in the hole on top of the new concrete and rope. You use the rope to lift the core after it is in place because you may need to add or remove concrete from the pier to get the core back to the perfect height to match the existing patio. You also want to be sure to have your broom line lined back up when you set the core. Also don’t worry the core will not sink into the new concrete.
After you have complete the piers let them set up for 24 hrs to insure the core will not move when working around it. After 24 hrs it will be safe to work on the area. You will need to use a concrete chalking to seal the cut lines so that water does not get under your patio. Then you are ready to put your deck on the piers.
Over all I have found this method works great and looks a ton better than trying to pour and finish new concrete it to match existing concrete. With concrete no matter how hard you try you will never get it to look like the old. The finish will be off. The color will be different. There is to many variables with concrete to make new concrete match old concrete exactly. The coring and capping with the core is the best method I have found.