Mark,
Putting one layer of concrete over another has many problems. For thick, uniform thickness concrete over old concrete, it sometimes works, especially if the new concrete is allowed to "float" above the old.
Let's ignore your question and look at your problem. You said that "one pad ... has drooped." If you have a section of concrete which is sinking at one end, then you can get some very successful results by having that one section raised in some manner such as having it "mudjacked."
Here is the first Google hit that I found, which may explain the process for you:
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Mudjacking is generally much less expensive than pouring new concrete, but it is still expensive. Also, I do not consider it to be a great long term solution since there is a tendency for the concrete to begin sinking again. This depends upon the quality of the mudjacking, the amount of cement mixed with the mud, and the condition of the soil below the concrete (which caused the original sinking).
If you have it mudjacked, you may want to negotiate for extra holes in the slab and extra mud jacked in. I would also trench on the sides of the raised slab and wedge treated timbers pieces under the perimeter as much as possible after the concrete has been raised.
If your concrete has only sunk 1.5" and if you aren't parking vehicles on that slab, you could get extremely good results with just basic mudjacking. How long did it take for the concrete to sink that much? If it occurred in just a few years, then you might expect more settling after the mudjacking. If it took 40 years, then there is a much greater chance that the raised concrete won't sink much or not at all.
I have raised smaller sections of concrete by brute force. The largest was four sections of sidewalk which were connected and raised as one piece. Chunks of treated timber where then driven in at several spots for primary support, followed by old bricks, stone and gravel shoved in to fill as much of the voids as possible. That four section piece of concrete was raised 18 years ago and it hasn't dropped a bit since.
I raised one end of that slab at least 6", which is actually easier than raising it an inch or so. Why so? It isn't much more work to lift it 6" than it is to lift it 1", but it is much easier to work with a 6" void than with a 1" void.
That took about 2 hours for a friend and me from start to finish, and it wasn't much more difficult than most heavy-labor yard chores. How would it fare with an automobile driving over it every day? Who knows for sure, but I would trust a driveway slab raised in this manner if it had sufficient material wedged underneath it. Of course, in many situations a driveway slab only offers access for digging and lifting along one edge. I wouldn't attempt raising it myself in that case, unless I only needed to raise one outer corner or edge. Obvious.
Back to your driveway: If I were having a concrete slab mudjacked, then I would personally excavate along the side(s) of the slab before the crew arrives to raise the concrete. I would attempt to negotiate a deal in which the workers would use their equipment to inject a huge volume of mud under the slab after they raised the concrete. My excavation on the side(s)s would provide optimal access for that task.
Pesonally, I would also negotiate on drilling the holes in the slab myself. I've got two SDS drills and I can put large holes in thick concrete rather quickly. Why pay somebody else for that part of the job? It would be very easy at the time of the estimate to mark the future hole locations with some quick shots of spray paint.
I would also investigate the option of having concrete shot under the slab after the slab had been raised by the mudjackers or by me. This may not work or there could be problems getting equipment into the work area. I know little about the process of shooting concrete. And, in your case, I doubt if concrete shooting equipment is designed for such a small access area. Once again, a 6" void is easier to work with than a 1.5" void.
Good luck, Gideon
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Mark wrote:
If anyone knows there way around concrete, I sure have a question:
I want to to some patching and touchup on my concrete driveway to try and postpone the inevitable replacement.
Does new concrete adhere to the older concrete? One pad at the entrance to the garage has drooped, and so I have about a 1.5 inch bump to drive over. I just want to smooth that with cement from the Home Depot, knowing that it will continue to drop a bit further, eventually crack off, and require full replacement of the pad one day. Just hoping to postpone the inevitable due to finance issues.
If I smooth that bump, will the concrete "stick" to the driveway, or will I just be creating a sort of angled piece of concrete that will slide off the existing surface?
Hope I explained that okay, as I don't know anything about concrete, other than I know that fixing my driveway will one day cost me a fortune :-)
Thanks,
Mark