miamicuse wrote:
My central AC compressor unit is outdoor resting on a concrete pad.
The pad is 48"x48" in size and 4" thick and it is reinforced with
steel rod. I believe the pad is original so that will put it back to
1980 when the house was built.

Recently the concrete pad started to fail. First a crack started
about 8 inches from one edge, then more cracks along the outside
edges, chunks of concrete started to fall off exposing the rusted
steel rods inside. The compressor has four legs and it's bolted to
the pad with concrete screws. Of course with the pad failing two of
the compressor legs will no longer be resting on anything solid real
soon.

Obviously I need to remedy this situation. But I have two questions:

(1) Is it normal for a pad to fail like this after 20 years? Why did
it not fail after 8 years or 12 years? If it waited 20 years, does
it mean there is some other factor involved? Like perhaps the soil
below is yielding due to a sink hole (house is in coastal south
Florida sandy soil high water table) or sewer line leak?

(2) If I replace the pad how would I go about doing it? I cannot
just lift and move the compressor aside which is very heavy and has
very rigid connections into the house's attic. A new pad will take a
day have the form done and the concrete poured, then a few more days
at least to cure. I will have to wait a while to put the compressor
on again...do I get a concrete repair place to fix this or the AC
place to fix this?

You don't "pour" AC pads. They come already poured, much like a huge
stepping-stone.

The "legs" you mention aren't legs (I don't think). They seem to be small
brackets to keep the unit from "walking" off the pad. The entire bottom of
the unit is resting on the concrete pad. In other words, the "legs" support
nothing.

Here's what you COULD do:
1. Cut the bolts/screws holding down the "legs."
2. Get a couple of 2x4s under the unit and raise the unit an inch or two.
3. Somehow manage to get the old block out.
4. Slide in a new, plastic, base.
5. Lower the AC unit back in place.
6. Use angle brackets to re-secure the AC unit.

Here's what *I* would do.
1. Ignore the whole thing.