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
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?
Bolted to pad??? How big is the damn thing? High "balsy" crime rate?
If you want to avoid the concrete redo, can you just slip a 4x4 metal
plate between the compressor and pad? Maybe that's too "ugly" for you
To answer some of your questions: Pouring a pad
is fairly common. Bolting a large compressor may
be common but smaller ones like 2-3 tons often are
not bolted (certainly mine isn't and most of the
ones in my subdivision that I can see simply sit
on the pad. Sure a pad can fail after 20 years,
especially if there is a lot of vibration. If
your machine shakes badly then something isn't
balanced. Failure with steel reinforced concrete
is probably do to the shaking.
The simplest solution is to simply get 1/8" to
1/4" thick metal 4" wide and long enough to extend
from one foot to the next (across the broken
edge). Simply unbolt, mark the bolt holes on the
two straps, drill the bolt holes, lift one end of
the AC(a 2x4 and a pivot point should do it), slip
the metal in place under the legs and rebolt.
If you don't want to lift the AC any higher than
the thickness of the metal straps, use a 6" wide
strap and cut a slot from one edge to the middle
the strap so it can slide under the legs.
Then get the AC fixed so it doesn't vibrate so much.
Good concrete should last a hundred years, but not all are good. My A/C pad
was poured in the 1970s and is still good. Slide something under it that is
strong enough and gently pry up and slip some blocks of wood, bricks or what
have you under to hold it an inch or more above while you remove and replace
all concrete cracks (eventually)
concrete is its hardest at around 100 years of age
consider using Quickrete Concrete Resurfacer on your existing base to repair
it, about $25 a bag
the resurfacer adheres to concrete something like 3 times better than
concrete adheres to itself to itself, and due to the polymers in it, it
resists water better than concrete (water seems to be the reason for the
spalling of your base)
trowel it on, instruction on how to use concrete resurfacer
My AC unit, installed 1977, has a
precast pad under it. The AC unit is a
Carrier round unit and the pad is also
round. Maybe you can get a precast
pad and slide it under the unit. Or,
they do have 2' x 2' precast patio block
that are only 1 1/4" thick or there
abouts. Four of these might work.
You don't "pour" AC pads. They come already poured, much like a huge
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
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.
Mine isn't bolted to the pad at all. I wish it were, becuase it keeps
moving maybe an eighth or fourteenth of an inch every year. Now it is
about 2 or 3 inches from where it was 27 years ago. (Maybe it moved
slower then and more quickly now. I don't know, and I'm not going to
mark it to find out.)
I'm afraid to put it back because I don't want to bend the pipes. But
it's only because I'm compulsive that I want to put it back at all.
It works fine.
People always ask questions like that. Why did my tv break now? The
question and answer are above my pay grade.
That might account for a crack, but not for crumbling, afaik.
Right. That's why I'd consider doing nothing until you need a new
compressor, now matter how long that is. I'm serious.
Or, since I mentioned how compulsive I am, I might make a wood form,
and pour cement under the compressor, just to fill up the empty space
where it crumbled away. Same height as current height or if you go a
quarter inch higher than the base, no problem. You won't lift the
compressor, and when you need a new compressor, you can replace the
pad then, if yours is still breaking or not flat after your patch.
Al's idea sounds pretty good too.
IIUC only the parts beyond the steel reinforcing are falling off.
Even if it's worse than that, the bulk of the cement pad will be there
another 40 years. There is no special reason to think there is a sink
hole. Cement cracks and crumbles some times. The AC isn't going to
fall to the center of the earth -- it won't fall farther down at all
--, and if perchance it does, you can fix it then. I wouldn't go
looking for trouble.
After looking at your picture, I agree with myself even more. I would
remove the chunks that are an inch away, that have fallen off
And maybe paint the base, if you think that would make it nicer, and
before that you could put some kind of spackle in the crack, but
unless you want to make it pretty for guests, I'd just remove those
totally loose pieces.
HomeOwnersHub.com is a website for homeowners and building and maintenance pros. It is not affiliated with any of the manufacturers or service providers discussed here.
All logos and trade names are the property of their respective owners.