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Old 02-15-2007, 02:37 PM   #1
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How do you measure poured concrete PSI?


To install a car lift in your garage, you need to know the PSI and thickness of the concrete floor. How do I figure out the PSI of concrete that's already poured?

I read here on how to tell the thickness --
http://www.mobiloil.com/USA-English/..._Lift_Now.aspx
Quote:
If you donít know your slab thickness, buy a 3/8-inch concrete drill bit at the local hardware store and slowly and carefully drill into the slab near where you plan to locate the posts of your lift. Donít drill at the corner of the garage; concrete thickness can vary near the perimeter. Use a light-duty drill rather than an impact drill, which could fracture the concrete when the bit breaks through. Once you hit dirt, clip a straight section of coat hanger and bend a little 90-degree elbow at the end. Stick it down the hole and try to catch the edge of the concrete with the elbow. Mark the wire where it emerges from the hole and measure the length. If the hole bothers you, fill it with concrete epoxy, which is strong enough to repair fractured holes as well.
But I can't find out anything on how to tell the psi of poured concrete, does anyone know?

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Old 02-15-2007, 02:56 PM   #2
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How do you measure poured concrete PSI?


Forget about the strength of the concrete that is there. Your eally do not know how consistant it is and how uniform the thickness is.

You will need a separate foundation for a lift unless you can spread the load over huge area without any part of the load being concentrated (virtually an impossible gamble). You may have to cut a few (4?) squares in the floor and pour 12" thick footings.

If you your lift is from a decent supplier, they should be able to give you some installation instructions or requirements. If they do not provide that, don't wander under it!

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Old 02-15-2007, 03:05 PM   #3
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How do you measure poured concrete PSI?


I've got the specs on the concrete
http://www.carguygarage.com/carlift.html

4 post lifts don't need that much support, 2 posts are more involved

I'm just trying to figure out how to measure the PSI of the concrete, can it even be done?
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Old 02-15-2007, 04:54 PM   #4
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How do you measure poured concrete PSI?


You cannot afford to measure the strength of the concrete in place. It is what it is.

If it is fairly new and appears to be done by a contractor, you COULD ASSUME it is 3000 psi, but that is no guarantee.

If it is possibly a DIY job do not assume it is the required 3000 psi. Even if a DIYer pours 3000 psi concret, if it is not placed correctly and cured properly, it could be weaker. Usually, a contractor knows what to do and does it, but there is no guarantee.

I do not know which unit you are considering. Your specs show 4" or 6" thicknesses. If you only have the typical 3 1/2" or 5 1/2" usually poured, the floor would be significantly weaker for a concentrate load. If you punch one leg through, the lift could be damaged and you would have a mess.

If it was me, I would do it right and saw openings and pour some 8" or 12" thick pads at the proper location and elevation.
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Old 02-15-2007, 05:06 PM   #5
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How do you measure poured concrete PSI?


Thanks - I'd hate to see a quarter million dollar car fall off a lift... eeshhh... or worse, if someone was below it.
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Old 02-15-2007, 06:46 PM   #6
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How do you measure poured concrete PSI?


Quote:
Originally Posted by head_dunce View Post
I'd hate to see a quarter million dollar car fall off a lift... eeshhh... or worse, if someone was below it.
not what i want to hear when i drop my quarter-million dollar car off to get an oil change

get some proper pads in there so you can sleep soundly at night.
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Old 02-15-2007, 07:29 PM   #7
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How do you measure poured concrete PSI?


Assuming a very generous 12,000# weight for the lift and vehicle and 4 posts, you could use 12"x12" plates on dirt, much less a concrete slab.

12,000/4 (posts) = 3000 (pounds per post)
Double that for eccentric loading to 6000# per post, for a point load of 6000#.

A 12"x12" plate (properly sized and fitted to the floor*) under the post would give you a PSI of 41.6, just about the same as your car sitting on it's tires.


*You can't just slap the plate onto the floor, as it is not perfectly level and flat. You will have to anchor the plates and shim them, and then fill with non-shrink grout. The plate also has to be strong enough not to deflect.

In answer to your actual question, you would have to rent a core saw, cut at least three cores and send them to a testing lab for destructive testing. Probably less than 2 grand, if you just want to do it for the heck of it.
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Old 02-17-2007, 02:00 PM   #8
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How do you measure poured concrete PSI?


I found another lift where you can put it on asphalt, but you can't work on the car
http://www.carguygarage.com/7hicecarstli.html
That works for the customer who just wanted to store his car.

So I guess there's no easy way to measure the PSI of concrete and cutting and pouring is the only thing to do. Can you give me some rough estimates of what it would cost for....

2 Post Lift = 2 holes, 2 feet deep, maybe 3' square?

4 Post Lift = 4 holes, 6 inches deep, maybe 3' square?

With 3000 PSI concrete?
Do know you of any good websites I can tell customers to go to to find contractors in their area that would do the work?

Thanks for all the help
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Old 02-17-2007, 02:30 PM   #9
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How do you measure poured concrete PSI?


Your best bet would be to find a small concrete contractor or someone that works for a contractor since it is not a big job. - Check around loacally or the yellow pages.

He should have saw to cut out the existing and remove the excess soil. Then he can pour using bagged concrete mix (3000 psi would be adequate).

If you really need them 3' square, put 3 or 4 rebars (1/2") each way.

Judging from the size of what you have got listed, a 3" thick floor would be a little marginal.
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Old 02-17-2007, 07:15 PM   #10
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How do you measure poured concrete PSI?


Properly placed concrete, 4 inches think, will support a tremendous amount of weight. I have seen a lot of dealership 2 post lifts installed with 3/4 inch wedge lock bolts at the corners of 12 inch or 16 inch square plates.

I do get a lot of requests for 6 inch floors in steel buildings, because of the misguided idea that 4 inches will fail under the load of trucks and tractors, and it just isn't so, providing the concrete was placed (poured and finished) correctly. I don't contradict their thinking, since I make more off of a thick floor, and I found a long time ago, to save my breath...the guys at the coffee shop usually know a lot more then we do anyway. By the way, trucks, tractors, and even machine tools can be supported by a 4 inch floor...providing there is no impact load. Crawlers can impact a floor, and even though the weight isn't a problem, per say, the grousers on the tracks can beat hard on a floor.
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Old 02-17-2007, 08:19 PM   #11
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How do you measure poured concrete PSI?


Since you have no idea of the thickness or quality of the concrete in place, it only makes sense to have a known support if you are going to work under the equipment and vehicle.

I believe the manufacturers specs refer to 4" or 6" thicknesses.

The 3' square was the posters choice. The extra width is no benefit unless you have very a thick concrete slab/footing.
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Old 02-18-2007, 08:02 AM   #12
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How do you measure poured concrete PSI?


Thanks for the help guys, here's what the specs say about pouring pads under the posts.

Quote:
The basic rule is that you need the pads to be 12" wider on each side from the base plates of the lift. For a 2 post, you'll need to go down at least 2 feet. For a 4 post, you'll need to go down at least 6" New concete must also cure at least 28 days.
So 3 feet square is a rough guess at the size needed. I just want to make sure people install the lifts as they are suppose to, I'd hate for something to go wrong because of an inch or two of concrete.
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Old 02-24-2007, 10:09 PM   #13
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How do you measure poured concrete PSI?


Do a search for Concrete Rebound Hammer or Shmidt Hammers. Very simple test to do.
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Old 02-25-2007, 09:18 AM   #14
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How do you measure poured concrete PSI?


Thanks! Looks like those rebound hammers are about $1500 -- is it common for contractors to own one?
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Old 02-25-2007, 05:51 PM   #15
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How do you measure poured concrete PSI?


The hammers for measuring the apparent strength are only for professionals (not contractors) since you have to interpret the hammer results.

They only give you the compressive strength (within 10 to 30%), but what good is that? You still have to know the minimum slab thickness for it to mean anything. The concrete can have any compressive strength, but if you do not know the minimum thickness, it is worthless.

Either plop down the lift and live with it or follow the manuafacturers recommendations. Just don't be under a moving or vibrating car and try to collect on life insurance if there is a problem.

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