# How do you measure poured concrete PSI?



## head_dunce (Nov 20, 2006)

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/MotorOil/Car_Care/DIY/Why_You_Need_a_Garage_Lift_Now.aspx


> If you don’t 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. Don’t 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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## concretemasonry (Oct 10, 2006)

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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## head_dunce (Nov 20, 2006)

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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## concretemasonry (Oct 10, 2006)

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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## head_dunce (Nov 20, 2006)

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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## elementx440 (Jan 24, 2007)

head_dunce said:


> 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 :laughing: 

get some proper pads in there so you can sleep soundly at night.


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## Tscarborough (Mar 31, 2006)

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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## head_dunce (Nov 20, 2006)

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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## concretemasonry (Oct 10, 2006)

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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## joasis (May 28, 2006)

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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## concretemasonry (Oct 10, 2006)

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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## head_dunce (Nov 20, 2006)

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



> 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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## Jeff4Sun (Feb 25, 2007)

Do a search for Concrete Rebound Hammer or Shmidt Hammers. Very simple test to do.


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## head_dunce (Nov 20, 2006)

Thanks! Looks like those rebound hammers are about $1500 -- is it common for contractors to own one?


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## concretemasonry (Oct 10, 2006)

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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## Jeff4Sun (Feb 25, 2007)

concretemasonry said:


> 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.


Used with other tools such as a drill to find the depth and by an experienced engineer it is a great tool. No one tool can give you all the information you need. And most contractors are professionals where I live. Most have a very strong knowledge of engineering principles I would hope. The hammers we use have a chart on them to convert the reading to PSI. If you can read directions you can figure it out. Or if in doubt call an engineer. Engineers specializing in structural inspections are more likely to have one than a contractor.


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## head_dunce (Nov 20, 2006)

Well so let's say the car lift spec says the customer needs a min 4" of 3000 PSI

First, what's a "normal" thickness for garage floor concrete?

If the guy found out he's got 6" of concrete, wouldn't it be a pretty safe bet that he'd be fine? It'd have to get tested before the lift goes in, but at least the customer would know it's worth the cost of bringing a pro out to get it certified.

---- can the floor get "certified" or how would that work?


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## concretemasonry (Oct 10, 2006)

As an experienced engineer, you are trying to beat a dead horse with details. The hammers are fun toys to play with, but I would not stake my life on the results.

It is difficult to get old concrete less than 3000 psi, but you have no idea of the minimum thickness unless you drill enough holes, when you eventually weaken the slab. - The minimum, not the average is what will control any failure.

There is no such meaningful thing as an average floor thickness. It could have been installed correctly or it could have be a "DIY week-end special" that was made trying to keep the cost to a minimum and fool the inspector for a shallow victory!

If you or someone else will be working on a vehicle held up by the lift and are injured you will get no satifiaction from a manufacturer's guarantee or an insurance company.

Just put in the lift according to instructions - it will be cheaper, easier and safer.


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## head_dunce (Nov 20, 2006)

:huh: Okay.


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