How is everyone measuring slab psi?

[darkbuddha]

First, please forgive me for starting this thread. Trust that I've searched and searched and read and read trying to find a tip on where to find someone to do a rebound hammer test on my slab. I've called several local lift installers, several concrete contractors, tons of tool rental places, Home Depot, Lowes, contractor supply joints, etc., all to no avail. NO ONE is doing slab testing! The only place I called that said they could do it was a materials testing lab that said they'd need a core sample, which I ain't gonna do. So now I'm looking for a bit of advice before I order a $150 Schmidt hammer off Amazon.

So, for all the folks that have installed lifts at home, who did your slab psi testing?

(Just to fuel discussion, my garage slab is 42 years old and has measured 4.25" thick in several locations.)

 

[GMCGarage]

What strength do you need, and why? 42 yo concrete is easily 3000psi.

 

[BillK]

I might be wrong but I would bet that 99.9% of lifts are installed without the psi of the slab ever being checked. I am thinking of putting one in my second warehouse unit and if I do all I am going to do is bore a couple of holes to make sure it is thick enough.

 

[59 wagon man]

2 or 4 post ? it makes a difference. I also agree with billk as I know of 3 instances where I installed a lift and never checked

 

[TommyK]

You need to find a local construction materials testing and inspection lab.

The test you are describing is called a Windsor probe. A more accurate way would be to core the concrete and have it broken at the lab.

 

[brownbagg]

core the slab and just patch the hole with grout

 

[rlitman]

What strength do you need, and why? 42 yo concrete is easily 3000psi.

It SHOULD be, but I've seen crumbly old **** too.

 

[darkbuddha]

Thanks for the replies. I kinda suspected most folks don't have the slab tested for hardness before installing their lifts themselves. But I was kinda surprised that none of the "professional" lift retailers/installers in my area even offer a hardness test as part of their $100-$130 "installation evaluation" service. All they do is check for cracks, drill for depth, and check for level of the site, all of which I did for myself for the cost of $5 for a 3/8" masonry drill bit.

What strength do you need, and why? 42 yo concrete is easily 3000psi.

I'm installing a Dannmar M-6 2-post lift (aka a MaxJax with the automatic locks). Installation requirements state the slab has to be 3000 psi minimum.
I'm 99% sure the slab will meet that minimum, but I'd like to have documentation just to reassure my wife and for insurance purposes.

2 or 4 post ? it makes a difference. I also agree with billk as I know of 3 instances where I installed a lift and never checked

2-post.

You need to find a local construction materials testing and inspection lab.

The test you are describing is called a Windsor probe. A more accurate way would be to core the concrete and have it broken at the lab.

I did finally talk to 2 labs today that offered both Schmidt rebound hammer testing and Windsor probe testing. The cost of the rebound hammer testing with a written report was several hundred dollars... nearly 25% of what the lift cost me. The Windsor probe testing was even more. Both companies said they could come do a quick "unofficial" test just to reassure me, but that cost was still $150. But both engineers I talked to said they felt that my concrete would easily exceed the 3000 psi minimum, so I did get that reassurance for free.

I think I've decided to just order a Schmidt hammer off Amazon... they're only $125, which is is still cheaper than an unofficial test, and I'll have the tool to lend/rent out to other locals in the same position in the future.

I'm still open to advice though... let me know what you guys think.

 

[darkbuddha]

BTW, the slab is in very good condition overall. No surface flaking or cracks. Not missing major chunks or anything. Still measures very level (not more than 1.5 degrees of slope any of the 18 different places/ways I tested).

 

[sberry]

Bolt it in.

 

[rlitman]

3000 PSI is nothing special for concrete. If the spec calls for 3000 PSI, then it's just calling for the bare minimum of sound concrete that isn't crumbling.

If it called for 4000 PSI or more, then you certainly would want to test.

If you've got a hammer drill (not an SDS rotary hammer), you should easily feel if the concrete drills like soft sandstone (that would be bad), or if it resists the drilling.

 

[SJW]

Dark, the engineers who said they thought your concrete would easily exceed 3000 psi, did they say why they thought that?

 

[ConCretin]

Without getting into a ton of detail, Windsor probe and Swiss hammer tests attempt to determine concrete compressive strength by measuring surface hardness. Accurate results require comparative tests on similar concrete of a known value. There are charts that seek to compare specific test results to theoretical compressive strengths but this is of little real value and is never used to determine the acceptability of concrete.

As limited as these tests are, the results from a $125 Chinese tester from Amazon is likely to be even less valuable. Honestly, I wouldn't put much stock in the results.

I tend to agree those who think you'd be fine to install the lift. If the slab is structurally intact and in good shape, its very likely to have achieved 3000 psi over the last 40 years. If you really want dependable results, taking and breaking a core is the only true test. If it's worth doing, its worth doing right.

 

[Lucid Moments]

I am going to install a lift here soon and will not be measuring my concrete for compression strength. To be fair I had 1 foot deep piers dug and poured where I am putting the lift and the rest of the slab is 6 inches so I am pretty confident of the strength of the concrete.

 

[Stuart in MN]

LLWillysfan and Brownbagg are our resident concrete experts, I'd follow their advice.

 

[mautotech]

I had the same concerns when i put my lift in.
"Should be good enough" won't save you when a diesel F350 falls on your head, so I poured an 8'x18'x12" 4500psi reinforced pad for my hoist. The cost was completely worth the piece of mind when I'm underneath something on the lift.

 

[brownbagg]

the winsor probe and the swiss hammer, i can make read anything you want, they are both a joke. I always core sample. I also test concrete for a living daily

 

[wssix99]

BTW, the slab is in very good condition overall. No surface flaking or cracks. Not missing major chunks or anything.

I will echo all of the above and would sleep well at night doing this install without testing for myself. If the cement ratio was such that you were to have less than 3000 psi, I would expect the slab to look a little rough.

Regardless, you should still not rely on the slab or the quality of your install for complete safety. Making sure your load is properly balanced and using stabilizing jacks at the end of the load (per the instructions) will get you the factor of safety your wife says she is after.

I'd like to have documentation just to reassure my wife and for insurance purposes.

Odd... when my insurance paperwork is in order and full effect, my wife usually asks me to do patently dangerous and deadly things...

 

[rharman]

I had the same concerns when i put my lift in.
"Should be good enough" won't save you when a diesel F350 falls on your head, so I poured an 8'x18'x12" 4500psi reinforced pad for my hoist. The cost was completely worth the piece of mind when I'm underneath something on the lift.

When I first read this, I didn't pay attention to the pad size. I was thinking mount points - like 24" sq or something. But, yowza! 8'x18' is a monster.

 

[lukedwag]

the winsor probe and the swiss hammer, i can make read anything you want, they are both a joke. I always core sample. I also test concrete for a living daily

Brownbagg is spot on. These tests have so many variables that it's almost in possible for the results to be useful. They are really only good for caparison testing on the same project with the same concrete.

Breaking a 4 or 6 inch core will give you a true understanding of the concretes condition.

 

[darkbuddha]

Again, thank you everyone for your replies. What I'm getting is most people don't check at all, a few do, and at least a couple just poured a new slab. Where does that leave me? I'm kinda wishing I hadn't asked now, but should probably be glad I did. I just wish this kind of safety thing was easier to sort out.

the winsor probe and the swiss hammer, i can make read anything you want, they are both a joke. I always core sample. I also test concrete for a living daily

I hear ya. I am curious though why the engineers I spoke to said the same thing but then still offer the Windsor probe and Schmidt hammer tests and stated that both tests are still "accepted standards." Talk about mixed messages! But just for reference sake, how much should a core sample test cost, because for what I was quoted, it would be as cost effective to tie a new 4'x12'x12" footer section into the old concrete to support the lift, which would likely provide a better reassurance per dollar ratio.

 

[rburke65]

By the time you spend the dough for the test, dig out the area, tie it into the existing concrete, and order a 4K psi mix and be done with it.

 

[darkbuddha]

Dark, the engineers who said they thought your concrete would easily exceed 3000 psi, did they say why they thought that?

They both said 3000 psi pour is very common in my area. Combined with the good condition of the slab and and the typical increase in hardness over time that concrete gets, they felt the slab would likely exceed that 3000 psi minimum.

 

[mtwaterguy]

2 or 4 post ? it makes a difference.

Really..... so what does either have to do with figuring out what the psi of your floor is?

 

[matt_i]

You know...the slab psi is only an important hair to split if you are using wedge anchors.

If you drill for epoxy anchors, prep the hole properly and make sure to cover the fastener completely, place the epoxy in proper temp range; I'd use B7 threaded rod at the same time, you're going to a place that a wedge anchor can't in terms of pullout strength.

 

[mautotech]

Yeah, I was only going to put pads in but I learned that the cost of the cement for 8'x18'x12" wasnt much more than the cement for just the pads, (due to cement load minimums), I figured I might as well make it one huge slab. I sleep well because of it.

 

[mrbc]

What about cutting out the areas where the posts will mount and pour higher pressure, reinforced footings in those spots? Or is that's what's meant by "pads?"

Sent from my SM-G920V using The Garage Journal mobile app

 

[brownbagg]

But just for reference sake, how much should a core sample test cost,

if you brought me the core $20

if I have to core it $120

Home depot rent core machines

 

[Orionrising]

When I first read this, I didn't pay attention to the pad size. I was thinking mount points - like 24" sq or something. But, yowza! 8'x18' is a monster.

concrete is cheap. Thats like 5 yards or $600 in concrete.

 

[wssix99]

Where does that leave me? I'm kinda wishing I hadn't asked now, but should probably be glad I did. I just wish this kind of safety thing was easier to sort out.

We won't tell you about rogue voids, anchor torquing, and all that other stuff then, lol. As long as you use the proper stabilizing jacks and keep things balanced, you don't need to worry about any of it. (including any marginal delta in your slab psi)

 

[willymakeit]

Do a core and have it tested. It will show psi and type of fracture.

 

[RVDan]

Geez rent a core drill and take that core sample to a shop that has a big press with a guage in it. Press it until I crumbles and note the pressure.

 

[brownbagg]

the core needs to be twice as long as diameter, 4x8, 3x6, 2x4, 1x2, then load is divided by area.

 

[darkbuddha]

if you brought me the core $20

if I have to core it $120

Home depot rent core machines

the core needs to be twice as long as diameter, 4x8, 3x6, 2x4, 1x2, then load is divided by area.

Yeah... not what they quoted me. I'd gladly pay $100-$120 for a core sample test, but the locals quoted me in the $300-$400 range with them drilling the core. And thanks for the info on the core sample size. Good to know.

 

[darkbuddha]

We won't tell you about rogue voids, anchor torquing, and all that other stuff then, lol. As long as you use the proper stabilizing jacks and keep things balanced, you don't need to worry about any of it. (including any marginal delta in your slab psi)

Oh boy... ignorance really is bliss! But yeah, don't be stupid, should be okay, right? Reminds me of that quote going around these days: "Play stupid games, win stupid prizes." Applies here too.

 

[darkbuddha]

You know...the slab psi is only an important hair to split if you are using wedge anchors.

If you drill for epoxy anchors, prep the hole properly and make sure to cover the fastener completely, place the epoxy in proper temp range; I'd use B7 threaded rod at the same time, you're going to a place that a wedge anchor can't in terms of pullout strength.

I did not know that. If that's the only real concern, I'd gladly pony up for the cost of epoxy anchors if that's all it is.

 

[brownbagg]

but the locals quoted me in the $300-$400 range with them drilling the core. And thanks for the info on the core sample size. Good to know.

our labor is fifty dollar a hour, take me about 12 minute to take a core, so couple hour for travel, equipment setting up, so two hour labor and $20 to break the core with paperwork

$20 is just a flat rate that we charge for breaking clyinders and cores

 

[curiousB]

I just installed my M-6 this past few days. All went well except for the anchors. I posted on another thread my issues. Not feeling too safe due to anchors spinning before torque level reached and some of them pulling above surface of concrete. Doesn't give you the warm and fuzzies of going underneath a 5,000 pound vehicle 40"+ in the air when the anchors are so suspect.

Concrete PSI is a much smaller potential problem than slab thickness and anchor "setting".

 

[wssix99]

If you drill for epoxy anchors, prep the hole properly and make sure to cover the fastener completely, place the epoxy in proper temp range; I'd use B7 threaded rod at the same time, you're going to a place that a wedge anchor can't in terms of pullout strength.

I did not know that. If that's the only real concern, I'd gladly pony up for the cost of epoxy anchors if that's all it is.

You can go back to not knowing it, because the assertion that epoxy anchors somehow use more strength in the concrete is not correct.

No matter what you use, the holding power is developed through a stress cone that projects through the slab, emanating from the lowest point of either anchor:

In fact, using epoxy is a more risky and odds (outside the hands of a trained and skilled professional) are greater that the anchor will be weaker because it requires much more skill to install. If the bond at the bottom of the hole isn't perfect, the stress cone comes up in the concrete and the pull out strength is much less:

 

[darkbuddha]

You can go back to not knowing it, because the assertion that epoxy anchors somehow use more strength in the concrete is not correct.

It's posts like yours make me love this forum.