PSI vs Thickness

[Overhaulin63]

I've seen posts where they talk about how anchor depth can compensate for PSI. I am not sure I understand that one, but my question is can psi, to some degree, compensate for concrete thickness? My garage has a block foundation and was built a few years ago. I built the floor with radiant heat and allocated two 30"x30" spots for a future two-post lift. The concrete was supposed to be at least 5" to 6" thick. I was finally able to save up enough to make my lift purchase and after clearing the garage and getting the first post located I finally got to drill the first hole. To my huge disappointment the concrete appeared at this hole to only be 4 1/2" depth.

My first reaction was to put everything down and walk out of the garage and get a really really strong drink before I set the place on fire. I eventually opted to pass on both reactions and just call it a night.

The lift manufacturer, TUXEDO 11K/2 post, calls for a minimum of 4 1/4" concrete thickness at 3000 psi and a 3 1/4" anchor embedment. While this first hole only showed a depth of 4 1/2", I am not sure where the others will fall. I was able to confirm the concrete I ordered was indeed 6 1/2 sack or 4000 psi.

My question is will the 4000 psi compensate at all for the lack of thickness? I've only drilled one hole so while I would really like to be able to use this lift, I am not going to sacrifice anyone's safety or property for it. I'll sell it and go buy a 4 post if that's what I need to do.

Most vehicle are light passenger car or a crewcab F150, but my heaviest vehicle hit the scales at 7,680 lbs.; 2000 F350 dually/7.3.

If I am overreacting, please pull me out of this hole, if not just let me hear your thoughts or concerns.
Thanks in advance for any thoughts.

 

[racecougar]

The larger concern, IMO, is the thickness of the pad outside of these 30"x30" areas. If this means that the rest of the pad is only 2-1/2" thick, that is a problem. Is it possible that the wells you requested weren't dug, and the whole pad is 4-1/2"? Were you able to witness the pour? Additionally, if the pad thickness isn't to spec, what guarantee do you have that the compressive resistance is to your spec?

 

[Rusted Nut]

Expansion anchor strength increases with higher PSI, check out manufacturers specs; Hilti has some good tables for this. However, I wouldn’t say that PSI can totally compensate for reduced concrete depth. Do you have reinforcement? That would help. Drill the other holes and see what you have before getting a drink.

 

[Overhaulin63]

The larger concern, IMO, is the thickness of the pad outside of these 30"x30" areas. If this means that the rest of the pad is only 2-1/2" thick, that is a problem. Is it possible that the wells you requested weren't dug, and the whole pad is 4-1/2"? Were you able to witness the pour? Additionally, if the pad thickness isn't to spec, what guarantee do you have that the compressive resistance is to your spec?

Thanks. Yes I witnessed the whole floor pour, about 30 yards, and it all basically the same thickness. The substrate was about 100 ton of recycle concrete with wire mesh. I worked and paid the concrete company directly and all the documentation shows 6 1/2 sack Portland cement. I also took test tubes from every truck, date and time stamped.

 

[Overhaulin63]

Expansion anchor strength increases with higher PSI, check out manufacturers specs; Hilti has some good tables for this. However, I wouldn’t say that PSI can totally compensate for reduced concrete depth. Do you have reinforcement? That would help. Drill the other holes and see what you have before getting a drink.

It was poured with heavy wire mesh.

 

[Professor Fate]

I've seen posts where they talk about how anchor depth can compensate for PSI. I am not sure I understand that one, but my question is can psi, to some degree, compensate for concrete thickness? My garage has a block foundation and was built a few years ago. I built the floor with radiant heat and allocated two 30"x30" spots for a future two-post lift. The concrete was supposed to be at least 5" to 6" thick. I was finally able to save up enough to make my lift purchase and after clearing the garage and getting the first post located I finally got to drill the first hole. To my huge disappointment the concrete appeared at this hole to only be 4 1/2" depth.

My first reaction was to put everything down and walk out of the garage and get a really really strong drink before I set the place on fire. I eventually opted to pass on both reactions and just call it a night.

The lift manufacturer, TUXEDO 11K/2 post, calls for a minimum of 4 1/4" concrete thickness at 3000 psi and a 3 1/4" anchor embedment. While this first hole only showed a depth of 4 1/2", I am not sure where the others will fall. I was able to confirm the concrete I ordered was indeed 6 1/2 sack or 4000 psi.

My question is will the 4000 psi compensate at all for the lack of thickness? I've only drilled one hole so while I would really like to be able to use this lift, I am not going to sacrifice anyone's safety or property for it. I'll sell it and go buy a 4 post if that's what I need to do.

Most vehicle are light passenger car or a crewcab F150, but my heaviest vehicle hit the scales at 7,680 lbs.; 2000 F350 dually/7.3.

If I am overreacting, please pull me out of this hole, if not just let me hear your thoughts or concerns.
Thanks in advance for any thoughts.

You state that you exceed the manufacturers minimum requirements (4 1/4" thickness and 3000 psi concrete) with 4 1/2" of thickness and 4000 psi concrete. What is the problem?

 

[Overhaulin63]

You state that you exceed the manufacturers minimum requirements (4 1/4" thickness and 3000 psi concrete) with 4 1/2" of thickness and 4000 psi concrete. What is the problem?

To be honest I thought I was going to have a bigger cushion for peace of mind and now that I am just meeting the minimum requirements, I am questioning the safety once I've got something heavy up in the air.

Professor Fate was the best!

 

[firebirdparts]

It all sounds good to me. Overkill would be nice, but 4" is enough and you have that. Concrete strength does compensate for depth, and you could also compensate using epoxy. I am no expert on epoxy, but you don't have the expansion forces so much.

 

[wssix99]

My question is will the 4000 psi compensate at all for the lack of thickness?

A little bit, but you should be fine. I expect that what you really need is 4" to meet the lift manufacturer's internal specifications and they added the 1/4" as padding so we don't cut things close. 10 years ago, most of the lift manufacturers specified 4" thicknesses for new slabs and gradually they have been changing their thicknesses to 4 1/2", 5" etc. There have generally been no changes in the designs - this appears to be just to ensure a quality install.

Since you are close to the limit for your lift, the best thing you can do is use a good hammer drill (rent one if you need to) and buy a NEW bit. (Don't be cheap.) Getting a clean hole is everything. Life will **** if you drill with a dull bit, grind on the holes, and ream out the holes.

Even if you don't get the full 4" or 4 1/4", or if your anchors don't bite like you want them to, its not the end of the world - just call your lift manufacturer. They have additional factors of safety and can work with you as to how far you can push things.


The depth of the anchors is needed for developing a stress cone in the concrete.


The surface area of this inverted cone is where the concrete resists tension of the bolt trying to pull out. The deeper the bolt, the bigger the cone, the more surface area and the greater tensile forces the concrete can handle. (That being said, there are some reasons why someone would not want to go deeper, even if its "stronger" for the concrete - but that's a different thread. Best to stay with the lift manufacturer's specs.)

Higher compression concrete does resist tension a little bit more, but the geometry related to the depth of the anchor bolt should be a much greater impact. (Cement/concrete is not good in tension, so the increase between 3000 psi and 4000 psi concrete should be negligible.)

Outside of the anchor bolt area, the thickness of the concrete doesn't matter so much. (This is why we can pour small repair slabs in the middle of thin, broken existing slabs and be OK.)

 

[WNYflyer]

I would be more concerned with the possibility of there being any concrete control joints between the posts as well as too close to the sides of the posts.

 

[Fatboy148!]

You might not have gotten what you paid for but you are more than meeting the minimum manufacturers requirements. First thing I would do is lay out where the holes go on the other leg of the lift and drill a hole there to check the slab thickness. If that checks out and you are still concerned, could you place a piece of 1/4” or 3/8” steel plate under the lift points? This might uniformly distribute the weight some more and offer some resistance to the pull out effect that was depicted in the drawings.

Another thing you could do is recalculate the amount of concrete you paid for on the load slips and see how many inches thick it will cover the square footage of your slab.

Good luck!

 

[racecougar]

Thanks. Yes I witnessed the whole floor pour, about 30 yards, and it all basically the same thickness. The substrate was about 100 ton of recycle concrete with wire mesh. I worked and paid the concrete company directly and all the documentation shows 6 1/2 sack Portland cement. I also took test tubes from every truck, date and time stamped.

If the whole pad is 4-1/2" thick, you're okay. I took the 30"x30" reference in the OP to mean that you were having those areas poured thicker.

 

[tom in nh]

If it bothers you that much....cut the floor open, dig down, add crushed gravel, rebar, etc. Add concrete and wait 30 days.
Then go to town.

 

[like2wheel]

Is drilling a hole with a rotary hammer an accurate way to measure the thickness?
I always assumed the last 1/2 to 1" would blow out the bottom from the hammering effect.

 

[Beemer]

How do you know the concrete is the strength that you ordered?
I have had a few jobs where the supplier cut corners and substituted lower design strength concrete so in your case if you are worried about less thickness then that might make the strength more critical.
There are tests that can check.

 

[jonesg]

If I am overreacting, please pull me out of this hole, if not just let me hear your thoughts or concerns.
Thanks in advance for any thoughts.

yes you are over reacting.