View Full Version : Plans to install 2-post lift - Incorrect slab poured


Jackob
06-12-2012, 11:39 AM
Hello everyone!

So I'm experiencing a very frustrating situation with a house I am having built right now. Long story short, instead of pouring 3000 psi concrete in the garage (to accommodate a 2-post lift), 2500 psi was poured.

The specifications for various lifts I've found call for 3000 psi minimum.

Putting aside the disgust I felt upon being told they did not at all do what they said they were going to do, what are the options to make this work? I figured this is the right place to ask!

Thank you for your time,

Jake

moserjj
06-12-2012, 11:44 AM
Do you have a contract or any other written estimate/specs that call for 3000 psi concrete? If so, have them rip it out and do it again correctly this time

Killer95Stang
06-12-2012, 12:32 PM
At the very least... have them cutout out two square holes in the slab.. and then pour deeper pads that are tied in to support the posts. If done correctly, it look like it was planned all along...

brownbagg
06-12-2012, 04:07 PM
first core the slab and test the concrete, its possible for 2500 concrete to break in the 3000 range

davedriveschevys
06-12-2012, 04:44 PM
The 3000 psi spec must be to hold the wedge anchors from uplift during an improper lift, because figuring roughly 3 sq ft of surface area for an average two post lift it would take 1,080,000 pounds to fail a 2500 psi slab.

snorky18
06-12-2012, 04:50 PM
first core the slab and test the concrete, its possible for 2500 concrete to break in the 3000 range

So you at least know that one core taken at that one place was around the 3000 range :thumbup:

Personally, for something that's going to hold a mult-ton vehicle over my head, I would not even consider doing that. And I'm an engineer. Who works with concrete. And concrete anchors.

Rip it out, put the correct stuff in (pay attention to psi and thickness and reinforcement requirements for the lift you want).

Alternately, you might, and I emphasize might, find a lift (4 post?) that will work with 2500psi. I have no idea.

On a side note, I haven't done much residential work, but I'm not sure I've ever known a 2500psi slab before. Seems like they are typically at least 3000. Is the contractor a lowest bidder kind of guy?

mercury26
06-12-2012, 10:01 PM
I had concrete footings put in for my two post MaxJax lift, 4' x 4' x 1' with 4000 PSI concrete. Footings were doweled into the existing slab. I believe this is the recommendation from a few different lift manufacturers.

Regards,

Chuck

brownbagg
06-12-2012, 10:28 PM
Personally, for something that's going to hold a mult-ton vehicle over my head, I would not even consider doing that. And I'm an engineer. Who works with concrete. And concrete anchors.

why not? Take two core and test as per ACI c 39. average the breaks and that will be the strength of the concrete. Since the slab is proberly over 28 day old this would get a good sample of concrete strength. If the psi is over 3000 psi there is no reason to demo the slab except to show your authority over something than doesnt exists. Demo slab due to incorrect concrete placement is rarely an option. First option is to see what exactly is in place. 3000 psi out of a 2500 is mix is very common if the finishing crew did not abuse the mix

tool-guy
06-13-2012, 12:37 AM
You could also consider tying the top of the hoist both forward and sideways into rafters or trusses of your garage.

mrobins297aaa
06-13-2012, 01:09 AM
I'd be more concerned that its at least 4" thick.
I'm going to get my balls busted here but there is a lot of over kill when it comes to these lifts and concrete on this site.

FunkyfullWidth
06-13-2012, 06:11 AM
I think that's due to the fact that it holds very heavy objects above our heads...

And out of curiousity... How thick IS the slab? If you really are worried about it.... I think tearing the whole pad out is over doing it. Best bet would be cutting squares for under the pads and pinning those to the existing slab.

Shadowdog500
06-13-2012, 07:31 AM
I see this is your first post, welcome to the forum!

Was 3,000 lb concrete spelled out in the contract? If so, I would have them rip it out and replace it.

If it wasn't then it's on you. You could either replace the entire floor, or dig out the area by the lift ant put a bunch of concrete in. You may want to go to the Morton website and look up their flooring requirements. They are the only ones with detailed guidance on concrete requirements.

You may look into getting a 4 post lift instead.

If you get a new floor poured you may want to increase the concrete to 4000 lb. it don't cost that much more to get stronger concrete. I put 4500 lb concrete in my shop. Also make sure you get rebar or mesh. The contractors like fiber because it is easy, rebar or mesh is better.

Chris

wssix99
06-13-2012, 08:02 AM
first core the slab and test the concrete, its possible for 2500 concrete to break in the 3000 range\

+1. 2500 psi is guaranteed to be at least that. You could be well in excess of 3000 in reality.

If your slab is thicker than the manufacturer specs, you should also still be OK with 2500. Just call the manufacturer and ask them how thick the slab needs to be for 2500 psi concrete.

jimp
06-13-2012, 09:37 AM
check you lift supplier for these. http://www.gregsmithequipment.com/Atlas-Baseplate-Extension-Kit

Jackob
06-13-2012, 10:41 AM
Wow, what a helpful group! Thank you so much!

The foundation is likely to remain as poured, unfortunately. Something I did not mention (!) is that this is a post-tension slab. Are modifications such as those described above (e.g. adding additional concrete where the posts would be located) even possible given the nature of a post-tension slab?

As for the thickness, I was surprised to see that even though they call for a 4" minimum, they poured a significantly thicker slab, as seen in the picture I attached.

As seen in the picture, though the slab appears to be pretty thick, it appears to be layered, so to speak...is this an issue?

Thanks for your time and help everyone, I really appreciate it!

Jackob
06-13-2012, 03:03 PM
Oh gosh, I just realized I embarrassed myself and made it clear how little I know about foundations. I was using the earth next to the foundation as a frame of reference for determining the thickness of the slab. Not at all the right frame of reference!

I suppose I'll have them test the slab.

Can the extensions found in the link jimp posted be used for any type of 2-post lift? I don't see why not but I might as well ask...

wssix99
06-14-2012, 10:39 AM
Something I did not mention (!) is that this is a post-tension slab.

I highly doubt it. Its slab-on-grade, right? What makes you think its tensioned?

If it is a tensioned slab, you won't be able to put a lift on it.

Jackob
06-14-2012, 11:34 AM
I highly doubt it. Its slab-on-grade, right? What makes you think its tensioned?

If it is a tensioned slab, you won't be able to put a lift on it.

I think the tension cables gave it away... :)

Why do you say you can't put a lift on a post-tension slab? Don't people position the posts away from the cables and carry on without any problems?

Bad Hammer
06-14-2012, 11:55 AM
I think the tension cables gave it away... :)

I'm probably wrong when I say this, but I don't think those are tension cables; I think those are snap ties...used to hold the forms during the pour.
But again, I could be wrong on that.

Jackob
06-14-2012, 12:49 PM
I'm probably wrong when I say this, but I don't think those are tension cables; I think those are snap ties...used to hold the forms during the pour.
But again, I could be wrong on that.

Guys, seriously. It's a post-tension slab, period. I don't understand why everyone is so thrown by this.

The real issue here is whether I can continue with the building and purchase of this house given that the garage foundation may have 2500 psi concrete.

I'll get it tested to see what we're really working with. But if there are problems with the slab being post-tension, then that's another problem - eek. My understanding that it's not a problem that the cables are there; you just need to be mindful of where they are.

Jackob
06-14-2012, 12:55 PM
Attached is a picture of the garage area.

willymakeit
06-14-2012, 02:51 PM
It has been mentioned several times. Check with your lift supplier. Bend pak has lifts that the min. is 3.5" of concrete at 2500 psi. They also don't want them on post tension.

UPSHIFT
06-14-2012, 06:18 PM
Guys, seriously. It's a post-tension slab, period. I don't understand why everyone is so thrown by this.

The real issue here is whether I can continue with the building and purchase of this house given that the garage foundation may have 2500 psi concrete.

I'll get it tested to see what we're really working with. But if there are problems with the slab being post-tension, then that's another problem - eek. My understanding that it's not a problem that the cables are there; you just need to be mindful of where they are.


The issue with the post tension slab is drilling it, there are many myths that the concrete can explode if you drill through a cable so beware. This explains the big "DO NOT DRILL" stamp in the middle of my post tension slab in my garage at home. We have drilled post tension slabs before but we used a metal detector and made sure we did not drill the cables. They have a plastic coating on them so you will see plastic when drilling to warn you. I would suggest a 4 post so you do not have to drill the slab. :scared: California and Texas have the bigger population of Post Tension Slabs. Good Luck :thumbup:

Gabe
Dannmar

UPSHIFT
06-14-2012, 06:21 PM
Attached is a picture of the garage area.

Knowing where the cables are makes this a non issue. :) Now just be sure the ceiling is built high enough to allow a 2 post lift and you will be in good shape. :thumbup:

Jackob
06-14-2012, 08:08 PM
Knowing where the cables are makes this a non issue. :) Now just be sure the ceiling is built high enough to allow a 2 post lift and you will be in good shape. :thumbup:

I will definitely know where the cables are so it should be a non-issue indeed (unless there is something else I'm missing here).

But still, my understanding is that with this darn 2500 psi slab they poured, I won't be able to put a "full size" two-post...totally screwed.

The 2-posts that I've researched all require 4" min at 3000 psi, so it appears I'm out of luck unless the builder tests the concrete and it turns out it is actually closer to 3000 psi.

Now, 3000 psi is what the 10,000 lb lifts appear to require. That's the type of lift I'm planning to install because I want a clear floor, etc. However, the car I'm mainly lifting is a Mustang, however I do want the flexibility of knowing I'll be able to safely lift a heavy truck.

Why couldn't they have just poured the 3000 psi. :(

brownbagg
06-14-2012, 09:05 PM
Why couldn't they have just poured the 3000 psi.


because contractors are cons with tractors

Bad Hammer
06-15-2012, 07:33 AM
Guys, seriously. It's a post-tension slab, period. I don't understand why everyone is so thrown by this.

I'm sorry (I wasn't actually thrown by the issue)...and I wasn't implying that it wasn't a post tension slab, just that what I was seeing in the picture appeared to be snap ties, not cables.
I even admitted that I was probably wrong.:D
I haven't worked around stressed slabs in a very long time, but I recall the cable ends and tensioning nuts, wedges, anchors, etc. being exposed along the edge of the slab.
Regardless, best of luck to 'ya.:thumbup:

mikefromme
06-15-2012, 07:40 AM
The real issue here is whether I can continue with the building and purchase of this house given that the garage foundation may have 2500 psi concrete.




Make them fix it or don't buy the house. You will always be pissed that it wasn't done right in the first place.

Why was 2500 psi poured in the first place. Honest mistake? Or were they trying to cut corners? Not a good way to start a new build.

buening
06-15-2012, 09:47 AM
I don't know that I've seen 2500psi concrete that tests out under 3000psi. Seriously, a 5 bag mix (at least in my area) may be specified as 2500psi but will likely test out higher than that at full strength. If you are nervous, cut a square hole where the two post pads will be, remove the concrete and dig deeper for a separate footing to be poured. This is common for retrofits on existing slabs and doesn't take that much work to do. There is a thread on here a few months back with pictures of a guy's slab that had just that done.

Shadowdog500
06-15-2012, 09:59 AM
I don't know that I've seen 2500psi concrete that tests out under 3000psi. Seriously, a 5 bag mix (at least in my area) may be specified as 2500psi but will likely test out higher than that at full strength. If you are nervous, cut a square hole where the two post pads will be, remove the concrete and dig deeper for a separate footing to be poured. This is common for retrofits on existing slabs and doesn't take that much work to do. There is a thread on here a few months back with pictures of a guy's slab that had just that done.


Can you cut a big hole in the center of a post tension slab?:headscrat

Chris

Shadowdog500
06-15-2012, 10:03 AM
...

Why couldn't they have just poured the 3000 psi. :(

Did you specify 3000psi in the contract?

Chris

buening
06-15-2012, 10:24 AM
Can you cut a big hole in the center of a post tension slab?:headscrat

Chris

Yes as long as you can identify the locations of the tension cables. I'm assuming his picture above is the tension cable ducts, which appear to have large spacing between them. If these tension ducts were at 18" centers or so, then yeah cutting a hole wouldn't be as easy.

Take the base plate dimensions and add a few inches on each side and that is your square that you need to cut. I don't recall the size of these two post lift baseplates, but you wouldn't need more than a 2' square. From the pics, the duct spacing is much larger than that.

Helpful link debunking the myth of penetrating post tensioned slabs: http://www.concreteconstruction.net/post-tensioning/cut-it-out.aspx

wssix99
06-15-2012, 11:22 AM
BTW - Your earlier post showed reinforcing mesh and I thought that was what you were referring to for post-tensioning. It looks like your tensinoing tendons are part of a larger grade beam system, which supports the house? You must be in a warm place with such a shallow foundation, right?

Putting lifts on engineered slabs (elevates, prestressed, post-tensioned, etc.) is problematic because lift companies design their specs for for simple slab-on grade situations. If you want to put a lift in this application, you'll need to consult the engineer who designed the slab to determine if you can put in a lift and where it needs to be placed.

For your situation, the manufacturer's spec on the concrete is irrelevant since they are assuming a different floor design. (A 3000 psi spec for a 4 inch simple slab assumes the slab has 0 resistance to tension in half of the slab and resists bending in the 2 inches of the slab seeing the maximum compression.) A post-tensioned slab has the ability to resist bending tension, so the calculations are not as straight forward. A post-tensioned slab can be stronger with less depth than a simple slab, but the complication is that your slab is also supporting stresses introduced in to the foundation.

You'll need to account for the total floor system, including the foundation stresses and the bending stresses that the lift will introduce on your engineered floor. Placing a lift in one location could be fine, but placing it in another location could overload the tendons in the slab. Only a professional engineer can confirm this.

Jackob
06-15-2012, 12:17 PM
All these posts makes me which so badly that I had time away from work to become educated on these issues...not to mention I'm freaked out by your post - this is getting complicated! I most definitely appreciate the information/advice.

BTW - Your earlier post showed reinforcing mesh and I thought that was what you were referring to for post-tensioning. It looks like your tensinoing tendons are part of a larger grade beam system, which supports the house? You must be in a warm place with such a shallow foundation, right?

You asked whether the tensioning tendons are part of a larger grade beam system that support the house. What do you mean by house? The house is a few feet to the side of the garage, not seen in this picture. This garage is basically a 4-car garage (2-deep) with a second story - it's a barn.

And yes, I'm in Houston, TX.

pstnbly
06-15-2012, 03:42 PM
Look at the photo, the tendons are the red lines. I'm having a hard time wraping my head around post tensioning a 2500 psi slab but so be it. I believe the 3000 psi requiement is for withdrawl resistance for the supplied wedge bolt anchors. Research or consultation into different anchoring methods is warrented.

mikefromme
06-15-2012, 05:02 PM
Make them fix it...

Unless this is was a favor by your cousins boyfriend from college old roommate kind of deal.

WNYflyer
06-15-2012, 06:32 PM
- My "educated" feel/guess is that if your slab is the same thickness of that recommended by the lift manufacturer then 2500 psi will be okay. Why? well..........

-Your slab design (post-tensioned) is a whole different animal than that assumed by a majority of the lift manufacturers. I am pretty sure many of the lift manufacturers are assuming "un-reinforced" concrete (i.e. any steel is only for crack control, etc.).

-In "un-reinforced" concrete design the concrete is designed to a certain thickness so as to keep the tension in the concrete due to bending, etc very low since concrete has very low tensile capacity.Your slab is probably prestressed in compression enough that the concrete would probably never see any tension due to bending, etc. when loaded by a lift.

-Don't know for sure and don't feel like looking it up but I bet a post-tensioned slab also has a higher shear capacity than an "un-reinforced" slab, for a given thickness of slab, due to the clamping forces/induced compression in the slab. I would venture that bolt embedment wouldn't change also even though you have 2500 psi concrete rather than 3000 psi. Even then you could get the tension load in the bolt from the lift manufacturer and then go to the bolt manufacturer as see what load it can take in 2500 psi rather than 3000 psi concrete. Might still be okay.

- I would put a big qualify statement on all of the above because from what I have seen it looks like many lift manufacturers give you minimal information on slab requirements so as to not scare you off. In otherwords why give the potential customer too much info upfront which might scare him off and thus lose sales to a manufacturer who hold his tongue until the lift is purchased. Be very careful about the slab requirement information published on websites and call the actual manufacturers to get a feel for the "real requirments" as well if they are trying to BS you.

- A professional in your local can probably check your slab for a lift ( actual one to be purchased) without too much effort if you provide the basic design/construction criteria for your slab. This is what I would ultimately suggest given that you have a post tensioned slab and as always it doesn't hurt to find out what that cost is no matter what you decide. Be informed/educated.

Good luck

wssix99
06-17-2012, 11:10 AM
All these posts makes me which so badly that I had time away from work to become educated on these issues...not to mention I'm freaked out by your post - this is getting complicated! I most definitely appreciate the information/advice.

The education required takes years to acquire because the behavior of reinforced concrete is very complex, particularly when you get in to pre-stressed and post-tensioned materials. Beyond having the education, one needs a good bit of experience actually designing this stuff. That's why you should consult a "Professional Engineer." They are licensed by your state to specifically tackle this kind of thing and make sure the design is safe. (Professional Engineer is licensed qualification, just like is given to architects, laywers, doctors, etc.) I'm not familiar with how plans are done in your area, but I'd expect that the plans for your house and your building permit will list the architect and engineer of record on your house. That engineer of record should be able to help you out with very little fuss and should already be familiar with your specific design.


You asked whether the tensioning tendons are part of a larger grade beam system that support the house. What do you mean by house? The house is a few feet to the side of the garage, not seen in this picture. This garage is basically a 4-car garage (2-deep) with a second story - it's a barn.

You can disregard my comment about house. I was referring more to the foundation. Whether or not the structure is a house or garage, they both will need a foundation for the walls. From the picture, it looks like you have a "shallow" foundation. Most of the USA needs to dig deeper foundations to combat frost heave, which I expect is not an issue in Houston. Slabs used with the deeper foundations typically "float" over the dirt and are not tied in to the walls or other structure like yours is.

When the trenches in the picture (that criss-cross the garage area) are filled with concrete, they make a beam. Those beams will support the weight of the walls, roof, etc. and spread the load across the ground. The post-tensioning could be used for a number of reasons. It may lock the grade beams in to each other to stiffen the whole system or it may also allow the foundation to be built with less reinforcing steel. (It may be more cost effective in your situation and consume less steel by using a few post-tenioning tendons instead of heavy rebar cages inside the beams.)


For the majority of the US, calculating lift loads on a floating (non-reinforced) slab is pretty straight forward and easy to do because its a homogenous system. (Even if a floating slab has rebar it it, the rebar is assumed to be non-structural and typically just controls cracking.) Slabs with structural reinforcement or stressing tendons are more complex as they make hetrogeneous system where the concrete and steel components interact in ways unlike other materials.

MoonRise
06-18-2012, 12:32 PM
+48 on 'you are not dealing with 'simple' concrete slab construction' there.

If the plans/contract (from the architect or Professional Engineer) call for 3000psi concrete and the concrete guys put in 2500 psi concrete, raise holy-heck with the architect/engineer/builder about that. And make them 'sign-off' on that aspect of the build.

If Building Codes in your area call out for minimum 3000 psi concrete, then you have a 'fail' right there.

Never mind your future plans of putting in a 2-post lift, if the original slab/foundation (in your area, the two are kind of combined) is inadequate, then you are looking at possible problems down the road.

2-post lift on a post-tensioned slab is not in the 'usual' category of 'typical' installation of the lift onto a 'plain' slab.

So you would need to contact your engineer (Professional Engineer) and/or the lift company for guidance regarding the installation of the lift onto the slab.

But for the overall original question/concern regarding the 'incorrect' slab:

If the architect/engineer called for 3000 psi minimum concrete for the concrete work, then the concrete guys flubbed it. Redo.

If the concrete guys want to go and try and do a whole bunch of core samples and 'prove' to the engineer/architect that the concrete they used was actually 'good enough' and the engineer/architect agrees to some sort of core sample testing (and no, just one or two core samples does not count as enough samples to take/do) and will 'accept' that, as well as the now pock-marked holes in the slab that then have to get filled in, then maybe all the parties involved can go that route.

If the plans and/or contract/bid/whatever with the concrete guys called for or specified 3000 psi minimum concrete and they used 2500 psi concrete, then the concrete guys either made a mistake or tried to 'cheat' you. Redo.

If the concrete guys used 2500 psi concrete 'mix' but ended up putting 'extra' water in to make it easier to slop the concrete around, then you didn't even get '2500 psi' concrete.

At the least I'd say you are looking at the architect/engineer reviewing the detailed plans and calculations regarding the slab and verifying whether the slab/foundation is 'ok' or not and if it meets Building Codes or not. Cause that is part of what they are/were payed for, not for you to really have to go and figure out. If they go and recheck all the calcs and Code-compliance and it is 'ok', then they have to document that.

Worse case, rip out the slab and have it redone. Again, not your problem and not your fault.

YMMV.

king nero
06-18-2012, 03:10 PM
Worse case, rip out the slab and have it redone. Again, not your problem and not your fault.
YMMV.

It has been asked several times whether or not the 3000 psi was asked for (and maybe even in a written contract) but the OP doesn't seem to confirm that.

Doesn't sound so good...

skamp
06-18-2012, 04:09 PM
I have a post tensioned slab as well and had no issues installing my lift. I used a metal detector and my orginal photos to locate all of the PT cables and made sure all the holes were 12" or more away from any cable. Looking at your photo's it looks ike the slab is deeper than 4". I would drill a test hole (in the area without a tendon) and see what your depth is. If you have more than 4" I can't see how you would not be ok. If you have 6" you may want to use an epoxy anchor to make sure you have no issues with a wedge anchor bitting. You can also get a core sample done to detemine the exact strength if you are really that concerned. There are more low tech methods like the hammer test that will give you a ballpark.

I also taked with multiple post tension engineers and they all said cutting out a piece between the cables is fine if you needed to pour a deeper hole provided you are 12" from a tendon. The only thing they said was to not pin the new pad to the existing slab to prevent it from pulling down the slab if any settling happened from the heavier slab. Hope this helps.

Steve

wssix99
06-19-2012, 06:41 AM
I have a post tensioned slab as well and had no issues installing my lift.

Without and engineer's blessing, this is kind of like having a pet rattlesnake and never being bit. Will it bite some day? Maybe, maybe not. If a load on a lift is perfectly balanced, then there is very little stress transferred to the slab. If that load is unbalanced, that's when the bending gets introduced to the slab. All of the design specs are detailed so that the lift will stand up in a situation where a load is badly unbalanced.


I also taked with multiple post tension engineers and they all said cutting out a piece between the cables is fine if you needed to pour a deeper hole provided you are 12" from a tendon. The only thing they said was to not pin the new pad to the existing slab to prevent it from pulling down the slab if any settling happened from the heavier slab.

That very well may be the case with the OP's slab but, unless the slab is a standardized design, only an engineer with specific knowledge of the slab can confirm. The grade beams that criss-cross the garage also need to be accounted for in addition to the slab.

LLWillysfan
06-19-2012, 09:36 AM
I'm probably wrong when I say this, but I don't think those are tension cables; I think those are snap ties...used to hold the forms during the pour.
But again, I could be wrong on that.

I'll take the OP's word for it but you are right about the metal rods - they are not PT cables.

For those who haven't come across it, post tensioned residential slabs are pretty common in some parts of the country. Plastic sheathed steel cables are run between steel plates across the slab prior to placment. After the concrete has reached a predetermined strenth, you hook a special hydraulic jack to the cable and apply tension. Wedges hold the cable once you release the tool.

Drilling holes in a PT slab is serious business. Cut a cable and it will fly out and can do serious damage - plus you've lost your post tensioning.

EDIT; Sorry. I missed a whole page of responses before my post - most of this has been covered.

skamp
06-19-2012, 09:40 AM
Without and engineer's blessing, this is kind of like having a pet rattlesnake and never being bit. Will it bite some day? Maybe, maybe not. If a load on a lift is perfectly balanced, then there is very little stress transferred to the slab. If that load is unbalanced, that's when the bending gets introduced to the slab. All of the design specs are detailed so that the lift will stand up in a situation where a load is badly unbalanced.

I am not sure why you state that. The issue with a post tension slab and a lift is not the reinforcement or strength of the concrete but the possibility of hitting a post tension cable and causing it to snap during the drilling. The PT cables serve the same purpose as rebar to provide the tension strength.

That very well may be the case with the OP's slab but, unless the slab is a standardized design, only an engineer with specific knowledge of the slab can confirm. The grade beams that criss-cross the garage also need to be accounted for in addition to the slab.

If you cut out sections in a PT slab for a lift you still need to meet the requirement of the lift vendor for a retrofit, ie. 4'x4' 12" 3000 PSI with rebar, etc. That is to support the lift and has nothing to do with the PT slab at that point. In addition it is highly advisiable to talk with the PT engineer that spec'ed the slab to make sure that those cutouts do not impact the slab for the structure.

I was lucky that I did not have to do any cutouts but after talking with the firm that built my slab they said it would not impact the slab to cutout 4x4 sections provided I was at least 12" from any tendon/beam. They also advised not to pin to the existing slab to prevent any sinking from pulling down the slab. They also mentioned that depending on the soil I may have to go down 24+" to hit solid ground as most of the time they bring in soil. These were all precautions to be safe.

Steve

Kevin54
06-19-2012, 10:22 AM
Didn't read through every single reply, but if it was supposed to be one type, AND it is down in writing, AND if they poured something of lesser quality and you can prove it, the Contractor should eat the cost of having it ripped out and redone correctly. If he won't, then it would be that you hold monies back from him. Then it would be going to court to get things remedied.

buening
06-19-2012, 10:38 AM
I question whether this slab was even designed by an engineer, so the OP should fill us in on that aspect as it will affect our recommendations. If it was designed by an engineer, then you MUST contact the engineer immediately to get this issue resolved. If it was designed by an engineer, then there are documents specifying the strength of concrete and he has the contractor by the balls and can make them rip it out. If it wasn't engineered and the concrete strength was a verbal request, it will be a fight to get anything done.

Much like a garage slab elsewhere (at least in my area), the slabs are not typically designed by an engineer but rather a contractor has a typical rebar size and spacing that he likes. These post tensioning strands are no different, they just take a bit more work but the end result is much better. Don't confuse these stressing stands as being structural like what is used on a bridge or suspended multistory slabs. This is a slab on grade and the strands are merely for temperature and shrinkage crack control and the strands were likely placed middepth of the slab. You don't want the strands placed outside the middepth, as this adds eccentricity to the compressive loads and will not control cracking as effectively. In fact, it could actually cause cracking if it is placed too close to either surface. A post-tensioned slab greatly reduces the control joint spacings and can often times eliminate the joints if the slab is small enough. Keep in mind there are no resistance to shrinkage cracks from the time its poured till the concrete has attained enough strength that the strands can be stressed. Proper mix design is crucial to minimizing cracking during this period.

The loads from a vehicle distributed on a slab on grade is generally a function of the slab thickness, subgrade modulus (controlled by the subgrade improvements most of the time), joint spacing, and concrete strength. Load resistance is not dependent on the reinforcement UNLESS it is a reinforced structural slab on grade, in which layers of reinforcement are placed at the top AND bottom of the slab. This is typically only used at industrial buildings. See signature for credentials.

Cutting into the slab is very serious, as cutting a duct will often cause injuries. I can't stress this enough. You should be able to track the ducts from the exposed edges of the slab as well as use a metal detector to mark the ducts. The 12" rule mentioned previously is a good rule of thumb!

buening
06-19-2012, 10:48 AM
Here is a good reference for those building a slab with post tensioned strands: http://www.concreteconstruction.net/Images/Building%20Better%20Post-Tensioned%20Slabs_tcm45-356807.pdf

more good info: http://www.concreteconstruction.net/concrete-construction/post-tensioned-slab-on-ground-foundations.aspx

wssix99
06-20-2012, 09:48 AM
The PT cables serve the same purpose as rebar to provide the tension strength.

In a non-tensioned, slab-on-grade situation, the reinforcement is not there for tension strength and is assumed as 0 strength in the design. It is only there to combat shrinkage cracking and is placed in the middle/neutral area of the cross-section. The stressed slab does have tension reinforcement, which is why its a different thing all together.

To your point, it does look like the tensioning is there more for the grade beams and the structure and not for the slab, itself. A slab on grade doesn't need any tension reinforcement if the cross section is deep/strong enough.