Please share your thoughts on this Best Practice.

I'm purchasing a new home to be constructed, which means that I have no rightto inspect and control the construction process etc. Anyway, I inspected the post-tensioned slab after it was poured and it has numerous long and narrow cracks radiating out from the plumbing pipes that pass through the middle of the foundation. Is this normal?? Could it be a defective condition??

Are you SURE it's a post-tensioned slab?

Post-tensioning usually involves steel cable tendons which run through the slab, and are jacked against the slab edges once the slab has cured, to create the tension. This requires some unusual know-how and special equipment, and, besides not usually being done in residential work, is unnecessary for a basement slab on ground. Post-tensioning is usually meant for long-span concrete structures such as parking garages.

I agree that it is probably not post tensioned. In any case the cracks are NOT normal and should not be accepted.

You say the cracks radiate from the points where plumbing pipes pass through the foundation. Are they passing horizontally, or vertially through the foundation?

In any case the cracks are probably due to two conditions. The soil around the pipes, and thus under the slab was not compacted properly and thus the slab has no support under it in those areas. Also assuming the slab was reinforced the reinforcing wire/rebar could have been interrupted by the plumbing meaning that there was no reinforcement in the area you see cracking.

I would not purchase this house until the slab is removed and repoured. I would also carefully check all other parts of the construction. Builders who are careless/cut-corners in one area have a tendency to do so elsewhere. If you do purchase the house make sure you have an iron clad warantee that covers ALL aspects of the construction. Check out the poster in the "Framing" forum who has a problem with "bouncy floors" in the new home he recently purchased. click here

Richard:

Interesting that you would say that post-tension slabs are not normally used in residential work. I am in Dallas, TX and MANY new home builders, in all size homes, use this system. I am in a 1963 home with rebar and so far no problems. I am courious though, your opinion on rebar slab, post-tension, and pier and beam. Which is best? I am planning a new home soon.

quote:

Originally posted by Richard Hetzel:
Are you SURE it's a post-tensioned slab?

Post-tensioning usually involves steel cable tendons which run through the slab, and are jacked against the slab edges once the slab has cured, to create the tension. This requires some unusual know-how and special equipment, and, besides not usually being done in residential work, is unnecessary for a basement slab on ground. Post-tensioning is usually meant for long-span concrete structures such as parking garages.

I'd be very much surprised if the concrete stuctures were actually post-tensioned. In my area, it would be completely unheard of for residential work.

As far as "which is best" is concerned, there is no general "best". Each method has its advantages and disadvantages, and in any specific case, only one method will likely be the "best", for reasons of structural appropriateness, structural integrity, and balance of cost vs. benefit.

Give me the exact problem to be solved, and THEN I can judge which method would be best.

Rich:Post tensioned slabs are widely used in the south and west in residential applications.As far as the fellow with the cracks in the newly poured slab go,I agree with David.This could be a major problem in the future.Poor workmanship,along with a lot of other facters[soil type,improper drainage,small plumbing leaks] can and do cause foundation failure,and major headaches for homeowners. Respectfully, Frank O'Pinion

Whu would soil type have anything to do with a post-tensioned slab, the purpose of which is to span long distances? If it's a slab-on-ground, what is the purpose of post-tensioning, since it spans nothing?

Somehow I think there's a difference in terminology here.

Richard:Clay soils can expand/ heave when wet, or shrink when abnormally dry. It's My understanding that " post tensioned" slabs are used in some areas to compensate for slight movement of soils under these residential foundations.Respectfully Frank O'Pinion

Frank:

You are right. Builders here advertise the post slabs regularlly. I saw only one actually in a friends house, but do not know how it held up.I am wondering though, how does ceramic tile hold up (cracks) in a pier and beam foor? I believe the slabs work best for this, but they are colder them insulated p & b. Just not to sure which I want, but am leaning towards pier and beam. Your thoughts?

quote:

Originally posted by frankopinion:
Richard:Clay soils can expand/ heave when wet, or shrink when abnormally dry. It's My understanding that " post tensioned" slabs are used in some areas to compensate for slight movement of soils under these residential foundations.Respectfully Frank O'Pinion

This thread baffles me. Yes, I am familiar with expanding soils in the southwest, even though I've never built there, so I understand the use of post-tensioned slabs to resist movement, that makes sense. But why could one not insulate under a post-tensioned slab, but can under a pier-and beam floor?

And if reinforced correctly, why would one crack any more than another? Or do the expanding soils also heave the piers?

Richard : I'm not sure if insulation would help.It's My understanding that changes in the moisture of the soil is the culprit. Too much[small plumbing leaks], or too little[the soil doesn't get wet in rains with the structure over it].If a post and beam foundation is excavated to " bearing ground" the movement should be minimal. I like You don't have experience in the south or southwest but I've read that poor plumbing workmanship by the original builder causes a large number of slab failures and problems. It only takes a very small leak in a sanitary connection to cause soil expansion. Rspectfully Frank O'Pinion

Frank O, I was talking about cold floors, not expansion, and my question was why a post-tensioned slab with proper insulation should be any colder or warmer than a pier and beam floor.

So what is proper insulation for slabs? How do you insulate a slab? When mine was poured in '63, it only had the required footings, pea gravel and vapor bearier. I guess they don't insulate here, or didn't back then when fuel was cheaper. Batts in P & B, but slab? Hmmmm, interesting!

quote:

Originally posted by Richard Hetzel:
Frank O, I was talking about cold floors, not expansion, and my question was why a post-tensioned slab with proper insulation should be any colder or warmer than a pier and beam floor.

Depending on the foundation design, there are a few methods, The usual method here in the northeast is 2 inches of extruded polytyrene board, extending 2 feet down on the foundation wall, and two feet hroizontally under the slab edges. That gives an insulating value of R-10.

Where the foundation does not need to extend so deeply into the ground, insulating the slab edges, or even the entire slab, is possible.

But recognize that concrete and ceramic tile are inherently cold to the touch, and such floors will FEEL cold to, say, bare feet, regardless of the insulation, unless such measures as radiant heating in or on the slab are employed.

It cracks me up that someone who doesn't know anything about the subject is an expert on it.

I live in NW Louisiana and nearly 100% of the foundation slabs on homes built since the early 90's are post-tensioned. In our expansive clay soil it is the only way to do it right. I can't think of a single licensed professional civil engineer who designs house foundations who doesn't use post tension. Sure, if the soil is mostly fill or if there were a lot of trees on the site which were removed, the civil engineer will call for pier shafts for the post tensioned beams to sit on. Every now and then a civil engineer will call for a spread footing or such but not often.

It's a matter of region. I daresay there is not a post-tensioned residential slab within 1000 miles of here. I have done a couple of post-tensioned parking structures where the slab is spanning long distances. That is the sun total of my experience with post-tensioning.
I don't know who claimed to be an expert in them, but it sure wasn't me...

quote:

Originally posted by Richard Hetzel:
It's a matter of region. I daresay there is not a post-tensioned residential slab within 1000 miles of here. I have done a couple of post-tensioned parking structures where the slab is spanning long distances. That is the sun total of my experience with post-tensioning.
I don't know who claimed to be an expert in them, but it sure wasn't me...

Properly designed and installed post tensioned slabs are almost always superior to rebar in expansive soils.

One advantage is nearly all of the surface cracks that appear in a slab right after its poured can be sealed up when the cables are tensioned to about 25,000 psi a few days after the pour.

Here is a photo of one of my house slabs with the post tension system in place:

Only now am I beginning to understand P/T slabs, thanks to you. I do have a couple of questions. Isn't there a water line in the lower part of the picture? I didn't think water lines should be in the slab except where it exits and I would think one would need rebar in the footongs. Shows ya how much I know!

John

A Boutique Home Builder?

Soooo, what is your take on pier and beam vs. slab foundations?