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conventional vs post tension slabs?
Posted on 3/12/14 at 12:32 am
Posted on 3/12/14 at 12:32 am
What is the general opinion? I always assumed that post tension was more expensive but it actually looks like it may be cheaper when you factor in that it is less concrete and steel. So why is it that most of the building suppliers tell me they dont trust the post tension? Has anyone had problems? Or are they just stuck in the past?
5
Posted on 3/12/14 at 5:52 am to vettegc
A lot depends on the soil under the slab. Having thick clay, I would go with the tension slab.
Posted on 3/12/14 at 7:09 am to fishfighter
I'm curious about this as well because I've heard arguments for both sides.
Bookmarked.
Bookmarked.
Posted on 3/12/14 at 8:24 am to vettegc
Structural engineer here. Conventional all the way. Post-tensioned done properly can be better, but it is never done properly, at least for residential structures.
We design commercial buildings all the time with grade-supported slabs and almost always use conventional.
However, that being said, ensuring a properly compacted subgrade and avoiding expansive soils are more critical in most cases.
We design commercial buildings all the time with grade-supported slabs and almost always use conventional.
However, that being said, ensuring a properly compacted subgrade and avoiding expansive soils are more critical in most cases.
Posted on 3/12/14 at 8:55 am to urinetrouble
I've had 2 homes. The one with a conventional slab I had lots of cracked tiles, the one with a post-tension slab, no cracked tiles. Cost wise, it wasn't that big of a difference.
This post was edited on 3/12/14 at 8:59 am
Posted on 3/12/14 at 12:21 pm to urinetrouble
quote:
However, that being said, ensuring a properly compacted subgrade and avoiding expansive soils are more critical in most cases.
This. If you have a good base, and get it tested a Post-tension is not needed for a residential property. Post tensions are a better more efficient design, but can be overkill for residential if done properly...
The subgrade is where most foundation issues tend to pop up.
Posted on 3/12/14 at 1:40 pm to vettegc
Keying into the "done right" argument for post tension, I had a buddy just have two cables snap within his slab and rocket out projectiles in his yard. Both put hefty dents into his fence 20+ feet away. In both cases the cable failed.
After that, I'll probably never do post tension. As mentioned, if a conventional is done right, it should not present a problem.
After that, I'll probably never do post tension. As mentioned, if a conventional is done right, it should not present a problem.
This post was edited on 3/12/14 at 5:21 pm
Posted on 3/12/14 at 5:11 pm to LSUtigerME
I build a few homes a year, but I’m not a civil engineer. I always get a bore sample and soil analysis done. Based on that I make a decision on which to use. As one poster said
This is very true. If the site passes the bore sample – conventional. But, if it doesn’t and involves cutting down 4’ and bringing in 20 loads of select fill to be brought up in 6” lifts with compaction tested after each lift, etc., etc.. I’m going with a post tension. While doing all that dirt work is viable on commercial property, the economics don’t work on spec homes. A properly designed post tension slab can design around expansive soils, high water tables, etc. And from what I’ve seen the cost of conventional vs post tension is about a wash. Conventional uses more steel less concrete, post tension uses more concrete less steel (at least on all of my designed slabs in East Texas where expansive soil is just part of the game).
quote:
“ensuring a properly compacted subgrade and avoiding expansive soils are more critical in most cases.”
This is very true. If the site passes the bore sample – conventional. But, if it doesn’t and involves cutting down 4’ and bringing in 20 loads of select fill to be brought up in 6” lifts with compaction tested after each lift, etc., etc.. I’m going with a post tension. While doing all that dirt work is viable on commercial property, the economics don’t work on spec homes. A properly designed post tension slab can design around expansive soils, high water tables, etc. And from what I’ve seen the cost of conventional vs post tension is about a wash. Conventional uses more steel less concrete, post tension uses more concrete less steel (at least on all of my designed slabs in East Texas where expansive soil is just part of the game).
Posted on 3/12/14 at 8:27 pm to vettegc
I'm not a fan of PT slabs at all. I steer customers towards conventional every opportunity I'm asked. We had one customer come back with the "VE the slab to post tension" and the cost savings was offset by the redesign fee. I also like to add a top layer of wire mesh above the tendons to eliminate shrinkage cracks, which also adds cost to the /SF number.
This is all in commercial applications. I have done several multi level cast in place projects and had great luck with post tension construction. Never on grade however.
This is all in commercial applications. I have done several multi level cast in place projects and had great luck with post tension construction. Never on grade however.
Posted on 3/12/14 at 8:51 pm to 4LSU2
Couldn't you use fiber in the slab to reduce cracking? Would this add much cost?
Posted on 3/12/14 at 9:19 pm to geauxnc0308
quote:
Conventional uses more steel less concrete, post tension uses more concrete less steel
Should be the opposite. (In southern LA anyway)
Problem is, most of the time, the footings are over dug in post tension, which eliminates any potential concrete savings.
Posted on 3/12/14 at 9:39 pm to TigerDeBaiter
It looks like I could save over $1000 doing post tension if I believe the estimates on the amount of concrete saving from the PT company.
Has anyone saw of someone having foundation issues on a PT slab?
Has anyone saw of someone having foundation issues on a PT slab?
Posted on 3/12/14 at 9:44 pm to TigerDeBaiter
The shape of the PT foudation has a significant effect on the amout of ready-mix used because of the grade beam layout. A square 10,000 sqft slab will require less concrete than an irregularly shaped slab of the same size. In some cases PT is more cost effective.
Installing both types properly is key to their effectiveness. Often times, structural engineers over-design conventional foundations.
This may be bragging but, I've had design pros compliment our foundations at pre-pour inspections and say that our photos should be put in textbooks.
Installing both types properly is key to their effectiveness. Often times, structural engineers over-design conventional foundations.
This may be bragging but, I've had design pros compliment our foundations at pre-pour inspections and say that our photos should be put in textbooks.
Posted on 3/12/14 at 10:03 pm to sostan
So how would this pertain to a place like New Orleans, below sea level, requiring pilings throughout? (I'm not really sure the exact type of soil we have here)
Posted on 3/12/14 at 10:17 pm to saderade
In Lafayette area. I've used drill shafts here but I'm not familiar with New Orleans area foundation design requirements.
I do know that PT designers do not like to tie a slab to shafts, caisons / pilings. They prefer to float the slab on top of them.
I do know that PT designers do not like to tie a slab to shafts, caisons / pilings. They prefer to float the slab on top of them.
Posted on 3/13/14 at 11:08 am to sostan
quote:
sostan
Shoot me an e-mail: ziggy.td@gmail.com
Posted on 3/14/14 at 8:22 pm to StinkBait72
I don't think it is needed. I believe the foundation is well packed. However, If it ends up cheaper due to less concrete and steel, why not do it? Unless people are having foundation issues?
They are saying I will save almost 20 yds of concrete. I don't know if I can believe that though. Anybody used Brewer Construction?
They are saying I will save almost 20 yds of concrete. I don't know if I can believe that though. Anybody used Brewer Construction?
Posted on 3/14/14 at 9:14 pm to vettegc
Post-tensioned done properly should not be cheaper than conventional.
Posted on 3/14/14 at 10:10 pm to urinetrouble
quote:
Post-tensioned done properly should not be cheaper than conventional.
Why? Here are my numbers from the quotes
Conventional material quote(wire,rebar)-$4000
Concrete 160 yds * $92/yd = $14,720
Total = $18,720
Post tention material+eng(cables)-$4,800
Concrete 140 yds * $92/yd = $12,880
Total = $17,680
Posted on 3/14/14 at 10:25 pm to urinetrouble
Conventional and make sure to prepare the sub-grade correctly, expansive clays are verboten. Sand is acceptable, aggregate is wonderful, a vapor barrier on top to be a boundary directly below the new concrete.
Someone mentioned fiber reinforcement. Two general types of fiber are available to be ordered from Ready-Mixed Concrete suppliers:
Steel Fiber
Steel fiber at a dosage rate of > 25 lbs. per cubic yard of concrete (adding at least an additional $20.00/ cu. yard) would address some structural issues but this is NEVER done.
Polypropylene Fiber
Added at a dosage rate of one or so pounds per cubic yard of concrete, cost is usually an additional $6.00-$8.00 per cu. yard. It does nothing structurally at this (or any practical) dosage rate but will minimize some surface cracking while the concrete is in a plastic (wet) state. After the concrete has cured, polypropylene fiber reinforcement at this dosage rate is pret near useless. The vast majority of fiber reinforced concrete sold is poly fiber at one pound per cubic yard.
There's a hybrid of steel and poly fibers sometimes used, if the dosage rate is right, they can be helpful.
Someone mentioned fiber reinforcement. Two general types of fiber are available to be ordered from Ready-Mixed Concrete suppliers:
Steel Fiber
Steel fiber at a dosage rate of > 25 lbs. per cubic yard of concrete (adding at least an additional $20.00/ cu. yard) would address some structural issues but this is NEVER done.
Polypropylene Fiber
Added at a dosage rate of one or so pounds per cubic yard of concrete, cost is usually an additional $6.00-$8.00 per cu. yard. It does nothing structurally at this (or any practical) dosage rate but will minimize some surface cracking while the concrete is in a plastic (wet) state. After the concrete has cured, polypropylene fiber reinforcement at this dosage rate is pret near useless. The vast majority of fiber reinforced concrete sold is poly fiber at one pound per cubic yard.
There's a hybrid of steel and poly fibers sometimes used, if the dosage rate is right, they can be helpful.
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