Slab Foundations: Rebar vs Post Tension (my Engineer’s Opinions)

Matt is with his engineer Witt Smith to talk about the differences between conventional and post-tension slab reinforcement. They review the theory behind both styles as well as the construction methods that make these two systems unique. This all takes place on the Risinger Build site which is an ongoing series available at BuildShow.com under the Originals tab. Be sure to follow along to learn more about this project and get more information on the decisions behind this slab construction which includes an original and new slab.
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[@mrsmegz]

I just poured my foundation back in January for my house here in Texas about 90 minutes from Austin. The house is next to a Creek in the engineer suggested we do post tension, and then drill 8 ft x1ft piers every 4 ft around the exterior beams. It's pretty interesting to see how the cables lay down deep in the bottom of the beams, under the plumbing and attach then live and dead ends about a foot below the brickledge. They then pull 40,000 lbs of tension on each cable. They are essentially a lifting Force on the beams in the middle of the foundation.

[@wakeupmofoers691]

thanks

[@THANATOSIXU]

What scares me is renovations or tensioning issues down the line. Also, having to have an engineer for repairs or renovations just to drill a hole in the slab sounds like a MAJOR PAIN, not to mention expensive.

[@davelindgren5245]

I have a post tension foundation in the San Diego area. Not sure if it was required because of the soil conditions in my area. It is a pain in the butt. Every time I think about doing a remodel on the first floor, the dollar signs go through my head on how to deal with it.

[@-BlackBird7]

All of my past houses where conventional rebar and all of them had several cracks some larger than I would like (all concrete cracks) Now I live in a house with a post tension slab with beams like in this video - there is not a crack anywhere. So I would guess post tension is better unless it fails of coarse.

[@alias177]

now you have to advertise for World of Warships, USS TX close by :)

[@philipvecchio3292]

This is a great look at slabs. New York State doesn't have many, but it's not completely unheard of.

[@HomesBuiltRight]

Post-tensioned does work well with expansive soils. I’d use rebar on stable / rocky soils. I did a video explaining the post tensioning thing. Post Tensioned Foundations - The Mystery Solved (How They Work) kzbin.info/www/bejne/iKClZGOmj75pi9U

[@MikeButton-s7d]

check with your bank before deciding to use post tensioned concrete for residential construction. For Calgary condos as an example you can not get a conventional mortgage for a unit in a post-tensioned building, it has to be "insured" with the extra fees involved

[@richardlittleton5705]

I like having opinions on topics I'm knowledgeable about.

[@LouieGrind]

When doing reinforced slabs, if you just do a proper job with the grading, you dig out all of the bio-mass, fill it with gravel, compact it and design your slab to not have any long term settlement and limit the crack width of the concrete with sufficient reinforcement, you will: 1. Never have to do these unnecessary "battleship beams", 2. You won't have a single crack in your floor, 3. The drainage will sort its self out because of the gravel, 4. You can use less concrete and do a much easier structure/form work, 5. It will be a cheaper slab. IMHO these slabs just seems to be a way to cop out of doing a proper geological survey, a proper preparation of the ground on the site and a way for the builder to charge more because it's way more work to prepare/form. In Sweden we never accept any settlement for buildings which makes this entire "the house should move like a ship" completely redundant. If I would've designed this slab I would demand a geological survey and proper preparation of the ground on site and just make a strip footing under load bearing walls big enough to achieve the ULS and SLS load (Pa) capacity of the ground. Everywhere else it would be either 100-150mm of concrete depending on loads and if the slab has integrated floor heating or not.

[@markvl11]

what about cellar walls? Is rebar still the best way to reinforce the walls or can post tension work there?

[@mostlyH20]

Do you grout the steel wire cable ducts in PT slabs. Curious if this is done in America?

[@Antonioprds]

Not in America

[@sense1510]

We did grout the tubes on new bay bridge project.

[@patrickkenny2077]

Isn't the big advantage to a post-tensioned slab that you get the same strength with a thinner slab, or is that just for suspended slabs?

[@LouieGrind]

Not necessarily. If you build on top of sand or any other stable soil it doesn't make much difference. Where it does make a difference would be on soil that's old river and/or sea bed, or any other settlement prone soils like clay, but then I would argue that doing a slab on top of the ground is the wrong way to go because of the settlements. Granted you could pre load the site with weights to take out the bulk of the settlements beforehand but then it wouldn't be necessary to have a post tensioned slab anyway.

[@DrJayToob]

Which system is best in Texas where your foundation perimeter can dry out if not watered/monitored regularly?

[@judefuselier]

Post tension is effective in riverine areas, i.e. in proximity to the Mississippi river, cause piles create route for water migration.

[@oldtimefarmboy617]

How about a slab with rebar and post tension cables. Maybe the best of both.

[@AndreyK415]

Would like to see some videos on non-slab foundations in seismic zones like California preferably with seismic isolators in residential (home) construction.

[@Patty-qh3lm]

Rebar for the win! No mussing, no fussing!!!

[@Isaacmantx]

Just switch to fiberglass or basalt fiber rebar to REALLY extend the life of that slab.

[@SmithintheBuilding1984]

I didn’t agree with the slap Will 😂. Great informational video! 🤌🏾

[@johnnichols2710]

I would use a pier and grade beam instead and have a crawl space. But thats my opinion and of course the labor would be free since I would be doing the work

[@mankind5709]

Post tension is very strong , very dangerous if you need to cut those rebars for any reason after slab cures , the slab can explode due to high tension. If you never touch it , no big deal.

[@rheuss1]

That’s an urban legend. I’ve never seen a tension cable blow a slab out. They’ll break right down the sleeve.

[@mankind5709]

@@rheuss1 rebar under tension is very dangerous.

[@rheuss1]

@@mankind5709 you’re taking cables not rebar I Assume. Cables usually don’t fail suddenly even if cut. They lose strands and as they do the pressure is released so that by the time the last pair of strands are cut through the pressure on them is not going to cause slab blow out.

[@Manohman-fx2jq]

Thank you for the clarification from your experience. good to know.

[@prepperdan]

Engineer the soil prior to slab for best performance

[@brucelance9766]

Post tension compression literally makes the concrete stronger. I’ve seen a Shapell home in the SF Bay area that the homeowner was adding a kitchen island and the hack remodel contractor jackhammered into one of the tensioned cables. Lucky he wasn’t killed. The cable pealed back 3’ of concrete floor flinging chunks through the ceiling.

[@andrewschafer7790]

Won’t the result of post tension be that the wires are put in tension and the slab in compression? That confused me.

[@billj5645]

The problem with PT is many engineers treat it like magic and don't put as much care into designing the slab. The design methods for PT slabs are much less conservative than the design methods for conventionally reinforced slab so you end up with less concrete and less reinforcing. To some extent the compression induced by PT reinforcing offsets the reduced amount of concrete but I think the PT slab can still be less stiff. There is no replacement for a well thought out layout of stiffening beams and extra care at projecting elements such as designs with breakfast nooks.

[@hopjo6356]

집기초에 이렇게 할필요가 있나? 철근 콘크리트가 중요함 그리고 여기에서 나온 철근보니 유리섬유 같은데? 철근 더넣어서 하며는 좋음

[@P_RO_]

The big problem with post-tensioning is a complete and instant loss of strength should it ever fail- and failure has been known to happen. If the soil engineering and slab design are good, a rebar-reinforced slab will perform very well and any failures will be gradual instead, allowing time for rectification. Alterations to tensioned concrete requires each one to be engineered, but with rebar slabs you can follow some general rules and guidelines with the alterations and bypass that extra engineering without problems. It's not that you can't do better than rebar; it's that you almost never really need to so why bother?

[@wakeupmofoers691]

in my cali exp there was graded repack ground and dig down bearing points slab pours not tensioned rebar grid throughout and specic concrete batch mix depending on aplications height load ob etc ive not seen any of that around my fave buiders chanels, like you describe the sole shoe of a house is primordial

[@matthunting1162]

There are only 2 types of concrete, concrete that has cracked and concrete that hasn't cracked yet.

[@David-bs7wy]

San diego cap ?

[@wakeupmofoers691]

lol ex pat

[@WolfBlauen]

There are many myths out there regarding concrete. The one I find most ridiculous is the saying that all concrete foundations will crack. If they do, then they haven't been engineered correctly for the site conditions. And ANY single layer reinforced slab will crack because the only thing either tension cables or a single rebar layer does is to provide a hinge. The idea behind proper steel reinforcement of concrete is to eliminate the concrete from being under tension in any application of force, either up or down. this can only efficiently be achieved by applying two layers of reinforcement. one close to the top and one near the bottom of the foundation. The larger the slab is, the more important is the application of a second layer of rebar. Foundations on instable soil need to be treated like free carrying floors in multi story buildings. If you apply this principle, you'll never have a problem with any foundation slab cracking. The downside is that it costs more and people cheap out on the most important part of the build. Especially track home builders.

[@ZimorKilled]

Every concrete slab is going to crack. Concrete has allot of moisture, and it dehydrates over time ... concrete is NOT flexible. Rebar is flexible. Thats why concrete and rebar and soil conditioning work hand-in-hand to effectively support a structure - a good engineer applies the proper formula for a long-lasting (30+ years) foundation.

[@colinstu]

There's two kinds of concrete slabs. The kind that have cracked and the ones that will.

[@bhbaker220]

No mention of soils study and clay content in video. Rather strange to me but wtf do I know.

[@hinesenpfefferfarms]

I second that.

[@ZimorKilled]

Unfortunately, steel wiring (aka cables) do have tension and stress relaxation over time. This affects the effectiveness of the post-tension foundations. Over time, the foundation becomes uneven and problems start to occur. Remedy for this is too re-stress the cable every 5-10 years, which is expensive. Otherwise, home owner will need to perform jacking and install piers - also expensive. A post-tension foundation is a short-lived cost effective solution to high-price home builds, and thats why they are sold to homeowners. The Builders and Engineer are long gone when a post-tension foundation starts to have problems. I am an engineer, and when we built our home in The Woodlands 3 years ago, we instructed our structural engineer to design a "commercial grade" foundation: concrete minimum 4200 psi, deep beams, 12" OC rebar, one size larger rebar, minimum 5" thick concrete flooring, including in the garage. You get what you pay for, and we expect to have no problems in the future. BTW, we also over-design our framing, and expect to be the only house in neighborhood to be left standing after all others are blown-down. Listen to Matt ... we implemented allot of his recommendations in our home-build.

[@GilmerJohn]

Post tension means you have long lengths of essentially unprotected and un-inspectable steel that are critical to your structure surviving. Failures might well be sudden and unexpected.

[@ferdafootie1338]

First -this is sick btw

[@tpep1693]

I don't consider those "foundations", just "slabs".

[@roypaulcarter4654]

No to both. Covering all your utilities with concrete is never a good idea. A repair could cost you 50 grand. For the long run it’s a bad idea. I concrete grid foundation with holes for running the utilities though would be my choice over a slab of any kind.

[@Mostviews111]

I will never own a home with slab on grade I dont get why people want it.

[@idealchanful]

You may not have the choice if you build in sistemic zones

[@robzilla69]

I would never want a crawl space that is a magnet for rodents and other critters. They always find a way in!

[@Mostviews111]

@robzilla69 shows your ignorance, how is a crawl space any different than your normal living space?

[@robzilla69]

​@@Mostviews111 OH it's very different. But if you had dealt with one tenth the crawl space infestations that I have you would know that. Yeah I'm ignorant! 🤣

[@anonymous..-]

My ex-wife gave me post tension.

[@seanm3226]

She gave me post tension as well.

[@anonymous..-]

Oh, hi Steve.

[@imtheonevanhalen1557]

Pre-tension is the best

[@wakeupmofoers691]

a head ache may aswell ?