Pre-Tensioned concrete

[Harley Monster]

Edit: I believe the proper term is prestressed concrete.

Do any of the members here have any experience with pre-tensioned concrete? Concrete that has reinforcing cables under tension when the concrete is poured?

I assume this is a substitute for using steel reinforcement. If so, is it effective and cost efficient? How would it compare to using rebar?

Is this an effective method to prevent cracking?

I plan on putting a 38,000 lb. motor home in this garage and I don't know how strong I need to the slab to be. I was told not to go thicker, 4 inches is fine, but just put more steel in it on 12" centers in the motor home bay and where I plan on having a lift and machinery.

We bought a rental, fairly new construction about 4 years ago, a tract home, and it had pre-tensioned concrete in the garage...maybe the slab too but I did not see that before the floor covering was down. In the garage there is a stamp in the concrete that warns not to cut the concrete because it is pre-tensioned. If they have done this in a tract home it must not be too expensive. I do live in earthquake country...SoCal.

Any advice?

Harley Monster

 

[Mike83]

I have no direct experience with slabs. I design prestressed girders, though. Prestressing is a very effective method to decrease or eliminate tension cracking. Shear cracking can still occur. However, it is more expensive than conventionally reinforced concrete. I don't know why you were told not to go thicker than 4". I think that a thicker slab would be a viable alternative.

I have never seen prestressed concrete cast in place. The structural element would be cast in a bed with jacks on either side/end of it to add prestress to the steel strands. After a curing time, the jacks are removed and the concrete is in complete compression - ready to go! The additional cost is a result of this step and also the increased transportation cost.

Just my two pennies - maybe others have direct experience with this scenario.

 

[Harley Monster]

Mike83...I wasn't told not to go to thicker concrete, what I was told that 4" is thick enough if I used rebar on 12" centers. They thought it would be cheaper. Probably the best is to go thicker with more rebar, but no sense in building it stronger than necessary.

 

[Harley Monster]

J...would it be possible to post larger photos or ones taken closer up? I am having a problem seeing what you are referring to.

Also, how many cables are used and how far apart are they. How do you calculate how many are needed?

 

[blkhonda1991]

Here's a picture before the pour:

The red is a sleeve that the steel cable slides in. After the concrete cures the cables are tensioned with a jack.

And yes, those are sewer cleanouts in the garage.

wouldnt that be POST tensioned concrete?

 

[Harley Monster]

J...the hi-res photos are perfect, thanks.

That slab looks like it will be pretty thick, what was it used for? How thick was it? How did the cost compare to using rebar and why did you go this route?

Just trying to learn.

 

[BooUrns!]

Are you sure you haven't confused the pre-tensioned precast modular slas with the post-tensioning system? My understanding is that the pretensioned slabs are meant to span across walls/girders and can actually break apart if they are supported incorrectly. I can't see them being used in a garage slab application unless there is an open cavity below.

 

[mikeceli]

The photo w/ cable in red sleeves........is POST Tension . My house was built that way..........12-14" thick concrete.

 

[petty4243]

I would recomend 6-8 inches of concrete with 1/2" rebar at 12"-18" centers..... when done pouring relief cuts in apx. 10'-12' squares (cuts should be about 1/4 of the depth of concrete)

once saw cuts and cured install backing rod(round foam) and then fill with SL1 caulk(brand of self leveling caulk).... note this is more important if you are in parts of the country where you recieve regular freezing... will also help prevent spauling of saw cut edges

relief cuts for crack control

 

[PAToyota]

For the most part, pre-tensioned slabs are meant to span across something. If you are pouring a slab on grade it is most often overkill. I have designed a number of truck garages and industrial facilities with 6 to 8" slab on grade reinforced with 6 to 4 gage welded wire mesh or with #4 rebar.

 

[Harley Monster]

Sounds like to be safe I should go with 6" with #4 rebar. The engineer is coming by today for his first visit, I will see what he recommends...he is both a soils and structural engineer.

 

[russlaferrera]

It's not mine, I took the photo because of the sewer clean out location. Wouldn't that be convenient for an oil change?

I would not put oil in a tank that's in the ground. EPA If there was ever a leak $$$$$$$$$$$ to have it cleaned up.