trishatreids
Structural
We are designing a greased and sheathed salb on grade. What is the max length and what are the pitfalls?
Tek-Tips is the largest IT community on the Internet today!
Members share and learn making Tek-Tips Forums the best source of peer-reviewed technical information on the Internet!
-
Congratulations TugboatEng on being selected by the Eng-Tips community for having the most helpful posts in the forums last week. Way to Go!
What maximum length slab on grade is feasible using 12.7mm unbonded ?
- Thread starter trishatreids
- Start date
- Status
I would not use an unbonded system. In an unbonded system, shrinkage cracks are free to open by the concrete sliding along the strand.
Large floors can be constructed using bonded post-tensioning. I know of one floor which was 165 metres long. It was cast in panels, but the entire floor stressing was done after all panels were cast. The prestress level was about 2.5 to 3.0 MPa.
One of the most important things is to provide a smooth surface on which to cast the slab in order to minimise restraint.
Large floors can be constructed using bonded post-tensioning. I know of one floor which was 165 metres long. It was cast in panels, but the entire floor stressing was done after all panels were cast. The prestress level was about 2.5 to 3.0 MPa.
One of the most important things is to provide a smooth surface on which to cast the slab in order to minimise restraint.
hokie66,
Agree on your unbonded comments, but you cannot tell Americans that, even if you were originally one!
I would not recommend those lengths for a single slab. Not much if any compression will get to the centre due to the friction. I saw a European Airport pavement document on this many years ago which went into the details.
Unfortunately I cannot remember their final conclusions on lengths. I would be interesting to get recorded slab shortening and comprsession records on a slab like this one to see how much it actually shortened compared to the calculated shortening assuming P/A gets into the whole slab length.
Agree on your unbonded comments, but you cannot tell Americans that, even if you were originally one!
I would not recommend those lengths for a single slab. Not much if any compression will get to the centre due to the friction. I saw a European Airport pavement document on this many years ago which went into the details.
Unfortunately I cannot remember their final conclusions on lengths. I would be interesting to get recorded slab shortening and comprsession records on a slab like this one to see how much it actually shortened compared to the calculated shortening assuming P/A gets into the whole slab length.
rapt,
By the OP's handle, I was guessing maybe a Kiwi, so we may have some chance.
The 165 metre long slab wasn't my job, but rather was described in a VSL publication, and I think was a warehouse in Sydney. It stuck in my mind as being extraordinary. They used a working slab, then a sand layer with a bitumen spray. Maybe VSL has some records of the tensioning and shortening. I think the longest PT slab on ground I have done was about 60 metres.
By the OP's handle, I was guessing maybe a Kiwi, so we may have some chance.
The 165 metre long slab wasn't my job, but rather was described in a VSL publication, and I think was a warehouse in Sydney. It stuck in my mind as being extraordinary. They used a working slab, then a sand layer with a bitumen spray. Maybe VSL has some records of the tensioning and shortening. I think the longest PT slab on ground I have done was about 60 metres.
- Thread starter
- #5
trishatreids
Structural
Would you do it bonded then? Would you do it in one or split it into two pours?
rowingengineer
Structural
If it was me I would do it bonded, 60m max length split so max stress length was 30m. But then again I would also review the pour size in SQ because i don't like getting over 800m2 for a pour, but this is generally dictated by local conditions.
maybe worth taking a look at the end of this manual by Structural systems, for detailing of joints ect.
When in doubt, just take the next small step.
maybe worth taking a look at the end of this manual by Structural systems, for detailing of joints ect.
When in doubt, just take the next small step.
- Thread starter
- #8
trishatreids
Structural
Our slab is 100m x 50m so 5000sq.
It is on highway stabilised soil and is not taking any great loadings. We have designed to the AS/NZ code to take a truck and trailer unit. Know that you guys LOVE unbonded, but here is a question for you? Why is it only Aussie and S/Africa who only do bonded while the rest of the world is happy to be unbonded?? I am concerned about the 100m myself hence why I have been asking others opinions. I think unbonded is fine for small slabs.
It is on highway stabilised soil and is not taking any great loadings. We have designed to the AS/NZ code to take a truck and trailer unit. Know that you guys LOVE unbonded, but here is a question for you? Why is it only Aussie and S/Africa who only do bonded while the rest of the world is happy to be unbonded?? I am concerned about the 100m myself hence why I have been asking others opinions. I think unbonded is fine for small slabs.
I would talk to the concrete contractors in your area about maximum pour size. I would prefer to have no joint dividing the 50m dimension. Stress from both sides. In the other direction, if you can pour 2500 m^2 at a time, pour the centre part first, then the two ends. That way you can stress the centre from both ways, and only have 25 metres to pull up at the ends.
As to unbonded systems, I don't have any experience with them. But as stated in my first post, if you get a shrinkage crack, the slab is free to move away from the crack by sliding over the strand. Not a logical way to do it, in my mind.
As to unbonded systems, I don't have any experience with them. But as stated in my first post, if you get a shrinkage crack, the slab is free to move away from the crack by sliding over the strand. Not a logical way to do it, in my mind.
trishatreids,
I would disagree with you on unbonded/bonded locations. Currently, the only major PT country/region that I know of that uses predominately unbonded is North America.
UK, Ireland, Spain, Middle East, India, Singapore, Malaysia, Thailand, Hong Kong, Australia, NZ and much of South America are all predominately BONDED. The rest of Europe is a mix depending on country and company. many places start as unbonded as it is seen as an easy, cheap, low training, low investment way to go. They often progress to bonded after several years.
Many of these countries started unbonded and have over time moved to bonded, same as they did from Imperial to SI units.
The only one that has not progressed is North America. I have not been to South Africa for several years but when I was last there, they were also unbonded, but a very small industry at that time. Maybe they have seen the light also if they are now bonded.
I personally think 100m without a joint is too far. The friction effect is too large and unpredictable.
I would disagree with you on unbonded/bonded locations. Currently, the only major PT country/region that I know of that uses predominately unbonded is North America.
UK, Ireland, Spain, Middle East, India, Singapore, Malaysia, Thailand, Hong Kong, Australia, NZ and much of South America are all predominately BONDED. The rest of Europe is a mix depending on country and company. many places start as unbonded as it is seen as an easy, cheap, low training, low investment way to go. They often progress to bonded after several years.
Many of these countries started unbonded and have over time moved to bonded, same as they did from Imperial to SI units.
The only one that has not progressed is North America. I have not been to South Africa for several years but when I was last there, they were also unbonded, but a very small industry at that time. Maybe they have seen the light also if they are now bonded.
I personally think 100m without a joint is too far. The friction effect is too large and unpredictable.
- Status
Similar threads
- Question
- Locked
- Question
