CMU FLOORS POST TENSION

[mlevario99]

Good morning. About a week ago we were contact to go look at some apartments. The apartments are a two story cmu walls. The floors are also cmu, they use some kind of post tension system, I am posting a picture. Well, the problem is, at the end of the cantilever the rods and anchor plates are corroded. I was trying to tell the owner that we do not know how far the steel rods are corroded, that it is dangerous. He came back and told us that he had removed some of the grout near the wall and it appears the steel is not corroded at the face of the wall. He wants to remove the cantilever and reattach the tension rods to and anchor plate. Have any of you guys have done something similar. I do not want to touch this job, but is something interesting to know, how where they designed, if any ones has knowledge of this type of construction please provide some input.

Thanks

 

[JedClampett]

Are you sure those aren't some kind of hollow core slabs?
I can't say there's never been any prestressed CMU floors, but I've never run into them.

 

[RHTPE]

Looks to me that Jed may be correct.


Ralph
Structures Consulting
Northeast USA

 

[mlevario99]

Actually that is what I thought at first, but there are some blocks that already broke, and you can see the cmu block, see this picture.

 

[RHTPE]

What a unique way to construct a floor system!

I'd find the engineer who devised this method and ask him/her how they anticipated the means to repair/alter the floor structure.

We always learn from methods that do not work well. Perhaps this engineer needs a crash course in repair methods.


Ralph
Structures Consulting
Northeast USA

 

[Jerehmy]

I looked at that last photograph you posted. I see there are different color stains at regular intervals, but when I zoom in at the where I would expect to see mortar, I don't see any. Maybe its the photo, maybe the it blends in real well, but I just don't see any breaks between blocks beside the color difference.

 

[mlevario99]

Jerehmy, when I first look at the floor, I was sure it was come kind of coreslab, but 16 inches wide, then when I saw closer it looked more like cmu, on the previous picture, the material looks like a cmu, it does not look like actual concrete, look at the attached pic, you can see the blocks.

I am not sure when was designed, and sure do not know who was the EOR, the owner does not have any drawings. I do not know how old is the building.

 

[port125]

The block looks like it was specifically created to be tensioned.(Each block has a hole in it to allow for a tensioning rod). The picture reminded me of an article I read a while back in structure magazine, Link The slab looks like the picture in this article of the NABCO Dox Plank system. Its interesting that this type of system had been standardized. It seems like the shoring during construction would have to be elaborate.

 

[msquared48]

Looks like a Block and Beam system to me, perhaps this, although I do not see the steel joists in the pictures. Perhaps a local variation of it...

See below... Also see the ICBO archive for ICBO Report 2692 from December of 1974.

Mike McCann
MMC Engineering

 

[Jerehmy]

Yes I can see it better in the 3rd photo you posted. I took your word for it anyways you were there, not me. Very peculiar but I guess it works in theory. It just seems impractical I wonder why they did it.

 

[msquared48]

...Trying to use CMU in a different application to try to create a market if it proved to be successful... ?

Now where have I heard that logic before?

Mike McCann
MMC Engineering

 

[paddingtongreen]

The cantilever should have reinforcing in the top holes but doesn't!

The rods in the bottom (at the sides) must be just to pull it together but any tension in them would be counterproductive to say the least. If that is their purpose, it looks like an over complication use to attach the handrailing system to them.

We are seeing the cantilever, not the slab itself, I wonder what that looks like.

Where is this job?

Michael.
"Science adjusts its views based on what's observed. Faith is the denial of observation so that belief can be preserved." ~ Tim Minchin

 

[Canuck67]

Attached is some old product literature for the Rapidex Segmental Precast Concrete Structural Slab System. Not quite the same as your photos, but close. This particular system is made with autoclaved concrete and the reinforcement appears to be grouted.

I have only looked at a couple of buildings using this structural system. Both were built in the mid to late 1970's.

 

[msquared48]

Paddington:

In the second photo, there is a rusted bolt end and bearing plate implying that the bolt could extend into the top portion of the center rib of a CMU unit.

Mike McCann
MMC Engineering

 

[tony1851]

May seem a dumb question, but could the steel strip at the front - which is clearly welded to the vertical square tube of the guard rail - be anything to do with support? If it's only two floors, maybe the vertical steel members are providing some support, and perhaps it is not a true cantilever?

 

[msquared48]

Possible, but I doubt it considering the size of the plate, unless the handrail verticals frame to structure above or below the deck surface and serve as supporting columns for the end plate.

Nevertheless, the Devil's in the details...

Mike McCann
MMC Engineering

 

[mlevario99]

The roof is made the same way, the cmu floor extends into to the room, I did not measure the actual distance, but in the middle of the roof there is steel beam supporting past of the floor. There is a top rod like every 48 inches, and the two rods as seen on the picture. The reason I was asking is as you saw on the pictures, it is deteriorating, and the owner wans to fix it, the ide was to remove the canteliver but in order to do that, you need to shore the floor (canteliver and inside room), then you need to make the thread to the tension rods, but I think is complicated and dangerous, so I am trying to stay away from it.

 

[mlevario99]

This job is in Las Cruces, NM, near New Mexico State University. I am not sure how old is this apartment, but is probably the 1970. The end steel plate is to allow the nut to the threaded and create the tension on the steel rods.

 

[Ingenuity]

In addition to the pair of threaded rods at each block joint, there appears to be a single rod and plate on every 3rd or 4th block that was possibly intended to handle "some" negative moments.

I doubt the system was "post-tensioned" in the sense of active pre-compression given that the threaded rod would have to be of high-strength to overcome concrete creep losses, and the magnitude of any prestress would be negligible given the very small rod sectional area and the small end-bearing "anchorage" plates.

What is the magnitude of the main span?

Is there any significant deflection in the main span?

In the topping slab non bonded to the blocks? because there appears to be a layer of some material at the exposed edge under the topping.

 

[concretemasonry]

As everyone sees, there are many different block flooring and roofing systems. - I think there must be over 100 in the world. Most are based on German systems (lack of international patent protection) in most countries.

Some are just floor filler systems that use either small prestressed beams or bar joists at about 16" o.c. because of the size of the filler elements (concrete block or clay tile)

Some of the DOX block systems (and varients) used block that were precision ground on 2 faces for complete bearing as part of the concrete unit manufacturing process. The DOX block were delivered to the site or block were stressed into a 16" wide beam and delivered and erected with a crane.

As you see there are any variations with different block, details and methods of construction. Most were prestressed.

How about the one and two story homes in some of Europe that had interior floors using a floor filler section with a 2" topping. Some even used small prestressed beams (made on 400' long beds, and cut to the desired lengths to achieve a sloped roof with clay or block infill. - The frown on wood for housing in many areas. In MI in about 1950, there was a store built using roof block trusses for longer spans. There seems to be no end of the "unique" systems around the world.

The project shown in the OP is probably some sort of a hybrid that was used by a local engineer working with a local supplier.

Dick

Engineer and international traveler interested in construction techniques, problems and proper design.