Hollow slab from the 50s, references? 2

[wth]

We've been asked to design an additional floor on a concrete building from the 1950s, I've never seen the slabs shown on the Architectural drawings, and I'm a bit hesitant taking this one on.

They're marked "H-Slabs 28 cm". As anyone ever seen or worked on similar slabs?
All floors have the same system, but they're not increasing the load on more than one. For that one underlying steel beams shouldn't be a problem for strengthening it. But I'm worried about the remaining lifespan of the slabs in general, as drawn the rebar cover can't be much.

 

[JAE]

Is that just a traditional hollow core precast plank system and the section you show is cut longitudinally down the span (showing one single core)?

 

[wth]

Good point about the cut direction, there is one span showing more "ribs".
But if it's a precast hollowcore slab it's not like something I've seen before, and it's from the beginning of the 1950s.
I've seen similar slabs from the same era, but without the bottom flange.

 

[jayrod12]

Yeah I don't see how they could get the void with the bottom flange in place without it being an extruded product.

 

[wth]

It's a weird one, might be inaccurate on the architectural plans. Didn't take any photos, but I'm pretty sure there wasn't any visible gaps between panels.
Could've be a done in 2 operations, but even then you'd have to leave plates under the top part.

 

[bones206]

I think it may have been cast with temporary void forms. The forms would not be full length, resulting in that one central transverse rib. This link describes it in a bit more detail:

https://hollowcore.org/historical-development-hollow-core-slabs/

 

[wth]

Thanks bones, that first image from 1912 looks very similar.

 

[JAE]

If it was cast with void forms, there would have been some means of holding the voids in place during concrete placement.
depending on weight - the void forms can, and will, tend to float upward during placement so there would be possibly some strap tie-downs which could still be visible on the bottom of the slab.

 

[bones206]

It does look from the sections that the H-slab was cast monolithically with the other framing members, so that would indicate a permanent void form. Temporary voids would only be practical for separate planks.

 

[wth]

I believe manpower were cheaper than materials at time of construction, so it might be constructed permanent void forms. Due to logistics concrete is still 150% at this location today, compared to central areas - can't imagine it was any better in 1950.

Regardless of construction method, I worry about the remaining lifespan - the plan is to convert it to apartments.

 

[bones206]

You could propose to do a load test. Are the service loads likely to increase with the new occupancy? I wouldn't worry too much about the reinforcement cover and durability unless it's been exposed to moisture/humidity for a long time and showing signs of corrosion.

 

[wth]

The service loads increase for the current roof, I have to look through former building codes for other floors and compare it to new loads - but I don't expect an increase.
The exterior walls are showing signs in limited areas of corrosion from what I've seen so far.

My main concern is carbonatation combined with the rebar cover. The building is 75 years old, and if retrofitting it should last another 50. But we can field test for it,

 

[bones206]

You could look at half-call potential mapping if there is someone in your region who has that expertise. Maybe test one area that shows the most signs of corrosion or aging, and then another typical area as a comparison. That should give you an indication of the probability of corrosion (low/medium/high/severe).