Formworks leaks 1

[releky]

In your practice (besides Ingenuiy), how often do you encounter formworks leak? Do you put putty all over the edge? How many contractors actually do this?

Without putty covering the leak, the bottom side edge of beams can form tiny honeycomb like in the following picture.

http://img560.imageshack.us/img560/7161/jn0p.jpg

Do you encounter it too? Is it common that you just cover it up with cement finishing? Does this occur for all formworks not totally sealed?

 

[hokie66]

With well fitted form ply and chamfers in the bottom corners, you should not get the amount of leakage shown in your picture. The mix design and amount of water in the concrete are probably also contributing factors. I haven't seen putty used to seal forms.

 

[releky]

Do all use wood for the form ply, or are there other materials used?

 

[hokie66]

Well, "ply" is just a shortened version of plywood, but other materials can be used. Metal formwork left in place is common, and in some areas, plastic forms are in use. Also, there are systems which use precast soffits, which form part of composite members. In addition to formwork, form liners are sometimes used for architecturally exposed concrete.

 

[releky]

I presume metal and plastic forms are more expensive than wood? Which material do you personally use (wood?)? You haven't encountered any leaked slurry or cement?

 

[TXStructural]

Well done formwork may leak a bit, but not much. Wood forms should have chamfer at corners and need to be oiled to avoid absorption of water out of the paste. If wood forms remain in place long after placement and the wood is not sealed well, it can desiccate the concrete.

Concrete that is not properly designed or mixed may have excess bleed water, which could carry away paste.

There are placements which use expanded mesh as forms or loosely spaced boards, and they work just fine with the right mix.

 

[releky]

Well done formwork may leak a bit, but not much. Wood forms should have chamfer at corners and need to be oiled to avoid absorption of water out of the paste. If wood forms remain in place long after placement and the wood is not sealed well, it can desiccate the concrete.

Slabs and beams bottoms needs to be shored for at least a month because concrete takes 28 days to gain strength so do you consider 28 days as long enough for wood to desiccate the concrete? And what is the context of "the wood is not sealed well"? The form works edges not sealed or the wood surface not sealed by oil?

What is the appearance of desiccated concrete?

 

[dik]

You want to 'loosen' the formwork after a day and it can remain in place after it is loosened as required. If not, the concrete may adhere to the formwork and make removal difficult and may damage formwork.

Dik

 

[hokie66]

It is rare for forms to be left in place for as long as 28 days. The usual practice is to strip and backprop a few days after casting. This time to release the forms can vary, depending on the design and the concrete strength gain, and the decision is best left to the design engineer.

 

[msquared48]

Most foundation and wall forms are pulled in 24 hours with temporary shoring applied if needed.

Flying formwork systems for high rises are a different animal though where shoring removal is usually staged over a month or so in phases as the floors below become able to support the load from above.

Mike McCann
MMC Engineering

 

[Archie264]

Msquared48,

Regarding your last point there is an interesting case study of when that principle wasn't adhered to properly. See link:

https://en.wikipedia.org/wiki/Skyline_Towers_collapse

 

[msquared48]

Interesting.

I, too, was in Alexandria from Jan to April of '73. Must have happened after I left. I was staying in a highrise then too - 18th floor of 24 if I remember correctly.
)

Mike McCann
MMC Engineering

 

[Archie264]

The article says it was March 2, 1973, if that's correct. In any case, glad you weren't near it at the time.

My understanding is that in the wake of it the governing municipality (Fairfax County) established a critical buildings department separate from their regular building department. I'm not sure what the threshold criteria is for what fits into that category but any high-rise building surely would.

 

[msquared48]

Musta been sleeping on that one.

The higrise I was staying in was on the west side of Jefferson Davis Highway, about a mile or two south of the river.

Mike McCann
MMC Engineering

 

[Archie264]

That was before internet and cable tv. Easily feasible if you were working hard, didn't get the paper, and missed the evening news. Quite a differnent world nowadays, eh?

 

[madmantrapper]

Quite a differnent world nowadays, eh?

Man that's the truth alright!