Concrete Balcony Detailing Practice 3

[bookowski]

Question regarding standard practice for durability of concrete cantilever balconies per ACI318.

I am talking about cantilever cast in place concrete, with at a minimum some type of surface applied finish/waterproofing and in some cases a raised surface (pavers etc.).

My interpretation of ACI would be that this is not 'exposed to weather' based on the commentary explanation of the intent being 'direct exposure to moisture changes..... wetting, drying etc.', essentially this is similar to a waterproofed roof. This implies 3/4" cover. Based on chapter 4 exposure categories it looks like the only requirement would be 0.45 w/c ratio, no increased cover.

The above would imply that it is sufficient to use 3/4" cover, W/C <= 0.45, and air entrainment. Is this standard practice? Knowing that balconies very often have corrosion issues eventually I am tempted to increase cover as added protection. Increasing cover to say 1.5" however has a decent impact on what kind of cantilever I can achieve (due to reduced 'd') and to stay competitive I don't want to be too conservative. Another option is epoxy coated bars but I'm not a huge fan.

 

[manstrom]

vis a vis the other balcony thread, you can see how critical the connection can be due to the sudden risk of failure.

In a case like this I would use epoxy coated bars with 3/4" cover. I wouldn't completely trust the waterproofing unless it was a very permanent assembly. A spray on waterproofing will erode over time. We don't design for the best case, we design for the worst case. How will the balcony perform in 45 years?



When I am working on a problem, I never think about beauty but when I have finished, if the solution is not beautiful, I know it is wrong.

-R. Buckminster Fuller

 

[hokie66]

The requirement for top cover should be considered for individual cases, depending on location. If it is near coastal, coastal, or very humid, the cover should be greater. I think the ACI code may be deficient in this regard, but haven't read it lately. I use 25mm as a minimum, 40mm near the coast, and 45 right on the beach. The concrete grade also needs to change as you approach the beach. An issue with top steel is getting the cover you want, not too much or too little. Contractors usually take the requirement as minimum, but this can lead to too little effective depth. They need to use the correct height chairs, and this can be assured only by getting out there and checking on site.

I would not decrease any cover requirement due to the promise of benefits from a membrane.

Bottom steel, if used, also requires similar considerations for cover. Much of the "concrete cancer" spalling you see on buildings is due to inadequate cover to bottom steel.

 

[bookowski]

This is roughly in line with ACI, with your 25mm being a bit higher than the ACI 3/4". Given the performance of balconies that I've seen and tendency for corrosion issues I was wondering if people were going beyond code as standard practice (and also assuming that membranes eventually fail and therefore push you into an exposed to wetting category). My informal survey of peers so far indicates that people are not providing anything beyond code min.

 

[Trenno]

The Gold Coast here in South East Queensland (AUS) is currently going through huge concrete cancer issues. Article

Here are some photos of a place I stayed at on the Gold Coast last year.

http://i.imgur.com/0DBajRb.jpg

http://i.imgur.com/wkE53WA.jpg

 

[darthsoilsguy2]

any chance the waterproofing damage from the connection to your guardrail posts make it a moot point?

 

[bookowski]

Trenno - Great pictures. This happens all over the place. Balcony corrosion and repairs are very common in my area and I also run across lots of articles/sales pitches about repair methods but not a lot of info suggesting that the detailing be changed to minimize some of these problems.

 

[BUGGAR]

Concrete decks be damned; look at those railing connections. Note only one or two? pop rivets holding main rail in place and skimpy lateral braces on the posts.

 

[KootK]

I've been specifying 25 mm cover like most everybody else.

Given the issues with bottom side corrosion, why do we include bottom steel? It seems like omitting it would be an easy partial solution.

I've often considered specifying galvanized rebar in balconies, particularly in combination with the isokorb thermal break strips as those would provide a natural start/stop for the galvanized / black bar. Isolating the galvanized bars is still a bit of an issue but I wouldn't lose much sleep over that.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[dhengr]

Well.... here’s another interesting thread which is no
longer worth the effort to try to read and follow, what with
all the scrolling required to read every single sentence in the
thread.

If we can’t limit the size (width) of sketches and photos, so
they don’t completely screw-up the width of all the printed
part of the thread, I think this photo insertion feature should
be removed from the forum system until all the bugs are
worked out.

Who needs a 20' wide photo, other than to screw-up the works?
Why not force a photo size which doesn’t change the rest of
the thread width, for reading purposes? Then, provide a sizing
feature within that photo or sketch block which allows it to be
enlarged if the reader wishes, without messing up the width of the
rest of the thread.

 

[BAretired]

It works fine with Chrome, dhengr. It doesn't work with Firefox.

BA

 

[JStructsteel]

I use firefox, and it works fine.

 

[bookowski]

By ACI it's 3/4" which is about 19mm, so about 20% less than the 25mm you guys are using outside US. Seems deficient to me based on observation of past performance.

 

[KootK]

More cover would surely be a benefit. But then, if you're the first guy in your market to start specing 10" balcony slabs, you're sunk. Darn free market economy...

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[TXStructural]

(My browser is also doing the w i d e thing, so I have broken lines manually - I am sure that will irritate some folks.)

In general, we intend that the durability provisions for cover and concrete quality
(which are being reviewed for changes) be applied in all cases. There are those who
reduce cover when using waterproofing membranes, but it is not the preferred method.
Surface coatings and sealers provide benefits, but cannot be relied upon for safety.

Significant salt deposits can accumulate on stored and erected reinforcement before concrete placement.
This layer of salt can provide sufficient chloride ion to result in accelerated rusting of rebar.
While normal, light rust is not a problem, the presence of chlorides on the steel results in a higher potential
for corrosion. Use of epoxy-coated reinforcement solves this problem by 1) protecting the surface of the bar from salt,
and 2) acts as a dielectric to slow the flow of electrons which is required to have corrosion. Even
with sufficient cover and good concrete quality, the presence of chlorides on uncoated bar in
exposed concrete can speed initiation of corrosion.

A commonly used preventative measure is to use epoxy-coated reinforcement in exposed balconies
(top and/or bottom exposures) and developing these coated bars into the main structure. Avoid
interconnecting coated and uncoated bars (non-contact laps, coated tie wire, plastic or coated bar supports.)
Avoid sealing the underside of exposed soffits with a coating that will hold moisture.
The top surface will eventually leak and the sealed bottom will act as a bowl.

ACI 318-14 does have expanded coverage on durability. I would expect to apply
exposure class C2 within 10+ miles of a coast, and even on balconies of buildings adjacent
to highways in areas where roads get salted frequently in winter. The amount of salt
available to collect and cause corrosion in surprising. The requirements really are not that
difficult to achieve economically. Salt does accumulate on the underside of exposed
surfaces near coastlines. The cycling of moisture in these slabs drives the water and chlorides
into all sides of the concrete (in some cases, the top gets washed off by rains, but the bottom
simply accumulates salt. When the dew point exceeds the temp of the slab (almost daily at some
times of the year along the Gulf coast) condensation on the bottom drives moisture deeper
into the concrete. Thus the fallacy of using shallow cover on the bottom resulting in
some of what those photos show.

Use of concrete with low permeability (strength and W/CM ratio are indicators/surrogates for permeability)
reduces the passage of moisture and chlorides into the concrete. This is a relatively inexpensive
measure that only requires that concrete be properly specified and placed. And NEVER use lightweight
aggregate concrete in balconies.

Lastly, attend to serviceability to keep strain in concrete and reinforcement low in chloride-exposed structures.
ACI 350 (environmental structures) uses a minimum reinforcement ratio of twice that for typical 318 structures.
By limiting strain in bars supporting cantilever balconies, cracking can be kept to a minimum along the
interface between balcony and building. This helps prevent intrusion of moisture and whatever junk is
carried by it.

With respect to the use of "concrete cancer", I would point you to the real deal in places like Curacao.
I can't find any of my photos, but they actually have tumor-like growths on some remote concrete
installations. Most likely due to use of very reactive aggregates and salt water.
What you show is the result of insufficient detailing and construction practices.

 

[IDS]

As I said in the other balcony thread, we need a new approach.

Yes, specification and design could be and should be greatly improved, but:

- Cover depth is often less than specified
- Concrete quality is sometimes less than specified
- Compaction is often less than it should be
- Buildings often have little or no planned inspection over time
- Buildings often stand for far longer than their nominal design life.

So even if specification and design are improved we can be sure that there will still be some cases
of severe reinforcement corrosion in critical locations.

Yet we design for the "Ultimate" Limit State assuming the full reinforcement area with a strength
reduction factor close to 1.

Structural elements such as cantilever balconies should be designed with sufficient redundancy so
that they do not collapse if the main support system fails, and design codes should spell that out.

Doug Jenkins
Interactive Design Services

http://newtonexcelbach.wordpress.com/

 

[hokie66]

I agree with KootK that where they are not needed, bottom bars should be omitted. I also believe in the use of galvanized reinforcement.

Doug, care to share any ideas as to how to give cantilevered balconies redundancy?

 

[KootK]

Structural elements such as cantilever balconies should be designed with sufficient redundancy so that they do not collapse if the main support system fails, and design codes should spell that out.

Would that effectively mean no cantilever balconies?

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[IDS]

Doug, care to share any ideas as to how to give cantilevered balconies redundancy?

The most obvious would be to fix the parapet to the building with a connection sufficiently strong to transfer the required load.
Other possibilities are:
Props at the front corners down to the ground
Ties from the top of the parapet up to next floor level
Secondary cantilever beams together with a ductile primary cantilever.

Or if the system has no redundancy to cope with degradation, an effective monitoring and overload alarm system.


Doug Jenkins
Interactive Design Services

http://newtonexcelbach.wordpress.com/

 

[Tomfh]

Note too how much of it is happening at the drip groove.

an inch of cover minus nearly an inch of drip groove equals not very much!