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dowelling extension of existing concrete tred

dowelling extension of existing concrete tred

dowelling extension of existing concrete tred

(OP)
If I extend the tread of an exterior step with a new 3" wide x 6" deep piece of concrete, dowelled to the face of the existing 6" deep riser, using Hilti threaded rod dowels 48" on centres, and provide a 15M (#5) horizontal bar, will there be shrinkage cracks formed due to the new concrete being restrained against shrinkage by the old concrete and dowels? If so, is the 15M sufficient to control cracking? The step is 70 feet long. It will be wetted after any rain, and I am worried that the cracks, even if hairline, will become visible after each rain, as they are the last area of concrete to dry out. I posted this question some time ago, but did not get a response.

I have again tried to attach my sketch, but it still does not seem to attach right since I cannot open it after attaching it. I don't know why. Maybe it is because I am using Windows 8.1. Has anyone successfully attached and opened a file to this system, using Windows 8.1? I cannot understand why I can't attached a file. I use to do it without problem before I had Windows 8.1.

RE: dowelling extension of existing concrete tred

The attachment works.

That's going to be a tough detail to make durable, no matter how you slice it. For controlling shrinkage cracks at interfaces with new concrete, I've had decent results with 2X T&S. Of course, I've also had very bad results with nosing bars in general (spalling). You don't really have a choice here, however. I've used FRP nosing bars in the past but SS ought to be as good.

Some radical ideas, 'cause that's what we do here:

1) Could you demo a wedge 11.5" into that existing step so that the reconstructed step has a bit more meat to it?

2) I worry that water will get between the pavers and your new step with all manner of freeze thaw nightmares ensuing. Could you construct a 1/2" gap between the paver and the step? Or, conversely, run the new step through the pavers?

The greatest trick that bond stress ever pulled was convincing the world it didn't exist.

RE: dowelling extension of existing concrete tred

(OP)
Kootk:

Normally we do not use nosing bars at all in stair flights because they do little and will corrode unless given large cover to both vertical and horizontal faces exposed to corrosive chlorfides. However in this particular case, I need the bar to control crack width, but I am making it stainless steel so I don't see it as being a significant problem.

Cutting 11.5" into the existing would be very much more expensive ... this is 70 feet long, and another 35 feet at each of 2 other steps for a total of 140 feet.

The calculations for 150 pounds per square foot vertical service (unfactored) load show that the dowels have a utilization ratio (V/Vr + T/ Tr) of only 0.20 based on 1.75" embedment, if the Hilti design manual is to be believed. I cannot see why you need more "bite" into the concrete, unless there is some other force for which I have not accounted, or I have made a calculation error. I have sent it to Hilti for their comment last week but have not yet heard back. I have assumed that the emdedment into the new concrete need not exceed the embedment of the adhesive anchor into the existing concrete. i.e 1.75" (although I called for 2.25" into the existing concrete to allow for field tolerances).

If I extend the concrete down to the bottom of the pavers, then water can get trapped there, whereas if it is only to the top of the pavers, then the sloped surface of the pavers can direct ater away from under the new concrete. Bug I do agree this is one weak aspect. I suspect though that in reality it will be an issue of settlement more than heave, because the soil is not the greatest here.

Can you help me with what 2X T&S means? Thanks



RE: dowelling extension of existing concrete tred

I believe he means if you double the required temperature and shrinkage steel at the interface he has had good results (2*[T&S])

RE: dowelling extension of existing concrete tred

@jayrod: you're the next best thing to Google Translate. Bang on.

@ajk: all of my comments pertained to durability. I think that strength is essentially irrelevant here. My comment regarding "more meat" was about improving aesthetics and durability. Something like the sketch below.

I worry that your new concrete will tend to shrink away from the existing as it will be restrained from shrinking into the existing concrete by the dowels. Water may get in that slight vertical joint and start frost jacking away. Maybe I'm just being paranoid, who knows.

I think that the key to your solution will be managing owner expectations. Your solution will be economical, visually apparent, and perhaps not all that long lived. As long as the owner is prepped for that, you should be good to go.

At minimum, I would change your 48" dowel spacing to something like 12".

The greatest trick that bond stress ever pulled was convincing the world it didn't exist.

RE: dowelling extension of existing concrete tred

I agree with KootK on the dowel spacing.

RE: dowelling extension of existing concrete tred

(OP)
yes I agree too about the dowel spacing now that I think about it further. Thanks.
To Kootk - that was another option, but is likely about 3 or 4 times more expensive. Doesn't that have an even bigger shrinkage concern with respect to opening of the joint and allowing water ingress and ice jacking, since that has a 14.5" width of concrete to shrink, rather than a 3" width to shrink, although the joint may be in a less visible location.

Assuming a long-term shrinkage of 800 microstrain, the joint opening should be much lass than 0.0024" over 3 inches. By comparison, a hair is 0.004" and the Code allows a max crack width of 0.013" for exterior exposure. Do you think that a joint that opens a maximum of 1/6th of the width of a hair is a problem? You guys are doing a good job of scaring me

RE: dowelling extension of existing concrete tred

Quote (ahk)

Doesn't that have an even bigger shrinkage concern with respect to opening of the joint and allowing water ingress and ice jacking, since that has a 14.5" width of concrete to shrink, rather than a 3" width to shrink, although the joint may be in a less visible location.

Maybe. My thinking was three fold:

1) Put the problematic joint where it's less visible.
2) Horizontal friction on the interface should promote more small cracks rather than one big one.
3) With a more secure feeling joint, I worry about frost action a little less.

Quote (Assuming a long-term shrinkage of 800 microstrain, the joint opening should be much lass than 0.0024" over 3 inches. By comparison, a hair is 0.004" and the Code allows a max crack width of 0.013" for exterior exposure. Do you think that a joint that opens a maximum of 1/6th of the width of a hair is a problem?)


Probably not. Based on your numbers, maybe shrinkage isn't the issue. I imagine that your little blob of concrete will tend to rotate and ratchet a tiny bit each of the thousands of times that the stair nosing gets stomped on. Over time, will that add up to a crack that will draw water in by capillary action? For this reason, perhaps it would be prudent to alternate your dowels top and bottom. Improve that lever arm a bit.

Landscaping BS in cold climates is one of my least favourite parts of the job. Boring, often intractable, and a magnet for client dissatisfaction and liability issues.

The greatest trick that bond stress ever pulled was convincing the world it didn't exist.

RE: dowelling extension of existing concrete tred

(OP)
ok, thanks. I will take your comments and ideas to heart and think carefully about it all.

RE: dowelling extension of existing concrete tred

Quote (KootK)

Landscaping BS in cold climates is one of my least favourite parts of the job. Boring, often intractable, and a magnet for client dissatisfaction and liability issues.

100% agree. The only thing worse than building architects are landscape architects in my eyes. And because it's not as tightly regulated (nor generally a life safety risk) they get away with using substandard structural designs and expect us to stamp them. Never a ultimate failure, but 9 times out of 10 they fail in serviceability at one point in their design life.

RE: dowelling extension of existing concrete tred

(OP)
Generally True. But there are structural engineers too who are not well versed in durability issues, and who do not know how to use LIFE 360 to assess the life in corrosive environments, unfortunately. Another structural engineer that the architect has now retained, dealing with the same issue, has proposed that just black steel be used, not stainless. I think that would be a mistake. (Notice that I do not mention epoxy coated bars because they are not as good at controlling crack widths (see Mitchell report of a number of years ago) and they are not as good at protecting against corrosion (MTO no longer uses thm in their barrier walls - they use stainless steel).

I notice that no one to-date has responded to my original question that I posted (please see the first item in this string). Does anyone have a response to that question about transverse cracking visibility (not longitudinal) and the 15M (#5) longitudinal bar shown? The tread is 70 feet long.

RE: dowelling extension of existing concrete tred

Quote (ajk)

Does anyone have a response to that question about transverse cracking visibility (not longitudinal) and the 15M (#5) longitudinal bar shown? The tread is 70 feet long.

This is what I was attempting to address with the 2X Temp & Shrink business.

The greatest trick that bond stress ever pulled was convincing the world it didn't exist.

RE: dowelling extension of existing concrete tred

(OP)
I see. The single 15M is a steel ratio of 0.017 that is [200 / (80 x 150)], which is much more than the 0.0018 shrinkage and temperature minimum. But I still wonder whether any cracks, irrespective of how fine they may be, will be disconcertingly visible as they are drying out after a rain. This is a grand main entrance walkway to the building.
Have you experience in that issue? I know that we locate cracks by wetting a surface down and examining after it is surface dry -- that makes the cracks stand out, as they dry more slowly and appear as dark lines.

RE: dowelling extension of existing concrete tred

I don't have much experience with that. I read an article a while back by Bruce Supernatant that dealt with restraint shrinkage cracks. That's where I've got the 2X T&S recommendation for highly restrained systems from. I've been using it for a few years now but can't say that I've had an opportunity to evaluate it in long term service.

The greatest trick that bond stress ever pulled was convincing the world it didn't exist.

RE: dowelling extension of existing concrete tred

(OP)
To Kootk, thanks very much. Much appreciated.

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