Concrete Wall - Minimum Reinforcement 3

[SkiisAndBikes]

Hello,

I have a project to design a 75ft. long, 12" thick curved wall for a cenotaph in Canada. Crack control is essential as no joints allowed and it will be an exposed, polished concrete surface. I have had a little experience with water retaining structures where minimum reinforcement ratio may be as high as 0.004, to reduce the chance of cracking. Very rough sketch attached. I would love to hear from any recommendations related to minimum reinforcement ratio for the horizontal reinforcement.

SkiisandBikes

 

[dik]

Also silica fume may reduce shrinkage... but don't know what the overall impact is... again a couple of test panels...

Dik

 

[hokie66]

JAE,
Here is another article about early age cracking. The terminology (chemical shrinkage, autogenous shrinkage, drying shrinkage, early age thermal shrinkage) is all double Dutch to me. Seems overly complicated for engineers, maybe not for concrete technicians. We know it shrinks, and if restrained, it cracks. This paper says that using higher w/c ratio will reduce autogenous shrinkage, and I find that hard to accept.

http://www.cement.org/tech/cct_cracking.asp

 

[hokie66]

Anybody have any comments about the idea in my post of 9 April, 0537?

 

[BAretired]

dik,

If tendons are located at the middle of the wall from end to end, post-tensioning will produce pure compression throughout the entire arc length (i.e. no bending). What better solution could there be to prevent cracking?

BA

 

[hokie66]

BA,
The horizontal curvature in the tendons has to be resisted. Just as vertical draping in a slab or beam balances gravity loading, horizontal draping has to be balanced by loading in the opposite direction, and that would be by bending or arch action. Without rigid supports at the ends, you don't have an arch.

 

[dik]

Thank, Hokie... I was thinking that they would want to straighten out the wall... different situation if they were bonded... like a sag and an equivalent UDL uplift...

Regarding your Apr 9 posting, the more cement powder you have the greater the shrinkage, in general.

Dik

 

[BAretired]

hokie,

A concrete cylinder prestressed with wires wrapped around the periphery is stressed in pure compression. There is no bending.

I am simply extending this to a sector of a cylinder stressed by tendons located at the centroid of the section. In such a case, I believe the stress in the concrete is pure compression...no bending.

Thus, it seems to me that post-tensioning is an ideal way to prevent cracking of the wall. I do not understand your objection.

BA

 

[hokie66]

dik,
1) Not different, whether bonded or unbonded. The force from the tendons wants to straighten the wall either way. Bonded tendons are unbonded before grouting, so impose the same forces on the wall, radially in this case.

2) I think you may be referring to my post of April 8, 2337. The graph which JAE posted April 8, 1845 supports my statement.

3) But enough of all this speculation about how to limit shrinkage. I would still like opinions on my proposed solution to the OP's problem, as stated in my post of April 9, 0537.

 

[BAretired]

hokie,

Commenting on your April 9, 0537 post...I would agree that if you are able to remove all friction between the wall and the lean concrete, there will be no resistance to shrinkage and the wall will not crack. However, in climates such as we have in central and northern Canada, it is doubtful that plastic sheeting would do the job. Perhaps there are other materials available which would work, I do not know.

BA

 

[hokie66]

Maybe you could build it on intermittent teflon slide bearings, with something like polystyrene between the bearings.

 

[jtx]

To be honest, I don't think you will avoid not have some degree of cracking (microcracking at the least). However, ACI 350 section 7.12 does require a minimum horizontal reinforcement ratio of 0.005 if movement joints are spaced 50' or greater (or in this case none at all). Also, keep an eye on the mix design and curing just as others have mentioned. I think ACI may also have recomendations on that somewhere in the Manual of Concrete Practice. Having the horizontal bars closest to the faces of the walls, as shown on your sketch, also helps.

Just a thought, maybe you can treat the concrete with Xypex to fill any cracking that may occur.

 

[BAretired]

It seems to me that the best way to prevent cracking is to prestress the wall prior to backfilling. A prestress of about 100 psi should be sufficient to prevent cracking. This can be achieved with concentric tendons placed along the center of the curved wall, anchored at each end.

BA

 

[hokie66]

BA,
I missed your post at 0017. A cylinder is a lot different than a part of a cylinder. As soon as you cut it, the hoop stress is not pure compression.

 

[Zambo]

Hope I'm not too off topic because I haven't read all of the above posts, but.......

I doubt post-tensioning will work here. As hokie mentions the wall is restrained at the base. If the base is restrained there will be no shortening while stressing, without shortening there is no prestress in the member.

The post-tensioning stressing force applied to the curved wall does not tend to cause a straightening. It acts more like the brake cable of a bicycle. Pull your brake handle and the brakes are applied, the cable (a wire in a sleeve) itself doesn't move from whatever curved shape it is in.

 

[hokie66]

Zambo,
I don't get your analogy with a brake cable. Surely a draped tendon causes deflection, therefore bending. Otherwise you wouldn't be able to balance deflections in a slab, and you wouldn't sometimes have upward deflection in bridge deck units. The only case where there is no bending is if the post-tensioning is completely around the perimeter, thus applying a uniform radial force.

 

[JAE]

If the cable is prestressed prior to concrete placement and cast fully bonded to the concrete there would be NO bending. Just axial compression.

Post-tensioned cables, where the bearing is at the ends, would cause bending.

However, it would be difficult, if not impossible, to harp the cables in place and tension on that kind of curve anyway - so a post-tensioned method would be required and there would be bending.

hokie - On your idea - I would just wonder on the practicality of doing that and ensuring that the friction would be minimial - there's a lot of weight on the base of the wall after all.

 

[Zambo]

hokie,

correct a draped cable causes an upward deflection i.e. a precamber. This is due to the tendon profile which is designed to provide the required precamber. But in this case it is as you state, the post tensioning is completely around the perimeter thus applying a uniform radial force.

Having said that you have made me think again - in fact prestressing often has that effect on me as it is not a straightforward concept.

So if you look at a "straight" post-tensioned beam the trumplates (end anchorages) will be angled upwards and then the tendon profile will result in a "drape" which will provide an upward precamber. If the end anchorages were angled downwards so that the tendon profile was a downward parabola I do not believe there would be an upward deflection - just compression.

Could be wrong and ready to be corrected.

 

[dik]

Hokie... I was thinking pre-tensioned for the bonded system, and not post-tensioned with conduits. Tensioning with conduits would be no different as you note. JAE is correct regarding the harping of the strand. You would require substantial and costly external buttresses.

Dik

 

[BAretired]

I think that the best argument against post-tensioning is the restraint at the bottom. Maybe using hokie's plastic sheeting or teflon slide bearings, the friction could be reduced to the point where post-tensioning could work.

Contrary to some views expressed above, a curved member subject only to internal prestress will have only axial force if the prestress is concentric at all sections. See the following link:

http://bridges4life.org/view_file.cfm?file=JL-97-JANUARY-FEBRUARY-10.pdf

I believe that elimination of all cracking is not possible without using control joints spaced at reasonable intervals.

BA

 

[SkiisAndBikes]

Thank you all for the interesting discussion and your suggestions. Below are some of my thoughts on moving forward with the project:
- minimizing cracks and crack width's in the exposed portion of the wall is primarily for asthetic reasons due to the proposed polished concrete finish.
- using supplementary cementing materials is frowned upon by the polished concrete industry for asthetic reasons.
- using a 0.6% reinforcement ratio with 2" concrete cover is possible (15M at 8" o/c each face).
- due to the project size, budget and local expertise in the area, I highly doubt we would be able to implement any form of post tensioning.
- I am in discussion with the landscape architect. There is a centrally located engraved granite stone being placed right in front of the wall, which if extended a few feet higher, could conceivably cover a vertical control joint at the centreline of the wall. Much easier with two 37'-6" wall sections.
- I am a huge fan of curing, curing and more curing.
- I am still pondering Hokie's comments re removing the base restraint. Overturning stability is not a significant problem with this wall.

I do not think hairline cracks will be a significant problem. Yes, me and the concrete polishers will know they are there, but to the casual observer walking by or spending time at the memorial, they likely won't even notice. Larger 1/8" or 1/4" cracks would be visually problematic as you would see them after repair. While I'm not a fan of external coatings in these types of situations, there has been some discussion by others that an external 'faux stone' coating could be applied in the event of an unsuitable result. I am recommending a test panel be created and approved, prior to construction of the wall so that the desired polished look is agreed upon by all involved.

Thank you all for your input, discussion and ideas.

skiisandbikes