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

 

[JAE]

Reinforcement doesn't reduce the chance of cracking. It only keeps cracks more tightly closed.

75 ft. long, restrained from shrinkage at the bottom but not on the top. Sounds like cracks are going to have a party.

The curve doesn't help either as my initial reaction is that there will be some kind of radial effect on the wall as well.

Other threads here have discussed the things to do to keep concrete shrinkage to a minimum.
1. Low w/c ratio
2. Avoid lots of cement in the mix.
3. Use larger aggregates
4. cure cure cure (water cure the best)

Other approaches include Type III cement or type K.

 

[hokie66]

Agree with JAE, but I wouldn't skimp on the cement. You can't prevent cracks unless you totally remove the base restraint, which is probably not feasible, although due to the plan shape and depth of embedment, it could be possible.

I would use at least 0.6% Ag horizontal reinforcement. The most likely place for objectionable cracking would be near the ends, and would slope from the end down to the footing as the top shrinks and trys to lift the end.

 

[JAE]

My item 2 was based on the fact that high cement content mixes shrink more than low cement content mixes. Aggregates don't shrink, cement does.

 

[BAretired]

How about 5. Post tensioning?

BA

 

[Brad805]

How about adding internal waterproofing such as Kryton or Xpex? It will protect you for cracks up to 0.5mm wide. It has a huge impact on your set time, so some like the surface applied options better.

Brad

 

[Bobber1]

Also, you can pour your wall in segments, then after initial shrinkage, which is usually the most, you go back and pour the infill walls. This is done regularly in reservoirs. Construction is difficult, though.

Bob

 

[hokie66]

Rebuttal,

JAE...that is a popular theory, but I think it is misleading. The paper which follows says that shrinkage of the paste in some cases is only 20% of the concrete shrinkage.

http://www.concrete.net.au/publications/pdf/DryingShrinkage.pdf

BA...with restraint from the footing and the curve of the wall, I wouldn't like to try post-tensioning. Might do more harm than good.

Brad805,
I don't think sealing cracks is his problem. As I understand it, the wall is exposed on both sides, and the products mentioned would just tend to make any cracks more unsightly.

 

[Brad805]

hokie, I see that now that I opened the picture. Somedays using the virtual PC in Windows 7 is a nuisance. The water comment might have sent me off on a tangent too.

Brad

 

[JAE]

hokie66,
Interesting overview article. The points I'd make in reply are that they are speaking entirely of drying shrinkage. There is also autogenous shrinkage which occurs in higher cement content mixes.

Also, they don't specifically say that cement content doesn't affect shrinkage. What they are saying is that with higher strength mixes (i.e. high cement content) there can be low shrinkage results. What is implied in their assertion is that for higher cement content mixes, there is the tendency to have lower w/c ratios, which indeed reduces drying shrinkage.

What they don't say is, for a given w/c ratio, higher cement doesn't affect shrinkage. I think it does.

See the attached table which represents cement content vs. drying shrinkage. For a given w/c ratio, cement content does make a difference.

 

[BAretired]

hokie,

I am not sure about the benefit (if any) of post-tensioning, but I cannot see where the restraint of the footing or the curve of the wall would have any deleterious effect, recognizing that a curved member, prestressed at its centroid results in a uniform compression throughout its entire length and that footing restraint is similar to a prestress.

I believe that post-tensioning is a valid way of reducing cracks in a curved wall.

BA

 

[JAE]

BA,
Just a question on that - if you post-tension something, don't you wait until 75% f'c before tensioning and doesn't much of the shrinkage already happen before that? Perhaps the full opening of the cracks occurs later and the tensioning does the job. What do you think?

 

[hokie66]

JAE,
My preferred interpretation of the graph you posted is "Within the range of practicable concrete mixes, for a given amount of water, higher cement content will reduce drying shrinkage".

BA,
Leaving aside the curved member argument for a while and just assuming that the wall is straight, footing restaint is the opposite of post-tensioning. Footing restraint tries to keep the wall long, while post-tensioning wants to make it shorter. Footing restraint introduces tension, post-tensioning introduces compression. The further the prestress is from the restraint, the more effective it is. But a large amount of mild steel reinforcement in the wall just above the footing restraint will keep the cracks together.

 

[BAretired]

JAE, Some shrinkage takes place prior to post-tensioning, but the subsequent compressive stress tends to close the gaps (perhaps not completely).

hokie, you are correct, the soil would tend to resist the concrete from shrinking, so perhaps this would suggest a greater prestress at the bottom to overcome soil friction. I have not used this solution, so I suggest it only as a possibility.

BA

 

[hokie66]

JAE,
According to my references, a 12" slab would have only reached about 20% of its ultimate shrinkage after 12 months. A wall exposed on both sides might be a bit different, but not too much. The thicker the element, the slower the shrinkage.

In typical post-tenioned structures, elastic shortening due to prestress only accounts for about 7 to 10% of the total shortening. The rest is shrinkage, creep, and temperature.

 

[hokie66]

SkiisAndBikes,

Back to my first post...removing the base restraint might not be so difficult. I would check the monumental wall for overturning stability with the bottom of the wall at ground level. Due to its shape, I doubt it will turn over. If that is OK, I suggest just digging a trench to the required depth for bearing and frost, fill the trench with lean concrete, and trowel the top level. Then provide some means of making the surface as frictionless as possible, maybe just plastic sheeting, but others may have better ideas. Build your circular wall, and if it is not restrained, it won't crack.

 

[kieran1]

Wouldn't cement replacement with ground granulated blast furnace slag help. It certainly helps against early age thermal cracking

Kieran

 

[hokie66]

kieran,
That might assist in limiting shrinkage, but as the monumental wall will be polished, it could affect the desired appearance.

Your terminology, "early age thermal cracking", seems to be typical in Europe, and in most other places we call it "drying shrinkage cracking". It is essentially the same thing, cracking caused by restraint of volume change.

 

[JAE]

hokie, "early age thermal cracking" I think is the autogenous cracking I spoke of earlier. This is different than drying shrinkage cracking.

 

[dik]

and a very low slump... if it's going to be polished, then you have to be careful with the concrete batches, they will be a slightly different colour that will be amplified by the polishing.

You may want to use HDG rebar to prevent staining and still have the bond... You might be looking at .5% or greater, even 1% reinforcing steel ratio and 2" cover. Rebar spacers to achieve cover will be a problem... in particular with the polishing.

Curing will be critical and as JAE suggested, the curved surface will likely increase flexural tension cracking. Any chance you can creatively introduce a couple of control (pour) joints.

As noted, post tensioning may help, but I'm not sure how this will shake out with a curved surface. Maybe some type of shrinkage compensating concrete.

Might want to do a couple of test panels...

Dik