Skin Reinforcing
Skin Reinforcing
(OP)
I have a large transformer base that is roughly 10' W x 15' L x 4' D.
I intend to reinforce it for flexure only (shear not an issue). I plan to use top and bottom bars, with hooks to ensure the bars are developed. I often see small 'masses' of concrete detailed with 'skin reinforcing' all faces. This includes walls with 'U' bars at the top, providing a continuity of reinforcing from the inside to outside face.
The work is in a corrosive salt environment and I've of the opinion that added rebar is just going to cause problems.
I've not been able to any reference to using skin reinforcing for this type of application, that includes pile cap design. Any suggestions about why the small masses would be have skin reinforcing?
Dik
I intend to reinforce it for flexure only (shear not an issue). I plan to use top and bottom bars, with hooks to ensure the bars are developed. I often see small 'masses' of concrete detailed with 'skin reinforcing' all faces. This includes walls with 'U' bars at the top, providing a continuity of reinforcing from the inside to outside face.
The work is in a corrosive salt environment and I've of the opinion that added rebar is just going to cause problems.
I've not been able to any reference to using skin reinforcing for this type of application, that includes pile cap design. Any suggestions about why the small masses would be have skin reinforcing?
Dik






RE: Skin Reinforcing
I would still use "skin" reinforcing, but make them epoxy bars. Or use a corrosion inhibtor.
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Dik
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or freeze thaw action) or soil related? You would I believe
specify appropriate concrete (air-entrained, minimum cement
content etc.)and could use epoxy bars for the top/exposed areas, if it is a case of surface salts. Otherwise, you may want to consider sulphate resistant concrete and epoxy
throughout. What about a precast structure or is this impractical? Jike & Kslee1000 are right and some minimum steel is required; how is the soil bearing and uniformity?
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If you are asking why use bars, I believe it is good prevention against the unpredictable nature of cracks especially in a transformer base. The recommendation to use epoxy bars and/or corrsion inhibtor is to counter the corrosive salt environment.
RE: Skin Reinforcing
I couldn't think of any cause for tensile stresses that would approach the modulus of rupture and the size is small for shrinkage stresses. I was thinking that thermal stresses from the size might be an issue, but it is likely small enough that the mass will only warm slightly.
I've seen this type of construction with face reinforcing all over and was wondering why.
Any other thoughts.
Dik
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I tended to treat pile cap as a deep beam, or slab, for which the state of stresses (non-linear) is not well understood, also localized stress concentration is likely to occur that may exceed typically assumed stress (average) used in the design. Given considerations to importance of its role in entire system, and consequences of something goes wrong, I always provide skin reinforcing to gain better sleep at night.
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This seems to be an impossible statement to believe. Would the same hold for tension reinforcement in the bottom of a concrete beam? Obviously not. Steel will take much more tension than concrete - which is the reason reinforced concrete exists at all.
What the skin reinforcement does is limits the crack widths (and therefore crack depths) and distibutes what could be a few major cracks into a network of smaller micro cracks that will be less damaging to the structure by limiting moisture penetration.
According to ACI, "for relatively deep flexural members, some reinforcement should be placed near the vertical faces of the tension zone to control cracking in the web. (See Fig. R10.6.7.) Without such auxiliary steel, the width of the cracks in the web may exceed the crack widths at the level of the flexural tension reinforcement" (R10.6.7, ACI 318-02)
While this is not specified explicitly for pile caps, it is required for beams. It does not seem to be such a stretch to believe that this would be helpful in a pile caps. ACI also has minimum horizontal and vertical reinforcement in the side faces of "deep beams" for the same reasons that are noted by kslee1000.
Depending on the exact layout of the pile cap, it might be a really good idea to have the skin reinforcement. If the loads and load distribution are more predictable, they may not be necessary.
RE: Skin Reinforcing
The Canadian code for reinforced concrete excludes skin reinforcing for foundations. They do have a requirement for skin reinforcing for deeper beams.
civilperson:
The concrete is completely buried except for the top surface. Might be that that surface could have skin reinforcing and/or flexural steel.
The other thought is, "What are the consequences of skin cracking?" Although I can't envision cracking forming, I can't see minor cracking as a problem if water is excluded from the surface.
There's no question that skin reinforcing will help minimise any cracking that forms, or distribute them more uniformly. I just don't grasp that this should be provided when considering that the mass of concrete will be exposed to chloride. I'm already using 3" concrete cover min to all rebar.
Dik
RE: Skin Reinforcing
If one were to provide the same amount of "skin reinforcement" as one provides at the bottom of a beam, then I agree, it wouldn't crack. But skin reinforcement much lighter than the reinforcement at the bottom of a beam.
If ACI thought about skin reinforcement for beams and walls, then I have to believe that they considered it in regards to footings. So, if ACI explicitly requires skin reinforcement in beams and walls, why not explicitly require it for footings and pile caps?
As I see it, the difference is this. A footing is much wider than a beam, so skin reinforcement, while effective in a beam or a wall, can't possibly be effective for a footing.
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that pile caps were involved. Still think some minimum
reinforcing is required.
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Here's on I did some time ago - about 12 feet thick and 26 feet wide - pentagon shaped on top of six drilled piers. Used vertical cages within the thickness.
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Shear can be a NON-issue if the pile cap is thick enough. See ACI 318 section 15.5.4.2.
It depends on the pile location, spacing and depth of cap.
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Dik...I essentially use "skin" reinforcement in small block foundations (3x3x2 feet up to 5x5x2')just for convenience. I see no need for it, but it helps the tie and placement process. These are reinforced for bending only...shear is not an issue. Columns are embedded, no pedestal.
I put extra cover on these for the corrosion issue....at least 3 inches all sides.
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In this depression - I've been pretty busy lately.
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On this matter I would think that AASHTO and ACI are similar in that AASHTO requires temperature and shrinkage reinforcing on a face that is "insert depth requirement here" and shall be no less than 0.125 sq. inches per foot.
Footings are an exception, perhaps because they are underground and not exposed to large temperature flucuations.
Ron/JAE - It's good to see your contributions. I hope all is well in your neck of the woods.
Regards,
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RE: Skin Reinforcing
I can't find this provision in AASHTO. Can you tell mer where it is?
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ACI 318-08 (15.5.4) requires computation of shear, the section considered with piles inside a distance of d/2 can ignore the reaction of those specific piles, CAN NOT IGNORE ALL PILES OR NOT CALCULATE SHEAR VALUES.
RE: Skin Reinforcing
However, your statement of Feb 10th above was a very broad, general catch-all statement that implied that you always have to check shear in a cap.
You even said that "interior" piles create shear and I was just noting that in some cases that isn't true.
In the picture I posted above, the "column" on top of the pile cap was over 15 feet across (compared to a 23 foot cap) and 12 feet deep.
So ALL my piers (piles) were within the given limit...thus, section 15.5.4.2 indicated that none these piers develop shear in the cap.
Despite that, we still added vertical stirrups within the cap simply based on our gut feel for providing some reinforcing within the mass concrete.
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I'm still not really convinced why the face reinforcing is necessary; I'm not sure, other than thermal and shrinkage issues, why there would sufficient tension in the face.
Dik
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Sometimes engineers do not soly act by conviction, but to the lack of certainty. (Many times concrete deterioration/defect is caused by intrusion of foreign substances through fine, but wide enough, cracks.)
Strickly to your original question on the need, or not, to provide "skin reinforcing" on mass footing elements, to me, it is a question on "certainty" that is to be judged by the designer/engineer.
RE: Skin Reinforcing
Regards,
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RE: Skin Reinforcing
I just can't find it. Could you please reference the Code Revision and the article number. The closest thing I find in the LRFD 3rd Edition is Article 5.10.8.2, where As≥0.11Ag/fy.
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You're point's well taken; and I concur to a large extent. I like to have a real understanding of things and then usually act accordingly. I have greater problems with things that I don't really understand the reasoning. I have great problems, sometimes, with "it's always been done that way" without understanding why... I'm now delving into thermal stresses to gain a better understanding.
Dik
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Well said.
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Thanks for your response. As mentioned before, "certainty" is more of a judgement call (against potential odds which may or may not occur), as opposed to "conviction", which is based on known fact(s), and/or knowledge(s). I applaud you for going through that route.
Personally I think, skin reinforcing would not be that important if the problem is merely the concern over temp. & shrinkage effect, which can be minimized significantly by exert controls over concrete mix and placement, curing method and duration. However, if either water, or chemical, or both are present, without reinforcing, the fine surface cracks may widen and get deeper into the concrete mass, which could leads to spalling and splitting of significant amount of the concrete.
For your case - pile cap/foundation for transformer, here is one more odds to be watch out for. I recall transformer spill is a hot topic, for which the acidic fluid with high temperature is quite harmful to the concrete, and not easy to clean up. Please consult with a mechanical engineer, or the supplier on this matter.
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Earlier transformers used PCB's for coolant/dielectric which over a period of time converted into toxins. I'm not aware of any corrosive effect and corrosion is not an issue for this condition.
On a recent project used a material called Sorbweb; this is a product that allows water to pass, but in the event of a spill, it dissolves and forms a liquid tignt membrane.
Dik
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Dik
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Dik