Skin Reinforcing 1

[dik]

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

 

[jike]

dik:

I would still use "skin" reinforcing, but make them epoxy bars. Or use a corrosion inhibtor.

 

[dik]

Jike: Any idea why? I don't like using epoxy bars in general.

Dik

 

[kslee1000]

The valid reason is to prevent surface(random) cracks to form and deeply penetrating the inner mass that would create paths for water and other substances to enter. You know the results from such attacks.

 

[miecz]

Why should cracks form?

 

[kslee1000]

Nature of concrete, it cracks.

 

[beton1]

Is the corrosion surface related (ie. salt to control icing
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?

 

[jike]

dik:

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.

 

[dik]

The mass concrete is supported on six friction piles spaced about 6' apart and the load is approx 160 Kips. I was planning on reinforcing it for flexure only as I've done with numerous pile caps. I haven't checked it, but the cap may work as plain concrete.

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

 

[enginerding]

ACI requires skin reinforcement on the tension half of beams greater than 36" deep. Maybe the designers are applying this requirement whether or not it is directly applicable - applying the principle. See 10.6.7 in ACI 318-02.

 

[jike]

I look at predicting stresses in foundations and pilecaps as potentially very inexact! One or more pile may not be seated properly (bearing pile) or in a softer layer (friction pile) which could produce stresses in the cap that you did not anticipate therefore I try to be always conservative in these cases.

 

[kslee1000]

Agree with Jike.

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.

 

[jike]

Often, the actual location of the piles may be quite a bit different than the drawings indicate.

 

[civilperson]

If the side face will be visible upon completion, use "skin" reinforcement to minimize the size and visibility of the cracks, (if buried then ignore). The mass pour may be successful in producing no thermal cracks by using a temperature probe in the center of mass and keeping the exterior face near that same temperature by heating or using insulating blankets.

 

[miecz]

I don't see skin friction helping the situation. If there is enough tension in the concrete to overcome the modulus of rupture, then that magnitude of tension will overwhelm any skin reinforcement. The reinforcement will yield, the concrete will crack, the steel will rust and spall the concrete. Are there any publications that call for skin reinforcement in pile caps? I can't find it in the CRSI Handbook. By the way, dik, you may want to take a look at slideruleera's foundation page.

http://www.slideruleera.net/FoundationsForCompressors.pdf

 

[kslee1000]

what is the stress required to cause a bar to "yield", say #3 (M-10)?

 

[enginerding]

miecz - "If there is enough tension in the concrete to overcome the modulus of rupture, then that magnitude of tension will overwhelm any skin reinforcement"

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.

 

[dik]

enginerding:
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

 

[miecz]

enginerding-

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.

 

[beton1]

Sorry, it was not apparent to me from the initial question
that pile caps were involved. Still think some minimum
reinforcing is required.