skin reinforcing

huangyhg

skin reinforcing
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
dik:
i would still use "skin" reinforcing, but make them epoxy bars.  or use a corrosion inhibtor.
jike:  any idea why?  i don't like using epoxy bars in general.
dik
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.
why should cracks form?
nature of concrete, it cracks.  
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?
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.
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
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.
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.