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rebar in mass concrete
how much rebar is needed in 15'x15'x10' deep mass concrete footing. it is carrying a 125' high chimney stack on top. do i only need minimum temp & shrinkage rebar mats located at top & bottom of 10' deep footing, or need more rebar mats thru out fooring depth? aci does not seem to indicate rebar requiremnents for "mass" concrete?
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only min. reinforcement will not be sufficient. pay attention to amount of heat of hydration, generated in the early age of concrete. this phenomenon is very important. as the heat generated is max. when the strength of concrete is min. follow aci 207 guide lines.
we designed a 26' x 26' x 12' pentagon shaped pier cap to carry a 300 foot tall, 15' diameter concrete mast for a domed stadium some years ago and had a similar design decision regarding the approach to analysis and reinforcing.
while we did everything we could to specify temperature control in the mass concrete, including monitoring instrumentation, we decided to approach the stress/rebar problem with a sledgehammer instead of a more precise design.
essentially, we looked at a combination of flexural behavior (yes, i know its a big mass of concrete that doesn't really behave in flexure) as well as some shear calculations to ensure that the individual piers (6 of them) wouldn't create a shear friction issue - see aci 15.5.
we ended up with a top and bottom mat of steel and a middle level mat. all of which were tied together with vertical stirrups which followed the path of interior "beams" within the mat spanning from pier to pier.
short of the above, the only other concept would be to model the mat with 3d finite elements. even then, you would get your stresses but it would be difficult to model the thermal stresses which might develop within the mass.
as far as thermal stresses are concerned, much will depend upon how u actually construct this structure. generally, chilled concrete is recommended (5-10 degree by adding ice flakes) and also important is the progress of concreting. if you pour a layer of concrete on one day, allow a 72 hour gap and then pour the second layer, you can control much of the damages threatened by thermal stresses. but, you will have to invest extra in terms of surface preparation, additional investment of time etc.
i dont see why any reinforcement will be needed apart from some surface rebars if it is a gravity type of a structure.

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