Reinforcement for mass concrete
Reinforcement for mass concrete
(OP)
There was a thread here regarding reinforcement of mass concrete a few months ago. I would like to open the subject again with hopes to get some input on a specific design scenario.
I am submitting a proposal for a contractor's bid on the installation of a pair of large (and tall) industrial tanks on a new foundation. Initial analysis included review of calculations done by another engineer. According to ASCE 7-10, with a seismic R factor of 2.0 (tank on legs) the result is a foundation with plan dimensions of 15 feet by 28 feet and a depth of 15 feet! The excessive depth is to answer to the significant overturning load due to seismic.
This is definitely mass concrete. My question is, how to reinforce this mass. It would qualify as Structural Plain Concrete (ACI318 Ch. 22) if we do not anticipate tensile stresses within the mass; however, this is not the case since we are depending upon the mass to withstand uplift forces due to overturning.
I am submitting a proposal for a contractor's bid on the installation of a pair of large (and tall) industrial tanks on a new foundation. Initial analysis included review of calculations done by another engineer. According to ASCE 7-10, with a seismic R factor of 2.0 (tank on legs) the result is a foundation with plan dimensions of 15 feet by 28 feet and a depth of 15 feet! The excessive depth is to answer to the significant overturning load due to seismic.
This is definitely mass concrete. My question is, how to reinforce this mass. It would qualify as Structural Plain Concrete (ACI318 Ch. 22) if we do not anticipate tensile stresses within the mass; however, this is not the case since we are depending upon the mass to withstand uplift forces due to overturning.
Richard L. Flower, P. E., LEED Green Associate
Senior Structural Engineer
Complere Engineering Group, Inc.






RE: Reinforcement for mass concrete
Designing the foundations as structural plain concrete was intentionally not even considered - not a time to look for short cuts. Pay special attention to anchor bolt locations, diameter, length and anchorage.
FWIW - Five years after initial operation, the plant took a direct hit from a Category 4 hurricane, with winds approaching design speed - no structural damage.
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RE: Reinforcement for mass concrete
I presume you will have tank hold down anchors which will have to be reinforced. How can you justify justifying reinforcing around the anchors and nowhere else? My opinion.
RE: Reinforcement for mass concrete
RE: Reinforcement for mass concrete
Just read up on mass concrete in ACI 207 and 301.
I have work on many structures within Power Plants and I have never seen anything designed as Structural Plain Concrete.
RE: Reinforcement for mass concrete
I would just make sure your structure can't pull out of the mass concrete. If the loads are so large that you need steel from a strut-tie perspective then OK, but surely there is no way a layer of steel at the top and bottom of a 15 foot mass of concrete is doing anything. I would also get advice from a concrete specialist (Ron?) to make sure the mix is spot on. Aggregates, binder, w/c ratio, admixtures, lifts, monitoring internal temperatures, SCMs, etc. That's what's important. Not a mat of reo.
RE: Reinforcement for mass concrete
For the 15' x 28' x 15' being asked about, I agree - my mistake. I should have clarified the example I gave. The free-standing 600' chimney foundation is an octagon, about 190' across the flats, 15' thick. My calling the rebar in the foundation a "top and bottom mat" is a gross simplification - it is a full rebar cage and it is essential. The foundation was made 15' thick to minimize deflection so that 200, or so, 4' diameter drill piers supporting it are equally loaded under high moment conditions. What I had hoped to point out is that this foundation is in a high seismic (and wind) area, and that reinforcing should be considered mandatory. Sorry for the confusion.
www.SlideRuleEra.net
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RE: Reinforcement for mass concrete
If I recall correctly, they added in a lot of temp / shrinkage reinforcing (i.e. total area of steel = 0.0018 * gross area in each directly) spread out through the thickness of the mat in 3 or 4 layers. I don't recall them talking about any problems with their project when it was built or the ensuing few years. So, I assumed that all went well.
ACI 2014 would, unfortunately, require rho = 0.0018 each for the top and bottom reinforcement. So, the new code more than doubles the amount of reinforcing required for those types of foundations.
RE: Reinforcement for mass concrete
Forming could be a challenge as well.
RE: Reinforcement for mass concrete
RE: Reinforcement for mass concrete
Richard L. Flower, P. E., LEED Green Associate
Senior Structural Engineer
Complere Engineering Group, Inc.