huangyhg

roller compacted concrete (rcc) question
i have a question that can probably be answered rather simply by a structural engineer. i'm a civil engineer, however, i have little experience with structures. i'm fortunate enough to have been put on a job where the schedule is lax and i can get a bit of structures experience (albeit with heavy oversight). recently on a project, we were designing a foundation that had to have very little displacement for extreme seismic events. one of the design alternatives was a solid rcc foundation that was ~10 ft thick.
herein lies my question. how can one pour a 10' concrete slab without using any rebar to resist the tensile forces that are developed in the bottom of the slab?? from my research, rcc doesn't provide significantly more flexural strength than standard concrete. wouldn't the tensile forces created in the bottom of a 10' thick slab be great enough to cause cracks to develop, starting the path toward failure??
furthermore, i remember seeing a dam being constructed on the history channel. it appeared that this was lift after lift of rcc without any steel reinforcement. how could the lower areas of this dam resist the tensile forces that evolve from all the dead load of the concrete resting on top of it?? why wouldn't rebar be needed in a huge monolithic concrete block?? aren't the tensile forces still present in the lower reaches of the structure?
as you can see, i'm basically confused as to why rcc doesn't need any reinforcing steel whatsoever. if anyone can shed some light on this to help me understand i would appreciate it....
peace,
nutbutter
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rcc is for pavement (picture asphalt concrete roads) and stack type dams.
if a foundation is uniformly loaded on the entire surface and the soil is uniformly compacted, then no bending of the foundation occurs,therefore no tensile stresses in the bottom. now if the soil was poor and or the load was not uniform (picture a column sitting in the middle of the foundation) then bending would occur to some degree and tensile stresses would form. now depending on the thickness of the foundation, you may or may not need reinforcing. picture a thin piece of glass sitting on grassy ground and you step on it. it will break due to bending. now picture a thick piece of glass and you step on it, the thicker the glass the less chance it will break. the grassy ground represents poor soil..not flat, and your foot represents non-uniform loading (the piece of glass has to be much bigger than your foot). put a piece of thin glass on a very flat smooth concrete floor and pour sand on top of it. as long as the floor is flat and there isn't little pieces of dirt of pebbles under the glass, it doesn't matter how much sand you pour on top of it, it won't break. the concrete represents good soil and the sand, uniform loading.
thanks! that was very well explained. so these giant 100+ ft rcc dams don't need rebar in excellent soil conditions and uniform loading.
nb
when you say 100+ ft, you mean length correct? height is pretty low compared to say hoover dam?
never designed a dam, but the way i understand it is that the rcc replaces an earthen dam. the are both wide at the bottom and narrower at the top. the are designed so they can resist the lateral load due to the hydrostatic pressure. i believe the main failure mechanism to design for is shear sliding. at the base of the dam, you need a very wide layer to resist the largest amound of lateral pressure. as the layers move up, less lateral pressure...therefore smaller width layers can be used. the shear plane between the layers have to be able to resist the cumulative shear from above. bending of the dam is not a design consideration.
again, i have never designed a dam.
now dams like hoover could not be rcc. it is a retaining wall, not an earthen dam.
hey twinnell,
thanks again.
i thought that the special i saw on history channel was a dam about 100' high. i don't know if it was made of rcc, but the concrete didn't appear to have any reinforcing steel. the concrete was poured in cells or blocks.
why couldn't a very tall dam be made of rcc?? is it just the economics of rolling that many lifts?? it would seem that the dam would be uniformly loaded vertically from the weight of the concrete. would regular unreinforced conrete with a very strong compressive strength be adequate in a situation like this where loadings are uniform.
nb
i don't have the answer. i would think that if you had a 100 ft high straight dam, as opposed to curved, and it was unreinforced; it would have to be very wide at the bottom to be able to resist overturning and sliding due to the lateral load from the water and to be sure that bending wasn't a design consideration. i think high dams such as hoover are thin relative to their height, therefore they are usually curved out into the retained water and reinforced. they are curved, i would suspect, to induce compression into the walls instead of tension.
sorry, i don't really have any answers. i am really guessing at this.
rcc dams go well over 100 ft. the small one we are building is about 70 m in height. they are a gravity wall, in essence. aci and international water power and dam construction web sites should be able to give you some good information. see the iwpdc magazine issue for july 2007 - rcc dam in portugal = 120 m in height. hickory log dam in georgia (usa) is designed to 180' high. also see:

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