Fiber-Reinforced Conc for Penstock Encasement

[BigH]

For our group:
Penstocks will be encased with massive concrete (10,000 m3 or so). Presently, call for large (32 mm dia) steel bars which we believe is for thermal cracking control. There is no real load on concrete other than compressive in our view.
We, at site, are toying with the idea to use steel fiber-reinforcement. One view is that steel isn't really needed - plane concrete would be suitable while others consider the steel fiber inclusion.
Cannot think that there is significant stresses induced in the concrete. Some think, though, that there may be some tensile stresses due to 60 m head running through the penstock - i.e., 60 m x 10 kN/m3 about 600 kPa or 0.6 MPa deforming the penstock's steel wall thereby adding tensile stresses on the concrete surrounding the round penstock. Using 25 MPa concrete suggests tensile strength approx 2 or so MPa > 0.6.
Any thoughts from our group (Ron?? SlideRuleEra??) Is conventional steel reinforcement needed? If not can we use steel fibres to mitigate thermal cracking (or minimizing opening up of cracks after initial cracking) - or why is steel bars or fibers needed?
Good answers will relate to beers/scotch if/when I meet up with you!

 

[Ron]

BigH...sounds like a good application for steel fiber to enhance the concrete. You might still need some linear steel for thermal crack control, but you could minimize it by using the fiber. Consider using the linear steel near the outside of the geometry, letting the entire inside area be controlled with the fiber.

Do the codes there require minimum steel for temperature?

 

[jheidt2543]

It is my understanding that fibermesh, whether steel, poly or frp, is used to reduce early shrinkage cracking (and may help with early thermal too), but to resist "structural" stresses reinforcing bars are needed.

It sounds like you have some pretty massive concrete pours that will generate a lot of heat, I think I would stay with the rebars. How are you planning to deal with the heat of hydration generated?

Do you have to worry about any dynamic forces due to the water running through the penstock?

 

[SlideRuleEra]

BigH - I agree that the use of fiber reinforcement is a good idea, as Ron and jheidt2543 have suggested... and for an additional reason:

My experience is with low-head Kaplan turbines, like this:

http://en.wikipedia.org/wiki/File:Hydroelectric_dam.svg

Sounds like you are using steel pipe penstocks, like this:

http://en.wikipedia.org/wiki/File:Ohakuri_Dam_Blue_Penstocks.jpg

In either case, a lot of JUNK passes thru a hydroelectric turbine over time (fish, logs, wrecked boat parts, etc.). This can impact the penstock at fairly high velocity. Intake trash racks minimize, but do not eliminate the problem. Fiber will help improve the concrete's impact resistance, whether the penstock is concrete or pipe encased in concrete. I would keep most of the rebar (at the surface and around the pipe). As Ron has suggested, the deep internal rebar probably does not contribute anything and can be deleted, if fiber is added.

http://www.SlideRuleEra.net

http://www.VacuumTubeEra.net

 

[BigH]

Thanks gents. Thermal heat of hydration will be monitored and strict control will be placed on concrete temperatures. Contractor thinks he can do it with chillers - we beg to differ - especially with the exceptionally large pours he plans to make. We are pushing the idea of an ice plant. He will, of course, balk . . . then the push and pull begins. I'll look at your links in a few hours, Rob, when I am at work. Time for bed now - it is nearly midnight and I have a 0430 alarm . . .