Concrete Retaining Walls - Rebar on Both Faces

[RFreund]

I'm curious to know when you would use concrete reinforcement on both faces of a retaining wall and/or basement wall?
As far as I can tell the Code (ACI) really only requires reinforcment on the tension face, but I typically like to distribute the tempature/shrinkage steel to both faces (2/3 to the front face and 1/3 to the rear) especially if the wall is 12" or thicker, but I don't know that it's neccesary.

Thanks!


EIT

http://www.HowToEngineer.com

 

[msquared48]

True, but I was looking at the picture above showing the spalled concrete with the rusting rebar on a non-earth face.

Mike McCann, PE, SE (WA)

 

[Agent666]

If galvanising you just need to be careful with hydrogen emnbrittlement of higher strength rebar.

I once had a job where galvanised bars were snapping at a bend when they placed down cages, you could reproduce the issue if you hit the bar lightly with a hammer (think like slowly swinging a sledge hammer like you were putting a golf ball). Turned out it was a combination of the galvanising and the contractors fabricator incorrectly bending the bars (used a press suitable for bending flat plate vs correctly bending the bars around an appropriate diameter former). Either way it was amazing how you could simply snap a 500MPa bar like it was a twig. I've been wary of galvanising since, especially discouraging bending after the bars are galvanised, it can crack the galvanising and increases the hard working of bars that may have had some hydrogen embrittlement during the galvanising process.

Never used epoxy coated bars, they simply are not available in this part of the world, so correct cover and concrete mix and compaction for the win.

Almost every issue I've seen regarding longer term durability in existing structures has been due to inadequate cover to reinforcement and tie wires, contractors don't seem to appreciate the fact that there is a -0mm tolerance on covers, maybe its because they won't be around in 10/15/25 years when the problem rears its head. Engineers don't seem to appreciate that they need to provide sufficient tolerance, designing and drawing to 0mm tolerance simply isn't achievable for insitu construction.

 

[hokie66]

Agree with everything you said, Agent666. Another thing...if you want a contractor to do the right thing, you have to inspect. Inspection seems to be a dirty word these days, but our designs won't be carried out correctly without it.

 

[TehMightyEngineer]

It's worth noting that even with the best concrete and attention to detail that carbonation, wear and tear, loss of passivization, and permeability of the concrete can all contribute to corrosion on the front face. Especially if the concrete isn't perfect (as is often the case) and the retaining wall is adjacent to a well-traveled roadway (carbon dioxide fumes, salt spray, etc.). Clear cover can solve most of these but, in the end, time wins out. Better to have a design that works on un-reinforced concrete if at all practical IMO.


Ian Riley, PE, SE
Professional Engineer (ME, NH, VT, CT, MA, FL) Structural Engineer (IL)
American Concrete Industries

https://americanconcrete.com/

 

[RFreund]

Thanks again.

@Hokie66 - to be clear we are referring to reinforcment on the compression face and whether or not you would spec it. Generally speaking there is really no need for it except crack control, but even then, it's the compression face...

EIT

http://www.HowToEngineer.com

 

[hokie66]

Whichever face, the horizontal bars are there for crack control. If you want good crack control on the exposed face, bars near that face will be more effective than bars on the far face.

 

[JAE]

RFreund - for a retaining wall the outside face is indeed the compression face but remember that compression is only in one direction.
Horizontally there is most likely very little, if any, compression.

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[RFreund]

Good Points.

EIT

http://www.HowToEngineer.com