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

 

[RickyTickyTavi]

Walls more than 10'' thick are required to have 2 mats of steel. I think there's part that says basement walls dont need the 2nd mat.

Personally I prefer putting 2 layers to help control shrinkage.

 

[Agent666]

Sometimes 2nd layer is required if you need compression steel to deal with failing balanced strain conditions in heavily reinforced sections.

Additionally if you have a propped cantilever wall (fixity at base, propped at the top), the moment reversal can dictate the need for reinforcement on both faces.

Limiting crack widths can also be another variable. For example, centrally placed bars might work for strength in propped cantilever wall, however crack widths at deep concrete cover might not work, moving to two layers at lesser cover can improve both strength and post cracking stiffness and crack widths.

 

[RFreund]

@RickyTickyTavi - Thanks for the response. You are correct about the 10" walls, but it doesn't apply to basement walls or cantilever retaining walls.
@Agent666 - Good points.

Where I'm conflicted is the case of a 8" or 10" thick cantilever wall or basement wall that isn't very tall. The wall really isn't thick enough for two curtains of reinforcment, but I'd still like to have it.


EIT

http://www.HowToEngineer.com

 

[Harste]

I agree with Ricky and Agent666 with regards to shrinkage cracking and cracking in general - it would be advantageous to have 2 layers of reo if your wall is thick enough.

Durability of concrete structures in my country usually needs to have a design life of 80 to 100 years. As soon as the concrete element is thicker than 150mm (~6") then each face must be reinforced.

An 8" or 10" thickness seems like enough for most applications to fit 2 layers, unless you're designing for high magnitude forces or require excessive concrete cover. You can always get the contractor to stagger the reinforcing bars in each layer to help with concrete flow.

 

[msquared48]

If you are designing a detention vault that has soil load and interior water detention loads, you can get stress reversa and periodic tension on both faces. Hence another circumstance for two layers of steel.

Mike McCann, PE, SE (WA)

 

[Ron]

Building codes are minimum requirements. If you want two layers, put them in.

 

[KootK]

For walls in that thickness range, I never include exterior reinforcement unless I see a primary flexural demand to justify it (multi-story etc). A few reasons for that:

1) Staying competitive in my market. The usual race to the bottom.

2) Aesthetically, the interior face is all that usually matters.

3) In a thin-ish wall, the horizontal reinforcing on the single face is enough to limit restraint cracking in my opinion. Moreover, any reinforcing your supply is surely more effective on the interior face (point #2).

4) In most cases there is a drainage and damp-proofing system in play on the dirt side of the wall such that I'm not especially concerned about exterior side durability issues.

5) Lots of experience seems to suggest that IF reinforcing only does not produce issues. It is my strongly held belief that the soil loads that we design our walls for never materialize in any meaningful way. If they did, we'd probably be checking our basement walls for inwards deflection which seems to pretty much never be the case.

 

[RFreund]

Good points, thanks for all the input!

KootK -> how about a cantilevered site retaining wall? In this case the reinforcment is on the opposite side that is "Aesthetically important"

EIT

http://www.HowToEngineer.com

 

[JAE]

For us, usually an 8 or 10 inch thick retaining wall has only reinforcement on one side (earth side obviously).
For 12" and thicker walls we go with two layers.

Check out Eng-Tips Forum's Policies here:
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[Guest262653]

I always use two layers as I like to control restrained shrinkage cracking near the wall surfaces with equal reinforcement on each side. The compression side vertical reinforcement in cantilever walls serves the only purpose of horizontal rebar support.

 

[TehMightyEngineer]

KootK -> how about a cantilevered site retaining wall? In this case the reinforcment is on the opposite side that is "Aesthetically important"

Not KootK but my rational is I avoid exterior reinforcement for cantilevered site retaining walls due to corrosion concerns and spalling of the concrete cover over the rebar. I see this failure far more than an un-reinforced wall with cracking. While you're correct that you may get shrinkage or other cracking I feel the interior rebar will help restrain the cracks and the compression from the flexure will help close the smaller cracks anyway (or keep them from forming to begin with).

I generally only use an external rebar layer if required by code (such as AASHTO), required for flexure, or if I have some localized forces that necessitate horizontal distribution reinforcement.

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

https://americanconcrete.com/

 

[RFreund]

Thanks again for the replies!

EIT

http://www.HowToEngineer.com

 

[KootK]

I don't generally put exposed face reinforcement in what I would consider "site wall" cantilevered walls. I'll do it when:

1) the exposed face is the inside of the building.

2) it's a sexy public art exhibit etc and we can spring for galvanized bar.

3) the client has expressed a need for limited jointing and cracking and we can spring for galvanized bar.

I usually follow the CRSI recommendations and temper that with some judgement regarding joint spacing, acceptable crack widths and durability.

To TME's point, I originally intended to post a picture of some old roman buildings adn point out that the most durable concrete on the planet tends to be that with no reinforcing at all. I didn't want to get myself sucked into a protracted debate over the admittedly different case of one sided reinforcement however.

 

[TehMightyEngineer]

Similar to KootK, though I'd probably go epoxy coated as the more cost-effective alternative to galvanized. As much as I dislike epoxy coated bar, it's perfectly suited for limiting corrosion jacking of the concrete cover and the subsequent spalling.

I also consider CRSI to be the go-to authority for retaining wall detailing.

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

https://americanconcrete.com/

 

[hokie66]

Maybe I misunderstand some of the comments, but I can't understand why anyone would build a cantilevered retaining wall without reinforcement on the tension face. As to corrosion, that is because of inadequate cover, poor compaction, or really bad concrete, not just the fact of reinforcement near that face.

 

[RickyTickyTavi]

Galvanized bar, Ive never used it. Slight side bar, but whats the general experience with it? I figure you can field bend, but it'll certainly look cool before the placement

 

[msquared48]

Hokie:

...or unaccounted for reveals in the tension face lessening the concrete cover, allowing water penetration and corrosion of the steel, and spalling of the concrete. The cover distance should be measured from the inside of the reveal to the nearest rebar.

Mike McCann, PE, SE (WA)

 

[KootK]

Slight side bar, but whats the general experience with it?

I'm used to epoxy being cheaper but, owing to some oddities in my particular market, you can get galvanized bar as cheap as epoxy, or so close that the difference is not a factor. I love it. You can field bend and are not at the mercy of a damage free installation as you sort of are with epoxy.

This is hearsay really but I've been told that galvanized is cheap in my area because the local municipality and provincial transportation department decided to spec it for much of their work. Then it became a self fulfilling prophecy: it's often cheaper to order what is often ordered. Kinda like with equal angles relative to unequal.

 

[hokie66]

Mike,
I agree, but don't know why you would have reveals in the earth face of a retaining wall.