Thickening a Gravity Retaining Wall 2

[bridgebuster]

I'm working on a project that involves rehabilitating four 75-year old gravity walls; total length about 1700. Each wall varies in height from 4-feet to 26-feet (from top of footing).

A major portion of the work is replacing the parapets and curb at the top of the walls. The upper 2-feet of each wall is spalled because the dowels from the parapet into the stems only had about 1/2-inch cover.

The other aspect of the project is that we're removing 2-inches of concrete from the face of each wall and installing a new overlay - it'll be at least 5-inches thick. The community wants a very elaborate aesthetic treatment on the walls.

My question: Is there any known method to determine the spacing and depth of dowels to hold the overlay to the existing wall? We're using a hooked No. 4 bar every 15-inches horizontally & 24-inches vertically. This is based upon what I came across in some specs and guidelines for shotcrete. However, the wasn't any information on how deep the dowels should be. We're embedding the dowels 6-inches.

We came up with the 6-inch depth by crystal ball and averaging out the embeddment assuming the dowels were resisting the full lateral pressure of the wet concrete.

I've worked in the field on jobs where we attached jersey shaped parapets to existing vertical parapets. In looking at old drawings there was no consistent rebar pattern.

Any thoughts?

 

[kslee1000]

I will treat it as a "Shear Friction" problem, and provide development length per ACI 11.7. Lookout for surface preparation, On top of roughening the existing concrete surface, I personally prefer to see scattered grooves type arrangement to further stabilize the finished product. Since this is out door installation, epoxy dowels and bonding agent should be in the mix.

Besides the dowels, are you going to use face mat (parallel to wall)? Drying shrinkage could be a problem, what is in your plan to handle it?

 

[kslee1000]

Further note:

Check anchorage manufactures, such as HILTI, for recommended development length of drilled and grout reinforcing dowels. Usually it requires shorter length to achieve the purpose behind code provisions.

You can also utilize the manuf's table to determine bar size and spacing.

Try:

http://www.hilti.com,

and download their's manual, it's used to be free.

 

[RHTPE]

bridgebuster: You mention "The community wants a very elaborate aesthetic treatment on the walls." To me, this implies the likelihood of using a form liner to impart a stone-like finish to your overlay. Be cautious about the relief on the liner and its impact on how the contractor will tie the form to the existing concrete. Form liners used on the "button-up" side of vertical formwork are difficult to install and minimize the visual impact of form ties. It will depend greatly on the type of forming system the contractor selects.

I agree with kslee1000 that you need to carefully plan for concrete shrinkage. Be careful about the resulting thickness of your overlay after factoring in the depth of relief on the form liner. It would be best to anticipate shrinkage cracks, provide a location for them to occur that will be inconspicuous, and can still integrate well with whatever "aesthetic treatment" is employed.


Ralph
Structures Consulting

 

[cvg]

if this is just for aesthetics, could you precast the facing panels and then attach them?

 

[bridgebuster]

I don't think the shear friction would apply here because it's a gravity wall and there isn't any bending.

Nevertheless, we are providing T&S reinforcement per AASHTO and matching the existing 30-foot vertical joit spacing.

The original aesthetic treatment was a series of horizontal lines in the walls and vertical corrugations in the parapets. At the 11th hour the community decided they wanted "something with arches". My architect is working on two schemes: A Roman aquaduct - arches on top of arches, and an arch bridge - at the low ends of the wall we'll use a stone form lines to suggest an abutment then a series 60 foot arches. The "arch effect" is annoying because we'll have to re-think the T&S rebar.

 

[RHTPE]

Good Luck!

Once they get the cost estimate for something like you describe, they'll be re-thinking it. The new concrete thickness must be ample enough to allow proper concrete placement and consolidation.

To keep the cost reasonable, you need to be able to utilize a forming system where the "base" form face stays in a the same plane. All of the desired "features" need to be built-up from that plane. This would be best done with a custom form liner or with some very creative form carpentry work. Do not lose sight of the fact that the forming system WILL need to be anchored to the existing concrete. Unless the formwork is custom-built for the application, you will NOT have a say in the tie locations - they will be established by the forming system chosen by contractor.

Ralph
Structures Consulting
Northeast USA

 

[kslee1000]

Gravity along causes shear friction between two rigid bodies, though minimal if the adds on does not have much weight. I agree, bonding is more important here.

 

[Ron]

Why not consider shotcrete? You can finish it with a faux finish, texture it during the finish process, or cut a stone-like pattern into it.

Your attachment spacing and doweling appear adequate. You might want to consider a bonding agent to be applied to the existing concrete, or even better, an epoxy bonding material, old to new.

 

[bridgebuster]

Ron - we thought about shotcrete but it's not common in NYC. There's only been two DOT project in the past six years that used shotcrete for repairs.

cvg - we could use precast panels but I don't think I want anymore gray hair. We have a project heading to construction involving installation of precast panels on the approach to a tunnel. The agency paying for it is not the agency that owns the structure. It took us more than two years to get the owner to accept the connection of the panels to the wall - they don't want any responsibilty in the event that a panel falls. Pretty much these panels can withstand a nuclear blast, truck collision, M10 earthquake, and Category 5 hurricane occurring simultaneously (OK I'm exaggerating a bit). It's the same situation with the project I'm working on now.

 

[miecz]

We came up with the 6-inch depth by ... averaging out the embeddment assuming the dowels were resisting the full lateral pressure of the wet concrete.

Are the dowels being designed to support the formwork? How tall are these walls? After construction, will the weight of the overlay hang off the gravity wall?

 

[bridgebuster]

The dowels do not support the formwork;

the walls vary in height: each starts at about 4' sloping upward to 26'.

the concrete overlay doesn't hang off the wall. It will rest on top of the footing.

I know the dowel embeddment will work; I'm just trying to find a rationale for my arbitrary choice of depth.

 

[RHTPE]

bridgebuster: Now I'm confused. Originally you said "We came up with the 6-inch depth by crystal ball and averaging out the embeddment assuming the dowels were resisting the full lateral pressure of the wet concrete." Then you say "The dowels do not support the formwork".

The contractor will have to use some method to restrain the form. At 4' high, bracing alone may do it IF you have the room. At 26' high he will have no choice but to anchor to the existing concrete. Hardware exists for formwork that will permit him to attach form ties to your dowels, assuming the spacings can be made to match and the thickness of the new concrete will allow it. This will help to minimize the number of holes that will have to be drilled into the existing.

Designing the reface around the means to build it makes for better results.


Ralph
Structures Consulting
Northeast USA

 

[bridgebuster]

Ralph - the dowels do not support the formwork. They're intended to tie the new facing to the existing wall. The dowels are holding the rebar mat.

We're not designing formwork as it is a means and methods issue. My original question had to do with justifiying the 6" embedment. It's one of those things that you know by gut instinct will work but you can't say that on the design calculations that will go to the owner.

It's like Potter Stewart's statement on porn: "I can't define it, but I know it when I see it."

I was looking for opinions on ways to justify it.

 

[RHTPE]

bridgebuster: I understood your question about justifying the size/spacing/embedment of the dowels. But with your first post your made the statement about the dowels resisting the full lateral pressure of the wet concrete, which implies some relationship to the formwork. If the formwork does not connect to the dowels, the dowels would sustain no forces from the fresh concrete, other than that of being immersed in fresh concrete (pressure against the dowel's surface).

While Means & Methods may be the contractor's concern, you will find them anchoring to the dowels (perhaps), which may become your concern.

I was just trying to offer some points that should be considered with this kind of concrete construction challenge.


Ralph
Structures Consulting
Northeast USA

 

[miecz]

If you're not supporting the formwork of the veneer, your only force is wind suction and seismic. With the dimensions you've given, you've got 10,000 of these anchors holding maybe 100 lbs each. I tend to think that a 3 inch embedment into concrete would suffice, unless the concrete is badly weathered. Drilling is expensive.

 

[bridgebuster]

Ralph - I'm sorry if the original post was unclear.

miecz - that sounds like a good approach. It's easy to overlook the obvious.