Extending an existing waste water tank concrete wall

[sengineering]

Hi,

I am working on adding another concrete tank aeration basin to an existing one. So the existing walls need to be extended. What do you recommend to consider in doweling into the old concrete? Since the tanks are going to be full, the tension in the corners add up, is simply doweling epoxy rebar into the old wall and extending it enough?

 

[SlideRuleEra]

Is simply doweling epoxy rebar into the old wall and extending it enough?

Depends on how many dowels can be used. Can't just install as many as you want... have to look at the problem the other way around. Using manufacturer's (e.g. Hilti) technical recommendations, calculate what size, where and how many epoxy dowels can be installed. Then check to see if that is enough to safely resist the necessary forces.

If not, another solution may be needed.

Even if enough dowels can (theoretically) be used, the challenge is to see how many can actually be installed. Field drilled holes in existing concrete that hit existing rebar can cause a change in plans.

Of course, use an appropriate water stop between old and new concrete.

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[JStructsteel]

Does the rest of the wall still check, that might limit how much steel you need.

 

[DaveAtkins]

Even if the bars with epoxy work, the bars in the existing wall may fail due to inadequate lap splice length with the new bars.

DaveAtkins

 

[sengineering]

The rest of the wall seems to be ok, using HILTIs calculations are ok. As long as the original rebars are constructed as planned. The old basin is more than 30 years old. Do you recommend using HILTIs or regular epoxy rebars are ok too? What can I do to address the rest of the wall & lap splices?

 

[SlideRuleEra]

sengineering - If you use Hilti calculations, suggest using Hilti products.

About your additional concerns... On a retrofit job, like this, you often have to put aside what are real concerns on a new design. That is why the manufacturer's recommendations are used for calculations instead of ACI, or similar specs. A reputable manufacturer, like Hilti, has researched the capabilities of their products to maximize probability of success. Make sure the existing concrete is in good condition and make conservative choices on the details of your design.

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[ATSE]

You should review the eng-tips discussions on post-installed anchors for retaining walls to footings. Very similar limit states and problems.
Keep in mind that simply developing the strength of a rebar into a concrete mass (a challenging task alone due to very deep embeds required) is not sufficient to ensure adequate joint performance.
A wall corner is the very worst location for a construction joint for a fluid-retaining structure, so a conservative approach is in order.
Design your wall plates so full continuity at the corners is not required (i.e. design for a pin, which will thicken your walls), because you won't realize anywhere close to even 80% efficiency. That is, your post-installed anchors should be shear-controlled.
And check shear-tension interaction.

 

[hokie66]

A detail of the points where you are extending would help. If you are adding another cell beside the existing, it may be better to build a new cell, independent of the old.

 

[sengineering]

Thanks for the replies. They are really helpful.

Can I use regular epoxy rebars intead of Hiltis? The client insists of extending the basin instead of building a new one next to it. Also, the extended foot print of the foundation needs to connect to the new concrete, do you recommend Hilitis there since the development length will be an issue? Is it safe to assume all the shear is going to the rebars in the corner and make sure they can handle the load?

 

[JedClampett]

I've done this pretty often. My advice:
[ul]
[li]Model the doweled connection as pinned. You get the choice, make it as easy as you can.[/li]
[li]If you stagger the bars, you increase the spacing. Every little bit helps. Of course, now the bars you need to lap to are staggered. Also see the next bullet.[/li]
[li]If it's a 12 inch wall, you're going to have issues with edge distance problems once you put in the retrofit waterstop. You might need to go with #4's or #5's.[/li]
[li]If you use the Hilti program, they'll give you the highest allowable capacities. They seem to have pulled some magic tricks with the bond capacity of their V3 epoxy. So you can't use other epoxies, they're not equal.[/li]
[li]The base slab on the drawing looks nice and even. It's not. Have the contractor saw cut it square.[/li]
[li]Joints on the retrofit waterstop might need to be detailed. There's a lot of 90 degree bends in each plane. Possibly ask for factory welded joints.[/li]
[/ul]