Attaching New Slab to Old Walls 2

[AminSaalaar]

Hi Guys,

I want to repair an old polygon shaped concrete manhole/vault that is 8' tall. The top slab is in a very bad shape as the rebars are exposed and corroded but the walls and floor slab are fine. I'v decided to replace the top slab by demolishing it and perform an in-place-pour or precast.
My question is....how do I attach the new slab to the existing walls? How do I deal with existing rebars in walls? Is precast preferable over in-place? I'll be using epoxy coded rebars in the new slab . Thickness for walls and slabs is one foot.
I appreciate any help in advance.
Thank you all

 

[hokie66]

You should try to preserve the reinforcing which projects from the wall, and tie it into the new slab. That probably rules out precast, and in any case, making a precast element for a one off application is impractical. What is the reason for using epoxy coated bars? What caused the top to deteriorate so badly? In most applications, a well designed concrete mix with plenty of portland cement should serve well to protect the steel and prevent spalling.

 

[AminSaalaar]

Epoxy coated rebars are to provide protection from underground gas elements and moisture. This is not the only manhole to be in this condition; they are about 40 years old and they contain steam piping within themselves which generates hot air inside despite the ventilation. I believe constant hot air and a bit of condensation inside manhole has caused this deterioration.

thanks for your comments hokie66

 

[dik]

Is the manhole used for s storm sewer or sanitary sewer? What is the condition of the rebar in the walls?

Dik

 

[dicksewerrat]

I'd consider using precast slabs with factory applied epoxy or coal tar coating to keep the water away from the concrete. Epoxy coated rebar may be overkill. I'd also test the walls and floor of the structure.

Richard A. Cornelius, P.E.

http://WWW.amlinereast.com

 

[AminSaalaar]

dik

the manhole is used for steam piping, expansion joints and anchors. the walls are in good condition, there is no exposed rebars.

 

[AminSaalaar]

dicksewerrat

How would you attach the preslab to the existing walls?

 

[kslee1000]

Amin:

He can provide threaded anchors in the existing wall, through sleeves or pockets in the precast pannel. As the weight of cover shuld be adequate to stay in place, the fastening effort could be minimal. Using such scheme, helps future maintenance that may require uncover the manhole/vault.

 

[dik]

It's likely a matter of cutting the old slab away to the soffit of the new slab, leaving the rebar from the walls intact, and bending them into the new slab as dowels. Ledgers can be secured to the existing walls and formwork constructed to support the slab (The ACI has an excellent spreadsheet for the design of forming for slabs and walls).
The slab is reinforced and cast... and, formwork removed.

Dik

 

[dicksewerrat]

Where would a 1 foot thick slab go? If you think the steam line will rupture, then bolt it down. Most manhole slabs are not 'attached' with anything. Look at how manholes are constructed. the ring and cover are cement mortared to the manhole.

Richard A. Cornelius, P.E.

http://WWW.amlinereast.com

 

[AminSaalaar]

Richard

One thing about precast for this application is that I won't be able to level off top of existing walls so that the precast lid sit on them nicely.

 

[cvg]

I agree, there may be no structural reason to attach the top slab to the walls. Grout to level and seal it in place is all that is required. Not sure why the top slab needs to be 1 foot thick.

 

[AminSaalaar]

cvg,

If precast lid is the way to go, is there a waterstop product to be applied to the joint. I can't have water finding its way in the manhole.

The reason for the 1 foot thick slab is because the existing slab is 1 foot and also it's out in the roadway with only 2 feet cover but I would go with 10" slab to take some weight off walls.

 

[kslee1000]

Try compressiblee/expandable sealers.

 

[cvg]

Hydrophilic waterstop

http://www.greenstreak.com/Div3/Waterstops/swellstop.asp

 

[dicksewerrat]

Expose the walls and saw cut the walls. Any decent contractor should be able to do this.

Richard A. Cornelius, P.E.

http://WWW.amlinereast.com