×
INTELLIGENT WORK FORUMS
FOR ENGINEERING PROFESSIONALS

Log In

Come Join Us!

Are you an
Engineering professional?
Join Eng-Tips Forums!
  • Talk With Other Members
  • Be Notified Of Responses
    To Your Posts
  • Keyword Search
  • One-Click Access To Your
    Favorite Forums
  • Automated Signatures
    On Your Posts
  • Best Of All, It's Free!
  • Students Click Here

*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail.

Posting Guidelines

Promoting, selling, recruiting, coursework and thesis posting is forbidden.

Students Click Here

Jobs

Increasing existing concrete slab thickness

Increasing existing concrete slab thickness

Increasing existing concrete slab thickness

(OP)
Is it practical to install a new layer of rebar and increase thickness of a concrete slab for the underside? The rebar at the underside of the existing has corroded and concrete spalled. Also, the top 2" of the 7" slab are to be removed and replaced due to extensive damage (50 year old slab). For these reasons I am wondering if it would be a good idea to reinforce from the bottom. I want to mechanically connect the two pours with SS U pins (see attached) and roughen the existing slab from the bottom and apply some sort of boding agent between the two pours. Does this seem like a practical approach

RE: Increasing existing concrete slab thickness

Possible, yes.  Practical, I don't think so.  Can't you just start over?

RE: Increasing existing concrete slab thickness

I agree with Hokie here.

Also, I would be concerned with what caused the corrosion and spalling in the first place and try to mitigate that prior to any repairs to the slab.

Mike McCann
MMC Engineering

 

RE: Increasing existing concrete slab thickness

Fix the reason.  Replace the slab.  Done and over.  Probably cheapest method.

RE: Increasing existing concrete slab thickness

No, not practical.  Have you ever watched someone try to jack hammer concrete over their heads?  How do you place concrete flush with the underside of a soffit?  How are the forms supported?  Most importantly, why?  Addressing the top and bottom of the slab suggests to me a full depth repair is in order.

RE: Increasing existing concrete slab thickness

I agree with all of the above re: practicality, especially the thought of jack hammering over head.

Assuming that the slab is already roughened and jack hammering wouldn't be required, I think placing the concrete would be possible: Traditional formwork, core drill at some spacing (5' oc), at every other core hole build a "hopper" to add a couple feet of pressure head to the fluid concrete and pour concrete until it comes out the next adjacent hole. Use superplasticizers and/or vibrate from below.

I don't think it would be feasible to use a bonding agent because anything out there would cure in the time it takes to build the formwork & pour.

If this slab is below grade - be mindful that if you remove and replace the slab, you're removing a support from the wall that the slab previously provided and significant temporary bracing may be in order.

RE: Increasing existing concrete slab thickness

In my experience the cost of a topping plus the cost of the soffit repairs would exceed the cost to drop the entire slab and replace. The design life of the cheaper option, i.e. dropping the entire slab, would far exceed that of the repair option you describe.

At 50 years old, I would be worried about possible chloride contamination or other deleterious issues there may be in the existing concrete you propose to leave in place. A new slab, with quality concrete, proper concrete cover and a waterproofing membrane is the way to go.

RE: Increasing existing concrete slab thickness

(OP)
I agree that it would be more practical and economical to remove and replace the entire slab. But the problem is with some existing limestone columns. See attached. The column sandwich the existing slab between itself and the brick pier below. I don't believe that there is any positive connection between the brick pier and the column. I am afraid that if I take out the entire slab, the columns and brick pier might be compromised because of bracing issues.

RE: Increasing existing concrete slab thickness

Nothing that a good scheme of needling and underpinning wont fix.

I agree wih the others, replacing top and bottom slabs wont fix the root cause and will normally cost more money.

Red Flag This Post

Please let us know here why this post is inappropriate. Reasons such as off-topic, duplicates, flames, illegal, vulgar, or students posting their homework.

Red Flag Submitted

Thank you for helping keep Eng-Tips Forums free from inappropriate posts.
The Eng-Tips staff will check this out and take appropriate action.

Reply To This Thread

Posting in the Eng-Tips forums is a member-only feature.

Click Here to join Eng-Tips and talk with other members!


Resources