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


slab on slab

slab on slab

slab on slab

I am looking for standards/specifications/recommendations/design guide for pouring a 4" concrete slab over an existing interior 4" concrete slab (foot traffic)...if one exists. Typically, we do not recommend or specify this, due to lack of knowledge of the existing slab and sub-base, however, a current client has specifically requested it, for financial reasons. Therefore, before I discount this option, I am wondering if there is any protocol that can be followed for successfully pouring a slab over a slab...with the confidence that the end product will perform. I have not been able to locate any information with ACI. Does a guide/protocol exist? If not, what are your thoughts/recommendations? I can provide more specific project information, if necessary. Thank you.

RE: slab on slab

Is the existing slab structural or on-grade?

RE: slab on slab

It is an interior slab on grade. Again, we know nothing about the material under the existing slab.

RE: slab on slab

What's the reasoning for the replacement/covering? If the old slab is heaving or overly cracked from anything other than shrinkage cracks then I wouldn't touch it with a 100 foot pole.

If it is just to have a nice new concrete floor and the old slab seems to be in generally good working order than I don't see why you can't just through the new slab on top of the old one.

RE: slab on slab

The existing concrete slab is in great condition. The reason for slab on slab is simply to raise the elevation of the existing slab area to connect spaces that are at varying elevations (4" difference) for accessibility purposes. The floor area to be raised 4" is approximately 380 s.f. (19ft x 20ft).
I am not able to locate any standards for "slab on slab" construction, if any exist. Should the new slab be independent from the existing slab?...slip sheet between....or
Should the new slab be bonded to the existing slab?...bonding agent and/or reinforcement ties.

RE: slab on slab

The real problem is that most slabs on grade develop soft spots, or areas where the materials underneath have settled away from the slab. These can cause serious problems, as well as any equipment driven onto the slab can cause temporary elastic deflection which can rebound and wreak havoc with your new slab.

I am speaking from personal experience. One of my jobs in New Zealand was nearly ruined by construction equipment causing a later rebound. This was likely due to an insufficiently compacted base under the existing slab. There is no way to ensure the quality of the new slab when you blindly place over old.

I have been told that the same preparation normally used for adding loads to an exising slab (ie: low pressure grout injection) works, but with no way to do any quality control worth a damn.

RE: slab on slab

19' x 20', I would just do it. A slip sheet would help with shrinkage cracking.

RE: slab on slab

Not all that different than a topping slab. You can try to match control joint locations and count on the adhesion between the old and new slab and with some luck get decent shrinkage cracking control. Or you can do something like a slip sheet to keep the two slabs from bonding together, but unless things are perfectly smooth you will be some interlock that could cause cracking.

RE: slab on slab

Either tie them both together COMPLETELY or separate them COMPLETELY. Any half-assed attempt at either will be a problem.

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! Already a Member? Login


eBook - Rethink Your PLM
A lot has changed since the 90s. You don't surf the Web using dial-up anymore, so why are you still using a legacy PLM solution that's blocking your ability to innovate? To develop and launch products today, you need a flexible, cloud-based PLM, not a solution that's stuck in the past. Download Now
White Paper - Using Virtualization for IVI and AUTOSAR Consolidation on an ECU
Current approaches used to tackle the complexities of a vehicle’s electrical and electronics (E/E) architecture are both cost prohibitive and lacking in performance. Utilizing virtualization in automotive software architecture provides a better approach. This can be achieved by encapsulating different heterogeneous automotive platforms inside virtual machines running on the same hardware. Download Now

Close Box

Join Eng-Tips® Today!

Join your peers on the Internet's largest technical engineering professional community.
It's easy to join and it's free.

Here's Why Members Love Eng-Tips Forums:

Register now while it's still free!

Already a member? Close this window and log in.

Join Us             Close