Replacing part of a slab-on-grade 1

[brokensail]

Need to replace part of a floor slab in an old warehouse. The area to be replaced is approximately 25' x 50' and the slab is 4" thick with wire mesh reinforcement.

Based on a variety of factors and people's input, the thought for this slab has become:

- #4 dowels at 12" (or 18"?) centers and 6x6 W2.9/W2.9 wire mesh reinforcement
- Non-swelling water stop (e.g., Sika Lockstop) at the interface between the cold/existing slab and the new section to be poured
- Control joints approx. every 10' (does not look like the existing slab has any control joints)
- Vapor barrier underneath the new slab-on-grade (shallow water table)

This seems like it might be overkill, but it's been awhile since I've done any real concrete work...

 

[Mjkkb2]

WHat I do is to draw an undermined area around the perimeter of the existing edge that is to be filled with concrete which creates a sort of saddle. Never specified dowels for just 4" slab unless there's a potential for forklift/heavy wheel loads in which case the slab should have been thicker anyways.
I think you got it covered....

 

[SlideRuleEra]

brokensail - For a 4" thick slab to perform correctly, both the design and construction details have to almost "perfect".

Is the subgrade suitable and will it be properly compacted?

I have low opinion of wire mesh, but even the Wire Reinforcement Institute has a stern warning on placement:

Will the contract documents and field construction management ensure that correct placement takes place?

Control joints at 10' spacing in a 4" thick slab is at, or near, the accepted maximum.

A 4" thick slab with rebar (dowels or otherwise) cannot meet ACI concrete cover requirements. It is too, "thin".

Regardless of thickness, concrete curing is of highest importance for slabs... especially those on a vapor barrier. Do the contract documents and field construction management plans address this?

Making a slab somewhat "thicker" than the bare minimum buys a lot of "wiggle room" for oversight and errors.

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[dik]

Other than residential stuff... never done a 4" slab...

Dik

 

[brokensail]

SlideRuleEra thank you for the information. Subgrade should be fine and will be compacted.

Makes sense that there would not be sufficient cover for the dowels (was thinking a similar issue might exist for the water stop). Is there an issue with not having some sort of reinforcement continuity between the existing slab and the new pour?

There will be detailed specs and oversight during construction.

Personally, I'm kind of surprised that it is a 4" slab, but that's what I've been told from.

 

[SlideRuleEra]

Is there an issue with not having some sort of reinforcement continuity between the existing slab and the new pour?
I'm kind of surprised that it is a 4" slab, but that's what I've been told from.

If the subgrade is good for both the old and new slabs, should be no problem for the slabs to be independent. In fact, if dowels were used it could cause problems. Dowels in a 4" slab are not going to stop differential settlement... IMHO, the settlement will most likely cause the dowels spall the concrete effectively "destroying" either the existing slab, the new slab, or both.

The existing 4" slab failed. Unless you can pinpoint a clear reason that can be corrected, why should a replacement 4" slab be any better? Even if there is a clear reason, as Dik mentioned, 4" is minimal "residential thickness"... not commercial or industrial.

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[Ron]

Will there be forklift traffic on the slab? If so, don't go back with 4". Make the new slab at least 6" thick and as previously noted, excavate a "shelf" under the existing slab for support and forget the dowels unless you can use small plate dowels.

As for welded wire fabric...not a fan of that either. If you need minimal reinforcement, use small rebar and support on chairs to end up at the center of the slab or slightly higher.

Use a minimum 4000 psi concrete mix with as large a coarse aggregate as you can place without voids (will help reduce drying shrinkage). Don't use a pump mix unless you have to for placement.

Make sure the subgrade is well compacted and control the grade of the subgrade to within about 5/8" in 10 feet with no short grade transitions. Control the thickness of the slab to within -1/4" to +3/8".

Cut control joints at 10 to 12 feet on center both ways. Make sure the depth of cut is about 25 percent of the slab thickness and make sure the cuts are made the same day as placement of the concrete.

Do a pre-placement conference to make sure all of these things are covered and the concrete sub is clear on the requirements.

 

[dik]

...and a maximum 3" slump mix...

dik

 

[Mjkkb2]

4" slab is a very common thing where I practice for commercial buildings. Industrial ones will usually be thicker (6" min) but only because it's a warehouse doesn't mean it needs a thicker slab. It all depends on the use and particularly on the concentrated loads the slab will be subject to. Maybe a better question to the OP would be -
what is the use of this warehouse, and
and, did the reason for failure in the first place get investigated.

The WWF placement has gotten better. It often doesn't end up just in the subgrade doing nothing. I always specify continuos chairs to be used for WWF support and even have a detail showing spacing. The mesh will get droopy from warkmen walking on it but it should still do a decend job of controlling the cracks.

 

[XR250]

I would add to use proper curing practices to prevent curling at the new/old interface

 

[SlideRuleEra]

Might as well mention my recommendations:

1. Use the concrete mix details per comments from Ron and Dik.

2. A nominal 6" thick, slab with continuous reinforcement. That is 0.65 to 0.80 % rebar. No joints needed. The slab will crack, but the rebar will keep the cracks held tightly together. For a 6" slab, #5 @ 7" o.c., each way is a combination I like. Number 5 rebar will stay where it should be during concrete placement without large numbers of "chairs".

3. I would not dig under the existing slab. You mention "shallow water table". Of course I can be wrong, but that often goes hand-in-hand with "poor soils". If the existing subgrade is "good", leave subgrade under the existing slab alone. The existing subgrade has likely had years to naturally compact during normal warehouse usage. Digging will cause disturbance, there is poor access for compaction of an excavated area under the existing slab, and getting low slump concrete to properly flow under a structure (the existing slab, in this case) is something I gave up trying over 25 year ago.

4. Warehouse floor finish in important. I was an early adopter (in 1991) of the F-Number System for our electric generating stations.
For a "general purpose warehouse" F[sub]F[/sub]50 / F[sub]L[/sub]30 is recommended. For high-lift forklifts, perhaps as much as F[sub]F[/sub]100 / F[sub]L[/sub]50... although that is extreme.

5. Repeating my comment on concrete curing made yesterday: "Regardless of thickness, concrete curing is of highest importance for slabs... especially those on a vapor barrier." Continuously wet cure the floor for 7 days.

The above recommendation are for what I consider an "upscale" floor... that is, exactly what I have done or specified on our heavy industrial projects where long term satisfactory success is far more important than initial lowest cost. If these priorities are not suitable for the budget... please ignore the comments.

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[brokensail]

All, I very much appreciate the feedback here! Incredibly helpful.

I should have clarified one thing--the reason for this replacement is not due to any existing structural failure. This portion of the slab is being removed so the owner can inspect the subgrade in this particular area because of a previous environmental investigation...it's a long story.