Yes. Most slabs on grade are designed as unreinforced. But getting the crack control steel in a location where it is effective in the slab has always been an issue. |
I like 2 options:
1. Use rebar on dobies. When you step on the bar, it comes back. You can also step in the spaces between the bars when placed at 18" o.c.
2. Use 4x4 mesh sheets on dobies. It is easier to walk on the closer spaced mesh.
I advise all engineers to wear their working clothes and boots, place a few slabs on grade, and learn what works and what does not work. I placed my first concrete slab on grade in 1965 inside a pole barn in Galt, Missouri with my father. We screeded with a straight 2x4, used a garden rake for consolidation, tooled all of the joints, broom finished the surface, and used wet burlap to cure the concrete. I would not do it the same way today, but that 46 year old slab is still in service.
I also decided at that time to avoid a career as a flat work contractor.
Regards, Harold Sprague
Subject: RE: warehouse slab
Date: Sat, 9 Apr 2011 11:35:49 +0000
Most typical distribution warehouse slabs are designed as unreinforced slabs.
D. Matthew Stuart, P.E., S.E., F.ASCE, SECB
Structural Division Manager
Pennoni Associates Inc.
One Drexel Plaza
3001 Market Street, 2nd Floor
Philadelphia, PA 19104
Office 215-222-3000 x7895 | Direct 215-254-7895
Fax 215-222-0789 | Mobile 908-309-8657
http://www.pennoni.com | mstuart(--nospam--at)pennoni.com
From: Scott Maxwell [mailto:smaxwell(--nospam--at)umich.edu]
Sent: Friday, April 08, 2011 10:09 PM
Subject: Re: warehouse slab
My first boss HATED welded wire fabric/mesh and would always insist on rebar at 18" oc. He got that way from going to sites and watching working trying to lift the WWF (more realistically themselves) up with hooked bars so that the reinforcement was actually in the top third of the slab like you typically want it to be. He descriptions of people "trying to lift themselves" amused me to no end and have stuck with me long after I left that company.
On Apr 3, 2011, at 8:04 PM, Harold Sprague wrote:
I have been off line. My son (who is serving in the US Navy) was married this weekend in Greensboro, NC. It was a wonderful event.
Now back to your issues. If saw cuts are to be used, require an early entry saw with anti-ravel skid pad. And the joints should be cut within 6 hours of placement. Any time later, and the saw cuts don't do any good. Any re-entrant corners should have supplimental reinforcement placed 45 degrees to the corner to intercept any cracks.
I prefer using rebar (#3's at about 18 inches O.C.) for concrete shrinkage control. Rebar is harder to screw up as far as placement height, chairs, etc. I also like to specify a fairly stiff mix and allow polycarboxilate high range water reducers to bring the slump up to about 5 or 6 inches.
You have already received good advice about concrete slab design for the applied loads. Pick a reasonably heavy rack load and a reasonably heavy fork truck to design the thickness of the slab. I also like to use about a 3 inch layer of crusher fines to provide a good compacted base, but the sand should provide a good base as long as it is compacted. This is a farily inexpensive, but good method of providing a good base.
I would advise using armored edges of any construction joints with heavy fork truck traffic. I also like using square bars with PNA clips to allow for differential longitudinal shrinkage at construction joints.
I would rather err on the side of a conservative design and if you get complaints, ask for more definition regarding loads, traffic lanes, etc.
Regards, Harold Sprague
Date: Thu, 31 Mar 2011 14:43:23 -0400
Subject: warehouse slab
Harold Sprague, et al:
Have a client who is unable to provide us any meaningful information about their light industrial and warehouse slab. They just are useless in this department.
Its been a long time since I did any type of warehouse slab.
Racks similar to "home depot" and could be that tall (b.o. steel around 26ft), storing rather lightweight boxes of vitamins, supplements, etc. Forklifts similar to home depot, we don't know about the traffic (I'd guess light to medium). We know no other details and at this time in regards to weights, rack post spacing, base plates, etc.
Conditioned space in Florida. Sawcut control joints probably around 10' o.c., not sure about WWF, rebar or steel fibers yet.
I was thinking of giving them a design for a couple of different slab sizes and reinforcing schemes, and give them my basis of design, and then they can choose. Maybe in table form on the foundation plan.
I welcome any opinions, especially on conservative rack point loads and base plate sizes, spacing of rack posts, and slab thickness. I have been told by those in the know that the "Cadillac" of warehouse slabs would be #3 @ 14-18" o.c. E.W. (I think Harold said this once upon a time).
Semi rigid epoxy to fill the CJs, any favorites?
And probably most importantly, I was not planning on providing dowels at CJs , maybe only at construction joints (pour stops for the day...)
Clean sand, we usually get 2500-3000psf for our foundations with basic compaction... Moisture barrier, no gravel, just a sand sub-base.
I am going to borrow or download ACI360R, I have been told this has pretty much all I need. I know how to run the numbers, though its been a long time.
Any do's and don'ts or recent success stories, etc.?
Thanks in advance.