Joints in PEMB slab with double mat

[CJBHM52]

I am working on a slab on grade design for a PEMB. Slab will have turned down perimeter footing and will be poured monolitically with column footings. Soils are not ideal and there will be heavy equipment loads (forklifts, loaders). Slab will be 12" with double mat of rebar. So how do the joints work? If I am reading ACI 360 right, it is recommended to install dowels or plates along each saw cut joint? That doesn't seem practical for a large slab (14,000 sf). What about leaving the bottom mat of rebar continuous across joints and breaking the top mat at each joint? There would be certain joints that would be doweled, but not every one. The main frames are spaced at 33', so I was thinking about putting one joint along the frame line and one between each frame (16.5' oc) and 15' oc perpendicular to the main frames. Are there any potential problems with a joint running into the column footing when the footing is poured monolithic with the slab? We wouldnt sawcut over the footing though, it would stop at the footing. Thanks in advance.

 

[msquared48]

I would not put the joints in line with the mainframes, but to the side by 8.25 feet, 2 per bay to get your 16.5 foot spacing between the sawcuts. You probably have tension ties or hairpins in the slab you will not want to cut.

As for the bars transverse to the cut, you should discontinue most of both the top and bottom bars, either providing 25 to 50% continuity of both across the joints, or greased dowels at the center of the slab with the bars totally discontinuous.

Mike McCann
MMC Engineering

http://mmcengineering.tripod.com

 

[hokie66]

For a 12" slab reinforced bottom and top, I would run all bars continuously through any construction joints you have, and would minimise the number of those to whatever size you can place in a day. If you place the whole thing, no joints. Provide .5 to .6%Ag crack control reinforcement. Joints are the biggest maintenance headache in industrial slabs with forklifts and loaders, so are best minimised. If you do use joints, use dowels middepth for shear transfer.

 

[CJBHM52]

So if you do not provide any joints, or a minimal amount, you rely on the top layer of rebar to control the cracking? So you would end up with tiny cracks all over the slab but no (ideally) significant cracking?

And if we were to go with joints, you would discontinue most of the rebar so it allows the joints to move, but leave some so you still get decent aggregate interlock? Thanks.

 

[paddingtongreen]

How big an area do you expect to place per day? You have nearly 560yds to place. Your expansion joints should be between the columns, not at them, as Mike says to do with the control joints. I've never tried to sawcut control joints in that thick a slab,just provided shrinkage rebar.

Michael.
Timing has a lot to do with the outcome of a rain dance.

 

[hokie66]

CJ,
Both layers of reinforcing act to control cracking. The subgrade drag, which is the force that causes tension in the slab, is at the bottom, so in many cases, crack initiate at the bottom. The total area of steel assists in controlling the cracks to an acceptable width. Yes, you would have tiny cracks all over the slab. In industrial applications, that is operationally preferable to joints (straight cracks). You should discuss this philosophy with your client. Jointless slabs are becoming the norm in industry, including high volume storage facilities.

No, I would not discontinue reinforcement at joints, but would carry it all through, and in addition, add dowels for shear transfer.

 

[CJBHM52]

Is the .5%Ag for each direction or total for both directions? And does it include bottom rebar or just top? ACI 360 recommends the rebar be placed as close to the top as possible, so it makes it sound like it would just be the top but a #8 @ 12" grid just on top seems excessive for crack control. I think #5 @ 12 on top and #6 @ 12 on bottom seems much more reasonable which would put me just over .5%Ag.

I am leaning toward doing two construction joints per ACI 360 so there would be three roughly square slabs. Would it present any problem to pour the end slabs day one and the center portion day two. Its a 70x200 slab, so the end slabs would be ~50x70 and the center would be ~100x70 to avoid joints at the frame lines. Or would you pour the center portion the first day and the outer portions day two. I guess that would be up to the contractor.

Thank you all for the info

 

[hokie66]

0.5 to 0.6%Ag reinforcing in each direction, including both top and bottom bars. I am not familiar with ACI 360, as I am not in the US, but that may be aimed at the thin slab commercial and residential market. Your #5@12" top and #6@12" bottom sounds about right.

Either scheme for your 3 pours would be fine.

 

[CJBHM52]

Ok, thank you all very much for the input.