Thread: OT- Yet another concrete question... re wire mesh placement

Sorry for so many concrete questions but I keep discovering new things as time passes. This commercial building I'm looking at possibly buying (or really part of the building...flexspace arrangement). Building is aprox 14,000 sq ft, I'm thinking of buying 6,250 sq ft on right side. The 5,000 sq ft on left side (middle spaces already sold) offers a unique opportunity to see how they did the slab as they didn't complete the slab. The slab is basically a rectangle around the perimeter of the inside aprox 15 feet with the middle area left open. This, apparently so that new owners could run pumbing to suit their needs.

Anyhoo, from this unfinished slab I can see it only 4 inches thick. Plastic sheeting on ground with wire mesh above. From what I have read, the wire mesh really should have been elevated so that it runs in the middle of the slab, not on the bottom. So the question is, does wire mesh on the bottom do any good at all ? I suspect if this was pointed out to original contractor he would say that if wire was in middle there wouldn't be enough thickness available to lay plumbing without concrete above the pipes being too thin. My answer to that is it never should have been 4 inches in the first place, should have been 6 inches. Thoughts ?

Likely they give the concrete guys hooks to pull the mesh up when they are pouring.

IMHO,

Low end = big wavy rolls of wire mesh set on the ground and pulled into place by precision concrete workers.

Higher end = wire mesh "mats" which start flat. Set on chairs or dobies to maintain their position.

I have heard the theory that the wire mesh at the bottom isn't all bad as it adds ductile metal to assist the concrete where its tensile strength is lowest when there's a point load on the surface above it. Prep work for my personal job would be choice #2 above.

Also...the solidity of the stuff underneath is verry important no matter how thick the concrete is nominally poured to be....

It doesn't matter where the wire mesh is on the edges. Its on the bottom, next to the plastic, for a large portion of the slab. The cost of the wire and installation was completely wasted. If the money spent had instead been spent on 6" unreinforced concrete there would be a much more stable floor.

Like Matt said, the subgrade is a structural part of the floor. The condition, composition, and compaction is more important to the integrity of the floor than presence or placement of reinforcing steel.

Go there with a 15#+ sledge hammer and holding the handle by the end thump the head in to the "compacted" base and see how dense it is, probably wasn't even compacted , where is the water table to close to the surface and you get a drenching rain loosens the compacted base and you end up with cracks or worse. My 8"+ 4,000# concrete slab with over than 700' of #8 rebar in it is setting on 24"s of compacted 1" to 1 1/2" crushed stone, 6 years of New England weather still no cracks or settling, no saw cuts in the slab 16' X 28'. Floor is flat and smooth enough I can do fab work on it .

[QUOTE=matt_isserstedt;1938924]

...............pulled into place by precision concrete workers.

..................
[QUOTE]

Thanks, I needed a good chuckle this morning.

Yes, they are supposed to pull it up as the concrete is poured. From testing done after the fact on poured slabs the level of "precision" isn't very high. Done correctly, it should be on chairs to hold it at the proper height.

Concur with making sure the base is properly compacted as being a critical factor.

4 inches thick is adequate for most needs. You will have to determine if your planned usage falls under "most".

Steve

when i had machine slabs poured the local building codes required that rebar be on chairs a min height off the ground, and an inspector came to assure that was so. i think the same rule applies to mesh.

Up, yes, too high, no.

The BOTTOM is in tension when loaded, and wire/bar takes the tension. If you pull it too high, it will be on the neutral axis or higher, and do little good. The slab will break when loaded and hinge around the bar if it's too high.

If it is something like 6 x 6 - 10/10ga, the wire adds so little strength to a 4" slab on grade that it barely matters where it is vertically. Ideally, it would be below center, to resist loads from above. But whether a wavey lay, generally 1/2" from the bottom would be any better than one 2" from the bottom (mid point) is about moot. Since small ga wire primarily is used to hold concrete toghether when it does crack, middle would be better for this app.

Far more important would be was the best mix ratio concrete placed with the ideal slump (minimal moisture) and compaction by skilled labor and finished & cured properly for the weather conditions at time of placement. Quite a bit of later concrete cracks are really shrinkage cracks initiated by wet mixes poured fast by unsupervised low bid subs since it "self"-levels and is easy to move around.

"wire" will help keep cracked portions of concrete from shifting & for that purpose should be near the middle of "thin" slabs (under 6") or about 1/3 down from the top in thicker slabs that have significant bar reinforcement in the lower 1/3 of the thickness. IME, it does not practically add much strength in the average installation or keep it from cracking. (unless you are getting down to 4 x 4- 6/6 ga mats) But if you are worried about loads from above, or soil subsidence (as opposed to swelling) then wire or rebar toward the bottom is better.

smt

I have always heard 1/3 in either direction, bottom or top. Middle offers no reinforcement strength at all. Worse, yet, as discussed above, where it ends up during and after the pour is anybody's guess. It floats, it sinks, the workers pull it too high, not high enough, etc... Just not generally worth the effort to even include it. Get your concrete fiber reinforced and let it ride.

I was advised by a wise engineer to not even bother with mesh, but to just go get rebar and cut/tie it myself. After pricing the mesh, I would have needed just over two rolls of mesh, leaving a bunch of leftover. The cost of two rolls was significantly more than the required amount of #3 rebar. You can also custom tailor your rebar to the conditions you will be putting on the floor. I spaced the rebar on 6" centers at the outside edges for the first few courses, then went to 8, 10, 12, 16 and ended up with the very center of the slab at 24" centers. If the edges never crack, the middle won't. I put all the leftover5-6ft pieces in the bay door area spaced between the other pieces, It's about 10" on center from the edge to 10ft inside that door.

For a light duty warehouse floor you "might" get away with a 4" floor. I'd certainly settle for nothing less than 6", especially if there's going to be machinery involved. Have to check again but I'm pretty sure here in B.C.--we have a provincial building code and, since we're in an earthquake zone, everything is based on seismic requirements--that 6" is minimum thickness for a commercial/industrial floor. The building we're in is about 35 years old and its floors are at least 6" thick. As others have said, reinforcing is important but if your base is bad the floor is ultimately doomed to failure..

Originally Posted by stephen thomas

If it is something like 6 x 6 - 10/10ga, the wire adds so little strength to a 4" slab on grade that it barely matters where it is vertically. Ideally, it would be below center, to resist loads from above. But whether a wavey lay, generally 1/2" from the bottom would be any better than one 2" from the bottom (mid point) is about moot. Since small ga wire primarily is used to hold concrete toghether when it does crack, middle would be better for this app.

Far more important would be was the best mix ratio concrete placed with the ideal slump (minimal moisture) and compaction by skilled labor and finished & cured properly for the weather conditions at time of placement. Quite a bit of later concrete cracks are really shrinkage cracks initiated by wet mixes poured fast by unsupervised low bid subs since it "self"-levels and is easy to move around.

"wire" will help keep cracked portions of concrete from shifting & for that purpose should be near the middle of "thin" slabs (under 6") or about 1/3 down from the top in thicker slabs that have significant bar reinforcement in the lower 1/3 of the thickness. IME, it does not practically add much strength in the average installation or keep it from cracking. (unless you are getting down to 4 x 4- 6/6 ga mats) But if you are worried about loads from above, or soil subsidence (as opposed to swelling) then wire or rebar toward the bottom is better.

smt

This is your best explanation. Go with it. Mesh is for crack control. If it required structural reinforcement it would be rebar.

For a warehouse minimum of 5 inches with #3 or #4 bars at 12 to 18 inch on cementer 6,6 ,10 is useless, concrete is going to crack, 6 to 8 inches for heavy traffic.
You need about an inch minimum of concrete around the steel.
concrete has good compression strength but not good tensile so generally you want the steel in the lower portion 1.5 to 2 inches usually the steel keeps the cracks from spreading.
The fiber added to the concrete helps with the spider cracks but isn't a substitute for the re-bar grid.

I found the plastic chairs/top hat spacers for the mesh were great at setting the height of the mesh, I can really recommend them:

My shed engineer specified the 6" mesh to be one inch from the 4-5" slab surface so I asked around and was told that's pretty normal. There is a constant downward force of many tonnes on the slab (and buried foundations) round the edges from the core filled concrete block walls.

I used plastic chairs to set the mesh height and they worked great, they hold the mesh at the correct height even with the concrete pumping hose resting on it. When we did the pour everyone was walking over the mesh but as their weight came off, the mesh bounced back to correct height. Using the plastic chairs took a lot of guesswork and hassle out of pouring the slab. Looking at the photos, I'd say at worst case the mesh is in the middle. The concrete is 4500 PSI as it didn't cost much extra to buy, just less water and more plasticiser additive to the mix.

After three years, there are two cracks width-ways right about where the expansion joins should have gone if the concretor wasn't so lazy to put them in along the 40' long slab.





Attachment 65606Attachment 65608

As others have said.

The mesh itself being seen before the concrete pour on grade is not a concern. This is typical practice for concrete contractors to pour the concrete, and while it is still wet to pull the mesh up as they pour, and as others have stated as the workers then move through the concrete before finishing it they step on it - so there is no firm answer to where it actually ends up (but core samples I have seen show the reinforcing steel usually ends up near the bottom of the concrete when done this way). Reputable contractors will place the reinforcing steel up on plastic or masonry (concrete bricks) spacers before pouring concrete to avoid this.

The reason the reinforcing steel needs to be approximately 1/3 of the concrete depth from the grade is 1) the steel absorbs all the tensile load that is applied to the slab (imagine the slab as a beam, when loaded in the middle it bends downward, putting all the tension in the bottom of the chord) which is in the bottom portion of slab when under load, and 2) the steel needs to be up from the grade far enough that it will not immediately encounter water that is wicked up through the concrete (typical concrete is not waterproof without treatment, but will provide a capillary action for water to climb up through it), hence why most DOT specs call for a minimum concrete cover over any reinforcement steel.

A 4" slab does seem to be lightweight for a commercial property. Realistically for a commercial use slab I would want to see 6-8" concrete thickness with a proper rebar #3-4 (rebar number indicates how many 1/8ths of an inch it is in diamter, so #3 is 3/8ths diameter) used in place of mesh, especially if you will be using the space to store heavy equipment that creates heavy point loads. As others have said the preparation of the subgrade counts heavily in how well your concrete slab may perform.

Really the best way to assuage concerns would be to get a material testing/consulting company to drill cores in the slab of the portion of the property you are looking to purchase. It may take more than one core to hit reinforcing steel, but it will show you what size and where it is - since if there is part of the building that isn't completed it may be that the entire property wasn't all done the same(concrete depth or reinforcing steel). It may also show if there has been settling of the subgrade under the slab, which would be the single biggest reason for any on-grade slab to fail. The testing company will probably also have a PE on staff that will be able to include in their report what the recommended loading for the slab is, although they will of course charge more for his or her time to analyze it.

Geoff

You are fortunate to be able to easily view details of the slab's construction, and based on what you've described the arrangement could be politely described as "minimal". And sadly typical of a lot of similar projects throughout the Sun Belt states in recent years.

The wire mesh-beneath-the-pour-issue was discussed in an earlier thread, with various opinions and theories. I'll keep mine to myself, other than to say "that ain't the way I'd have dunnit".

OT - Question about Improperly Poured Concrete Slab, What To Do?

~TW~

Originally Posted by stephen thomas

If it is something like 6 x 6 - 10/10ga, the wire adds so little strength to a 4" slab on grade that it barely matters where it is vertically.

I assumed the mesh was heavy enough to be a help. If not, then who cares? In that case it is just as mentioned, to keep the broken piece together for a short time..... I say a short time because anything that doesn't offer tensile strength also will tear, break, or rust through pretty quick after the concrete cracks.

The small wire does add significant strength for crack resistance and to keep cracked portions tight. Even in exterior work, this matters. But the point is that in a 4" slab, for "strength" (resistance to deflecting with external loads) it barely matters where it ends up vertically because if everything else is not right, there is not enough depth of section to resist significant external loads. No matter where you put it in the slab, the geometry is not going to give a high gain in the overall tensile properties of that slab with a load on top. Worse, as has been more or less "argued" in this thread, you don't knwo if the loads on a floor are going to be due to a machine or vehicle "pressing down" in the center (or soil subsidence); or the earth "pressing up" due to perimeter subsidence or, or hydraulic pressure in the center or a combination. I think on a cost/performance basis, wire is better than fiber, which I personally think does not have a good cost/benefit ratio for applications like a shop floor.

As in any other structural application, the individual project needs to be "engineered" for the app and for the budget. In some cases, say a level sidewalk, it might be better to "take" the money for the cost of wire and the cost of placement & control, on merely pouring the sections an additional 1-1/2" thicker, and making good cut joints. In a suspended floor, with "perfectly spec'd and placed concrete" the wire in the moulded sections might allow those sections to safely be 2-1/2" thick and span larger areas, instead of say 4" thick in combination with the way the grid is laid out; saving money on overall concrete volume and weight & accumulated loads elsewhere in the structure. ETC.

I find it astonishing how long metal lasts in concrete even in exterior use after it cracks. But it does begin to set up a vicious cycle of rust and spalling especially if salt is present. Inside without a lot of moisture, it barely degrades.

As others noted, I prefer rebar for adding "strength" to structure, especially in exterior work. If you get into wire in the 6 x 6-6/6 & heavier mats or more closely spaced grid, say 4 x 4, use mats that lay flat & place it carefully and use chairs or masonary support, it can begin to add a lot of strength to the structure. But it still takes some ciphering to decide where to "use" the allocated funds: more reinforcement, or a thicker slab or a "richer" mix to optimize performance per budget? The labor costs can't be ignored in the material calculations, either. Having guys do a perfect job with 4 x 4 - 6/6 wire in a 4,500lb 3-1/2" thick slab may save a lot of concrete cost, but considering the "extra" (labor and steel) is the in-place construction "superior" for the app than a sloppily poured 6" thick 3,000 lb slab with rolled 6 x 6 - 10/10 wire sloppily pulled up from the bottom if it is a garage floor at a similar cost? For a suspended slab with foot traffic, you might want the first. For a garage floor, maybe the latter. If costs are no object, do both (tons of rebar, wire on top for crack control, and a 10" slab)

Many of us in our own garages or add-on shops are reduced to using a 5 bag mix, 4-1/2" or 5" thick, and hope it does not crack too badly. I would work really hard on a compact base, & add wire to the concrete.

Following on my own "economic" ponderings above, I guess what I am saying is that rebar (in the past, it's been a few years for me) seems to add better cost-benefit to a project if it is necessary to add significant "strenght" to a given thickness slab. But wire is useful, and if you can't pour the perfect no-cost-worries slab the thinner wire adds very useful crack control. It won't change whether the slab does or does not crack when the thrash truck accidentally backs over it. Which is closer to the question Milacron seems to be asking.

smt

I don't think this building was built with placement of heavy machine tools or operation of forklifts in mind. As most usual, it was built to look good enough and be mostly good enough for typical commercial uses, not necessarily for industrial use.

As to proper concrete construction methods, as far as I know the only good answer is rebar placed appropriately per the engineer's specification, which is based on the desired characteristics the engineer was given to design to. Wire doesn't do much but it will help hold together a typical light duty commercial or residential garage floor.

The link below gives you the correct answer. Basically it should be in the middle 1/3 or about 2 inches from the top in a 4-6 inch slab. It is used for crack control, not reinforcing. I have evaluated many slabs and almost always find it at the bottom. As others have said, contractors like to pour the concrete on top of it and try to pull is up to the middle-seldom works. If the slab is not badly cracked it should not matter too much.

The slab that the Tod Engine sits on is 8" thick with No. 4 (1/2") rebar placed in a 12" square grid. It was placed between 2 to 3" from the bottom. The portion of the slab directly under the crankshaft has about 230,000 lbs. of weight on it, and that has been enough to flex the concrete down about an inch and a half. The concrete was placed over 18" of large sized steel slag which was then covered with smaller steel slag and the slag was then allowed to hydrate. The aggregate itself is now nearly as hard as the concrete.

We will shortly begin building forms for adding an additional 36" of heavily reinforced concrete in the space between the slab and the bottom of the engine bedplates to spread the load out and tie both sides of the engine together. I will be using nothing smaller than no. 8 (1") rebar placed as Joe Michaels has specified in a set of plans that he drew up for me. One addition will be adding eight full lengths of 100 lb. railroad rail all the way across appropriately spaced and about 3" up from the current slab. Putting rail in heavy concrete is an old school technique to add a lot of strength to concrete that will be functioning as a beam. My forms will be 1/4" to 1/2" steel plate which will become part of the foundation. The rebar and rails will be welded to the inside of the plates. The plates will act as girders as well, providing additional rigidity. I plan to use 4,500 psi concrete with a water reducer additive to keep the water content as low as possible.

Yes this is getting way off your initial question about wire mesh... sorry.