I’ve done a search and found that the general consensus is that a 4″ slab under a flagstone patio is considered solid construction provided that the base is compacted and prepared properly. My question is whether a mesh is required. Whenever we’ve done basement or garage slabs, a 4″x4″ grid was laid down and pulled up into the slab as it was poured.
I’m using the same mason who did my fieldstone veneer for my patio. He does great work on walls and veneers (see attached pic) but looked at me like I had 2 heads when I asked if he was using a mesh for the patio slab. I know he wont argue the point if I push it but I’m curious as to whether it’s common practice _not_ to use the mesh for any type of slab. In this case, the patio will be 12′ deep by 44′ long.
As always, thank you in advance.
-Jonathan
Edited 5/1/2007 7:08 am ET by Mojo
Replies
what can it hurt,one roll of wire would do that area and you know it's down there. is he going to pin the concrete along the porch and garage wall? larry
hand me the chainsaw, i need to trim the casing just a hair.
" is he going to pin the concrete along the porch and garage wall? "
Good point. We are setting footings where some steps will be and he did say he would pin those. Makes sense to pin along the whole slab.
Thanks
Makes sense to pin along the whole slab.No it doesnt, unless you have bad soil, pinning the slab is not going to help. The slab is held there by gravity. I dont like pinning because all structure setting to some intend. a house might settle uniform and no cracking will occur but if a patio is pinned it will pop the slab. The owner will start going crazy. I like patio, sidewalk, driveway to be isolated from heavy loaded structural. the patio is a floating slab, it designed just to lay there.
On the mesh you can do whatever you feel good with, I have stop using mesh years ago, everything with fiber.
Mesh will not stop the cracking. Dividing and pouring your patio into 12' X 11' slabs will stop all cracking. An "L" shape slab poured around your steps will crack.
Just don't pour half the section then pour the remaining minutes later. On large slabs without joints, you can see the cracks locating where each section of pour is placed. When cement is poured, crystals start forming, the next section crystals form in a different directions so it doesn't bond. Tamping, vibrating or high forms help the cement not crack.
Ok. So would you advise lining up a row of flagstone along each section of slab? Otherwise, I'd expect the flagstone bridging two sections would crack as the slabs move underneath. Thoughts?
Thanks.
"Ok. So would you advise lining up a row of flagstone along each section of slab? Otherwise, I'd expect the flagstone bridging two sections would crack as the slabs move underneath. Thoughts?"A little more work for the stone setter. But better jobs do it. Seen some jobs saw cut the slate. Saw cutting the slab doesn't always work.
I wouldn't pin/dowel unless there was a threshold nearby that needed to be not misaligned.
In this case, barring huge soil movements, the steps ought to "even out" any changes.
I would install 1/2" expansion joints against all the existing work before setting the new flagstone-supporting slab. I'm in Brownbag's camp on WWM being about useless, and that fibremesh & fly ash will "do" you better.
I'd be inclined to extend some expansion joints around the steps, and dimensioned to "hit" the flagstone module. The the flagstone man can use a flexible sealant at the joints in the flags, same as in the expansion joint. Which ought to keep any support slab cracks from telegraphing through your expensive flags. Ought to.
Pick one. Purple is expansion joints, red is #4 rebar set in middle of slab. Push it in from top after pour.
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Place 1/2" isolation tarred fiber board around steps. I would put it around house and garage too. No pins in slab.
No WWM either, cover slab with visqueen ASAP after finish and leave for 14 days, keeping 'crete damp under visqueen. Seal edges down with dirt or 2x so it won't dry out.
SamT
Praise the Corporation, for the Corporations' highest concern is the well being of the public.
Boy, thanks for all the responses. I thought this was going to turn into a "You asked the question but you already know the answer" thread. I almost didn't post it.
This is good info. I've done more research on expansion joints and the diagram is very useful.
Thank you all for your patience and explanations!
I know he wont argue the point if I push it but I'm curious as to whether it's common practice _not_ to use the mesh for any type of slab.
Common practice, yes, good practice, no.
The purpose of wire mesh is to limit the expansion of cracks that form. Some people claim that fiber replaces mesh, but it does not. Fiber helps prevent microcracks from forming during curing, but provides no structural strength and will not support the cracks that develop.
"Good practice" for a slab is to use fiber (because it compensates somewhat for curing issues - and it's hard to keep the slab soaking wet constantly for a month) and wire mesh for crack integrity. If the slab is structural, rebar should be used. Rebar is also a good idea if the substrate is likely to settle - but you are far better off to make sure the substrate is stable before you pour the slab than to try to compensate with rebar.
Edited 5/1/2007 1:05 pm ET by woodturner9
The purpose of wire mesh is to limit the expansion of cracks that form
If it were more often in the center third of slabs, I'd be more comfortable with that statement. But, in my demolition experience, it's usually right at the bottom of the slab, or just looped up in the bottom inch, and then down to the bottom again in about 1-2' "swoops." Or, if I did not find that last end of the roll snipped off where it was curling up out of the cream.
The big squares, laid flat for concrete flooring over steel decks are better, but not ideal--at least they are less "wild" than the unrolled WWM.
Almost all of it is way too rusty for best adhesion, so the scale around the mesh does nothing but make a tube around th mesh.
But, that's my experience; others' differ.Occupational hazard of my occupation not being around (sorry Bubba)
If it were more often in the center third of slabs, I'd be more comfortable with that statement. But, in my demolition experience, it's usually right at the bottom of the slab, or just looped up in the bottom inch, and then down to the bottom again in about 1-2' "swoops." Or, if I did not find that last end of the roll snipped off where it was curling up out of the cream.
All good points. To be effective, the mesh must be clean and centered in the slab. It doesn't have to be too far from the middle to defeat the purpose.
Ever since I found out about using fiberglass fibers in the concrete, I've completely given up on wire mesh for slabs (I've only had 4-5 of them done for me in the past few years).
Ever since I found out about using fiberglass fibers in the concrete, I've completely given up on wire mesh for slabs (I've only had 4-5 of them done for me in the past few years).
Fiber is better than nothing, and better than improperly placed mesh, but it cannot perfrom the function of the mesh. You really need both. Take a look at concrete failure analysis test results and you will see what I mean.
Based on a few years experience, placing concrete, a few years inspecting concrete, a year of demo work on old concrete, and what I learned in engineering school: WWM is a waste of time and effort to install. If I want reinforcement in a slab I spec bars, on dobeys or chairs.
Back to the topic at hand. The slab on grade, to act as a base for a flagstone patio, shouldn't require any reinforcement provided: the subgrade is stable and well compacted, the base aggregate is well compacted, and the slab is thick enough.
Sidewalks aren't reinforced, and residential driveways typically aren't either. I can't imagine that a patio will see more load than a driveway. They are designed with proper joint spacing for the thickness and strength of the concrete. For 3,000-psi concrete the spacing is roughly 15 times the depth. That is where the five foot joint spacing comes from on sidewalks.
I am not sure that a concrete slab is really necessary in this instance. I think that if the flagstone were laid on 4-in of compacted clean sand, it would work just fine, (crushed limestone would be better). This was the method of construction for centuries. The one at the last house was laid on sand, with geotech fabric, (weed blocker fabric is a readily available alternative), around the perimeter to keep the sand from migrating, and sand swept into the joints and hosed in until it wouldn't take any more. It was still holding up well when I sold the house eight years later.
Saw cuts and or expansion joints will stop concrete cracks is false. Nothing will stop concrete from cracking, that's what it does. You can only anticipate and "control" or lessen visible cracks.
As for the wire mesh, fiber mesh, or rebar debate going around my background is in heavy commercial and institutional construction (ie. universities, hospitals, etc..) with lots of concrete, and even these guys vary in their specs on how to do it right. (for slabs on grade)
I've seen slabs on grade with rebar matts or wire mesh and fiber mesh, or no steel and only fiber, exterior walks that require to be doweled to the structure and others to be independent of each other. I've poured slabs that require "slip" dowels and expansion joint in the same spot to adjoing slabs. (Idea being the rebar is not in structural tension but keep the TC of each slab even and still allow it to naturally expand and contract. ) From one extreme I know Michigan State University requires all exterior door stoops even on a 6 story building have a 4' footing and than the side walk will generally be doweled into the stoop. On the other extreme is all city walk and drive approaches around here call for no reinforcement to be added. (So it's easier to remove or repair as needed.)
Sorry, not much help on the reinforcement end, however as for it "being hard to keep a slab wet for a month" one added step to help aid a "wet cure" is to lay burlap down under your plastic. It will help to retain the moisture so your not watering your concrete every couple of hours. Also be careful you don't "shock" your concrete and dump gallons of cold water on it as soon as it's hard enough to add water. Sort of like putting a hot frying pan in the sink, you can wait several hours if not the next morning even and still get a wet cure. If done incorrectly you can actually be doing more damage than good because you don't want to shock it cold nor do you want to trap the concrete's heat under a piece of clear plastic that will act like a green house baking the internal temp. even higher in the afternoon sun.
I will add a question about curing slabs. What is the consensus (if their is one ) or input ( which I hope there will be ) about the substituition of arcylic or other formulas of cures spray or roller applied to slabs in lie of covering and keeping damp? Not something I have seen discussed here. No I didn't do a search.
"Poor is not the person who has too little, but the person who craves more."...Seneca
it works ok, but if you are staining the concrete it will react with the stain
I don't know for sure, but I think:
A water hose and burlap|plastic will keep it wet for as long as you want.
Curing compounds help keep it wet for the first some odd days. A week maybe.
In dry summer weather, the really good and experienced mudders who taught me always used burlap wetted three times daily.
We did really huge slabs, 80'x100'x6', 120'x160'x12', 100'x8' tank bases with no visible cracks.
(n') = depth in feet.SamT
Praise the Corporation, for the Corporations' highest concern is the well being of the public.
SamT and Brownie, Thanks for the input. The spray on cures are the most common place way to address the issue here.
I was taught to cover a slab for a house or garage with 15# felt just as soon as one could safely walk on it. We then just build right over the felt.
After the job had progressed to the point of trim work we would pull the felt up . Slabs were always still "green" beneath the felt.
"Poor is not the person who has too little, but the person who craves more."...Seneca
I myself always cover with burlap and a water hose but that method is dead in commerical work. spray the chemical and start walking on it. Highway dept, its law, burlap. no other method accepted
Actually burlaping isn't dead in commercial, that's where I've learned it as standard practice and have seen it called for in the job specs. I've done it on a military installation that required it both at the ground floor with a vapor barrier and on the upper floors too that were composite deck. They also required a squeegee on acrylic curing compound prior to burlaping, wetting, covering with plastic and than thermal blankets too since it was the dead of winter for 30 days. Man that was fun 400' of fire house ran into the middle of the building and a dozen 3/4" hoses split off in every direction to keep the slab wet. As if construction in Michigan during the winter isn't muddy, wet, and cold enough already. (That's also where I learned the residual effects of chloride and concrete burns while man handling also those wet layers of material that soaked it for a month. Never had a problem with raw concrete, I've mixed it bare handed in a pinch with no effect but messing with that burlap you're hands would be in pain and raw in 20 minutes.)
I realize that was a federal project just like the DOT but I've done it regularly for other projects that aren't public.
Sort of like putting a hot frying pan in the sink, you can wait several hours if not the next morning even and still get a wet cure.
Thanks for posting a lot of good information. The issues with shocking the concrete are often overlooked - thanks for bringing that up.
The problem with waiting to add water is that if the curing reaction progresses to the point that it is starved for water, it slows down or stops, and adding more water doesn't solve the problem once that happens. On a hot summer day around here, I wouldn't be comfortable waiting more than an hour or two to flood the slab with water, given the risk of corrupting the curing reaction.
FWIW, when I poured my garage floor, 16'x27' with no expansion joints, I did wet cure it for a month. Years later, there are still no cracks. So concrete doesn't HAVE to crack, but often does, for a variety of factors generally not fully within the contractor's control.