I enjoyed Mike Guertin’s article on pouring a slab, and had two questions.
First, he talked about pulling the reinforcing mats “into the upper third of the slab”. Reinforcing steel generally needs to be low, right? (remaining code-approved distances from the grade, of course). I guess he’s assuming the wire will settle a bit, so start high for a low final placement? Or have I had it backwards all along?
Second, in the magazine, the only finishing tool used is a bull float. I was taught that a steel trowel phase was neccesary for strength, even if you’re going to drag a broom across it for the final finish. That always seemed like a lot of hard, stressful work if you don’t need the glassy finish . If it’s not neccessary, I’ll never do it again. Is it neccessary?
Replies
In MI, the old Italian concrete guys always troweled before the broom. In the 90's the younger guys claimed that the troweling invited more stone pops from winter water intrusion and broomed after bullfloating.
Done well, both jobs looked good and were acceptable.
Bob's next test date: 12/10/07
Yeah, my young helper always want to skip that step too, although I think that has more to do with being thirsty than with concrete performance issues. I'm starting to think he might be right. I'm in temperate No. Cal., I don't know if that negates the stone-popping issue.
The way I see it is that the older guys wanted to "close" the surface with the steel trowel. The newer guys just broomed it and there was no "surface" for water to get under and scale. Bob's next test date: 12/10/07
That's what I was taught also, that the steel trowel compacts the fines and cream, making a harder, more durable surface layer, even if you broom it. I guess my question remains; is that necessary, and when and why? I can see if you're bouncing bowling balls on it, it probably is, but what about a driveway, or a sidewalk? Is there a rule of thumb?
The trick is too bull float it well,
so the aggregate is worked down from the surface. To " Bring up the fat" as they say. A troweled surface is less porous, which does two
beneficial things. One is to slow the curing process,
so they slab will be harder.
The other is to keep water out, so it will
weather better. So no it isn't necessary but rather
a good thing to do. Make it cure slow and seal it right away.
Anyone know what effect a power screed has? It sure does leave a nice finish, even though they keep working it from there.
Is the finished product a little different? What I mean is that I've seen broomed finishes that have sand in them so the lines are kind of furry.
Does the troweled/broomed finish have less sand in it? Are the lines finer?
Dunno. I always trowel, because it was drilled in my head that you had to. That's part of my question about to trowel or not - I don't know from personal experience how much it matters. I wish I'd experimented with not doing it, just to see how it turns out.
Let me pose the question like this- If there's no explicit mention of steel troweling in specs, is it assumed that it won't be done?
I don't know. If a lot of finishers are doing it, not troweling, and it isn't in the specs then I guess so. I have noticed that overall concrete finishes have gone down hill over the years. Maybe the safety of a ruff finish has become more important or maybe it is just an excuse.
Maybe the safety of a ruff finish has become more important or maybe it is just an excuse
- isn't broom finish a code requirement - of most codes - on exterior concrete now? Here we can finish the garage floor with a trowel finish, but the driveway has to be broomed.View Image “Good work costs much more than poor imitation or factory product” – Charles GreeneCaliforniaRemodelingContractor.com
You're right, Huck. My original question though, was if it is advantageous to steel trowel before brooming, or seeding aggregate, or whatever the final finish is. I was taught that troweling was important for durability even if the final finish isn't a smooth one. (But then i've been taught a lot of unnecessary stuff.) Is troweling a waste of time if you don't want the glassy finish?
k
You're right, it probably is code. I guess there is code, and then there is code. Or beyond code.
My point is that I've seen finishing a lot of material workmanship go down hill. Not just concrete. I've seen a lot of broomed finishes that are sloppy. A heavy broom finish can be used to hide sloppy work.
I can be pretty critical of work though. I ride a road bicycle and i feel every bump in the road. I've always thought that if you were hiring someone in charge of quality control of roads or paths hire a bicyclist. Those skinny tires and small seats are pretty unforgiving.
One "Roady" to another-
Lets keep the Spandex talk to a minimum. Pure speculation here, but other then making sure to
raise the fat I don't see how troweling would make
any difference if your going to open the surface
with a broom. sorry about the run on sentence.
I guess the theory is that you pack that top layer real good, compressing the fines, working out air bubbles etc. and that the top 1/4" to 1/2" is extra strong, even if you scratch the top 16th open with a broom. Only theoretical, though.
I know commercial work (parking garages, etc.) is troweled, even though the final surface is usually left rough- not broomed, but you can see the swirls from the power trowel. Any union concrete finishers out there?
k
By the time your set up enough to
trowel, I doubt your compressing much at all.
I'd give ya an 1/8" to be generous.
Lets face it, the broom is going to open it up
quite nicely. No, lets say you float it enough that the broom stays in the
fat. Certainly what it looks like.
Then the question is how rich do you pour.
You're probably right. I'm not arguing either side, just relating a theory. It sure sounds like the consensus is for not troweling unless you're leaving a troweled finish. Is there a definitive source on this or is it years of experience?
k
Not claiming to know the definitive answer,
just my humble opinion.
Yeah I've poured a good bit of concrete, but this would
require some real studies to find out. All in all, I'd say if it's sealed well what's the difference?
Redi-mix drivers dont get that much fun so filling boots up/overloading wheelbarrows and telling u not much when u ask.."how much concrete is left?"..is a real fringe benefit for them.
Plus us in the Gambino family have no problem with someones boots getting filled with concret every now and then !
I have some vivid memories of myself;
twelve years old, skinny as a bean pole trying to
handle an overflowing wheelbarrow caught on the mesh.
My dad yelling-" keep the handles down!"
Me cussing like a sailor
The driver rolling around, busting a gut.
FHB editors decided to make the article a 'tips' article rather than an all about pouring slabs. That's why some things aren't clear to pros.
Wire mesh does not technically 'reinforce' concrete. The purpose it's used in a slab is to prevent concrete spread when cracks occur. In supported slab work (over good base), putting the mesh in the upper third is most effective.
IF you are creating a true reinforced slab - best done with cross network of #4 or #5 rebar - you'd want the steel in the lower third so the steel is in tension when the slab is loaded.
There will undoubtly be a ton of LTE in next issue knocking me on the reinforcing issue.
If you go to the online extra for the slab article, I think it shows me steel troweling, brooming and brushing.
There's little benefit to steel troweling if you're going to brush the surface. Mag floats (like a bull float) bring up the fines (cream, paste.... cement particles) and push down the aggregate (stones and sand). The steel trowel closes off bubble holes in the fines and as other posters noted, make the slab more durable and surface 'harder.' By minimizing the water that gets into the slab over time, the weathering is reduced. BUT you end up with a super smooth finish that isn't the best thing for exterior slabs. So when you run a broom over a steel troweled finish you end up opening up pores again. End result is there's no benefit to hitting the slab with steel.
I find that for patio slabs in my cold/mixed climate that mag floated surfaces give the best balance of durability, hardness, cleanability and slip resistance. For a sidewalk, I'd probably use a fine broom and driveways might get a course broom (lots of ice in winter here).
HOpe this answers what was left on the cutting room floor.
MG
Thanks MG. I probably ask stupid questions a lot, but only to people whose answers I respect. (did that make any sense? it's meant as a compliment).
lots to learn,
k
I didn't think the questions were stupid. I just wish we could cover subjects in enough detail so everybody 'got it' right away.I'm by no means an expert at any of this stuff. I ask a lot of questions so I can figure out what works best under what conditions.Keep asking questions. The trick is deciding which answers are stupid and which ones are valuable.Mike
"There will undoubtly be a ton of LTE in next issue knocking me on the reinforcing issue"
The joys of writing for a national periodical. These days it seems you need to have a pretty thick skin.
So to start the ball rolling: I've always wondered whether reinforcing in patio slabs wouldn't be just as effective in the top third. My reasoning is that the forces that act on the slab leading to cracks are usually not from excessive loading as they would be on a suspended slab, but rather the result of differential settling creating point loads acting from below. Depending on how well the substrate supports the slab, there may be just as many areas where the top of the slab is in tension as the bottom.
As an aside, the one tip I would have added was to wear rubber boots with a draw string top to avoid pouring a couple of pounds of mix straight from the chute onto your ankles.
You give the idea of concrete boots new meaning. I've had a couple pairs of old concrete overboots that have rips in them - not much good for slogging through soup but if I put them on my brother when he's taking a jobsite nap and fill them up - He'd sink faster....
Wire mesh does not technically 'reinforce' concrete
The editors left it so that seemed to hit my common gripe about WWM. Sure it gets yanked up to some vague sort of depth during placement, then to be walked right down to the bottom during screeding. In ripping out concrete with WWM, I've seen almost as much mesh out the bottom of the pour as in.
It's a good thing we have fibre mesh and fly ash in my book.
That article has been nagging at me, and I really had not ciphered why, until you brought this up. (Can be illuminating, to read an author's diaries, with their unexpurgated commentary on the editing they had recieved--definitely a thing best left to posterity, though <g>)Occupational hazard of my occupation not being around (sorry Bubba)
Studies I've seen on fibers haven't shown them to either reinforce a slab structurally nor reduce shrinkage cracks. They mostly reduce micro cracks that lead to slab deterioration when water gets in.And as far as I understand about fly ash, it mostly makes the concrete more plastic so you can pour a stiffer mix. No need to add water. That does help reduce shrinkage.You are right on about wading back through the mix during screeding. I try to be careful and minimize the actual steps by reaching the screed out. One reason why I like a jitterbug screed rather than a two person saw screed. In commercial work you'll see guys use creeping scaffolds that hover over the slab by inches.I think one of the reasons that the ICC included a requirement for permanent mesh supports (chairs) to elevate the WWF is to try to prevent just what you experience from occuring - the mesh being driven back down by walking. Even though it may be a nice idea, I haven't found them effective. The legs sink right into the bed. And you trip all the time. It's funny they way the code language is written. Something like "When slab reinforcement is used, it must be supported by....." There is no requirement TO use reinforcement, but if you choose to you MUST support it. Heck, I'll just skip it.MG
And as far as I understand about fly ash, it mostly makes the concrete more plastic so you can pour a stiffer mix. No need to add water. That does help reduce shrinkage.
That's more Johnny's balliwick than mine, but my general understanding is that using fly ash as an admixtured meant the pour tended to wind up on the higher side of the "possible" strength range than not. Which might give a stronger pour, or maybe, a "tighter" surface finish.
The "small job" stamp article would likely garner a lot of Table of Contents interest. The trick of that will be in getting the parties to agree to what "small" is. I know of a local 'crete contractor who considers anything under10 yards as small (often prefixed with "too"). Client might think half a driveway, or an 8x8 patio as small. Client starts from the slightly flawed point that, small = easy. Contractor is used to yards placed (and, maybe, manhours per yard).
Now, I really like the effects you can get with stanping and coloring concrete. (It's also kind of cool seeing "artist" come out of a PM/Super, too.) It's an under-used technique mostly because it does not get thought of enough--and I'm as guilty of that as any.Occupational hazard of my occupation not being around (sorry Bubba)
"It's funny they way the code language is written. Something like "When slab reinforcement is used, it must be supported by....." There is no requirement TO use reinforcement, but if you choose to you MUST support it."In thirty years of watching slabs poured in MI, I saw very little mesh used on residential slabs. None in basements; occasionally in garages; rarely on the driveways. I suspect that MI has a relatively stable soil base because structural problems aren't common with concrete unless the installers didn't give it a proper base. Usually the base was 4" of compacted sand. Bob's next test date: 12/10/07
for outdoor stuff that doesn't get a slick (steel) finish i screed (power or hand) and bull float, mag float then broom.... things like dumpster pads ect.... but now i've gotten to where i stamp alot of concrete... i can screed it, bull float it, hit the edges... then stamp it... faster than if i tried to get a really nice floated surface... if it's just a small patio... say 10x12 i can color (liquid color) stamp it and touch up the edges in 30 min max... pretty great effect for the time spent... which is about zero extra time... the most time is waiting for it to set so i can stamp it... even with sets of 6 stamps i can do most with just 2 stamps... walk on one while i stamp the next... plus it's fun...
p
Your the first person to ever admit that it's faster to stamp the concrete rather than to finish it. The guys that stamp it act like it takes so much more time. So, I get to wondering...how much more time can you put into the slab when it's going to cure and harden in the same amount of time whether you stamp it or finish it? Thank you Ponytl for exposing this concrete scam! Bob's next test date: 12/10/07
i'm one of those weird guys who likes finishing concrete... more than a few times i was the last one there at 3am on the machine put'n a slick hard finish on ... then i found out if i was going to score and stain it the flaws are what gave it the character i wanted... then i got stamps... wow... float it to a finish you wouldn't have for POS driveway... and wait... when you get to where you can see your finger print and get on lift up... start stamp'n... i use liquid release (mineral spirits with color added... liquid color or the dry powder color mixed in with the mineral spirits...) it's really really hard to screw up stamping... I'm sure i'll get bored with it but i still have stamp patterns i haven't used yet... all my stamps have come off the net or ebay... and i've never paid more than $50 for a stamp... like i said you need 2... more for large projects where more than one person is stamping... but for projects under 500sf i think 2 are plenty...
i think you thought on the time thing is dead on
p
The FHB editors would probably like an article on stamping small slabs. I think you have a few techniques that are simple and effective. The best one is that you only need two stamps. I see guys show up with a dozen, and like you noted in a previous post - make it look like a lot of work. An editor will visit you on a jobsite and take photos. You just do a brain dump and they make it look pretty.MG
I would love to read an article on stamping. Now that we are slowing down, we are doing our own foundations again, and hope to do some flatwork.
I have 6 small patios aprox 8x10... and an entry walk.... 8 x 12 and a 30 x 4ft walk and a 5x 30ft patio that i need to do sometime soon... and i'm think'n i might try to do a stamped grid drive... where i make the stamped concrete 2ft wide with maybe an 8ft sq center to be filled with packed rock... I like the idea of not have'n so much storm run off... and was thinking with this grid system i could contain the rock and still control runoff with some natural drainage... and it'd look good... if it didn't work out figured i could always comeback and asphalt or cocncrete in the squares... still in the thought stage of this... i hate the look of a parking lot... and not fond of the run off... maybe someone has some ideas on this?
P
fly ash has the same properities as cement, it will get hard on its own. flyash is free. so by adding flyash to the mix, the cement can be cut back. Flyash also is a round particle so it will have a more flowable mix with less water. also because the cement is cut back the mix is not as hot(heat) so shrinkage cracks are least. alot of cracking is flash setting of the cement.
because the cement is cut back the mix is not as hot
That's a lot of benefits, makes a person wonder if there are any drawbacks (excluding things like cheapskates wanting 6-sack from 4-sack mix and the like).
So, do you have a stand on "sharp" versus "washed" aggregates? Opinions locally seem to vary. Middle ground (NPI) likes washed for where it "shows" and sharp elsewhere. Pump guys seem to prefer washed; bunch of supers who swear only sharp fro strength--all of which determines who gets that nod for supplying the mix locally.Occupational hazard of my occupation not being around (sorry Bubba)
fly ash has the same properities as cement, it will get hard on its own. flyash is free. so by adding flyash to the mix, the cement can be cut back.
Around here, fly ash has a bad name - to the point you can't even find a readymix plant that will mix it in. Is that likely for the reasons you mentioned - that you can skimp on the concrete, so that it got a bad name - or are there other reasons?
Fly ash has a lot of methylmercury in it, no?
k
One of the cement contractors I've used says that now in CA fly ash is actually expensive. He says that Cal-trans has spec'd it in their concrete mixes and now its hard to get!
Daniel Neumansky
Restoring our second Victorian home this time in Alamdea CA. Check out the blog http://www.chezneumansky.blogspot.com/
Oakland CA
Crazy Homeowner-Victorian Restorer
What are your thoughts on porous concrete?
http://www.wisegeek.com/what-is-porous-concrete.htmView Image “Good work costs much more than poor imitation or factory product” – Charles GreeneCaliforniaRemodelingContractor.com
i get some trade mags waste water treatment, pavement today, and roads & bridges, all of them address storm water, drainage, and new products... I've read about porous concrete... also looked at a process where you leave alot of 4" holes honeycomb style and plant them which... i like the idea of...
p