Concrete, the most consumed product hot rod

Duncan_2

Great information, hot rod - concrete is one of those things we just kinda take for granted, you know? - concrete is concrete?

I've seen batches so hot, it's half-set by the time it gets to the site. Instead of slushing around in the drum of the truck, it's thumping from side to side like a glob of Play Dough inside the drum, the truck's bouncing from side to side as the wad is heaved about.

In places like this, where the trucks have to travel as much as an hour and a half to the site, I think the batch operators might be the ones that add the water in hopes of slowing the setting time. But I think we've all seen the finisher look at the crete coming down the chute and decide to add more water.

It sounds like the heating contractor has to communicate directly with the batch plant operator, and be on the site when the finishers arrive. I think most heating guys are on site during the pour in case a pressurized piping air leak somehow occurs.

I've been considering trying an inch and a half concrete thin pour over a wooden subfloor, using an mixture with drying agents (like the "Youker mix" John Siegenthaler mentions). I'm concerned with the inconsistency of some of the Gypsum-based floor pours by installers in this area - dusting and things like that - as well as floor product adhesion and reaction to water leak soaking.

But stuff like you mentioned about concrete makes me nervous.

I'm curious, hot rod - have you ever done a concrete thin pour over structural framing (like TJIs or BCIs) and wooden subfloor?

Were you happy with the results?

Duncan

hr

A typical mix is 1800 lbs of stone, 1400 sand, 500 cement, 250 gallons of water. About 3900 lbs per yard.

If the mix leaves the yard in this range you should not have to add more water. Just enough water to properly consilidate the mix. However, many concrete finishers add water to ease the placement. Bad idea.

The "water of convience" will have to leave the top of the slab when vapor barriers or "tight insulation is used. Vapor barriers can aggrevate cracking, as the top of the pour dries too quickly. Shrinkage is the number one reason concrete cracks. However some consider a crack to be a "free joint"

Every gallon of water added to the typical mix will reduce the strength by 200psi. One gallon added to a 3000 psi mix will change that to a 2800 lb mix.

There are numerous water reducing admixes and other additives to adjust the mix to the application and weather. Windy dry days would dictate a spray fog or plastic sheets to slow the cure. The slower the better.

A good batch plant operator can help select the best mix for your application. The concrete installers or finishers are not always up to speed on the products available to get a top notch slab.

These are some of my notes from this years RPA seminar presented by Alan Sparkman from the Tennessee Redi Mix Concrete Association.

hot rod

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hr

Thin pours

The floor covering will play heavily into your choice of concrete/ gyp choices. There are admixes available to alter the set time for long distance jobs. A good batch plant should have info on these. Do a web surf for concrete additives.

I once poured a pea gravel mix 1-1/2" on my own home in 1985. It cracked everywhere. We used a glue down parquet floor tile and the cracks did not seem to effect the floor covering. The smaller the aggregate, however the less strength. ACI suggests three times the aggregate size over the tube, or rebar.
This speaker highly recommended doubleing up the fiber mesh for thin pours. Use 3 lbs per yard instead of 1-1/2 lbs. A small torch will burn off any fibers after the pour.
Remember with concrete over wood you will want a slip sheet between the wood and pour, but this brings you back to the water out the top issue again. I would say a release agent or sheet against the bottom plate would be wise also.
Any admixes you add to concrete will effect the finishability of the pour keep in mind, also.
In your dry climate a monomolecular film (fog sealers, or curing compounds) will make a difference in the cure and cracking, especially in a thin pour. A dry windy day is the hardest condition to pour and finish in!

My thoughts, if you plan on a thin pour as a finish floor, plan on seeing some cracks. Of course the framing under the floor will effect the cracking also. If the floor is springy before the concrete is added- you know what is next.. I used 16" deep TJIs under my floor to assure there wasn't any bounce or deflection. Concrete is always a crap shoot when considering it for finish floor use. Good luck.

hot rod



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Duncan

Floor deflection, slip sheet, and curing

You're absolutely right about the admixtures for setting time... shows you what I remember about concrete.

Deflection is definitely a key consideration when it comes to cracking. Any concrete is gonna crack... there are curing cracks, shrinkage cracks, control joints, and probably other kinds of cracking I haven't heard about. Fact is, when the water evaporates, it goes out of the concrete and the concrete shrinks.

I really don't care about shrinkage or curing cracks, what I DON'T want to happen is to have a bunch of pieces like peanut brittle on the floor. I was happily encouraged when you said "it didn't seem to affect the finished flooring".

>>> So tell me, hot rod... do you remember the SPAN of those 16" deep TJIs? Did you space them 16"o.c., or closer together?

Another thing that occurred to me is that with a slip sheet, if you staple down the tubing, you're defeating the whole purpose of the slip sheet. This means you need to a use free floating tube-on-mesh installation. I can already picture a nightmare of tubing popping up out of the pour with a free-floating install like I mention.

>>> Any observations on that aspect?

>>> What about curing? What are your thoughts about how to handle that? I don't think there's a builder or homeowner in the world that really looks forward to waiting 28 days for an initial cure of concrete!???

I wonder if the concrete countertop, decorative concrete guys have any ideas on thin pours?

Hope I'm not being a pest asking all these questions, hot rod. It's just that, while I'm not 100% happy with my gypsum-based crete experiences, it also sounds like concrete has its own set of issues. ;-) But if John Siegenthaler recommends it, I'm confident it has some merit.

Thanks, hot rod!

Duncan

hr

More thin slab thoughts

I would use the smallest diameter tube you are comfortable with 3/8 pex would be my choice. Perhaps consider a 3/8 in OD copper and make Ken proud! Of course watch your loop lengths, but this would handle the diameter issue as well as the fastening down question. When I poured my radiant concrete counter top I used straight length 3/8" copper and heated and bent the loops. It did lie perfectly flat, without any fasteners.

The fellow that does concrete tops and decrotive mantels, etc around here also beefs up the plastic fiber. He does it more by sight than weight, but swears this is the key to LIMITING cracks. It also will prevent a small crack from traveling as I discovered on my concrete counter top. A crack developed months after I installed the top. It tried to go from the sink hole cut out to the faucet mounting hole. It only traveled 1 inch and stopped. I hope that's it, and if so I buy into the fiber theory.

Once again shrinkage is the main reason concrete cracks. Check W R Grace for their shrinkage reduction admixes. Plastic Shrinkage Cracks are the ones you see soon after placement generally the are shallow and not entirely thru the pour, often several parall to one another, and they tend to stop and start.

The 28 days to cure is under scrunity these days. Many so called concrete experts agree full strength is met in 7 days! In less of course the "typical" mix has been souped up to change something. Concrete becomes frost resistent at 500 psi, generally in a day of two depending on the mix and weather conditions.

I only used 16" TJIs because I got a deal on some rems. I would guess a 12 or 14" TJI would handle most residential spans and loads. Maybe go up a series for greater load bearing and less deflection. Check with the joist manufactures for advise. Truss Joist has great tech support on the phone. I'm a big fan of engineered lumber for weight bearing members, based on the quality of dimensional lumber available these days you will agree

hot rod

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nick z.

concrete

Having a Grandfather that was an engineer for a Bridge building company I think I have some useful knowledge of concrete. If you are concnered if concrete has cured enough for const. to continue have test cylinders made from what you pour with. Many big concrete co. have the equipment to check them and see what psi there breaking at.As afr as adding water, of course it affects the srength . Talk to your salesman maybe Plastisizer (not sure of the spelling)would work for your job. It amkes the concrete very wet and easy to work with,without affected its srenght.But it does amke it set VERY fast and might lead to more shrinkage cracks.
Covering concrete with plastic to cure it was something we were never allowed to do as you get a grenn house affect.Wet burlap coverd with plastic is the way to go.There is a product called Burlean which is heavy plastic with burlap on one side of it.

Duncan_2

Am I being too fussy?

Maybe I'm just a little too nervous because I had a few piddling little problems with my first thin pour job [GRIN]

Duncan_2

Maybe I'm making too much of this...

Just because I had a few cracks in my first thin pour...

Duncan_2

Am I making too much of this, hot rod?

I mean, just because I had a few piddling little cracks in my first thin pour job...

Thinset mortar should fill in most minor cracks, don'tcha think? [GRIN]

hr

perfectly acceptable

Use tile grout or Dap to fill in the cracks! They should be able to supply Dap in a tank truck quantity.

hot rod

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hr

A mistake in my formula

make that 250 lbs of water about 30 gallons per yard. Thanks to JJ for the watchful eye:)

hot rod

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Ken

concrete cracking and plastic covering

> make that 250 lbs of water about 30 gallons per

> yard. Thanks to JJ for the watchful eye:)

> hot rod

>

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> 144&Step=30"_To Learn More About This Contractor,

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Ken

concrete cracking and plastic covering

we had our upper floors poured one and a half inches with regular concrete and then was instructed by the floor finisher to cover it with plastic and cardboard the next day. It stayed covered except for the areas uncovered by the framers to place the stud walls on the plates which had been placed prior to concrete pouring. I covered these back up at the end of each day. It was almost three months by the time every thing was done up to the baseboard stage (including a shutdown for a holiday) before we took the plastic off. the floor finisher came in with his router tools and staining equipment and finished the floors with a crew of 4 in 5 days. This cost about $ 7600 canadian (cheap) dollars to do 3000 sq. ft. The other contractors in the area and the mixing plant have been bringing people by to see it. Most people seeing it that haven't been told about it usually look at it for awhile and then ask if it is slate or marble. They can't believe it is only finished concrete. there are some small wandering cracks but most people agree that that adds to the antiquing look and made them more convinced it was a stone. The finisher said that some people have asked if he could give them more cracks because they really do add to the effect. Chipping or flaking is not happening. A once a year coat of a watery sealer spread on with a mop is all it needs for upkeep. A burnisher can be used but then it can be too shiny and shows every speck of dust. If you people on the wall could get together with a finisher like this you could increase the money available for heating systems because the flooring costs are lower.I am trying to get him on the wall to answer some of these questions. He started doing radiant floors over 20 years ago but now is kept very busy doing the finishing. If the wall had been up then he would have been on it as he did belong to both a canadian and an american radiant heating association back then. He is now begining to have more of his routering machines built and is wanting to get more people trained. This may be an opportunity for concrete finishers that you know that take pride in their work and would like to diversify. I didn't plan to write a book. I will get off now.

Duncan

The after photo...

Thanks hot rod, I took your advice. ;-)

Did you know that thinset and DAP develop shrinkage cracks, too? Photo below after patching. [g]

I just don't know what all the fuss is about a few little cracks. Thanks for the laughs.

Nick and Ken... excellent information, thank you. Ken, we gotta get that guy on here!!! I really liked your story. I did a short search, and it confirms what you both said. I firmly believe there's nothing that compares to years of experience, eh Nick?

I'm gonna continue the quest, and share the results. For starters, here's one site I found.

http://www.prmconcrete.com/plastic.htm

Chris_2

Thick and thin slabs and cracking ! (checking)

Ken has has threatened me with an "irresponsibility" charge if I don't get on here say something and since I have much respect for the man so here I am here!(smile)

We've(we are Sunise Systems Ltd. of Kamloops BC) been designing and installing radiant floor heating since 1980 and placing tubing in both 4" slab-on-grade floors and suspended floors with 1 1/2" of concrete overpour since then. About 14 years ago we started doing decorative work on both thick and thin slabs and have now done some hundreds of jobs. We're starting to believe that we have a handle on the do's and don'ts and how to's. We know also that we don't know it all!

The concrete suppliers all have their own slightly different ratios for mixes, placer/finishers all have their favorite suppliers and we have our favorite placer/finishers. We rely on both of them to provide the finest of materials and workmanship possible and they usually do! Sometimes, in remote circumstances, something go's awry and the challange for us is to apply the "silk purse/sows ear" attitude and go for it anyway.

Here are some of our guidelines:

1. We always suggest that NO fibre mesh be used in any slabs as we have found that checking seems to be inhibited not a whit. One of the most badly cracked slabs ever encountered by us had plenty of fibre and had still cracked everywhere. Fibre is perhaps great for structural monolithic pours.

4" (or more) slabs seems to be held together best with 6" X 6" wire mesh that has been faithfully "pulled up" into the slab during the pour. Mesh is incidentally the best for placing tubing accurately. Rebar is seldom placed at close enough spacing(like 12 inches), is expensive to buy and to place but is mandatory where structural qualities are required. Most slabs-on-grade are not structural!

1 1/2" slabs should be 3/8" aggregate on suspended floors and should be approved by the engineering services normally supplied by floor joist suppliers or by a structual engineer!!! The slab should be shown on the building plans. NEVER RISK THIS ONE! Wet concrete is alive and heavy and can get one into deep trouble(like falling into the basement)! Cured concrete adds additional spanning strength and beautiful thermal mass for low temperture heat distribution. Ensure that suppliers know of your desired end results.


2. We always suggest the placement of a layer of 2mil poly on the wooden deck surface prior to placing the tubing and the slab. Quick loss of moisture seems to be the single most serious condition leading to cracking and a thin slab on a dry wooden deck on a warm day can can create both workability problems for the placers and early determination by the slab to check randomly and profusely!
I truly believe that a slab determines it's own checking pattern within a few hours of placement.

3. We always suggest that 2" stucco mesh be placed over the tubing and secured to the floor with a staple gun which is set to stop about one half inch short of the floor deck so that the mesh remains up in the concrete when placed. This seems to allow placement of say 24' by 24' slabs with little evidence of checking and certainly inhibits slab curl which occurs when one side of a check line raises higher than than the other side.

4. We always suggest that the placer/finishers stay with the slab until they have brought the surface to that fine marblized quality which will allow us to create the most elegant finish with our stains, line cuts and top finishes.

5. We always suggest that if the floor does crack one accepts the fact and that the filled and repaired cracks will add a touch of antiquity and charm that becomes even as much as "desirable"! (final smile before I depart to my bed)

I feel that I have just begun my ramblings on these subjects and hope to have said something useful or even questionable to someone.

hr

Great info, Chris

I still disagree with your point about fiber mesh, both from my years of experience and others in the industry. All of the articles I have seen in FHB, JLC, etc highly recommend fiber mesh, especially in thin pours, as do many of the concrete associations, and my local concrete counter top contractor.

True it will not prevent cracking, but it really does seem to limit the travel of the crack IMO. All in all concrete has a mind of it own, and mix quality and ingrediant quality probabaly varies from location to location. Thanks for your input.

hot rod

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Duncan_2

Some radiant floor crowd perceptions.

Chris, thanks for stopping by! Twenty years of experience brings a lot of knowledge.

I've worked on a few jobs with a friend who does stone masonry and flatwork, and have picked up a very little knowledge of concrete over the years He definitely agrees that it's desireable to stay with the 'crete and work it - he calls it bringing the cream to the top.

Of courseThe radiant floor biz has it's own set of concerns and issues, some shared with the decorative concrete biz, and I think some different ideas evolved in the separate culture of radiant floor world. If you don't mind, I'd like to get your thoughts ona few ideas.

We've always been told by rfh (radiant floor heat) common knowledge that the plastic sheet under the thin pour on wooden decks was there as a slip sheet. It's purpose (we've been told) is to allow the thin slab to shear from the wooden deck. The reason is that concrete and wood expand at different rates when heated, and if the concrete is bonded to the wood subfloor, it can let loose with a very loud BANG when that bond is broken by expansion.(Absolutely no question that it helps with moisture and curing. Definitely!)

Anyway... what you said about stapling 2" stucco mesh to the subfloor kinda contradicts this explanation. If the slip sheet is there to allow the 'crete to (more or less) float over the subfloor, don't the staples, which now are part of the slab, prevent this movement by bonding the slab to the subfloor? Any experience with this aspect of "floor expansion noise" on wooden subfloors?

Quarter inch pea gravel is the recommended aggregate I've heard mentioned for thin pour. Any pros or cons to this versus 3/8" aggregate?

I find your ideas on fiberglass mesh interesting, especially in light of what others say. I don't doubt you, because twenty years of experience means a lot of pours! The way I figure, If I were doing a finished decorative concrete floor, I probably wouldn't want those hairs in there, epoxy or other finish coat notwithstanding. For a floor that's gonna be covered, hey, if fiber doesn't hurt and it does add strength, why not?

What fiber brings to mind (besides strengthening) is adobe brick used in the Southwestern United States and Mexico. This may be way off base, but for whatever it's worth.... When making adobe bricks, straw is added to the mixture. I've been told that this is somewhat for strength, but most importantly, it acts as a moisture wick. It distributes the moisture content evenly throughout the brick, to aid curing and prevent cracking. Anyway, that's mud, not concrete.

Hey, I really appreciate your sharing your knowledge and experience, Chris. Much appreciated. My questions are purely in a quest for knowledge, please don't think I'm challenging anything you said. I'm glad you take the time to share what you know!

By the way... I have a very fond memory of Kamloops back in the mid 70's. A friend and I were bumming across Canada, and having a very difficult time hitchhiking, so we thought we'd hop a freight train. We got to the yards, and looked at all the cars on the siding, and wondered how the hell we'd know where we'd end up! We stopped into a little coffee shop and started talking about it. I mentioned that my Grandfather worked for the Erie Lakawanna Railroad for fifty years, he'd know. A guy sitting at the next booth heard that,and asked us where we were headed (Vancouver). He told us where the travel manifests were found in the cars on the siding. They had schedules and destinations. After chatting a bit more, he mentioned that he was an engineer for Canadian Pacific (?) and said we coul;d ride in the third unit (engine)!!! I wondered for a long time about that extremely generous act of kindness, and even asked my Grandad about it. He said the guy probably could just tell we were good kids and not bums. Talking to other railroad folks since then, I know now that he probably just didn't have the heart to see two kids get robbed, beat up, and maybe killed by real railroad bums. I thank him, wherever he is now.We had a great time in beautiful Western Canada. Ah, the good old days of youth and adventure...

OK, I'll stop rambling now......

DanHolohan

Duncan

PLEASE don't stop rambling. Great story. Thanks.

Earthfire

Thincrete

Back in the late 70's when we were doing a lot of rehabs around Philly, one of the dreaded jobs was taking out the tile walls and floors in the old houses. They were all wetbed, Sand and cement mix, and some of the floor beds were 4" thick . There were so tough that some times we damaged studs and joist getting them out. Now a days when we do a cement over pour I try to keep it at 1-1/2". 1 part portland type 1 cement & 4 parts course concrete sand and just enough water that it looks like crumbly oatmeal in the mixer. We use a 6mil poly slipsheet staple the tubing down and pour our mud. compact it and screed it of. Maybe I'm just lucky but have had no problems with cracking of tile or other floor finish problems. Minimum floorsheating is 3/4" plywood haven't gotten up the courage (?) to try osb under thin pour