# Slab Advice



## Tom R (Jun 1, 2004)

I'm going to be doing a patio slab for a regular customer in the next week or so, - - it'll be 9' X 18', - - right up against the back of the house, - - running (longways) parrallel to the house.

It'll be 2" of 3/4" crushed stone, then 4" of 3500# concrete with a metal control/key joint across the middle, - - effectively making two 9' X 9' slabs, - - that can be poured simultaneously. I'll provide a slope away from the house of 1/4" per foot.

Along the foundation wall I plan on using (what I call) 'zip-strip' foam, - - so after it's done I can peel off the top of the strip and fill it back in with self-leveling caulk.

So here's the question, - - should the slab be 'tied-in' to the foundation wall with 're-bar shorts' or should it just be allowed to 'float'??

I would think just let it float, - - but want to make sure that's the right way . . . 

Appreciate any and all input . . .

I'm in central Jersey.


----------



## 6stringmason (May 20, 2005)

I would let it float. Sounds like you have it pretty well put together. I use that same type of exp joint as well for alot of things.


----------



## Tom R (Jun 1, 2004)

Thanks 6, 

Yeah, I would think 'tying it in' could possible CAUSE a problem down the road . . .

But yet I have heard some say it's better, - - or even recommended.


----------



## 6stringmason (May 20, 2005)

Ive heard the same thing about both as well. My thinking is that if you tie and have some heaving during the winter it could possibly mess things up. I dont see any problems with not tieing it in. Possible seperation between slab and foundation if you get heaving but nothing structural. You wont have to worry about cracking the or ruining the foundation. 

You could also always put down a few inches of sand on your base.


----------



## maj (Mar 13, 2006)

I like to dowel slabs to whatever it is they are abutting. I have seen too many garage approaches & sidewalks that have settled or pulled away. Usually because of the overdig. Maybe you don't have that on this project Tom? (overdig, I mean)

I will epoxy #4 rebar every 2', then tie a bar perpendicular out about 1'.
Right or wrong, I've not seen any problems since I started doing it this way. Maybe if they did it here,http://www.contractortalk.com/showpost.php?p=103534&postcount=1, this wouldn't have happened.


----------



## Tom R (Jun 1, 2004)

Thanks maj,

No, no overdig scenario, - - but interesting point about the 'pulling away' possiblity . . . 

As far as that porch that Ron's talking about, - - sounds like it was a combination of loose fill and no wire or re-bar that may have caused that . . .


----------



## Tom R (Jun 1, 2004)

Here's what I would view as a possible 'pitfall' of tying in . . .

Where it's tied in is the 'warm' side of the slab (against the house) . . . 

The outside edges are all 'cold' (subject to frost heave) . . . 

If the outside edge and the side were to 'heave', - - and the house edge were to 'stay', - - seems something would have to give . . . 

Also, - - maybe a potential problem of causing a 'reverse-slope' after several years of the outside edge heaving and not going back down as far each time . . .

Just thinking aloud here . . .

Anyone??


P.S. Oh, just saw your post (#4) after I wrote this, 6string.


----------



## Tom R (Jun 1, 2004)

6stringmason said:


> You could also always put down a few inches of sand on your base.


You mean just sand, or both sand and the stone??

For what, - - curling prevention, maybe??


----------



## Glasshousebltr (Feb 9, 2004)

Tom....just be sure to tie up any rogue goats.

Bob


----------



## 6stringmason (May 20, 2005)

I guess sand isnt really a must. I just use it since it helps with drainage and reduces the chances of getting a frost heave in my neck of the woods. Its not needed, just something I have always done since I was a grunt and still do today.


----------



## Joasis (Mar 28, 2006)

I know it is different up north..we don't have frost heave in Oklahoma...but I'm with maj, we would pin it..only I don't epoxy, I angle the pins alternating and tie a bar out.


----------



## maj (Mar 13, 2006)

joasis said:


> I know it is different up north..we don't have frost heave in Oklahoma...but I'm with maj, we would pin it..only I don't epoxy, I angle the pins alternating and tie a bar out.


Hey, THAT'S a good idea, why haven't I thought of that!!!!:thumbup: 
I hate messing with epoxy!!! Thanks Jay, I'm going to angle 'em from now on...


----------



## Tom R (Jun 1, 2004)

joasis said:


> I know it is different up north..we don't have frost heave in Oklahoma...but I'm with maj, we would pin it..only I don't epoxy, I angle the pins alternating and tie a bar out.


Never heard of that angling 'angle', - - that's a good one. :thumbsup:


----------



## jmic (Dec 10, 2005)

6stringmason said:


> I guess sand isnt really a must. I just use it since it helps with drainage and reduces the chances of getting a frost heave in my neck of the woods. Its not needed, just something I have always done since I was a grunt and still do today.


 If I might throw 2 cents in I'd prefer the 3/4" stone ( about 4" ) under the slab for drainage and added protection from frost. I might be mistaken but Tom do you get much frost down in Jersey ? Or very minimal?


----------



## Tom R (Jun 1, 2004)

jmic said:


> If I might throw 2 cents in I'd prefer the 3/4" stone ( about 4" ) under the slab for drainage and added protection from frost. I might be mistaken but Tom do you get much frost down in Jersey ? Or very minimal?


We get a good amount, I guess, - - our 'frost-line' depth is actually 2', - - so our footing depth requirement is 3' . . .

2" of stone is pretty 'standard' around here, - - at least for those of us who even 'put it', - - most guys will just dig the 4" and lay the slab right in.

What's your frost-line there??

And what's your opinion on 'tying-in' to the foundation wall??

I'm still undecided, - - good points from both sides . . .


----------



## Glasshousebltr (Feb 9, 2004)

I'd let her float Tom. It may prevent movement problems down the road.

In fact, just did one.....a floating.

Bob


----------



## Tom R (Jun 1, 2004)

Glasshousebltr said:


> I'd let her float Tom.


Thanks, Bob . . . 

Now, - - we are talking about SLABS, and not EX-WIVES, right?? :whistling


----------



## Glasshousebltr (Feb 9, 2004)

I'm pretty sure not a one of those brauds would float.:laughing:

Bob


----------



## tkle (Apr 15, 2006)

Doweling through the expansion joint(as compared to a control joint)defeats the purpose of the expansion joint.More so with the use of epoxy.There would be no slippage to allow any expansion.You also create a place for water to enter creating a rust problem,leaving possible voids in the slab.That said,I usually don't put expansion against the foundation as long as the slab isn't locked in and has a place to move.If there is a frost heavage problem in you're area,I would consider doweling.I know when I worked in Lake Tahoe we never doweled any exposed slabs and never had a problem.Just pay attention to your base as advised.A good base will allow water expansion under the slab as jmic pointed out.


----------



## jmic (Dec 10, 2005)

Tom R said:


> We get a good amount, I guess, - - our 'frost-line' depth is actually 2', - - so our footing depth requirement is 3' . . .
> 
> 2" of stone is pretty 'standard' around here, - - at least for those of us who even 'put it', - - most guys will just dig the 4" and lay the slab right in.
> 
> ...


 Tom,
We use 42" for frost protection. As Maj pointed out I would pin it if overdig was an issue, but I gather what you're doing the site has had many years to settle, so I would think it has good compaction. I vote let her float.


----------



## Tscarborough (Feb 25, 2006)

They will, but the tensile strength of the dowel is much greater than the concrete. Therefore, when the forces act on the unit, they will act first upon the concrete, i.e. cracking it or displacing it if the generated forces are out of plane with the bar. The dowel acts as a load transfer device, transmitting loads from one slab to another. Those loads, in two concrete slabs abutting each other, are perpendcular to the face of the joint and parallel with the surface.


----------



## Tom R (Jun 1, 2004)

Tscarborough said:


> They will, but the tensile strength of the dowel is much greater than the concrete. Therefore, when the forces act on the unit, they will act first upon the concrete, i.e. cracking it or displacing it if the generated forces are out of plane with the bar. The dowel acts as a load transfer device, transmitting loads from one slab to another. Those loads, in two concrete slabs abutting each other, are perpendcular to the face of the joint and parallel with the surface.



You pose a good argument, there, - - I'll concede more to the theory than the practice (so far), though, - - as Joasis states in post #22, he hasn't had a 'related' failure . . .

I do note, though, - - that he also pointed out his 'location' (which is somewhat near yours) as compared to others (mine, or say, maj's).

Great stuff either way, - - much to think about . . . :thumbsup:


----------



## Tscarborough (Feb 25, 2006)

It is somewhat theoretical for this application, but it is certainly not for highway and structural concrete buildings. Call it a "best practices" argument, and I have never been one to accept anecdotal, "I have done it this way for years with no problems" practices over engineered ones.


----------



## Tom R (Jun 1, 2004)

Tscarborough said:


> It is somewhat theoretical for this application, but it is certainly not for highway and structural concrete buildings. Call it a "best practices" argument, and I have never been one to accept anecdotal, "I have done it this way for years with no problems" practices over engineered ones.


My guess as to the 'whether or not it would work' answer would depend on the size of the job, - - highway NO, - - patio YES . . .

Either way, - - thanks for all your great knowledgeable info . . . :thumbsup:


----------



## tkle (Apr 15, 2006)

Tom R said:


> If it was a 'square' 2-yard placement, - - I wouldn't be putting a cold-joint, - - but I think most would agree that in a 9' X 18' rectangle, - - it's best to have one . . .
> 
> Though I appreciate the opinion, - - personally, - - I like the look of the key-joint better, - - less visible, less obtrusive . . .
> 
> ...



If people don't disagree I don't learn anything.
A cold joint is a new placement against an old one,what you want is a simple control joint.Use a 2x down the middle to screed off of with a stake to support it halfway.Put a wooden stake ear at the top of one end of your screed board .Use that to ride your middle screed.The only time expansion is needed against the existing is if the slab is locked in,such as a driveway between the house and the curb.It's sometimes hard to fit against old rough footings and tends not to be straight plus it does leave an unsightly gap that collects dirt.That will make your customer happy and you'll be back.Walk around your neighborhood and look see how it's done there.Keep troweling to a minimum as overtroweling can cause to much soup to rise causing a scalding problem.But you won't get a good finish without.Overfloating is also a common problem a wood float is better than a mag .If you have no trowel experiance hire someone who knows what to do when.Timing is very important.


----------



## Tom R (Jun 1, 2004)

tkle said:


> If people don't disagree I don't learn anything.
> A cold joint is a new placement against an old one,what you want is a simple control joint.Use a 2x down the middle to screed off of with a stake to support it halfway.Put a wooden stake ear at the top of one end of your screed board .Use that to ride your middle screed.The only time expansion is needed against the existing is if the slab is locked in,such as a driveway between the house and the curb.It's sometimes hard to fit against old rough footings and tends not to be straight plus it does leave an unsightly gap that collects dirt.That will make your customer happy and you'll be back.Walk around your neighborhood and look see how it's done there.Keep troweling to a minimum as overtroweling can cause to much soup to rise causing a scalding problem.But you won't get a good finish without.Overfloating is also a common problem a wood float is better than a mag .If you have no trowel experiance hire someone who knows what to do when.Timing is very important.


Yes, disagreements are a good thing, - - allows everyone else to make their own choices . . 

Sorry, - - from one of your previous posts, - - I thought it was 'you' who was calling a cold joint and a control joint one and the same, then I went and called it a cold joint, - - my mistake . . .

I'm 'hearin' ya on the expansion joint and may re-consider . . .

No lack of troweling experience, was going with 'wood' as a happy medium between whether to use steel or broom . . . 

When I look at all the neighbor's patios, - - that's what makes me decide I better do this one myself . . .

Thanks for all your knowledge, tkle, - - in helping make this such an interesting thread!! :thumbsup: 


P.S. Can't say I'm really 'gettin' what you're saying on your method of screeding, - - but I'll read it a few more times, - - maybe it'll sink in. I'm actually pretty set on doing it the way I already said I would, - - but I'm still interested, as I may give your method a try another time.

P.P.S. My experience is all with crawl space slabs, - - not with patios, - - that's why I'm asking.


----------



## Tscarborough (Feb 25, 2006)

A control joint and a cold joint are not always the same thing, though there is overlap in what they are designed to do. Against the house slab, there is no question that using an expansion material with a zip strip is the best method. On a 9x18 slab, I would use a keyway for screed out of habit, if nothing else. It is cheap, effective and for sure will not effect the integrity of the slab.

As for the finishing method, it is my understanding that in a freeze/thaw zone that you want as much air entrainment as possible. A wood float will promote this, while a hard steel trowel will decrease air entrainment at the surface (although it will create a more dense, thus harder surface). Regardless, it should be jitterbugged, then bullfloated, and finished how you see fit.


----------



## Tom R (Jun 1, 2004)

Tscarborough said:


> A control joint and a cold joint are not always the same thing, though there is overlap in what they are designed to do. Against the house slab, there is no question that using an expansion material with a zip strip is the best method. On a 9x18 slab, I would use a keyway for screed out of habit, if nothing else. It is cheap, effective and for sure will not effect the integrity of the slab.
> 
> As for the finishing method, it is my understanding that in a freeze/thaw zone that you want as much air entrainment as possible. A wood float will promote this, while a hard steel trowel will decrease air entrainment at the surface (although it will create a more dense, thus harder surface). Regardless, it should be jitterbugged, then bullfloated, and finished how you see fit.


Sheesh!! 

Tkle just talked me 'out' of the zip-strip, - - now you're bringin' me back in!! :laughing: 

Could you two 'duke it out' all night and I'll check back later?? :laughing:


----------



## tkle (Apr 15, 2006)

Thats just the way I learned it in school and have done it and seen it done for the last 30 years.Sometimes the best method is overkill.
Your middle board that is used to screed off of rides above your forms 4".The ear on top of your screed board rides on top of this dropping your screed down to slab height.


----------



## Blackhawk (Nov 12, 2005)

Not tryin to stir the pot here ..but angle doweling is the way we do it here in kansas to..and i have seen this first hand cure the seperation problem.. soooo i say pour the dam concrete lol just kidding have fun ..


----------



## Tscarborough (Feb 25, 2006)

When you angle dowel, do you use slick dowels or rebar?


----------



## Joasis (Mar 28, 2006)

#3 rebar pins ...and I place them like this: / \ / \ / \ with a long bar tieing the rebar. A lot of what I learned about concrete was on the job. I didn't see an engineering manual on conrete construction til much later. The angled dowels work for the applications I have used them. 

Now another point: You advised jitterbugging the concrete. That is a practice that is not done around here anymore. I bet I am one of the few contractors that own a jitterbug, and we use it only on footings, and don't ram it hard. School of thought here is the reason all the old concrete is surface popping is the aggregate was "jitterbuged" down, the cream up, and it also weakened the floor. made for great finishing. 

When we pour now, we place, rake, screed, hand float the edges, bull float the area, wait, wait, wait, steel trowel and edge, machine trowel the area. Collect the check.

I bought a vibrascreed a few years ago...now the concrete guys in this area are leaving it at home in favor of the oldschool 2X4 with the broom handles. One guy insists the surface will crack everywhere we use the powered screed.

I agree, tkle, if not for these threads, I would not learn anything either.

The only for sure, indisputable bit of knowledge I have about working concrete is: You work concrete when it is ready, not when you are ready! You can work yourself to death rushing mud, and it can kill you if you let it get ahead. It is fun pouring in the summer, wind blowing, 95 in the shade..and all it takes is somethig going wrong..


----------



## tkle (Apr 15, 2006)

95 in the shade and no shade...


----------



## Joasis (Mar 28, 2006)

I have nearly lost a slab in the summer heat...I always carry 2 power trowels...just in case we have to run a race. You know, maybe we should hijack the thread and get opinions on slump for summer pours....the crew I use now for finishing really like it to run...and every bit of literature on concrete strength says this weakens the concrete. Really weakens the concrete...I have only pulled cores maybe 3 times and none were rejected...so?


----------



## Tscarborough (Feb 25, 2006)

Now it is clear. Rebar is not a load transfer device, nor does it allow movement. In those terms, it doesn't really matter how you place it, even when you are using it to pin across slabs.

I had heard that jitterbugging was out of fashion, and it makes sense, although we usually only worked the edges and joints anyway.


----------



## Tom R (Jun 1, 2004)

tkle said:


> Your middle board that is used to screed off of rides above your forms 4".The ear on top of your screed board rides on top of this dropping your screed down to slab height.


Okay, - - now I understand what you're saying . . .


----------



## Tom R (Jun 1, 2004)

joasis said:


> You know, maybe we should hijack the thread and get opinions on slump for summer pours....


Feel free to hijack in any direction you want, - - sometimes it's best to start a new one, though, because it makes it easier for someone to find later . . .


----------



## Tom R (Jun 1, 2004)

I was going to go ahead and tackle this job today and get it out of the way, - - but had to go ahead and call it off til next week, - - they're callin' for rain here every day this week, - - doesn't really matter to me, - - I've got several weeks worth of 'built-ins' 'n such to do in my shop either way . . .


----------



## tkle (Apr 15, 2006)

joasis said:


> I have nearly lost a slab in the summer heat...I always carry 2 power trowels...just in case we have to run a race. You know, maybe we should hijack the thread and get opinions on slump for summer pours....the crew I use now for finishing really like it to run...and every bit of literature on concrete strength says this weakens the concrete. Really weakens the concrete...I have only pulled cores maybe 3 times and none were rejected...so?


Concrete retains the water,it doesn't bleed off or evaporate as some think.The water combined with cement goes through a chemical change to cure.You want to place it as dry as possible and still have it workable..Wetting it just gives them a longer break between screeding and floating,once it starts to set it all goes at about the same rate anyway.Too much water and you will get evaporation causing shrinkage and also segregation


----------



## Joasis (Mar 28, 2006)

We used to do storm cellars inground...you know, a form box floating in concrete....the trick is a 3-4 inch slump.....and vibrate carefully...very carefully since the form pressure can easily reach 1000# a sq/ft with 7 feet of mud....so one day we pull a core and I send it off...just to see what we are really getting...and it tested 2950# at 25 days...great...3500# mix....a year later, we did a pour and the finisher poured really wet...self leveling...and the stem wall cracked out...we had to re-pour it....so I was telling my buddy at the lab about it and he said bring him a chunk to check...didn't have to be a core..and he smashed it and 30 days had a strength of 3300# or so.....I presume it cracked our since it was really wet and shrank.


----------



## Tscarborough (Feb 25, 2006)

First, you can't take field applied mud, core it and expect it to reach mix design breaks. You can sample the mud at the site and make cylinders which have to be properly cured, then broken. There is a procedure for this, and it is done on commercial jobs as a matter of course.

Second, adding excess water doesn't make the concrete weaker, per se. It does make it a lower quality concrete, as it has effects on various properties of the concrete aside from compressive strength. As a rule, you should add no more water than required, if any. Adding water to make the concrete more flowable is a bad idea in all circumstances, and adding it to increase working life is futile.

Plan your pour so the mud doesn't have to be moved laterally as much as possible to prevent segregation of the aggregate, and the same goes for using vibration to move the mud.

The best thing to do in hot dry weather is to start your pour early. Try to plan it so that you are finish troweling before noon, and you will be OK 9 times out of ten. We always kept keyway on hand in case issues arose, and would cut the pour short rather than losing the slab.

Of all the concrete I have poured, I can remember only 2 slabs that we lost, one to rain (not a big one, but it was a tilt-up panel bed, so it had to perfect) and one to traffic congestion (@#[email protected]#$ Houston traffic). That does not count forms blowing out, though; those happen a little more often.


----------



## Joasis (Mar 28, 2006)

Tscarborough said:


> First, you can't take field applied mud, core it and expect it to reach mix design breaks. You can sample the mud at the site and make cylinders which have to be properly cured, then broken. There is a procedure for this, and it is done on commercial jobs as a matter of course.


If I didn't have a buddy that worked for the company that does testing for the state, I would have never pulled a sample anyway...to me, it was just curiousity. I was surprised at the yield strength of the concrete. I had some sample cylinders he gave me..might still have a few. What impressed me was taking a small piece of the footing and having him crush it (I didn't see it done) and telling me the properties. I do not know exactly how they do it..if it works like hardness testing on steel or is it a 1 sq/inch pin press, or what?

I try to avoid ever pouring more then a 6 inch slump for flat work, 4 inches for form. It always amazes me the number of finishers who will add lots of water...like adding 30 isn't uncommon.(in 9 or 10 yards)..and literally having a floor self level....

The reason I went to a vibrascreed was to be able to pull mud without working your tail off....and the last to concrete crews I hired out used the 2 handled board screed and wet mud...both floors look great. We don't do concrete that often unless it is a house floor or 2000 sq/ft max...then we hire it out since I usually don't have the extra help to get it done...I am never gutsy enough to pour a 40X60 floor with 2 people.:no:


----------



## Tscarborough (Feb 25, 2006)

He would have had to cut cubes to get a test, but it would not be one that would hold up in court.

It is worth whatever effort it takes to set up your pour so you do not have to pull mud more than a foot or 3.

I might pour a 24x100 driveway with 2 people, but no way would I do a 2400 SqFt house slab, or even patio with less than 6.


----------



## lukedogg (Apr 16, 2006)

*Don't tie*

Don't tie it in as it will crack due to heaving. You should however use expansion board between the house and the patio. Talk to your concrete supplier and they will send it on the first truck.


----------

