Garage project help 3

[Cleron]

Hello,
I just found this site and I hope I can get a little guidance on a personal project now in progress.
I am building a detatched garage. So far the site is prepared for the concrete floor/slab. The building will be 30 ft. deep X 40 ft. wide. We have had to build concrete walls to contain the floor. We filled with a slate mixture and tamped each load with the bulldozer. The top few inches is crusher-run and smoothed as close as possible to insure an even concrete thickness. The top of the walls (back wall and 1 side) have metal rods extending out a few inches for the concrete floor to be poured over. I have asked for a 5" floor thickness and would really like to have this done without expansion joints. In other words, a smooth continuous floor. The contractor is currently going to use the fiber filled concrete mixture instead of metal reinforcemnet. I have had some experience with this on outside concrete but would this be any problem with an interior floor?
(1)-Am I asking too much to try and get a smooth (30 X 40)floor without joints? I do know in my industrial experience we poured floors up to 50 ft. X 100 ft. without joints or metal reinforcement. But that concrete was 10" or more thick and 5000 lb.
(2)-Will 3000 lb. concrete be enough or should I specify 4000?
(3)-The slab will be over a vapor barrier.
(4)-The floor will be poured on top of the back and one end wall and poured against forms near grade for the other end and front.
(5)-I'm not sure yet but in the near future I might use either an epoxy finish or other type of floor finish. Of course I must wait for a minimum 30 days cure before attempting any of these. Is that amount of time sufficient?
Thanks for your input.

 

[JAE]

Use 3000 psi - more cement means more shrinkage.

Use joints at max 20' o.c.

Concrete is NEVER poured. It is placed. If you are POURING concrete it is too wet and will definitely crack.

If this is a garage for vehicles, I would not use a vapor barrier. The VB will inhibit uniform setting times across the thickness of the slab and possibly cause the slab to curl at the corners and edges. The VB will also trap moisture falling off wet cars. The purpose of a VB is to ensure a dry slab for a "dry" space in an occupied room with carpets, furniture, adhered tile, etc. For a garage it is not needed. An epoxy finish would not like a vapor barrier below if there is trapped moisture on top of the VB

For an epoxy finish, check with the manufacturer, but generally 28 day cure-out is specified. You can hurry this a bit if you get a testing lab to check on the moisture coming out of the slab periodically during the cure. Duct tape and a clear plastic sheet over a 10x10 area works too.

 

[Taro]

Here's an alternative take...

30'x40' slabs without joints can be done. Shrinkage-compensating concrete can be used for this. Make sure your contractor has experience with this type of slab. Post-tensioning is also an option but might be overkill.

Every concrete contractor I have worked with "pours" concrete. Maybe it's a regional term, but it doesn't necessarily mean that you have a soupy mix.

A vapor barrier should be used if an epoxy finish is used and there is a chance of groundwater migrating up through the slab. Otherwise, you will get bubbles. In fact, many epoxy manufacturers will not warranty their product if a vapor barrier is not used. The preferred location for the vapor barrier is beneath the crusher fines so the slab will dry more uniformly and reduce curling.

Do not allow the crusher fines to get wet before pouring concrete. Use the lowest water/cement ratio possible that allows the finishers to do their job. Water-reducing admixtures help. Make sure the slab is very dry before the epoxy is applied. Dehumidifiers will help if the schedule is short.

Use rebar each way to keep crack widths tight and to resist curling. Fibers can help with initial plastic shrinkage, but can actually make cracking worse. There will be fewer cracks, but the ones that do form will be wider.

 

[Cleron]

Thanks for the advice.
Regards using a vapor barrier, I know from previous experience that one is required if any coating that truly "seals" is put on the concrete. I have assisted in putting moisture cure urethane (in the 70s & 80s Dupont had an industrial Imron for this) on industrial floors. We always had to etch or power sand the concrete for this. If the floor didn't have a vapor barrier the film wouldn't last more than a month or so before it started peeling. We learned to do the tape and plastic film test to make sure. I also know that some finishers don't like to put a vapor barrier down because it extends the time they have to work the concrete.
It is beginning to look like I will have to just grin and bear it and put at least one joint in. I would rather have that than cracks. I do intend to ask about shrinkage-compensating concrete though.
My floor is scheduled to be installed early this week. I now know some important questions to ask and specifications to insist on before they start.
I hope my only worry now will be keeping anyone from marking it after everyone leaves.
And, I promise to keep it sprayed with water for days to come.

 

[jheidt2543]

One thing to keep in mind with shrinkage compensating concrete is the requirement for higher than "normal" amounts of reinforcing steel. When a shrinkage compensating cement is used, what actually happens is the concrete volumn increases during initial set. Then, as the concrete continues to cure it shrinks (all concrete shrinks). The added reinforcing inhibits the shinkage and the slab is actually put under compression ( a "kind" of prestressing).

My first job using a shrinkage compensating cememt was about 1980 using "Chem-Comp" cement. The ready mix concrete was about $5.00/cy more and the reinforcing was two layers of 6x6-6/6 mesh in sheets instead of one layer of rolled 6x6-10/10. The first layer was placed on the subgrage on chairs and the second layer was "walked" into the fresh concrete. It worked great, 10,000 sf pours with no control joints or boxouts around building columns. That warehouse floor is still in good shape today. I want to say that the total cost for that floor was about 20% more than using normal concrete and remesh design.

 

[Laser28]

Boy, are you guys making this complicated.

First of all, a bulldozer is not a compactor. It has those wide treads on it to REDUCE the load on the soil. You must have uniform compaction in order for the floor to function properly.

Secondly, you are restraining the slab by tying it into the walls with reinforcement. if you let it float inside the walls, there will be less restraint, and the slab will be less likely to crack.

3000 psi concrete does NOT shrink less than 5000 psi concrete, especially if properly cured. Use the vapor barrier on the TOP of the slab to CURE it.

Shrinkage compensating cement is difficult to get and use. It would be preferable to design the floor with reinforcement (steel fibres, wire mesh or deformed bars) using one of several good and simple design methods such as the Wire Reinforcement Institute method. See "Designing Floor Slabs on Grade" by Ringo and Anderson from Aberdeen Group. Yes - you CAN have a 40x50 slab without joints.

Moisture penetration of the slab can affect impervious toppings, the vapor pressure can lift these toppings, especially if they are placed on a weak (3000psi) slab. A vapor barrier does not STOP moisture, it merely retards it. One inch of sand will reduce the capillary action that allows moisture to reach the bottom of the slab. if you are truly concerned, use a proper waterproof paper or fabric under the slab - cheap insurance.

My credentials - I gice workshops on SOG design and construction and am a certified concrete finishing technician - not an armchair quarterback.

 

[boo1]

Laser28,
reading your post: "My credentials - I gice workshops on SOG design"
What does gice mean?

 

[Cleron]

Hello again and many thanks.
Hey, Laser28, I appreciate your post. And, I knew immediately what gice means. I think everyone knows how close the c and v are on the keyboard. If I didn't miss one every now and then I'm typing too slow (or thinking too fast!).
I've already increased the concrete strength and asked the contractor to saw cut it at least half way through. We also are going to put a vapor barrier on top as soon as possible after finishing. I asked the contractor and finisher to please check the trucks before unloading to make sure the water content is not too high. I feel pretty good about both of these men as I have worked with them before. They both have worked with the concrete supplier for years and have a good relationship with them.
This is a great site. I have scanned over posts for the past couple of years and know a lot more about concrete than I did.

 

[Taro]

Laser28 wrote "3000 psi concrete does NOT shrink less than 5000 psi concrete".

For typical mixes, 3000 psi concrete most certainly DOES shrink less than 5000 psi concrete. Higher strength mixes usually use more cement than lower strength mixes. More cement requires more water in the mix to hydrate the cement. More water means more shrinkage. When specifiying slab mixes, never use higher strength concrete than you really need for strength and durability requirements.

 

[Laser28]

Boo1:

"Gice" is civil engineering shorthand for Give, Inform,Conduct, Elucidate and now that you know, I'll have to kill you.

Taro:

You are, of course, right,at least in the simplicity of the arguement. Work performed at MIT showed that for each 1% increase in mixing water, shrinkage could be expected to increase by about 2%. However, this statement is only true if nothing else changes. It is meant to be illustrative of the effect of adding additional water to a mix in order to increase workability.

In fact, Tremper and Spellman noted in "Shrinkage of Concrete - Comparison of Laboratory and Field Experience" HRR 3, Properties of Concrete, TRB/NRC that the type, fineness and composition and cementing materials content have very little effect on the drying shrinkage of normal-strength concrete.

I also gice seminars in concrete mix design and fundamentals...

"Never wrestle in the mud with a pig, you are going to get dirty and the pig loves it."

 

[jheidt2543]

Yes, but how else are you going to bring home the bacon?

 

[JAE]

"Never try to teach a pig to dance....you won't succeed and you'll end up annoying the pig."

 

[Zambo]

the vapour barrier (damp proof membrane) under the slab will stop loss of water from the mix during placement and curing. Some subbase mterials can soak up a lot of water. It will also protect from sulphate attack, if this is an issue.

If you do still get a crack after all your precautions it will most likely be a surface crack only which you can repair by applying cement with bonding agent using a sponge to force the material into the crack. This can be carried out while the concrete is still "green" i.e. the cement has not fully hydrated (proper clean up needed after to avoid a dusty surface). Of course this method is no replacement for carrying out the job as well as you seem to be planning it.

Concrete mix design is a very technical subject especially with all the modern admixtures available. No need to overspecify as additional cement does not mean an improved job. Most importantly for the final finish is that the concrete comes with a slump that the workers can deal with and that they take the time to trowel the surface properly. They should not need to apply any water to the surface to carry out the final finishing.

Zambo

 

[AlohaBob]

The Ringo Anderson book has excellent suggestions. One I like is a double layer of plastic VB over sand. He instructs how to calculate the proper amount of steel to match the stress. More smaller bars is better at narrower spacing.

Curing is most critical.