What Spec. of Concrete Strength for a 6" Thick RV pad?

[safrog]

I went with 6000 psi on the pad with rebar and 4000 psi on the drive. Probably overkill but it didnt cost much more. I also put a drain in the middle and ran extra conduit under for future wiring.

[Hexnut]

I can assure you that there are as many thoughts on this matter as the number of folks that are asked. I can offer a few more from over 40 years as a structural engineer...

It is the nature of concrete to crack, we can only control it. Concrete shrinks as it cures, so that is done either by providing control joints that are usually tooled-in or saw-cut, or restraining the shrinkage with an adequate amount of rebar. The rebar in slabs on grade will not prevent cracks, but keep them tightly closed so as not be a problem. If rebar is used, it should be close to the top with about 1.5" of cover.

I feel the better option is to use control joints to provide a place for the concrete to crack. A rule of thumb spacing is not more than 20x the slab thickness and closer is usually better, otherwise a random crack could form in between. If the joints are saw-cut, be sure this is done as soon as the concrete can stand it, usually the next morning (within 24 hours). Otherwise, the concrete has already determined where it will crack.

A someone mentioned, 4000 psi at 28 days is a good choice. Ask for a light broom finish for skid resistance and apply a membrane forming liquid curing compound as soon as possible after finishing. If this can not be done, it must be kept continuously wet for several days. Proper curing is an absolute must for durability. Since this will be exposed to freeze/thaw cycles, require an air entrainment admixture be added to prevent spalling later on.

As for a plastic sheet (vapor barrier) beneath the slab, that is normally used for interior slabs to retard moisture migration into the floor finishes and should not be used for exterior applications. When the slab has begun curing, excess water has to leave because only a small amount of water is needed for the chemical reaction to take place. Only the least amount of water should be used to make it workable, it should never flow on it's own.

Excess water leaves the slab from the top and bottom, otherwise the slab may curl upwards at the joints as seen in dried clay mud puddles. For this reason, crushed stone is necessary under the slab to provide a place for water to go. If at least 4" of compacted aggregate base course is used, a 6" 4000 psi slab should be fine. Approximately 2/3 of the 28 day strength will be seen in seven days of curing which should be ok for moderate wheel loads, but if it can wait that's even better.

Those are my main points and others have some as well based on their experience, but concrete design and placement is as much an art as a science. At times it seems to have a mind of it's own regardless. Good luck.

[itat]

In Canada, we use 35 MPa concrete for structural applications. That equates to around 5000 psi. I wouldn't use anything less than 4000 psi.

Rebar is used for tension loads in a suspended slab so for a slab on grade I'd say you can do without the rebar and just use the steel mesh, although my FIL used prestressed steel in his garage floor just because he worked for a contractor that does prestressing.

I agree with other posters about drainage and good strength in the granular base materials. Your granulars may be a lot different than ours but I would look for a well-graded crushed material that drains well, not a sandy material. If that's cheaper, use 6" of the crushed material immediately under the concrete slab and use the sandier material below that.

If you live in an urban area, you may be able to get some free advice from the road designers at your town/city hall on what they use for an industrial road. A Class A MH will need a strong base to sit on for extended periods.

Good luck with your project!

[Steveboe]

Ample expansion joints are the only way to ensure it wont crack anywhere else. Fiber and 3500 are good insurance too. The base that the slab rest on is key.

[New2aBus]

WOW someone who knows what he is talking about, REFRESHING!

[New2aBus]

Hexnut!

[HAMMDOWN]

First off the base is important remove all organic material, I like to use 6 inches of compacted rock (2 -3" lifts water and compact) of 411 crushed stone
I would not use rebar but go with the fiber only the rebar is a waste of your money period
Use a 6.5 bag mix or what is called class C
One thing that is that is a for sure concrete WILL crack you cut it in 10 x 10 sections so it cracks where you want it.
U ope this helps

[MillerTime]

Our drives were poured with our heavy equipment company and trucking needs in mind.
We used 6", 3500psi cement with another 6-8" of thickened edge, piers anywhere there was fill and then 1/2" rebar mat that was no more than 18" between bars. The rebars were also drilled and epoxy filled into the existing shop slab. We used a broom finish with minimal saw cuts and picture frame edge. We have sandy soil naturally, but we wet it and compacted it. No slab cracks in the 6 years since it was poured.

(Our shop has 7-8" slab with 11.5" thickened edge, 1/2"rebar and 18"x18" footing. No saw cuts and 22 chain floor pots. I think there are 2 or 3 hairline cracks in it. Pretty good considering we had tons of rain right before that pour.)

(I hate fiber mesh, go crawl under your bus or truck on that and you will too.)

[itat]

What is this fibre mesh everyone is talking about? We use either rebar or steel wire mesh reinforcement. We've experimented with loose fibreglass fibre in concrete and it didn't work very well. Any kind of a synthetic fibre mat would be a geotextile product?

Many posters have mentioned a type of granular material but my experience is that the material has different names depending on where you are. The OP may not be familiar with the names that some folks are using to describe the granular material they're referring to (e.g. "411 crushed", "6-6-6", we call it "Granular A" in Ontario).

I'd go with RubberNail's recommendation on the concrete.

[markb422]

You could pay and engineer to write a spec and test the concrete for air, slump, etc. at the site. They can also pour cylinders and do test. That would give you confidence that is was done correctly and have some insurance.

[MillerTime]

Originally Posted by itat

What is this fibre mesh everyone is talking about? We use either rebar or steel wire mesh reinforcement. We've experimented with loose fibreglass fibre in concrete and it didn't work very well. Any kind of a synthetic fibre mat would be a geotextile product?

Many posters have mentioned a type of granular material but my experience is that the material has different names depending on where you are. The OP may not be familiar with the names that some folks are using to describe the granular material they're referring to (e.g. "411 crushed", "6-6-6", we call it "Granular A" in Ontario).

I'd go with RubberNail's recommendation on the concrete.

Its chopped fiberglass thats added to the mix at the plant.

In my opinion fiber mesh is a crappy way of doing concrete. I've only seen it used on mini storages, and it has a place there i guess. I'd use remesh personally in that case though. I had a buddy that used it (fiber mesh) on a shop he was finishing on a house he had for sale....it was done as a hack job instead of using steel and he had no piers, thickened edges etc and had a mild amount of fill.

[ShermanD]

Originally Posted by markb422

You could pay and engineer to write a spec and test the concrete for air, slump, etc. at the site. They can also pour cylinders and do test. That would give you confidence that is was done correctly and have some insurance.

Unfortunately, the added expense will make your jaw drop to the ground.

[Mr Havercamp]

Originally Posted by MillerTime

In my opinion fiber mesh is a crappy way of doing concrete. I've only seen it used on mini storages, and it has a place there i guess. I'd use remesh personally in that case though. I had a buddy that used it (fiber mesh) on a shop he was finishing on a house he had for sale....it was done as a hack job instead of using steel and he had no piers, thickened edges etc and had a mild amount of fill.

Fibermesh = absolute garbage IMHO. It's the lazy and cheap way to do something. That's coming from a commercial and industrial construction project and general superintendent with 34 years experience. Lots of opinions in this thread. Almost as many as recommendations from the American Concrete Institute which seems to change their minds every time they have a convention, conference, or the wind changes direction. But it is good for publishing changing recommendations and charging you through the nose for them. They throw a good shindig out at the World of Concrete expo in Vegas every year. I highly recommend it.

Concrete mixes and designs are very much a regional business. What works here in Pittsburgh may not work so well in the heat of Tucson AZ. Get a good concrete guy and don't 2nd guess him. You pay for a good concrete guy. Its when someone shops absolute price for the Craigslist professional who is the lowest irresponsible bidder that you get raked over the coals. Just get some references and check him out. Let him choose the design. A lot is based on temperature, humidity,and workability needed on the day of the pour. Let him choose the admixtures based on the plants design. That way when something go's bonkers which it sometimes tend to do with concrete its on him. Once you start dictating specs is when you can get your butt in a ringer if the poop hits the fan and it takes off on him.

Since its a regional thing IMO, the only thing I'll say about a spec on an exterior 6" slab design on grade is what I would do if doing it for myself. I'd use 4000 with air, on a base of 12" #2B limestone. No rebar, its an unneeded expense. 6X6 4/4 W.W.F. on chair, epoxy coated. I'd saw cut (within 24 hours of pour with a softcut saw) control joints and a lot of them. If I was going 12' width the panels should be 6'X6'. If 10 ' width, I'd go overkill @ 5'X5' but 10'X10' would suffice. The idea is to get the panels as close as possible to a square, and not rectangular. Along those same considerations I would put a hard joint with key or smooth dowels @ every 30' if 10' wide, at every 36' @ 12' wide etc. If I had anything coming into the slab, such as a corner of a garage, I'd lay 3-3' long pieces of bar in on a 45 degree angle off of that to stop any re entrance cracks and try and cut a control from that corner. No vapor barrier under an outside slab. Depending on soil conditions some geo textile under the gravel base may not be a bad idea.

Concrete, it comes in a truck, cracks, and gets hard.........you hope. I hate it almost as much as drywall and paint.

[41ford]

Have a brother in law that is a retired concrete finisher, he has always poured my concrete patios and driveways 4 1/2 inches, minimum thick, no rebar and guaranteed the concrete.

He guaranteed the concrete to get hard and crack.

My shop, the concrete is 4 1/2" thick, except a 10 foot section where the 2-pole car lift is installed, it's 6 1/2" thick.

[MillerTime]

Originally Posted by Mr Havercamp

Fibermesh = absolute garbage IMHO. It's the lazy and cheap way to do something. That's coming from a commercial and industrial construction project and general superintendent with 34 years experience. Lots of opinions in this thread. Almost as many as recommendations from the American Concrete Institute which seems to change their minds every time they have a convention, conference, or the wind changes direction. But it is good for publishing changing recommendations and charging you through the nose for them. They throw a good shindig out at the World of Concrete expo in Vegas every year. I highly recommend it.

Concrete mixes and designs are very much a regional business. What works here in Pittsburgh may not work so well in the heat of Tucson AZ. Get a good concrete guy and don't 2nd guess him. You pay for a good concrete guy. Its when someone shops absolute price for the Craigslist professional who is the lowest irresponsible bidder that you get raked over the coals. Just get some references and check him out. Let him choose the design. A lot is based on temperature, humidity,and workability needed on the day of the pour. Let him choose the admixtures based on the plants design. That way when something go's bonkers which it sometimes tend to do with concrete its on him. Once you start dictating specs is when you can get your butt in a ringer if the poop hits the fan and it takes off on him.

Since its a regional thing IMO, the only thing I'll say about a spec on an exterior 6" slab design on grade is what I would do if doing it for myself. I'd use 4000 with air, on a base of 12" #2B limestone. No rebar, its an unneeded expense. 6X6 4/4 W.W.F. on chair, epoxy coated. I'd saw cut (within 24 hours of pour with a softcut saw) control joints and a lot of them. If I was going 12' width the panels should be 6'X6'. If 10 ' width, I'd go overkill @ 5'X5' but 10'X10' would suffice. The idea is to get the panels as close as possible to a square, and not rectangular. Along those same considerations I would put a hard joint with key or smooth dowels @ every 30' if 10' wide, at every 36' @ 12' wide etc. If I had anything coming into the slab, such as a corner of a garage, I'd lay 3-3' long pieces of bar in on a 45 degree angle off of that to stop any re entrance cracks and try and cut a control from that corner. No vapor barrier under an outside slab. Depending on soil conditions some geo textile under the gravel base may not be a bad idea.

Concrete, it comes in a truck, cracks, and gets hard.........you hope. I hate it almost as much as drywall and paint.

You have a good point in your second paragraph.
Op is in Austin TX.
I'm in cental OK where we have a lot of clay. We need the steel in our cement here. The ground heaves too much. If someone around here recommended no steel, you are seriously getting screwed, and getting a very poor job.

Im not familiar with the soil type that is around Austin, so i cant speculate. It sounds like op has everything done the way we would do it up here in OK though.

To Op's origional question.....
Op, for our area, 3500psi and #3 or #4 steel (3/8" and 1/2") should be fine. I like to over build stuff, so i would use 1/2" steel. And i highly recommend thickened edges, as it helps prevent washout and helps reinforce the edges.

[RubberNail]

Mr Havercamp had some good observations. I don't care for, and don't spec. the fiber concrete either, but have used it when asked to do so, and it's better than wire mesh that that the contractor leaves in the sand. (Owner didn't have the best contractor.) I had 50 years of mixed office and field and had some extremely knowledgeable concrete contractors, and then again some that had been doing it wrong for 30 years. The fact that concrete always cracks allows for a lot of wrong to go undetected.

My comments inadvertently omitted the curing compound. Should do that, or wet cure with burlap for 3 to 7 days. If you use, as I recommended, a good superplastizer you'll get your concrete strength much faster and make it easier of the concrete crew. On a hot day it's really hard to tell them they can't add water to the mix. With the superP they will not need it.

As for slab thickness, much depends on how good your base preparation is, and how many load cycles the concrete will see. Concrete has a fatigue strength of about 1/2 of the spec strength. Hence, for highways, with millions of axle cycles, the thickness has to be greater than for a driveway with only a few thousand cycles, perhaps on a few hundred heavy axles.

Testing concrete cylinders is only good for telling you if the concrete WAS good. It does little to assure you that your concrete WILL be good.

[Shayne]

Thanks everybody for the many excellent and detailed replies, I really appreciate it.

Regards

Shayne

[mnoland30]

You probably have more than you can digest, but here are a couple of more thoughts. Here, we call the base material ABC (Asphaltic Base Course). Local gravel companies will make that up for the road pavers, so it is usually easily available.

With an RV, you know exactly where the load is going. I just added two pieces of 1/2" rebar the length of the slab, right where the tires will go. The mesh works, but it is extremely hard to get it at the right depth. I've torn apart many slabs where it was laying on the base course. Rebar is easier to get 2" up, where it won't be affected by rust.

Think of all the driveways out there that were done with low strength concrete, no reinforcement, and they are still good 20 years later. We're probably overthinking this, which is what engineers do for a living.

[GulfHusker]

Originally Posted by conceptumator

BBQGuy hit it on the head. After 40+ years in general contracting, that's more than adequate and a good solution. What's UNDER the concrete is more important. Make sure the ground is compacted. Put a layer of 6 mil polyethylene under the concrete before placing it. You can proof roll the dirt by driving your RV over it and looking for soft spots that actually squeeze out when you driver over them. Replace any bad spots you find.

I would just add that for slightly more, you can go to a 4,000 psi concrete mix. This is the normal for driveways in our area....(home contractor here). I agree with adding the fiber, but not in place of 6x6 wire mesh. For optimal protection, the combination of re-bar, 5/8" by the way, wire mesh, and fiber, and the suggested minimum 10' expansion joints, will be the best protection against cracking. No guarantees, since, as others have pointed out, the actual mix and sometimes the adding of too much water as it's being poured, will drastically alter the mixture. Water can be added simply to make the finishers job easier in moving the concrete around.

Keep in mind, if you add fibers to the mix, you will have exposed fibers showing through the top of the concrete. Many will wear off if traffic patterns, but untouched areas, you'll see the fiber "hairs". Some customers of mine didn't care for the look.

Also, they are going to use 2x6 for form material, which will have your drive depth measure closer to 5-1/2". This is normal practice, but if you insist, he may keep the fill down below the bottom of the form boards slightly.