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Concrete: Rebar or Fiberglass?

04/10/2010 7:10 PM

I'm planning to pour a concrete driveway. The base is sand, about 15' deep. I'll be driving my truck, car and camper over it. The camper is a fifth wheel, about 10,000 lbs when loaded. Being a fifth wheel, some of the load is transferred to the truck. Is it acceptable to use the cement mix containing fiberglass fibers instead of wire mesh/rebar? What thickness and psi cement should I use?

Thanks,

Don

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#1

Re: Concrete: Rebar or Fiberglass?

04/10/2010 10:22 PM

Around here we typically do 4 -5 inch slabs for driveways with rebar at least every 2 feet on center and some go more or use wire mesh. Adding fiberglass to the concrete is even better and is almost standard practice now.

Don't go cheap on the concrete work. It will cost far more to take it out and do it over ten years from now and you or someone else will regret what little you saved in comparison.

Do it right the first time and over do it if possible! It will be apreciated later, I promise!

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#4
In reply to #1

Re: Concrete: Rebar or Fiberglass?

04/12/2010 8:11 AM

I agree rebar and the fibers are a good combo with a 2500-3500 psi mix. My concern is the sand base.

Being that concrete isn't flexible you should have an open graded aggregate like an ASTM #57 ~ 6-8" depth underneath it to allow for water drainage.

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#2

Re: Concrete: Rebar or Fiberglass?

04/11/2010 10:57 PM

I woulf recommend the 3 m fibers. They have support for your project. My engineer was asked the same questiom and he thought it ok for a driveway. I think its better than mesh because you should have less cracking

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#3

Re: Concrete: Rebar or Fiberglass?

04/12/2010 2:39 AM

I have one with microfibers in it. And cut in sections, otherwise cracks will be inevitable.

I used only 1500 psi about 6 inches thick, because I still want to finish it later with a harder technical colored cement type plaster. (with print)

I should have gone for 2500 psi if this hadn't been the case.

The dry concrete has also a different color cement proportion -wise.

If it freezes there I should certainly go for plasticizing additives, like e.g Compactuna.

Don't worry about the weights of your vehicles, you come not even close to problem situations.

With 2500 psi you can park your car on 4 domino blocks without tires after 20 days.

And without rebar no rust problems too. I am pro fibers.

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#5
In reply to #3

Re: Concrete: Rebar or Fiberglass?

04/12/2010 8:31 AM

Thanks to everyone for the replies. You've supplied with a lot of good information. As far as the sand base--when I built my house 5 years ago I had a soil evaluation done by an engineer. I was concerned about building a slab house on sand without pilings. He told me the sand base (15') would be fine for the load without pilings. I was a little surprised because I had heard those old quotes about people building a house on sand asking for trouble. Back to the driveway. I have seen several of those DIY home TV shows talking about the positive aspects of fiber reinforced concrete and I thought I'd get some more expert opinions. Oh, we don't have any problems with severe freezes and frost heave down here in S. Louisiana. Thanks again.

Don

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#6
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Re: Concrete: Rebar or Fiberglass?

04/12/2010 8:45 AM

Quick question, I'm not familiar with Louisiana, how high is the water table?

I'm only asking because sand becomes fluid in water situations and you may end up with pockets under the concrete which will lead to the concrete failing. I've seen enough of similar problems along the NJ shore and the Florida pan handle around Pensacola.

Good Luck

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#8
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Re: Concrete: Rebar or Fiberglass?

04/12/2010 8:50 AM

Don,

I did my driveway about 5 years ago with no rebar or wire mesh only fiberglass. I regret the heck out of it. The problem I see with the fiberglass is once the concrete cracks it just keeps on spreading apart. I have chunks that cacked off that have a one inch gap in them. Had I gone with wire mesh even without rebar I think the peices that cracked would at least be held in place. You might even consider going a little thicker on your driveway since your hauling that camper across it. Mine is just 3.5" I also regret that. I had a 5.5 inch slab poured in front of my shed and it's held up nicely.

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#7

Re: Concrete: Rebar or Fiberglass?

04/12/2010 8:48 AM

I have done several driveways and a 24 X30 slab for a garage using fibermesh. I live just outside Chicago where weather can be a problem. the oldest drive is about 10 years old with no cracks. The garage floor has one small crack that has not opened up for more then 2 years. I did use rebar in the corner footing of the garage slab but the rest is strictly a six bag mix with fibermesh. The drives are all 4 inches thick but the garage floor is 5 inches thick.

The good part about fibermesh is that You know You have reinforcing thruout the slab. I have seen many slabs where wire was used but in the heat of pouring the wire mesh was not pulled up into the slab but left lying on the ground where it does no good. also you are not tripping over the wire when pouring.

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#9

Re: Concrete: Rebar or Fiberglass?

04/12/2010 9:06 AM

Here's the thing about concrete - it NEVER stops "curing" and thus NEVER stops shrinking. What causes cracks is that the tensile strength of the concrete system cannot withstand the friction of the slab on the surface under it. Re-bar and mesh will keep the cracked pieces in place. Fibers in the mix will help avoid micro surface cracks which develope due to differential curing. Post tensioning will keep the system together by providing a compressive pre-load on the concrete.

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#13
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Re: Concrete: Rebar or Fiberglass?

04/12/2010 12:15 PM

A post tensioned slab for a driveways? seriously. LOL

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#10

Re: Concrete: Rebar or Fiberglass?

04/12/2010 9:30 AM

Good Morning Don,

I am willing to help you out, but will need some more info please, if possible.

1). What is the maximum axle load (Rated)? I'm assuming you have tandem axles under the trailer, correct? Spacing between the axles? Tire size? These three factors will be critical in designing a structurally sound driveway slab.

2). Size of your driveway? I'm assuming a long rectangle, W x L. You are going to need to install concrete construction joints if a long drive + concrete contraction joints, ie, "raked" joints to control shrinkage cracking in each direction.

3). Is the seasonal high groundwater depth within 24" below existing ground surface line?

Just based on the 10,000# load provided, I would construct a slab no less than 5.5-inches, because the critical shear locations will occur in 3 different locations: slab corners, along the slab transverse joint or construction joint, and along the longitudinal edge.

Per ACI, the maximum rebar spacing cannot exceed 18" o/c. Also per ACI, the minimum Shrinkage & Temperature reinforcement (Ast) is 0.16 in^2/Ft each way.....so provide #4 bars at 15" o.c. each way. Make sure that they conform to ASTM A615, Grade 60 (Fy=60Ksi). You could also use Welded Wire Mesh (flat sheets only), but to get the correct wire size and wire spacing in WWM may prove to be difficult (not normally stocked), even if you use 2 layers of WWM. Using the rebar you can also use it as "dowel bars" @ the Construction Joint to hold the entire slab together at that location and to help wheel load transfer from one panel to the next. Make sure that all rebar in held at least 3" above your gravel subbase material with plastic or Nylon rebar chairs-----don't use pieces of brick or rocks as that will induce weakened stress lines in the slab. Anyhow, the rebar chairs are relatively cheap. Provide 21" minimum rebar splice laps. Minimum concrete cover over (and to the sides) of the reinforcing bars is 1 1/2-inches, whereas the minimum concrete cover under the rebar is 3-inches

Provide Construction Joints (ie, transverse joint) @ 40'-0" intervals if you have a long driveway. You don't want to place concrete for slab lengths greater then this distance because when the slab cures the larger the joint spacing the larger the "slab creep" will be and you'll have many cracks in the slab.

Also, the Concrete Control/Contraction Joints (@ 90 degrees to one another) should be raked into the slab surface at 10'-0" maximum spacings each way.....these are needed to reduce shrinkage cracking that occurs during the curing process. You'll need to provide 1 1/2" deep raked joints (D = H/4 as a rule of thumb). As an alternative, you could saw-cut 1/8" wide x 1 1/2" deep joints when the concrete is still in it's early curing stages (green concrete). Just make sure you use a long straightedge to follow with and use a fairly strong corded circular saw with at least 11 to 14 Amps. Avoid wimpy cordless circular saws as they'll not have the adequate power to saw deeply.

By all means use the poly or Nylon micro reinforcement fibers as they will reduce the concrete shrinkage cracking significantly (only if you cure the slab correctly) as well as the amount of "micro" cracking and crazing of the surface. Use the manufacturer's written recommended amount in pounds per cubic yard of concrete.....usually this ranges anywhere from 3 to 5 # per C.Y., depending on the manufacturer and the ready-mix concrete supplier.

Concrete: My suggestion to you is to use a dense concrete that will be strong and equally resilient against loads and wear. Use a ready-mix concrete design as follows: Concrete Compressive Strength (f'c) = 5,000 psi minimum @ 28 days, cement content (dry weight) = 650#/C.Y. minimum, but you can substitute up to 15% of the Portland Cement weight with Fly Ash Type F or G (Pozzolans).....this will cut the cost of the concrete and give you an early high strength concrete/ Only drawback is that it will give you a more gray-ish color concrete and not Alabaster White as normally encountered. I really like using Pozzolans because they make the concrete more denser and wear resistant...hey, the ancient Romans used it in their civil works and many are still around today with some still fully functional. Use a Maximum Water-to-Cement ratio (W/C Ratio) = 0.42; use a maximum slump of 3" and Air-Entrainment Admixture. I would also recommend a Mid-Range Plasticizer Admixture that the ready-mix truck driver will need to add at your project site (not at the batch plant) to bring the slump level up around 6.5" to 8" for workability-----DO NOT ADD WATER TO THE MIX NOT ALLOW THE DRIVER TO DO SO...as it will weaken your concrete. Too much water is the enemy of a good strong and resilient concrete. During finishing operations, use sprinkled water as little as possible and try not to wash away the paste in the upper surface of the slab.

I prefer wet-curing or flooding the slab during the initial curing period. For wet-curing, use garden soaker hoses and cover the entire slab with at least 6-mil poly plastic sheeting. For flood-curing, cover the slab with the same poly sheeting and maintain a minimum of 1 to 1.5" water depth atop the slab. In either method, provide and continually maintain the water curing process of choice for a period between 10 to 14 days...the long the better, then strip the form-work.

Try to avoid chemical concrete curing compounds. I feel that they can be adversely affected by the weather. Also, if applied incorrectly (too much) it can destroy the finished surface by eating into the cement paste, thus leaving pock-marks.

I hope this gets you started or at least how you want to build the driveway slab. I've used this type of slab for over 25 years now in upstate NY and many look almost new if maintained and no de-icing salts are used.

Good luck.....if any questions, then just ask away, okay?

===CaptMoosie, SE/LPE

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#11

Re: Concrete: Re bar or Fiberglass?

04/12/2010 10:42 AM

I will weigh in just because of my experiences. I put fiber-mesh in my garage, instead of the re bar, solely because of a thin topper slab being poured on top of the Spancrete planking I used for the upstairs portion.

THE BIGGEST MISTAKE OF MY CONSTRUCTION LIFE.

It has cracked up, spalled, and generally been an ugly installation. Some day I have to go in and break it up and get it the heck out of there. I am hoping that eventually the cracking gets bad enough that it comes out without too much effort, since the planks are under it, and I can't possibly afford to break them up.

If I was ever going to trust fiber-mesh again, it would ONLY be in conjunction with real re bar - or at the very least mesh. All projects that I have done otherwise have been with re bar and I have had NO problems with any of them. Each time I have looked at the mesh alternative, but they usually come out to the same or lower price for the re bar (MY installation, not paying a contractor - who usually marks up the "superior" product), and I don't have to trip over or pick up the mesh. On occasion I have done both, but that is with cheap or free mesh, to allow for a lightening up of use of the re bar.

I know it exists as an alternative, because some other guys have luck with it, but I did not. The cost of doing it twice precludes the cost of doing it right the first time. Also, as a "for instance", I always use 2x6 (5.5" deep) framing for the forms on any driveway (I always have a 3/4T truck and tend toward large trailers), I usually use #4's @ 24" or #5's @ about 30/36 inches, and I have placed 3 aprons, A shed floor, and a complete driveway (garage to curb - 100') and never had any of the problems that I have with the fiber-mesh. I often (can't say always) throw in an extra #5 at the leading edge of the apron or anywhere there is a transition from gravel/dirt/sand to concrete to help hold the lib together with the inevitable "banging" load that will occur as you drive onto it.

If you do all this and judicially place the expansion/control joints you will never have any trouble and won't need the fiber. If you have the money to spend and feel unsure, pay for a stronger mix or more re bar.

Most importantly: DON'T SKIMP ON MATERIALS due to cost considerations. Put the project off, skip a couple nights at the bar, or rob a bank, but you don't want to short yourself on the project. If you are doing the work yourself, you are saving a bundle on the labor, so spend a small portion of it on getting it right the first time. If you save 10% now, you will spend the same money next time for the replacement slab, plus the 10%, plus inflation, plus the labor/equipment to bust up this slab, plus the tongue lashing from the wife (perhaps the most expensive item!). If you want cheap, leave the driveway gravel!

Best of luck and remember to enjoy the beer while you watch it cure the evening after you place it!

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#12

Re: Concrete: Rebar or Fiberglass?

04/12/2010 11:18 AM

Hard to improve on what Capt. M. said. I might add however that I have always heard that the fiberglass does not really help once the slab is set up. It seems to be really helpful during the curing period...which of course, never really ends, but if you can keep it from lifting, cracking or otherwise shifting in the first couple of weeks, it will be much more solid, and the fiberglass will aid substantially in that respect.

(reference...my college foundations course textbook)

The big problem might be your base. If it is only soft runny sand, you have a problem just waiting to happen. However, I have successfully built upon sand by making sure that the object (slab or footing) was solid unto itself. Like a ship floating on the ocean, your slab should float upon the sand. That means lots and lots of rebar. It may be possible to have too much rebar. But I doubt it. I double meshed for my workshop slab, and pushed it to 6 inches, with an 18 inch heavily re-barred rim all around the edge. The base was rocky sand. Apparently thats the worst. I used a lot of scrap iron (sandblasted and painted...) in that rim! I wish I had spent the extra forty bucks or so for a third layer of mesh...it IS a blacksmith shop after all! But two layers (spaced an inch apart) made it hard enough to walk on as I was constructing it.

I think the only real tricky part was to remember to not make the rim too thick. Seems if it is too thick, it will lift in the frost differently than in the middle. But if you don't have frost to worry about, well, then you are golden.

Good luck. A few extra bucks spent on rebar and mesh will save thousands down the road. So don't skimp.

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#14
In reply to #12

Re: Concrete: Rebar or Fiberglass?

04/12/2010 5:54 PM

Here's some of the answers to the questions:

-Axle load GAWR is 5,080 per axle. I'm not sure what the weight distribution is between the camper and the fifth wheel hitch is. I'm thinking that the hitch carries about 30% of the load.

-The tires are 225/75/15.

-The axle spacing is 34".

-The driveway size is about 12' X 50'.

-The water table here varies a lot depending on the river level. The river is about 300' from the end of my driveway and the average river level is about 12' lower than the low end of my driveway. The water table is average is about 4' at the low end of my driveway.

-We don't have any problems with frost heave and severe, extended freezes. This last winter was the worst in 20 years. We had a low of 18 degrees F for three nights in a row.

The slab for my RV garage (attached to my house) is 4" thick, 3,000 psi with rebar and 6 X 6 mesh. It was approved by an engineer. I was a little concerned about the thickness. About 4 months after the slab was poured, when the house was finished, I parked a small fire truck (not sure of the weight: 8,000 lbs.?) with a full 500 gallon water (8.3 lbs/gallon) tank on the RV garage slab for about an hour and then I parked my pickup truck with on it for two days. I had put a tarp in the 8' pickup truck bed and filled it with water (about 500 gallons?). I didn't see any difference in the slab. It seems to be holding up OK. Just the normal hairline cracks. From what you guys have told me I will probably go with the 6' X 6' mesh along with glass fibers. I'm also going to install one or two water pipes under the drive to provide a water fountain for my grandkids. I've been encouraged to have either a broom finish or a pebble finish. Is the pebble finish actually pebbles added to the finished cement or a washed finish?

Don

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#17
In reply to #14

Re: Concrete: Rebar or Fiberglass?

04/14/2010 9:32 AM

If You don't have experience doing a pebble finish I would not attempt It.

It involves pebbles instead of stone mixed with the concrete, an inhibitor is then sprayed on the surface to keep it from setting and then this is washed off to expose the pebbles. If not done right it really looks ugly. Save Yourself a lot of cussing, stick to a broom finish. If You need to make it look a little more fancy use an extra wide edging and grooving tool after brooming.

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#18
In reply to #14

Re: Concrete: Rebar or Fiberglass?

04/14/2010 11:32 AM

You might also consider a salt finish. A broomed finish can still be quite slick if wet as it is completed after the surface has been finished and allowed to set some, also pebble finishes can be slick too. Pebble or exposed rock finishes tend to be hard on bare feet. A salt finish adds some texture, is slightly rougher than a straight broom finish, and pools water from the surface into small depressions from the salt. It does not feel much different then walking on flat concrete as the salt depressions can be kept relatively few and small with a flat broomed finish between. There is also stamped concrete as a possibility. There are a lot of finish options for surfacing stamped concrete.

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#15

Re: Concrete: Rebar or Fiberglass?

04/13/2010 12:46 AM

I worked two years in Mississippi after Katrina and have been a contractor in Florida for awhile... Listening to your description of what you have and what you want to do I would pour the slab at least six inches thick with a footer on the sides with two sticks of 5/8" rebar if there is a curb cut they will probably want at least 8 if not 10 inches at the street. If you have a finished approach I would wrap the footer around the front and continue the rebar tying it all together from the front and the sides... You need to make a few cuts in the slab for cracking and also when you pour it keep it wet for a week or ten days which will slow the drying time down and assist with a minimal amount of cracking. Also the idea of the footer is to have it act as a hat assisting in keeping the slab in place and using the footers to keep it in place.. Good Luck..

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#16
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Re: Concrete: Rebar or Fiberglass?

04/13/2010 11:46 AM

Actually check the City or County Standards if there is a curb to be replaced. Curbs are frequently doweled together, and in areas where soils are weak they may have cut offs along the curb and gutter to stop subsurface drainage from getting into the AC subgrade. You can in some jurisdictions, have curbs extending 24 inched into the base/subgrade. The depth of concrete and aggregate base are dependant upon the R value of your native subgrade materials. Also the reinforcement requirements are also dependant on the R-value and the anticipated loading. Frequently heavily loaded concrete pavements will have WWF to reinforce. #5 (5/8 inch dia.) is probably overkill, you might get away with #3 or #4, if a footing is required, depending on design. It is good practice to have a cut off, a deepened section along the perimeter, not necessarily a footing, that extends deeper than the agg base and upto at least 1 inch into the native soils adjacent permeable areas like lawn.

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