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Old 07-17-2016, 10:41 AM   #1
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What Spec. of Concrete Strength for a 6" Thick RV pad?

We have engaged a contractor to pour a new driveway and parking pad suitable for a Class A or fifth wheel to drive over and park on that will not crack.

Our spec is described as:

Provide and use sufficient rebar and concrete to a depth of 6” suitable to allow heavy Recreational Vehicles of type Class A and 5th wheel to drive across without cracking or damaging concrete

We have agreed a height of six inches plus rebar and he is using a road base of crushed fines that looks like sand.

Given that I have just six inches of concrete depth, what specification of concrete should he be providing to sufficiently meet the requirement please?

I'm looking for specific guidance on type of concrete to pour and rebar please, (not recommendations to increase the depth of concrete).

Thank you
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Old 07-17-2016, 10:56 AM   #2
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your concrete contractor should know exactly what mix is required to meet your needs. If not, you need a new contractor.
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Old 07-17-2016, 11:17 AM   #3
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Quote:
Originally Posted by davel1971 View Post
your concrete contractor should know exactly what mix is required to meet your needs. If not, you need a new contractor.
Yes agreed. That is why I Am asking - to check before the job. Do you know the answer please?
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Old 07-17-2016, 11:27 AM   #4
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Rebar, wow. Thats some heavy duty driveway. As the others stated the contractor should be able to rattle off PSI numbers based on, concrete strength and thickness. That are the two factors that determine load ratings. If money doesn't matter, then go with it, otherwise look into mesh, cheaper and serves a similar purpose, without the xtra cost.
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Old 07-17-2016, 11:31 AM   #5
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3500 psi with fiber mesh. As said above the contractor should know. If it wasn't on stone I would go with 8" for that much weight. Control joints every 10' but the rebar should be continuous. Make sure the stone is compacted. Pull a string across the forms in several places and measure down to vetify it is 6" in depth before pouring. Watch the amount of water being added, should be kept to a minimum. verify the mix with the driver before pouring. Recommend a slick finish (float) not a broom finish. Don't drive on the concrete for at least 7 days to allow it to cure and develop full strength.
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Old 07-17-2016, 11:32 AM   #6
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We are happy to be paying for 6" plus rebar and road base but I want to ensure the psi strength is enough. Some of our friends have huge tag axle class A buses and I don't want to concrete to crack if they come and visit.

Just keeping the contractor honest by seeing if others have had this experience and what they used
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Old 07-17-2016, 11:35 AM   #7
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Thanks BBQguy.

We have to take a trip for 2 months after the pour so it will have plenty of time to set and harden.

With rebar in there what is the purpose of the fiber mesh? Would just rebar be sufficient?
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Old 07-17-2016, 11:39 AM   #8
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I'm betting with the proper mix, you don't even need rebar or wire mesh reinforcement. Would be overkill on a residential driveway
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Old 07-17-2016, 12:28 PM   #9
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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.
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Old 07-17-2016, 12:30 PM   #10
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First off, you need a good base and drainage. It is impossible to determine what needs to be done in your case without knowing what kind of base you currently have. If you have a clay base and suffer from water issues, you need to build a good base and drainage before you even think about concrete strength.

lets as an example say you have a sandy loam condition that is currently a grassy area and you want to make a concrete pad to park your motorhome on. First you need to excavate all the sod and decomposing matter so you can get down to a solid undisturbed base. Then you are going to want to bring in 6" worth of 1" crushed stone or crushed concrete and have it vibrator compacted. Now you are ready for rebar, mesh, etc, make sure it is installed on stands, just having the concrete guy pulling it up into the concrete as he pours it won't do because it will get pushed back down as they walk around in the concrete. Once the rebar is in, you are finally ready for concrete, be forewarned though, that the amount of water and the length of time the concrete has had a chance to cure while in the cement truck will make a difference in the final yield strength.

Here is an article that will give you some idea on what is involved in determining concrete yield strength.

http://www.ce.memphis.edu/1112/notes...mix_design.pdf

What I would suggest is find out how much weight you plan on putting on the driveway. i.e. the weight of your fully loaded class A motor home,

Get apples to apples detailed bids from several REPUTABLE concrete conractors on what strength concrete they will be using, what size rebar and on what centers, how many layers of rebar, etc.
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Old 07-17-2016, 12:40 PM   #11
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"Hairline" cracks should be considered acceptable. Reinforcing doesn't function until the concrete cracks unless it's pre-tensioned. I'd go with #4 bars at 16" each way centered on the 6" slab and a minimum concrete strength of 3500 psi. Also, keep heavy loads off it for a couple weeks.
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Old 07-17-2016, 12:58 PM   #12
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6-6-6 6 bag mix, 6% air-entrainment admixture, 6 inch slump. Recommend 6" steel mesh reinforcement. Good granular base - sand or gravel compacted.
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Old 07-17-2016, 01:01 PM   #13
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3500 PSI concrete, make sure it is air entrained. Re-Bar per standard engineering practice for this type slab. Stone Sub base minimum 8" 12" is better. Control joints to create approximate squares, this will minimize cracking, elimination of cracking is impossible. Once concrete is poured, and bull floated DO NOT trowel or broom finish UNTIL free water is off surface and concrete has dull sheen. Troweling prior to this will drive free water down into top of setting concrete ruin surface strength of concrete. (Surface chipping and flaking will result.
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Old 07-17-2016, 01:19 PM   #14
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If you are in cold country use 4,000 PSI air entrained concrete. 3 inch slump. Fiber mesh is essentially temperature cracking control and is good if you don't use rebar. Make sure they don't add water at the site as that reduces strength. Additionally if it sets too fast don't allow spaying with a hose or flashing with a large brush full of water. That will cause pasting on the surface and spalling later. Saw joints or tooled joints every 10 feet is good,but try to keep squares if possible. Sounds nice, good luck.
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Old 07-17-2016, 01:21 PM   #15
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Retired Civil/Structural Engineer here. As others said 6" 3500 PSI minimum, (I use 4000 psi min.) 6 sack mix, air entrained, FLAT wire mesh reinforcement, CONTRACTION JOINTS at 12' centers, and I would add a "superplastizer" additive in the mix. Contractor and concrete plant will know what this is. WATER in concrete is the enemy of strength and a promoter of shrinkage cracks, BUT water makes it MUCH EASIER to spread/finish the concrete. Superplastizer will give you the benefits of water without adding excess water.

Concrete plants have MANY concrete mix designs, and in general, your can ask for a "highway department" mix matching the above requirements and have some assurance of decent results. I suggest you pay for the concrete yourself, or reimburse the contractor for the concrete plant invoice. That way the contractor has no incentive to short the mix design.

You do not need rebar unless you intend to space joints in excess of 12' (pvt thickness " x 2). If so, #4 bars at 9" ea way (1.5x pvt thick) is all that is useful. Flat mesh is preferred as mesh chairs can be used to maintain the mesh in the center of the slab. Pulling up rolled mesh into the slab always leaves part of the mesh on the ground, and hence, does no good. If your concrete plant has an "fiber mix concrete" available, that works in place of the wire mesh.
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Old 07-17-2016, 02:00 PM   #16
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3500PSI 5.1 bag mix. 3to4 inch slump and 4% air should do it. If ten. Is above 90 degrees don't pour in afternoon. Keep covered with wet burlap for three days.
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Old 07-17-2016, 02:26 PM   #17
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You really don't need concrete to be super strong, but a lot can happen between the plant and the pour. The most common is that they add water to make it more workable. Cement is cheap. I always specify a stronger mix, knowing I may not actually get it. Also, the longer you wait after the pour to put your RV on it, the stronger it will be. We use 3 days minimum to cure. If it is covered so it doesn't dry out, it will continue to get stronger.
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Old 07-17-2016, 02:41 PM   #18
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Rubbernail covered it all but with supper it should be added at site not plant, will cause the mix to flash set and hard to finish. Ask about fly ash in mix to make it workable longer in hot weather. Road mesh is 6x6 I think is all you need.
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Old 07-17-2016, 03:25 PM   #19
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My father was a concrete contractor for 30 years and I worked with him for 14. Some good and erroneous info here. You have to be careful when using higher strength mixes as they generate more heat when curing. Personally I wouldn't go above 3500 psi. or more than 6" thick. The most important thing is not driving on it for 30 days, which is how long it takes to achieve the needed strength. Saw lines or grooves don't prevent cracking, they control it, hence the name "Control Joints". Fly Ash in my experience made the mix less workable and more prone to cracking. Whatever you do you will probably see some cracks over time, unless you want to pour it like a section of interstate highway.
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Old 07-17-2016, 03:33 PM   #20
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Concrete Pad

I went with 6000 psi on the pad with rebar and 4000 psi on the drive. Probably overkill but it didnt cost much more.
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