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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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