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Slab thickness for new shop


ivanuke
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What thickness would you guys recommend for a truck repair shop? I will be building a shop soon to service my fleet of dump trucks. Ive outgrown my 35x50 shop that has a 12" slab. That to me seems overkill. I am thinking 6" slap for the new shop. Any recommendations so the slab holds up for many years? Will also be adding a gantry crane in the future to lift dump beds and other items so I would most likely put grade beams for that. 

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I’m far from an expert so keep that in mind, but I was looking at a finishing pouring a concrete slab in the back 16’ of my shop and the concrete guy told me 6” was enough for semis and equipment. So that’s what I’m planning on doing. Hope this was helpful. 

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The problems we face today exist because the people who work for a living are outnumbered by the people who vote for a living.

The government can only "give" someone what they first take from another.

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I am no expert either but I went with 6" on my 40 x 60 shop. No dump trucks but some large farm tractors and backhoes no cracks yet. I used some  wire mesh that I had in the middle. Went with fiber in the concrete. I do not remember the rating on the mix. Pour it as dry as you can and stay off it for as long as you can.. 

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It really comes down to good subgrade prep.  Up north we have to do a good subgrade of stone because as the ground freezes and thaws the slab needs a "cloud" to float on so that there aren't any pressure points against the underside of the slab.  A good base of gravel acts like a mattress to allow the slab to not break during ground movements.  In Texas you won't get the freeze thaw cycles we get that are very destructive. I would guess 6-8 inches would be adequate.  Rebar on 1ft grid is a great idea, you want it in the middle of the slab depth. I don't care for fiber, I've been told it actually weakens the concrete's actual strength, but I haven't done my research on it.  Pour it during cooler temperatures, 60-70 degrees Fahrenheit. Don't pour it wet. A 3.5 inch slump should be workable for a skilled crew if you are pouring a minimum of 6".  If they need more workability than that then order the concrete with a mid grade plasticizer.  Water is the #1 killer of concrete.  It is a catalyst that causes the cement powder to crystalize, thus glueing everything together.  The water will leave, and if there is a lot in it you will get shrinkage cracks, you can get spalling, dusting, etc.  Plus your floor will not be as dense.  3500 psi is adequate, but 4000 psi will give you a higher cement content that will prolong the durability of the surface.  The PSI rating indicates how much weight the concrete can hold before failing. (After a standard 28 day cure). 

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If you are using barrel trucks your contractor should specify how much time between truck loads showing up. If the trucks are loaded and dispatched all at once, the first load will be nice, the second will be harder to work with, and the third will be a ball buster.  They should be loaded and dispatched at a frequency that the contractor can handle.  Otherwise they will add water to subsequent loads to try and maintain workability, and your floor's integrity will suffer.  Every time I drive through new neighborhoods I can see where the crew took too long to unload a truck and they added water, causing the last concrete out of the truck to be visibly inferior to the first concrete out.

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Wow thanks guys for all the awesome advice! I will be doing the work myself. Forming and doing the rebar only. There are 4 concrete plants in the same section of commercial property where my shop is. Two gunite companies and two companies with barrel trucks. Cemex being one of them. I think I will budget for the 6" slab. Thanks for everyones advice especially JoeH for the detailed info!

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Will you be hiring a concrete finisher? Unless you have a crew that knows what they're doing I strongly recommend having an experienced crew on hand to screed, float, and power trowel the surface.  You want your floor flat and smooth and durable.  1 cubic ft of concrete weighs about 146 lbs.  That's a lot of weight and material for an inexperienced/out of shape crew to handle under the pressure of barrel truckloads getting hot.  Consult with them on truck access/reach to decide if you need to rent a concrete pump or if the trucks can  reach and unload without a pump.

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9 minutes ago, ivanuke said:

Wow thanks guys for all the awesome advice! I will be doing the work myself. Forming and doing the rebar. There are 4 concrete plants in the same section of commercial property where my shop is. Two gunite companies and two companies with barrel trucks. Cemex being one of them. I think I will budget for the 6" slab. Thanks for everyones advice especially JoeH for the detailed info!

Are two of the concrete plants volumetric mixers?  My dad and I own meter mix trucks, personally I'm partial to them.  But you want a reputable company with good drivers! Meter mix drivers can't just be drivers, they have to know what good concrete looks like and care enough about their career to want make the best concrete they can for every job!  Barrel trucks, drivers don't have to know anything about concrete, but the company as a whole has to be reputable and use quality materials.

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11 minutes ago, JoeH said:

Are two of the concrete plants volumetric mixers?  My dad and I own meter mix trucks, personally I'm partial to them.  But you want a reputable company with good drivers! Meter mix drivers can't just be drivers, they have to know what good concrete looks like and care enough about their career to want make the best concrete they can for every job!  Barrel trucks, drivers don't have to know anything about concrete, but the company as a whole has to be reputable and use quality materials.

Two are gunite companies (not sure the proper term for the truck they use) The other companies have readimix that they bring to you in barrel trucks. Shop will be 50x80 and is accessible at all sides, can most barrel trucks reach half way? I will get pros to do the finish thats for sure, but I have experience with framing and rebar so I will do that myself (I still have somewhat of a good back lol)

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Gotcha, thought you said 2 gunite companies and 4 concrete companies.

Gunite trucks looks similar to meter mix trucks, but they produce different products.  Not sure how far barrel truck chutes reach.  My meter mix truck can reach 18.5 ft straight off the back of the truck. A barrel should be able to reach a bit further.  If you break the pour lengthways in half you should be able to reach in 25 ft from either side to cover the 50 ft, assuming the truck can get his tires right up to your forms without hitting any form braces.  You may consider using a "screed key" down the length in the middle (along that 25 ft line)to prevent a cold seam and to keep the two halves from being able to shift vertically from each other.  Screed keys notch the face of the concrete with an indent that will get filled by a slab being poured against it. It effectively locks the two slabs together so they can't shift vertically. Google should give you pictures of them. 

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Ivanuke is in Houston I believe. It doesn't get cold enough there to justify in-slab heat. I would not do Type III "high early" cement. It'll shorten how long the crew has for working with it. Type I cement will be hard enough in a week to roll trucks across.

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Yep I'm in Houston, we rarely see freezing temps. I don't think we got any this past few years. Maybe a day or two. The majority of my property is covered with 4-6" of crushed concrete that my trucks are parked on. I was planning on bringing in select fill as a base for the slab and compact it. Would it be a good idea to add a layer of crushed or would it not make a difference.

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4000lb. would be best. If you can afford the extra 2" of concrete for 8" total, do it now,it will greatly help for resale. Non fiber will also give you a smoother surface to sweep  easier and not worth the money. Wire mesh is sufficient at that thickness. In front of the truck doors, lay rebars across to help take the blow when trucks first hop up and in to help disperse the weight. I'm a concrete contractor, so that's my two cents on that! good luck.

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Twenty years ago I built my garage and it is a pole barn so the guy that did the floor poured it a good 8" and put a heavy haunch around the sides I have about 30 tons off machinery setting on the floor and my b model one crack at the overhead door  

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Obviously the thicker the better if you can afford it. But 6 inch thick is plenty for loaded trucks and excavators and anything else. Everyone I know has 6 inch slabs and have never had an issue.

And do yourself a favor. Put down poly vapor barrier and a layer of rigid insulation under your pad. Your floor will stay warm and dry. Makes a huge difference. No damp cold floor to lay on.

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I ran black plastic drain pipe in gravel around three sides and I used double the layer of plastic but I didn't do the insulation under the floor but I spent it on the walls and ceiling  and door. for thierty plus years I never had heat worth a dam where I worked so what I built was fantastic.

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