Concrete Thickness Recommendation

[curb guy]

We have no problems burning floors in black with macro fiber. If there are any fuzzies sticking up afterwards, we just wave a brush burner over them and they disappear. It really is superior to rebar, in my opinion.

 

[td25c]

No rebar = no concrete .

Won't pour without it in place . To many reasons & examples to list .....

 

[DIYDAVE]

Don't have to use rebar, if you have some wore out drag harrows, to get rid of...

 

[td25c]

Look at it from the opposite direction ...

Concrete demo job with rebar or without ? With rebar it takes longer , it's tougher .

That's why they mix steel & concrete together for strength when building .

 

[doublewide]

Fiber is a good product in its place and I would be happy to replace the mesh with it but not the perimeter rebar. :thumbsup:

 

[Georgia Iron]

I believe the surface the concrete is placed on is more important than the concrete. Tree roots and trees with in 25 feet can ruin the slab. Removing top soil and building on a good solid base is needed. The best way to check your bases is take a tandem axle dump truck load it and drive it on your base material. If it does not rut or sink you are good to go. Personally I don't think gravel is any better than good fill dirt. In some cases I have seen clay pack better than gravel does. I have spoke to different engineers that spec work. Some claim more than 3000 psi drys to fast and will crack in the heat. Some claim you need gravel and then I see what I tear out and the age of it. I notice ruts in the pour from pour soil quite a bit. I don't use calcium and I don't over finish the surface or trowel it to soon, keep as much rock as you can near the top..

I have seen 4" concrete hold up to years of abuse with the right base and drainage around it. I have seen 8" concrete bust to pieces because the dirt it was on was soft. Keep the soil dry under the slab is important so water run off and drainage is at the top of the list. A heavy forklift can place more weight on a smaller area than a dozer or other tracked machine. I would recommend on the min. a 6" 3000 psi slab with number #3 rebar placed in a grid on 18" centers. Proper turn downs based on your freeze line should be used along with rebar in the turndowns (or footings). Grade beams can be laid across the slab in heavy use areas for more strength. Wire can be ok if you dont tail gate the concrete and push it to the bottom.

Concrete seems to always want to make it self square so weird corners and joints can cause cracks. Expansion joints or cuts are a good idea. Maybe even using a separate pour for the area of heavy use would be a good idea. That way it is easy to remove and replace if you damage it over the years. Concrete is a wear item and should be considered such.

Consistency is the key. Every thing should be as close to the same as you can make it. It all should settle in the same.


A loaded tandem dump truck will tell you all you need to know, always proof your ground...

 

[repowerguy]

A 4000 or a 4500 psi mix is only a few dollars more around here and in reality if you place it at more than a 4" slump without using a water reducer and using just water, you will be at a 3000 to 3500 psi mix. There are a few contractors here that want a 4500 straight cement mix and will chute it in at a 10" slump for easy placement knowing that it will end up a 3500 psi when all is said and done.
I suppose what I'm getting at is don't order a 3000 mix unless you are placing it at a 4" slump, over design the mix and allow for the loss strength when you wet it up so it is workable.

 

[Georgia Iron]

A 4000 or a 4500 psi mix is only a few dollars more around here and in reality if you place it at more than a 4" slump without using a water reducer and using just water, you will be at a 3000 to 3500 psi mix. There are a few contractors here that want a 4500 straight cement mix and will chute it in at a 10" slump for easy placement knowing that it will end up a 3500 psi when all is said and done.
I suppose what I'm getting at is don't order a 3000 mix unless you are placing it at a 4" slump, over design the mix and allow for the loss strength when you wet it up so it is workable.

I have had many conversations with guys about this. We have noticed that 2500 psi concrete seems to have more rock in it and it is harder to finish without adding water. I don't test every pour but I have heard that a lot of 2500 psi concrete and 3000 psi concrete turns out much stronger than what it is "ordered at". I am not saying use 2500 but how it is placed is very important and a lot of finishers WILL weaken the mix by adding too much water and floating it to soon.

I have never poured anything with a 10" slump. Maybe a 6 when it is pumped. I notice that concrete that is too "wet" gets super white when it drys and it also wants to dust and loose powder over time. We often don't add water and the days must be cool. At this point in the game I don't like to pour on hot days and cold days. I had a job go south last year pouring in 100* heat. Cracked later. Most flat work pours I do are under 40 yards at a time. I don't have enough experience with super large jobs, I am sure they must add more water to get it all finished in time.

We poured a good many jobs this winter. I waited for non freezing days. I went to the plant to pay a bill and they asked me how many jobs went south due to pouring in 10 to 20* weather. I said NONE. I did not pour. They smiled and went into all the problem jobs that happened and how a builder poured a home slab and then framed it and it is still wet and moisture is not leaving the slab... and several have had the tops flake off....

I don't like pouring on super dry dirt. I like to mist it first. I dont like pouring on plastic. I hate plastic cracks. Inspectors here require it. I dont like pouring on windy days. I often wonder why some concrete that is 50 years old is still working well and some that is only 10 years old is ruined. It seems that more often than not it is what is under it that has made the difference. Pouring in the wrong conditions is not good. Having a poor base is not good. Adding too much water is not good. 5 to 10 gallons per 9 yard truck should be max. Tree roots have probably ruined more concrete than anything else.

 

[8V149 Detroit]

I forgot how much rebar my dad used he had most of it laying around but I remember him telling me more than most buildings its size, we will be doing the ground under the slab and the rebar in the slab and have a company we used on his pour the concrete, I'm not sure what concrete they use but its holding well at our shop very little cracks after these 8 or so years

 

[old-iron-habit]

Properly mixed, placed, and cured concrete will strengthen for 50 years. In 100 years it should be back to its original 30 day cured strength. In my 32 X 36 work space I compacted 2 ft. of sand under my floating slab. The edge is 12" thick in one foot and then a six inch slab. I put 2 rounds of 3/4" bar on the perimeter. I spaced number 4 bar at 18" on center. I used heavy gauge 4" sq. mesh over the number 4s. Lastly I used a 4,000 lb., fiber reinforced concrete, after thoroughly wetting the sand. We poured with a low slump, (3"?) no extra water added. No vapor barrier on the sand. I went this way at the advice of one of the better structural engineers I have ever worked with. The mesh and umber 4 bar are fairly cheap. It has been 16 years ago now and still no shrink cracks even. The edge only has to carry the building, which even in a 2 story is not many lbs. per lineal foot. A last word of advice for any concrete placement. The squarer you can keep your slabs between expansion joints, the less cracking you will have.

 

[Georgia Iron]

Properly mixed, placed, and cured concrete will strengthen for 50 years. In 100 years it should be back to its original 30 day cured strength. In my 32 X 36 work space I compacted 2 ft. of sand under my floating slab. The edge is 12" thick in one foot and then a six inch slab. I put 2 rounds of 3/4" bar on the perimeter. I spaced number 4 bar at 18" on center. I used heavy gauge 4" sq. mesh over the number 4s. Lastly I used a 4,000 lb., fiber reinforced concrete, after thoroughly wetting the sand. We poured with a low slump, (3"?) no extra water added. No vapor barrier on the sand. I went this way at the advice of one of the better structural engineers I have ever worked with. The mesh and umber 4 bar are fairly cheap. It has been 16 years ago now and still no shrink cracks even. The edge only has to carry the building, which even in a 2 story is not many lbs. per lineal foot. A last word of advice for any concrete placement. The squarer you can keep your slabs between expansion joints, the less cracking you will have.

Sounds like a nice slab. How did you get the concrete in the slab?

 

[old-iron-habit]

Sounds like a nice slab. How did you get the concrete in the slab?

Local ready mix supply for the mix. For placement I left out 12 ft. of form on one end and backed the trucks in on two layers of plank, staggering the joints. With the stiff mix it was a bit of work to get it down the long chute and a bit of edge pulling with concrete rakes. As we pulled the truck ahead we pulled the planks, lifted the mat and placed adobe blocks to hold the reinforcing up. Borrowed a 2 blade riding power trowel from work and hired one of our company finishers to help get it right. I had lots of placement help so it was not a bad job tp place. I'm pouring a 20 by 32 ft. elevated deck this summer in front of the garage outside the 2nd level. I will use a conveyor ready mix truck for that placement. I'll post pics of that build when I do it.

 

[repowerguy]

OIH, is there a particular reason you placed at such a stiff slump? If you are trying to maintain a low water cement ratio, a high range reducer (super p), sure makes it a lot easier to put down.
BTW, I noticed you said it was delivered in rears, does anyone run fronts like Advance or Phoenix mixers up there?

 

[old-iron-habit]

OIH, is there a particular reason you placed at such a stiff slump? If you are trying to maintain a low water cement ratio, a high range reducer (super p), sure makes it a lot easier to put down.
BTW, I noticed you said it was delivered in rears, does anyone run fronts like Advance or Phoenix mixers up there?

The larger plant 40 miles away has front dumpers. They also have the truck with the conveyor. I probably should have got that. It's about a $50 a hour premium when one buys a few yards. The two smaller plants near me have a few rear dumps each. I poured it stiff to reduce shrinkage. A plasticizer would have been nice but gets expensive when I have to pay for it myself. I don't allow dragging concrete with the vibrator either. I guess after 44+ years in the business you want it done the same way at home as on the job.

 

[Randy88]

It depends on how wet your ground is naturally, my 34x80 shop has right at 2000 feet of dual walled drain tile under it, set up on multiple layers, anywhere from 9 feet deep to 4 feet deep clean crushed rock packed as we filled it up, the service pit walls are 10 inches thick, the footings under it are 12 inches thick, rebarred beyond belief, with both tile and crushed rock under the pit floor. We used pilasters all the way around the pit walls and then two levels of rebar set on 18 inch centers for the floor, staggered for the 8 inch thick floor, with insulation board under the floor for in floor heat. If I recall, the mix was six bag mix, the local concrete company wasn't willing to do this for me, so I got someone else to mix it that rich, for just a few bucks more per yard.

When done, the flow of water under the building started out dumping into a 24 inch dual walled tile set on end, 8 feet deep under the pit floor, the float switch for the sump pump was set to pump out 2 feet deep water each time it kicks in, at first it kicked in every 5 minutes 24/7/365 then after about five years it backed down to kicking in every 10-15 minutes during the winter and then back up to every five minutes the rest of the year.

Anyone can argue if rebar or fiber is better, I'd argue on the quality of concrete is far more important than anything to you use to reinforce it with. I've broken up old concrete my grandpa poured 60 years ago that was much harder than anything with rebar or fiber in it that the local concrete company poured anytime since, I think its called getting what you actually paid for, or scamming the customer, but that's a different story completely.

I use rubber belting on the floor as we drive tracked machines into he shop.

Been around many shops over the years, where they scrimped on something, either fill under the concrete, thickness, rebar or whatever to only find out years later they got to do it again, my advice is to do it once, go overkill and forget about it for decades to come and not worry what your going to drive on it in the future with larger machines that may or may not be purchased.

 

[repowerguy]

Randy, a 6 bag mix is approximately a 4000 lb mix. Maybe it was a 6000 lb mix? A 6000 mix is a potent mix that makes for a high early strength. Why your local company didn’t want to make a special mix is odd, more cement in a mix is a billable item so maybe they just didn’t have a jmf for the mix.

 

[Randy88]

The local concrete company has a habit of billing for a lot of things that never actually made it into the mix, they wanted to sell me a four bag mix, claiming it was strong enough and also claimed they'd never sold a six bag mix for a shop floor.

I could type pages of issues this company has had over the years in the area, so I just got someone else to pour me a six bag mix, which I think is what I actually got, but I think it was called much more than a 4000 lb mix, but its been about seven years now to recall what the mix was called exactly.

 

[Jumbo]

The slab in my shop is 8" with two mats of #5 bar on 8" centers. 5000# mix. Burke (now Meadows Burke) has a whole array of inserts to place in your slab. I went with 1" ACME threads because the rod is easy to handle and screws in fast. Of the two dozen I placed when the slab was poured, I have only use three or four. But you put them in for the "maybe." I placed two runs 8" apart of 3/8 x 10" flat stock
so that pretty much the whole 3/8 thickness exposed. I used 3/4" Nelson studs every 18" to hold them in place. A little 90 weight on top of them works wonders sliding things around especially the older I get. All of this was done though with parts left over from other jobs so the concrete was my only expense. When things are free, you can go crazy if you want.

 

[colson04]

The slab in my shop is 8" with two mats of #5 bar on 8" centers. 5000# mix. Burke (now Meadows Burke) has a whole array of inserts to place in your slab. I went with 1" ACME threads because the rod is easy to handle and screws in fast. Of the two dozen I placed when the slab was poured, I have only use three or four. But you put them in for the "maybe." I placed two runs 8" apart of 3/8 x 10" flat stock
so that pretty much the whole 3/8 thickness exposed. I used 3/4" Nelson studs every 18" to hold them in place. A little 90 weight on top of them works wonders sliding things around especially the older I get. All of this was done though with parts left over from other jobs so the concrete was my only expense. When things are free, you can go crazy if you want.

Do you have any pics of your shop floor and how you laid out the inserts and flat stock? What kind of work are the inserts good for? I've poured a lot of concrete but all residential and they just want it flat smooth (and cheap).

 

[Jumbo]

Do you have any pics of your shop floor and how you laid out the inserts and flat stock? What kind of work are the inserts good for? I've poured a lot of concrete but all residential and they just want it flat smooth (and cheap).

No, the shop is buried in "stuff" scattered about. and right now, I am phone free. The inserts were way over sized, half inch would be more appropriate for a normal shop, I just liked the 1" acme threads because of their ease in using. And, the price was free which improves my opinion of things immensely. You cannot do this for a normal residential person, like you said, price is everything.
My shop is my playpen, my wife says it is cheaper than daycere for me and/or a bar bill.