Pushing the budget..

[EddieWalker]

I can't say for sure if concrete works that way or not. More volume adding more strength as a direct measurement. My understanding is that a 3 1/2 inch slab with the right mix can be rated for 4,000 psi pretty easily. A 20 percent increase in thickness might raise the psi, but I don't know if it's a 20 percent increase or not. What is more important is what it is resting on, how much water was added to the mix so you don't weaken the psi strength of it, and how well it's held together. If you do everything right, then you can get away with something lesser then rebar. I've seen plenty of slabs poured without any rebar in it, or anything else holding it together. When I poured the slab for my house, either I, or my guy who did the finish work, got the math wrong and I had about five yards too much left over. I dumped it where I could create a parking pad close to the house without any prep work, or anything to hold it together. It's not consistant in it's thickness either, but close to 3 inches in some places, maybe a bit over in others. In the last ten years, I've driven my backhoe over it a couple dozen times. It weighs 14,000 pounds. I have two cracks in that pad that are big enough for grass to grow in it in a couple of places, but overall, it's not bad. The concrete is plenty strong enough to support that much weight, but without anything holding it together, it cracked, I think it would have cracked with rebar anyway. That's what concrete does.

Instead of paying for more material to be hauled on a cement truck, and paying more for labor to work it, and build taller forms, I would ensure that the concrete is poured at the ideal temperature. Do not add anything to it to work it in cold temps because that will make it a lot weaker. I would also make sure that it is so dry it barely comes down the chute. Most guys who do this for a living know that water makes it easier for them to spread it, but also that it makes it weaker. If you are not there to watch them, odds are good that they will add water to the mix. I always watch them to make sure this doesn't happen.

And of course, make sure your soil underneath the pad is compacted. I used to work for a guy driving a water truck in CA after getting out of the Marines. One of my jobs was to add water to the dirt being used to build up a pad. Not enough and it wouldn't compact, too much and it was mud. Getting it just right was fairly simple, but it was a process that required me to spray the pile while a guy in a wheel loader mixed it up before loading it into the dump truck and hauled to the pad. Then it was spread and compacted with a sheepsfoot roller. Once built up, the inspector would put a machine on top of the pad that measured compactness. I think it was supposed to be 98% or better to pass. Most of the time we got it on the first try. Twice they had to dig it out and do it again. In those cases, we didn't have enough water in the mix. So it was hauled back to where we where mixing it, and I sprayed it again while the operator picked it up, dumped it, picked it up and dumped it over and over again until he felt it was ready.

 

[s219]

Really, I think the best way to manage cracks in concrete is to eliminate the reasons they happen. Re-bar and screen are there to provide tensile strength, not manage cracks. Fiber is there to increase structural integrity on a fine scale. It doesn't add much overall strength but can help re-bar and screen do a better job, and it can help corners and edges from developing stress concentrations. It can prevent small cracks and help the concrete cure crack free, but it won't avoid big cracks that happen due to settling, or improper slab size. None of the "reinforcements" will do that. The only surefire way to avoid cracks is to have a properly compacted base and to put in keyways or use a saw to cut expansion joints, and allow room around the edges for the concrete to move independently from other structures that settle or move.

For my barn, we used keyway to divide the slab into 10x10' sections. Though I don't often see it done, I made sure to put expansion strips around all the poles, that way if the poles settled or moved seasonally, they wouldn't pull on the floor slab. So really, I have a bunch of 10x10 squares that are more or less free floating and sitting on a compacted fill and gravel base. If they do crack, it will be within the 10' sections and it will invariably be due to the base support. All the other sources of cracking have been eliminated.

 

[paulharvey]

As Eddy said. I've poured concrete, supervised crews pouring, and inspected concrete work. The typical residential crew, if no one is watching, will normally try to pour at a 8" slump. It's not At All uncommon for the to add 20 gallons to a 5" mix, before even starting a pour, and add another 5-10 gallons as they go. I've poured concrete, and sure, I like a 6" slump, but really anything beyond that is Lazy.

Typical, 3000 psi mud, is normally ordered at 5" slump, and there is a margin of error from the plant, so anywhere from 4 to 6" could be on the truck. I've got no issue with adding 10 gallons to some 4" mud for slab work.

4" of good mud, poured over a good, compacted, no organics or clay base will be better than 6" of 4000 psi with wire and rebar, poured on mud, or muck.

 

[USAFpj]

I'm checking with the contractor now to see what the psi rating is of the 4in mix. That may be the deciding factor on whether to bump it up to 5in or not. I do believe that site prep/compaction will be the key, either way.

 

[AGRIMAN]

I do believe that site prep/compaction will be the key, either way.

I agree. My newer shop is a 50x50x16 red iron building. It has a 6" concrete floor with fiber only, there is no rebar in my floor and after 7 years there are no cracks anywhere. We did pour 24"x24" footers under each door to reinforce the edges of the pad against the weight of heavy equipment driving on and off it.

i don't have a hobby shop, mine is mainly used for the farm. We normally drive tractors, combines, backhoes, skid steers, and semi trucks in and out of it.

It's also true that you can never seem to build it big enough. I'm already looking at adding on to mine. Good thing I spent the extra money on the expandable end wall.

Will there be any fill under your floor? It so, how much?

 

[USAFpj]

Builder says that the 4in is a 3,000psi mix.

Is it safe to say, that if the earth is compacted the same either way, that 4in concrete with fiber would have less life longevity than 5in with fiber? Or not enough to justify the financial difference?

 

[paulharvey]

Builder says that the 4in is a 3,000psi mix.

Is it safe to say, that if the earth is compacted the same either way, that 4in concrete with fiber would have less life longevity than 5in with fiber? Or not enough to justify the financial difference?

I am a bit leary of this question. 6" is of coarse stronger than 4"; that's why we always spec it for drive ways, and 4" for sidewalks and house slabs. Is it 'worth it' that is a hard one to answer. Many many drive ways and garages use 4" and are just fine, but that doesn't mean the 6" wouldn't have been better. If you where to do 4" and it cracks up, then I would say 5" or 6" would have been better, but no proof it wouldn't have busted up as well.

 

[USAFpj]

If you where to do 4" and it cracks up, then I would say 5" or 6" would have been better, but no proof it wouldn't have busted up as well.

But at least if I went with correct compaction, solid control cuts, and 5in concrete, I could at least tell myself that I tried as much as possible...

 

[ovrszd]

For me, I want a smooth floor free of cracks and imperfections if possible. So I did everything reasonable to improve my odds of ending up with that kind of floor. So far it's paid off.

Think about this for a second, then answer....... What is the most used part of a building???

 

[Citydude]

For me, I want a smooth floor free of cracks and imperfections if possible. So I did everything reasonable to improve my odds of ending up with that kind of floor. So far it's paid off. Think about this for a second, then answer....... What is the most used part of a building???

Floor?