3500psi vs 4000psi slabs?

[Hobby_Man22]

I thought 4000 was the standard for concrete foundation slabs. My contractor wants to use 3500psi for my 3000 sq ft steel building. He says with the footings it's not necessary to have 4000 psi and it will just crack more. I can't remember if he said it will crack more or dry too fast. It didn't make much sense. I'm a little irritated because when I called I asked for a quote for a 6" slab then when he came out to look at the lot to see how much dirt I would need to bring in he said he quoted me a 4" slab over the phone $9/ square foot. Then he says it would be $12 for a 6" slab. Opinions? I'm sure 3500 would be fine, but I mean what happened to doing what the customer wants? This is a common theme I see with contractors. I guess he's right honestly. I'm sure just about everything is 4" parking lots etc.

 

[MerlinsBeard]

I believe that 3500 and 4" is the standard for normal vehicular traffic, that's what my basement and garage slabs are and no real issues except I have done some small crack repair after 8 years with slabweld that's held up for 2 years and counting. You planning on storing something heavy duty?

 

[Hobby_Man22]

I have 3 vehicles that weigh in around 7_8000 pounds. I'd just like the extra insurance

 

[wssix99]

The higher the psi, the higher the cement content. The more cement in the mix, the more prone it will be to shrinkage cracking.

The reason people typically choose 4000 over 3500 is for a smoother finish on the floor. If you are going to coat the floor with expoxy or something else, then spending the extra money for 4000 would be a waste. (You do not need it for strength or structural purposes.)

6" is overkill unless you have forklifts or have equipment that specifically requires the depth. Most vehicle lifts used to require 4" but now they have moved up to 5 or more inches, so you might want to check that if you plan to install lifts.

Either way - if you have a specific depth that you really need for something, you should have a mechanism to check for it. It's really easy to pour a 4" slab and only get 3.5" in some areas.

 

[racecougar]

Do you plan to install a lift?

 

[wssix99]

I have 3 vehicles that weigh in around 7_8000 pounds. I'd just like the extra insurance

This doesn't matter so much! The pressure they put on the slab equals the tire pressure. So, if you have trucks with tires filled to 55psi, then that's the pressure they are putting on the slab. (The tires deform to a contact patch that spreads out a vehicles load and matches the tire pressure.) That pressure decreaces with the square of the depth, so 55psi tires would impart around 55/(4*4) = 3.5ish psi to the ground.

^ These numbers are nothing for the (3500 psi) concrete. They aren't anything for 3000 psi concrete either. The only thing additional depth does for you is it lessens the pressure at the ground. So a 6" slab would impart 1.5 psi to the ground vs the 3.5 psi on the 4" slab. (But, you don't need this.)

Pavements where vehicles are traveling at speed (like roads) and slabs that suppport machinery and equipment have different concerns and rules. For a slab you are just parking vehicles on - even heavy ones - you don't need to worry. You would just be throwing your money away for extra thickness.

 

[Lassen Forge]

I thought 4000 was the standard for concrete foundation slabs. My contractor wants to use 3500psi for my 3000 sq ft steel building. He says with the footings it's not necessary to have 4000 psi and it will just crack more. I can't remember if he said it will crack more or dry too fast. It didn't make much sense. I'm a little irritated because when I called I asked for a quote for a 6" slab then when he came out to look at the lot to see how much dirt I would need to bring in he said he quoted me a 4" slab over the phone $9/ square foot. Then he says it would be $12 for a 6" slab. Opinions? I'm sure 3500 would be fine, but I mean what happened to doing what the customer wants? This is a common theme I see with contractors. I guess he's right honestly. I'm sure just about everything is 4" parking lots etc.

Depends on whats going into your shop. For a "normal" parking slot, then 3500 should be fine. If you're putting in machinery (such as a lathe or shaper) where you have both a smaller "footprint" to cover the weight, you may want to go higher.

Regardless, if YOU specced the concrete to 4000 psi for WHATEVER reason, thats what they should put down there. I had a contractor who decided he knew better than I and decided what I needed or wanted didn't matter, that he knew what I wanted (and why)) better than I did. Most of it was so he could cut corners and make more $$$ (while billing us full boat) or because he wanted it to be built his way regardless of what we wanted, from putting a (factory second) sheet metal bathtub when we specced (and paid for) CI, Decided that replacing the overhead light in the shower was too much of a hassle, so it became "unsafe" (even though there had been one there since the place was built) and then refused to tile the ceiling of the tub enclosure as "You can't do that" (um, it was like that before, so why not?), rerouted heating ductwork because "You want the heat to come out -here- (under the vanity near the door) instead of there (the far end of the room where it would heat the entire room)", the **** went on and on and on with this guy. He called his business "custom changes", mainly because when we caught him going off plan, he would charge us for a "change order" to build it the way we'd specced...

Bottom line, YOU'RE the one paying the bill. If you specced 6500 lb concrete with lavender pigment and a water moat along the edge to control oil spills, then unless there is a code violation or a valid STRUCTURAL reason not to (and has been discussed), and YOU'RE paying for it, then they do what YOU want, not the other way around. Like I told the guy numerous times (before we finally fired his incompetent ***) "When YOU'RE paying for it in YOUR building, you decide, until then, you're being hired as LABOR and do it as the plans state."

 

[Leaflessshadetree]

I agree with everyone above. As far as the 4" to 6" cost they all charge "per yard". Customers expect labor will be less because the labor to finish is the same. Reality the time spent floating and trowling isn't much.

 

[Hobby_Man22]

Do you plan to install a lift?

Yes. The contractor did say he would pour a 6" 3x3 footer where I plan to install the lift.

 

[Hobby_Man22]

I must have 3500psi in my current building then. I thought the smoothness of the finish was based off of how well the contractor levels it all out. He'll in my current building I can feel the highs and low spots by just walking around.

 

[topp64]

It may depend on location, some areas maybe 3500 psi is common, but 4000 psi has been the standard around here for 30+ years in flatwork and walls, probably 20 years in footings. I did hear about a small time home builder a couple of years ago that used 3500 psi for everything. The last time I ordered a 3500 psi mix, except for footings, was in the 80’s. More cement, in 4000 psi vs. 3500 psi, should result in a mix that is more workable for the finishers and make it easier to leave a smooth floor. With that being said, I think the 3500 psi mix back in the 80’s finished better than the typical 4000 psi floor mix we use now. The reason for that is because modern mix designs are substituting fly ash and ground granulated blast furnace slag for up to 40% of the Portland cement used in the mix, along with water reducer and other admixtures, and reaching the desired strength.

 

[dcg9381]

Looks like less than 10% price swing on the materials side, everything else the same. (figure out your yards of concrete, add about 8%).
I'm sure he'll do what you want, he just may charge more for it - more than actual material swing, but you can push on that.

If you want to know if you'll need it, you'll need an engineered foundation... Otherwise, you're just guessing based on what the rest of us have done...

 

[ConCretin]

I agree with everything wssix99 said in his previous posts regarding concrete strength and thickness. The only thing I'd add is that higher strength concrete has more cement, which does provide a stronger, more durable surface. It also provides a little more margin for error if bleed water becomes an issue. These are a few of the reasons I generally spec 4000 for exterior, working or parking surfaces. Would you ever notice the difference from 3500 psi mix? Probably not. It's relatively cheap insurance but not a hill worth dying on.

 

[Professor Fate]

This doesn't matter so much! The pressure they put on the slab equals the tire pressure. So, if you have trucks with tires filled to 55psi, then that's the pressure they are putting on the slab. (The tires deform to a contact patch that spreads out a vehicles load and matches the tire pressure.) That pressure decreaces with the square of the depth, so 55psi tires would impart around 55/(4*4) = 3.5ish psi to the ground.

^ These numbers are nothing for the (3500 psi) concrete. They aren't anything for 3000 psi concrete either. The only thing additional depth does for you is it lessens the pressure at the ground. So a 6" slab would impart 1.5 psi to the ground vs the 3.5 psi on the 4" slab. (But, you don't need this.)

Pavements where vehicles are traveling at speed (like roads) and slabs that suppport machinery and equipment have different concerns and rules. For a slab you are just parking vehicles on - even heavy ones - you don't need to worry. You would just be throwing your money away for extra thickness.

This could not be any more wrong. Sorry.

 

[bigdav160]

This could not be any more wrong. Sorry.

Is that why it hurts when I get run over by a car?

 

[Renegade1LI]

Here's a good guide to go by for minimum requirements by NYC DOT

TABLE 1904.3 MINIMUM SPECIFIED COMPRESSIVE STRENGTH (f 'c)

TYPE OR LOCATION OF CONCRETE CONSTRUCTION	MINIMUM SPECIFIED COMPRESSIVE STRENGTH (f 'c at 28 days, psi)
Basement walls, foundation walls, exterior walls and other vertical concrete surfaces, basement slabs, interior slabs on grade except for garage floor slabs, and foundations not exposed to the weather	3,000
Driveways, curbs, walks, patios, porches, carport slabs, steps and other flatwork exposed to the weather, and garage floor slabs	3,500
Structural concrete exposed to weather	4,500

 

[brownbagg]

the standard is 3000, most house slabs are 2500. now understand, what ever you order the finishers going wet it up and you will end up with a 1800 and it will crack every where

you can only do concrete once so order what you want, if it was me 6 inches of 4000 at a 4 inch slump, maybe some super p

 

[jrsavoie]

What are you getting for reinforcement and base?

That is as important as the concrete mix.

 

[ozyborn]

My driveway is 5000 psi, no fly ash, 3/4" rebar and 5" thick. 8 inch's wide along the long edges and back is 12" thick. That is what I wanted. Any contractor that tried to change it in the quote never got called back. Been great for 12 years now. No cracks with a very good sub base.