What PSI concrete for a slab? 2500?

[Onewolf]

We are going to have a 41'x68' detached garage/workshop built here in Central Florida. Because of the slope of the property it will be built with a foundation, stem wall, and slab. The GC we are probably going to use has specified 2500 PSI concrete with fiber mesh for the slab. The other two proposals we got specified either 3000 or 4000 PSI concrete for the slab.

I don't really know what type of concrete should be used for a slab around here. Would 2500 PSI concrete with fiber mesh be recommended for a slab in central florida?

Thanks.

 

[Bob C]

Do you plan to install a car lift? Personally I would never ever use less than 4000 PSI concrete in a new garage build.

 

[JWVan]

First of all is his proposal less than the others? I would not use anything less than the 4000 psi. If his quote is considerable less, then I would ask him to bid the 4000psi. In my area it is minimal savings for the reduction of 1 sack of cement per yard. I am an engineer and have been accused of overbuilding my own home projects.

 

[lynnbilodeau]

I have never used less then 4000 psi even for sidewalk and patio pours.

All the trouble of getting the job together, and then go with weaker concrete to save 100 (or even 300) bucks? I don't think so.

 

[ACDNate]

3000 psi for slabs/footers and 2500 for flatwork is common here.

 

[Backlight]

In Ontario (Canada) they seem to use 32MPa (about 4600psi) for slabs..

 

[tlmartin84]

When I priced my concrete last week it 111 for 4000psi, and 107 for 3000psi. 5$ a yard is worth the extra insurance.

I would never pour a slab under 4000psi.

 

[nehog]

IMHO: 3500 minimum, with rebar. Mesh is not enough.

 

[A**holes Garage]

IMHO: 3500 minimum, with rebar. Mesh is not enough.

yup

 

[ACDNate]

What he said, rebar and wire shouldn't be optional.

 

[KenC]

The concrete strength would be good enough if the subgrade is well done.


A solid base will make or break any slab. Maybe I should say: a weak base will break any slab.

But, 2500 would be really hard to finish well. Just not enough cement to provide a nice smooth, dense surface. DAMHIKT!! But, I'll tell you: I tried to save a buck long time ago, never again!

 

[mrpowderkeg]

You can have a 1000psi slab hold up, if the subsurface is solid. BUT IMO build it right and strong. My shop has a foundation, walls and then the slab. I chose to go with 4000psi with a true 4 inch slab (not just a 3.5 inch like a normal 2x4 would be) and I did excavation, back filling and slab setup, I went overboard on the rebar. I never planned to have a 2 post lift, as for my personal preference is a 4 post lift or maybe someday and inground 2 post.

 

[Trey T]

be careful how the GC advertise his concrete 3000psi rated vs 4000psi tested, bc they can be the same concrete mix. Ask them to provide some info if you're curious about the concrete strength otherwise, just stick w/ the 3000psi concrete that everyone uses.

concrete should have enough steel, within a certain %, for maximum strength; too much will weaken the concrete.

 

[wssix99]

I don't really know what type of concrete should be used for a slab around here. Would 2500 PSI concrete with fiber mesh be recommended for a slab in central florida?

It all depends on what you are going to do with the slab. PSI, reinforcing, fibers, all have their place - but they all have their costs, too.

Are you going to put a lift on the pad? Are you going to finish the floor? And/or what do you want the appearance/finish to be? How far will your control joints be? Will you have any other machinery installed? Is this going to be treated more like a shed and place to park cars, or are you going to do work in there and have it more like a work space? etc.

 

[Onewolf]

It all depends on what you are going to do with the slab. PSI, reinforcing, fibers, all have their place - but they all have their costs, too.

Are you going to put a lift on the pad? Are you going to finish the floor? And/or what do you want the appearance/finish to be? How far will your control joints be? Will you have any other machinery installed? Is this going to be treated more like a shed and place to park cars, or are you going to do work in there and have it more like a work space? etc.

Thanks for all the replies so far. When I meet with the GC on Saturday I will discuss why he chose to spec 2500 PSI concrete for the slab.

No lifts. The garage portion will be used to park an Excursion (7500 lbs) and a 21 ft bay boat (4000 lbs).

I plan to do an epoxy/flake/clear floor finish.

No heavy machinery. Just wood working tools.

 

[wssix99]

I plan to do an epoxy/flake/clear floor finish.

Then, you'll want 4000 psi. The higher strength concrete is not needed here for the "strength" but for the finishing qualities. The higher strength concrete has a higher cement content. (That's what will also make it a little bit more expensive.) I'm told that the higher cement content allows for a smoother surface, which should make sanding down the surface easier for the epoxy to take. (I had mine professionally done and the coating company wouldn't touch the slab unless it was 4000 psi.)

You don't want fibers - they will throw off your epoxy finish.

You can get away without reinforcing, but you'll want to space your control joints closely together to minimize the chance of large cracks forming. If you want to spread those joints out or get additional insurance, you can reinforce the slab with rebar to control cracking.

 

[MoonRise]

In addition to specing the 'right' concrete mix/strength (2500 psi concrete??? wtf! cheap, cheap, cheap), watch out for a GC who does the concrete work and then slops in 'extra' water into the mix so he and his 'crew' can slop the concrete around a little bit faster/easier.

Adding 'extra' water in a batch of concrete weakens it. A lot.

Just an FYI.

hmmm, starting with a batch spec of just 2500 psi concrete and putting in some 'extra' water into the mix and you'll end up with weak, sloppy concrete. If the GC/crew put in that extra water into the mix and work the crete while it is still 'wet' (bleed water on top as they trowel it around), you'll probably end up with an even extra weak, dusty **** layer on the top of the slab. **** material and **** workmanship and you end up with an extra crappy slab.

And as mentioned, the price difference from the ready-mix company is usually something like $10 a yard more for the 'better' concrete.

 

[Fastfish]

Howdy,

Just finished my floor and 2 post above ground lift install. Very happy with all. I stronglyl recommend 4000 psi concrete, 4 inches, haunched to 8 inches under the lift uprights.

 

[Backlight]

Just had my floor poured yesterday, 32MPa with fiber, 6" thick. They will saw cut and seal today. Came out fairly nice, I'll post a photo when complete.

 

[senegalyouw]

I am an engineer and have been accused of overbuilding my own home projects.

 

[Highland doc]

Getting ready to do mine here in central FL too. Mine is 3000 with fiber, rebar in the footers

 

[SeanConklin]

I am a plant manager at a large ready mix company in Indiana. I can't tell you what to do but from my experience, we won't even use 2500psi in a footer. We will use 3000psi. Most of our slabs are poured with either 4000psi (around 550 total cementitious material including cement and ash/slag) or a 6 bag (564lbs of cement).

Fiber is commonly misconceived as reinforcement. Fiber is actually to reduce plastic shrinkage cracks in the surface of your concrete. Proper rebar reinforcement is the way to go.

As far as water goes, ask for the constituent weights for the different mixes you are being quoted. You want a water/cement ratio of around 40% From my experience, a 4" slump is ideal for reducing bleed water and achieving maximum strength. Don't let the contractor add 20-30 gallons of water just because he doesn't want to work the mud.

I apologize for rambling, but if you or anyone has any concrete related questions, let me know, shoot me a PM.

 

[Onewolf]

We had a meeting this morning with the GC and he had no objection to changing the slab to 4000 PSI with fiber mesh and wire mesh.

Thanks for all the advice!

 

[creativecars]

I am a plant manager at a large ready mix company in Indiana. I can't tell you what to do but from my experience, we won't even use 2500psi in a footer. We will use 3000psi. Most of our slabs are poured with either 4000psi (around 550 total cementitious material including cement and ash/slag) or a 6 bag (564lbs of cement).

Fiber is commonly misconceived as reinforcement. Fiber is actually to reduce plastic shrinkage cracks in the surface of your concrete. Proper rebar reinforcement is the way to go.

As far as water goes, ask for the constituent weights for the different mixes you are being quoted. You want a water/cement ratio of around 40% From my experience, a 4" slump is ideal for reducing bleed water and achieving maximum strength. Don't let the contractor add 20-30 gallons of water just because he doesn't want to work the mud.

I apologize for rambling, but if you or anyone has any concrete related questions, let me know, shoot me a PM.

Sean, This is good information and not rambling. And if you want to load up a truck and come my way next weekend that would be great... I'll bring the beer.

 

[wssix99]

with fiber mesh

If you are going to properly prep the slab (ie: grind it) to take the epoxy finish, then you won't want the fiber mesh. It will leave a bunch of little fiber fingers sticking out of the slab when you go to coat it.

If you are just going to do the paint-on stuff, then you probably won't notice, but I hear that stuff doesn't always adhere well.

 

[Onewolf]

If you are going to properly prep the slab (ie: grind it) to take the epoxy finish, then you won't want the fiber mesh. It will leave a bunch of little fiber fingers sticking out of the slab when you go to coat it.

If you are just going to do the paint-on stuff, then you probably won't notice, but I hear that stuff doesn't always adhere well.

When prepping for epoxy floor coating, if I choose to acid wash the slab rather than diamond grind it, will the fiber mesh still be a problem?

Thanks.

 

[KenC]

Seems the common thing to do to fiber mix slabs is 'treat' with a weed burner (BIG propane torch) after curing. Singes the little buggers off!

 

[wssix99]

When prepping for epoxy floor coating, if I choose to acid wash the slab rather than diamond grind it, will the fiber mesh still be a problem?

Thanks.

I would think so. The prep will etch the top of the slab, which would leave you with more visible fibers. As KenC points out, even with a virgin slab, you could still see some fibers sticking out.

I'm not sure what the fiber will be getting you here. If you are going to coat the floor, that coating should fill in any of the fine little cracks that the fibers are meant to control.