Consensus on concrete slab specs?

[GarageGuy]

Hello,

We're in the Chicagoland area and we were wondering if there's any consensus in the garage community as to what the best/strongest/longest-lasting/least-cracking concrete would be for a garage floor (new construction).

We've hired a reputable, well-researched garage company but they sub out the concrete (as pretty much all of them do). Our garage includes the basic concrete floor that everyone gets (I believe 4 inches with wire mesh). We asked about thicker concrete, adding fiber mesh for strength, etc. and were told anything beyond the basic floor (additional walks, fiber mesh, thicker slab, etc.) is between us and the concrete contractor when he shows up.

Does anyone have any recommendations as to what we should ask for - psi, how many bags, fiber mesh, etc. and also how much extra should that "ideal" concrete cost over the basic, included concrete slab? Thanks so much for any help/advice you folks can provide.

 

[like2wheel]

https://www.garagejournal.com/forum/showthread.php?t=382626


Unrelated:
How is it possible that your screen name was still available on this Garage specific website 250,000 members in??

Mind boggling...

 

[mrobins297aaa]

that was my first thought also.......lol

I don't know if this is true or not but adding the fiber may give you some problems with finishing the floor with some type of coating at a later date, maybe it's true and maybe it's not but be something worth checking out.

 

[ConCretin]

You can give my Guide to Floor Slabs a read. You can access it below. Achieving the best/strongest/longest-lasting/least-cracking concrete is the result of many factors from preparation of the soils below to curing the concrete after it is done. The concrete itself is the easy part. Good luck with your project.

Edit - Like2wheel beat me to it.

 

[RPH]

Adding fiber has been known to stick up like bristles in a brush. The fix was to use a torch to burn them off. There only fiberglass.

 

[GarageGuy]

Thanks all for the replies. LLWillysfan, wow, that's a very comprehensive guide you've got there. Maybe too comprehensive for me (j/k). It sure would be nice if there were *less* options and variables, and someone could just say, "Here's an extra $500, give me the deluxe floor please." Looks like I've got some reading to do!

And I was as surprised as you guys that the screen name was available. It was just the first thing that popped into my head when I was registering, and I was fully prepared to be GarageGuyXYZ123 or whatever, lol.

 

[Armorpoxy]

Totally correct. Adding fibers to concrete adds a challenge to get rid of them if you want to coat the floor as the epoxy creates a 'hair gel' effect. Burning them off is really one of the only options but using a landscaping torch inside and along the walls is a risky thing! Keep fire extinguishers handy if you ever do this!

 

[Kaizen]

I stepped up to 4k psi and six inch slab. If you have the money insulate and put in pex floor heat. It was not in my budget. I got fiber only and steel around the thickened edge. Site compaction is very very important. I doubt they will do as good a job tamping the base. I would learn about base fill and tell the builder what you want. Talk to concrete guy ASAP. Never wait for the day he shows up. That’s dumb. Plus he has to order better mix if you want it. Love my floor. I cut joints myself. Use zip strips if in budget. I did epoxy do ground floor and no glass sticking up after. You know garage dimensions so go use a concrete calculator and see difference in yards. Call concrete company and get prices for 4K vs 3k. That will give you rough idea of price. This time of year you will pay for no freeze additives already


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[NUTTSGT]

Unrelated:
How is it possible that your screen name was still available on this Garage specific website 250,000 members in??

Mind boggling...

And I was as surprised as you guys that the screen name was available. It was just the first thing that popped into my head when I was registering, and I was fully prepared to be GarageGuyXYZ123 or whatever, lol.

Actually there's about 20 variants of garageguy in some form, I was curious so I checked the active members. I didn't check "banned" side though.


Back on topic, it really boils down to two things, what are your doing in the garage and what can you afford ?

 

[dcg9381]

I'm in the south, so my climate may be different.
Around here, it's relatively easy to find the guy that IS doing your concrete work, not a GC subbing it out to someone else.
In terms of surface cracking - that seems to be - at least to a large degree - a function of letting the concrete dry at a reasonable rate and keeping it wet. But your climate may be different.

I've found that laying concrete foundations are like building a wood frame. It's not that you're just straight pouring 4" of concrete over a (well packed) surface - my shop had a significant "beam" around the edges and beams spanning across the garage. Just like any steel or wood structure. Having huge beams doesn't mean **** if they put concrete down when it's 95 degrees out and don't cool it.

The structural design for the concrete will depend on what you're going to put on it. If you really want to get serious about it, you dig (or drill) core samples of your soil, submit those to a geo-tech company and call for a slab engineer. This is likely under $2000 total.

You end up with a stamped drawing for concrete / foundation design that will specify rebar, beams, concrete psi, thickness.

Or you can just put that money back into materials.

Whomever is telling you that you can just switch from a 4" slab to a 5 or 6" slab at pour time is wrong. These things are framed to height and the contractors order a specific amount of cement.

Here's a less scientific answer - in my area (very hard ground) - a 4" slab over packed roadbase with a wire mesh got the job done for a simple 1000 sqft garage that contained "normal" cars/motorcycles/tools. Never had a problem. I think it was 3000 or 3500 psi concrete.

Building a new shop that might hold 18k RV - added a lot of rebar, still 4" min depth, but with 2' beams on a 40x60 slab.

I'd find some concrete contractors and pick their brains on how they'd do it.
In some parts of the country, you need to run piers down... It's going to vary quite a bit.

 

[GarageGuy]

Thanks all for the advice/input. The way it was explained is that the concrete crew will come the first day to remove soil, prepare the ground, set up forms, etc. This is when you can discuss extra walkways, a different mix, fibers, custom requests beyond what's included, etc. Then the second day, after working out any special requests the day before beyond what's included, they'll come back for the pour.

I'm sad to say it but we just don't have the time/luxury to become concrete experts and talk to different contractors. We're sort of under the gun in that we need the garage by winter and it's getting late in the season for concrete. So we're gonna go with "what's included" and hope for the best. We've looked at multiple garages done by this contractor and spoken with his past customers (unbeknownst to him - they were not references) and everyone's been very happy and would do it again without hesitation.

I'll come back in a few weeks and let you guys know how it turned out. Thanks again for the help.

 

[Toomanytools?]

Don't know if this is same area as you but thought it was funny.
http://www.chicagolandconcrete.com/what-is-5-bag-mix-6-bag-mix-3000-psi-3500-psi-4000-psi-ect

Without knowing what your intended shop/garage is used for, say parking a heavy RV, putting in a 2 post lift, it's hard to say what "extras" are needed.
The fact that he is doing wire mesh is a plus, 4" slab is more that adequate for most peoples needs. Sounds like this contractor does good work from the past customers reviews so he most likely will choose 3500 psi for interior work (common). You could add fiber mesh it helps hold surface cracks to a minimum (still can happen) makes for a good slab, cost is about $10 per yard.
I think it's going to be fine, if you are there when they pour watch to see if they add more water to the truck mix. A squirt or two to get the mix moving in the chute is one thing but a lot is not good. Good Luck.