| just an observation on this , rebar is actually most effective when as far as possible from top load surface , but there are other issues
Why you see galvanized deck plate with only 2-3” of concrete on it in high rise deck floors .( depends on span) .
think about a wood beam ( or concrete ) with a heavy load tight in the middle of say a 10 foot span . The bottom of the beam is trying to split open upward as the bottom is trying to be stretched out . It is in tension — If You saw a slot in bottom at middle beam will fail . The top is in compression (being squeezed together ) This is also how an open truss beam of steel or wood works too , middle is open air , bottom in tension . also , like old wooden covered bridge So concrete is very strong in compression the PSI number , but also weak in tension . So rebar is at bottom , the concrete is assumed to crack at bottom but steel is very strong in tension ( 40,000 psi) so crack can’t happen all is good . however if too close to bottom , on grade , water / oxygen can get at it and rust it. So 2” off bottom has been sort of the compromise . . I put 4x4 mesh in mine we are good and yes you need slits in a big slab as thermal expansion will crack it . Call it pre cracked where you want it in straight lines . Slab guys know how . a more subtle question is water vapor barrier or not , insulation or not . I opted twice for no water barrier as water will rise up— and then what with a barrier ? sounds like lots of trouble to me . 1000 year old cathedrals have no water barrier .
i Put 2” foam under . but we all know heat rises not fall, it rises . from experience i find that in summer evaporative cooling from the water evap off slab is very effective no ac needed , put vent in roof that can be closed in winter . I add a dehumidifier in summer as i had a hassle with mildew , with or without water barrier . keep it at 50-55% One day mildew gets on rubber , hoses , leather , vinyl etc Really hard to get off in millions of small places on stored cars . The microfiber is a great thing I agree but won’t do much for spreading loads IF it cracks . Mesh will spread it out 4” or more , the fibers are not that long . Use Both is best . $ get into it too 4-5”probably ok,6” used in industrial slabs with mesh Just info … I mean 3000 psi concrete under 4x4 baseplate is 16 sq in ,so 50000 + lbs theoretically , 20 % still 10000 . per leg No problem .. If bottom of slab does not crack fyi jkg
Sent from my iPhone On Dec 2, 2023, at 7:32 PM, 'dave mason' via Chrysler 300 Club International <chrysler-300-club-international@xxxxxxxxxxxxxxxx> wrote:
To tack on to Marshalls comment about floor strength, I’ve been meaning to throw in the following about moving a lift with a car on it.
If you have a lift on casters with a car on it (I.e. John Grady’s point of having an inop car), all the weight which would otherwise be spread out over a larger surface area of footprint from the posts is now on the very much reduced surface area of only where the caster wheels contact the floor. This is a lot of force on a small area. It’s spread out over the multiple wheels but the weight on each wheel is pushing down on small “line” where the curved wheels contact the floor. So is 4” of concrete enough? Not sure what the magic number is but something to consider. Sent from my iPhone On Dec 2, 2023, at 17:35, Nick Taylor <nicksgaragesd@xxxxxxxxx> wrote:
Here is an album on my workshop. If you go to the bottom you'll see the slab work. I have them make it thicker in the area where the two-post lift was going. About 6 inches thick there. I'd have to look up in my records for the strength. I had the floor epoxied and haven't had any cracks. I've had problems with the outside concrete, including having to have a big section replaced.
I put in my new concrete floor and 2-post lift 2 years ago after much research concerning the concrete. My lift brand called for 5.5" of reinforced concrete with 4000# test compression.
What I found out in several conversations with flatwork companies, and also through independent online queries, is that today's recommendations regarding reinforcement and thickness have changed considerably from the old days. First, a number of years ago they came out with microfiber reinforcement. The recommendation then was to put a certain amount of microfiber, leave out the wire mesh, but still use rebar. Then a few years ago they came out with macrofiber reinforcement, and it was a real game changer. Testing showed that macrofiber inclusion in the concrete mix eliminated the need in most cases for either wire mesh or rebar.
The problem with wire mesh had been that even though it is intended to be held up in the pour on "chairs", the truth is that a lot of it would get mashed down to the bottom by the guys laying the pour instead of ending up in the upper third of the thickness where it might do some good. Rebar also has the problem of rusting and busting up the concrete over time if it's used in wet locations. With macrofiber those problems are history as the reinforcement is consistent throughout the mix from bottom to top.
Testing has also shown that macrofiber reinforced concrete can even be laid thinner than wire-mesh and rebarred concrete and yet achieve equal performance characteristics. My recollection is that the thickness reduction can be about 20%.
The bottom line is we put down 7500 square feet at 5" thickness using only macrofiber reinforcement right in the mix. We also cut control slots. This was two years ago and I have yet to see even a single hairline crack anywhere. It troweled out to a fairly slick surface and we hit it with a coat of densifier too to eliminate dusting of the surface.
This new development is fantastic and the flatwork companies are all using it.
Keith Boonstra -
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