Educate me on garage slabs, please

[XS29L9B]

I’ve heard a bunch of threads about garage slabs, there’s a bunch of information about concrete thickness and also fiber reinforced concrete and also the rebar and mesh. Trying to make heads and tails of it all.

My present garage, the slab is about 26 x 34 wide, I seem to recall rebar, but I don’t remember the separation between each bar. It’s at least 4 inches thick, but I seem to recall it may be being slightly thicker. I honestly don’t remember. Anyways, I have a few four post lifts, and the garage floor has been fine - meaning there’s no sinkhole or cave-in yet.

Building the new garage, the slab will be at least 36 wide by 44 deep, or maybe 56 feet deep, still undecided. It’s whatever the budget will allow.

The question is, how thick should the slab be. The builder is saying that it’s structural concrete and would have rebar on a 4 inch thick slab. Is that enough? Seems like there’s gonna be a lot of weight on that concrete

What does fiber reinforcement do? And is there a psi or some other rating that I should mention to the builder? Thank you in advance, so far, this is all Greek to me

 

[Don1357]

Trust your cement guy, he knows more about cement that you'll be able to google in a few hours. Tell him how many cars and lifts you plan on having there.

Having said that go to the website of your lift manufacturer and pull the slab requirements for the lift, give this to your cement guy. Why? Because just like you should trust your cement guy, your cement guy should trust the lift manufacturer guy.

 

[like2wheel]

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

 

[XS29L9B]

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

Long read, but interesting.

Still not sure on what fiber reinforcement really means, other than it works like rebar/mesh, but does not "up" the strength, overall (If I read right).

 

[ConCretin]

Long read, but interesting.

Still not sure on what fiber reinforcement really means, other than it works like rebar/mesh, but does not "up" the strength, overall (If I read right).

First off, neither steel reinforcing or fiber makes a slab 'stronger' if you are defining it as resistance to cracking under load. The average slab isn't thick enough to span very far over soft ground and depends on uniform support from the base to carry loads.

Secondly, fiber 'reinforcement' isn't really reinforcing at all. The fiberglass strands in the concrete are intended to reduce and/or delay what are called plastic shrinkage cracks. It provides minimal value once the concrete has cracked.

Keep in mind that almost all concrete has cracks even if they are hidden in the bottom of control joints.

Steel reinforcing can actually increase the likelihood of shrinkage cracks but it will hold cracks resulting from shrinkage or overloading together once they occur thereby maintaining the integrity of the slab.

Two different products that perform two different functions. If money is no object, you could use both or conversely it's not absolutely necessary to use either.


Btw, I used to have a '68 Dodge Coronet R/T with a 440 and a 4 speed. I miss that car!

 

[XS29L9B]

First off, neither steel reinforcing or fiber makes a slab 'stronger' if you are defining it as resistance to cracking under load. The average slab isn't thick enough to span very far over soft ground and depends on uniform support from the base to carry loads.

Secondly, fiber 'reinforcement' isn't really reinforcing at all. The fiberglass strands in the concrete are intended to reduce and/or delay what are called plastic shrinkage cracks. It provides minimal value once the concrete has cracked.

Keep in mind that almost all concrete has cracks even if they are hidden in the bottom of control joints.

Steel reinforcing can actually increase the likelihood of shrinkage cracks but it will hold cracks resulting from shrinkage or overloading together once they occur thereby maintaining the integrity of the slab.

Two different products that perform two different functions. If money is no object, you could use both or conversely it's not absolutely necessary to use either.


Btw, I used to have a '68 Dodge Coronet R/T with a 440 and a 4 speed. I miss that car!

Thanks. The thing I am looking for, is how thick should the concrete slab be poured, to carry the load of the 4-post lifts.

Let's say the garage is 36' x 44' and the earth beneath was properly compacted. The builder's specs are 4" thick Concrete Slab, reinforced with #4 Rebar 12" o/c EW on Mil poly on fully compacted fill, sloped 2" towards overhead doors.



(This, compared to basement slab which is 4" conc slab reinforced w/ 6x6 10/10 woven wire mesh on 6 mil poly)


Coronets are sweet, and similar to the Chargers, overall. I've had the pleasure and good fortune to own four of them so far.

 

[ConCretin]

Thanks. The thing I am looking for, is how thick should the concrete slab be poured, to carry the load of the 4-post lifts.

Let's say the garage is 36' x 44' and the earth beneath was properly compacted. The builder's specs are 4" thick Concrete Slab, reinforced with #4 Rebar 12" o/c EW on Mil poly on fully compacted fill, sloped 2" towards overhead doors.

(This, compared to basement slab which is 4" conc slab reinforced w/ 6x6 10/10 woven wire mesh on 6 mil poly)

Most lift manufacturers seem to require a min slab thickness of 4" but the safest thing to do is check the literature of the one you are most likely to buy. After that I'd add and inch to the desired thickness just to be safe. The elevations of the base and concrete will vary a bit and if you happen to be a 1/4" high on the base and an 1/4" low with the concrete......

The other option a lot of guys do is to thicken the concrete a few inch inches under the lift posts if you know where they will be.

Objectively, #4 bars at 12" centers is overkill. It's also true that many, including myself have succumbed to overkill. Other than cost, there's no downside but the reality is that welded wire fabric or even spacing the bars out 2x will provide more than adequate crack restraint.

I'm not sure which was your slab will pitch but even if it's the short way, that's only a 1/16" of an inch per foot. This will help move water but you'll still have a lot of puddles. It takes several times that to actually drain water reliably. Personally, I'd rather have a flat floor.

The only other advice I'd offer is to consider a 15 mil vapor barrier if you think you'll ever epoxy your floor. Poly is fine for general moisture control but a 15 mil product will help ensure your expensive new floor covering stays down.

 

[matt_i]

Spec as "4 inch minimum"

I like a crushed stone base, 3/4" nominal, washed, #57 is one regional term. Plate compact in 2 directions.

6 mil vapor barrier is OK but a 10 mil will actually stand up to people that have to walk on it.

Very important to either wet-cure or apply curing-sealer to the concrete so you get good strength.

Very important to cut the control joints the next morning after the pour.

If you are using rebar I would set them on chairs or dobies and wire-tie. If using mesh panels you will probably struggle to get enough support so that someone can walk on it. Typical solution is to "hook into place" which is extremely variable and might as well be sitting on the bottom, but it does have some affect even there.
Are you just pouring a floor or do you have footings to also be poured?

 

[XS29L9B]

Footings to be poured, and the concrete walls, too.

Good point on the thickness of plastic. Will ask for thicker. Thanks,

 

[dcg9381]

>The question is, how thick should the slab be. The builder is saying that it’s structural concrete and would have rebar on a 4 inch thick slab. Is that enough? Seems like there’s gonna be a lot of weight on that concrete

Here, to answer that question, we get our slabs "engineered". Then the concrete guys pour the slab per engineers spec. Often a post-install inspection is involved. It's pretty reasonable, around $1000 for an "engineered" slab.

Anything else is perhaps subjective. I have a shop slab, it's 4" thick, but has 2' footings all the way around and two across spanning across. It has held a 5th wheel and 1-ton dually, about 20,000 lbs empty. Part of why I didn't engineer it is that I know that the strata under the slab is very solid.... The home built on the same property - I had engineered.

 

[XS29L9B]

>The question is, how thick should the slab be. The builder is saying that it’s structural concrete and would have rebar on a 4 inch thick slab. Is that enough? Seems like there’s gonna be a lot of weight on that concrete

Here, to answer that question, we get our slabs "engineered". Then the concrete guys pour the slab per engineers spec. Often a post-install inspection is involved. It's pretty reasonable, around $1000 for an "engineered" slab.

Anything else is perhaps subjective. I have a shop slab, it's 4" thick, but has 2' footings all the way around and two across spanning across. It has held a 5th wheel and 1-ton dually, about 20,000 lbs empty. Part of why I didn't engineer it is that I know that the strata under the slab is very solid.... The home built on the same property - I had engineered.

So you had it "engineered" to a weight or a spec? Please explain a bit more, thank you

 

[Firebrick43]

First, everything LLwillysfan says is spot on. I will add that 12” of compacted 3/4 minus clean crushed stone is a sure fire way of ensure a solid sub base and preventing cracking. Properly compacted means going over 3 or 4 times with a walkhind plate compactor in 4” lifts or at least twice with a larger vibrating roller in no more than 6” lifts.

Many contractors will state and try to say properly compacted is running their skid steer over a few times and that is BS.

As far as an engineered slab goes it something typically done in locals with unstable soils. The include things like grade beams that will support the slab of the ground underneath settles and leaves a pocket under the slab.

As far as thickness, a 4” slab that is wet cured for a week and has 4000+psi concrete mix will be stronger than a 5” that is allowed to just cure in hot summer sun. 5+” is something usually only needed in shops with very heavy equipment such as fully loaded semis, 500 hp stiegers or 10k forklifts

 

[jeffyhog]

Also looking for all the information I can on slab construction, as I'm just a couple weeks away from starting my 34x58 shop construction. The OPs avatar photo caught my eye and then I noticed your username. I used to have a XS29L8B many years ago. It's the one I wish I still had.

 

[ConCretin]

So you had it "engineered" to a weight or a spec? Please explain a bit more, thank you

There really isn't a whole lot of engineering that goes into a 4 or 5 inch slab on grade since they aren't structural. This type of slab depends almost entirely on the ground below for support. Adding concrete thickness increases the slabs ability to span localized 'soft spots' but no reasonable amount of concrete will make up for an inadequate base or span any kind of distance. Slab thickness requirements for lifts are more about anchorage and uplift concerns than load bearing.

The soil pressures exerted by a slab on grade even when loaded with heavy equipment are actually quite low for most soils. As long as the base provides uniform support, the slab will perform just fine.

Slab quality issues are far more likely to involve shrinkage and surface defects than structural failure. I'd make sure I had a good base, figure on a 4 or 5 inch slab and focus my attention on these things.

 

[matt_i]

Just another data point. I poured my slab at 6" thick with the idea that I was going to use forklifts carrying heavy machinery. If you think of the forklift as a counterbalance (see-saw) the front tires bear more than 2X the weight of the machine being carried. I planned for machines around 8000 lbs, so the front tires load can easily be 16,000 lbs split between the two. Any road vehicle short of a loaded-up medium-duty truck is going to have a tough time equalling an 8000 lb concentrated load on a pneumatic tire. Fitted as duals-Load Range E tires are ~2500 lbs per tire, and I don't think the rear axle of a dually is good for a lot more than 5000 lbs fully loaded.

I did spend a lot of time with a jumping jack compacting the areas where I previously excavated and then backfilled inside the foundation. I also setup a 6" thick washed stone base that's leveled as best I could....using "coarse" setting on the rotary laser level...its tough to use "fine" as its more sensitive than 0 or 1 more stone. I plate compacted the stone base in 2 directions.

So far no issues.

 

[XS29L9B]

First, everything LLwillysfan says is spot on. I will add that 12” of compacted 3/4 minus clean crushed stone is a sure fire way of ensure a solid sub base and preventing cracking. Properly compacted means going over 3 or 4 times with a walkhind plate compactor in 4” lifts or at least twice with a larger vibrating roller in no more than 6” lifts.

Many contractors will state and try to say properly compacted is running their skid steer over a few times and that is BS.

As far as an engineered slab goes it something typically done in locals with unstable soils. The include things like grade beams that will support the slab of the ground underneath settles and leaves a pocket under the slab.

As far as thickness, a 4” slab that is wet cured for a week and has 4000+psi concrete mix will be stronger than a 5” that is allowed to just cure in hot summer sun. 5+” is something usually only needed in shops with very heavy equipment such as fully loaded semis, 500 hp stiegers or 10k forklifts

OK, so it's the "MIX" and not the thickness?

I know concrete is expensive, so trying not to overrun costs on stuff unnecessary.

So a 44x44' shop with 4 post lifts, how best to floor with the slab?

Understand on the plate compactors, and will definitely demand that be done. Thanks!!

Other tips appreciated.

Thanks again

 

[XS29L9B]

Also looking for all the information I can on slab construction, as I'm just a couple weeks away from starting my 34x58 shop construction. The OPs avatar photo caught my eye and then I noticed your username. I used to have a XS29L8B many years ago. It's the one I wish I still had.

I've had four of these cars, most of them XS29L9B cars. You have good taste, clearly

 

[XS29L9B]

Just another data point. I poured my slab at 6" thick with the idea that I was going to use forklifts carrying heavy machinery. If you think of the forklift as a counterbalance (see-saw) the front tires bear more than 2X the weight of the machine being carried. I planned for machines around 8000 lbs, so the front tires load can easily be 16,000 lbs split between the two. Any road vehicle short of a loaded-up medium-duty truck is going to have a tough time equalling an 8000 lb concentrated load on a pneumatic tire. Fitted as duals-Load Range E tires are ~2500 lbs per tire, and I don't think the rear axle of a dually is good for a lot more than 5000 lbs fully loaded.

I did spend a lot of time with a jumping jack compacting the areas where I previously excavated and then backfilled inside the foundation. I also setup a 6" thick washed stone base that's leveled as best I could....using "coarse" setting on the rotary laser level...its tough to use "fine" as its more sensitive than 0 or 1 more stone. I plate compacted the stone base in 2 directions.

So far no issues.

Good to hear on no issues

No industrial use, or even a light forklift. Just vehicles, heaviest ones, maybe 10k lbs. But for parking, and light work.

 

[WNYflyer]

OK, so it's the "MIX" and not the thickness?

I know concrete is expensive, so trying not to overrun costs on stuff unnecessary.

For structural plain (structurally unreinforced) concrete design which a majority of slab-on-grade (S.O.G) are designed as, with all things being equal the slab bending strength typically varies with the square root of the concrete strength and the square of the slab thickness.

Thus square root of 3000 psi = 55 psi

square root of 4000 psi = 63 psi.............63/55 = 1.15 therefore 15% bending strength increase.


4" thickness............... 4^2 = 16

5" thickness................ 5^2 = 25 .............25/16 = 1.56 = 56% bending strength increase.

I am assuming any spread sheets, computer programs, charts used for the design of structurally plain concrete slab-on-grade/pavements would reflect similar changes is bending strength for concrete mix strength and thickness variables.

 

[Firebrick43]

For structural plain (structurally unreinforced) concrete design which a majority of slab-on-grade (S.O.G) are designed as, with all things being equal the slab bending strength typically varies with the square root of the concrete strength and the square of the slab thickness.

Thus square root of 3000 psi = 55 psi

square root of 4000 psi = 63 psi.............63/55 = 1.15 therefore 15% bending strength increase.


4" thickness............... 4^2 = 16

5" thickness................ 5^2 = 25 .............25/16 = 1.56 = 56% bending strength increase.

I am assuming any spread sheets, computer programs, charts used for the design of structurally plain concrete slab-on-grade/pavements would reflect similar changes is bending strength for concrete mix strength and thickness variables.

As Wnyflyer post stated Thickness exponentially increases strength. However a properly done 4” slab can hold tremendous weight already.

The problem is most concrete men are lazy/ignorant or the customer won’t pay.
3500 psi concrete is “if” it’s cured correctly. Most concrete will end up significantly less strength than 3500. Number one issue, is concrete must be kept moist for at least a week or hydration stops and when it does the strength is where it’s at for good. Curing sealer helps but it’s best not only to keep the top wet but the heavy plastic underneath keeps the soil from pulling the moisture out. Also if the top of the slab to dry but the slab bottom keeps shrinking the slab will curl and then crack to relieve the stress.

An example of perfect curing is ICF walls. The foam form protects against moisture evaporation. A cured 6” icf wall will be as strong or stronger than a formed 8” wall simply because most forms are stripped after 3 days and never kept moist after that.

Second, most concrete men want soupy mix as it spreads easy. 4” slump is ideal but it’s actually pretty damn hard to move. 5” to 6” slumps is moveable but many concrete men will go much more than this and it reduces strength and the aggregate will also settle out making the mix inconsistent. You order and pay for the concrete, allow no more than 6” slump and get a slump cone and be there with it. Watch for the crocheted men wanting the driver to add more water and stop them. Better yet get some 4” pvc by 8” long with a cap glued on. Get some concrete in that at the beginning of the pour. Even if you don’t have it tested everyone involved will realize you want the concrete to meet specs and if they have any brains will ensure it does.

Adding some extra cement typically just makes sure you get to the 3500 psi if other factors effect the pour.

Luckily the things that effect the concrete the most the homeowner has/can have the most effect on. It’s relatively easy to place the sub base and compact, as the equipment is a one day rental and cheap. Place the gravel evenly and pack it 3 or 4 time before putting 4” down again.

The home owner can place a heavy plastic barrier such as steggowrap making sure it goes up the wall to decouple and also lay rebar/wire with enough chairs(my slabs have no reinforcing except some 1/2 hair pins in the corner).

And the homeowner can spray down the slab twice a day(or more in heat) for at least 7 days.

My 28x50 and 28x32 slabs have no visible cracks. As they strink, they pull away more than a 1/4 to 3/8” from the walls all around. If they were restrained by uneven sub grade, uneven thickness, or post/walls coupling them the will crack.

 

[alia176]

As Wnyflyer post stated Thickness exponentially increases strength. However a properly done 4” slab can hold tremendous weight already.

The problem is most concrete men are lazy/ignorant or the customer won’t pay.
3500 psi concrete is “if” it’s cured correctly. Most concrete will end up significantly less strength than 3500. Number one issue, is concrete must be kept moist for at least a week or hydration stops and when it does the strength is where it’s at for good. Curing sealer helps but it’s best not only to keep the top wet but the heavy plastic underneath keeps the soil from pulling the moisture out. Also if the top of the slab to dry but the slab bottom keeps shrinking the slab will curl and then crack to relieve the stress.

An example of perfect curing is ICF walls. The foam form protects against moisture evaporation. A cured 6” icf wall will be as strong or stronger than a formed 8” wall simply because most forms are stripped after 3 days and never kept moist after that.

Second, most concrete men want soupy mix as it spreads easy. 4” slump is ideal but it’s actually pretty damn hard to move. 5” to 6” slumps is moveable but many concrete men will go much more than this and it reduces strength and the aggregate will also settle out making the mix inconsistent. You order and pay for the concrete, allow no more than 6” slump and get a slump cone and be there with it. Watch for the crocheted men wanting the driver to add more water and stop them. Better yet get some 4” pvc by 8” long with a cap glued on. Get some concrete in that at the beginning of the pour. Even if you don’t have it tested everyone involved will realize you want the concrete to meet specs and if they have any brains will ensure it does.

Adding some extra cement typically just makes sure you get to the 3500 psi if other factors effect the pour.

Luckily the things that effect the concrete the most the homeowner has/can have the most effect on. It’s relatively easy to place the sub base and compact, as the equipment is a one day rental and cheap. Place the gravel evenly and pack it 3 or 4 time before putting 4” down again.

The home owner can place a heavy plastic barrier such as steggowrap making sure it goes up the wall to decouple and also lay rebar/wire with enough chairs(my slabs have no reinforcing except some 1/2 hair pins in the corner).

And the homeowner can spray down the slab twice a day(or more in heat) for at least 7 days.

My 28x50 and 28x32 slabs have no visible cracks. As they strink, they pull away more than a 1/4 to 3/8” from the walls all around. If they were restrained by uneven sub grade, uneven thickness, or post/walls coupling them the will crack.

Thank you for the detailed explanation, really helped visualize the process. I'll be having a contractor make me a 10' x 20' pad outside for a 12k 2 post hoist.

I had no idea how to do conduct a slump test so I turned to YT:

another one

Other than looking at the ticket from the cement truck driver, I don't see my concrete guy having a slump test cone like the one in the video. So, what is my recourse to make sure that I'm getting what I paid for?

According to challenger, a 12k 2 post lift requires the following:

Minimum requirements for concrete are 4 inches
minimum depth, with steel reinforcement, 3500
psi, cured for 28 days per local commercial
practice. Floor should be level within 3/8 inch over
the installation area. No anchors should be installed
within 8 inches of any crack, edge, or expansion
joint. If these conditions cannot be met, a pad may
be poured to accommodate the lift.

Should I ask for 3500 psi concrete mixed with fiber and put down a 5" vs 4" pad? I'll ask for a wire bed that comes in a roll vs rebar. Cost shouldn't be much more, I'd think.

I spoke to the gent at the lift company who removed the 12k lift from a Chevy dealer today and he told me go with 4" 3500psi, fiber mixed on a roll of steel. He said 5" would be fine as well. He also advised that I NOT sink the anchors while the concrete is wet, even if I have the perfect template. He said this is risky because the columns may not be perfectly level or the bridge may not connect to the columns up top correctly or squarely.

Another thing that the gent told me is that I shouldn't be making the concrete thicker under the columns from the rest of the pad. He said this creates a stress point. He said an even thickness pad is better. I'll defer to this gent's experience and wisdom as I'm not a structural engineer!

Thank you for any inputs.

 

[ConCretin]

Other than looking at the ticket from the cement truck driver, I don't see my concrete guy having a slump test cone like the one in the video. So, what is my recourse to make sure that I'm getting what I paid for?

According to challenger, a 12k 2 post lift requires the following:

Minimum requirements for concrete are 4 inches
minimum depth, with steel reinforcement, 3500
psi, cured for 28 days per local commercial
practice. Floor should be level within 3/8 inch over
the installation area. No anchors should be installed
within 8 inches of any crack, edge, or expansion
joint. If these conditions cannot be met, a pad may
be poured to accommodate the lift.

Should I ask for 3500 psi concrete mixed with fiber and put down a 5" vs 4" pad? I'll ask for a wire bed that comes in a roll vs rebar. Cost shouldn't be much more, I'd think.

You won't be able to determine slump from the batch ticket unless you have the mix design and know how to interpret the information. The batch slip will tell you how much was added to the truck at the plant. The mix design will tell you how much is allowed. If water is added on site, that must be added in. Kinda complicated for those who don't do it everyday.

I'm going to suggest you ask the plant for a mid range water reducer. This is a chemical admixture that increases the flowability of the concrete without adding water. You should be able to get a 5-6 inch slump with less water than a traditional mix with a 4" slump. A 6" slump should be fine for any decent finisher. It's not a perfect solution but it b buys you a margin for error.

A couple additional thoughts. I wouldn't be worried at all about a thickened area under the lift. It's done every day on thousands of jobs. Just create a gentle slope where the concrete gets thicker and there won't be any stress risers or whatever this expert told you.

If you opt not to thicken just the areas under the lift, I think I already suggested adding an inch of depth to the minimum allowable thickness. So if Mohawk wants at least 4", I'd go with 5". It wouldn't be shocking if the base and the finished concrete varied by a 1/4" each. If you got unlucky, you could end up with 3 1/2" of concrete. That would be bad.

Finally, I'd buy sheets of wire mesh instead of rolls. They will be heavier wire and lay much flatter. The rolled stuff is junk. Make sure you support the mesh at mid slab using bolster, chairs, dobies or something similar. I'd skip the fiber. It's not much value on a slab of this size.

 

[alia176]

You won't be able to determine slump from the batch ticket unless you have the mix design and know how to interpret the information. The batch slip will tell you how much was added to the truck at the plant. The mix design will tell you how much is allowed. If water is added on site, that must be added in. Kinda complicated for those who don't do it everyday.

I'm going to suggest you ask the plant for a mid range water reducer. This is a chemical admixture that increases the flowability of the concrete without adding water. You should be able to get a 5-6 inch slump with less water than a traditional mix with a 4" slump. A 6" slump should be fine for any decent finisher. It's not a perfect solution but it b buys you a margin for error.

A couple additional thoughts. I wouldn't be worried at all about a thickened area under the lift. It's done every day on thousands of jobs. Just create a gentle slope where the concrete gets thicker and there won't be any stress risers or whatever this expert told you.

If you opt not to thicken just the areas under the lift, I think I already suggested adding an inch of depth to the minimum allowable thickness. So if Mohawk wants at least 4", I'd go with 5". It wouldn't be shocking if the base and the finished concrete varied by a 1/4" each. If you got unlucky, you could end up with 3 1/2" of concrete. That would be bad.

Finally, I'd buy sheets of wire mesh instead of rolls. They will be heavier wire and lay much flatter. The rolled stuff is junk. Make sure you support the mesh at mid slab using bolster, chairs, dobies or something similar. I'd skip the fiber. It's not much value on a slab of this size.

Thank you VERY MUCH for the education and hand holding. This is beginning to sink in, all be it slowly.

I'll need to find out which cement plant the contractor uses then, somehow, I need to find out which truck load will contain my truckload. I don't know the feasibility of this but I'll dig into it.

Just to clarify, a 5" or 6" slump is the vert distance from the top of the metal slump creating cone height to the top of the wet cement height after the metal cone is removed, correct?

This stuff is fascinating!

 

[ConCretin]

I'll need to find out which cement plant the contractor uses then, somehow, I need to find out which truck load will contain my truckload. I don't know the feasibility of this but I'll dig into it.

Just to clarify, a 5" or 6" slump is the vert distance from the top of the metal slump creating cone height to the top of the wet cement height after the metal cone is removed, correct?

Your concrete will come out on it's very own truck so no worries there.

You have it right on measuring slump. A tapered cone is filled with concrete and rodded in several lifts and then slowly pulled off. The cone is flipped upside down, the rod is placed on the inverted cone and a tape measure is used to determine how much the concrete 'slumped'.

While often used as a measure of workability, slump is really a measure of mix water. Projected compressive strength is generally based on the amount of water that results in a 4" slump. A slump higher than that is an indication of excessive mix water.

 

[Ozz316]

I'm going to throw my 2 cents in. I honestly wouldn't worry about buying a slump cone and disrupting the pour. I personally believe the slump test to be antiquated because it really tells you nothing in today's concrete world. If you are concernced about the quality of the concrete the water cement (w/c) ratio is much more important. You may have to ask for the batch weights on the ticket from the concrete supplier, but that number will be on there. Most residential floors are poured around a .48-.50 wc ratio. Most suppliers already use Mid Range in their concrete. If you are pouring on plastic, ask for a lower w/c ratio and more Mid Range, or pay the extra for High Range, or Super P. This will allow the contractor to place the mix at a lower wc ratio, around a .45 without any ill effects on the concrete. Also, if pouring on plastic, it is recommended to put a base of sand on top to help with the bleed water. This is rarely done, because it is much harder to place the concrete without disturbing the sand. At minimum, make sure all the bleed water is off the surface before they start troweling it. If bleed water is still present, they will just be working it into the surface, essentially raising the wc ratio on the top and could cause delamination. I just read a post where the guy posted pictures of the slab with the trowel machine on it, and there was obvious bleed water still present. Then a few posts later, he was asking why he had spots that had peeled and popped. For mixes, I think most people over cement their mixes. I am a fan of an approximate 5 1/2 sack mix with fly ash. Fly ash helps the mix be more workable with keeping the wc ratio low. In addition it gives higher strengths after fully cured than a straight cement mix. Adding too much cement to a mix causes more paste, and paste is where plastic cracking occurs. Use a good fiber mesh and plastic shrinkage cracks shouldn't be a problem. I like it better than wire mesh, since contractors tend to just step on it and not pull it up into the concrete and then it is useless. I am also not a fan of homeowners wet curing their concrete. Wet curing involves constantly keeping the top wet, either by constant sprinklers or wet burlap that is never allowed to dry out for at least 7 days, if not 28. Constant wet and drying of the concrete is just as bad, if not worse than not curing it at all I good high solids Acryilc Cure will do very well if applied properly and can be tinted too make sure adequate application. Most are a cure and seal so you can go back in 28 days and seal the concrete again with it. DO NOT buy a straight sealer and immediately apply it. Another option is a water based cure that after 28 days, a penetrating sealer can be used without having to peel the topical acrylic off. If your going to epoxy, use the water based cure also. As far as a lift is concerned I would actually isolate out the area where the posts are going to be sitting and pour that later. That way the static load differences won't cause a problem. They make products that you can isolate it while pouring the whole thing, PM me and I'll get you the information. Other wise, just box off the area and pour it later. Hope this helps.

 

[alia176]

Could you please elaborate on this statement please? What is the purpose of separating the area under the lift columns and pour it later? Will this be the same thickness as the rest of the pour?

As far as a lift is concerned I would actually isolate out the area where the posts are going to be sitting and pour that later. That way the static load differences won't cause a problem. They make products that you can isolate it while pouring the whole thing, PM me and I'll get you the information. Other wise, just box off the area and pour it later. Hope this helps.

 

[Ozz316]

You can pour it a separate thickness, but essentially you want to take the constant load off the pad in those areas. With a lift there is a constant pressure that is different than the rest of the floor. As the concrete expands and moves the pressure differences can cause an issue. We have a contractor here that sells iso piers that are made just for this purpose, to separate out the concrete.

 

[skulldrinker]

Worried about 4" or 6"? Not much difference in price to go 6" at that point.

Just watch them with t h e rebar or wiremesh whichever you go with that they actually lift up on it as they pour so that it lays in the center of the thickness.

I cant tell you how many laborers forget about doing that when t b e boss is yelling them.

Drove a concrete mixer for over 20 years. Ive seen it all.

Sent from my SM-N960U using Tapatalk

 

[alia176]

Thank you for the explanations gents. The concrete contractor will be at the house with their "concrete specialist" so I'll talk to them about the things discussed here.

Topics to discuss:
- Iso piers or pouring the column to be poured later, not sure how much later. The concept of separate pour vs pouring at once is something new to me.
- sheets of iron mesh is preferred over rolls
- pull up on the iron mesh during the pouring
-5" or 6" thick pour all over vs the standard 4" that Challenger suggests
-slump test won't be discussed
- Anything else?


According to challenger, a 12k 2 post lift requires the following:

Minimum requirements for concrete are 4 inches
minimum depth, with steel reinforcement, 3500
psi, cured for 28 days per local commercial
practice. Floor should be level within 3/8 inch over
the installation area. No anchors should be installed
within 8 inches of any crack, edge, or expansion
joint. If these conditions cannot be met, a pad may
be poured to accommodate the lift.

 

[GMCGarage]

I’ve heard a bunch of threads about garage slabs, there’s a bunch of information about concrete thickness and also fiber reinforced concrete and also the rebar and mesh. Trying to make heads and tails of it all.

My present garage, the slab is about 26 x 34 wide, I seem to recall rebar, but I don’t remember the separation between each bar. It’s at least 4 inches thick, but I seem to recall it may be being slightly thicker. I honestly don’t remember. Anyways, I have a few four post lifts, and the garage floor has been fine - meaning there’s no sinkhole or cave-in yet.

Building the new garage, the slab will be at least 36 wide by 44 deep, or maybe 56 feet deep, still undecided. It’s whatever the budget will allow.

The question is, how thick should the slab be. The builder is saying that it’s structural concrete and would have rebar on a 4 inch thick slab. Is that enough? Seems like there’s gonna be a lot of weight on that concrete

What does fiber reinforcement do? And is there a psi or some other rating that I should mention to the builder? Thank you in advance, so far, this is all Greek to me

A slab is only as good as the sub base. Spend the time on that, and you can go with no reinforcement.