Minimum thickness for concrete floor slab?

[SecondMonkey]

Originally Posted By: Amkeer
Sounds like you do not intend on doing the job properly. It is better to do nothing.

That was exactly the purpose of this thread. I know how to do it properly: Plenty of gravel, rebar, a truck to come pour 6" and a bunch of mexicans to smooth it out.

My point being, I want to do it cheaper, and I don't care if it lasts very long. Even a 1/2" thick, cracked up peice of concrete would be better than the nasty gravel that is there now. I've been trying to rinse it but the water pressure is so low it's taking forever...

I have decided for now to rake up all the gravel (not very deep) and put plastic there, replace the gravel, and add another pickup load or two of new gravel. I still want to work on the concrete thing, cheaply, and "one peice at a time".

 

[Amkeer]

No you were told how to do it properly and you came back with another reason or excuse to do it IMPROPERLY.

1. You do not need rebar. It is not going to carry walls or load bearing columns. 4 inch thick with wire mesh or fibermesh.

2. You do not need gravel.

3. This sets you up for failure before you even start....
"My point being, I want to do it cheaper, and I don't care if it lasts very long. Even a 1/2" thick, cracked up peice of concrete would"......

Do it correct the first time or do not do it at all!

 

[Audi Junkie]

He said he was looking for alternatives to a full procedure.

 

[Boatowner]

I have done a little concrete work, which means I have done some wrong. One mistake was no plastic under the slab. It not only seals moisture from coming up, it also seals the water from getting sucked out of the setting concrete. The weight of the concrete presses the plastic into the dirt or gravel just fine. It is heavy stuff.

Concrete is a mixture of portland cement (named based on its color which is similar to the limestone of the island of Portland in the English Channel), sand, gravel and water. Concrete does not need to dry, it needs to set. Concrete continues to get harder for years as it sets. Concrete setting is a chemical reaction and it gives off heat as the water, cement and sand form a complex of crystalization. Too much water weakens concrete. Too little water weakens concrete. Drying too fast weakens concrete, and can cause cracks, especially when the concrete is too wet.

For your application you will find 4" of concrete will be fine, with or without rebar. 4" of concrete is poured with 2 x 4 forms. This means your thickness is actually 3 1/2" and this works fine. I would use a 2 x 2 or 3 x3 grid of # 3 or # 4 rebar. Sometimes you can get # 4 cheaper than # 3.

Since you are pouring inside, you should not need to cover with plastic to slow drying, but you may want to wet it a few times to make sure it doesn't dry too fast to set well. If you are going to cover with plastic, don't do it until the concrete is hard, a few hours after you finish.

I would not cover the concrete with straw unless you want straw marks on your concrete.

Get proper tools. A bull float is necessary, as is a power trowel. Brooming doesn't cover flaws. The best job is troweled to perfection, then broomed. However, you are talking about a shop floor, so I would trowel it to the consistency of a sheet of ice for easy sweeping.

Although too much water weakens concrete, you still need enough water in it to make it workable for amateurs. Let the driver advise you on this.

Use redi-mix in the big truck. Overall, it is the cheapest was to go for any large size pour. I have mixed with cement, sand and gravel in a gas powered mixer, but that was for a series of small jobs (1 yard or less), and I have lots of kids to haul the materials to the mixer.

I calculate 7.1 yards for your job. Order 7.5 yards and have some waste. It is a lot cheaper than ordering 1/2 yard to finish the job off. You will probably end of with 1/2 yard left over, so you might want to figure out what you want to do with the extra rather than sending it back to make ecology blocks.

For the best job, pay an experienced person to help you. That is what I do when I want it done right.

Those 12 x 12 blocks make a lot of sense for your application too.

 

[crinkles]

Originally Posted By: Boatowner
I have done a little concrete work, which means I have done some wrong. One mistake was no plastic under the slab. It not only seals moisture from coming up, it also seals the water from getting sucked out of the setting concrete. The weight of the concrete presses the plastic into the dirt or gravel just fine. It is heavy stuff.

Concrete is a mixture of portland cement (named based on its color which is similar to the limestone of the island of Portland in the English Channel), sand, gravel and water. Concrete does not need to dry, it needs to set. Concrete continues to get harder for years as it sets. Concrete setting is a chemical reaction and it gives off heat as the water, cement and sand form a complex of crystalization. Too much water weakens concrete. Too little water weakens concrete. Drying too fast weakens concrete, and can cause cracks, especially when the concrete is too wet.

For your application you will find 4" of concrete will be fine, with or without rebar. 4" of concrete is poured with 2 x 4 forms. This means your thickness is actually 3 1/2" and this works fine. I would use a 2 x 2 or 3 x3 grid of # 3 or # 4 rebar. Sometimes you can get # 4 cheaper than # 3.

Since you are pouring inside, you should not need to cover with plastic to slow drying, but you may want to wet it a few times to make sure it doesn't dry too fast to set well. If you are going to cover with plastic, don't do it until the concrete is hard, a few hours after you finish.

I would not cover the concrete with straw unless you want straw marks on your concrete.

Get proper tools. A bull float is necessary, as is a power trowel. Brooming doesn't cover flaws. The best job is troweled to perfection, then broomed. However, you are talking about a shop floor, so I would trowel it to the consistency of a sheet of ice for easy sweeping.

Although too much water weakens concrete, you still need enough water in it to make it workable for amateurs. Let the driver advise you on this.

Use redi-mix in the big truck. Overall, it is the cheapest was to go for any large size pour. I have mixed with cement, sand and gravel in a gas powered mixer, but that was for a series of small jobs (1 yard or less), and I have lots of kids to haul the materials to the mixer.

I calculate 7.1 yards for your job. Order 7.5 yards and have some waste. It is a lot cheaper than ordering 1/2 yard to finish the job off. You will probably end of with 1/2 yard left over, so you might want to figure out what you want to do with the extra rather than sending it back to make ecology blocks.

For the best job, pay an experienced person to help you. That is what I do when I want it done right.

Those 12 x 12 blocks make a lot of sense for your application too.


good advice. it may be more expensive but this is the better way. i would suggest that some form of rebar is used (the least expensive) to ensure no cracking. that way the serviceability of the floor would be maintained for many years.

 

[Amkeer]

No rebar is necessary for a 4 inch thick slab. You actually could create issues by using it in such a thin slab.

To prevent cracking you use the least amount of water. Usually the concrete company will set the slump, and do not add any more water. You then cut saw cuts every 10 feet or so as soon as the concrete is cured enough to stand on. Any later, then you have waited to long. The cuts in the concrete are to control where you want the concrete to shrink and crack.

 

[SecondMonkey]

I just noticed there's a bunch of people giving away broken up sections of sidewalk and such on craigslist. I might pick up some of that as an alternative to the brick paver idea. Pre cracked, so no worries!

Another thought: Asphalt. How much do they generally charge for a small job of that?


Originally Posted By: Amkeer
No you were told how to do it properly and you came back with another reason or excuse to do it IMPROPERLY.

You seem to be missing the overall point here. I don't care if it's done PROPERLY. It's not my building, I'm not going to stay here forever. I don't like giving things to my landlord.

 

[crinkles]

Originally Posted By: SecondMonkey
I don't care if it's done PROPERLY. It's not my building, I'm not going to stay here forever. I don't like giving things to my landlord.


i missed that part sorry. if i were you i would do nothing, at the most a very thin layer of gravel strewn over it may help a bit. don't add value to your landlord's asset wihtout being reimbursed for it.

 

[Amkeer]

No, I did not miss that point. I have witnessed many problems and safety hazards with people that make excuses for not doing things properly. If you are going to take the time to do something, do it the proper way. That is all.....

 

[Win]

Your landlord may or may not look at the concrete as a gift.

As a property owner, the last thing I want on one of my properties is a surprise "improvement" that has to be removed or brought up to code after the tenant is gone.

Concrete slabs are somewhat expensive to bust up and remove, so if I were left with a surprise slab of unknown quality, I might be tracking down the responsible party (ies) to recover the cost of removing it.

OTOH, for a good tenant wanting to make a worthwhile and durable improvement to the property, I would cover some or all of the cost to do the job right, and just build it into the rent.

Four inches will probably be fine, and you don't need (or want) rebar. I poured a four inch slab for my little shop building, and it wasn't very expensive. Even if your LL is a cheap jerk, he/she may help you with the cost to do it right

 

[SecondMonkey]

Originally Posted By: Amkeer
I have witnessed many problems and safety hazards with people that make excuses for not doing things properly.

Oh, it's a SAFETY issue now? LOL! My point has obviously gone way over your head.

 

[SecondMonkey]

Originally Posted By: Win
Your landlord may or may not look at the concrete as a gift.

As a property owner, the last thing I want on one of my properties is a surprise "improvement" that has to be removed or brought up to code after the tenant is gone.

Concrete slabs are somewhat expensive to bust up and remove, so if I were left with a surprise slab of unknown quality, I might be tracking down the responsible party (ies) to recover the cost of removing it.

OTOH, for a good tenant wanting to make a worthwhile and durable improvement to the property, I would cover some or all of the cost to do the job right, and just build it into the rent.

Four inches will probably be fine, and you don't need (or want) rebar. I poured a four inch slab for my little shop building, and it wasn't very expensive. Even if your LL is a cheap jerk, he/she may help you with the cost to do it right


My landlord is one of the most clueless people I have ever met. She knows nothing about her industrial building, she inherited it. In my last shop I brought her in and asked if I could repaint the wall on one side because there is a hose hookup there, and I was worried about possible water damage to the drywall. When I said that she got this confused look on her face and said "what do you mean water damage?". I said You know, 'cause there's a hose there, and she goes "OH! That's an OUTSIDE hose! I didn't know that was there".

I had to turn my instant laugh into a cough so as not to offend her.

When I moved into this other place it was disgusting, especially the gravel in question. Former occupant was doing meth back there, so far I've found several little plastic bags and a broken meth pipe. Plus it's dirty and stinky, like an old cow barn, even though it's never housed animals. (except the meth addict that lived thre for a few months) My point being, he left the place a total mess, and she didn't seem to care. Didn't even bother getting rid of his garbage before trying to rent it back out. She's also perfectly happy knowing almost nothing is up to code, and letting me fix it up however I want/need.

My main concern for covering, and especially SEALING off the gravel is that I don't want my dog to find any meth, or broken glass, or old rusty nails, etc, that are scattered throughout. I will ask her permission first, but if I just said "I'm gonna dump an inch or two of concrete in here", she'd probably say ok.

Like I said above, for now I'm just going to wash the gravel as best I can and lay down some plastic, and put new gravel on top. I still want concrete eventually though...

 

[SecondMonkey]

Also, as a side note about doing the job CORRECTLY...the "foundation" on this building is a mound of concrete somebody hand formed around the outside maybe 4" thick, with a 2x4 on top. Somehow I don't think that's the "recommended" way to do it...

 

[MNgopher]

If you want to do it cheap, then clean gravel or clearance pavers. 1" thick of gravel means you need about 2 yards - 4 reasonable loads in a 1/2 ton truck, 2 if you don't like your truck very much. 1" thick is very hard to spread effectively and keep in good shape - I'd do more if it were me.

If you insist on concrete, others have told you what it will take. Less could be a hazard down the road - not an improvement, and if I were you, I'd clear it with the land lord, in writing, before doing anything. As a landlord, I'd be PO'd when I find out what a cheapie job ends up costing to remove and pursue it court. 4" is standard practice for general, non structural flat slab work.

If it were me, 4" thick 4000 psi concrete with rebar (my preference), wire mesh, or fiberglass (also worked very well for me on garage slabs). Plastic underneath if the building is heated space. Your local ready mix operator can get you the correct mix for your needs.

 

[Audi Junkie]

Maybe put down some lime on the old gravel and then hose it in a few days.

 

[Amkeer]

Originally Posted By: SecondMonkey
Originally Posted By: Amkeer
I have witnessed many problems and safety hazards with people that make excuses for not doing things properly.

Oh, it's a SAFETY issue now? LOL! My point has obviously gone way over your head.


Second,

Your point has not gone over my head as you suggest. I keep coming back to make sure "others" that may be considering doing such a thing would do the job correctly, as has been recommended here.

 

[tom slick]

I'd use rebar because the ground isn't being prepped/compacted and we don't know the soil type. rebar will keep the numerous cracks from opening wider. Rebar in anything bigger than sidewalk is standard around here.