New concrete not as flat as I expected...
- Thread starter Vintage Veloce
- Start date
Well I sit corrected. I finally got a chance to review the code criteria and as it turns out it was an adopted standard for this particular municipality. Also I was 1/16" shy, the actual was 1/4" in ten ft and to boot was a title 15 ADA compliance standard. Sooooo my bad
Anyhow, any luck with something from the Contactor OP?
^ ^ ^ Still a great thing that might come in handy.
New concrete pours could set "SPEC" at the ADA one listed above so that "grandpa" can come over and roll around garage in wheelchair !!
OP . . . hope you get satisfaction by having the contractor pay for grind down and re-sealing the concrete.
New concrete pours could set "SPEC" at the ADA one listed above so that "grandpa" can come over and roll around garage in wheelchair !!
OP . . . hope you get satisfaction by having the contractor pay for grind down and re-sealing the concrete.
OP
Vintage Veloce
Well-known member
I thought I'd follow up with some of what I've learned...
On the high end, you can have "super flat" concrete installed, they do this particularly for warehouses that have high lifts and robots that run in the isles. However, this requires laser guided machines than smooth and flatten the concrete. I don't think many of us are going to do this for our relatively small backyard garages or shops.
There are some standards for flatness and levelness and these are measured in FF (flatness) and FL (levelness) numbers. And these are not easy things to measure. The sell special gizmos to do the measuring. Essentially, measuring with a long straight edge isn't really acceptable, because each person who measures will measure differently. The other obvious problem with using a straightedge is that what you really want to do is hold that edge level and then measure below it, you don't want to just lay it on the floor. Companies make cool gizmos to actually measure FF and FL, and they are pretty sophisticated (see http://www.dipstick.com/).
I called a bunch of concrete guys. Essentially, none were willing to commit to a standard like "less than an 1/8" dip below 6' straight edge anywhere on the floor". A couple grinding and polishing companies sounded interested in the job and said "oh we can definitely make it flat", but as soon as I mentioned the straightedge they lost interest. No one had a plan or a method to make the floor flat. No one said anything about flooding the slab and marking high spots, or anything like that. No one owned an electronic dipstick to measure flatness. I suspect this stuff just doesn't happen in standard residential construction.
I did find one company that seemed pretty competent at flattening concrete floors and their info mentioned they do "slab correction". Guy does lots of work including warehouses that use high lifts that really need flat floors. Essentially, he said my floor was "normal" for work these days (sadly). He could make it better and wasn't wildly expensive, he'd charge just somewhat more than a regular floor polishing for the extra time and effort. But he questioned making an effort to get it really within 1/8".
I found this chart online:
(source: http://www.concreteconstruction.net/how-to/floor-flatness-number-conversion_o)
"Although there are no direct equivalents between F-numbers and straightedge tolerances, ACI 302, "Construction of Concrete Floors and Slabs," gives the following table of approximate values:
F-number Gap under an unleveled 10-foot straightedge
(fraction of an inch)
FF12 1/2
FF20 5/16
FF25 1/4
FF32 3/16
FF50 1/8
Apparently typical concrete floors are in the FF20 to FF25 range. I did find this statement: "Although "1/8th inch in ten feet" has been used to specify billions of square feet of concrete, it was seldom, if ever, achieved. The typical industrial floor, for example, is closer to a 5/8th inch deep envelope, rather than a 1/8th inch deep envelope."
(source: http://www.faceco.com/docs/40 Questions June 2012.pdf)
A big issue is that typical concrete guys never check their work. How many visit the site 30 days after the pour and measure the surface with a proper dipstick style gauge according to the standard? So essentially, your residential concrete guys do not really know how good or bad their work comes out!
I'm a geek, so my initial post came I went out and tried a 6' straight edge all around the floor in a grid pattern. I really tried about 40 placements. Only one area was out of whack. Mine may not be that great, but I also suspect very few people have garages that would really meet the FF50 (1/8") standard.
On contracts. I've been around the track a couple times and I understand these things. It sounds great to have everything specified in a contract and to have it be enforceable. In my case, I didn't know enough to ask the details about flatness, let alone specify something. But at least around here, it wouldn't be easy to even find a residential contractor with a dipstick gauge who would commit to an FF number. The other problem is that the residential finisher is usually a subcontractor who makes a modest wage. Even the head contractor , unless he is a big outfit, isn't in a financial position to "replace your foundation" if it doesn't work out. Certainly, the finisher, who is maybe making $250 to $1000 is going to disappear before buying you a new slab. I personally think you are just far better off trying to understand what you can get, checking references, and looking at similar work than expecting the contract to make it "all right". In my case, yes, I wish the floor was better in that one spot, but I can live with it and I'm not going to make others pay for something I didn't understand well enough to demand better.
So, in conclusion...
I'm no expert on concrete, but having done this once, and read some others advice; if someone where to ask me, here are my recommendations:
- You aren't going to get a "perfectly flat" floor, especially with high PSI concrete poured on a warm dry day.
- It seems harder to make a floor with a curb (stem wall) around the edge flat (especially with just one pour) than a slab without a curb, as the finisher cannot use the top of the form as a guide when there is a curb. If you are going to have a curb/stem wall, perhaps look into doing the floor first as a separate pour.
- Talk with the actual finisher, not your general contractor, about what he can do. Make sure he has a wide bull float and will run it both ways, North-South and East-West. Is there enough room to run the float both ways or are other buildings in the way? Go see similar work he has done and if it matters bring a dipstick or at least a straight edge. Does he own a dipstick? Has he ever returned to a job after it has cured and tested the floor's flatness?
- During the job, have him show you it is flat, while it is wet. Maybe they can go out with skis and show you a straight edge on the surface.
- Lastly, If you can get it close to1/8" gap under a 10 foot straight edge, you are doing amazingly well. 1/4" is probably more reasonable.
Like I said in the beginning, I can live with my floor. But I wish I knew a bit more beforehand.
I hope maybe someone else can learn from my experience.
C
On the high end, you can have "super flat" concrete installed, they do this particularly for warehouses that have high lifts and robots that run in the isles. However, this requires laser guided machines than smooth and flatten the concrete. I don't think many of us are going to do this for our relatively small backyard garages or shops.
There are some standards for flatness and levelness and these are measured in FF (flatness) and FL (levelness) numbers. And these are not easy things to measure. The sell special gizmos to do the measuring. Essentially, measuring with a long straight edge isn't really acceptable, because each person who measures will measure differently. The other obvious problem with using a straightedge is that what you really want to do is hold that edge level and then measure below it, you don't want to just lay it on the floor. Companies make cool gizmos to actually measure FF and FL, and they are pretty sophisticated (see http://www.dipstick.com/).
I called a bunch of concrete guys. Essentially, none were willing to commit to a standard like "less than an 1/8" dip below 6' straight edge anywhere on the floor". A couple grinding and polishing companies sounded interested in the job and said "oh we can definitely make it flat", but as soon as I mentioned the straightedge they lost interest. No one had a plan or a method to make the floor flat. No one said anything about flooding the slab and marking high spots, or anything like that. No one owned an electronic dipstick to measure flatness. I suspect this stuff just doesn't happen in standard residential construction.
I did find one company that seemed pretty competent at flattening concrete floors and their info mentioned they do "slab correction". Guy does lots of work including warehouses that use high lifts that really need flat floors. Essentially, he said my floor was "normal" for work these days (sadly). He could make it better and wasn't wildly expensive, he'd charge just somewhat more than a regular floor polishing for the extra time and effort. But he questioned making an effort to get it really within 1/8".
I found this chart online:
(source: http://www.concreteconstruction.net/how-to/floor-flatness-number-conversion_o)
"Although there are no direct equivalents between F-numbers and straightedge tolerances, ACI 302, "Construction of Concrete Floors and Slabs," gives the following table of approximate values:
F-number Gap under an unleveled 10-foot straightedge
(fraction of an inch)
FF12 1/2
FF20 5/16
FF25 1/4
FF32 3/16
FF50 1/8
Apparently typical concrete floors are in the FF20 to FF25 range. I did find this statement: "Although "1/8th inch in ten feet" has been used to specify billions of square feet of concrete, it was seldom, if ever, achieved. The typical industrial floor, for example, is closer to a 5/8th inch deep envelope, rather than a 1/8th inch deep envelope."
(source: http://www.faceco.com/docs/40 Questions June 2012.pdf)
A big issue is that typical concrete guys never check their work. How many visit the site 30 days after the pour and measure the surface with a proper dipstick style gauge according to the standard? So essentially, your residential concrete guys do not really know how good or bad their work comes out!
I'm a geek, so my initial post came I went out and tried a 6' straight edge all around the floor in a grid pattern. I really tried about 40 placements. Only one area was out of whack. Mine may not be that great, but I also suspect very few people have garages that would really meet the FF50 (1/8") standard.
On contracts. I've been around the track a couple times and I understand these things. It sounds great to have everything specified in a contract and to have it be enforceable. In my case, I didn't know enough to ask the details about flatness, let alone specify something. But at least around here, it wouldn't be easy to even find a residential contractor with a dipstick gauge who would commit to an FF number. The other problem is that the residential finisher is usually a subcontractor who makes a modest wage. Even the head contractor , unless he is a big outfit, isn't in a financial position to "replace your foundation" if it doesn't work out. Certainly, the finisher, who is maybe making $250 to $1000 is going to disappear before buying you a new slab. I personally think you are just far better off trying to understand what you can get, checking references, and looking at similar work than expecting the contract to make it "all right". In my case, yes, I wish the floor was better in that one spot, but I can live with it and I'm not going to make others pay for something I didn't understand well enough to demand better.
So, in conclusion...
I'm no expert on concrete, but having done this once, and read some others advice; if someone where to ask me, here are my recommendations:
- You aren't going to get a "perfectly flat" floor, especially with high PSI concrete poured on a warm dry day.
- It seems harder to make a floor with a curb (stem wall) around the edge flat (especially with just one pour) than a slab without a curb, as the finisher cannot use the top of the form as a guide when there is a curb. If you are going to have a curb/stem wall, perhaps look into doing the floor first as a separate pour.
- Talk with the actual finisher, not your general contractor, about what he can do. Make sure he has a wide bull float and will run it both ways, North-South and East-West. Is there enough room to run the float both ways or are other buildings in the way? Go see similar work he has done and if it matters bring a dipstick or at least a straight edge. Does he own a dipstick? Has he ever returned to a job after it has cured and tested the floor's flatness?
- During the job, have him show you it is flat, while it is wet. Maybe they can go out with skis and show you a straight edge on the surface.
- Lastly, If you can get it close to1/8" gap under a 10 foot straight edge, you are doing amazingly well. 1/4" is probably more reasonable.
Like I said in the beginning, I can live with my floor. But I wish I knew a bit more beforehand.
I hope maybe someone else can learn from my experience.
C
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Wow, you did some homework. But something I need to point out. You commented that setting up a screed method on a your type of pour is not true. They could have easily set up screed stakes to compensate for this situation. Anyhow getting that one location ground down is pretty easy as long as you can live with having exposed aggregate in that area.
skulldrinker
Well-known member
If I remember correctly from my driving a ready truck 3500 psi is only a 5 bag mix. 4000 would be a 6 bag mix. 3500 is what they use for sidewalks. Personally I would of went with the 4000#. 1/2 inch does seem like a bad spot.
A 12' bump cutter would of come in handy for those amigos but chances are they would of had better luck knowing what a power trowel with a pan is, and how to start and run one. If you say you can live with it, then that crew has a real future in concrete. By the way, they had enough labor on that job they could have ate the concrete and still been hungry after the last shovel full was gone! Just kidding.
Diesel Dan
Well-known member
How can you use screed (grade) stakes without piercing the vapor barrier?
I'm in talks with a commercial concrete place, even they don't have an 8' float. I might buy one myself so they can use it. A USA made 8' magnesium float can be had for about $200, w/o handle, cheaper than the price of a good air tool for a mechanic and I'd bet that float will last longer. Just have to see about delivery time.
I've been in a relatively new (2002) 400K+sq/ft warehouse where the individual slabs curled so bad the pick vehicles would drag their undercarriages. Those joints were ground down and some had to be mud jacked due to settling. Pretty sad for a green field construction.
BlackTalon
Well-known member
That was a nice write-up, Vintage. It also should be noted each increase in the F# results in a big jump in cost. And many/ most concrete contractors will not be willing to tackle the flatter floor requirements. You really do need a specialized concrete guy, and then you need to convince them to work on your small garage slab vs that 200,000 sf slab for the new warehouse a couple miles away. So that means you need to be willing to pay a lot more then for a typical slab, and you also need to convince those guys to work directly for you vs a GC. Most of the commercial guys do not want the hassle of dealing with residential customers, as those customers are generally not too savvy when it comes to construction items, and are quick to refuse payment.
BlackTalon
Well-known member
Curling happens, unfortunately. What was the consensus on why it was so severe in this case, though? It is pretty normal to need to grind at least some of the slab edges before putting the slab into service in a warehouse environment.
Diesel Dan
Well-known member
It was "fixed" before I was there, so don't know what it was blamed on. They had Sandy soils, low clay to deal with in that area but most of the work was done in the winter I think.
Talked with another fairly large and known residential concrete place today. Was told in no uncertain terms to pour it myself. Looks like I'll be using the commercial place, they mentioned using a laser screed.
Talked with another fairly large and known residential concrete place today. Was told in no uncertain terms to pour it myself. Looks like I'll be using the commercial place, they mentioned using a laser screed.
Diesel Dan
Well-known member
Looks like I'm still looking for a contractor.
The commercial place has changed their quote.
Initially it was $4K labor to pour the slab, no pump truck needed and laser screed. Now I'm required to supply a pump truck($1K), they are using a hand screed, should get "close to" a FF25 spec and "won't be back to rework things that are considered unreasonable."
Maybe my first clue should have been the estimator showing up in a BMW M3 and heels?
The commercial place has changed their quote.
Initially it was $4K labor to pour the slab, no pump truck needed and laser screed. Now I'm required to supply a pump truck($1K), they are using a hand screed, should get "close to" a FF25 spec and "won't be back to rework things that are considered unreasonable."
Maybe my first clue should have been the estimator showing up in a BMW M3 and heels?
OP
Vintage Veloce
Well-known member
Diesel Dan, Sadly, it isn't as easy as some people think. Please keep us informed on your efforts, I'm really curious.
larry4406
Well-known member
Below is the contract excerpt that the company I work for imposes on our concrete contractor:
The grade of the finished surface shall be within 1/480 of level (i.e., one inch in 40', half inch in 20', one-quarter inch in 10'). Under no circumstances can the slab vary from level more than 1/4" in 10' in any direction (except garage). Any slabs found to be exceeding this tolerance shall create extra work for the carpenter and this cost will be back charged to the concrete Contractor.
The garage exception noted is due to our garage slabs having a 3" pitch to let water drain. That said, they still work to this criteria in terms of flatness recognizing it is not level.
Our contractors pound pieces of rebar vertically and periodically (every 10-15') throughout the area to be poured. These are grade pins and checked with a laser level. The slab is then troweled to these grade pins. As the concrete sets up, the pins are hammered down about 2-3" and covered with cream from the pour.
The grade of the finished surface shall be within 1/480 of level (i.e., one inch in 40', half inch in 20', one-quarter inch in 10'). Under no circumstances can the slab vary from level more than 1/4" in 10' in any direction (except garage). Any slabs found to be exceeding this tolerance shall create extra work for the carpenter and this cost will be back charged to the concrete Contractor.
The garage exception noted is due to our garage slabs having a 3" pitch to let water drain. That said, they still work to this criteria in terms of flatness recognizing it is not level.
Our contractors pound pieces of rebar vertically and periodically (every 10-15') throughout the area to be poured. These are grade pins and checked with a laser level. The slab is then troweled to these grade pins. As the concrete sets up, the pins are hammered down about 2-3" and covered with cream from the pour.
VintageVeloce
Active member
- Joined
- Mar 15, 2015
- Messages
- 34
Larry, this discussion is about flatness, not levelness. You can be level and not flat. Is there a flatness clause in your contract?
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Sent from my MotoG3 using Tapatalk
OP
Vintage Veloce
Well-known member
Also your company's existing flatness spec allows +/- 1/4" in 10 feet. That could allow a 1/2 wave or ripple... like the one I have in one spot. I guess that is just more evidence, this floor isn't too far away from normal.
Oh, the rebar idea is a great one to set a grade, I'll be doing that in the driveway!
Oh, the rebar idea is a great one to set a grade, I'll be doing that in the driveway!
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larry4406
Well-known member
The garage exception noted is due to our garage slabs having a 3" pitch to let water drain. That said, they still work to this criteria in terms of flatness recognizing it is not level.
I think the above is pretty clear
I think the above is pretty clear