Another lumpy/wavy slab question - grinding?

[tinyrobotman]

So I just had a 1000 sq. ft. monopour slab done for a red iron shop building. The building fabricator highly recommended a concrete contractor who could do the pour and finishing. Had done over 50 slabs for them, been in business for 15 years. I did the excavation, prep and forms. 4" of graded and compacted 3/4 clean stone, 2" of 20psi XPS, 15 mil vapor barrier, forms set within 1/8" in elevation with a laser. #4 rebar, 18" OC and 2 bars top and bottom in the footing. Even did CNC'd plywood bolt carriers to make sure the anchor bolts were dead on. The building inspector said it was the most meticulous slab prep he's ever seen.

I met with the concrete contractor and walked the site and checked my prep. He was a bit rough around the edges but a lot of concrete guys are and he assured me that everything looked good and they'd use a laser during screeding it. I emphasised that I needed a high level of flatness for rolling carts, CNC machine etc. Well, despite having a laser setup during the pour I ended up with a slab that has a 1/2" crown in the center (13 ft. from the edge) and has a lot of closely spaced waves and lumps. I understand that slabs aren't going to be ever dead flat but this is a mess. The saw cut guy who came out later even said it didn't look good. The trowel finish is pretty good and tight but the flatness is terrible. On top of it they left a bunch of rough concrete under the bolt carriers I had to chip and grind off with a cup wheel.

I'm considering getting a scarifier and a grinder/polisher to try and even things up after the building is framed. Is that just crazy or should I work on my zen acceptance? I paid the concrete and pump separately so the only refund I could ask for would be from the finisher. I definitely let the building fabricator know that they shouldn't be recommending this guy.

Photo is pretty typical all over the slab so that's a 3/8" dip in only 3 ft using a 9 ft. track saw track as a straight edge.

 

[The Cobbler]

grinding that flat will be an exercise in futility and frustration . is that the worst part of the pad?

 

[tinyrobotman]

grinding that flat will be an exercise in futility and frustration . is that the worst part of the pad?

That's pretty typical. If there was just one spot like that I could deal with it, but it's like that basically all over the pad.

 

[The Cobbler]

Is there any thing with more complaints on Here than poor results with concrete pads ?
It seems to top the list for sure.

 

[tinyrobotman]

Yeah, I agree. Seems to be a common complaint. I think it's one of those things where you have a contractor once out of the blue for a job where there's basically no way to fix it afterwards. Also what's acceptable for a horse barn is very different than for a machine shop.

 

[Innovate1]

I've met very few people who said they did other than great work but their actual work many times tells a different story. And concrete is the worst because it's very difficult to do anything to correct things.

 

[tinyrobotman]

Yep you get references and recommendations but with concrete what's acceptable for one person or use could be very different. As you said with concrete what's done is literally cast in cement after only a few hours. Since most marginal flat work won't be actually ripped out and redone after a pour and the contractor will never be back to the site there's a lot of incentive to cut corners. Almost every other trade mistakes can be caught along the way and remedied or the sub even fired half way through if stuff is really bad.

I think I errored by not learning enough about the finishing process and quizzing them about what equipment and techniques they were using to ensure flatness. I've since learned that a vibra screed rather than a manual screed (faster and flattens the aggregate in the mix), good bullfloat technique (both ways to reduce waves) and using a pizza pan on a trowel machine will help ensure a flat floor. They did none of those things. I should have also put on my rubber boots and went out and double checked the center grade with my own laser during screeding. I also could have asked if they needed me to pull the bolt templates at some point.

Another lesson is ask how many guys they're bringing. They had three plus the pumper for 1000 sq. ft. I think one more guy could have allowed the lead to supervisor closer rather than scrambling. Weather was perfect and trucks were all on time so that wasn't an issue.

That and have a flatness value in the contract. I've since learned that's a thing. I paid for a 25 cu yard, 3500 psi monopour slab, trowel finish without any other specifications in the contract. Evidently that's not enough of a spec to ensure performance.

All a tough lesson to learn after spending so much money, blood and sweat doing the prep.

 

[Innovate1]

On our basement floor pour someone on the crew said they were a man short that day because they had to visit their parole officer. Alrighty then....

 

[tinyrobotman]

chnique (both ways to reduce waves) and using a pizza

On our basement floor pour someone on the crew said they were a man short that day because they had to visit their parole officer. Alrighty then....

Yeah in general concrete in general doesn't attract the highest talent in the trades. Obviously there are some true meticulous craftsman but the general labor pool isn't great. A friend had a patio done and one of the guys was wearing an ankle monitor.

 

[bluedog225]

I put a lot of prep and engineering time into my concrete. Still got somewhat poor results in a couple of respects. And in my whole project, the concrete guys told the most straight up lies.

Sorry OP. I’d still be tempted to give it a go with the grinder and a bunch of patience.

 

[thunderalley3]

We have a concrete guy that has done 20+ pours for us in the last 15 years, mostly driveways but he did all of my patios and walks at my house as well and I have never had a complaint or even been slightly unhappy with his work. He has even left his skidsteer for me to use for a few days and I have let him borrow my jack hammer many times.

With saying that I almost sent him packing on the first job I had him look at. He was referred by a good friend and he gave me the typical rundown of things such as if they show up after lunch they will all have been drinking, they will look rough in the morning but will be there on time and will get things done etc. I can deal with that but the first time we met I asked if he offered any guarantee on his work and he replied "most definitely, I guarantee that it is going to crack and that no one will steal it."

I am glad that I gave him a shot after that remark because he runs a tight ship on his work.

 

[Rusted Nut]

Do you have a written contract with the finisher, specifying a flatness tolerance! If so, you might prevail in court to have it torn out and re done, although doubtful. In all reality, a few bird baths are not that bad to deal with most of the time. Talk to your finisher about coming back and fixing some of the bad areas.

 

[tinyrobotman]

I put a lot of prep and engineering time into my concrete. Still got somewhat poor results in a couple of respects. And in my whole project, the concrete guys told the most straight up lies.

Sorry OP. I’d still be tempted to give it a go with the grinder and a bunch of patience.

I've got a grind and polish guy coming by to take a look. From what I saved doing all the prep myself I've got some room in the budget to fix this if possible.

I just finished stripping the forms and there's a little bit of a horizontal cold joint between the slab pour and the thickened footing right at the very last part of the pour. I don't think it will be an issue since it's 4" down and I hooked the end of my rebar down into the footing at the edge. However it does show that they were running behind when they started finishing. Explains a lot. They needed more guys.

 

[tinyrobotman]

Do you have a written contract with the finisher, specifying a flatness tolerance! If so, you might prevail in court to have it torn out and re done, although doubtful. In all reality, a few bird baths are not that bad to deal with most of the time. Talk to your finisher about coming back and fixing some of the bad areas.

No, something I learned after the fact that you can specify the flatness. One of those thing I assumed they would just do. Like isn't concrete just supposed to be flat. Well evidently not always.

Not an option to tear out. Partly because of the stuff that was put underneath and that I need to get it weathered in before winter. Full bathroom drains stubbed in, XPS foam and PEX radiant tubes. Footing is 18x18 with two #4 bars top and bottom. #4 rebar 18" OC in the entire slab. Thickened interior footings as well. Welcome to seismic zone 3.

 

[gba2331]

I’m guessing a lot of self-leveling concrete isn’t an option? IDK anything about that other than the name

 

[djbmw]

I’m guessing a lot of self-leveling concrete isn’t an option? IDK anything about that other than the name

Self leveling could work for OP but thats a huge expense too as each bag is $45 and you'll need tons of primer as well. Also, self leveling is good to do under a static floor (with a sub floor installed on it, for example). If the floor has live loads (like in an auto shop with things rolling directly on it) then self leveler will usually crack and flake

 

[Rusted Nut]

No, something I learned after the fact that you can specify the flatness. One of those thing I assumed they would just do. Like isn't concrete just supposed to be flat. Well evidently not always.

Without an “F” spec (flatness), generally accepted standard, at least in my part of the country, is 1/8’ in 10’. If your slab is beyond this, then I would meet the concrete finisher and see what they will do. There are several ways of fixing some of this. Or maybe finisher will pay for grinding; although grinding out a 3/8” dip can be a mess. If it’s just a few bad areas, they can be saw cut and bushed down an inch, then patched with a bonding agent and highway patching compound.

 

[tinyrobotman]

Without an “F” spec (flatness), generally accepted standard, at least in my part of the country, is 1/8’ in 10’. If your slab is beyond this, then I would meet the concrete finisher and see what they will do. There are several ways of fixing some of this. Or maybe finisher will pay for grinding; although grinding out a 3/8” dip can be a mess. If it’s just a few bad areas, they can be saw cut and bushed down an inch, then patched with a bonding agent and highway patching compound.

Yeah, there needs to be a public service campaign about "F" specs. I had no idea until after this fiasco.

Well we're definitely way out of 1/8" in 10'. Just the crown in the middle is 1/2" in 13'. These idiots screeded in a slope off a center post base bolt carrier that they I guess just assumed was a reference grade or something. Before the pour I told them specifically to check that point with the laser because that could move up or down (it was tied to the rebar mat) . They had a $2000 rotary laser all setup and I showed them where the reference TOC grade was but I guess just chose to ignore it.

Although actually I'm more concerned with what I'd call waves and lumps. Where it's 1/4" to 1/8" within 3 ft. There's one that's 3/8" in 3 ft. Just using my laser and sampling the ponds there aren't many low spots, more high lumps. compared to the edges. Forms were dead on and didn't move a bit (my work) so the edges are at least level. My thought is I'll have to live with the general rise to the center but maybe could map the high lumps/waves and grind those off a bit before doing a general grind and seal?

I haven't talked to the finished yet other than about the 1/2" crown. He was somewhat dismissive. I'm giving myself some time to calm down before talking to him again because I'm so pissed. Anyway, right now I need to get the building moving again so I stop thinking about this screw up.

 

[ConCretin]

It doesn't do the OP any good but this seems to be a common discussion. You can specify F numbers but for them to be of any value you have to hire a testing company to come out and roll the floor. I don't know what it costs because the concrete contractor generally doesn't pay for testing on our jobs but my guess is that's not cheap.

For those who don't know Ff is a way of measuring the waviness of a floor, which is the result of finishing operations while Fl is a measurement of the overall levelness of the floor which is determined by formwork, screeding, etc. It requires specialized equipment and computer programming to determine F values.

Trying to get a contractor who's willing to sign your contract with F numbers is going to limit your options and although there is some value in letting the contractor know you will be checking his work, what do you do if it fails. Sure you have a contract and can sue but unless the contractor has assets, it's probably futile. At the end of the day, most of us are at the mercy of the skills and dedication of the guy finishing the floor and sometimes all the due diligence in the world isn't enough.

I'll just add that it's is incredibly difficult to repair a slab that failed it's Ff and or Fl or even what the OP is facing. You can just run a grinder over the whole floor because it will just follow the existing profile. You have to survey the floor in a grid pattern, identify the highest areas and spot grind them. You repeat this process over and over as the areas requiring grinding get bigger each time, constantly checking progress. It's doable but brutally time consuming.

OP, if you can live with what you have it's probably best to move on. You probably will stop noticing the waviness as much as you start using your new shop. If you can't live with it due to your equipment or operations, grinding is an option.

 

[Sumboodie]

Have a multilillion $$ building for our warehouse at work. The ~40ft of foor trench drain is the highest point of the floor.
To the point that an office 60ft away will flood if I park the truck with a bunch of snow under it.
It's lumpy to where over 5mph in the forklift will have me almost smashing the old twig and berries under myself. I've spot used to high center till it wore down.

 

[tinyrobotman]

It doesn't do the OP any good but this seems to be a common discussion. You can specify F numbers but for them to be of any value you have to hire a testing company to come out and roll the floor. I don't know what it costs because the concrete contractor generally doesn't pay for testing on our jobs but my guess is that's not cheap.

For those who don't know Ff is a way of measuring the waviness of a floor, which is the result of finishing operations while Fl is a measurement of the overall levelness of the floor which is determined by formwork, screeding, etc. It requires specialized equipment and computer programming to determine F values.

Trying to get a contractor who's willing to sign your contract with F numbers is going to limit your options and although there is some value in letting the contractor know you will be checking his work, what do you do if it fails. Sure you have a contract and can sue but unless the contractor has assets, it's probably futile. At the end of the day, most of us are at the mercy of the skills and dedication of the guy finishing the floor and sometimes all the due diligence in the world isn't enough.

I'll just add that it's is incredibly difficult to repair a slab that failed it's Ff and or Fl or even what the OP is facing. You can just run a grinder over the whole floor because it will just follow the existing profile. You have to survey the floor in a grid pattern, identify the highest areas and spot grind them. You repeat this process over and over as the areas requiring grinding get bigger each time, constantly checking progress. It's doable but brutally time consuming.

OP, if you can live with what you have it's probably best to move on. You probably will stop noticing the waviness as much as you start using your new shop. If you can't live with it due to your equipment or operations, grinding is an option.

I was thinking that someone should invent a floor flattening machine. Basically a robot that maps the floor with a laser and raises the grinding head up and down automatically based on some +/- tolerance value. I need to put a tool cart on the slab next week and see how it rolls. If it's like a rollercoaster then maybe I'll thinking about hunting down the worst spots with a grinder, if it's not too bad I'll just spend the time and money on therapy. Ha ha.

 

[ConCretin]

I was thinking that someone should invent a floor flattening machine. Basically a robot that maps the floor with a laser and raises the grinding head up and down automatically based on some +/- tolerance value.

Hmmm. You might want to get ahold of the patent office sooner rather than later cause that's a brilliant idea. If I start posting from a beach somewhere y'all will know where I got all my money.

 

[tinyrobotman]

Hmmm. You might want to get ahold of the patent office sooner rather than later cause that's a brilliant idea. If I start posting from a beach somewhere y'all will know where I got all my money.

Ha, go for it as long as you can make me a flat floor. I mean it seems obvious to someone who works in the CNC world. Like flattening a piece of stock on the CNC but just upside down on concrete. I'm used to working down to about +/-.002"of the Z axis so this concrete stuff where they talk about +/- 1/8" in 10 ft. being fine just seems bonkers. Also probably explains my neurosis over the floor.

 

[RPH]

Is there any thing with more complaints on Here than poor results with concrete pads ?
It seems to top the list for sure.

Ya, Snap On, no matter which way you lean. It’s like Pavlov’s dogs drooling over the bell ringing.

 

[djbmw]

Hmmm. You might want to get ahold of the patent office sooner rather than later cause that's a brilliant idea. If I start posting from a beach somewhere y'all will know where I got all my money.

You can just hose down the floor, give it 5 min to settle, then use marking paint to map out the high spots before grinding.
Alternatively, get a 10' straight edge and run it down the floor while circling high spots.

Sure, its not automated, but quick none the less.

 

[Rusted Nut]

They have laser grinding machines. Many warehouses have very tight flatness criteria, most are lazer ground after curing.

OP - if you have some high spots, those can be ground. Tell your finisher to do it. In all reality you probably won’t notice the floor that much once you start using it. Warehouses need the flatness so forklifts and other machines don’t bounce and can run faster.

 

[Ricky Joe]

An alternative fix might be to level the floor with tile or pavers. Find the high spot, determine the layout, and put your first tile there. Let it set up and then tile to it, using mortar thickness to achieve a level surface. I’ve done that successfully before.

 

[tinyrobotman]

So the finisher is suggesting that the lumpiness is due to the PEX radiant tubing telegraphing through during the finishing. I had it ziptied to the rebar which is #4 18" OC. Tubing is 12" spacing but I tied it at every spot it crosses a bar. Has anyone heard of that where PEX caused finishing issues?

Unfortunately I don't think any kind of leveler or tile is going to work since this space has vehicle traffic and heavy machinery.

 

[RPH]

Was the pex filled with fluid or air?

 

[Rusted Nut]

We do slabs with pex, elect conduit, air lines, Smurf tube, PT tendons, etc… none of it has ever telegraphed through the finish.

 

[tinyrobotman]

Was the pex filled with fluid or air?

Had 100 psi in it during the pour in case of damage.

 

[tinyrobotman]

We do slabs with pex, elect conduit, air lines, Smurf tube, PT tendons, etc… none of it has ever telegraphed through the finish.

Good to know. That sounded kind of like BS to me when he suggested it. I guess I could see how the tubes could spring back during floating making a hump but it seems like he's trying to put the blame on my pex.

They hand crawled the slab and didn't use a power float pan before troweling could that be part of the issue with the lumps? Ironically the trowel finish is glass smooth and tight. Just lumpy and wavy everywhere.

 

[RPH]

I doubt that was the reason for the poor quality. Just a question I had. I wonder if charging the system with water under pressure would have mattered. One of the guru’s here should chime in.

 

[tinyrobotman]

With water it seems like if you get a leak during the pour and have water spraying out into your concrete that would make a bigger mess?

 

[The Cobbler]

air pressure or water pressure on pex will not cause the issue of waviness.
and water spewing out of a break when the concrete hasn't cured would make a huge mess . at least with air the concrete wouldn't be compromised as it would be with water

 

[PCustoms]

I was thinking that someone should invent a floor flattening machine. Basically a robot that maps the floor with a laser and raises the grinding head up and down automatically based on some +/- tolerance value. I need to put a tool cart on the slab next week and see how it rolls. If it's like a rollercoaster then maybe I'll thinking about hunting down the worst spots with a grinder, if it's not too bad I'll just spend the time and money on therapy. Ha ha.

Hmmm. You might want to get ahold of the patent office sooner rather than later cause that's a brilliant idea. If I start posting from a beach somewhere y'all will know where I got all my money.

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[tinyrobotman]

Damn it, there goes the patent.

I talked to a guy with a big multihead grinder today. He looked at the slab and claims it will flatten the lumps. If his quote isn't too bad I'll probably just go that route and polish to a matte polish.

 

[Rusted Nut]

Damn it, there goes the patent.

I talked to a guy with a big multihead grinder today. He looked at the slab and claims it will flatten the lumps. If his quote isn't too bad I'll probably just go that route and polish to a matte polish.

Just a heads up, once the lumps are ground off, those areas may have a different looking finish to them, as the grinder mat take things down to the aggregate. Not a big deal, just beware. I’d take the finish difference over lumps.

 

[tinyrobotman]

Just a heads up, once the lumps are ground off, those areas may have a different looking finish to them, as the grinder mat take things down to the aggregate. Not a big deal, just beware. I’d take the finish difference over lumps.

Yep, then grinding guy said that will be an issue unless he grinds all the way into the aggregate on the entire floor. Depending on the cost I might have him do the entire floor to expose the stone. I like that look anyway.

 

[Innovate1]

Filling PEX with air to make sure it doesn't get damaged is the standard way it's done. As long as it's tied to rebar at close enough intervals I can't see it coming up to the surface. More likely to get mashed down from the concrete guys walking on it.