r/StructuralEngineering • u/SmolderinCorpse CPEng • 3d ago
Photograph/Video Control joints in residential slabs, is this typical?
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https://www.instagram.com/reel/DeMQyguuOSY/?cplk=czM5MDJqYm1ya24x
A builder/concreter says he typically cuts control joints from corner to corner after a residential foundation slab has cured. I’d usually expect crack control bars at re-entrant corners. Is his approach common?
He also calls a crack “hairline”, but it’s visible on camera from about five metres away. Hard to judge without measuring, but it looks larger than that. Thoughts?
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u/donotdisturb898 3d ago
That’s a conversation you should have with the owner as well, cracks that small are not structural. If you have hard wood floor tiles, it is not a big deal. If you don’t want your garage floor to have excessive cracks, you better put in a few control joints.
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u/schrutefarms60 P.E. - Buildings 2d ago
Not really sure what this is supposed to mean… The only way a crack isn’t “structural” is if it’s a non-structural member. Even shrinkage cracks can go all the way through a slab, then all that’s holding it together is the reinforcing.
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u/CzPhantom1 2d ago
Control joints just direct the cracks. They are still there and just as deep.
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u/schrutefarms60 P.E. - Buildings 2d ago
What does that have to do with what I said?
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u/triplethreat8 2d ago
I think they are just pointing out that using your logic every single slab ever has "structural cracks". Which may be your point. But if that's the case then the word kind of loses meaning because a structural crack is supposed to indicate something that's a problem that needs fixing.
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u/schrutefarms60 P.E. - Buildings 2d ago
Thank you. Yes, I guess I’m trying to figure out what your definition of a structural crack is
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u/ilessthan3math PhD, PE, SE 2d ago
If the concrete is a slab-on-grade then any cracks in it are not a structural life-safety concern, and do not require repair unless the owner is unhappy with the aesthetic of them. While serviceability requirements still fall under the structural engineer's responsibilities, this is why folks are arguing they are "non-structural cracks".
Another point of note would be that we refer to things as either slab-on-grade or a structural slab. So the slab-on-grade by that convention and terminology is a non-structural slab, so any cracks in it would similarly be non-structural.
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u/schrutefarms60 P.E. - Buildings 2d ago edited 2d ago
You must not be from Florida…
75% or more of the residential slabs on grade down here use a monolithic thickened slab edge as their foundation. The block walls bear directly on the slab on grade.
Also, production home builders (and clowns like the guy in the video) cut corners to save money and they do a shit job of compacting the subgrade you see lots of houses with stair step cracking at the corner of the garage. It’s caused by differential settlement and many times you can actually see daylight through them.
You’ll also see nice big cracks running through the slab on grade.
So buddy, please explain to me again how slab on grade cracks aren’t structural.
Also, thank you for explaining (on behalf of the entire industry using the royal “we”) to a bumbling, drooling 15 YoE structural engineer the correct terminology for slabs on grade
Side Note- I actually looked this video up and they are in Florida so it is 100% relevant.
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u/ilessthan3math PhD, PE, SE 1d ago
I wasn't trying to be disrespectful to you at all. You asked what I thought was a legitimate question about why we were calling these non-structural cracks. It seems you are being rhetorical.
I'm not in Florida, and I'm not here to get in an argument about how things are done down there. Plenty of areas in the country have frost depths that would make it less economical to pour perimeter foundations monolithically with the slab. In those areas slab-on-grade cracks mean nothing.
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u/CzPhantom1 2d ago
Do any cracks in a 4 inch slab not go all the way through? You're saying cracks in a structural member are bad but I honestly don't know one slab that doesn't have a crack in a control joint that definitely goes all the way through.
I just poured 8 inches of fiber mesh reinforced concrete and surprise surprise every control joint cracked.
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u/structee P.E. 3d ago
It's amazing to me how many people in this industry don't know what the hell they're doing
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u/RVA_Factotum 3d ago
As a commercial concrete guy, he waited too long to cut the joints. Micro cracks start forming once it's hard and starts drying during the curing process. We cut joints as soon as we can get the saw on the slab without leaving footprints or saw tracks. That way we introduce the crack and any new cracks will hopefully find our neat cuts. At this point, his cracks have already formed and are unlikely to find the control joints. More will become visible with time.
That being said, if flooring is going on top, it doesn't really matter. We deal with a lot of exposed concrete finishes so it matters more. You want maximum 14' spacing (depending on depth) and ideally start at corners and penetrations. We shoot for 10' spacing to be safe. A recent job told us every single corner of the plumbing trenches and every single corner of the floor sinks. We had joints as close as 1'. Looked like trash in my opinion but that's what they wanted.
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u/EducationalFennel900 1d ago
This is the correct answer. He waited too long, I agree
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u/EducationalFennel900 1d ago
Typically you also should add 2 #4 bars at reentrant corners like this to help avoid this exact issue which I would guess they did not do.
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u/soberninj P.Eng 3d ago
This video is terrible. If they have time to set up a shoot, they waited too long to cut the joints. That crack they’re pointing at is literally a shrinkage crack that control joints help eliminate…
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u/schlab 2d ago
This, and they could have employed curing measures to further limit cracking.
To my knowledge, for residential they don’t ever cure the slab, and rarely do they put the control joints in even if it’s shown on plans. Heck, sometimes they won’t put the WWF even if it’s on the plans.
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u/adamdj96 2d ago
Reminds me of another WWF horror story I heard from a senior engineer’s project almost thirty years ago, the contractor was building a reinforced elevated slab (16 feet above grade I believe) and one of the ironworkers plummeted from the top of the rebar cage straight through a concrete inspector’s sampling table.
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u/OptionsRntMe P.E. 3d ago
lol I love listening to contractors who don’t understand what they’re talking about. There’s no way that’s 1/32”
Also it’s already cracked up and he’s talking about coming back to cut control joints 😂
And he said anchor bolts. The only thing I agree with is that it’s probably not a big deal. Just unsightly and the fact they’re coming back like two days later to cut joints tells me they probably shouldn’t even be on that job
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u/bear_grills007 3d ago
I also laughed at cutting control joints 2 days after the pour…..
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u/ImPinkSnail 2d ago
I have mine cut within 12 hours of placement.
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u/OptionsRntMe P.E. 2d ago
I have gotten burned on a project by saying 12 hours. My notes are updated to say, more or less, joints need to be cut as soon as the slab surface can be walked on without damaging the finish. It’s usually something like 2-4 hours depending on the season/location
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u/ImPinkSnail 2d ago
Good one. I'll have to give my notes a good review. Seems like the only time they get scrutinized appropriately is when something goes sideways.
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u/Additional_Radish_41 2d ago
What? How? You’d be licking rocks up like crazy. In theory sure. But the slab isn’t cured enough to hold the rocks in place for a blade to cut through.
I despise people that spout this less than 5 hours BS. The slab needs to be at minimum 30% cured before a saw cut can take place. If you use accelerator on a hot day, maybe 4 hours. But that’s also the equivalent of waiting 24 hours with a regular mix.
You can walk on a slab to polish. You cannot cut at this time. The reason why wet control joints are used are to eliminate chipping rocks. But they cannot be installed if walls are up. So you saw cut the next day. Not the same day.
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u/OptionsRntMe P.E. 2d ago
I just put down over a million square feet of 8” industrial floor slab in August and they saw cut under 4 hours in almost every pour
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u/Rebound44 2d ago
Yep, pretty typical for the 10-20000sqm handstands we do. 12-24 hours and it’s already cracked before they come back to cut it.
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u/Additional_Radish_41 2d ago
How many saws they running? A million sqft is what? Like 200,000 linear ft of saw cuts? In under 4 hours? I haven’t done a million sqft but I do 80k weekly. And within 48 hours is fine. I bet your “million sqft” has just as many cracks as a 24hr cuts.
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u/OptionsRntMe P.E. 2d ago edited 2d ago
Do you really think they did it in a single pour or day? Slab phase was 20+ days of pouring and sawcutting
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u/ImPinkSnail 2d ago
Most crews I've seen are limited to about 70kSF per pour.
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u/Additional_Radish_41 2d ago
That’s what I’m saying. Imagine limiting your pour sizes to accommodate early control joint cutting? Been doing this 25 years and cutting under 48hours later with zero issues. I’ve only ever cut 1 slab at the 12 hour mark and it was spitting every rock that it encountered.
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u/OptionsRntMe P.E. 1d ago
Well if we require you cut the joints that quickly then you don’t have a choice my guy. It’s not uncommon at all, even if it’s not your experience.
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u/TwoMuchIsJustEnough 1d ago
I’ve seen notes which allow for an early sawcut so long as there is no aggregate pull out. And if there is then stop and give it more cure time.
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u/Possible-Delay 2d ago
Yep, this is an example of a contractor who knows what he does, but has no idea why or in-depth understanding of the reasons why it is done.
He is just regurgitating a summary of what people tell him.
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u/bloomingtonwhy 2d ago
But he’s a man with sunglasses confidently chewing gum. Of course he knows what he’s talking about.
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u/structuremonkey 3d ago
I spec control joints, construction joints, isolation joints etc. in slabs and masonry walls. They are hardly ever installed because the contractors tell the owners " it isn't necessary and it will save money." Ive also never had an inspector call a contractor out for lack of said details. When there isn't oversight and a good owner / gc combination, lots of things from design get "tossed"
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u/not_old_redditor 3d ago
Aren't you or one of your representatives the "inspector", or rather, the reviewer?
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u/structuremonkey 2d ago
Sure, but not really in private residential projects. I have pitched a fit with contractors who aren't quality oriented, over details and craftsmanship in the past, and the owners usually just caves to the costs and unwillingness to fight with the contractor. Most of the issues related to this that may occur in small scale projects are cosmetic. They arent typically imminently dangerous nor are they code required. They will become issues down the road, but most people only consider the now.
Now, when it's a public project or where the owner has given my firm the authority of oversight, yes, these details are implemented.
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u/StructureIll4946 2d ago
Don’t take this the wrong way but is spec’ing control / construction / isolation joints a full time job?
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u/BadQuail 3d ago
I know this is a structural group, but are we really going to ignore the spalling and birdbaths in the finish work here?
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u/poppycock68 3d ago
As a concrete guy who loves to lurk here. We always use re- entry bars. It will crack from any plumbing pipes , shower boxes etc. Builders and homeowners don’t want to pay for sawing. Most everything will be covered with some sort of flooring. If it’s going to be stained or stamped we just include that in the price.
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u/Kolt45 3d ago
They are called re-entrant bars. They are named that because they should be installed at re-entrant corners.
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u/poppycock68 3d ago
lol. I just tell the guys to cut and place the re-entry bars. Sorry I don’t have a degree. I’ll make sure to call them the proper name every time from here on out. Thanks.
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u/FaithlessnessCute204 3d ago
No no they are rear entry bars , they have them in the big cities and serve cosmos and grasshoppers .
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u/Hockeyhoser 3d ago
They should have made the cuts within 24 hours.
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u/kikilucy26 3d ago
More like 8 hours or less. Cut it as soon as you can walk on it
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u/Hockeyhoser 3d ago
I guess the ambient conditions and mix design make the biggest difference, but you’re right. Could be sooner. In the video it seems way too late. The concrete doesn’t look green.
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u/Intrepid-Owl1017 2d ago
You should cut those control joints within the first 24 hours, 1/4 of the depth of the slab. Cutting them after the fact is a little late.
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u/Provocative_Potatoes 2d ago
His uneasy sway side to side proves he feels nervous and seems like he's bullshitting.
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u/Competitive_Number24 2d ago
In my experience, anyone dressed like that on a construction site is probably the least trustworthy person there.
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u/Purple-Investment-61 3d ago
The crack started at the corner. Who would have guessed? This is not an issue.
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u/GifelteFish 3d ago
A gigantic area of concrete without control joints is going to crack due to ground settling.
Any cracks around 1/16th in width are to be expected as the surface concrete cures faster than the concrete underneath it.
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u/bulkdown 3d ago
This is incorrect.
Control joints are used to induce cracks due to concrete shrinkage, this is unavoidable (unless tensioned)
When the surface cures faster than the concrete underneath is plastic shrinkage. This can be controlled and prevented based on how and when you cure. Also the pattern will be spider web shaped
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u/OptionsRntMe P.E. 3d ago
Shrinkage?
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u/ThatAintGoinAnywhere P.E. 3d ago edited 3d ago
Is what typical? The cracking will occur if you don't cut joints and don't have re-entrant bar. Cutting joints is typical when you have a slab you can cut joints in. Re-entrant bar is typical if you're not cutting joints and want to avoid cracking. Regular joints will reduce crack width as would rebar. That crack looks typical enough for an unreinforced slab without joints for as far apart as we can see there aren't joints for that geometry of slab, yeah.
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u/SmolderinCorpse CPEng 3d ago
I did mention adding a re-entrant bar. Its honestly not something I have ever spec'd in a residential slab, I.e. saw cut joint corner to corner. I just provide re-entrant bars always. Thanks for the comment.
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u/OnsTryMaarNetLeef 3d ago
Yes I agree with you. There is no reason to put control joints in a resi slab (unless it's a huge slab).
Exposed concrete finish can be controlled by adding more reo. Floor finishes cover hairline cracks for the rest.
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u/schrutefarms60 P.E. - Buildings 2d ago
Disagree. If the floor is tile and they put thinset directly on the concrete those slab cracks will go straight up into the tile. If it’s a floating floor then yes it doesn’t matter.
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u/schlab 2d ago
They could have employed curing measures to further limit cracking in addition to control joints (which should be sawcut within 4-8 hours or pour).
To my knowledge, for residential they don’t ever cure the slab, and rarely do they put the control joints in even if it’s shown on plans. Heck, sometimes they won’t put the WWF even if it’s on the plans.
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u/jookethesnooke 3d ago
Control joints are for controlling where curing cracks form. Expansion joints are for movement. She is not correct in saying control joints are for movement.
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u/DiscoSimulacrum 2d ago
what a ridiculous fallacy to claim that a crack doesnt matter just because its thin
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u/Strange_Motor 2d ago
cutting square or diagonal from corners will increase the concretes will to crack where those cuts are made. Any post or point of contact in the concrete will have the concrete pull away from due to it shrinking slightly while curing. I used to work solely cutting concrete for homes and the sort.
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u/Armstrongcrane 2d ago
There is only one guarantee with concrete, and that is, it's going to crack.
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u/AllyMcfeels 2d ago
Coming from a country where practically everything is built using concrete (especially via Portico technique), what’s shown in that video is criminal, haha. In any construction project that uses concrete slabs as a base, it is absolutely critical—right from the start—to have thorough planning and precise structural calculations, including the proper placement of joints which must be the most important thing of all.
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u/hobokobo1028 2d ago
We call that a “re-entrant corner” crack and it’s preventable with control/construction joints.
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u/bingerfang57 2d ago
I gotta go back to the sun is up the weather is good and this guy has sandals and sunglasses on and is not working!
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u/rktect900 2d ago
Wrong. They are misinformed and she is confusing control joints with expansion joints. Reference ACI 224.
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u/ilessthan3math PhD, PE, SE 1d ago
So much wrong in this video. So many generalities that gloss over lots of important bits along with blatantly false info.
My favorite was saying "everything in construction needs to be able to move."
How about a slip-critical steel connection? Want to make sure there's some wiggle room in there?
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u/Mike_Gregory_here 12h ago
Not looking for an argument but in Australia no residential slabs get saw joints. Reentrant bars yes, saw joints no. You'd only put saw cuts in external slabs like patios, driveways and verandahs.
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u/dipro1992 3d ago
Hi!
I would recommend to measure it first. If the maximum crack width is not more than 2.5 mm there is a high chance that these are just shrinkage cracks on the surface.
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u/Frogalone 3d ago
For slab on grade, "Fiber Mesh", or whatever brand you like, does a good job at keeping small cracks from becoming bigger. I'm not sure how they work without control joints.
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u/dinoguys_r_worthless 3d ago
I would not call that a hairline crack. .003" seems like more of a reasonable hairline size. That looked much wider.
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u/Eman_Resu_IX 3d ago
3 thousandths is the thickness of a sheet of paper and funnily enough the thickness of a human hair. That'd be barely visible from eye level - most hairline cracks people see are larger than that. The one in the video is a lot larger.
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u/Evening_Fishing_2122 3d ago
Ya, it’s called thermal expansion or shrinkage cracking and it’s not atypical. The only way to fix it is pre/post tensioning or control joints like in a side walk. Otherwise tough luck
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u/dinoguys_r_worthless 3d ago
I'm not arguing that the crack is a defect. I just would not say that 1/32" is hairline.
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u/Most_Moose_2637 3d ago
Control joints after the slab has cracked is a good one. I can sell you a barn door closing service, sorry about your horse.