1 yr old concrete floor cracks - advice please

[whm_fab]

Hi Guys,

im looking for some advice please from some of the far more knowlegable people on here than me on these things.

last summer, i had the floor of my workshop poured 6in of 35kn concrete. approx 9 months later, these cracks started to appear, which i am let to beleive a shrinkage cracks? but im not a concrete expert, so i dont know.
my plan is to gdiamond grind, then epoxy coat the floor this summer, but of course i need to repair the cracks first.

my questions are basically:
are these likely to get worse or is that it? it looks like they havent grown in the last 5 months or so.
whats the best repair product/approach?

a bit of back ground for info that may have led to this:

concrete was laid following a 2 month heat wave (for us) which we are not used to here... 25-30c with very little rain so the ground was hot and dry.

there is no steel in the concrete since its min 6in. more at the sides and under the portal frames.

the size of the slab is 30ftx35ft with one control joint down the middle of the 35ft.

it rained heavily about 6hrs after the concrete went down


So, should i be worried, or is this just unfortunate shrinkage due to the weather, fix it and move on? thanks for any info!

 

[ConCretin]

You are correct in that these appear to be shrinkage cracks. They probably occurred shortly after the concrete was placed and only became noticeable later on. They shouldn't have very much movement going forward.

In hindsight, one control joint probably wasn't enough to ensure the cracks were hidden but they would be there whether you could see them or not.

Normally it's pretty difficult to 'repair' shrinkage cracks without making them look worse but since you are planning to apply epoxy, I'm sure our resident experts will suggest something.

 

[whm_fab]

cheers thanks for the quick reply! yes so i've been told - i didnt actually lay the concrete myself, i got a builder to do the pouring/screeding it as i was afraid to mess it up. ive been told since there should have been at least one more control joint across the floor - ironically right where those two cracks appear perpendicular to the control joint in the third last photo

the cracks are quite small, you can see my shoe in one photo for persepctive, so it will be inpossible to get any sort of epoxy repair paste etc in there. id want some sort of liquid resin, or something in a tube with a fine nozzle...

how deep do you think these go? is it ambitious of me to hope that they will come out of it after grinding down the floor?

 

[ConCretin]

how deep do you think these go? is it ambitious of me to hope that they will come out of it after grinding down the floor?

Unfortunately, they are likely full depth. Depending on slump and environmental conditions concrete shrinks as much as a 1/2" in 100'. There is almost certainly a crack at the bottom of the one control joint leaving two 35' by 15' sections. A crack where you have one was almost inevitable.

 

[whm_fab]

cool thanks for the advice really appreicate it!

so what are my options for a crack free epoxy finish? i really dont want to just ignore it and epoxy over it. im a bit of a perfectionist and that would really really bug the hell out of me.

 

[JoeMcGov]

The manufacturer of the epoxy material/system that you're going to use likely has suggestions for small crack preparation that they recommend for compatibility with their epoxy system. You may have to dig a bit into their technical literature (known in The States as: manufacturer's written installation instructions) to find it. If it is there.

 

[spudley]

i didnt actually lay the concrete myself, i got a builder to do the pouring/screeding it as i was afraid to mess it up.

I'd be sure to send out a Thank You card to the expert contractor who didn't take the extra 15 minutes to cut more control joints.

 

[kram71]

They look pretty normal to me.

 

[dcg9381]

To me too...

If your expectations of a slab are "no cracks" - it's worth mentioning to the contractor as they may cut more control joints or engage in some additional care (READ: costs).

I've got similar to the OP in my 40x60 foundation. Not the end of the world and pre-epoxy, probably reasonably easy to patch up and cover.

 

[whm_fab]

cool ok, thanks for all the advice guys.
i would have insisted on more cuts had i expected this outcome, but of course, i had naively left it to the 'professionals' which i dont normally do.

I am also led to believe by someone i was talking to recently, that its never too late to cut another control joint where it should have been, and this will still help now at this stage, and just repair the small cracks that have appeared. Is this true?

Any recommendations for a product that will flow into the small cracks?

 

[ConCretin]

I am also led to believe by someone i was talking to recently, that its never too late to cut another control joint where it should have been, and this will still help now at this stage, and just repair the small cracks that have appeared. Is this true?

I wouldn't bother with another cut now. Shrinkage cracks form early when tension from drying shrinkage is high and tensile strength is low. The crack you see relieved that tension and at this stage additional cracking is pretty unlikely.

 

[Angelfire]

I had a similar experience with my contractor. I had asked for control joints every 12'. What he gave me was every 12' in one direction but left the other direction without cuts. I ended up with very similar results to yours. I filled them with an epoxy filler after they had sat for about 4 years (while I was routed to Ireland for work ). Ultimately I did porcelain tile after weighing the pros/cons of epoxy, plastic tiles, etc... Very happy with it. I think the key is to make sure the slab is done moving. Take some measurements and check in a few months. Cutting control joints after a few days of curing is futile and won't buy you anything.
Cheers.

 

[Armorpoxy]

We have a really nice Crack Repair Epoxy Putty that gets mixed like Bondo and you apply with a putty knife. It's gray or clear and would do a nice repair. It's stronger than concrete when cured.

 

[kram71]

cool ok, thanks for all the advice guys.
i would have insisted on more cuts had i expected this outcome, but of course, i had naively left it to the 'professionals' which i dont normally do.

I am also led to believe by someone i was talking to recently, that its never too late to cut another control joint where it should have been, and this will still help now at this stage, and just repair the small cracks that have appeared. Is this true?

Any recommendations for a product that will flow into the small cracks?

I would get some in person opinions before moving forward on doing anything more. It is possible that nothing needs to be done. Surface cracks in concrete are pretty normal.

 

[Honolulu]

Cracks in concrete are "normal" but failure to provide control joints is not. If your contractor knew you were going to coat the floor, I'd hold his feet to the fire and let HIM do the crack repair.

Nine months post-pour, your slab has stopped shrinking/moving unless part of it is exposed to hot sunlight (do you have that in Ireland? I'm in Hawaii...) so you can go forward.

You could consider laying down flexible tiles from various suppliers that post here. Any of those would bridge the cracks and you'd never see them again. But that might not be your cuppa tea.

I assume that 9 months later you KNOW the slab is stable. A relatively thin, brittle coating will never withstand any differential movement from one side or the other. I'd tend to look for a flexible repair product, but not with any conviction that if the slab moves later, it won't avoid cracking of a thin brittle topcoat.

There's a lesson for us all here.

 

[benwah]

Hey OP, the preferred method is to open the cracks up using a 4.5" grinder with a masonry diamond blade. Be sure to wear eye and a respirator (not dust mask). I prefer to fill joints with a 100% solids epoxy joint filler, that is applied with a putty knife, then struck clean and level with another, larger putty knife. I prefer plastic.

Sometimes I use a polyurea joint filler as well. It has a much shorter pot life so if you're inexperienced I'd stick with the 100% solids joint filler.

After this cures you can top coat with the epoxy of your choice.

 

[brickG-man]

cool ok, thanks for all the advice guys.
i would have insisted on more cuts had i expected this outcome, but of course, i had naively left it to the 'professionals' which i dont normally do.

I am also led to believe by someone i was talking to recently, that its never too late to cut another control joint where it should have been, and this will still help now at this stage, and just repair the small cracks that have appeared. Is this true?

Any recommendations for a product that will flow into the small cracks?

With garage and driveway slabs I have always gone by the rule of thumb that control joints should be appropriately in 10ft x 10ft squares. My opinion is like some others here that your contractor did not provide you with adequate control joints for a slab of that size.

While it’s too late for that crack, I agree that it’s not too late to saw cut additional control joints in order to alleviate future cracks in other areas.

 

[whm_fab]

hi Folks, sorry i've been away for a while, i really appreciate all the suggestions and comments, many thanks!

Shortly, il be tackling this project, and i was just wondering what order would be the best to do this?

What i want (ideally) is a flat smooth crack free epoxy floor.

The floor is not powerfloated as you can see, and has bullfoat marks etc so i will be grinding it.

What im wondering is, so i put down the crack and imperfection repair first, then grind the whole lot? or do i grind, repair any imperfections/cracks/etc that show up, and then just zip over these with a 4in grinder afterwards?

this is the product i plan to use (not being in the U.S unfortunately we dont have a massive range of products available

https://www.watco.co.uk/products/watco-fine-crack-filler

thanks!

 

[glanafosha]

You can roam this wide world over, but you'll only find two types of concrete: already cracked, or going to crack. You could try to ignore the cracks.

According to my wife, I'm extremely good at ignoring things. Your results may vary.