It is normal for concrete to crack. There are ways to prevent cracking and ways to mitigate cracking.

If you ask the internet, you often get one-off anecdotal replies.

Concrete has been around for thousands of years.

You got what you paid for. If you did not want your concrete cracking, you would have paid more.

Due diligence.

I gotta say, I haven't seen fresh concrete crack like that while curing unless there was something amiss. Like if it's too hot and dry in the summer it can happen, I've been told watering the concrete can help mitigate that.

But if there are radiant pipes in that concrete.. I'd be concerned with the cracks being sealed up and the pipes being in correct working order before starting to lay down flooring on top.

normal, unfortuately, but not "right".

Relief cuts should help control the cracking , but usually this crack that you have is an "over time" thing caused by settling or driving something heavy on it. Also those relief cuts don't look deep enough - they are supposed to be 1/3 of the slab thickness IIRC.

We poured our floors and garage and honestly they ended up not being as thick as I would have liked due to the insulation underneath them and some front to back slope we needed to build in, but, there were no cracks visible (maybe in the relief cuts) after the slabs sitting for a year and having all sorts of equipment on them - fork lifts, scissor lifts, pallet jacks with 600 lb freezers, etc.

There were 2 cracks exposed when the floor people ground the surface to prepare for the epoxy but they had not "separated".

Do you have rebar in addition to the radiant heating tubes?

Concrete cracks appear within days due to shrinkage. We had some during the head wave, especially on large interior slabs without relief cuts. They also appear when flatwork is poured around hard unforgiving objects, like those stem walls. The relief cuts are made to hopefully control the cracks.

Concrete cracks. Birds sing. Dogs bark. Duh.

Sorry, but additional due diligence wouldn't have altered the cracking of your slab.

Cracking is an inherent characteristic of concrete. The use of control joints--which it appears you have in your slab-- are intended to encourage cracking along them, but concrete doesn't always do what we'd like it to. We use keyed metal concrete joint formers in most of our slabs which serve as forms and remain in place after placing concrete to serve as control joints. If those have been used you won't see a deep joint as you would in the case of a saw-cut control joint.

Ours also cracked, I was also alarmed, I also asked here and was waved off. I let it go and suppose than in a decade or two I’ll know if that was wise! 😂

I'm looking to replace the flooring in my rec room on my first floor.
While doing that, I found a crack in my concrete subfloor, and I'm pretty concerned... Online, I've read that hairline cracks are normal, since concrete shrinks when it dries but this is no hairline crack.

The floor on either side of the crack is flat, but it does stretch all the way across the room. It looks to be 4mm large at the largest point, and it stretches in one direction. At the deepest point I was able to measure, the crack was 4cm deep Precast inspection Melbourne. From the outside, right outside the door, there is no visible crack in the foundation. The room is about 10ft by 22ft. I don't see any other cracks. (so the crack stretches about 10ft)

The townhouse is about 20 years old, 3 stories high, and an earthquake has rocked the region maybe 5 or 6 years ago. The crack might be from that.
Is the crack a cause for concern? Does this need to be repaired? Sealed?
Thanks for any advice you can give.

No to all 3 questions.

There is a very high probability that Joseph is correct.

You can caulk it if you want just so you don’t get and bugs from coming up.

"Is the crack a cause for concern?"

It may be. It could be an easier route for insects to enter your home as Kate noted above, and it could be an easier path for soil gases such as radon to enter your home. Either concern can be mitigated by filling with a high-grade caulk.

Control joints are cut with a saw into the concrete after the concrete has cured a short period of time. The purpose of the control joint is to control where cracks in the concrete occur. Something went wrong here and cracks have occurred in locations other than at the control joints. Either the control joints were not cut deep enough or cut after the cracks occurred.

Sometimes I feel like this image illustrates the story of my life.

@Mark Bischak. Architect,

If it illustrates the story of your life, we'd need additional information to understand what the story is. Consider, for example:

The pessimist looks at the failure to, well, fall into line as a failure cast in concrete.

The optimist sees it as an expression of creativity; coloring outside the lines, if your will.

The non-conformist sees it as an everyday experience. Meh.

The QC manager sees it as acceptable if it's within some number of standard deviations.

The HR manager sees it as an unmet need for additional training and development

The musician realizes that the first cut may not be the deepest after all

The nurse sees a wound that needs an application of something

The concrete contractor sees it as a reason to get his check and cash it promptly

The architect sees it as a reminder to add something new to the standard specifications

The janitor sees it an another thing to clean while bending over or on his knees

The chiropractor sees it as another janitor annuity stream in the making

The homeowner sees it as an opportunity to find fault with the builder

Today, Charles Ross wins the Internet.