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Building a new workshop, had part of it concreted a few weeks ago. The guys did a very nice job and have a nice 70'x40' slab to start with. I was disapointed to see the other day that a crack was starting to form from one of the end wall posts and is already out 2m into floor and is also branching out in 2 directions. I know with concrete its luck of the draw but still disapointing to see.
We had good weather when it was done but did get some late afternoon sun as the doors aren't on yet, job was all but finished by then though. The apron across the front of the shed for the door track has about 8 cracks across it and hasn't even been driven on yet.
Could we have done anything different or is that just concrete?

Our old shop at my Dads was concreted 30 years ago and doesn't have a crack anywhere.
 

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Did you saw cut the slab? If you can get that done right away should stop the crack from traveling beyond the cut. I know the concrete guys I use have been able to repair a fresh slab so you cant see where the issue was.

Was told by an expensive so called General Contractor, when they didn't do a great job on the concrete on our community center, that he only guaranteed two things about the concrete, it would get hard, and it would crack.
 

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Hi Rusty.

Judging by your location I'm guessing it wasn't poured on frost or freezing conditions. :).

My first guesses would be either too much water in the mix or settling of the base. I know the driver makes a note of how much water was added to a load on the ready-mix tickets up here. It does decrease the strength a fair bit. And cause more shrinkage as the pad cures. If they used a pumper sometimes they get a little carried away with adding water. Had to have a discussion when I poured my basement.
 

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Concrete will crack, so either saw down an inch into approximately 10' squares and have controlled cracks, or don't saw it and watch it crack where ever it decides. This is what I was told by the guy that poured the floor in my Quonset 25 years ago. Mine was sawed in 12' squares, and I don't have a single uncontrolled crack.
 

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As Dan mentioned a big cause is too wet of concrete for sure. That's what happened when I had my shop poured years back. Hired a guy to place and finish it and he got water happy. Even the guys on Inland's trucks thought something was screwy. Long and short, floor took over two weeks to setup with the heat on in early November, and in about April I noticed hairline cracks starting to develop, emanating from the center of the shop outwards like a spiders web. Talked to a few guys in the concrete biz and that was their conclusion that it was due to shrinkage from being too wet when poured.

It was the high MPA rated mix and it is very hard which is what I wanted, is actually 9 inches thick for most of the floor, makes rolling stuff like when splitting tractors quite nice, but if you drop something on it it will chip. I had the placer make a cut all around the outside walls about a foot in, nary a crack anywhere along the walls to this day, but those ones from the center ended up being open anywhere from 1/8 to 3/8 of a inch after the first year. They extend outwards from the center to about four feet from the walls in a radial pattern. No question is was shrinkage. Sucks cause I gotta make sure I always position stuff so I can work around the cracks. It's a mistake I will never let happen again that's for sure:mad:
 

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There is a T shaped piece of plastic they place in concrete when you pour it (where you want your controlled crack). At trowelling time you peel off the top of the T and the crack forms beside the remaining plastic piece. The cracks are much narrower and tidyier than saw cuts.
 

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We poured a feed bunker a couple years ago nice day 80 degrees poured 6-8 in of concrete with rebar on 2' grids. Go to stip the forms couple days later and noticed cracks. Apparently on low humidity days with deeper pours it causes cracks. I was always told there are 2 types of concrete... cracked and will crack.
 

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My shop floor is a year and a half old with no cracks. The crew cut the floor about 10 hours after the last cement slid down the chute. They cut it about 7/8" deep on ten foot centres both ways. As an added bonus it's nice to use the saw cuts to measure things out on. When the two guys that owned the crew doing the work asked if I wanted it cut, I asked their opinion. One brother said to cut it and the other said not to.
 

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For just slabs I think adding fiber into the mix... It is supposed to take the place of rebar or hog panels, I usually use both . For not really thick I use hog panels and fiber and thicker or footers walls rebar and fiber. The fiber you can get hair like fuzz sticking out though....

Just curios how they did the base. When I worked in Alaska we would take d1 and compact the holly crap out of it and pour on it. When I moved back to montana I was shocked most guys here will pour footer directly on dirt as long as it was not over dug and filled back in. Most slabs here are poured on washed 1 inch mines drain rock with no fines in it. And it will not compact . Basically have a floating slab. I have seen both systems work great . And have seen wreack with both . I think the two most important things with concrete is to make sure the base it is going on is uniform . Ether hard as heck or if doing a floating slab to try to make that it has uniform give in the entire base and two keep water out of it before and after...rain gutters drain tile to make sure it does not get under part do the slab. The last but not least important is the mix design. Itself. A lot of guys will order a lesser mix that is cheaper to beat the other contractors price and the buyer is unaware of the change ... If you do not know your concrete guy well I would talk to the guy that is going to batch your mix tell him what you are doing and ask him what he recommends .

I am surprised that most guys do not do a slump test anymore. I would just to try to cover my butt . I also would save a sample or two so if there gets to be issues with it .you can send it off to a lab and have it tested . So if plant screws up you might be able to get them to replace the slab. Had a subcontractor on a big gym poured the slab three times before it passed. It looked great every time . However when the inspector would send off core samples to the fed lab. The failed. The cement provider had to pay for the removable and re pour ....needles to say they lost there butt.
 

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Discussion Starter #10
Thanks for the reply's. The guys that did the work did a good job and I cant see what they could've done differently, I am aware that concrete is very prone to cracks I just wondered how and what other guys were doing. It seems we are all in the same boat.
I do have a cut up the middle of the floor, we talked about doing more across it but decided against it, in hindsight it may have been a good idea. Having said that some guys put more cuts in and it cracks anyway so nothing is bullet proof. It was poured on a pretty hard base as we had to cut into a hill to level the site.
Polished concrete floors are becoming popular in houses, I wonder how they pour them because people would be pissed if their living room floor cracked.
Anyways, sounds like I'm not alone. I will soon get used to it.

Building the doors tomorrow.
 

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Rusty it might be to later now but when ever we pour concrete we water it to slow the drying.the blokes I've had do concrete say if it dries to fast it will crack badly. We haven't had to many cracks but we water it for probably 2 weeks after pouring.
 

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Since we are talking concreate stories,they were building a new house next to one of my farms twenty years ago .They poured the basement floor before they put the sub floor on ,as they lifted the troweling machines out the floor was cracking it wasn't hot at all ,almost cold spring day .But what cracked it the top cured to fast because of high wind,the cement floor was just a few hours old with cracks in it.
 

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What you are describing sounds like shrinkage cracks. As concrete hydrates(hardens) the water is used up and this forms the cracks. I also think it has sometning to do with concrete companies replacing some of the portland cement with flyash. Large slabs should be scored no larger than 20ft squares and 10ft is best. The score line needs to be at least 1/4 the depth of the slab. If it is hot, high wind or low humidity the top needs to stay moist or you will get spider web cracks from the surface drying out to quickly.
 

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Building a new workshop, had part of it concreted a few weeks ago. The guys did a very nice job and have a nice 70'x40' slab to start with. I was disapointed to see the other day that a crack was starting to form from one of the end wall posts and is already out 2m into floor and is also branching out in 2 directions. I know with concrete its luck of the draw but still disapointing to see.
We had good weather when it was done but did get some late afternoon sun as the doors aren't on yet, job was all but finished by then though. The apron across the front of the shed for the door track has about 8 cracks across it and hasn't even been driven on yet.
Could we have done anything different or is that just concrete?

Our old shop at my Dads was concreted 30 years ago and doesn't have a crack anywhere.
Please go to magicmortar.com we can help you out. You can call me at 816 813 5053
 

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We mfg and sell a high grade epoxy mortar you mix yourself in a small bucket. The powder Mix is a special blend with flint added to it. You can see more about it at magicmortar.com
We sell to the military for their concrete repairs on hangers, flightlines, and aprons. I’m very comfortable in saying if you will cut the crack out now and fill with this product it will not get any worse.
call me if you have any questions
816 813 5053
 

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Some contractors add water to the mix to make screeding easier but adding plasticizer is the correct way to lower the slump without sacrificing the quality of the concrete. Contractors that don't have or aren't using the engineers specification won't use plasticizer because of the cost.
 
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