Garage floor crack free

[andyvh1959]

I had the slab poured for my 24x28 garage back in mid Oct 19 up here in Green Bay. By early Nov the temperature went way cool, single digit highs for about a week. The slab is 4" thick laid on 2" of pink styrofoam/heat tubing, laid on 6ml vapor barrier, over 8" of recycled packed asphalt, on very sandy soil that was graded and packed before the ground up asphalt fill went down. The concrete guys cut reliefs into the slab within a week of the pour. The garage was closed in by Jan 1st 2020. So far the floor does not sweat one bit in hot humid weather, and to date not a single crack has shown anywhere on the garage floor. I think all the prep under the slab makes all the difference. So far the garage is not heated, have to install the boiler for the in floor heating system later this year.

 

[like2wheel]

Ok good job jinxing yourself.
I may or may not have a similar situation with a similar outcome, but I'm not saying.

 

[nadogail]

My concrete guy guarantees only two things when he pours a slab. 1. Nobody will come steal it and carry it away and 2. They will crack.

 

[wssix99]

The concrete guys cut reliefs into the slab within a week of the pour.

This is too long. ... but glad you lucked out. (Or maybe you also had some reinforcing that held down the fort while the slab was waiting for the control joints to be cut?) Reinforcing is great for an insurance policy.

You should kick your concrete guys in the nuts. The relief cuts are called control joints. They are the thing that controls the cracking the cracking.

 

[TractorJeff]

Mine was poured early November 2019 or 18(?). 30 by 32 by 6 inches thick with big thick/wide mono slab piers poured on heavy Plastic. Control joints were cut that afternoon. Concrete never saw Sun for 4 weeks. After 2 weeks it sprinkled every day. Concrete was Black when they finished and it stayed Black! Never got Sun faded or Gray looking. Now it is still a very dark gray'ish/black.
NEVER CRACKED!!!!!!!!!!!
Same thing - Excellent base, good drainage

 

[Wes Tex]

My 30x40 garage floor was poured almost 20 years ago and has one very tiny crack about 3 feet long. The specifications and procedure was the same as Tractor Jeff's pour. The only difference was mine was never covered. The city inspector told me it looked like a commercial job. The control joints were cut the same day as the pour.

 

[txvwnut]

Mine is 20x26 5 inches thick with 18”x 18” cross beams ******** in the mix while the pour was going on and no control joints. My concrete guy double dawged dared it to crack. That was 19 years ago and it hasn’t cracked.

 

[andyvh1959]

Thanks guys. My comment about the "control" cuts being within the week of the pour just lazy memory. Now that I think about it, they poured by midday and had the control cuts the next day in mid Oct of a very wet fall 2019. No cover or water applied on top after the pour, in fact it was a drizzly rain day when they poured. And it was wet many days after the pour. Its a 4" thick floating lock slab to 12" deep at the edges. It was done by a professional crew between other jobs. I got a broomed finish as I didn't want a polished smooth (sweaty) floor. I agree, like death and taxes, concrete cracks in some way some day, to relieve internal stresses. So my post is to get a feel for when it does crack, not to boast it will never crack.

However, base prep and base support has to make the biggest difference.

 

[rburke65]

In 2012 I had my 32x56 placed and cuts made same day..?.. 12x12 approx. areas. No cracks on the surface but I can see cracks IN the saw kerfs….that’s where I wanted them

 

[rayra]

Oh it cracked for sure. Just in the cuts, where it is supposed to. Regardless of your ground prep.

 

[c39er]

My 6" slab never cracked either for over 10 years....
Until the 5.7 earthquake...one 4' long crack.

 

[JOsmund999]

Dad would always say... " there are two types of concrete, concrete thats cracked and concrete thats gonna crack "

 

[Musky_Hunter]

32x40 7” thick 4500 psi with fiber. Poured in march of 21. Control joints cut same day. Still not cracks at the end of the joints. I expected it to crack when we moved the mill in with a fork truck but none of them did n

 

[TractorJeff]

I am not a Concrete Expert; BUT! I often wonder if the overcast, drizzly, cool weather let the Concrete cure so slowly that the internal stresses never developed?
As far as Base Prep, I had lots of time to wait before even considering Concrete. This allowed the Base lots of settling/compaction time before the Concrete guys showed up and did the final prep.

 

[wssix99]

I am not a Concrete Expert; BUT! I often wonder if the overcast, drizzly, cool weather let the Concrete cure so slowly that the internal stresses never developed?
As far as Base Prep, I had lots of time to wait before even considering Concrete. This allowed the Base lots of settling/compaction time before the Concrete guys showed up and did the final prep.

No, you can't escape the shrinkage cracking. A high water content can heighten the risk, but this is a function of chemistry and physics.

There are different types of cracking. Some of the other types, like plastic shrinkage cracking and (what this article calls) offset cracks can be caused by the slab surface drying too quickly or by a bad base: https://www.concreteconstruction.net/how-to/construction/identifying-cracks-in-slabs_o

 

[andyvh1959]

All interesting to learn, concrete is not as totally "solid" as we think it is. Interstate 41 up here in the Green Bay area was totally rebuilt over the past ten years; all old surface and substrate taken out, all new substrate and newest technology concrete laid back in place, and its surprising how bumpy it can be. But then, it gets a LOT of heavy truck traffic daily.

Some interstate areas in Wisconsin get the tie bars in the concrete that connect sections together, and those are much smoother for many years. With heavy truck traffic as the truck moves onto a leading edge of concrete I guess the bending "wave" action in the concrete must eventually move the substrate forward to the path of travel until it starts to lift the other end of the concrete. So steps are formed over time.

 

[wssix99]

With heavy truck traffic as the truck moves onto a leading edge of concrete I guess the bending "wave" action in the concrete must eventually move the substrate forward to the path of travel until it starts to lift the other end of the concrete. So steps are formed over time.

Correct. The wave also puts different stresses on the concrete than we would see in a garage or driveway. 0(So, the reinforcing, pins, and design are very different.) Driveways are much less dynamic. Roadways also have added stresses when overloaded trucks use the roadways and when traffic volumes exceed the design.

 

[Montyx5]

So if the base is solid, well packed and stable the concrete will not crack if poured properly. The problem is that with varying moisture levels and temperature changes the base will move leading to areas of tension in the concrete. For my shop in early June I had the site prepped for the building, set the concrete post in the ground and tied together with 2x6's along with the other work that needed to be done before pouring the concrete. With the vapor barrier, insulation, wire panel reinforcement, and hydronic tubing left to do the concrete installer was pushing to pour do to scheduling or wait till end of season and hope the pouring season didn't end early. I opted to to let the summer rains do the final base packing. In mid August I had areas of the pad sink nearly 3/4 of an inch. I brought the pad back up to level and had very little settling in late October when the rest of the building went up and the pad was poured. I used 2" foam panels under a 5.5" 6000psi pour with heavy wire reinforcement panels on chairs and rebar in selected areas all wired tied together. I opted to not have control joints cut into the concrete and in 6 years no cracks yet in a 60x64 pad. I believe that with the solid base, attention to the concrete wire reinforcement and the give and take of the foam bed should allow the pad to float with reduced stresses. I have paid a lot of attention to the water drainage around the building to prevent it from migrating under the pad. I am not suggesting that this pad will not crack, but I am curios to see if it does while I am in possession of it. Should it crack, it should hold tight with reinforcement near center of the slab.

 

[wssix99]

So if the base is solid, well packed and stable the concrete will not crack if poured properly. The problem is that with varying moisture levels and temperature changes the base will move leading to areas of tension in the concrete.

A good base eliminates one avenue of cracking. There are still other causes and things that need to be addressed.

As the concrete cures and water evaporates, the pad shrinks. As the pad shrinks, it wants to crawl over the base to shrink towards the center, which it cannot do because it's heavy and the base creates friction. Eventually, things give way and the pad breaks into sections that can shrink locally. (The saw cut joints entice this to happen exactly where the cuts are placed.)

heavy wire reinforcement panels on chairs and rebar in selected areas all wired tied together. I opted to not have control joints cut into the concrete and in 6 years no cracks yet in a 60x64 pad.

All concrete cracks, but we can reinforce the pads so much that the cracks are super-thin and not visible. I imagine your reinforcing is over this threshold. (The American Concrete Institute also has equations where one can calcualte the exact amount of rebar needed to space out saw cut joints or eliminate them entirely.)

 

[Montyx5]

As the concrete cures and water evaporates, the pad shrinks. As the pad shrinks, it wants to crawl over the base to shrink towards the center, which it cannot do because it's heavy and the base creates friction. Eventually, things give way and the pad breaks into sections that can shrink locally. (The saw cut joints entice this to happen exactly where the cuts are placed.)

I agree, but this shrinkage is controllable to the point where cracking is extremely unlikely. When I referenced concrete being poured properly I had things like: cement to aggregate ratio, aggregate size and quality, water ratio, curing temps, water bleeding vs evaporation rate, etc.. The shrinkage rate is controllable, even reversible while the concrete is wet. Once it is dry you have no more control.

The one thing I would suggest for some one that is looking to have concrete poured, especially if it is a large pour, is to take the time to talk to the material engineer at their local concrete plant. This cost nothing and it will ensure that you get the proper concrete composition for your situation and time of the year. The engineer I talked to suggested I increase depth of my pour by 1/2" to compensate for the hydronic tubing and to move up 1000psi to 6000psi, an increase in aggregate size and additional tri-calcium silicate to slow the cure rate. We also had a lengthy discussion about the requirements for curing after the surface had set. This is only part the battle, if the people pouring the concrete fall behind or try to reduce their work load by authorizing additional unspecified water to the mix this will decress the streangth of the concrete. Once the surface is finished and sets the work rarely is done and this work can make big difference in the end result.

 

[wssix99]

I agree, but this shrinkage is controllable to the point where cracking is extremely unlikely.

... and one uses reinforcement.

Most people will not use a lot of reinforcement in their slabs or pay for high-cement ratio mixes with admixtures that can be finished dry.

If planned propely, control joints aren't a big deal and can be grouted in after the cure. (Under expoxy coatings, they can be made invisible.)

 

[jives]

The 32 x 42 slab poured 5 years ago in my pole barn has not cracked one bit. Control cuts, 5" thick slab with wire mesh. Slab was poured over compacted fill, in some places 4' of bank run, topped with ~12" of gravel. Rigid foam insulation surrounds only the edge (except at overhead doors). Compaction was by a multi-ton road roller, not some rental place compaction tool.

Oh, and not heated. Not one sign of heaving. BTW, it was -11 F the other day. . .

 

[Sumboodie]

At my folks, if it's not heated, it'll bust up from the frost.

My Dad's main garage, which he built in the 90s is cracked all over. To the point that it could almost need a repour.