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New shop concrete slab

Carol Reed

Carol Reed

In Memoriam
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Coolidge, AZ
Inundated with concrete information. Assuming a monolithic pour with a smooth finish for a 20'x24'x4" slab in central Arizona. No ground frost issues. Thoughts, suggestions, gotchas, etc.?
 
My concrete guy down here used the fiberglass fill mix on three jobs for me. My shop addition required a footer around three outside edges that included rebar, but that was all the rebar required. I had a few trees removed to the tune of about $800. Several yards of sand were required to fill the area. Overall cost for the 20' by 16' slab was about $3500.
 
My dad was a civil engineer who specialized in concrete, soils, and asphalt for over 50 years. Towards the end of his career, he told me the technological advancements in concrete mix designs and chemicals had outpaced him, and he allowed as how the younger guys in his office knew more about it than he did. My personal experience as a concrete tester/inspector was about 30 years ago, so what I knew back then might no longer be relevant, either.

Two of the three things my dad taught me about concrete are probably still true: It gets hard, it cracks, and it's gray. The latter isn't necessarily true with the popularity of dyed concrete mixes, but unless you really need an orange or blue shop floor, gray is probably still the least expensive way to go. :D

I'd suggest finding a reputable local concrete contractor to see what he recommends for your specific use case. Let him know what's important to you. Different locales call for different measures, and a local guy will likely know best what works locally. The Internet is just going to be a lot of local opinions that may or may not apply to that part of Arizona. Weather and temps are one consideration, but local soil conditions, as well as what mixes the local plant produces with the aggregates available to them also play an important role. (The properties of the locally-available sand and gravel are key when designing a concrete mix. What works with gravel from Abilene may not work with gravel from Albuquerque.) The local guys should already know what works. Same thing with stuff like fibers, reinforcing mesh, and other add-ins. Usually there are some trade offs. If low cost is the biggest concern, it's quite possible you may get some minor cracks. If cracks are a big issue, you're likely to pay more money.

As an aside, I saw you mentioned Tuff Sheds in another post. We've been very happy with ours. We went with the Premier Pro line. The extra cost for more robustness seemed justified. I doubt I could have built it myself for much less than what we paid, and for sure I wouldn't have gotten it stood up and finished in a day.
 
Plan to use the Tuff Shed concrete guys. They should get it right. Mr friend Phil loves his. Had it done a number of years ago and still looks and functions as new. I agree with the cost being little more than I could do myself (if only I was 40 years younger!). And NEVER in the same time frame. I too am looking at the Pro Ranch line. Did you get the Tall or the regular?
 
We got the regular, since it was only going to be for storage and we didn't want it to stick up too much higher than the backyard walls. If I was planning to spend time working in it, I think I would have opted for the tall version.
 
Your slab is only as strong as what is supporting it. Make sure you have a good solid subgrade before placing the aggregate or granular base. On an aside I would recommend installing plywood over the concrete slab. Easier on the knees, also the tools when dropped.
 
Have plastic sheeting ready, and after it's been poured, cover it with the sheet. The longer you can keep it from drying quickly, the harder the concrete will be. I've also removed the sheet daily and sprayed water on the surface, returning the plastic as soon as I had wet the entire surface. In about a week you will be able to remove the plastic and build your shop. If the concrete mix was good and you have a solid base under it, I doubt that you will ever have a problem. If it dries too fast it will develop shrinkage cracks and not be very strong. Slowing the curing and drying will give you a good strong floor.

Charley
 
Charley's advice about curing is very good, although (at least in this part of the country) most if not all concrete contractors use a spray-on membrane curing compound instead of plastic sheeting these days. The result is the same or better, assuming the spray coverage is sufficient. The idea is to seal the surface to slow the chemical reaction where the cement forms chemical bonds with water molecules and become hydrates or hydration products. Air can get between the plastic sheets and the slab surface and you can get dry spots. A sprayed membrane cure avoids that.

Side story: As a general rule, the longer concrete stays wet, the stronger it gets. Years ago I worked for an earthwork company, and we sometimes partnered with a demolition company to tear down old buildings. (They'd tear things down and we'd flatten the building pad.) I remember once when they won the bid to tear out an old car wash. Their estimator had allowed two days to remove the slab. In the end, it took them nearly two weeks because it was so tough from being wet most of its life.
 
:thumb: Good one, Vaughn!

Thanks for the heads up on a couple of points. I feel better prepared to talk to the concrete guys. Keep the suggestions coming. The joys of building a new shop should be shared!
 
I trust that when you talk monolithic slab, it includes turned down/thickened edges with anchor bolts to secure your frame and support the load, right?
 
Another suggestion is to have the contractor cut a control joint on the center line in each direction. One of the only guarantees about concrete is it will crack, control joints are intended to control where it will crack. Rule of thumb is 1/4 of the slab thickness or in your case cut 1" deep. If your contractor does not have the capabilities to saw cut control joints find another contractor. Preferably they would be able to green cut the slab the same day it is poured otherwise cut it first thing the next morning.
 
If you will have any underground services, or think you might one day, make sure you block out a place for these services where they will enter before the concrete pour. Don't forget drains, etc. Instead of doing this it is sometimes better to install conduit and drains in their proper places before the pour. Consider a couple of extra conduits for the phone, video, and LAN cables too. You may never need them all, but it's better to have extras than to leave them out and have to break the floor up to add them later.

If your building floor will be at ground level, make certain that the floor is slightly higher than the surrounding ground, so it doesn't become a wading pool during heavy rains. I would rather buy a load of soil to build a slight ramp up to floor level after the floor is poured than risk the chance of the floor becoming a wading pool. They slope garage floors just slightly toward the door so fumes and snow melt will flow out of the garage under the door. You may or may not want to do this for your shop because a floor with any slope is frequently a pain when leveling machinery and benches.

My next shop will have a poured concrete or concrete block wall so the wood wall is above the rain run-off splash point (18 - 24"). Maintenance of exterior wood walls at this level is a major headache. I'll let the splashes hit the masonry and not the wood in my next shop. The sill plate and any other wood in contact with the masonry will be treated lumber as well and I will size the siding so it hangs down over the joint between the masonry and the wood. I'll also install anchor bolts in the masonry so that I can bolt the wood frame to the masonry. Doing this while the concrete is being installed is easy. Drilling holes in the masonry for anchors after the masonry is cured is very hard to do.

Charley
 
steve ramsey said:
Another suggestion is to have the contractor cut a control joint on the center line in each direction. One of the only guarantees about concrete is it will crack, control joints are intended to control where it will crack. Rule of thumb is 1/4 of the slab thickness or in your case cut 1" deep. If your contractor does not have the capabilities to saw cut control joints find another contractor. Preferably they would be able to green cut the slab the same day it is poured otherwise cut it first thing the next morning.
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A couple other things about control joints: Concrete wants to be square. A 6' x 12' slab will almost invariably break into two (roughly) 6' x 6' slabs without a control joint. The rule of thumb I was taught for control joint spacing is to take the slab thickness in inches, double that number, then turn it to feet. So ideally a 4" slab would ideally have control joints to turn it into 8' x 8' squares. This rule isn't written in stone (or concrete, lol), and it's a pretty conservative number. For the 20' x 24' slab Carol's looking at, dividing it into four 10' x 12' "squares" should be just fine as long as the subgrade is properly prepared and compacted. (Especially considering the relatively light loads the slab will be subjected to.)

I used the "double the inches and turn it to feet" rule of thumb when I was given the task of designing the control joint placement for the aircraft parking aprons at the ABQ airport in the mid '80s. Those slabs are unreinforced and get a lot of heavy wheel loads daily, and after 30 years, the only places where they've cracked other than the control joints are where odd shapes prevented me from being able to make squares. (Triangle shapes are problematic, for example.) For the most part, those slabs are 18" thick, so we kept the CJs no farther than 40' apart, with 36' being the target. Where the slabs are 9" thick, we shot for 18', and lived with 20' as a max. The subgrade below the 18" slabs was phenomenal, though. Below the concrete is 6" of asphalt compacted to 95%, below that is 18" of aggregate base course compacted to 98%, and below that is 24" of engineered soil fill compacted to 95%.
 
OK. I will be on a red clay caleche type soil. How deep should the substrate be and what size rock? Assuming gravel substrate. Building will not a be a garage. There will be 6' opening for a pair of 3-0 doors. There may also be one 3-0 people door. Single story with overhangs to match the house.
 
Carol Reed said:
OK. I will be on a red clay caleche type soil. How deep should the substrate be and what size rock? Assuming gravel substrate. Building will not a be a garage. There will be 6' opening for a pair of 3-0 doors. There may also be one 3-0 people door. Single story with overhangs to match the house.
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Short of hiring an engineer (which I wouldn't do for building a shed/shop building), I'd talk to the local concrete guys to see what others have used on that type of soil for that type of building. Caliche/clay mixtures are generally pretty stable as long as they don't get saturated. I've helped build river channels and dams out of soil/cement mixtures that are cheaper than concrete and hard as a rock when compacted and cured. Caliche soils are nature's own soil cement. And they can also be pretty impenetrable to water. That's a good thing.

If the building pad is already flat and level, there's a possibility that as long as you have good drainage to keep it from getting wet under the building (or collecting water on the edges of the building), you may not have to do much more than put up forms and pour the concrete on lightly moistened native soil. If you do end up moving soil to level the building pad, or if you add material for a subgrade, it will need to be compacted. Here's an informative article about subgrades and compaction:

https://www.concreteconstruction.net/how-to/site-prep/proper-subgrade-prep_o

Locals will also likely have suggestions about what material to use if you do need to add any. Either crusher fines or crushed rock base course, moistened to a workable moisture content and compacted, would probably be a pretty safe bet. The gravel size isn't as important as the number of fractured faces on the rock particles. Crushed rocks have jagged faces that help them grip adjacent rocks when they are mashed together (compacted). Smooth rocks don't stick together as well. So you can use small particles (crusher fines) or a mix of small and larger larger ones (base course) and get much the same result/benefit. As a rough rule of thumb, smaller rock is easier to work with, larger rock is less expensive. (Or at least that's how things were here in NM 30 years ago. I suspect it's still much the same.)
 
Another little aside: In a lot of areas, "caliche" is a frequently misused term for "hard dirt". True caliche is a mixture of calcium carbonate binding other materials like rock, sand, silt, or clay. The calcium carbonate itself is whitish in color, so "red caliche clay" might just be "hard red clay", in which case my comments above about soil cement may not apply at all. Yet another good reason to simply find out what building methods have stood the test of time for the locals and run with it. ;)
 
Vaughn McMillan said:
... I'd talk to the local concrete guys to see what others have used on that type of soil for that type of building. ...
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Shortly after we moved to Thomasville, I met a man whose company builds commercial metal buildings, including some that required perfectly flat concrete floors. He recommended a local concrete guy who had done a lot of work for him, but also worked on small jobs. All it took was one conversation with the concrete guy to know he knew his business.

Since we live outside the city limits, rules are fairly open for homeowners. I didn't have to use and architect or engineer or pull permits for any of the concrete work I had him do.


Vaughn McMillan said:
Another little aside: In a lot of areas, "caliche" is a frequently misused term for "hard dirt". True caliche is a mixture of calcium carbonate binding other materials like rock, sand, silt, or clay. The calcium carbonate itself is whitish in color, so "red caliche clay" might just be "hard red clay", in which case my comments above about soil cement may not apply at all. Yet another good reason to simply find out what building methods have stood the test of time for the locals and run with it. ;)
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In Texas, 'caliche' is basically crushed limestone. The soil here in Georgia around where we live has a high clay content. Unless we've had a ton of rain, you can drive over our lawn with no problem. With that kind of base soil, we didn't have any grading to do.
 
I certainly plan to ask the local guys. However, I ask these questions because I really want to know, especially the 'why' of things. Secondly, guys would not believe how often girls are shined on because it is assumed we don't know anything. I enjoy popping the bubble!
 
Carol Reed said:
... Secondly, guys would not believe how often girls are shined on because it is assumed we don't know anything. I enjoy popping the bubble!
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Now c'mon Carol...we all know you're just a delicate little flower. Don't you worry your lil' head about the details and let us men-folk take care of things for you. :rofl: I pity the fool who tries to baffle you with bull. ;)
 
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