concrete theory

[kalakoa]

I've dabbled in concrete and am now looking at some larger projects, so have questions that aren't well-documented in any of the books I have.
1. The guy who poured my slab/footings says you should always have a layer of crushed rock at the bottom of the formwork -- even when pouring on pahoehoe plate -- because the slab needs freedom of movement in an earthquake.
2. Slabs tend to be at least 3-1/2" thick because it's convenient to make formwork out of 2x4. How thin is practical for foot traffic?
Basically I want a patio, and it would be "really easy" to pour a 2" slab directly onto the pahoehoe. So easy that it seems like it can't possibly be right, hence the above questions...
Thanks in advance, don't take any wooden nickels, etc.
.

[Seeb-2]

1) No - moving bad
2) if it’s basically decorative you can go thinner- richer mix, smaller aggregate, can use plaster mesh for reinforcement

[terracore]

1) I've never heard of crushed rock having anything to do with earthquakes. I thought it was for leveling, and to keep the soil from eroding under the concrete, or to facilitate drainage.

2) I've heard of using 3" for backyard sheds, but never anything as small as 2" except for pavers etc.

Also, I have very little experience with concrete so can only regurgitate pedantic knowledge.

[MarkP]

Both modern and ancient architects have employed mechanisms to let buildings stay still while the ground moves underneath them thereby sparing the building some of the abuse from earthquakes. I have long thought that a healthy layer of gravel would serve as a bond breaker between the moving pahoehoe bedrock and the slab at least as far as movement in the horizontal plane goes. I know nothing about the specific details though. I assume that letting the patio slab float would be good if the attached house is also floating and if the patio slab is firmly attached to the house slab. If not you just get each part floating off on its own.

If the pahoehoe is really right there at the surface then realistically all you are doing is building up the low spots to make things smooth and I would put the concrete right onto the pahoehoe. They sell topping mix for this purpose.

I can see the slab over gravel working very well in a new house build. It is the practice in areas with expansive soil to build a very strong slab, either a foot thick with rebar near both the top and the bottom or using pre-stressed post-tensioned reinforcement. In these areas the ground rises and falls something like a foot or more as the ground alternately dries and soaks up water throughout the seasons. If all this happened uniformly there would be no problem but what actually happens is that one corner of the house or another would drop a couple of inches relative to the rest if it were not for the super-slab. Since this is a yearly problem in some areas it makes sense to spend the money there. Here it might be an unknown number of decades before you get an earthquake where these expensive measures pay off. I wonder if you could even get contractors to do the necessary work since it is so out of keeping with how things are done here.

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https://www.homedepot.com/p/Quikrete-60-.../100318505

[dobanion]

I have had very good results with basalt mesh in thin slabs. Lay out formwork, lay down mesh, cut to shape and zip tie the pieces together. Now put the mesh aside, pour half the concrete, lay mesh down, and pour the rest on top. No worries about corrosion/rust either.

It's about $0.70 sq/ft.

[Seeb-2]

Is there a source of the mesh here

[eightfingers2.0]

Concrete theory in a nutshell:

Works better under compression.

Not so well under tension.

[dobanion]

Basalt mesh must be shipped in, far as I know. It is light, for what it does. 500 sq/ft of coverage in under 50lbs.