> Thanks, Tim.
> Final pour? I don't understand. As I foresee it, I will be mixin and
> pouring all until it is done. I planned to put the wire and rebar on
> supports where they would be at half of the final thickness.
If they're right at half the final thickness, they'll contribute *nothing*
to preventing initial cracks. However, they will contribute considerably to
the final bending strength.
You can look this up for specifics but the general idea is to use the
reinforcement close to the side that's in tension, with just enough concrete
covering the mesh and rebar on the tension side to minimize corrosion. That
means that you want it concentrated near the low side.
If you're into structural design, the point is that concrete has great
compression strength but almost no tensile strength. And the center of any
section -- half the final thickness of your lid, in this case -- is known as
the "neutral axis" in a member that is under a bending load. Structural
material on or near the neutral axis contributes nothing to initial bending
resistance. But once you get some small cracks on the down side, the steel
reinforcement is loaded, and it prevents further cracking. In effect, as it
appears to tension loads on the downside resulting from compression loads on
the topside, the steel is now the "bottom" of the section.
Still, the best strength is obtained by putting your reinforcement near the
bottom of a concrete slab of any kind that's going to have loads imposed
only from the top side. That's the way that most well-made sidewalks and
other concrete structures are reinforced.
>>> I drove my (new to me), big, diesel backhoe over the last tank in my
>>> two-tank septic system. Still, not too smart but I thought that I had
>>> cleared it by a foot or two. NOPE! So, one of the rear tires fell in
>>> the hole, not all the way but it competely broke the concrete lid on
>>> this concrete tank.
>>> Now, not only being dumb but being cheap, too, I want to make my own lid
>>> of about 6 inches thick and about 4 feet in diameter. I have only worked
>>> with concrete just a bit by pouring it into some holes for a few t-posts
>>> and a few vertical landscape timbers.
>>> I do have a small (unused by me) concrete mixer.
>>> Do I simply lay out some plywood on solid ground, build forms around it,
>>> put in some welded wire and some rebar and pour away? Yes, I will keep
>>> a center hole open and install some hooks to lift it on the tank.
>>> Does this sound like a plan?
>> Good thing you have a backhoe to put it in place with!
>> I suspect you need to suspend the rebar where you want it in the final
>> pour, although I understand that you can lift up reinforcing net by
>> reaching down through the glop and pulling up on it.
>> Surely someone's done this on a web page before. I know there used to be
>> a Popular Mechanics article on this -- my dad built my grandmother's
>> septic tank from the afore-mentioned article. Of course, to use _that_
>> one you'll need a Popular Mechanics collection that goes back to 1940 or