[LouisVanDerWright]

I'm not an engineer, but my understanding of that question is as follows:

Yes there is always a permanent "fault line" of sorts where two pours come together, but that the concrete actually does seal together fairly tightly. Remember that concrete is basically just aggregate (i.e. sand and gravel) with portland cement holding it together. So what is the difference between gluing together a bunch of little pieces of rock (i.e. the aggregate) and gluing a bunch of little pieces of rock (i.e. the aggregate in the fresh pour) to one big piece of rock (i.e. the hardened previous pour)? Not much, other than the fact that there is likely to be more air pockets along that line. So of course there is some degree of a fault there permanently, though it is bonded fairly well.

However, none of this matters when you understand how we use concrete. Concrete is all about compression, as was mentioned earlier. This is why they pour all of these floors in one big go. The rebar exists solely to makeup for the shortcoming of concrete which is that it is brittle. Each pour bonds so tightly together that the sheer forces would have to be immense to sever the rebar embedded through the fault between the pours. This wouldn't work so well if you had faults aligned with the pull of gravity (i.e. if you poured half of one floor one day and the other half after the first half dries, the vertical fault would be significantly weaker).

So short summary: concrete actually binds to itself fairly well, but we never use it in situations where how well it binds to itself would be a factor. Rebar is used in concrete to account for the same shortcomings that would make leaving a vertical joint in concrete a problem. You can think of it almost like legos, the block is concrete, the pegs are rebar. If you put compressive forces on a lego wall, it's extremely strong, but if you held the same wall horizontally and applied force, it would quickly crumble. The bricks are bound together quite tightly, but the pegs really carry any sheer force.