I have poured several slabs of cement using premix that I mixed with my cement mixer. I've noticed that when I am pouring it next to a existing slab that over time the new slab always seem to shrink even though it was level when I finished the job. I'm wondering if the reason is because I did not compact the dirt enough before I poured the slab and that it is just a matter of settling, or if maybe the cement was too wet when I pour it? Any ideas?
Its just a property of concrete. It occurs as the concrete cures and does so at an exponentially decreasing rate. You can use type K concrete which is supposed expand if anything but in most cases it just doesnt shrink. Water does increase the inital shrinkage.