Advice on my first masonry block project

First post on this forum, looking for some feedback on a mailbox I plan to build. I’m pretty handy with woodworking but I’ve never worked with concrete before. So I’m hoping I can layout my plan and see if anyone has tips or sees problems. One main question: Say I pour some concrete one day and let it cure, then pour more on top of it. Will the two adhere just as well as if I'd done one complete pour? Or is there a major loss in strength? Here's what the finished product will (hopefully) look like. Attachment 44640 (http://www.doityourself.com/forum/attachments/bricks-masonry-cinder-block-paving-walking-stones-asphalt-concrete/44640-advice-my-first-masonry-block-project-complete.jpg) I plan to dig a 8-12" diameter hole about 2'6" deep and put 6" of gravel in the bottom and fill the rest with concrete to support a 2" diameter steel pipe. The pipe will run right up the center of the blocks. Attachment 44641 (http://www.doityourself.com/forum/attachments/bricks-masonry-cinder-block-paving-walking-stones-asphalt-concrete/44641-advice-my-first-masonry-block-project-interior-support.jpg)