I'm looking to do a concrete countertop and would like some advice on where to make the seams.I was thinking either where the red or the yellow lines are. Thanks, greg
you dont need to seam it anywhere. just pour it in one shot, and make sure you have steel reinforcing mesh and the glass fibre additive in your concrete. its only 12 s 5 total size, so its an easy lift for 6 men. this way you dont need to worry about fit up or future cracking.
this is one of the reasons concrete countertops are used. they are no where near as durable as granite or silestone, but they don't need seams.
Use your "second thought" comment for your placement of the joints. With your faucet in mind you can lay a 3/16" or 1/4" diameter stainless steel rod at the front edge and two on the back edge oriented around the faucet layout. DO NOT use the standard steel rods you get at the big box stores. They will rust and pop the concrete! Make sure the rods go past the sink cutouts by at least 6". More is better.
I'd consider slicing at the right-hand red line, and then at the left edge of the sink opening (or just inside a radius corner). My reasoning is that you eliminate about 40% of your total seam length this way.
Symmetry is good, but with your countertop installed and in use, with various appliances and other objects distracting the eye, this is less of an issue.
Full disclosure: I've never built a concrete top, and therefore my answer doesn't account for practical issues. I'm responding merely from an aesthetic standpoint.