How do I handle the control joints on this T-shaped section?

The biggest mistake people make in these wierd tie-in situations is trying to match up with every single crooked line from the surrounding slabs. If you chase those old joints, you're going to end up with a mess that looks intentional but wrong. You need to focus on the **re-entrant corners** first. Any inside corner where the new T-shape wraps around an existing edge is a guaranteed crack waiting to happen. You have to cut straight out from those corners to relieve that tension, regardless of where the old lines are.

I usally aim for a clean grid that makes sense for the new section while still acknowledging the general flow of the traffic. For a T-shape, you shold prolly run a long center cut that divides the main body of the T, and then cross-cut it to keep your individual panels as close to square as posible. Avoid making any panel more than 1.5 times as long as it is wide. If you exceed that ratio, the concrete will just decide where it wants to crack for you, and it won't be a straight line.

Since your concrete is dark grey and the old stuff is weathered, the contrast is allready going to draw the eye. Trying to force a connection between two lines that don't actualy line up will just highlight the wonkiness. Parallel lines look much more professional than diagonal or 'searching' cuts. Keep your horizontal cuts parallel to each other and perpendicular to the main length of the pour.

And don't wait too long to get that saw in there. If you're already at the point where the broom finish is set and hard, you're in the prime window for early-entry sawing. If you see those tiny 'spider' cracks starting to form near the mailbox or the corners, you've already lost the battle. Cut it now, keep the lines clean and logical for the new shape, and let the old slabs be thier own thing.

You relly ought to consider the isolation of that mailbox post with a pre-molded expansion joint or at least a perimeter wrap of closed-cell foam, as the differential movement between the rigid post foundation and the new slab will inevitably cause a radial crack if you simply pour against it without a proper buffer. Furthermore, while the previous advice regarding re-entrant corners is technically sound, you must ensure your saw-cut depth is precisely one-quarter of the slab's total thickness—not a fraction less—beacuse anything shallower fails to create the weakened plane necessery to consolidate the tensile stress during the hydration process.

This is all way more technical than I realized! I'm just starting to learn about this stuff and I didn't even know what a re-entrant corner was til now. If I'm trying to follow that 1.5 ratio rule for the panels, does the thickness of the slab change how far apart the cuts shoud be? Also, since the concrete is alredy dark grey, will the saw cuts show the lighter color underneath and look wierd, or does it all eventually blend in?

Quick update—focusing on the re-entrant corners first relly helped simplify the layout, so thanks for that tip. It looks way better than trying to match the old slabs. I’m still a bit nervous about the mailbox area, though. Since I missed the foam wrap during the pour, would cutting a small diamond around the post now still help prevent those radial cracks?