How do I fill a wide expansion joint between my driveway and the street?

For a gap that wide, standard liquid sealants like Sikaflex are definitly going to fail beacuse they can't handle the sheer volume of material or the amount of movement in a two-and-a-half-inch span. I had a similar situation where the city street was pulling away from my driveway apron, and the best DIY solution I found was using a pre-formed **expansion joint foam** or a product specifically designed for oversized gaps like SlabGasket. Since you're looking at a 6-inch depth, you need to fill most of that void before you even think about sealing the top. I'd suggest grabbing some closed-cell backer rod that is at least 25% wider than your widest point. You'll probably need 3-inch diameter rod. Shove that down into the gap untill it's about half an inch to an inch below the surface. This creates a solid base and prevents the sealant from sinking to the bottom of the earth. But honestly, even with backer rod, liquid sealant at 2.5 inches wide often sags in the middle during the summer heat. A better move for these massive joints is a **compression seal**. These are ribbed rubber strips that you hammer into the gap with a mallet. They stay in place through friction and can handle way more expansion and contraction than a bead of caulk ever coud. They also look alot cleaner than a giant, messy puddle of self-leveling goop.

If you're dead set on a pourable product, look for a professional grade two-part polysulfide or a heavy-duty highway grade sealant. These are ussually sold at specialty masonry supply houses rather than big-box stores. Just make sure the backer rod is tight. If there are any gaps around the edges of the rod, the sealant will leak right past it and you'll waste fifty bucks of product in seconds. Keep the area bone dry before you start, or it won't bond to the concrete edges at all.

I had this exact gap size on my last place. I skipped the liquid stuff entirely and went with SlabGasket. It’s basically a hard rubber insert you trim to fit and pound in with a rubber mallet. It’s been five years and hasn't budged or cracked, plus I didn't have to wait for anything to cure before driving over it. If you go that route, just make sure you scrape the sidewalls of the concrete really clean with a wire brush first so it seats flush. It looks way more professional than a giant bead of caulk anyway.

Don't forget to check the sidewalls for any old asphalt or oily gunk before you drop anything in. If you're gonna use those big rubber gaskets, those walls gotta be clean or they'll just pop out when the temp shifts. I tried just jammin' 'em in once without a good scrape and they're a pain to redo. Also, if you go the backer rod route, maybe throw some sand over the top once you're done? It helps with traction so you don't slip on the giant rubber strip when it's wet out.

This is so cool to find! I just moved in and I’m obsessed with fixing up my driveway, so seeing these fixes for huge gaps is a total lifesaver. That compression seal sounds like a blast to install with a mallet! Speaking of the street edge, does anyone know if it’s okay to pressure wash the curb area before sealing, or will the water soak into the ground and mess up the new joint later? I'm worried about trapped moisture under the slab!

Spot on about the compression seals. They realy are the only thing that holds up against the sheer amount of road salt and heavy vehicle weight at the street edge. One thing people often miss when using those rubber gaskets or even the heavy-duty highway sealants is checking for 'lip' or height difference between the city asphalt and your concrete. If the street has sunk a bit, I ussually recomend slightly beveling the edge of the gasket with a utility knife before hammering it in. It prevents snowplows or tires from catching the edge and ripping your hard work right out of the joint.