Bricks, Masonry, Asphalt and Concrete - Sidewalk Joint filler

The original wood 1 x 4's used as expansion joints in my sidewalks have rotted out. I would like to use a pourable self- leveling material to replace the wood. I have only found caulking cartridges of material to use at a cost of $5 per cartridge. I will need perhaps 50 cartridges. That's more than I want to pay.
Can you recommend a good material that is sold in the bulk such as gallon cans that is cheaper? I will use backer rod and sand to fill the bottom of the crack and want to use the pourable material to seal the top 1/2 inch or so of the crack.
I assume that leaving the crack open (not sealed) exposes me to problems of water getting under the side walk slabs and causing washouts or heaving?

I assume that leaving the crack open (not sealed) exposes me to problems of water getting under the side walk slabs and causing washouts or heaving?

If this is an older side walk and roughly 3 feet wide then your assumption is incorrect. It won't change a thing here filling it up at all. The side of the walk is a BIG void for water to get under the walk. Even a hair line crack will allow water in.

Your walk sounds like the old ways we used to do expansion joints for swimming pools. That was because "tar" expansion was all that was available at the time and hot "tar" and bare feet / pool doesn't mix well. The best and cheapest way it to plane down treated lumber and re insert

The gallons cans needs a old fashion caulk gun that you actually fill. I hate to do those jobs. A " Pour Rock" would be to expansive also. keep it simple and cheap, Treated Lumber...

been replacing them for years around pools..