Concrete – Preserving cracked concrete

Looking at your pic, I think you have a couple of problems to solve. the reason the bricks broke in the first place is that the base under the edge of the bricks is too low or uneven. This cause the edge of the bricks to break under pressure. I also see that the crack has traveled further to the inside brick and that one as well as the mortar joint is cracked.

I think you really need to remove the broken bricks, clean off the mortar best you can, then fix the base and replace the bricks and mortar. Simply use a cold chisel, and hammer to remove the broken bricks and mortar. Lay in some fine base, (stone dust, or similar), compact it well with a block and mallet. Moistening the base material helps a bit here. Dry fit the bricks to check the base height, then butter the replacement bricks, place them and point the joints.

I would also be checking the other edge bricks to assure there are no voids under the edge, or the same thing will happen to other bricks. If there are voids, try packing fill under them and along the outside border. The edges of these bricks need to be on a solid base or you will continue to have broken edges and cracked mortar.

You should check the label/product literature for the cement products you intend to use. From what I remember self leveling cement is not recommended for filling cracks deeper than 1/2". If you want to use it then you should patch any deep cracks first. Self leveling cement is also about 10x the cost of a bag of regular cement mix.

If the ground is still settling underneath it's best to just break up the section, compact the ground underneath and then pour in that new section. That's going to be your longest strongest fix.

If you're not sure if it's done settling I would just fill in the cracks and see if they patches crack again.

A lot is going to depend on the size of the cracks as far as the best product to use but for most cracks I like to use a fast setting concrete patching mix. Can fill large gaps and can also be feathered out at the edges. It sets up in less than an hour which I like for walkways. You can also use a regular cement with an acrylic fortifier added to it if you can keep people off the repair for 2-3 days as it will be much cheaper.

With the fast set you need to clean out any loose chunks, wet the concrete, make a thin mix of the cement that you paint on the crack to aid in bonding, then mix the regular thickness mix, trowel in the crack, trowel it smooth then use the paint brush to smooth out the edges. I like to build up the level a little higher and then scrape it off with the trowel after a few minutes.

All the concrete repair products I've seen are polymer modified but not to the same extent that self-leveling cement is. Adding an acrylic fortifier to the mix will give you a stronger bond.