Concrete – Fixing broken piece of 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.

From the description and photograph, it appears that the crack is the natural result of the sort of stresses one would expect at a location with:

• Drastic change in slab width to extend the slab through the door opening.

• Drastic change in thermal stress where the slab transitions from interior temperatures to exterior temperatures.

• Drastic change in slab stiffness. Dynamic vehicle loads are applied sequentially from the rigidly supported portion directly above the wall below to the spanning portion where they create deflection in the supporting structure.

All of which is to say that repairing the joint does not mean filling it in. There will always be a joint there. Instead, repairing the joint probably consists of keeping water out using backer rod and a flexible traffic sealant rated for wide joints...and perhaps a metal joint cover.

There's no magic in a can that it going to defy the laws of physics or change the internal stresses of a concrete slab under dynamic vehicular loads. Any repair needs to be premised on acceptance that there will be a joint across the opening where the slab narrows.