When it comes to structural crack repair, anything that doesn’t repair the structure itself is useless. Putting epoxy into a crack doesn’t repair the structure, it blocks the hole, however, the structure is still damaged and will continue to suffer from what’s causing the damage.
Epoxies have issues when it comes to repairing concrete cracks, mostly, that they do not penetrate the crack and they aren’t part of the physical structure, so as cracks continue to widen, epoxies break away from the concrete and leaking continues. It doesn’t take long for that to happen, usually within the first year or less.
Applying a filler to a crack requires penetration, the filler has to go inside the crack and get out the back of it, penetrating the crack and becoming one with the edges of the crack where the separation occurs. Concrete is used to repair concrete, there shouldn’t be anything else applied to fill the crack.
The other issue, and this is where compression comes into play, is that cracks don’t just appear and stay there. Cracks have pressure behind them that causes them to grow wider, longer and even shift. The only way to stop that force, as Einstein shares, is with an equal or opposite force. That opposite force is compression.