I have a concrete floor in my garage with some ugly cracks that keep getting bigger. I tried silicone caulk a year ago but it opened up again. What product actually works for this? Do I need to do something special before applying it, or just clean it and go?
I've had the same thing in my garage and silicone doesn't hold up because the concrete keeps moving, especially with temperature changes or if the floor gets a bit wet. What actually works is using a polyurethane or epoxy sealant specifically made for concrete - the kind that expands and contracts without cracking.
Before you apply it you have to clean the crack really well with a wire brush or compressed air to get out the dust and dirt, and if there's moisture dry everything thoroughly. Some sealants want you to leave the surface slightly damp but with no standing water, so check the instructions on whatever you buy. Apply it in layers if the crack is deep and let it cure for the time the manufacturer says before people walk on it or the floor gets wet.
The problem is that caulk is flexible but can't handle the constant movement of concrete, especially in garages where there are temperature and humidity changes; you need a polyurethane sealant or an epoxy compound that adheres better and is more resistant to those cycles.
What most people don't do: before sealing, widen the crack a bit with a tool (like a screwdriver or chisel) to give it an inverted V shape - that way the sealant anchors better into the sides and doesn't pull out as easily when the concrete moves. Clean out the dust well, stuff a foam backer rod into the crack first if it's deep, and then apply the sealant. That makes it way more durable than just running something over a flat crack.
Don't trust silicone for cracks that expand, because concrete moves constantly and it just can't handle that. What actually works is a polyurethane sealant or an epoxy compound, but before you get your hands dirty you need to do what most people skip: widen the crack a bit with a tool (like a screwdriver or cold chisel) so the sealant grips better, clean it thoroughly with a vacuum or compressed air to get all the dust and debris out, and let the floor dry completely for at least 24 hours. If the crack is wide or deep, some polyurethanes come with a low-density foam that goes in first to fill the space, and then the sealant goes on top. Concrete keeps moving with temperature and humidity changes, so you need something flexible but way more resistant than silicone.
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