No, no, not weed killer. Please don't use dangerous chemicals that seep into the water table.
Much easier and cheaper and less dangerous is salt. Use the cheapest you can find. Pour boiling water over the cracks, and then gently pour salt on top.
Works really well.
Not tried this yet myself but will be doing (I have hedgehogs so can’t use anything noxious) a friend said cheap economy cola, spray/pour it on weeds when its sunny and they die!! Like I say will be trying it very soon.