Cracks in the Roof

http://usa.sika.com/en/concrete-products/concrete_home.html

I would consider Sikaflex SL for the job. It's a self levelling sealant made for the task that you mention.

As an aside, if you hate throwing away the remaining portion of a partly used tube of silicone because it has gone off, or even if you hate cleaning out the nozzle because that bit has gone hard, then put the tube in the freezer after opening.

The product will remain workable right up to its use by date, and even the bit in the nozzle will stay soft, no need to plug it, just wrap a bit of glad wrap or similar over the end to prevent it getting on other stuff in the freezer.

When you want it againg, just give it a few minutes to come up to ambient and go for it.