One way to turn a crappy bomb into a really powerful one is to constrain it in something that can be fractured.

There are 'blast blankets' used in demolition and highway construction that are balasted but still open enough that the explosion itself can vent. Basically the effect is to reduce the slope of the build-up of pressure which significantly reduces the amount of damage an explosion will do.

The concrete would have to cure properly to have any significant effect, which takes time on the order of weeks.

I obviously don't know, but I'm hard pressed to believe that the people involved didn't consider simpler methods before determining the method most likely to succeed was to use C4.

All I'm saying is that there's probably a good reason for the route they went with.

It wouldn't work because the bomb was full of nitroglycerin. Concrete is strong but it would only possibly dampen the explosion. Also recall that the bomb was set to detonate without any delay if it was subject to even slight vibration. I wouldn't have volunteered to cover it in concrete.