For instance, when Carthage was converted into a large parking lot (!), how did the Romans get rid of the buildings there in a safe yet timely manner? I am interested in the Roman's demolition methods because, as conquerors, I figured they would often be left with the task of demolishing buildings that they did not construct- and that might not of always been easy.
A couple of things: if you think that Scipio Africanus "salted the earth" of Carthage, he didn't. It's been a persistent myth from the 19th century, but Polybius and other ancient sources make no mention of it whatsoever. It may be based on the destruction of Shechem in the Book of Judges.
But, regarding the demolitions of structures/cities: They did it by hand. Several thousand, or tens of thousands, of fit men can accomplish far more demolition (perhaps de-construction would be a better word) in a few days than you might think. This was a military force that routinely built a fort to stay in every night while on the move. They had shovels, pickaxes, hammers, and men used to heavy labor. The structures they were demolishing, after all, were built by hand as well.
In any case, there would not have been a need to rush, particularly.
The destruction of Carthage would of course have been helped along greatly by the fact that the city burned extensively during the battle.
Sources: Complete Roman Army, Polybius I guess (how do you source a negative?), The Bible, Carthage Must Be Destroyed
Additional non-academic source: I watched six guys pull down a small house by hand and stack it in a couple of dumpsters in a single day when I was a kid.
What is the strategic value in razing a city? Seems like the empire in full swing was all about expansions and, once all the inhabitants are dead or enslaved, I figure it's a waste of a good city. Why destroy all the infrastructure if you're going to establish a colony or keep the old establishment from coming back?