Yes, slaves (almost all of subsaharan African ancestry) built many things in this country, but so did free laborers (mostly of European ancestry). If slavery were so productive that slaves' contribution outweighed that of the far more numerous free laborers, the loss of the Confederacy, which by and large had far better generals than the Union, in (take your pick, the Civil War, the War Between the States, or my favorite the Late Unpleasantness) would not have occurred.