Just a word of warning: When concrete is heated (anything but really quite slowly) to high temperatures, such as by a torch, it is likely to exhibit explosive spalling. Power washers can, however, not wash through concrete in the sort of timespan people are prepared to be there watching, unless it is unworthy of the name (i.e. it is about to crumble anyway), no matter how powerful the washer. Paint stripper application followed by wire brushing, repeated several times, followed by washing, should do the job.