I'm still shocked they demonstrated over concrete like that. You know it's permanently damaged now. I HATE doing hot work over concrete if I can't cover it with steel or catch the slag in a bucket. Burnt / spalled concrete in the wake of a job is the sign of an amateur.[/QUOTE]
Often, classes like that are held at locations which are about to be demolished. Not for sure, but it may explain why damaging the concrete wasn’t a concern.