I ran a layer of self levelling compound over my garage floor and it was a disaster; its all erupted and burst. I can only assume the existing garage floor was contaminated, despite being swept and washed down before the self levelling.
When the builders laid the screed on my garage a few years ago they had a large roller spike which they ran over the screed before it set
I wonder if this was to let air or gas out - it was on fairly new concrete
