Been chasing out a channel in concrete floor in a house built in the 60s. some of the concrete broke away in the bottom of the channel about 4 inches down, revealing a very very thin layer (under 1 mm thick) of what looks like tar/bitumen. Is it a 60s dampcourse? The concrete under this layer is undamaged. I'll be filling in later with cement so take it i just need to patch the holes up with some tar like substance before cementing it over?