Detergent as an additive to concrete?

My father used to add a dash of washing up liquid (detergent) to his concrete. He is sadly departed and I never thought to ask him why he did this: perhaps it reduced surface tension and made the concrete mix more easily? Would such an additive affect the strength of the concrete at all? I'm sure he was not doing this without having been taught it himself by someone else, although he was a chemistry graduate and teacher and so had a fair degree of scientific knowledge.