It is mainly related to the water content as the more is the water content the weeker is the concrete. Hence if you notice that the concrete mix is harsh and not easily workable that's mainly a sign of low water content which is a good sign in general.

You can read the following paper which is open acces in Rilem and in researchgate

Cazacliu B., Mixing self-compacting concrete: mixers, mixing methods, mixing time, RILEM Proceedings pro090 - Rheology and Processing of Construction Materials, pp.57-64, 2013