What's bleeding in concrete, how to control it?

Bleeding is primarily the result of a too high water saturation level in the mixture, such that the aggregate particles cannot absorb the excess moisture. Instead of soaking into the rocks and sand, the water floats up towards the surface of the recently poured concrete  because it has the lowest density of all the ingredients in the mix.

adding more fine aggregate . Smaller particles have larger relative surface areas and are more likely to have a larger percent of their pores exposed, making smaller aggregates like sand much more absorbent than larger rocks. Adding more fine aggregate to the mixture ensures that excess water will be absorbed rather than allowed to float up to the top of the poured layer. Conversely, reducing the water content of the mix would also prevent concrete bleeding

Another way to prevent bleeding in concrete is to entrain the concrete mixture with air bubbles. These pockets of air allow for water to be absorbed into when temperatures drop below freezing (thus preventing the concrete from cracking ) but also allow for excess water to be trapped in the mix rather than separated from it.