Bleeding isn't always bad. It lowers the water-cement ratio and densifies the concrete. But concrete that bleeds too fast or too long can cause a number of problems. Even if bleeding isn't excessive, finishing concrete at the wrong time causes a different set of bleeding-related problems.
THE BLEEDING PROCESS
Almost all freshly placed concrete bleeds. As aggregate and cement particles settle, they force excess mixing water upward. The total amount of bleeding or settlement depends on mix properties, primarily water content and amount of fines (cement, fly ash, fine sand).
EFFECTS OF EXCESSIVE BLEEDING IN DEEP SECTIONS
Sometimes bleedwater can't entirely evaporate because it has been trapped near the top surface by setting. This raises the water-cement ratio, increases permeability, and lowers strength. Excessive bleeding also causes some other problems in deep sections: heavy laitance accumulation at horizontal construction joints; bond loss at aggregate and rebar surfaces; and unsightly sand streaks.
BLEEDING PROBLEMS IN FLATWORK
Never float or trowel concrete while there's bleedwater on the surface. That's the cardinal rule of finishing. Finishing before bleedwater has evaporated can cause dusting, craze cracking, scaling, and low wear resistance. Working bleed-water into the surface also increases permeability.
HOW TO CONTROL BLEEDING
Excessive bleeding can be avoided. Don't add too much water to the concrete. Place concrete at the lowest possible slump. Add additional concrete fines to reduce bleeding.