What causes concrete to bleed and what can be done to avoid problems?
After concrete is placed and before it begins to harden the aggregate and cement particles tend to sink, which causes water to appear on the surface of the concrete. The amount of bleeding is effected mostly by;
• Slump ‐ High slump concrete will bleed more than low slump concrete if the slump is caused by excess water and not by admixtures.
• Air‐Entrainment – Non air‐entrained concrete will bleed more than air entrained concrete.
• Aggregate Gradation – Well blended aggregate mix designs will bleed less than those with gap‐graded aggregates.
• Standing water on the sub‐grade before placement.
All the bleed water should disappear or be removed before finishing operations begin. This will avoid damaging the surface by diluting the paste on top. One way to remove excess bleed water is by holding a garden hose on either side of the placement and dragging the water off.
How do I know when it's time to start the finishing process?
After straight edging and bull floating, a slight stiffening of the concrete is necessary before proceeding further. Floating and troweling a concrete slab too soon is likely to cause problems later. These operations should not be started until bleeding has stopped and the concrete is firm enough to permit the craftsman to walk on it without leaving footprints more than about ¼ inch deep.
The time that must elapse between bull floating and floating depends on many factors relating to bleeding, including ambient temperature, concrete temperature, relative humidity, and whether the material is in the sun or shade.