• Clean: no foreign materials on surface, no excessive dust, no curing compounds or sealers

• Dry: depending on type of floor, adhesive or installation procedure used, maximum permissible concrete moisture level is 3 to 5 lbs/1,000 sq. ft./24 hours when tested using the calcium chloride test method

• Smooth: Steel trowel finish, no ridges, roughness or pitting

• Level: +/- 1/8 inch in a 10 foot radius maximum permissible deviation

Flooring material is unusable in this condition.
The image to the right is an example of excessive moisture in the concrete, accordance subfloor has caused bubbling in the material. The concrete is too "wet" for the adhesive and flooring system that was installed.

• Allows certain point- or mixed-elastic sports floors to be installed on concrete slabs with moisture levels of up to 10 lbs./1,000 sq. ft./24 hours
• Insurance against future moisture infiltration
• Relatively inexpensive when compared to other concrete moisture solutions

• Loose lay installation will not telegraph certain sub-floor defects as readily
• Sports floors can be installed sooner since the concrete does not need to be fully dried

• Properly sized, positioned and spaced continuous reinforcement (rebar) in the slab
• Practical low water-to-cement ratio concrete
• No purposely entrained air - Measured air < 3.0%
• High-tolerance placement & finishing techniques
• 7 day cover cure immediately after final finish
• Elimination of joints with proper design

Uneven concrete subfloor will lead to uneven playability 

Source - Peter Craig