The next progressions in strength generating mixes, after CDF’s, are lean mixes. A lean mix is measured in PSI (lbs/square inch). A Lean mix is used in applications where a solid surface or sub-base is needed, never for walls or structural applications. Generally developing 2000 psi or less and placed in thicknesses of 2 inches or more. The mix can use up to 4 sacks of cement per cubic yard and contains both fine and coarse aggregates. Lean Mixes will pump if the cementitious value is high enough and there is Fly Ash in the mix to aid in pumpability. If lean mixes contained a higher content of cement they would become regular concrete.