Lean Concrete Base

Lean concrete base is constructed in essentially the same manner as conventional concrete. The contractor can utilize the same equipment for construction of PCC and the base, thereby distributing the mobilization cost to a greater scope of work. In addition, it may eliminate the need to hire a subcontractor if a different type of base (e.g., asphalt-treated base) is to be constructed. The keys to achieving a successful lean concrete base material are as follows:

• For reasons explained earlier, the strength of lean concrete base should be targeted tobe between 750 and 1,200 psi. Lean concrete material meeting this requirement does not need joints. Shrinkage cracks will develop but will not reflect through the PCC slabs.
• Best control of surface grade is achieved by lean concrete base. The finished surface should be within ± ¼ inch of the design profile grade.
• The surface of lean concrete base should be left untextured to prevent bonding to the PCC slabs. Application of a bond breaker,such as two coats of wax-based curing compound, can further prevent bonding.
• Lean concrete base gains strength overtime, similar to conventional concrete. As such, lean concrete base strength development must be factored in project sequencing, such that the lean concrete base does not become excessively stiff at the time of PCC placement.
• In situations where lean concrete base strength is estimated to be too stiff (not necessarily due to misapplication of the specification but due to elapsed time), the lean concrete base surface should be notched at the same locations where the joints will be cut in the PCC.
• Current practice in Germany is to place a 0.2-inch-thick polypropylene geotextile interlayer as a bond breaker between lean concrete base and PCC. The German experience has shown that this practice eliminates the need for notching of the lean concrete base [Germany 2012, Leykauf and Birmann 2006].