Contractors in cold weather regions know that adequate air content in concrete is required for freeze/thaw durability. Most concrete contractors also know that the air content can be as difficult to control consistently and to predict as the weather. Any number of construction activities can affect air content. Addition of water at the jobsite, admixtures, qualification of the testing technicians, and different air testing procedures (ASTM pressure or unit weight test methods) can affect resulting air content measurements.
Even in concrete air content studies—when the tightest possible control was established at the batch plant, jobsite, and test site—specified air content was found difficult to maintain with consistency. Air-voids in hardened concrete may be either higher or lower than determined by ASTM standard test methods depending upon many factors.
This Concrete Perspectives discusses the real world problems associated with hardened air-voids in concrete.