A research study was conducted at Twin City Testing Corporation in 1973, in cooperation with the North Dakota State Highway Department and the FHWA. The purpose of the study was to determine the effect on properties and performance of paving concrete with lignite fly ash substituted for various percentages of portland cement. The test program included both laboratory and field evaluations of compressive and flexural strength and freeze-thaw durability. The good performance of this concrete after 15 years of field exposure supports the original laboratory findings. The fly ashes used in this study do not conform to certain chemical and physical requirements of the current version of ASTM C618, the national consensus specification generally used as guidance for fly ash procurement; however, the nonspecification fly ash was used in pavement construction only after laboratory testing indicated its potential for providing satisfactory performance.