Semi-flexible pavement surfacing, or grouted macadam, is an alternative to conventional flexible and rigid pavement. It is constructed by injecting cementitious grout into the voids of an open-graded asphalt surfacing. The cement used in cementitious grouts has adverse environmental effects because of the carbon dioxide emission in cement production. The objective of this study was to investigate the potential of using irradiated waste polyethylene terephthalate (PET) and fly ash (FA) as a (partial) cement replacement in cementitious grouts for semi-flexible pavement surfacing. This study sought to assess the stiffness modulus and fatigue properties of the semi-flexible specimens prepared with control grout, regular PET (2.57 PET + 10 FA) and irradiated PET (4.75 PET + 10 FA)-based grouts and compares the stiffness modulus and fatigue properties of semi-flexible specimens with the conventional hot mix asphalt (HMA) concrete. The semi-flexible surfacing specimens showed superior performance, higher stiffness modulus, and better fatigue life than the hot mix asphalt. The difference in fatigue cycles was apparent at lower stress ratios of 25 and 30. The semi-flexible pavement mixtures exceeded 100,000 cycles at the lowest stress ratio of 25, while the HMA fatigue cycles were less than 100,000 cycles. Furthermore, the semi-flexible specimen with irradiated PET (which contain a higher amount of waste PET than the regular PET) showed similar stiffness modulus and fatigue life as the specimens with regular PET and control grout. The irradiation technique offers a sustainable solution for recycling higher amounts of waste PET in highway materials for semi-flexible pavement surfacing. © 2022 by the authors.