1 STATE OF KNOWLEDGE REPORT ON RUBBER MODIFIED ASPHALT BY FINAL REPORT MAY 25, 2021 WILLIAM G. BUTTLAR, PH.D., P.E. PUNYASLOK RATH, PH.D. Professor and Glen Barton Endowed Chair in Flexible Pavement Systems Director, Missouri Center for Transportation Innovation Director, Mizzou Asphalt Pavement and Innovation Laboratory Department of Civil and Environmental Engineering University of Missouri-Columbia Research Scientist Department of Civil and Environmental Engineering University of Missouri-Columbia ON BEHALF OF