About the research
Iowa has over 110,000 miles of roads, and they frequently support local residential vehicles as well as heavy trucks. Traditional bituminous surface treatments (BST) could be employed to maintain these roads properly, but these treatments typically require high-quality materials and standard construction equipment. Otta seal is a relatively new BST that allows for the use of more economical local aggregates and commonly available equipment. To evaluate the feasibility of Otta seal implementation, a Phase I study was launched to design and construct Iowa’s first Otta seal-surfaced road in Cherokee County in 2017. The successful construction of the first Otta seal road in Iowa provided valuable experience and knowledge for local public agencies, and a follow-up Phase II study was initiated in 2018. The Phase II study described in this report aimed to establish the recommended specifications through comprehensive laboratory evaluation and characterization and additional field implementation projects. In the Phase II study, more than 50 Otta seal sites constructed since 2017 were evaluated. These sites had various local aggregate and binder types, as well as various application rates. Five consecutive years of field measurements were executed in this Phase II study, including lightweight deflectometer-derived elastic modulus, International Roughness Index, dust generation, and skid resistance. The field evaluation in 2023 indicated that all Phase II Otta seal sites still exhibit satisfactory performance. A detailed laboratory investigation was conducted to establish a rational methodology for designing Otta seal road surfaces utilizing locally available materials. Four distinct aggregate types were considered in this study: limestone, recycled concrete aggregate (RCA), slag, and river aggregates. The McLeod method was modified to determine optimal rates for aggregate and binder application. The laboratory study primarily assessed Otta seal performance through a sweep test exploring the impact of material type, aggregate gradation, binder type, and application rate on aggregate loss. Key findings include the significance of aggregate gradation in reduced aggregate loss, the comparable performance of HFMS-2s and MC 3000 binders, the suitability of recycled materials (especially slag), and the effectiveness of the modified McLeod method. A life-cycle cost analysis revealed that the lowest cost was associated with Otta seal designed using the modified McLeod method. These findings contribute to a more efficient Otta seal design, enhancing road surface longevity while utilizing locally sourced materials.