Assessment of the Causes of Rutting in Asphalt Layers on the Morogoro-Gairo Road, Tanzania

Abstract: Rutting in flexible pavements is the gradual accumulation of irrecoverable deformation due to repeated traffic loading. In bituminous pavement layers, rutting can result from various factors, such as low shear strength or inadequate field compaction. Rutting that exceeds the allowable limit before the pavement reaches its design life poses safety risks for road users and leads to increased maintenance costs. In Tanzania, the causes of rutting in asphalt layers have not been thoroughly established or documented. This paper investigates the causes of excessive rutting observed in asphalt layers on sections of the Morogoro-Gairo road, using both field and laboratory studies. The studied road section is located in a moderate to dry climatic zone, with a maximum 7-day average pavement temperature of 61°C. Severe rutting, reaching depths of up to 140 mm, was recorded in this section. In some newly rehabilitated areas, in-situ air voids were found to be less than 1%, which is below the critical minimum value of 3% specified in design standards, indicating that the asphalt is unstable in the field. The aggregate grading complied with the specified job mix formula. This paper identifies an inappropriate mix design method as the primary cause of the observed rutting, emphasizing deficiencies in material selection and testing. To improve rutting resistance, we recommend adopting a performance-related asphalt mix design approach, shifting from empirical methods to advanced mix design techniques that could directly link asphalt mix design with pavement design for both new construction and rehabilitation projects. The paper also calls for improvements in construction quality control and advocates for increased investment in local research on asphalt and binders to establish appropriate criteria and specifications.