Kenya Bituminous Materials Study

Abstract

The causes of premature failures in asphalt surfacings in Kenya have been identified. Two heavily trafficked sites had suffered top-down cracking, associated with severe hardening of the asphalt in the surface of the wearing course, whereas others had deformed plastically. Tests confirmed that reduction of voids in the mix (VIM) to less than 3 percent resulted in a high risk of plastic deformation under heavy traffic. Difficulties arise in designing asphalt mixes for heavily trafficked roads in countries where laboratory equipment is limited. These difficulties include selection of appropriate levels of compaction in the Marshall test and predicting the combined effect of asphalt hardening and secondary compaction. Use of a refusal density test is recommended to determine a reference density to ensure that 3 percent VIM will be retained. The importance of preconstruction field trials to establish the workability of such a mix is emphasized. Acceptable results were most easily obtained with aggregate gradings for binder course or roadbase layers that fell below the Superpave restricted zone. Such materials must be sealed to prevent premature top-down cracking associated with age hardening of asphalt in the layer surface. Performance tests, such as those used in this study, are ultimately required to ensure good performance of asphalt surfacings. Until developing countries can acquire the equipment required for these tests, it is recommended that the procedure described here be used to reduce the large number of premature failures such as those currently occurring in Kenya.