Investigation of causes of failures of flexible pavements: a case study of Nairobi-Thika highway

Failures of flexible pavements (bituminous roads) have been prevalent in many developing countries, a trend causing concern to both the road users and the roads authorities. In this regard, it was found necessary that an investigation be initiated to determine the possible causes of such failures, and a case study was carried out on Nairobi-Thika Highway in Kenya. The investigations were conducted in several categories to determine the pavement characteristics such as structural strength by Benkelman Beam Deflection Test (BBDT), skid resistance by British Pendulum Tester (BPT). Traffic loading by Axle load measurements, Traffic volumes by traffic counting and ageing characteristics of the type of the bitumen used by cutting cores and extracting the bitumen for the standard tests, such as penetration, ductility, bitumen content, air voids, softening point and CBR using the Light Drop Weight Tester (LDWT). The visual inspection of the pavement showed that due to the hardening of the bitumen and overloading, cracks and potholes were very common for the whole stretch of the road. Axle load data revealed that, there has been a tendency to exceed the maximum allowed axle load limits, especially until 19th august, 1999, after that the enforcement of axle load limits was tightened. This fact together with the revelation that the road is a very busy one as depicted in the traffic counts, could be one of the major causes of the road's fast deterioration. (Journal of Civil Engineering, JKUAT: 2002 7: 13-26