This paper summarises the performance of 60 experimental overlays constructed on six sites in different climatic areas of Kenya. The overlays varied from 30 to 170 mm thickness on roads carrying up to 0.75 million standard axles per year. The majority of overlays were asphaltic concretes but the experiments included both hot rolled asphalts and dense bitumen macadams.
The predominant form of deterioration was premature cracking which began at the top surface of the overlay and propagated downwards. The techniques of failure time analysis were used to identify the signficant variables on which the cracking depended. The cracking was always associated with age hardening of the bitumen, especially in the top few millimetres of the surfacing. No strong correlations could be established between cracking and deflection, radius of curvature, overlay thickness or traffic.
The characteristics of overlays which have not cracked to date have shown how mixes can be designed for long life but the tolerances are likely to be too strict for normal production methods. Other techniques for improving performance are being tested and further analysis is being carried out to try and define the critical degree of ageing.
Proceedings of the 2nd International Conference on Bearing Capacity of Roads and Airfields, Plymouth, 15-18 September 1986. TRL - Crowthorne, UK. pp. 25