Studies Of Flexible Road And Airport Pavements In Kenya

Abstract

This thesis presents results of a long-term study initiated in the late 1960s on flexible pavements in Kenya. The study was conducted through field and laboratory studies. It was prompted by the need for continuing research and development of rational design, construction and maintenance of a large network of paved roads in Kenya and increasing runway and taxiway pavements for rising air traffic. The geotechnical properties of some subgrade soils in Kenya have been presented. In addition a method for determining the optimum moisture content and maximum dry density from bulk density and percentage of water added to specimens during experimentation has been developed. The use of locally available construction materials has been found to be acceptable with good compaction and selection of suitable materials. Timely maintenance of flexible pavements and in particular protecting the pavement structures from ingress of water has been found to enhance their supporting capacity. The properties of the entire pavement structure have been shown to change with age and environment. The bituminous surfacing hardens with age while the base and subbase materials strength characteristics change upon loading. The net effect of all this is a continuously changing structural capacity of flexible pavements. The study of bituminous materials has shown that the voids content of the bituminous structure is a critical consideration. A minimum voids content is required to give the mixture the necessary elasticity while at the same time the voids contents should be minimised for the reduction of hardening of the binder. The hardening of the bituminous materials has been shown to be associated with increasing asphaltene content in the binder. The penetration of bitumen recovered from thin asphal surfacings has been found to follow a hyperbolic function with age. For the thick bituminous concrete surfacing, as the air voids content increase the bitumen penetration and ductility reduce with a power relationship. Evaluation of the pavement structures can be done by quantification of surface condition distress indicators. These include cracking, rutting, roughness and present serviceability index (PSI). A multiple regression equation has been developed showing that PSI is a function of rutting, roughness and cracking. A new maintenance criterion has been developed using a new total rut concept. The current roughness at which maintenance for road pavements was required was found to be high. In effect even when other surface conditions namely cracking, rutting and the present serviceability index indicated the pavement required maintenance, the roughness would pass the road as well performing. A new roughness criterion has therefore been proposed. The structural capacity can be established using deflection measurements and the dynamic cone penetrometer. Deflection data together with finite element analysis has been shown to be a method of determining pavement deflection and pavement stresses. When finite element is carried out with varying pavement moduli the modulus, which gives the same deflection as the field deflection is regarded as the pavement resilient modulus. A more relaxed specification for construction of pavements carrying light traffic has been proposed. For the more heavily loaded pavements including runways and taxiways, maintenance is a key element, which seems to have escaped the attention of those responsible for maintaining them. This is because timely maintenance and especially clearing of water under the sealed pavement layers is crucial in maintaining the structural capacity of the entire pavement structure.