Strength Of Flexible Road Pavements In Kenya

Abstract

The paper considers strength parameters for the materials generally used for pavement construction in Kenya. Laboratory tests were carried out to establish deformation modulus of common subgrade soils and to establish the stiffnesses of bituminous mixes of varying formulations used in road surfacing. The plate bearing test was carried out on crushed stone embankment to establish modulus of crushed stone layers used in road base construction. The evaluation of pavement performance was carried out on the basis of the field measurements of various factors including measurement of deflections, cracking and pavement distortion. Traffic studies were carried out to establish traffic loading of the selected test sections. Results of laboratory studies show that deformation modulus of subgrade soils tend to decrease with time under constant stress conditions to almost constant moduli values. For example Murram specimen showed deformation modulus values which fell from 2800 kg/cm2 to about 250 kg/cm2 after about 2 weeks under constant stress conditions. The stiffness values obtained on the basis of Marshall test for various bituminous mixes used in Kenya were found to be about a tenth of expected values. Results of the plate bearing test on an embankment of crushed stone gave moduli values which were considered lower than expected. For example tangent moduli obtained ranged from 696 kg/cm2 to 1371 kg/cm2. The results of the evaluation of pavement performance showed interesting trends. For example it was found that high pavement deflections are associated with periods of high rainfall and high air temperatures. Pavement cracking and distortion were also found to be associated with high deflections. (a) (TRRL)