| Abstract: |
Over the past few years, instability rutting has more circulation in many tropical countries as a result of the drastic changes in truck axle loads as well as .tire pressures. Thereby a considerable numbers of researchers have devoted their efforts to study rutting problem all over the world. The results concluded that instability rutting was the result of lateral distortion or shear flow of the asphalt layer. Subsequently, increasing the shearing resistance of the asphalt mix can effectively reduce this shear flow (rutting). The results demonstrated also that tire pressure has more significant effect on rutting tendency of surface asphalt layer than wheel loads.The inability of Marshall stability test to predict asphalt paving mix . susceptibility to shear deformation increases the need for more fundamental test procedure that will permit this to be done. So, the Strategic Highway Research Program (SHRP) has adopted the Superpave shear tester (SST), under the believe that the shearing resistance of asphalt paving mix is the most related property to mix performance in rutting.The primary objective of the current study IS to establish new rational strength criterion, namely, Shearing Resistance (SR) for mix optimization against shear flow hased on triaxial test. Furthermore, setting of guide, maximunl permissible tire pressure (P max) to be used as stability limits forasphalt paving mixes to control the pavement rutting. Secondary objectives include : development of statistical models for more simplification of determining shear strength parameters (C & <1», developing statistical models relating the rational criterion to the mix and test variables, and study of the effects of different variations in mix component and testing conditions on the shear strength parameters and SR of paving mixes.To accomplish these objectives, the current study involved developing an extensive laboratory-testing program to define key mix parameter most related to permanent defomlation of paving mixes. The laboratory-testing program included standard Marshall test, Shell creep test, triaxial test, indirect tensiJe test and unconfined compression test. These tests were conducted on different paving mixes that simulate those mixes that are used in actual construction practices. Asphaltic concrete mixes with different aggregate type and gradation, difl’erent asphalt type and content were evaluated for their shear strength haracteristics under different test conditions.A statistical analysis computer program (SPSS) was used to study the elationships between the rut depth and the different mix properties, including arshall stability, Marshall flow, Marshall stiffness and the shearing resistance, weJl as developing statistical model relating shearing resistance to the mix d test variables. The same program was used to develop possible relationships etween the shear strength parameters (C & <1» from triaxial test and those pproximately measured from ITT and VCT. The relationships between C & <1> d Marshall stability & flow, respectively were also developed. In addition, the. flects of mix composition and testing conditions on the shear strength arameters (C, <1> & SR) were also investigated.
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