BEHAVIOR OF BITUMINOUS PAVING MIXTURE WITH VARYING Binder VISCOSITY

Abstract

This report presents the results of an investigation which was carried out in order to study the behaviour of bituminous paving mixtures when prepared with varying binder viscosity combi nations at mixing and compaction. All the existing specifications for using bituminous binder are based on empirical tests. It is being realized that these tests' are incompetent to specify the optimum temperature combination,adequate to employ for varying aggregate grading, bitumen type and prevalent climatic and traffic conditions. For eacn binder and aggregate grading, there exists an optimum vis cosity combination which improves, if adopted, the performance of ' the paving mix in its load-carrying characteristics. Studies by now have mainly used Marshall test criterion. From the wide variations of test procedures in current mix design methods, it is evident that tnere is alack of understanding regarding the fundamental behaviour of bituminous mixes. It is also being accepted that these mixes when placed in layers exhibit viscoelastic characteristics. Their load-carrying characteristic may be even qualitatively different from those predicted on the basis of laboratory (specimen) testing, when placed as an overlay. The structural action of this overlay would largely depend upon the mobilized lateral confinement and the support provided by the under lying layer system. To be able to concentrate more on the scientific and rational approach in defining the influences of varying binder viscosity combinations on the strength characteristics of the resulting paving mixes, the variables like aggregate type, grading Xlll and bitumen type and grade etc., were minimized. The present study was divided into three major parts. In the first part, the viscous characteristic of the binder used was established by using three different equipment for varying temperature ranges. Besides the sliding plate micro-viseometer and Brookfield viscometer that were available, the vacuum capillary viscometer was designed and fabricated in the laboratory. In the second part, the bituminous mixtures Type I and II which were used for the laboratory specimens were characterized with the influence of varying binder viscosity by performing several series of conventional laboratory tests. Included were the Marshall test, Hveem Stabilometer and Cohesiometer tests. Further tests were performed on laboratory specimens using unconventional test i.e. flexural strength test. Special equipment was designed for this purpose. In order to study the strength characteristics of the paving mixes prepared with varying binder viscosity combinations under the realistic conditions of (i) lateral confinement (ii) support of underlying layer system and (iii) viscoeiastic charac teristics of the materials, tests were carried out on the model bituminous (concrete) slabs in the third part. It was anticipated that the results would verify whether the variations in trie physi cal strength characteristics of slab mixes as observed in second part of this study, have any practical significance in the struc tural behaviour of the same when used as an overlay. For this part, the bituminous concrete slabs of 60 inches x 60 inches size and of varying thicknesses were prepared. XIV In all 39 slabs were investigated. Since the main objective was to study che structural action of these overlaps, the underlying system of base-subgrade was maintained constant. For casting slabs, a special bituminous mixer and the laboratory roller simulating the 8-Ton steel roller (as used in the field paving lobs in India) were designed and fabricated in the laboratory. Instruments like thermo-couples, pressure cells, deflection dial gauges etc., were also installed for the slab testing. The load-deflection responses of the slabs under rigid bearing plates of varying diameters were studied. Besides this, measurement of transmitted stresses under the surface loads was made at the subgrade interface. The results so obtained were critically analysed. With the limitations of materials, mixtures, and methods used in the present study, the following are main derivations : i) The quality control on the basis of scientific analysis of viscous characteristics of the bitumen binder as presented in this study, results in improved bituminous paving mixtures for their load carrying characteristics. This improved stability/strength of the paving mixes would result in appreciable savings in road construction costs. Even with wide variations in the testing concepts employed in Marshall test, Hveem and Cohesiometer tests and Flexural strength test, there exists a close agreement in defining the optimum temperature combination for a given bituminous mix. This observation is of significance to the field engineers who may thus employ any test procedure. XV ii) The model slab testing exnibits an excellent agreement with the results obtained on the laboratory test specimens. This suggests that the laboratory test procedures as used here are self sufficient for the scientific designing of bituminous paving mix based on binder viscosity control, iii) The load-deflection responses of the bituminous concrete slabs prepared with varying binder viscosity combinations under the rigid bearing plates are represented by a two constant hyperbolic equation. Using this, it is now possible to predict the load-deflection characteristics of a bituminous overlay for its structural design. iv) Based on layered system theory, elastic moduli! of the bituminous pavements, Ep were computed to represent the variations due to the binder viscosity combinations used. It is interesting to observe that the usual assumption of constant modulus of elasticity with varying sizes of loaded area and thickness of rein forcing layer is found invalid. Values of Westorgaard's subgrade modulus K were w computed to represent the supporting capacity of the pavement slabs. The observed variations in these values were mainly due to viscous resistance offered by the different slab mixes,- and these variations are critically analysed and reported in this work. v) The perimeter-shear theory is observed to be suitable for judging the excellence of the bituminous slabs. Based on the parameter of rigidity modulus R, nen design ' XV1 equation is developed. The possible correlation between elastic modulus Ep and rigidity modulus R is also worked out. With the developed equation, it is now possible to predict the structural behaviour of the bituminous slab for its design. vi) On the basis of load transmission tests conducted on the bituminous slabs, the relationships between the surface load and the transmitted subgrade pressure are defined. This approach seems to be of practical significance and with further work in this direction, there exists a possibility of developing a realistic method of pavement analysis. Attempts have been made to study the basic inferences of viscoelastic behaviour of bituminous pavement slabs. The results of rigid plate bearing tests carried out with varying time-function are analysed. The measured values of maximum transmitted pressures are compared with the computed values of Boussinesq's elastic theory. Based on the observations it has been established that critical stress magnitudes are induced in bituminous pavements when the wheel loads are static. It is hoped that the results of this investigation would help in reviewing the present empirical specifications of the bituminous paving jobs for better quality control and economy. Tne analysis of the test pavements as presented here would further help in better understanding the complexity of the problem of flexible pavement design.