PERFORMANCE PREDICTION MODEL FOR BITUMINOUS MIXES

Abstract

Bituminous mixes are used as wearing and base course layers in a pavement structure to distribute stresses caused by wheel load and tire pressure also to protect granular material layers (base, subbase) and subgrade layer from moisture damage which accrue from the effects of water. In flexible pavement analysis, a load on the surface of the pavement causes two strains which are considered the main critical factors for design purpose. These are the horizontal tensile strain (εt) at the bottom of the bituminous layer and vertical compressive strain (εz) strain at the top of subgrade layer. These strains depend on properties of materials used in the different layers of a flexible pavement. Performance of bituminous mixtures can be defined by its ability to resist permanent deformation, fatigue cracking, moisture induced damage, thermal cracking, and the mixture’s overall stiffness. Aggregate gradation can affect these and other properties such as skid resistance, field constructability, and the asphalt binder aging characteristics. This thesis presents a study of the effects of aggregate gradation and type of bitumen binder on the properties and performance of bituminous mixtures. It also explains the effect of aggregate gradation and binder type in bituminous mix layers on life of a flexible pavement using a 2D finite element solution. The program is written in FORTRAN to predict the displacement in each pavement layers, tensile strain (εt) and the compressive strain (εz). Two types of mixes are selected in this study, Bituminous Concrete (BC) Mix for surface layer and Dense Bituminous Macadam (DBM) Mix for binder course layer. Three types of aggregate gradation (upper limit, mid limit and lower limit) for these mixes are selected as given in MORTH – 2001 specifications, and two types of bitumen binders; conventional bitumen binder (VG-30) and Polymer Modified Bitumen (PMB 40). In total 12 different mixes are studied. Optimum mixes are designed as per Marshall method of mix design which is currently used in India. Durability of bituminous mixtures was studied through retained stability, indirect tensile strength, and tensile strength ratio. Rutting potential of bituminous mixtures was studied through static and dynamic creep test as well as wheel tracking test. The static triaxial tests and resilient modulus tests were conducted on all the twelve mixes. The results of experimental work indicate that the performance of BC mix is better than DBM mix in moisture susceptibility tests and the moisture susceptibility of mixes with PMB-40 mixes is low when compared with mixes using VG-30. The reduction in retained ii stability, and indirect tensile strength (ITS) and increase in creep are evaluated for finer, coarser and normal gradation of aggregates to observe the effect of gradation on moisture susceptibility of mixes. The retained stability is least affected by gradation or binder type when compared with other moisture susceptibility parameters. The result of rutting potential indicated that the accumulated strain and rut depth in PMB-40 mixes is lower than in mixes using VG-30. Two gradation parameters σ1 and gradation ratio (GR) are introduced in this study to correlate moisture susceptibility and pavement performance of a mix with its gradation. These parameters can be determined from the grain size distribution of the curve and later can be used to predict the durability or rutting potential of a mix according to binder type. Rut depth models are developed using the results of static and dynamic creep tests and gradation ratio (GR). These models can be used to predict the rut depth of an asphalt mix at 60 0C, without actually doing the test. The static triaxial tests conducted at three confining pressures of 1.5, 3.0 and 4.2 Kg/cm2 provided the necessary data to determine the cohesion C and angle of friction φ. The Mohr's circle was used to obtain cohesion and friction angle. The shear strength of the mix determined from static triaxial tests is correlated with rut depth in wheel tracking test. Since shear strength of a mix depends on the confining pressure also, separate relations are developed between rut depth and shear strength at confining pressures of 1.5, 3.0 and 4.2 Kg/cm2. The relationship between rut depth and shear strength at 4.2 kg/cm2 confining pressure is found highly significant with very high value of coefficient of correlation (R2 equal 0.93). The relation is simple and can be used to predict rutting value of a mix from static triaxial test data. A linear and nonlinear finite element program is developed for the analysis of flexible pavements. The input data included number of layers, elastic moduli values of the various layers in kPa., Poisson’s ratio, thickness of the layers in mm excluding the subgrade thickness. The program provides stress, strain and deflection at desired points. The software is written in FORTRAN 90. The results indicated that vertical compressive strain and the horizontal tensile strain are very sensitive to the HMA layer thickness but the vertical compressive strain is more sensitive to the base and subbase layers thickness. The horizontal tensile strain and the fatigue life of mix prepared with PMB-40 was 1.6 to 1.9 times more than the mix with VG-30. Utilizing SPSS software with nonlinear regression model, fatigue model for flexible pavement section having bituminous base course (DBM) and wearing course (BC) was developed. This model shows that the fatigue lives are iii strongly related to the bituminous layer thickness and the gradation ratio of mixes. The elastic modulus of BC layer has more effect on compressive strain than tensile strain whereas the elastic modulus of DBM layer has more effect on tensile strain than compressive strain, but both εt and εz values are more influenced by DBM elastic modulus than BC elastic modulus.