DEVELOPMENT OF HIGH PERFORMANCE BITUMINOUS MIXES FOR HIGHWAYS

Abstract

India has an ambitious plan for GDP growth at 7-8% annually in the coming years. It is essential that transport infrastructure is improved and expanded in tune with the economic growth. Road transport in India is carried by a vast network of about 3.3 million kilometers of roads. The design life of a bituminous overlay has reduced from an average 5-6 years in the past to about 3-4 years in recent years. The premature failure of bituminous pavements is due to rapid increase in the number of commercial vehicles coupled with substantial increase in axle loads and extreme variations in climatic conditions. In order to increase the life of a bituminous pavement, quality of bitumen needs to be enhanced. Quality of bitumen can be improved by incorporating additives at the laboratory level and their subsequentapplication in the field. Two types of high performance bituminous mixes comprising of Polymer Modified and Stone Matrix Asphalt mixes were studied in this thesis. Two types of polymers; Linear Low Density Polyethylene (LLDPE), a plastomer and Styrene Butadiene Styrene (SBS), an elastomer were considered. The performance characteristics of bituminous mixes prepared with bituminous binder modified by SBS, LLDPE and combinations thereof were studied. The rheological aspects of conventional and modified binders were also studied. Stone Matrix Asphalt (SMA) is considered to have high resistance to permanent deformation combined with durability. Synthetic fibers are normally used but they are not manufactured in India and are imported at a higher cost. Therefore, a study was conducted to find out the feasibility of alternate materials like jute fibers or crumb rubber modified binder (without fiber) for construction ofSMA in India and other tropical countries. Abstract The dose of polymers was optimized on the basis of Indian and SHRP specifications. The strength and permanent deformation characteristics of polymer modified bituminous mixes were evaluated by dynamic creep test and Hamburg wheel tracking device. The fatigue life of polymer modified mixes in tension (Indirect Tensile Fatigue test) and in flexure (Beam Fatigue test) was estimated. The effect ofdifferent types ofstabilizers (synthetic fiber, natural fiber and crumb rubber modified binders) on strength parameters like draindown, stability, tensile strength ratio and performance parameters like rutting, fatigue life and aging characteristics of Stone Matrix Asphalt (SMA) was evaluated. Theeconomic evaluation of all the above mixes is carried out using the values of tensile strain in bituminous layer calculated by finite element ANSYS programme and results are presented in terms of Traffic Benefit Ratio (TBR), initial cost of construction (ICC) and life cycle cost (LCC). Polymer modification increases the elastic response and complex modulus of the bitumen at high temperatures. The degree of improvement generally increased with the polymer content, but varies with the polymer type. It also reduces the temperature susceptibility of the bitumen. The viscosity of the modified binder was slightly higher than specified range in IRC: SP 53-2002 but meets the SHRP specifications. The elastic recovery of the plastomer binder before and after Thin Film Oven Test (TFOT) was lower than prescribed in IRC: SP 53-2002. There was improvement in the elastic recovery of combination binder both before and after TFOT. The strength parameters like tensile strength and Marshall stability ofpolymer-modified mixes are considerably higher than those of unmodified bitumen mixes. There is improvement in the strength characteristics of combination mix as compared to plastomer-modified mix. The resistance to permanent deformation of modified mixes is also higher as indicated by lower accumulated strain, lower creep slope andhigher creep modulus at high temperatures in Abstract dynamic creep test. Among modified binders, the elastomer modified mix performed better than plastomer-modified mix. The trend of results obtained from dynamic creep test was same as obtained from Hamburg wheel tracking test. The fatigue lives of SBS, LLDPE and combination modified mixes determined by indirect tensile fatigue test at 25°C were 4.6, 2.2 and 4.0 times higher as compared to conventional mix. The fatigue life of combination mix is slightly lower than SBS mix, but significantly higher than plastomer modified mix. The number of repetitions to failure decreases with an increase in temperature. The beam fatigue life of a mixes for a given tensile strain follows a patterns: SBS > Comb > LLDPE > 60/70. The results ofSMA mixes are compared with those of conventional bituminous concrete (BC) mix. SMA mixes are more resistant to moisture damage than BC mix as indicated by higher tensile strength ratio of former. However, Marshall stability of SMA mixes is less than that of BC mix. The dynamic creep test was not found reliable for SMA mixes due to nonconfinement of the Marshall specimen in this test. The wheel-tracking test indicated that rutting potential of SMA mixes was less than that of BC mix. Among SMA mixes, creep slope was minimum for SMA without fiber and with crumb rubber modified binder (referred as SMA-CRMB) and maximum for SMA with 60/70 bitumen and syntheticfiber (referred as SMA-SF). The resilient modulus of SMA-SF was higher than that ofSMA with 60/70 bitumen and naturalfiber (referred as SMA-NF). The increase in stiffness and tensile strength after aging was considerably higher in BC mixtures than in SMA. The dense graded mixes are more susceptible to cracking than SMA after they are age hardened. For a given tensile strain, the fatigue life of the mixes in the descending order was: SMA-CRMB > SMA-SF > SMANF > BC. There was substantial improvement in the fatigue life ofSMA mix after fibers were replaced with CRMB. The results of strength tests on SMA mixes obtained with natural fibers were comparable to the synthetic fibers. The mix prepared by using aggregate gradation as in Abstract specified for SMA and CRMB without fibers is expected to perform similar to or better than conventional SMA although it does notfollow the definition of SMA. The economic evaluation is worked out basedon performance parameters of fatigue life. The thickness of unmodified bituminous concrete layer was varied to study its effect on tensile strain below the bituminous layer using ANSYS programme. The tensile strain corresponding to a designed thickness of40mm was used to calculate the number ofcycles using beam fatigue equations for respective mixes. The number of cycles so obtained was multiplied by a shift factor to get the number of cycles corresponding to field conditions. Traffic benefit ratio (TBR) of the SBS modified mix was substantially higher than unmodified mix. The initial cost of construction of this layer was also substantially higher, but its life cycle cost was low. TBR ofcombination mix is substantially higher than that ofunmodified mix and close to SBS modified mix. The initial cost of construction is higher, but their life cycle cost is lower than unmodified mix. SMA mixes with fibers are slightly better in their TBR, but their initial costs are very high especially for SMA with synthetic fibers. There was significant improvement in the TBR ofSMA with CRMB mix. The initial and life cycle cost ofSMA mix also reduces with the addition of modified binder. The initial cost of SMA-SF is the highest amongst all mix. There was significant reduction in the initial cost with the use of natural fiber in place of synthetic fiber, butTBR of natural fiber was slightly low. Inorder to further reduce the initial cost of construction, crumb rubber modified binder was used in place of fibers. There was substantial improvement in TBR ofthis mix as compared to others mixes. From fatigue considerations, dense graded mixes containing SBS and combination polymer have higher fatigue life than other mixes. But the initial cost ofconstruction ofSBS IV Abstract modified mix is considerably high. The combination mix is considered the best from initial cost of construction and TBR points of view.