LAB STUDY ON CHEMICAL AND RHEOLOGICAL CHANGES IN MODIFIED BINDERS

Abstract

India having 3.34 million Km of road network is the second largest comuising of 65569 Km of NHs and 2000 Km of Expressway. Traffic on road is growing at the rate of 7 to 10% per annum while vehicle population growth is of the order of 12% per annum. India is having widely varying climate, terrain, construction materials and traffic both in terms of loads and volume. The traffic on Indian highways is mixed and quite different in character as compared to many other countries. In India, about 98 percent of the roads are of flexible type, therefore bitumen as a binder plays a key role in the performance related properties of bituminous mix. Most of the roads of India, affected due to increase in traffic density, overloading and significant variations in daily and seasonal temperature have been responsible for early development of distress conditions. Bitumens are viscoelastic materials behaving as an elastic solid at low service temperatures showing high stiffness and brittleness. At high temperature, it behaves as a viscous liquid exhibiting high ductility. This classical dichotomy creates a need to improve the performance of asphalt binders by addition of polymers. Binder modification results in improvement of properties of binder viz, fatigue resistance, stiffness modulus, rutting resistance, stripping potential, temperature susceptibility, oxidation potential etc. In India, the rheological characterization of bituminous binders is based on the properties such as penetration, softening point, ductility and absolute viscosity, which are inadequate to characterize the bitumen. The Dynamic Shear Rheometer (DSR) is used for determination of the rheological properties of bitumen over a wide range of temperature. The changes in rheological properties of 60/70 (VG 20) and 80/100 (VG 10) grades bitumen modified with different percentage of CR, EVA and SBS (2 to 8%) were studied. The rheological properties of the bituminous binders in terms of their complex iii modulus (G*), stiffness and overall resistance to deformation, storage modulus (G'), binder elasticity, loss modulus (G"), viscous behaviour and phase angle (5), viscoelastic behaviour were measured by using SR5 Ashphalt Rheometer. Testing was performed at temperatures ranging from 46°C to 70°C in increments of 6°C at a frequency of 10 rad/sec. Short term ageing was simulated by Thin Film Oven Test and long term ageing by Pressure Ageing Vessel. The changes in values of complex modulus were measured after both the ageing. Rutting resistance (G*/sin 8) was also determined at 60°C. The physical properties of bitumen improved with addition of the polymers and • crumb rubber. SBS modified binder gave higher softening point and better elastic recovery than EVA and CR modified binders. Following the ageing process, higher softening points as well as lower penetration values are found. The loss of volatile fractions contributes to the difference in weights between original and aged sample. The complex modulus of the modified binders is higher as compared to neat bitumen. The phase angle values illustrate the improved elastic response (reduced phase angles) of the modified binders as compared to their respective base bitumen. The phase angle is observed as 84.8° for 60/70 bitumen at 46°C. However, the phase angles of bitumen modified with 2% CR, EVA and SBS are 79.5, 71.1 and 72.7° respectively. The complex modulus increases with increase in percentage of modifier and decreases with increase in temperature. However, phase angle decreases with increase in percentage of modifier and increases with increase in temperature. The value of complex modulus of 60/70 bitumen at 46°C increased 1.5 times by adding 2% CR, 2.9 times by 2% SBS and 5 times by 2% EVA. The increase in complex modulus and decrease in phase angle of the modified binder indicate higher resistance to deformation as compared to neat bitumen. The parameter shear modulus (G*/sin 8) is an indicator of stiffness or resistance of binder to deformation under load at specified temperature. EVA modified binders were iv found to have higher rutting resistance as well as higher loss and storage moduli compared to CR and SBS modified binders. The increase in complex modulus after pressure aging vessel (PAV) aging is understandably greater than that after TFOT aging due to the prolonged aging process in the PAV. From the results, it is seen that CRMB can be used in low cost roads. EVA modified binders can be used in very high temperature and heavy traffic areas but it cannot be used at lower temperature as its ductility value is less. SBS modified binders can be used in all climatic conditions of low as well as high temperature. From the relationships of G*/sin 8 with test temperature before and after ageing, it was seen that 60/70 and 80/100 penetration bitumen meet the PG 58 specification requirements of S1-IRP. The optimum dose of the SBS and EVA was found to be 2% and of CR as 5% on the basis of performance grade 70. Compared to the nominal mix, there is slight increase in cost of 7.75%, 4.5% and 3.3% for SBS, EVA and CR modified mixes respectively. The chemical properties of polymers and CR as well as modified binders were also analysed using spectroscopy techniques and thermal analysis. Infrared analysis of modified bitumen indicates retaining of covalent bonding between modifier and bitumen. Electronic spectra studies in case of EVA modified bitumen indicate partial degradation after ageing effects. For SBS modified bitumen, it was observed that the some degradation occurred during preparation of modified samples. However, the quality was maintained even during ageing wherein high temperature and pressure was applied. The thermal analysis results for pure samples indicate that residue obtained after decomposition are 30%, 0% and 1.6% for CR, EVA and SBS respectively. The spectral and thermal analysis results suggest that SBS is the best material amongst three for bitumen modification to enhance the chemical properties during service life.