Study on the rheological properties of polymer modified bituminous binder and mixes

Abstract

Conventional asphalts perform well on the majority of roads. However, demands made upon roads increase year by year. The increase in traffic volume, axle loads and tyre pressures has led to the need for road structures and materials to withstand the heavier loads. To enable pavements to meet this growing challenge, there now exists a wide range of proprietary asphalts made with polymer modified bitumen and a range of polymer modified bitumen for generic asphalts, all of which have been proven in service. Conventional bituminous binder is highly susceptible to both low and high temperatures. Its behavior is greatly influenced by temperature and other climatic factors like water, air solar radiation etc. These detrimental environmental agents in accompany with high traffic loading causes manifold deteriorations such as fatigue cracking, stripping, raveling, bleeding, undesirable deforming, rutting, potholing, age hardening etc in bituminous pavements even when the pavement is designed taking into consideration of anticipated traffic loads and soil parameters. To maintain traffic serviceable in these pavements, extensive and costly maintenance and rehabilitation works are needed. Many researchers in different countries have shown that polymer materials can be used to overcome the problems of low temperature brittleness, high temperature fluidity and to ensure improve binder behavior to fatigue cracking with substantially improved elastic qualities in flexible pavements. Additionally, polymer modified binder greatly reduces the stripping tendencies in bituminous mixes. These improved rheological properties of polymer modified binder increase the durability of pavement structure and thus require lesser maintenance and rehabilitation works. Though initial cost of polymer modified bituminous pavement is more than that of pure bituminous pavements, the incorporation of polymer materials in the bitumen can save money in this way that it provides longer life span than that of pure bituminous pavement and thereby decrease lifecycle costs of conventional pavements. The study presented in this investigation is an attempt to overcome the limitations of the study of previous researcher, Islam, M. S. [2003] who has worked on the almost similar topic. In this research, initially the laboratory tests of conventional and modified binder like ductility and penetration at varying temperature proposed for the previous study has been carried out because these tests weren't performed by the previous researcher. In this stage, Low Density Polyethylene (LDPE) is used as modifier to prepare the samples required for performing those tests. Secondly, a thermostatically and mechanically controlled blending device was prepared to try with different modifier other than pure polymer. It is mentioned here that the previous researcher was unable to blend waste polymer other than pure polymer for want of blender like this. Other objective of the research is to study the rheological properties of modified binder and mixes through laboratory experimentation. The qualitative improvement of polymer modified binder and mixes is studied accordingly by comparing their properties with that of conventional bituminous binder and mixes by conducting the tests regarding rheological properties so far as carried out in Transportation Engineering Laboratory of Bangladesh University of Engineering and Technology (BUET) as per laboratory facilities. It is also noted here that some important tests like Dynamic Shear Rheometer (DSR), Bending Beam Rheometer (BBR) tests etc for evaluation of rheological properties were not made possible to conduct for unavailability of required equipment for those tests at BUET Transportation Engineering laboratory. To fulfill the objectives of the study, first of all, a total of four modified binders and mixes are prepared with 2.5%, 5.0%, 7.5% and 10% LDPE contents to perform the test of penetration and ductility at varying temperature. For making this test sample, blending operation is done by the manually controlled blender prepared by the previous researcher. While performing these tests, the proposed mechanically and thermostatically controlled blender was being prepared. After completing the preparation of the proposed blender, trials are made to find out possible procedures and techniques to blend waste polymers with bitumen and through this trial a compatible recycled polymer (scrap tyre) is found. Then a total of four modified binders sample are prepared with 2.5%, 5.0%, 7.5% and 10% tyre polymer. After this, the rheological properties of fresh binder and tyre modified binder are evaluated by comparing concern parameters like penetration, ductility, elastic recovery, softening point, loss on heating, viscosity etc. In addition, a non-standard film thickness test is carried out to compare the binder film thickness on aggregate coated with fresh and modified bitumen. The performance of modified bituminous mixes is evaluated by determining stability, flow, density and void in the mixes. The study results conclude that properties like penetration, ductility, and specific gravity of the LDPE and tyre modified bitumen decrease with the increase of polymer concentration in bitumen while elastic recovery, softening point and viscosity increase with the increase in concentration of the polymer in the bitumen. Experimental results indicate that like LDPE polymer, tyre polymer also improves the binder's temperature susceptibility and consistency by significant amounts. The film thickness experiment conducted with solid steel spheres shows that the binder coating thickness increases significantly with the increase of the tyre content in the bitumen. With 10% tyre content, the increase of film thickness was about 101% as compared to that of the conventional binder. The Marshall stability results reveals that tyre polymer increases the stability values of the compacted mixes significantly with increasing tyre content in the bitumen. It is indicated further that the addition of 10% tyre in the binder increases the resulting mixture stability by about 32%. The flow values as obtained in the Marshall tests show slightly increasing pattern with the tyre content, whereas unlike stability, the density of the compacted mixes slightly decreases with the increase of tyre content in the bitumen. The effect of tyre on air void (Va), void in mineral aggregate (VMA) and void filled with asphalt (VF A) is found to be not substantial. The study also shows that the blending of bitumen with recycled polymer can be done by using mechanically controlled blender. The limitations, which is observed in this investigation has been mentioned at the last chapter so as to make possible to overcome that limitations for the researchers who are interested to work in this field in future. It is expected from the study that the findings of it would encourage the practical application of polymer modified binders in pavement construction in Bangladesh in near future.