Rheological properties of modified asphalt and resistance to moisture damage of asphalt concrete mixes using polymer and portland cement additives

Abstract

Bangladesh highways are experiencing a remarkable rise in axle loads, pavement temperatures and significant period of inundation due to monsoon flooding and poor drainage induced water logging with manifestation of severe pavement deterioration such as rutting, raveling and stripping as a consequence. Highway Authorities are striving for modified asphalt mix, especially, on wearing course which can be effective to resist permanent deformation and moisture damage as above. In this research, Styrene Butadiene Styrene (SBS) and Ordinary Portland Cement (OPC) will be explored as potential additives in modified asphalt to achieve durable pavement in heavily loaded pavement of tropical climates. The main goal is to evaluate the effectiveness of SBS modifier in the context of rutting and moisture damage resistance. In addition, the effect of OPC as an anti-stripping additive is investigated to achieve pavement with improved moisture damage resistance. Initially, an attempt has been made to study the rheological properties of unmodified asphalt and polymer (SBS) modified asphalts with varying percentages of SBS. Various rheological tests viz, Dynamic Shear Rheometer (DSR) and Multiple Stress Creep Recovery (MSCR) tests are conducted and assessed; the obtained data from tests are analysed in order to find out suitable SBS percentage for permanent deformation resistance. The DSR test results for SBS modified binder shows that complex shear modulus is increased by about 172% which represent the increased resistance to permanent deformation; and phase angle decreased by about 23% showing increased elasticity at high temperature upto 840C. The rutting parameter (G*/sin δ) is significantly improved by about 200% for 6% SBS polymer. The MSCR test data shows that the highest recovery value is found as 82% for 6% SBS polymer whereas unmodified binder shows recovery value of 33% only at 3.2 Kpa stress level. Therefore, SBS modifier, particularly in the range of 4% to 6%, can be very effective in improving the elasticity of binders that can sustain extreme level of traffic at 400C to 700C temperature. For experimentation with asphalt mix using asphalt modified with SBS and OPC two separate aggregate gradations representing mid-gradation of AASHTO T11 and RHD wearing course aggregate gradation envelope are used. Using Marshall Mix design, optimum asphalt contents for unmodified and 6% SBS modified asphalt mixes are determined and comparative performance of 6% SBS modified and unmodified asphalt mix is evaluated. All specimens made of both AASHTO T11 and RHD mid-aggregate gradation using 6%SBS modified asphalt and unmodified asphalt meet the minimum stability value criteria for heavy traffic. Using 6% SBS modified binder, a significant increase of 60% in stability value is observed for AASHTO T11 mid-aggregate gradation and a considerable increase of 15% in stability value is observed for RHD mid-aggregate gradation respectively, when compared with unmodified binder mix. Moisture susceptibility test for unmodified, SBS and OPC modified asphalt mix is undertaken using Indirect Tensile Strength (ITS) test. SBS modified asphalt and OPC modified asphalt mixes satisfy the required tensile strength ratio (TSR) (above 80%) but unmodified asphalt mixes show TSR value lower than 80%. For treated samples of 6%SBS modified asphalt using both AASHTO T11 and RHD mid-aggregate gradation showed that the ITS value is increased by 38% and 37% respectively compared with unmodified binder. For OPC modified treated samples using 2% and 3% OPC the ITS values are increased by 33% and 37% respectively, compared to unmodified mix. Therefore, modifying conventional asphalt with SBS in the range of 4% - 6% or OPC in the range of 2% - 3% can be a potential solution for mitigating permanent deformation and moisture damage susceptibility of asphalt pavement in heavily loaded tropical climatic condition such as Bangladesh.