PERFORMANCE CHARACTERISTICS OF WARM MIX ASPHALT

Abstract

India is a vast country, having widely varying climate and traffic in terms of both load and volume.. In general, roads in India are mainly bitumen-based roads. During construction of the bituminous pavement, the temperature of the bituminous mix must be high enough to ensure the workability of the mix. To counteract reduced workability of bituminous mix, generally the temperature is increased to reduce the viscosity of the bitumen binder and hence to improve the workability and flow of the bituminous mix. This increase in the temperature of the mix often results in increased plant emissions and fumes at the construction site. With the increasing prices of crude oil and depleting reserves of natural non-renewable resources over time, the need for adopting sustainable approaches in various construction activities is strongly required. The hot mix asphalt industry in India is constantly exploring technological improvements that will enhance the materials performance, increase construction efficiency, conserve resources, and advance environmental stewardship. These goals can be achieved by involving methods to reduce production and compaction temperatures of Hot Mix Asphalt (HMA). With the reduced production temperature of the bituminous mix the additional benefit of decreased emissions from burning fuels, fumes, and odour generated at the plant and the paving site as well as comparatively lower quantities of fuel consumption can be achieved. Warm Mix Asphalt (WMA) is a technology that allows significant lowering of the production and paving temperature of Hot Mix Asphalt (HMA). By reducing the viscosity of bitumen and/or increasing the workability of mixture, some WMA technologies can reduce the mixing temperature to 100ºC and even lower without compromising the performance of asphalt. When asphalt mixture is produced at lower temperatures, there are many potential benefits achieved such as reduced energy consumption (fuel) in the plant and reduced greenhouse gas emissions, improved working conditions, better workability and compaction. The mechanism of WMA is to use some additives or technologies to modify the rheological behaviour of asphalt binders, and thus improve the workability of the mixture at lower temperature. Several technologies and additives are available in the country today to produce WMA. The present study investigates the effects of three different types of warm mix asphalt additives on the properties of asphalt binders and mixtures. Two types of binders are considered in this research viscosity grade bitumen VG-30 and polymer modified bitumen PMB40, and three types of warm mix additives Sasobit®, Evotherm™ and Rediset® are used. The study is carried out in two parts; Part 1 is the Characterization of Warm Asphalt Binders, and part 2 is the Characterization of Warm Asphalt Mixes. Warm asphalt binders were prepared using two virgin binders (VG 30 and PMB 40) and three warm mix additives at three different doses, so as to create a total of 18 warm asphalt binders. The rheological tests were performed to evaluate the response of warm asphalt binders at different temperatures and different loading frequencies. It was reported by many researchers that the SHRP rutting parameter G*/sinδ is not very effective in predicting the rutting performance of binders, especially in case of modified binders as its value is highly sensitive to shear rate. To explain how the asphalt binder contributes to the rutting behaviour of the pavement, zero shear viscosity (ZSV) seems to be adequate. Moreover bituminous binder in an asphalt mixture is mixed with mineral fillers not as bitumen alone but, forming bitumen-filler mastic. The ZSVs of bitumen-filler mastics, as opposed to those of plain bitumen, may be more appropriate in establishing a correlation with the permanent deformation properties of asphalt mixtures. Therefore in this study bitumen-filler-mastics were prepared by adding four different concentrations of limestone filler to warm mix binders. Filler was added to get the filler-bitumen ratio of 0.5, 1.0, 1.3 and 1.5 by weight of the binder. Thus a total of 72 formulations of bitumen-filler mastic were prepared. Frequency sweep tests were conducted to determine Zero Shear Viscosity of warm mix binder and bitumen-filler-mastic. Conventional rutting parameter G*/sinδ is also determined over a temperature range to evaluate Failure temperature of warm mix binder and bitumen filler mastic. Various other properties like temperature susceptibility, aging index, short term and long term aging, creep stiffness and m-value of warm asphalt binders were also evaluated. Fourier Transform Infrared Spectroscopy was performed to find the aging characteristics of warm mix asphalt binders. Performance properties of Warm Asphalt Mixes were evaluated and compared with Hot Asphalt Mixes. Indirect tensile strengths (ITS), tensile strength ratio (TSR),moisture induced sensitivity test, resilient modulus, dynamic creep, fatigue strength, and rutting depths of WMA mixtures were found and compared with that of HMA mixtures. The effects of aging of warm asphalt mixes were evaluated by artificially aging the mixture samples in the oven at 85ºC for 120 hours. The aged samples were then tested for all the performance properties. The Viscosity-temperature susceptibility test results showed that the addition of warm mix additives make VG30 and PMB40 binder more temperature susceptible in the temperature range of 60°C to 135°C, which shows that addition of these additives can reduce the mixing and compaction temperatures. Rutting factor G*/sinδ of control binders improves with the addition of Sasobit® at higher dosage, whereas it reduced with the addition of Evotherm™ and Rediset®. Temperature sweep test results showed that failure temperature of VG-30 improved with the addition of Sasobit®, while it showed very marginal change with Rediset® and Evotherm™. In the case of bitumen – filler mastic, addition of warm mix additives improved the failure temperature values of VG 30 based mastics. Zero shear viscosity values indicated that Sasobit® is effective in enhancing the rut resistance of bitumen-filler mastic at its all doses with VG30, but it showed improvement at the dose of 3 percent and above for mastic prepared from PMB40 binder. Bitumen-filler mastics prepared from Rediset® and Evotherm™ modified binders showed improved ZSV only at higher filler-binder ratio of 1.3% and 1.5% which indicates improved resistance to rutting. The results showed that the Evotherm™ and Rediset® do not alter any property of bitumen, but are effective only when mixed with bitumen in presence of aggregate/filler part. It was observed that reduced mixing temperatures significantly reduced the aging index of the binder containing warm mix additives. Long term aged warm mix binders with Evotherm™ and Rediset® additive showed better fatigue cracking resistance, whereas Sasobit® modified warm mix binders showed low fatigue cracking resistance as compared to control binders. Fourier Transform Infrared Spectroscopy (FTIR) for asphalt binders is used as an indicator of oxidation (aging). FTIR analysis confirmed that binders containing warm mix additives age less as compared to the control binders. The long term conditioning of the mixtures was done by aging them in an oven for 5 days (120 hours) at 85ºC. The effect of aging on moisture resistance and permanent deformation of warm mix asphalt was compared with that of hot mix asphalt. It was observed that the warm asphalt mixes have better TSR values than the control hot mix asphalt. However after the MIST conditioning of the samples, warm mix samples containing Sasobit® additive were found to be more susceptible to moisture induced damage in comparison to control VG 30 hot mix asphalt samples. Evotherm™ and Rediset® based warm mix samples showed improved resistance to moisture induced damage in comparison to control hot mix asphalt. Warm asphalt mixes showed better resilient modulus values than the control hot mix asphalt. Un-aged control HMA mixes showed more permanent accumulated strains than WMA mixes. The results showed that un-aged warm asphalt mixes will have more resistance to permanent deformation than the hot asphalt mixes. Whereas in the case of aged samples, the WMA mixes showed almost similar extent of permanent accumulated strain as in the control HMA mixes except the Evotherm™ modified mixes. Wheel rutting test results indicated that the warm asphalt mixes had significantly lower rutting depths than the control hot asphalt mixes. The results of four point bending beam test at different strain levels showed that the addition of warm mix additives improved the fatigue life of warm asphalt mixes.