Evaluation of different laboratory methods for simulaton of pavement wear and road dust generation from studded tires

In several large cities in Norway the traffic volume is high. The use of studded tires and other friction enhancing measures during winter leads to significant pavement wear, which in turn leads to an increase in the amount of airborne particulate matter, often exceeding the limits set in the ambient air regulation. This represents a nuisance or health risk for people being exposed to the pollution. According to regulations set by the European Union particulate matter is measured and regulated through mass concentration of particles less than 10 μm in diameter (PM10). However, studies of health effects related to exposure to particulate matter have shown that particle properties like morphology and composition are essential regarding toxicity and in considering risk of developing disease. There seems to be a more complex relationship between dust pollution and health effects, which cannot be explained with just the mass concentration alone. This makes it necessary to characterize the physical and chemical properties of the road dust. Characterization of road dust particles is important to provide basis for a better understanding of the pavement wear and production of road dust and its influence on human health in urban areas. The overall purpose of this study has been to characterize particles generated from pavement wear qualitatively and quantitatively.Five papers are included in the thesis. Paper I and II present field investigations of road dust sampled in the vertical direction close to an urban road in Trondheim, Norway. In Paper III different laboratory methods have been used for simulation of pavement wear and road dust generation by studded tires. Paper IV and V describe how aggregate size, driving speeds and vehicle tires affect pavement wear and particle properties.The main conclusions from the study are:Studded tires used in the winter season in Norway are the main cause for generation of road dust from pavement wear. If studded tires were not used, there would be much less suspended particulate matter originating from wear of pavements in urban air. Studded tires produce much more dust particles from wear of pavements compared to nonstudded tires. For instance, at 60 km/h studded tires produce 30-40 times more TSP compared to non-studded tires on an SMA 8 pavement measured in an indoor test facility. The airborne road dust is composed of almost 90 % by weight of mineral particles under winter conditions, and approximately 50 % of the particles have size smaller than 25 μm (D50). Based on all samples analyzed, D10 is 3 μm and D90 is 60 μm. However, agglomeration of particles seems to make the measured particle size distribution coarser than it really is. Studded tires generate dust with finer particle size distribution than non-studded tires. The main fraction of the particles can be described as rounded particles, with average roundness value around 0.6. The specific surface area of the sample is dependent on the amount of organic material present in the sample and the particle size distribution. In general, field samples have higher specific surface area than dust particles generated in laboratory because of higher organic content. However, finely ground particles produced in laboratory may exhibit even higher surface area than field samples of road dust because of finer particle size distribution. Surface area is an important factor in health considerations since the reactivity of particles increase with increasing surface area.Other factors affecting the generation of particles from pavement wear are the driving speed and type of rock material used in the pavement. Test results show that the PM10 concentration measured under laboratory conditions is reduced by 32-49 % when reducing the driving speed from 70 to 50 km/h, 52-83 % when reducing the driving speed from 50 to 30 km/h, and 76-89 % when reducing the driving speed from 70 to 30 km/h. The driving speed affects the particle size and the particle shape distributions. Increasing driving speed generate particles with finer particle size distribution and more irregular particles. The rock material used in the pavement affect the amount of dust generated, the composition, the particle size distribution, the shape distribution, and the specific surface area. Some mineral types are regarded harmful to health, for example quartz and asbestos which are classified as carcinogenic. The total amount of airborne dust (TSP) and PM10 can be very different; a high TSP does not necessarily lead to a high PM10 concentration and vice versa.This study has shown that it is possible to produce dust comparable to studded tire wear by use of simple laboratory techniques. This has significance with regard to cost because it is not necessary to build expensive test sections when the purpose is to generate and characterize the dust from pavement wear. The small scale asphalt testing procedures, Prall and Troger, are the methods best suited to give fine material which is comparable to particles generated from the Pavement testing machine (PTM). It also seems that one can test the aggregate alone to get reasonably good samples for analysis of dust from wear by studded tires. Among the aggregate testing procedures, the Los Angeles (LA) method gives the best correlation with the PTM. The dust produced by Troger and Prall is more similar to the dust produced in the field because the dust is generated from asphalt mixtures, while the dust produced by LA, Nordic ball mill (KM) and micro-Deval (MD) comes from the aggregate only. However, none of these methods include/simulate the effect of the car tire, only the studs.The results have shown that apart from the use of studded tires, the rock material used in the pavement has a significant influence on the airborne dust generated. It is therefore important to carefully select the rock materials for use in urban road pavements. The aggregate type affects both the amount of dust generated and the particle properties. Existing knowledge shows that the finer the particles, the greater will be their potential effects on health. Since driving speed influences both the amounts of road dust generated and the particle size distribution, one may have to consider the use of speed restrictions in urban areas in winter time to reduce the potentially hazardous effects of road dust. However, this has to be balanced against other traffic conditions such as congestion.