Road Surface and Tyre Interaction: Functional Properties affecting Road Dust Load Dynamics and Storage

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Particulate matter is a problem for human health, where several relationships between negative health effects and air pollution has been found, including, but not limited to, respiratory diseases, lung cancer and cardiovascular diseases. In countries where studded tyres are used, for example Sweden, Norway and Finland, and where traction sanding is used, particles from abrasion wear of pavements and crushing of traction sand contribute significantly to PM10.

The thesis has several objectives, where a broader aim is to investigate the complex road surface and tyre system regarding abrasion wear of pavements and the impact on abrasion wear particles and road dust. The thesis also aims to put these aspects in relation to other, equally complex, aspects coming from or affected by the road surface and tyre interaction which include noise, rolling resistance and friction. This is done through some more specific objectives and limitations described in the thesis. The thesis also has the fundamental aim to act as a starting point to reach a more holistic approach to understand the functional performance of the road surface and tyre interaction which has been done in cooperation with Vieira and the results he publishes in his thesis.

The road surface and tyre interaction consist of a complex contact system which is affected by both tyre properties and the road surface course properties, including both its inherent material properties and the road surface characteristics, as well as the surrounding environment and any interface consisting of for example water, slush, snow, ice or sand and so on.

The surface wear course has several functions which is dependent on the inherent material properties. The wear course must resist several degradation processes, including chipping, different types of deformation, different types of cracking as well as abrasion wear due to studded tyres to mention some.

The surface course construction and the traffic characteristics affect the particle generation, where the surface course properties that govern the resistance against abrasion wear also affect the generation of wear particles.

Other aspects which are affected by the road surface and tyre interaction is the generation of noise and the rolling resistance. Noise has, as for particles, an negative impact on health and the road surface and tyre interaction is the dominating source from about 15 km/h to 25 km/h for light traffic and from about 30 to 35 km/h for heavy traffic. Several mechanisms generate or amplifies the noise and is connected to the surface characteristics such as the macrotexture. Rolling resistance is the conversion of mechanical energy to heat for a rolling tyre and is affected by both the road surface and tyre deflections and deformations and are affected by the surface characteristics such as unevenness and the macrotexture. The rolling resistance is linked to fuel consumption and in extension to exhaust emissions. Another functional property is the friction which is affected by the road surface characteristics by the micro- and macrotexture.

There are several measures to reduce road dust loads and PM10. The measures can be either preventive or mitigative. Measures aimed at changing the traffic situation and the tyre usage, changing of the road surface wear course, cleaning of the road surface and dust binding are described.

Several methods has been used in the studies discussed in the thesis and consist of a large-scale road simulator, the usage of laser measurement systems for determination of road abrasion wear and texture respectively, a prediction model for studded tyre abrasion wear and the NORTRIP model for modelling of non-exhaust particle emissions from road traffic. Also used was a commercial system for traffic measurements and a method for determining the proportion of studded tyre usage. Road dust was sampled and quantified using the WDS (Wet Dust Sampler) method and the collected dust was quantified and characterised using a laboratory method and by using laser granulometry.  Turbidity was used as an approximation of the road dust load.

Five papers are appended to the thesis. The first paper describes the calibration of the Swedish studded tyre abrasion wear prediction model and the effect it has on the NORTRIP model, in which the abrasion wear model is implemented. The second paper describes the macrotexture of different surface wear courses and how different texture measures could be used to describe the potential dust storage capability. The third paper investigate the WDS-method regarding its performance regarding water and how the water performance theoretically affects potential dust losses. The fourth paper describe the spatial and temporal variation of road dust for six winter and spring seasons in Stockholm, Sweden, for several streets with SMA (Stone Mastic Asphalt) pavements. The fifth paper describe a similar investigation performed in Linköping, Sweden, during one winter and spring season for a double layered porous asphalt and for an SMA which acted as a reference. When applicable, the results from Linköping was compared to those from Stockholm.

The results showed that the abrasion wear modelling overestimated the abrasion wear by approximately 50% which caused the NORTRIP model to overestimate the contribution from the abrasion wear to the particle emissions, which was not surprising. However, it is not likely that the NORTRIP model gets a decrease of the emissions 50% since the road surface and tyre interaction is complex and several aspects affects the abrasion wear and the resulting generation and storage of road dust, including, but not limited to, polishing of the road surface, increased abrasion wear for wet surfaces.

The results from the WDS investigation showed that the method seems to function well, given the limitations of the study. The largest water loss was the water retained on the road surface. It also seems like most of the dust is collected. The discussions also consider how the WDS method uses water and the strengths and weaknesses this has compared to dry sampling methods.

The results from the spatial and temporal variation of the road dust loads in Stockholm showed that there are differences between seasons and there is a difference between the dust loads in the wheeltracks and between wheeltracks. In some cases, differences were seen between the streets with large variations, which could be expected since the road dust load is dependent on the traffic characterization, road operation, deposition of material on the surface and the meteorology.  Another result was that an increasing macrotexture seemed to result in an increase in dust loads. The macrotexture was, generally, lower between the wheeltracks and higher in the wheeltracks, which was not surprising due to the traffic impact on the texture development. The macrotexture was, however, only measured at a single occasion. The repaving of a SMA surface course to a more abrasion resistant SMA surface course resulted in a higher dust load compared the before the repaving, while visual observation of the road surface implied a rougher macrotexture. This could, however, have been affected by an increased abrasion wear which occur during the first winter season due to a higher initial abrasion wear. The results in Linköping showed similar temporal and spatial variations as in Stockholm for the investigated SMA surface course. It was also discussed how the double layered porous pavements construction affect the particle transport processes. In the comparison between Stockholm and Linköping, it was suggested that the dust binding and cleaning in Stockholm affect the dust load since these measures are not performed in Linköping which is possibly reflected in the dust loads in and between wheeltracks.

How different texture measures could be used to characterize the road surface texture and its connection to the dust load storage was also discussed, including a discussion of which measures that could be used. It is, however, also noted that the measures discussed the measure that should be used is not necessarily discovered yet.

The discussion also mention the lack of a holistic approach regarding the road surface and tyre interaction which simultaneously consider effect such as abrasion wear particles, noise and rolling resistance. Some measures seem to be of interest to improve at least two aspects simultaneously, for example the usage of a double layered porous pavement or texture optimisation. Different strength and weaknesses are discussed for the different mechanisms affecting the different aspects as well as how some mechanisms should be further studies from other perspectives, for example noise mechanisms which may be interesting from a particle perspective.

The thesis ends with giving some suggestions for continued research to increase the knowledge. This concern abrasion wear modelling and road dust emission modelling where the road surface texture should be considered. Also suggested is that mechanisms from other aspects of the road surface and tyre interaction, for example those affecting noise, also should be investigated and be used to explain mechanisms related to road dust generation and suspension. Several combined investigations are suggested for studying several aspects from or affecting the road surface tyre interaction simultaneously, including noise, rolling resistance, the road surface characteristics, road abrasion wear, abrasion nwear particles, the road dust loads, the suspension of particles and friction which is required to finally achieve the holistic knowledge required to at least minimise conflicts of interest between different functional properties for road surface courses.

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