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Long Heavy Vehicles (LHV) are considered more efficient and environmentally friendly transportation of goods compared to conventional trucks. Thus, the maximum allowable gross vehicle weight (GVW) in Sweden was increased on part of the road network from 64 to 74 tons in 2018 by increasing the vehicles' length and the number of axle groups per vehicle but not the axle load limits. This change in loading conditions is expected to lead to changes in the structural response and degradation rate of thin pavements on the low-volume road network. To improve our understanding of thin pavements behaviour exposed to multiple axle loadings two thin pavement structures located in the north of Sweden were instrumented with road response and climate sensors. Four measurement campaigns were carried out within one year by in-situ stress and strain measurements from the built-in sensors as LHV passes over at normal speed. The recorded response was compared with numerical calculations based on multilayer elastic theory (MLET). Values of stresses and strains showed a generally good agreement with high values of coefficient of determination R-2 during different seasons when the asphalt stiffness values were adjusted based on temperature and granular layer stiffness values based on moisture.

It is well established that both traffic-related loading and environmental conditions influence the structural behaviour of pavements. Pavement design methods aim to consider the effect of traffic loading and environmental variables on pavement structure, foresee their changes during the lifetime of the pavement and predict the resulting distresses and pavement life. Newer models are required to further advance the development of pavement design methods. Validations using reliable and representative data are required prior to incorporating these models in pavement design methods. The impact of environmental factors such as temperature, moisture content and freeze-thaw cycles on pavement behaviour have been examined in this doctoral thesis. Furthermore, the impact of increased loading by new long heavy vehicles on low-volume roads subjected to large variations of the environmental conditions has been investigated. The findings presented in this thesis have been based on field data collected on roads that have been and currently are in day-to-day operation. The collected data on mechanical response, temperature, moisture and frost was used to validate models on mechanical behaviour and thermal behaviour, as well as the effect of their interaction in pavement structures. The models developed and validated in this study are aimed to be integrated into a new mechanistic-empirical pavement design framework that is currently under development in Sweden. The work done for this thesis is presented hereby in the form of 5 papers and a short summary part. This thesis is a continuation of a licentiate thesis previously published at KTH Royal Institute of Technology. Part of the material published in the licentiate thesis has been included in this doctoral thesis., Det är väl känt att både belastning från trafik och klimat påverkar vägars strukturella beteende. Strukturella dimensioneringsmetoder för vägar tar hänsyn till båda dessa belastningar och förutser deras inverkan på nedbrytningsförloppet under dimensioneringsperioden. Nya metoder och modeller är under utveckling som syftar till att utöka och förbättra förutsägelserna av vägars tillståndsförändringar och därmed nedbrytningsprocessen. Dessa modeller behöver valideras mot uppmätta data innan de börjar användas i allmänna dimensioneringsprogram. Denna doktorsavhandling undersöker påverkan av klimatfaktorer så som temperatur, fukt och frys-tö cyklar på vägars strukturella beteende. Dessutom har det undersökts hur långa och tungt lastade fordon påverkar tunna vägkonstruktioner vid olika klimatförhållanden. Avhandlingens slutsatser baseras på jämförelser av fullskaliga försök med tunga fordon som används på vägnätet i dag med beräkningar. Insamlad last- och töjningsdata från sensorer i vägkroppen som belastas med tung trafik samt vägtemperatur, fukt och frost har använts för att validera modeller för att beräkna den uppmätta mekanistiska samt termiska responsen i vägar. De modeller som har utvecklas och validerats har inkluderats i en ny mekanistisk – empirisk vägdimensioneringsmetodik som är under utveckling i Sverige. Resultaten presenteras här i fem artiklar samt i en kort sammanfattande del. Doktorsavhandlingen är en fortsättning av en Licentiatavhandling som tidigare har publicerats vid KTH. Delar av det arbetet är även inkluderat i denna avhandling.

Fine-grained materials are associated with a higher water retention capacity due to their higher surface energy in comparison with coarser materials. That characteristic is connected to why fine-grained materials also yields more suction when drying. In addition, seasonal variation of the water table subjects fine-grained subgrades to different moisture contents, varying the suction built in the subgrade, consequently affecting its mechanical characteristics. As fine-grained materials are frequently frost susceptible, they can cause issues to the whole pavement cross-section related to Freeze-Thaw (F-T) actions, widely known as frost heave and thaw weakening. This study will seek to investigate if subsequent cycles of closed-system F-T can permanently alter the unsaturated behaviour of fine-grained materials, using as background their Soil Water Retention Capacity obtained using a pressure plate apparatus. Such issues may become more common with permafrost areas being subjected to freeze and thaw cycles due to climate change or extraordinary events creating the circumstances to F-T where it is currently not observed. Two silty sands with low plasticity were tested, and the results show a reduced water retention capacity (WRC) after closed-system F-T cycles. Considering that matric suction changes the state of stress of soils, the findings suggest an impact in the resilient modulus (Mr ) not only seasonally, as it is well established, but also after seasonal freezing and thaw events.

Longer Heavier Vehicles provide an improvement in energy efficiency and environmental performance compared to traditional Heavy-Duty Vehicles. In Sweden, the maximum permissible vehicle gross weight has been increased from ∼64 to ∼74 tonnes without increasing the axle load limits. The consequence of this is investigated in this study. Response from two instrumented thin flexible pavements subjected to loading from three types of heavy vehicles (∼64, ∼68 and ∼74 tonnes) has been measured and the recordings were compared with numerical calculations based on 2D multilayer elastic calculations. Pavement damage contribution by the three vehicles was thereafter investigated. As long as the number of axles is increased to compensate for the increased vehicle loading and dual wheels are used, ∼74 tonnes vehicle are not more aggressive to the two thin pavement structures compared to the lighter vehicles with fewer axles but higher average axle loads and tyre pressure.

The seasonal variations of the climatic factors such as temperature, moisture, and freeze-thaw cycles are known to influence the material properties and structural behavior of flexible pavement structures. Mechanistic models are required to predict the behavior of the structure throughout the entire year including the winter frost and spring thaw periods. In this study, the mechanical response of an instrumented flexible pavement structure located in the north of Sweden has been investigated at four different times during a year under loading by falling weight deflectometer and three different long heavy vehicles (~64, ~68 and ~74 ton). The mechanical response values recorded by the sensors embedded in the structure have been compared to the numerical model values obtained by 2D multilayer elastic calculations. It is shown that multilayer elastic theory provides a reasonable prediction of the mechanical behavior on the condition that the stiffness of the asphalt concrete is adjusted according to the temperature variations of the layer and the stiffness of the unbound granular layers is adjusted according to moisture content levels., QC 20210226

The determination of frost penetration is one of the main requirements in considering environmental effects in pavement design in cold regions. At the present time, the frost depth of pavements in Sweden is estimated computationally using computer software which approximates the heat equation by finite difference. Due to the geographical positioning of Sweden, a wide range of air freezing index and frost penetration depths were observed with lower values in the south and higher values in the north. This paper introduces a simplified design chart which is obtained by empirically correlating the air freezing index estimated from temperature measurements by 44 local meteorological stations to the maximum frost penetration depth obtained by 49 RWIS Road Weather Information Station data. The results are classified depending on their location and the climatic zones defined by the Swedish pavement design codes. Nonlinear prediction intervals are implemented to provide a range of possible frost penetration depths since local site conditions are not taken into account. Further research is required to consider local on-site effects such as frost susceptibility of pavement materials, the thermal conductivity of layers, access to water and snow covering., QC 20210225

QC 20221123