Farhad Salour Doctoral Thesis

7.SUMMARY OF THE PAPERS ............................................................................................................ 35 
7.1. 
P
APER 
I
 AND 
II ................................................................................................................................................ 35 
7.2. 
P
APER 
III
 AND 
IV ........................................................................................................................................... 42 
7.3. 
P
APER 
V ............................................................................................................................................................ 44 
7.4. 
P
APER 
VI .......................................................................................................................................................... 46 
8.SUMMARY AND CONCLUSIONS ...................................................................................................... 51 
8.1. 
A
SSUMPTIONS AND LIMITATIONS
 ................................................................................................................ 52 
8.2. 
R
ECOMMENDATIONS FOR FUTURE WORK
 .................................................................................................. 53 
9.REFERENCES ..................................................................................................................................... 55 

---

xiv 

---

1 
1.
 
Introduction 
1.1.
 
Background 
The road network is an important element of the national infrastructure and its 
construction, operation, and maintenance constitute a large part of the national annual 
budget. Roads are built to provide road users a safe, comfortable and robust ride 
throughout their service life. They must basically fulfil the two major functional and 
structural requirements. The functional requirement or serviceability is to provide skid 
resistance and a smooth ride experience for the users, and structural requirement is to 
provide an adequate level of structural adequacy over a reasonable time period. 
Pavement structures are primarily designed to distribute the traffic induced stresses and 
strains over the load bearing layers to an intensity level which the material can 
withstand. In mechanistic-empirical pavement design, inputs such as material 
properties, traffic loads and structural layer thicknesses are related to the pavement 
response such as stresses and strains, using mechanistic principles. The pavement 
response is then used to predict pavement performance using laboratory and field based 
data and measurements. Pavement design dependency on field and laboratory 
performance and observations is mainly due to the complex nature of pavement 
systems and the many system boundary conditions which affect its performance. These 
boundary conditions are mainly the interaction of climatic factors such as temperature 
and moisture (water) content, the mechanical behaviour of bound and unbound 
materials, and the traffic load spectrum and frequency. Due to the complexity of 
pavement systems, the theory alone has not yet been able to realistically predict 
pavement performance, and pavement engineering is still dealing with fundamental 
difficulties in many aspects of the design process. 
To overcome the limitations of empirical pavement design, the pavement engineering 
research community have been moving towards a so-called mechanistic approach in 
which the pavement structure and environment are treated as a system and the 
pavement mechanical response is analysed, taking into account the external traffic 
loading and environmental conditions simultaneously. Even though great developments 
have been achieved with the enhancement of computers that allows for more advanced 
analysis and design procedures, the transition towards a fully mechanistic design with 
improved design reliability and distress prediction models still requires a better 
understanding of the factors and mechanisms involved. This will then allow for feasibly 
relating the limited laboratory and field based data to the design process. The 
mechanistic pavement design method will evidently be more flexible to accommodate 
various loads, materials and climatic conditions. 

---

2 
In thin flexible pavement structures, unbound layers such as granular base and subbase 
materials, as well as the subgrade material, play a significant role in the overall pavement 
performance. In order to establish a more robust and rational pavement design and 
analysis, a thorough understanding of the factors involved, such as material behaviour 
and response to traffic-load induced stresses and environmental factors, is essential. 
Flexible pavement structures with a thin HMA layer (≤ 100 mm HMA layer thickness) 
and unsurfaced gravel roads constitute a large part of the road network in Sweden. 
Depending on the governmental policies and available funds, these secondary low 
volume roads may not receive a sufficient construction and maintenance budget. In 
addition to the traffic load distresses that can be considerable due to the heavy axle 
loads of the forest industry, these roads are often exposed to significant environmental 
effects. These pavement structures usually experience considerable temperature and 
moisture variations since they are exposed to rain, snow, frost and freeze-thaw cycles 
during a significant period of the year. The cold region-related climatic and 
environmental factors can expose pavements to intense loading that can result in 
seasonal and long term loss of bearing capacity (structural shortage), or loss of surface 
smoothness due to differential frost action, crack propagation and surface ravelling 
(serviceability shortage). Furthermore, these environmental factors, when accompanied 
with traffic loading, can accelerate the traffic-related deterioration mechanisms.