Farhad Salour Doctoral Thesis

Figure 10.
Conceptual pavement stiffness variations due to freezing and thawing. 

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5.
 
Mechanical Behaviour of Unbound Materials and Subgrade Soils
5.1.
 
Laboratory investigation and measurements 
The mechanistic pavement design and analysis often require substantial data on the 
properties of pavement materials and their mechanical response to external loads. For 
pavement unbound materials and subgrade soils, these material properties are mainly 
related to the stiffness, durability and stability of the material. The test methods often 
used consist of investigating the mechanical behaviour of the material under repeated 
loading, simulating the effect of the traffic loads under controlled temperature and 
moisture conditions that can be experienced by the material in the field. Similar to the 
traffic induced stresses, the applied cyclic loads are rather small compared to material 
strength; however they are usually applied for a large number of times. 
The mechanical deformation of pavement unbound materials under repeated small 
loading can be divided into two parts: the recoverable deformation and the accumulated 
plastic deformation. After each load application, the material usually experiences some 
deformation that does not recover. The permanent deformation, often known as the 
plastic deformation, is generally observed during the early stages of load application. 
After a sufficient number of load applications, the deformation almost completely 
recovers and the contribution to plastic deformation becomes very small or negligible. 
The deformation that is totally recovered during the load application is usually used to 
determine the stiffness properties of the material. Figure 11 shows the typical behaviour 
of unbound materials under cyclic loading. 
Figure 11
. Deformation behaviour of unbound materials under repeated load (left) and 
stress-strain curve during one cycle of load application (right).