Static creep

The stiffness modulus of bitumen (S) can be determined in the laboratory by apparatuses applying static (creep) or dynamic loading. In both cases, the shear modulus, G, may be determined, which is defined as... 

It follows that the stiffness modulus under static (creep) or dynamic conditions, S, is... 

By combining static-creep and dynamic tests, a range of stiffness modulus can be obtained. When measurements of stiffness modulus as a function at time, at various temperatures, are carried out and the results are plotted in logarithmic scales, a graph of the type shown in Figure 4.16 is obtained. 

The device may be used when the test is carried out under static-creep loading, which is the one developed by Shell laboratory at the end of the 1960s (Fenijn and Krooshof 1970), similar to the sliding plate viscometer or the improved version known as AARB elastomer tester (Tredrea 2007). 

In addition to the above, the maximum permissible bitumen content is determined by executing the triaxial cyclic compression test according to CEN EN 12697 (2012), Method B, or the unconfined static creep test (see Section 7.6.3). This ensures sufficient resistance of the CAM to permanent deformation. 

The creep coefficient may also be determined by executing the static creep test. In this case, the creep coefficient, b, is the slope of the linear fit on the log 5, versus log 5 ix,creep-The stiffness of the bitumen, is determined from the Van der Poel nomograph, and the static creep stiffness, 5 is determined from the static creep test, at various stages of loading. More information regarding the static creep test is given in Section 7.6.3. 

Two specimens per bitumen content (use five levels of bitumen content), after curing for Marshall testing, are subjected to either the triaxial cyclic compression test or the static creep test at 40°C 0.5 C. 

The inability of the static creep test to simulate the real loading conditions on site and to reflect better the improved performance of binder modifiers led to the establishment of the dynamic (cyclic) loading test at the beginning of the 1980s (Finn et al. 1983 Valkering et al. 1990). 

The use of the static creep test without confining lateral pressure, which was the first to be introduced, is currently almost obsolete. 

The total duration of loading in case the uniaxial static (creep) test is employed is 1 h. 

Although the static creep test is not included in ENs, some countries still use it. 

The apparatus used is much simpler than the apparatus used in the cyclic compression test (Cabrera and Nikolaides 1987 Jongeneel et al. 1985). It is mentioned that the CRT-UTM-NU (Cooper Research Technology Ltd. 2014) apparatus is also capable of performing the static creep test. 

By conducting the static creep test, it is possible to estimate the oncoming permanent deformation (rutting) of the asphalt layers after n years in service. 

Under long term stress conditions, in eithercyclic (fatigue) or static (creep) loadings, intrinsic morphological features can become areas of weakness, e.g., cracks can form in polyethylene at the boundaries of spherulites and within them, suggesting that long term failure is due to separation between lamellae. 

Thereby, a careful consideration of the limit of the linear viscoelasticity is of particular importance and will be shown based on both dynamic and static (creep, relaxation) experiments. 

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