Issue 45

C. Huang et alii, Frattura ed Integrità Strutturale, 45 (2018) 108-120; DOI: 10.3221/IGF-ESIS.45.09 112 3 2 0 1 1 2 0 1 1 3 2 ( ) ( ) t a b t t ct t e t      − − + − = + + +   (9) For the unloading stage, 3 2 ( ) 0 0 0 0 0 0 0 1 2 0 (1 ) 3 2 ( ) t t t a b t t ct t e e t       − − −   − +   − = + +        (10) The creep speed during the loading stage is 2 0 1 2 0 1 ( ) ( ) t e at bt c t       − − + = + + (11) For the creep accelerator during the loading stage is 2 0 2 2 0 2 ( ) ( ) t e at b t       − − − = + (12) where t 0 is the total loading time. In Eqn. (12), if (t)=0, then 0 2 2 2 2 2 t b t e a a     −  − = (13) * t in Eqn. (13) can be solved by Newton iterative method. Obviously, if * 0 t t   , then 0   , 0   and 0   . If * t t  , then 0   , 0   and 0   . So * t is the onset time of inflection point in the curve of creep strain and time, and is the demarcation point between the stable creep and the accelerated creep. Also, the creep rate become the minimum at * t . After this point, the creep speed gradually increases and the rheological time of FRAC F t = * t . This model theoretically constructs the demarcation point between stable creep and accelerated creep, which cannot be seen in Burgers model and modified Burgers model, and it matches the characteristics of typical creep deformation and time curve, as shown in Fig. 1. FRAC V ISCOELASTIC A NALYSIS AND V ALIDATION OF T HE M ODEL P ARAMETERS n the model with five units and eight parameters of FRAC, the resistance to elastic deformation is stronger as E1 gets larger. In the condition of same load and same time with increases, the permanent deformation decreases, and the resistance to viscous flow deformation tends to be stronger. When the relaxation time becomes larger, the increase of viscous flow deformation over time becomes slower, and then FRAC has a greater ability to resist rutting deformation. When the delay time becomes larger, the deformation development over time becomes slower, and then the resistance to viscoelastic deformation of FRAC becomes greater. When b is larger, a becomes smaller, while b becomes larger resulting in a larger rheological time, and then the resistance to viscous flow deformation of FRAC becomes stronger [12]. To verify the validity of the model expressed in Eqn. (9) and (10), the creep test of FRAC was carried out. A-70 asphalt and polyester fiber (PF) with various aspect ratios and volume fractions were used in this test. The length of polyester fiber is 3mm ， 9mm and 12mm respectively, the ratio of length to diameter is 162 ， 486 and 649, and the ratio of fiber to mineral mass is 0.2%.The volume ratio of fiber is 0.35%; For polyester fiber with length of 6mm, the ratio of length to diameter is 324, and the fiber content is 0.1% ， 0.2% ， 0.3% and 0.4% respectively, and the corresponding fiber volume I