Issue 45

C. Huang et alii, Frattura ed Integrità Strutturale, 45 (2018) 108-120; DOI: 10.3221/IGF-ESIS.45.09 109 viscoelastic mechanics elements such as spring and dampers [1-4]. Generally, the model parameters can be obtained by uniaxial compression creep test, triaxial compression creep test and beam bending creep test [5-11]. Besides, the influence of stress level, temperature and fiber content on the model parameters and the viscoelastic behavior of AC can also be achieved by above mentioned tests. Over the years, many models have been developed to study the properties of AC and FRAC. For instance, the Burgers model [2] reveals that the permanent deformation of FRAC has a linear positive correlation with loading time and a linear negative correlation with viscosity, and that the viscosity increases with the extension of loading time. However, some of the modelling results differ from the deformation features of FRAC during creep, such as the negative correlation between permanent strain and loading time, the absence of constant velocity creep, and the accelerated creep process. In the four-element five-parameter model [2], the viscosity increases with loading time, and the permanent strain increment exhibits an opposite trend; under the infinite loading time, the permanent strain increment gradually approximates zero; in this case, the deformation features are close to those of constant velocity creep rather than those of FRAC during accelerated creep. Reference [8] examines the effects of fiber volume ratio and length-diameter ratio on AC viscoelasticity, but fails to characterize the overall effects of the two ratios with one parameter. Thus, the understanding of the viscoelastic behavior of AC and FRAC is still very limited. In this paper, a new viscoelastic mechanics model with five units and eight parameters for FRAC was employed by analyzing the viscoelastic behavior of FRAC. The parameters of this model were obtained by beam bending creep tests with different fiber volume fraction and aspect ratio. The efficiency and validity of the present model were demonstrated by comparing the results of the model with those of Burgers model and modified Burgers model. At last, the effects of fiber volume fraction and fiber aspect fraction on the model parameters and the viscoelastic properties at loading and unloading creep stage of AC were studied by using the model. V ISCOELASTIC MECHANIC MODEL WITH FIVE UNITS AND EIGHT PARAMETERS t is known that AC exhibits high time-dependent deformation process which called creep process and can be divided into two stages: loading creep and unloading creep. The curve of the whole creep process of FRAC beam bending in Fig. 1 shows that the creep deformation in loading is similar to that of AC and it also includes three stages [1, 3]: (1) The stage of deceleration creep during the beginning of loading (migration period). Polyester fiber reinforced asphalt concrete (PFRAC) beam produces instantaneous elastic deformation under creep loading in this period, and the span deflection increases with time, while the creep rate gradually decreases with time. (2) The stage of constant speed creep (stable creep period). Creep deformation continues to increase with time elapsing, while the incremental rate of creep deformation keeps constant approximately and the deformation-time curve is a straight line. (3) The stage of accelerating creep (undermine period). With the deformation gradually increasing, the slippage among the beam internal aggregates begins to take place and the cracks found in the bottom of test specimen begin to propagate gradually. It is noted that creep rate increases gradually and the deformation-time curve deviates from the straight line. An inflection point can be easily observed in the transition from the stable creep to accelerated creep in the deformation- time curve, and the corresponding loading time at the inflection point is named rheology time Ft of FRAC [11]. Obviously, Ft is the time that the creep rate reduced to the minimum and the start time that FRAC begin to enter the stage of the accelerated creep. The creep rate increases after the inflection point of time. During the stage of creep unloading, the elastic deformation immediately recovered and the viscoelastic deformation gradually restored with time. However, the plastic deformation cannot be restored and become permanent deformation. Both Burgers model and modified Burgers model are widely used to describe the viscoelastic deformation characteristics of AC in engineering. Burgers model is a combination of two springs and two dampers [2], as shown in Fig. 2. The creep equation of the model in loading stage is: 0 1 1 2 1 1 ( ) ( ) t t e t     − − = + +   (1) The creep rate is: 0 1 2 1 ( ) ( ) 0 t e t      − = +  (2) I