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• Four different layers were found in the asphalt phase of SFP through microscale tests. • A micromechanical model considering interlocking and stiffening effect was proposed for SFP. • The equivalent modulus of asphalt phase considering interlocking effect was cross-checked with NI test. Semi-flexible pavement (SFP) has been promoted to apply in pavement engineering mainly for its excellent performance in rutting resistance. However, due to the material characteristics of three-phase heterogeneous phase and the structural characteristics of dense skeleton interlocking structure, the microstructure of semi-flexible pavement material is complicated, greatly influencing the mechanical properties of the material. This research aims to identify the two characteristics of SFP employing micromechanical models as well as nano-techniques, including nanoindentation (NI), Scanning Electron Microscope (SEM) and energy dispersive spectrum analysis (EDS). Two interlocking factors were introduced in the three-phase micromechanical model to characterize the interlocking effect. Identification of the asphalt mastic phase in SFP was implemented as the basis of stiffening correction in the micromechanical model. Because four different layers were found in the asphalt phase of SFP that was difficult to simulate, a general stiffening correction was used in the improved micromechanical model. The final model considering the interlocking and asphalt stiffening effects can characterize the property of SFP well. It was found that grouting cement has an enhancement function on the viscoelastic interlocking effect while does little on the elastic interlocking effect. Meanwhile, more grouting cement will lead to more asphalt-cement stiffening effect. An equivalent dynamic modulus of the asphalt phase-phase from the micromechanical model was proposed in the end, which has been proven corresponding to the experimental results of NI test. [ABSTRACT FROM AUTHOR]

Graphical abstract Highlights • The 2S2P1D model is suitable for SFP. • A larger porosity in PA means a greater effect of skeleton interlock on loss modulus. • Grouting cement had a negative effect on the loss modulus at low temperatures. • Physiochemical stiffening effect and imperfect interface exist between PA and cement skeletons. • A larger porosity leads to a more stable interlock effect with the frequency increasing. Abstract Semi-flexible pavement (SFP) has been promoted to apply in pavement engineering mainly for its excellent performance in rutting resistance. Since the reinforcing mechanism of its stiffness and strength is still ambiguous, this paper aims to investigate the reinforcing mechanisms of SFP at meso-scale. SFP was regarded as a double skeleton structure (open graded asphalt skeleton and grouting cement skeleton) and three main reinforcing mechanisms were characterized on the basis of the Mori-Tanaka model (M-T), including matrix modulus, cement filling effect and double structure interlock. During the process, the matrix of SFP was redefined as asphalt mixture without effect of cement-filling air voids. The behavior of the imaginary matrix was obtained through back-calculation in M-T model. The two springs, two parabolic elements, and one dashpot (2S2P1D) model was utilized to characterize viscoelastic behavior and two interlock factors were established to amend interlock effect. The results may demonstrate the existence of physiochemical stiffening effect of grouting cement and imperfect interphase between the two skeletons. SFP with matrix of three different original porosities were compared during the prediction process. It was found that the reinforcing rate was positively related with original porosity. While the dynamic modulus value of SFP, meaning specially storage modulus value, depended more on the matrix gradation than porosity. And grouting cement had a negative effect on the holistic loss modulus at high frequencies. [ABSTRACT FROM AUTHOR]

• The heterogeneity and interlocking effect were evaluated in SFP's stiffness and strength. • Strain related strength theory should be used for SFP in engineering. • The addition of fiber can restrain the trend of accelerated shear failure along interface. Semi-flexible pavement (SFP) material has been promoted to apply in pavement engineering mainly for its excellent performance in rutting resistance. But its mechanical behavior is complicated due to its heterogeneity and interlocking structure. According to the present study, the damage process of SFP (with and without fiber), as well as its porous asphalt (PA) matrix, under uniaxial compression test was detected employing acoustic emission (AE) technique. The heterogeneity and the interlocking effect were varied by changing the matrix porosity and the gradation, respectively. The stress-strain curves reveal that cement grouting could decrease the material's failure strain distinctly, while the addition of fiber made no effect on the results. Nevertheless, both the cement grouting and the addition of fiber caused medium and high amplitude (50–65 dB) AE signals during the AE detection. Besides, the Rise Angle (RA) value and energy distribution both illustrate that there was increasing trend in the amounts of shear cracking in SFP during compression, which was different from asphalt mixtures. And the addition of fiber could restrain the shear cracking to improve the crack resistance. Quantificationally, heterogeneity was characterized by standard deviation of compositions' modulus and an interlocking index was extracted from a modulus micromechanical model to characterize the interlocking effect. The Weibull distribution was adopted to characterize the damage evolution, where the cumulative AE energy was chosen as the damage variable. The result shows the two Weibull parameters m , n had obviously positive correlation with the matrix porosity and the interlocking index, respectively. [ABSTRACT FROM AUTHOR]

• Effective coarse aggregate-grouting cement contacts were distinguished from non-effective ones. • A fast contact calculation method was proposed. • A good linear relationship was found between the mechanical and morphological interlocking indexes. Semi-flexible pavement (SFP) material has been proven to have more interlocking effect than its Porous Asphalt (PA) matrix because of the secondary skeleton consisting of interconnected grouting cement. However, because of the irregular, elongated and concave shapes conflicting with the requirement of aggregates, together with a stiffness much higher than that of asphalt binder (which can bear and transfer most of the load), it is hard to define whether cement grouted into air voids works mainly as a role of another aggregate or binder. Based on the X-ray computed tomography and digital image process method, this study aims to distinguish effective coarse aggregate-grouting cement contacts (cement working as aggregates) from noneffective ones (cement working only as binder). A contact classification method, as well as a fast contact calculation method, was proposed. The characteristics of different types of contacts were analyzed and two morphological interlocking indexes were established based on the concept of contact points and contact lines, respectively. A mechanical interlocking index was also back-calculated from a micromechanical model as a reference. The result reveals that there exists a good linear relationship between the mechanical and morphological interlocking indexes, verifying the validity of the contact classification method. Additionally, with a larger quantity than that of contacts between coarse aggregates, only one-sixth of aggregate-cement contacts are effective ones, the number of which is half of the coarse aggregates' contacts. As for contact lines, while the average length of effective aggregate-cement contacts is the same as that of aggregate contacts, the length of the noneffective ones is only the half. [ABSTRACT FROM AUTHOR]