2.3.1. Polymers

Polymers are the most traditional admixtures used in GSP, including latex, resin, SBS, styrene-butadiene rubber (SBR), and waste rubber [74,84]. Latex and resin are efficient to smooth the high rigidity of HCM and reinforce the tensile strength of the whole structure because of their hydrophilicity, which results in bonding at interfaces between materials [56,76]. However, they have little or no influence on the high-temperature performance of GSP. That is because adding polymers into cement mortar will impair the compressive strength of HCM.

In contrast, GSP with rubber as an admixture has little promotion to the low-temperature cracking resistance, due to the weak interface between rubber and cement [48,75], although the fatigue life of rubber-modified GSP is prolonged with the increase of rubber content [85]. In this category, waste rubber is thought to be a cost-effective and environmental-friendly admixture as a substitute to replace raw materials with a high blending ratio up to 20% [75]. Its weakness of insufficient strength can be enhanced by adding an interface modifier, which will be introduced later in this paper [86].

### 2.3.2. Fibers

Fiber plays a role in reinforcing tensile strength in GSP [87]. It can be blended into GSF in two ways: adding into OMA alone or into both OMA and HCM. However, the effect of fiber mainly works in OMA structure, as there is no obvious advantage showed by the latter method [78].

Nevertheless, it is possible that fiber may harm GSP. GSP samples with 0.3% loose cellulose fiber show a high abrasion loss, which is difficult to meet the requirement as a surface course [46]. Moreover, the tensile strength of fiber-modified GSP may decrease sometimes, even though the fatigue life was increased [83]. The reason is unknown.

### 2.3.3. Interface Modifiers

Interface modifier commonly refers to silane coupling agen<sup>t</sup> (SCA) or other additives, which can enhance the cohesive strength of interfaces between asphalt and cement to protect weak interfaces from cracking [79]. With the increase of the dosage of SCA, all properties of GSP are improved, especially shear resistance [88,89].

However, SCA needs to be combined with other additives such as rubber or latex to loosen the inner stress in HCM, due to its high stiffness after reaction. It is found that the workability and fluidity of HCM modified by SCA are improved, while the dry shrinkage rate and bleeding rate are greatly alleviated. Therefore, the following GSP samples are strengthened in the cracking and rutting resistance [90].

### 2.3.4. Emulsified Asphalt

Anionic emulsified asphalt and cationic emulsified asphalt are two types of emulsified asphalt which are always blended with cement to produce cement asphalt (CA) as grouting material for GSP. Cationic CA efficiently increases the flexibility of grouting material, compared to anionic CA and average cement mortar [80]. In addition, CA (especially Cationic CA) has better cohesion with asphalt than average cement mortar, which can also act as a kind of interface modifier [76].

Despite advancement in anti-cracking resistance, CA-modified GSP will be inevitably sabotaged in both the compressive strength of grouting material and the rutting resistance of GSP. Accordingly, Xu et al., presents a method using SBS in OMA to offset the decrease in strength of CA-modified GSP [76].

### 2.3.5. Complex Admixtures

The purpose of complex admixtures for GSP is to combine advantages of different admixtures. However, some combinations are not always as expected, because of underlying contradictive influences of combined admixtures on GSP. Therefore, in its composition design, more factors and more procedures should be considered in a comprehensive way to achieve desired outcomes, comparing to the individual components [83].

### 2.3.6. New Functional Admixtures

Some novel admixtures are applied in GSP to extend functions of a road, such as environment conservation and road life extension. For example, water-retaining material is mixed into grouting material to produce water-retaining GSP. This pavement can cool down 8 ◦C–10 ◦C for eight hours for the surrounding area by evaporating the water absorbed from rain, while it maintains a high anti-rutting performance [91,92]. Phasechange material (PCM) is also added into GSP to adjust the ambient temperature. The PCM-modified GSP can be protected from the damage of temperature fluctuation by the thermal storage capacity of PCM. It is proved that GSP mixed with 5% PCM can relieve the temperature of pavement by 11.5 ◦C for 4 h [93,94].

### **3. Evaluation Methods of the Performance of GSP**

*3.1. Common Laboratory Testing Methods*

Common traditional laboratory tests are used for the evaluation of GSP, including: the Marshall test and the rutting test to assess high-temperature performance, splitting test, semi-circular bending test, small beam bending test at −10 ◦C to measure low-temperature performance, and Immersion Marshall Residual Stability test to estimate moisture resistance, as shown in Table 10.


**Table 10.** Traditional laboratory tests and performance of GSP.

### 3.1.1. High-Temperature Performance

The Marshall test is most commonly employed to evaluate high-temperature performance of GSP and then results can be easily compared by this index from different projects. GSP has greater Marshall stability and lower flow value than traditional AC, due to its enhanced strength and fewer residual voids by grouting cement mortar. Additionally, the compressive strength of GSP continues to improve with growth of the OMA void [6]. GSP containing reclaimed asphalt pavement (RAP) is also evaluated by the Marshall test, which shows a promising stability value of 53.9 kN [105].

The Rutting test is another popular method, and shows the grea<sup>t</sup> anti-rutting resistance of GSP. Average dynamic stability of GSP can be up to 15,000 times/mm, compared to the value of AC-16 at only 2000 times/mm [49,106]. GSP samples can keep their excellent function even at a high test-temperature of 70 ◦C [103]. Additionally, the Static Indentation Creep test is also adopted to illustrate these characteristics of GSP [107].

### 3.1.2. Low-Temperature Performance

Evaluation methods for the low-temperature performance of GSP include the small beam bending (SBB) test, splitting test, and semi-circular bending (SCB) test. Additionally, the creep test and indirect tensile strength (ITS) test are also employed at −10 ◦C to inspect the anti-cracking resistance of GSP [6,108].

Tensile strength of GSP can approximated to AC in the SBB test, while tensile strain is found lower than AC. This is because the excess stiffness of GSP limits the deformation of whole structure. However, admixtures can be efficient to accomplish a significant enhancement in tensile strength of GSP [38]. That is, the modified GSP shows improved low -temperature performance in the SCB test, creep test, and ITS test at −10 ◦C [108].

However, the laboratory behavior of GSP in splitting tests is far different from that in SBB tests, and both tensile strength and strain are significantly lower than AC's [70]. Modifiers also had a mild effect on GSP in this test. Therefore, these methods need further investigation and selection to uncover properties of GSP concisely. Further, Ding et al., investigated recycled Semi-flexible Pavement material by splitting test at 20 ◦C, which indicated its anti-cracking resistance was positively linked with viscoelasticity of the binder [109].

### 3.1.3. Moisture Resistance

GSP is considered as a promising anti-moisture pavement as it prevents water permeation by its high density and low residual void. This property can be assessed by freeze-thaw test and Immersion Marshall test. The freeze-thaw test is divided into the freeze-thaw splitting test (ST) and freeze-thaw indirect tensile strength (ITS) test. Thus, the results of retained Marshall stability (RMS) and tensile strength ratios (TSR) are summarized and illustrated in Figure 3 [6,38,49,70,77,103,104,110,111].

**Figure 3.** Moisture resistance of GSP.

Moisture resistance of GSP is significantly enhanced with high void rate OMA. The reason is that a high void rate can lead to void interconnection and saturated grouting [112]. From the figure, results of Immersion Marshall testing are commonly higher and easier to reach design requirement than those in freeze-thaw tests, because of the more rigorous experimental environment of the latter. There is another interesting phenomenon where some RMS results exceed 100%. This phenomenon may be triggered by further hydration of cement in GSP. Therefore, from this aspect, the freeze-thaw test may be more accurate than the RMS test in the evaluation of moisture resistance for GSP.

### 3.1.4. Oil Corrosion Resistance

GSP is made to possess an excellent oil corrosion resistance by its good density. Hao et al., adopted oil corrosion resistance test to assess GSP samples. Marshall samples were soaked in #90 gasoline for 24 h to test its retained Marshall stability (RMS), which was 88.5% twice bigger than AC [6]. Hirato et al., also immersed Marshall samples in kerosene for 48 h to attain RMS values, which exceeded 80% [111].

### 3.1.5. Impact Resistance

Impact resistance of GSP was just tested for some airport lanes by Split Hopkinson Pressure Bar equipment, which was raised by Kolsky to measure stress pulse propagation [113]. The peak stress and failure modes of samples were collected and analyzed by Dong-Hua Test Real-Time Data Measurement Analysis Software System under various air pressures and different OMA void rates. It was found that GSP with 25% void rate had the best impact resistance in the low-pressure areas (0.25 Mpa and 0.30 Mpa), and the 27%-GSP sample could resist the peak stress value of 19.67 Mpa in the high-pressure areas (0.35 Mpa and 0.4 Mpa) [113].

### 3.1.6. Anti-Weather-Exposure Ability

Anti-weather-exposure ability of GSP was tested by conserving samples in an exposing environment for 7 days, 90 days, 180 days, and 240 days. Its strength and fatigue life were shown no decline in Marshall stability test and cyclic wheel load test [114].

In addition, thermal cracking equipment was adopted to evaluate long-necked specimens at −5 ◦C. It was found the thermal resistance of GSP was positively related to the content and viscosity of binders [115].

### *3.2. Fatigue Life Performance*

The fatigue life of GSP has raised more concern in recent years, because of its different fatigue life behavior from asphalt and concrete [116]. The results of fatigue life of GSP are summarized in characteristics and correction factors, which are attained by the Indirect Tensile Fatigue test (ITFT), Fatigue Bending test, Cyclic Wheel Load test, Proportion-scale Accelerated Road test, and Full-scale Accelerated Road test. Test methods and fatigue equations of these studies are also shown in Table 11.



### 3.2.1. Characteristics of Fatigue Life of GSP

Raw materials have effects on the fatigue characteristics of GSP. To assess the fatigue life of GSP, the indirect tensile fatigue test (ITFT) and two-point bending test are commonly used under stress or strain control mode. Fatigue curves illustrate that modified asphalt (polymer asphalt or rubber asphalt) and low-shrinkage cement mortar dominate the fatigue life behavior, while binder content has a slight influence [123]. Moreover, cement may be the

most effective material for fatigue life in GSP, compared with asphalt and aggregate [120]. The fatigue life also increases with the growth of the void rate in OMA, which means more cement mortar will be grouted in OMA [77]. Additionally, from the aspect of test conditions, the fatigue life decreases with the rising test temperature and cannot be changed with different loading frequencies [119,124].

The fatigue life of GSP has a linear relationship with stiffness modulus, of which the equation is expressed as below [125]:

$$
\log N\_f = \log a + \beta \cdot \log \sigma + \gamma \cdot \log E\_{100} \tag{1}
$$

*α*, *β*, *γ* are the regression parameters. *E*100 is the stiffness parameter on the 100th cycle of the fatigue test according to the European standard, and R<sup>2</sup> could reach 0.79.

In general, GSP shows a better fatigue life than traditional AC, especially in a lowstress level [120,125]. However, the fatigue life of GSP decreases more rapidly than that of AC and more slowly than that of semi-rigid material at the high-stress level. Due to this reason, GSP may be inferior to AC in anti-fatigue performance with a higher stress [38]. The cumulative fatigue life of GSP will be also shorter than that of AC based on MINER's Linear Fatigue Damage Accumulation theory, according to the traditional fatigue failure standard (50% of initial stiffness) [126].

However, far different from the pattern of fatigue curves of AC, GSP does not have a sudden drop after loss of the 50% initial stiffness, and thus can still maintain its work capacity instead of failing with loading time [117]. Consequently, 10% residual stiffness is presented to be a new failure standard for GSP, and then the fatigue life of GSP is found to be greatly underestimated [127]. Therefore, failure standards are important and can lead to different results for the evaluation of fatigue life. Additionally, the strain control mode is recommended for fatigue life test of GSP due to its thin course structure applied in field, which shows a better fatigue life than the stress-control mode [123,128].

In 2015, Yang et al., adopted the Cyclic Wheel Loading test on GSP with different OMA void ratios (20%, 23%, 26%, and 30%) [121,122]. The failure standard was defined as a 20 mm crack on the surface. Finally, the repeated wheel loading times *Ne* was derived, which was close to the actual fatigue life in the field [121].

### 3.2.2. Fatigue Correction Factor

The correction factor or shift factor of fatigue life is an experiential effective coefficient expressing the relationship between the fatigue life in test and that in field. According to complex test conditions including material types, test methods, loading mode, and testing temperatures, the shift factor is difficult to be unified and determined through different studies.

Ling et al., calculated the fatigue correction coefficient of GSP as follows [119]:

$$N\_f = \frac{1}{5} \times \frac{1}{3} \times 0.50 \times \frac{60}{365} \cdot N\_c = 5.48 \times 10^{-3} N\_c \tag{2}$$

Specifically, the intermittent time coefficient was selected as 5; the stress-reduction multiple of the fatigue life was 3 times; the transverse distribution coefficient was adopted as 0.5; and the unfavorable season days was 60 [119].

Similarly, in another test, Ding et al., considered different factors from Ling: the intermittent time coefficient was 7; the crack propagation coefficient was 20; the Days number of the unfavorable season was 60; and the transverse distribution coefficient was 0.5. The result was expressed as follows [109]:

$$N\_c = \frac{1}{0.5} \times 7 \times 20 \times \frac{365}{60} \cdot N\_f = 1.703 \times 10^3 N\_f \tag{3}$$

Olivia et al., indicated the fatigue life correction factor of GSP should be calculated based on ITFT data, considering the intermittent time and loading mode, as shown in Table 12 [117,118].


**Table 12.** Fatigue Correction Coefficient of GSP [117,118].

Due to an additional factor of 1.1 for the lateral load distribution, 45 was finally determined as the correction factor [117,118]. However, this factor was still thought to be a conservative value, because the loading intermission in field was much longer than that in experiment.

### *3.3. Computationand Simulation Method*

3.3.1. Finite Element Method under Various Contact Models

The Finite Element Method (FEM) is widely utilized in study of material engineering. FEM software such as ANSYS, ABAQUS, and BISAR are practiced in computation and simulation for GSP. Physical properties of GSP are calculated under various contact models according to different computational hypothesis and parameters from experience and tests, as shown in Tables 13 and 14.


**Table 13.** Computational parameters of GSP in FEM.

**Table 14.** Results of FEM Calculation.


FEM can calculate load status of GSP structures using the Elastic Layered System model in three dimensions (3D) and two dimensions (2D). This model easily describes the relationship between the traffic load and the whole structure of GSP. It is found that the bonding of interlayers can reduce the shear stress, bottom tensile stress, and rebound deflection in GSP [133,134]. Thus, the structure shows a better anti-shear capability than AC, which results in a high rutting resistance [120–131]. This method also fits well and gains consistent conclusion in many circumstances, such as airports, which have different structures and requirements, [122,132].

The toughness of GSP is calculated using the Visco-Elastoplastic model as the contact model by the ABAQUS software. It is found that the toughness is affected by the OMA void: when it was 25%, GSP has the highest toughness of 11.358 kJ/m2, showing the best anti-cracking capacity [135].

A 2D sectional image model is established by a CCD digital camera and CAD software to analyze the position of cracking for GSP. OMA is assumed to be visco-elastoplastic, and cement mortar is defined as elastic in the computational process. Then, expansion force and contraction force in GSP are calculated by ABAQUS. The results show that expansion of cement mortar has little or no effect on GSP; but contraction can dramatically increase stress on asphalt-cement interfaces to lead to cracking [71]. In short, low-temperature shrinkage of cement mortar is the main reason causing cracking and asphalt-cement interfaces are the weak interfaces.

### 3.3.2. Compressive Strength Prediction Model

Compressive strength of GSP is assessed by Cube compressive strength test. The values have a linear relationship with that of grouted hydrated cement mortar [136]. The expression is shown as below:

$$y = 1.3619 \text{x}^{0.4736}, \; R^2 = 0.8525 \tag{4}$$

#### *x* is the compressive strength of cement mortar; *y* is the compressive strength of GSP. And the relationship can be illustrated in Figure 4:

**Figure 4.** Relationship of compressive strength between cement mortar and GSP [136].

In addition, Wu et al., indicated that void rates of OMA and compressive strength of GSP had an empirical regression relationship as follows [137,138]:

$$y = 8.3 \text{x}^2 - 319.7 \text{x} + 5181.4 \tag{5}$$

*x* was the void rate of OMA (20%~30%); *y* was the resilient modulus of GSP.

### 3.3.3. 2S2P1D Model

Cai et al., evaluated the viscoelastic behavior of GSP according to the 2S2P1D model (composed of 2 spring units, 2 parabolic units, and 1 clay pot unit). It was found that the 2S2P1D model had a good correlation with the test results. The equation was expressed as follows [139]:

$$E^\*(w) = E\_\varepsilon + \frac{E\_\mathfrak{g} - E\_\mathfrak{z}}{1 + \mu \left(iw\tau\_0\right)^{-k} + \left(iw\tau\_0\right)^{-h} + \left(iw\beta\tau\_0\right)^{-1}}\tag{6}$$

*<sup>E</sup>*<sup>∗</sup>(*w*) was the complex modulus; *Ee* was the equilibrium modulus; *Eg* was the glassy modulus; *w* was the angular frequency; *μ* was a calibration constant; *i* was the complex number; *τ*0 was the characteristic relaxation time; k and h were constant values defined as 0 < *k* < *h* < 1; and *β* was a constant that depended on the viscosity of the dashpot.

The reinforcement effect of GSP is positively linked with the OMA void ratio, and the dynamic modulus is associated with the gradation [139]. In other words, cement can play a role to enhance the strength of GSP under high-temperature and low-frequency loads.

### 3.3.4. Weak Interlayer Model

A cracking model is specially introduced to calculate cohesion strength of interfaces to determine the position of cracking and explain the internal factors through a full-scale Heavy Vehicle Simulator test [42,140]. Then, the strain energy of distortion (SED) is defined as the response parameter to predict weak interlayers. A high SED value presents a higher vulnerability to to cracking at this position. The equation is illustrated as follows:

$$V\_0 = \frac{1}{2E} \left(\sigma\_x^2 + \sigma\_y^2 + \sigma\_z^2\right) - \frac{\nu}{E} \left(\sigma\_x \sigma\_y + \sigma\_y \sigma\_z + \sigma\_x \sigma\_z\right) + \frac{1}{2G} \left(\tau\_{xy}^2 + \tau\_{yz}^2 + \tau\_{xz}^2\right),\tag{7}$$

$$SED = V\_0 - \frac{1 - 2\nu}{6E} \left(\sigma\_x + \sigma\_y + \sigma\_z\right)^2\tag{8}$$

*SED* is the strain energy of distortion (N·m/m3); *V*0 is the total strain energy per unit volume; *E* is Young's Modulus (Mpa); *ν* is Poison's Ratio; *G* was shear modulus (Mpa); *σ* is compressive or tensile stress; and *τ* is shear stress (Mpa).

From the traditional perspective, thicknesses and Poisson's ratios of GSP are selected from experiential values referring to traditional AC pavement, rather than the values from actually measuring. Therefore, though specialized parameters are adopted in tests for GSP, these values vary widely. For instance, the U.S. Air Force recommends that the elastic modulus of GSP should be 12,000 Mpa at 20 ◦C, following the given modulustemperature correlation curve (Figure 5), and Poisson's ratio should be 0.27 [27]. However, the design manual in the United Kingdom suggests the elastic modulus should be 8000 Mpa and Poisson's Ratio should be 0.25, according to an Indirect Tensile Stiffness Modulus (ITSM) test at the frequency of 5 Hz and the temperature of 20 ◦C [118,141]. In contrast, Pozarycki et al., back-calculated the in-situ GSP modulus by a Falling Weight Deflectometer, ˙ of which the value reaches to 23,700 Mpa [142]. For this reason, the full-scale pavement test is suggested to obtain the more precise property parameters [140].

**Figure 5.** GSP Resilient Modulus Versus Temperature Design Curve.
