**1. Introduction**

Conventional flexible pavement, namely asphalt concrete (AC) pavement, suffers from many distresses including rutting, moisture damage, and fatigue failure after longterm operation, because of weather wearing and traffic volume growing worldwide [1,2]. With soaring demand of transportation, the costs of highway construction and pavement maintenance have increased dramatically over time. As a solution for these problems, Grouted Semi-flexible Pavement (GSP), as known as Resin-Modified Pavement, is a type of high-performance pavement that was derived in France in the 1950s. GSP consists of an open-graded matrix asphalt (OMA) mattress with a 20–35% void rate, grouted by high-fluidity cement mortar (HCM) [3–8]. GSP has grea<sup>t</sup> resistance to rutting, top-down cracking, oil corrosion, and fatigue damage, combining advantages of both asphalt and concrete [9–11]. It is cost-effective and has been used in many fields, which may be under unfavorable or complex traffic environments that are slow-speed, heavy-duty, or hightemperature, such as airport runways, factory field pavement, bus rapid transit lanes, and high-performance reclaimed asphalt pavement [12–14].

Construction of GSP is normally a two-phase operation. First, the OMA mattress is prepared and paved by equipment lighter than or equal to that used for AC. After the asphalt is cooled, HCM can be spread on the surface. Due to the good connectivity of voids in OMA, HCM penetrates the whole layer to obtain a very low residential void rate for GSP by rubber scrapers and light vibratory rollers. The GSP eventually forms the required strength for traffic through curing for a few days. The beneficial properties of this material rely on careful construction process control, and recent related challenges are problems for mechanical mechanism investigation and distress prevention, such as cracking.

In Europe, GSP was originally invented as a patent of heat-resistant pavement, namely "*Salviacim*", by French company Jean Lefebvre at Cognac airport in 1954 [3,15]. In 1987, two

**Citation:** Guo, X.; Hao, P. Influential Factors and Evaluation Methods of the Performance of Grouted Semi-Flexible Pavement (GSP)—A Review. *Appl. Sci.* **2021**, *11*, 6700. https:// doi.org/10.3390/app11156700

Academic Editor: Amir Tabakovic

Received: 22 June 2021 Accepted: 19 July 2021 Published: 21 July 2021

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**Copyright:** © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

School of Highway, Chang'an University, Xi'an 710064, China; gangxo@outlook.com **\***Correspondence:pwhao@chd.edu.cn

companies, Densit and Phoenix, jointly developed and promoted the second generation of GSP, called "*Densiphalt*", of which the residual void rate was reduced after being grouted to improve in-field performance [15–17]. In 1989, Roffe, the researcher belonging to the Jean Lefebvre Company in France, published a monograph on the "*Salviacim*", which was still considered a special construction method by the public [3,18].

The U.S. Army issued a series of evaluation reports of GSP and practiced this technology on airport aprons and tank runways from 1976 [19–21]. The Departments of Transportation of states and the U.S. were also concerned for the performance of GSP and conducted many projects to prove its performance [22–24]. Consequently, the specification of GSP by the U.S. Army was established and applied in many fields, and were updated several times [25–28].

Wang et al., conducted a trial road of GSP on Huishen pavement in the Guangdong province in China in 1986 [29]. In 1995, Xu et al., introduced laboratory tests for GSP and presented the bending tensile strength as an indicator to control its quality [30]. Subsequently, Zhang and Pan adapted the Main Aggregate Filling (MAF) method and the orthogonal tests to design GSP in 2000 [31,32]. Further, Hao et al., proposed that the optimal asphalt content of OMA should be determined by the Cantabro test and the Schellenberg Binder Drainage test, while the low-temperature cracking of GSP was found to be the main problem affecting its operational life [4–6].

Although GSP was just used originally as a construction technique for special pavements such as airport aprons, it is now considered as a rather new technology because it is found to be strongly different from rigid pavement and flexible pavement for their working behaviors. With the theory of perpetual pavement arousing and traffic load increasing in the recent years, GSP has been welcomed into a rapidly developing period and has become a potential high-performance and cost-effective pavement which has a higher complex shear modulus than AC. However, standards of this technology are seldom published in the world. There are still many unsolved problems in GSP design, such as low-temperature cracking, unreasonable testing parameters, and undeveloped laboratory methods, etc., which need intensive research [6,33]. The poor understanding of these problems limited further development and application of GSP.

The objectives of this study are to summarize the findings in GSP over the past two decades and analyze the relationships of various factors and the methods of laboratory tests on the performance to identify knowledge gaps, promote this technology and deliver recommendations for future research. 143 papers have been selected and reviewed in total on three principles: first, the study should illustrate a qualitative and quantitative evaluation for GSP; second, the research should come from a new perspective for factors and properties of GSP; third, new findings and different conclusions are included for more discussion. Finally, relevant recommendations and future studies are proposed.

#### **2. Influential Factors on the Performance of GSP**

### *2.1. Microstructure*

Raw materials and structures of GSP are the essential elements affecting laboratory performance and also yielding technical problems. In a microscopic view, the relationships among asphalt, aggregate, and cement particles build the macro-strength of GSP. Indeed, the micro-analysis method is most commonly used in laboratory tests to demonstrate the intuitive influential factors of micromorphology on the behavior of GSP.

In recent years, resolution of micromorphology techniques, especially the Scanning Electron Microscope (SEM), has reached to the nanometer level, which can capture the microstructure views between different particles to analyze their relationships [34,35]. The GSP samples are observed in various instruments, resolutions, and observing ranges. The methods and results are shown in Table 1 and Figure 1.


**Table 1.** Methods of microscopic observation for GSP.

**Figure 1.** The GSP microscopic images observed by SEM: (**a**) Microstructure at 90-day age; (**b**) Interface and fibrous structures; (**c**) Wrapping phenomenon of asphalt film; (**d**) Generation of micro-cracking and bridging effect of the fiberlikenetwork.

GSF has higher density and solidity than traditional AC, which contributes to grea<sup>t</sup> complex shear modulus and compressive strength [36]. The strength of GSP is reinforced with increase of age (at ages of 7 d, 28 d, and 90 d), and the inner hydrated cement forms a fibrous-like network that penetrates through asphalt films, as shown in Figure 1 [39]. This exclusive phenomenon, refered to as three-dimensional lattices or a mosaic spatial network, bridges asphalt and cement, which increases the thickness of interface transition zones and enhances the bonding force between asphalt and cement [36]. Meanwhile, the un-hydrated cement plays a role as mineral powder to strengthen the adhesion between asphalt and aggregate.

The propagation of micro-cracks in GSP can be also investigated by SEM (Figure 1). The observation results indicate cracking emerges from the bottoms of samples and expands along with the interface between asphalt and cement [38,40]. This reveals that the asphalt-cement interface is the weak interface existing through the whole depth of GSP structure, approved by the Heavy Vehicle Simulator test and the Dynamic Cone Penetrometer test [41,42]. Additionally, the reaction of internal among raw materials is mainly physical, because no chemical or asphalt aging processes are identified at the interfaces by the Fourier Infrared Spectrum Analyzer (FISA) (200SXVFT-IR) [37].

In general, these characteristics of micro-structure determine the macro properties of GSP, which has high rutting resistance and low cracking resistance. Moreover, the micro-analysis method can effectively and intuitively gain the relationships of each of these components and raw materials. On the other hand, this method is also limited, owing to the subjectivity of corresponding researchers and the complexity of GSP's structure. For example, some different fibrous structures can be observed from GSP samples, but which has a better coherent strength between materials is unknown because it is hard to ge<sup>t</sup> quantitative analysis from SEM images.

### *2.2. Raw Materials and Admixtures*

GSP is a complex composite, combining multiple materials, such as asphalt binder, aggregate, cement, and admixtures. Generally, the effects of them can be separated into two independent steps: intermediate mixes (OMA and HCM) and final grouted or combined mix (GSP). In other words, the properties and the formation of raw materials and admixtures can directly determine the performance of intermediate mixes, which further affects the quality of the following GSP.

OAC is an asphalt mattress with high void rate similar to the open grade friction course (OGFC). According to design principles of OGFC, aggregate gradation can contribute to the volume of void and binder is able to provide inner cohesive strength in OAC. Likewise, strength and fluidity are two main properties of HCM, and are controlled by two factors: formation and types of raw materials.

### 2.2.1. Aggregate Gradation

Open-grade aggregate is designed primarily as a skeleton of OAC for GSP. It mainly maintains compressive pressures from traffic loads and forms inter-air voids of OAC. Therefore, a good gradation can not only provide a good compressive strength but also construct an even and interconnective space of void.

An optimal gradation mostly can be realized by two design methods: an experiential method and a volumetric method called the main aggregate filling (MAF) method. The latter assumes coarse aggregate is supporting the main structure of OAC and can achieve the required void volume by filling fine aggregate and binder into the compacted coarse aggregate. Due to back-calculation of the void volume, this method is concise to control the final void rate of OMA [43,44]. However, interconnectivity of void is overlooked in this method, which is another important factor for permeability of OAC equally to void volume [45]. Therefore, factors affecting particle sizes and consecutiveness of aggregate gradation are considered to establish the relationships between the given characters above, illustrated in Figure 2, Tables 2–4.



**Table 3.** Effects of gradations on physical properties of OAC [47].



**Figure 2.** Some used gradations with different particle sizes and proportions.

The main coarse aggregate used in GSP normally falls in a range of 4.75 mm–13.2 mm particle size. Several combinations of different tiers in this range can form OMA with the requested void rate. Under the similar void rate, gradation with a large particle size will benefit the whole structure. Moreover, the water permeability of samples which have a large proportion in larger particles (14~10 mm) is significantly higher than that of average samples (Table 2) [46]. A lower residual void rate of GSP will be obtained after grouting [50,51]. Overall, a coarser aggregate gradation can be more conducive to forming a larger void space, which has better interconnection, to make the grouted cement mortar more easily able to penetrate the OMA mat [46,52]. Subsequently, flexural tensile strength and compressive strength of the following GSP are also be improved, which are positively linked with low-temperature performance and high-temperature performance, respectively [49]. On the contrary, some other properties may be slightly harmed with the

increase of particle size. Cantabro loss rises to a certain value, which indicates that the cohesion between aggregate in GSP is weakened.

From a different perspective, Ding et al., proposed that uniform gradation could conduct smoother void spaces and less stress concentration than continuous gradation. He compared two types of gradations: uniform gradation (one tier: 10~5 mm) and continuous gradation (three tiers: 13~10 mm, 10~5 mm, and 5~3 mm) [53]. It was found that GSP with uniform gradation was superior to that with continuous gradation in the properties of Marshall stability, splitting strength, compressive resilience, and low-temperature bending capacity [53].

Saboo et al., evaluated seven types of gradations using the hierarchical ranking strategy considering parameters as void content, permeability, abrasion resistance, and tensile strength of OAC; BSI-4% and Densiphalt12-4.5% were selected as the optimal gradation which had a main tier of 4.75 mm~12.5 mm [47]. In general, gradation with the main particle sizes in the scope of 4.75 mm~12.5 mm is appropriate to improve the performance of GSP to form a better structure (Figure 2).

However, a conflict exists in previous hypotheses that mainly concerns coarse particles. For example, a given continuous gradation may also contain larger-tier aggregate. The reason may be that the influence of gravel morphology and fine aggregate is overlooked which needs further research [52]. In our opinion, the two viewpoints are all correct, though only the applying scope of these principles is different. The definitions for describing gradation types are difficult to quantify, due to the existence of few samples with untested properties, such as residual void rate and water permeability. Large particle size and uniform gradation can both contribute to the formation of evener and larger void space.

### 2.2.2. Asphalt Binder

The types of asphalt binder in OMA include base asphalt, rubber asphalt, SBSmodified asphalt, and high-viscosity modified asphalt. Due to various characteristics at different temperatures, the optimal type and content of asphalt binder as two factors varies in OMA design, as shown in Table 5. Orthogonal experiment uncovers the effects of binder on the relative OMA or GSP sample, as illustrated in Tables 5 and 6.


**Table 5.** Optimal binder and high-temperature performance of OMA.


**Table 6.** Optimal binder and low temperature performance of OMA.

Asphalt binder with high viscosity and low penetration, such as 50-pen straight-run asphalt, SBS asphalt, rubber-asphalt, and high-viscosity asphalt, can increase strength and reduce Cantabro loss for OMA which will result in good high-temperature performance of GSP. However, binder types have little impact on void space structure in OMA, as well as strength and moisture resistance of GSP [46,54].

OMA samples using SBS asphalt as binder can obtain favorable water permeability. Setyawan et al., explained that modified asphalt protected OMA from drainage that caused void-blocking [46]. Therefore, the performance of following GSP samples was also enhanced, especially in low-temperature cracking resistance [46,54,55].

Rubber asphalt also promotes high-temperature performance of GSP. It is considered as a potential candidate due to its excellent cost-effectiveness and environmentfriendliness [48].,High-viscosity asphalt also shows advantages of flexural tensile strain and stiffness modulus at a low temperature, which is associated with better anti-cracking performance [54].

Optimal binder content of OMA can be determined by three main parameters in orthogonal testing: compressive strength, tensile strength, and Cantabro loss. The value of it mostly falls in a range of 3%~5%, due to different test methods. However, it can be argued that a high content rate of binder in OMA can increase toughness of OMA, leading to good anti-cracking performance of GSP [57], because the Marshall stability of OMA continues to rise with the increase of the binder content rate, even up to 9% [58].

Besides these two factors: binder type and content in OMA, bonding force for the asphalt-cement interface is overlooked in steps of influential factors analysis, which can contribute to low-temperature performance of GSP. To achieve a criterion for AC (the flexural tensile strain in small beam bending test should exceed 2000 με at −10 ◦C [59]), the factors for anti-cracking characteristics of GSP should be considered of both OMA and HCM to bridge two materials in whole structure in design.

### 2.2.3. High-Fluidity Cement Mortar

High-fluidity cement mortar (HCM) is a specialized grouting material which can penetrate OMA due to its high fluidity to construct a fibrous-like network structure inside the asphalt mattress. Strength and shrinkage rate are the two main factors of HCM controlling the performance of GSP. Therefore, to investigate their effects, various formulas of raw materials and different additives are compared, as shown in Table 7.


**Table 7.** Formulas of raw materials and performance of HCM.

The water-cement ratio is the critical factor affecting both fluidity and strength of cement mortar. The fluidity of HCM is positively associated with the water-cement ratio and negatively related to the sand-cement ratio [65]. However, an excess water-cement ratio or a short sand-cement ratio will lead to high dry shrinkage [60,61]. In addition, with the increase of water-cement ratio, the bleeding rate of HCM shows an upward trend, which may cause void-blocking and slurry leakage [66,67]. Therefore, Cheng et al., recommends that the water-cement ratio should be less than or equal to 0.55 for HCM, while Saboo et al., advises a ratio scope of 0.4~0.6 [45,66]. Additionally, 10~14 s is a recommended range for HCM fluidity [45,49,67].

The additive type is another factor in determining the performance of HCM. Superplasticizer and fly ash are commonly used to increase fluidity of HCM and minimize residual void after grouting. Some studies indicates that silica fume, mineral powder, and ultrafine sand also have good effects on fluidity [43,64,68,69]. Expansion agen<sup>t</sup> UEA can reduce the shrinkage of HCM and furtherly enhance the anti-cracking ability of following GSP [60–63,70]. However, the strength of cement mortar is inevitably underestimated by fluidity improving in the methods above. Therefore, to achieve the balance design between fluidity and strength, a range of strength value is proposed as 10~30 Mpa, and furtherly narrowed to 20~25 Mpa for HCM [45,49]. Although other factors such as the shrinkage and flexural strength of HCM have important effects on the low-temperature performance of GSP, their values are not considered as parameters by recent studies comprehensively.

Cai et al., used ABAQUS software to simulate the shrinkage and expansion of cement mortar in GSP to estimate the inner stresses [71]. Index values of the deformation were calculated and determined to protect GSP from cracking that is caused by stress concentration of shrinkage, as shown in Table 8.


**Table 8.** Recommended deformation indexes of cement mortar in GSP [71].

### *2.3. Admixtures*

Admixtures for GSP refer to specific materials adding to OMA or HCM to improve workability or strength of following GSP. Resin is the admixture early used in GSP which can be traced back to 1976 in the U.S. Amy [72], and polymers have been used to enhance the anti-cracking performance of GSP from the 1980s on the Huishen highway, China [73]. Over the past 40 years, more and more types of admixtures have been developed into a big family for GSP.

According to different ingredients, GSP admixtures can be roughly classified into five categories: polymers, fibers, interface modifiers, emulsified asphalt, and other new functional admixtures. New functional admixtures have been recently explored for extension in road functions such as emission-reducing, water conservation, and weather cooling. In addition, two blending methods are usually used for admixtures of GSP: firstly, admixture is blended into cement mortar to reduce rigidity and dry shrinkage of HCM, which furtherly improve cracking resistance for following GSP; secondly, it is mixed within asphalt to enhance the strength of OMA, which achieves a high rutting resistance for the whole structure. Admixtures and their effects on GSP are illustrated in Table 9.


**Table 9.** Admixtures and their effects on GSP.


**Table 9.** *Cont.*

1 A = Asphalt; C = Cement; P = Polymer; E = Epoxy; F = Fibre; I = Interface modifier; M = Modifier.
