**1. Introduction**

Semi-flexible pavement is a new type of pavement structure formed by replacing one or more layers of the traditional asphalt pavement surface with a semi-flexible composite material formed by pouring cement grouting material with porous asphalt mixture. It has been highly valued in road engineering in recent years. The annual application area of SFP in China is close to 1 million square meters. The application of SFP covers Jiangsu, Shanghai, Guangdong and other provinces [1–3]. SFP mixture is a composite pavement material formed by pouring specific cement slurry into macroporous asphalt mixture, which is named because its stiffness is between asphalt concrete and cement concrete [4]. Laboratory tests and engineering practices show that the SFP mixture have excellent shear strength, rutting resistance, high temperature stability [5,6], better durability than traditional asphalt pavement [7], and may help with mitigating urban heat islands in city centres [8,9]. Although SFP shows excellent rutting resistance and comprehensive road

**Citation:** Wang, S.; Zhou, H.; Chen, X.; Gong, M.; Hong, J.; Shi, X. Fatigue Resistance and Cracking Mechanism of Semi-Flexible Pavement Mixture. *Materials* **2021**, *14*, 5277. https:// doi.org/10.3390/ma14185277

Academic Editor: Krzysztof Schabowicz

Received: 26 August 2021 Accepted: 9 September 2021 Published: 14 September 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/).

performance, SFP in many sections has different degrees of cracking, and a few sections have premature pavement failure due to rapid crack development [10,11].

By finite element calculation, it is found that the maximum stress in the structure does not reach the ultimate stress that SFP can bear, so the influence of fatigue on cracking needs to be considered. In order to improve the material and structure for crack resistance, it is necessary to judge the weak position of the SFP mixture and the crack control stress. In order to find the weak parts, Ding built a SFP finite element model through digital image processing technology, characterized the distribution of materials through three-phase material structure, inferred the weak points of materials in cracks through tensile strain, and considered that the probability of cracks appearing at the interface between asphalt and grouting materials was greater [12]. A. Setyawan believes that the strength of SFP mainly depends on the strength of cement binder, and the compressive strength of cold mix grouting composite is lower than that of hot mix grouting material [13]. For the judgment of control stress, Cai applied acoustic emission technology to detect the failure process of the SFP and its porous asphalt mixture in uniaxial compression test [14]. RA (quotient of rise time divided by amplitude) value and energy distribution show that the number of shear cracks of the SFP increases during compression [14].

For the improvement of structure, Chen believes that the cracks of the SFP are mainly reflection cracks [15]. The structural model of the SFP mixture is established through finite element method, and the shear force is the main factor for crack development by applying the principle of fracture mechanics and judging through the stress intensity factor [15]. Temperature also has an important influence on the crack resistance of the SFP mixture [16,17]. Based on the above research, the methods to improve the crack resistance of the SFP include: adding materials to strengthen the bonding between three-phase material interfaces [14], reinforcing three-phase materials [18], adjusting gradation or material mix ratio [19], optimizing pavement structure and layer position [20–23].

Phenomenological method is a more traditional fatigue performance research method. It is considered that fatigue is a phenomenon caused by strength attenuation under repeated action. There are many kinds of laboratory tests used in phenomenological method, mainly including splitting fatigue test, four-point bending fatigue test, semicircular bending fatigue test, etc. The local deformation of splitting fatigue test is large, and it is difficult to control the strain [24]. The loading mode of the four-point bending method is closer to the vehicle load of the actual pavement. The four-point bending operation is simple, and the theoretical cracking point expands into a region, which is suitable for uneven materials [25,26]. The SCB fatigue test can establish a fatigue prediction model with good correlation and high accuracy [27]. The specimen of SCB fatigue test has a notch, which is more suitable for the structure with initial crack. The test specimen is easy to make, the test process is easy to control, and the result parameters are stable. It can be compared and verified in combination with the experimental results of indoor preparation and field sampling samples, which is of great significance for engineering practical detection and indoor experimental research [28].

For the cracking phenomenon of SFP, there is little research on the mechanism of fatigue failure starting from fatigue cracking, and the calculation method of fatigue performance has not formed a system. It is often considered that the weak point of the SFP mixture is the interface between asphalt binder and other materials, while the impact on the integrity of the material caused by the brittleness of grouting material and the property difference between it and asphalt mixture is ignored.

In order to understand the fatigue performance and cracking mechanism of SFP and provide reference for the material and structural design of SFP, the specific objectives of this study are as follows:


the difference between asphalt binder and grouting material on the fatigue cracking mechanism under different conditions.

#### **2. Materials and Methods**

*2.1. Materials*

SFP is prepared by pouring cement grouting material into porous asphalt (PA) mixture. The PA mixture is composed of asphalt binder, aggregate, lignin fiber, and MA-100 (Modified Agent-100) modifier. The cement grouting material was provided by Sobute New Materials Co. Ltd. (Nanjing, China).

Styrene–butadiene–styrene (SBS) modified asphalt was used as the basic asphalt binder. Its main technical indicators are shown in Table 1, and they all meet the technical requirements. Basalt was chosen as aggregate in this study and the gradation of PA is referred to Gong [29]. MA-100 modifier is a kind of asphalt interface reinforcing agent produced by Sobute New Materials Co. Ltd., and its dosage is determined by the weight of asphalt binder. Lignin fiber was included as the stabilizer of the mixture, and its dosage is determined by the weight of PA mixture. The optimal asphalt content was calculated according to the contribution from asphalt binder and modifiers and determined by Cantabro and drainage tests. The material composition and quality of PA mixture is shown in Table 2.

**Table 1.** Main technical indicators of SBS modified asphalt.


**Table 2.** Material composition and quality of PA mixture.


JGM®-301 grouting material is adopted in this study. Its main properties are shown in Table 3. The water cement ratio is 0.34. It is a kind of early strength cement, which only needs curing for 14 days in the standard curing room.



The preparation process of the SFP specimen for the experiment is as follows. Firstly, the cylindrical specimens of PA mixture is prepared using shear gyratory compactor (CONTROLS, Italy), and were sealed by tape after cooling to room temperature. The

grouting material is poured into PA mixture, and the specimens after grouting are cured for 14 days in the environment with temperature of 25 ◦C and humidity of 90%. After curing, the specimens were cut into semicircles. The thickness and diameter of the specimens were 50 mm and 150 mm, respectively.

#### *2.2. Experiment*

The semicircular specimens were used for monotonic SCB test and SCB fatigue test [30,31]. Universal testing machines (IPC, Australia) were used to load the specimens. The specimens were slit with the target notch depth of 15 mm and width of 1 mm for the SCB fatigue test. In this paper, the fracture of the specimen was taken as the fatigue failure criterion, so the stress control method is adopted in the SCB fatigue test. Temperature and stress ratio were selected as the variables of the test and the test under each condition was repeated three times to reduce the error. The tensile strength of the SFP at different temperature values was obtained by monotonic SCB test. The calculation method of tensile strength is as follows [32]:

$$
\sigma\_l = \frac{4.976F}{BD} \tag{1}
$$

*σ<sup>t</sup>* is tensile strength, MPa; *F* is the value of peak load, N; *B* is the thickness of the specimen, mm; *D* is the diameter of the specimen, mm.

The tensile strength of the SFP is the average of three parallel tests. The fatigue load of different temperature values and stress ratio tests is calculated by tensile strength. The loading frequency of the fatigue test was 10 Hz. The scheme of SCB fatigue test is shown in Table 4.

**Table 4.** Scheme of SCB fatigue test.

