The Influential Factors of Semi-Flexible Pavement Cracking Performance

XIONG Zijia, GONG Minghui, HONG Jinxiang＊, DENG Cheng

(1.Jiangsu Sobute New Materials. Co. Ltd, Nanjing 210000, China; 2. State Key Laboratory of High Performance Civil Engineering Materials, Nanjing 210000, China)

Abstract: The cracking performance of semi-flexible pavement (SFP) was investigated by using the semi-circular bending (SCB) test in this paper. Thirteen grouting slurries were prepared. The compressive strength of these materials ranges from 3 to 100 MPa. The relationship between the compressive strength of the grouting slurry and the cracking property of SFP was obtained at different loading rates and different temperatures. The peak load, fracture energy (E), flexible index (FI), and cracking resistance index (CRI) were calculated to determine the material performance. The results show that the compressive strength of the grout influences the cracking behavior. With a higher comprehensive strength grouting slurry, the FI value of SFP decreased initially and then increased slightly at 25 ℃ in 50 mm/min. The CRI value decreased at the same time. E values changed just according to the test temperature and loading rate. The damage paths of SFP are different. The damage path of the SFP sample appears as diffuse damage at 1 mm/min at 60 ℃ or clean damage at 50 mm/min at 25 ℃. These findings indicate that there is a correlation between the compressive strength of grouting slurry and SFP cracking behavior. The cracking form is influenced by loading rate and temperature.

Key words: semi-flexible pavement; cracking performance; SCB; grouting slurry; compressive strength

1 Introduction

SFP is contrasted by porous asphalt mixture filled with suitable cement-based grouting slurry[1]. The asphalt mixture skeleton’s void is located in the range of 20% to 28%[2]. As a composite material, SFP displays a rigidity-flexibility characteristic with the flexibility of the asphalt mixture skeleton and the rigidity of the cement-based grouting slurry[3,4]. This composite material can be used to resist the rutting in vehicle flow and heavy traffic[5,6].

As a new kind of asphalt pavement[7-9], SFP was first developed in France in the 1950’s as a surface protection course to address the oil and fuel spillage[10,11].Since then, many studies have been performed to investigate the performance of SFP[12].

The pavement structure engineering properties of SFP have gained much attention. Dinget al[13]investigated the recycled asphalt mixture of SFP material to determine the failure performance and the mechanical behavior at multiple scales. The major determining factor in the performance of the recycled SFP material is the recycled asphalt’s properties. Nikhil Sabooet al[14]obtained the grout suitable proportioning used in SFP by using a mathematical approach. Fanget al[15]looked into the critical performance quality of SFP affected by the cement slurry formulation. A high-performance cement grout was important to the SFP materials. T Nhan Tran[16]concluded that the significant advantages of SFP are the combinations of AC and PCC by analyzing the rutting performance and IDT strength. Yang[17]analyzed the mechanical properties of SFP material. The suitable values of the void content in the asphalt mixture skeleton are confirmed by using the cyclic wheel load test. The result assesses the impact of loading frequency on the variable of damage. Zhanget al[18]evaluated the strength of the SFP material and excogitated the optimal formulation of grout. Suhana Koting[19]investigated the mechanical properties of SFP material.The results propose that the water-cement ratio and the void content in asphalt skeleton are the critical factors regarding the workability and strength of SFP materials. A Setyawanet al[20]studied the relationship of bitumen type, filler addition oil content, and aggregate grading on the performance of asphalt mixture skeleton for SFP.

The cement-based grout material is a major part of SFP. As a rigid material, the grout increases the stiffness of SFP. Depending on this performance, it is more cracking risk of SFP than a normal asphalt mixture.Several studies have investigated the cracking performance of SFP. Gong[21]evaluated the SFP’s cracking performance with three different kinds of asphalt. The results show that adding MA-100 and fiber can increase the fracture energy and flexibility index. Sohrab[22]observed that the cement-based grout with asphalt emulsion has the same cracking resistance as the hot asphalt mixture at low-temperature. Zhang[23]pointed out that the low temperature cracking resistance performance of the cement-asphalt mix was preferable than that of the conventional asphalt matrix. Hu[24]found that the low-temperature cracking resistance of Fe-doped TiO2nanoparticles semi-flexible pavement was lower than the OGFC-16 asphalt mix.

Based on the previous studies, it can be concluded that the SFP has high resistance to rutting than the conventional asphalt mixture, while the cracking performance of SFP is inconsistent according to different researches. The main factors of cracking performance of this composite material are undefined for the present.The current studies are mainly focused on the grouting slurry proportion and the mechanical performance of SFP. As a rigid skeleton, the compressive strength of the grouting slurry may be a valuable factor to be taken into account.

It is well known that the adhesion of the interface affects the material crack development[25]. In SFP, interfaces are asphalt-asphalt, asphalt-grout, asphalt-aggregate, and grout-aggregate. Asphalt is a viscous material at a high temperature, while it transforms to viscoelasticity at a normal temperature. Thus, the adhesion of the interface is changed by temperature. Cracking is essential for interface failure. Most studies focused on low-temperature cracking performance, but the cracking properties at normal and high temperatures are rarely considered.

This study is aimed at investigating the factors of the cracking performance of SFP. The SCB test method is used. The test temperature and loading rate are distinct from the traditional. The experiment parameters are concluded in two temperatures and two loading rates. Thirteen compressive strength cement grouting slurry were prepared. The interface failure states of SFP in the above conditions are observed by scanning electron microscope (SEM).

2 Experimental

2.1 Materials

2.1.1 Maternal asphalt mixture

The porous asphalt mixture was constructed by SBS modified asphalt and basalt aggregate. Related studies[26,27]have shown that the suitable performance of the SFP material is with the void content of the asphalt mixture between 23% and 25%. Therefore, the void content of the porous asphalt mixture skeleton is selected at 24%. The maximum size of aggregate is 10-13 mm, which is often used as the top layer of the pavement. The aggregate curves are shown in Fig.1.

Fig.1 Maternal asphalt mixture gradation GRAC-13

The void content of the asphalt mixture has been calculated by the volume method. The Marshall test results are shown in Table 1. The test was based on the Test Methods of Aggregate for Highway Engineering,China (JTG E30-2005)[28]. The design index is based on the Technology Guide for Application of Semi-Flexible Pavement of China[29].

2.1.2 Cement grout

The cement grouting slurry materials of SFP mainly consisted of cement, fly ash, mineral powder,water, and some essential admixture. Based grout is acommon commodity grout named JGM-301 at serial tenth. Others are created by adding the mineral powder to decrease the compressive strength or by adding cement to increase the compressive strength. The materials are premixed by mixer, then water is added to the mixer. Thirteen grouting slurry samples of different compressive strengths are constructed. The mix proportions of thirteen cement grouting slurry are shown in Table 2.

Table 1 GRAC-13 Marshall test results

Table 2 The mix proportions of thirteen cement grouting slurry

The compressive strength was tested based on the Test Methods of Cement and Concrete for Highway Engineering, China (JTG E30-2005)[30]. The cement can meet the quality requirements. Generally, the grouting slurry of SFP should have suitable fluidity.

2.2 Test methods

2.2.1 SCB test

The SCB test was proposed to evaluate the fracture toughness of brittle materials in mode I failure[31].Later, it was adopted by scholars to study the fracture performance of asphalt mixture[32]. The SCB test is based on the AASHTO TP 124-16[33].

In the SCB test, a semi-circular sample is loaded of three-point bending. The specimen is cut to a straight notch measuring 1.5 cm, as specified in this paper. The notch is parallel to the loading axis. A sample preparation process of the SCB test is illustrated in Fig.2.

In this test, a semi-circular sample is placed on the two-point holder. The distance between the two supports isL, which is 120 mm, or 0.8 times the specimen’s diameter. The force is from up to down. The load direction and the straight notch are on the same axis.The test mode is illustrated in Fig.3. The sizes of the semi-circular specimen are shown in Fig.3.Ris the radius of the SCB specimen, and the thickness is 50 mm.These dimensions generally satisfy the requirements in the suggested method[33]. During the test, the load (F)increases continuously with the displacement control until the specimen begins cracking. In this performance, a fracture initiates from the notch tip as the load drops from the peak.

Fig.2 A sample preparation process of the SCB test

Fig.3 Specimen dimensions and loading scheme of the SCB test.F is the applied load. R and L are the radius and the distance between two branches. The length of the pre-existing notch is 15 mm

Fig.4 The performance index of the SCB test

AASHTO (TP124-16)[33]are the specifications of SCB tests. Some test elements are described, such as loading rate, sample size, bracket form, fracture index,and energy calculation for asphalt mixtures. Some test parameters are obtained and calculated. The performance index of the SCB test is illustrated in Fig.4.Ef0is the fracture energy in the whole procedure, and Ef1is the fracture energy in the pre-peak period. The slope at the inflection point ism.

Fracture energy is the area below the load-displacement curve, as expressed in Eq.(1). It means a load of energy in the form of a crack[34]. TheFIis the whole fracture energy divided by the slope at the inflection point of the post-peak load-displacement curve, as expressed in Eq.(2).FIvalue is defined as the cracking resistance of the asphalt mixture[35]. TheCRIis the SCB cracking performance parameter, as expressed in Eq.(3)[36]. The formula is as follows:

In these formulas,Ais the ligament area which is found by multiplying the thickness and the ligament length of the semi-circle specimen. Theuis the displacement at the failure time.Pis the load.Pmaxis the peak load. Themis the tangential slope at the inflection point. Themis the ability of the asphalt mixture to resist crack propagation. A highEvalue and/or a highFIvalue are desired for a good cracking performance.LowEand/or highPvalues will have a lowCRI.

Generally, the experiment loading rate usually is 50 mm/min, and the temperature is 25 ℃. While in this condition, the cracks will expand rapidly. This phenomenon is inconsistent with the actual crack evolution. For the SFP material, it is difficult to observe the process of cracking from when it first occurs to when it expands in the normal experimental mode. The procedure is very important when evaluating the anti-cracking ability of materials. Therefore, this study also considers a low loading rate (1 mm/min) and a high temperature (60 ℃).

2.2.2 Fluidity test

The flow time is the index of cement grouting slurry flowability. A flow cone is used. Firstly, 1 000 mL of grouting slurry was poured into the flow cone with the outlet closed. Then, the outlet was switched on, and the flowing time was recorded until the grouting slurry was completely removed from the cone.The test was performed based on the Test Methods of Cement and Concrete for Highway Engineering, China(JTG E30-2005)[30].

2.2.3 Compressive and flexural strength tests

A rectangular steel mold was used to prepare the test specimens. The dimension of the rectangular is 40 mm×40 mm×160 mm. One mold can form three specimens. All of the specimens were cured in a curing room. The curing humidity is 95% at 20 ± 2 ℃ for 28 d.The compressive strengths of the grouting slurry were at the rate of 2 400 ±200 N/s. The flexural strength of the grouting slurry was at the rate of 50 ± 10 N/s. The test was completed based on the Test Methods of Cement and Concrete for Highway Engineering, China(JTG E30-2005)[30].

3 Results and discussion

3.1 Grout materials tests

3.1.1 Fluidity test

The flowability was increased by the water/cementitious grouting slurry (w/c) ratio. The grout would bleed when there was too much water. The suitablew/cof cement grouting slurry were different to ensure a good fluidity, which ranges from 0.29 to 0.39. The appropriate fluidity time is between 12 and 16 seconds.The fluidity time andw/cratio are present in Table 3.

3.1.2 Compressive and flexural strength tests

Compressive strength properties are routinely used to characterize concrete materials. This type of test was taken to be the main indicator of concrete quality[37]. The compressive strengths of grouting slurry were shown in Fig.5.

Fig.5 shows the grouting slurry compressive strengths. It is seen that thirteen different compressive strengths of cement base grout were used from 3 to 1 00 MPa. When the flexural strengths of the cement base grout are in a narrow area, it is difficult to make a distinction between the grouting slurry. Otherwise, the grouting slurry in the SFP mixture filled in the voids of the maternal asphalt mixture, which is between theaggregate[38]. The stiffened grouting slurry was under the action of pressure forces. Accordingly, the grouting slurry’ compressive strengths were discussed in this study.

Table 3 The fluidity time and water content

Fig.5 Grouting slurry compressive strength and flexural strength

3.2 Load

Fig.6(a) and 6(b) present the load-displacement curves of typical grouting slurry SFP samples. There was a clear change in the load-displacement curves between 28 and 69 MPa. It is observed that the maximum peak loads significantly increased when the grouting slurry’ compressive strength increased. The slope of the post-peak curve represents the rate of crack growth.The post-peak slope became steeper when the grouting slurry’ compressive strength increased, corresponding to the progression of fast and brittle crack growth. The lower grouting slurry’ compressive strength of SFP has a smaller slope than the higher grouting slurry compressive strength.

The grouting slurry compressive strength had a significant influence on the peak loading. Based on the analysis, the low grouting slurry compressive strength could delay the crack initiation time in the same load environment.

In SFP specimen with the same grouting slurry,the peak load was affected by the temperature and loading rate. According to the results, the peak loads increased with the temperature decreasing or the loading rate increasing. The effect of the loading rate was more remarkable when the temperature was 60 ℃. At 25 ℃, the peak load at the loading rate of 50 mm/min was about twice as much as that at the loading rate of 1 mm/min. Moreover, at the temperature of 60 ℃, the value was about 2.5 times more.

Fig.6 (c) and (d) show the 1stderivative of the load-displacement curves. When the loading rate was 50 mm/min at 25 ℃, the load-displacement curve had a first derivative extreme point. Whereas when at 60℃, or there was a low loading rate of 1 mm/min, the 1stderivative-displacement curve is an inexistence trend curve. In other words, the load-displacement curves had no inflection point after the peak load. It means that the SFP materials had no obviously breaking transition at a high temperature or at a low loading rate. Therefore, there were noFIvalues.

Fig.6 The load-displacement curves of typical grouting slurry SFP specimens: (a) at 25 ℃; (b) at 60 ℃; (c) 1st derivative of typical curves at 25 ℃; (d)1st derivative of typical curves at 60 ℃

At 25 ℃, the fracture energy accumulated and reached the load limit rapidly. Whereas the specimens were destroyed slowly. That is because the cement-grouting slurry endues the SFP material with stiffness. In contrast with the high temperature of 60℃ or the low loading rate of 1 mm/min, the fracture energy accumulated slowly, but the peak forces were rapidly got, and the specimens were destroyed rapidly.It reflected the process of energy accumulation and destruction. The results show that at a high temperature or low loading rate, the failure of the specimen has certain continuity. The form of the destruction is different with different load modes.

Fig.7 Grout compressive strength and load curve

Fig.7 shows the effect of the grouting slurry’ compressive strength on the peak load of the SFP material specimen. It is obvious that the peak load increased with the grout compressive strength increasing, and the strength-load curves were linear. It means that the grout was the skeleton of the SFP. The effect of the grout skeleton was obvious in the load limit of the SFP material.

Moreover, the slopes of the strength-load fitting curves were different, which was partly because of the temperature. The slopes of the fitting curve at 60 ℃were smaller than the slopes at 25 ℃. It means that the peak load increased rapidly at 25 ℃ with the grouting slurry compressive strength increasing. It is because the asphalt matrix skeleton was effective at 25 ℃, while it lost efficacy at 60 ℃. The grout and the asphalt matrix form a double network construction. However, due to the viscous state of the asphalt matrix, the maternal asphalt mixture fails at 60 ℃. As shown in Fig.7, the peak load of the maternal asphalt mixture was just 0.36 kN at 60 ℃ and 3.19 kN at 25 ℃. It means that the SFP material had a short slab at high temperature. SFP material was destroyed due to the asphalt mixture skeleton losing efficacy. Therefore, it is necessary to strengthen the stability of the asphalt binder in the high-temperature area.

3.3 Flexibility index

TheFIis used to investigate the cracking potential of the SFP material[35]. The FI parameter is based on fracture mechanics, which is helpful in investigating SFP materials with different cracking characteristics.TheFIis related to the fracture energy and post-peak slope. It reflects the cracking behavior of the whole process.

Fig.8 Grout compressive strength and flexibility index curve

Fig.8 shows the effects of the grouting slurry’compressive strength on theFI. By calculating, the fitting curve presented an increasing trend as the grouting slurry compressive strength exceeds 69 MPa. In order to investigate the obvious trend, two compressive strengths (80 and 100 MPa) of grouting slurry were added. TheFIdecreased with grouting slurry’ compressive strength increasing and then increasing.

The results also illustrated that theFIis significantly correlated with the grouting slurry’ compressive strength. In the fitting curve, the extreme point of the quadratic function was 73 MPa. In other words, this extreme point may be the main destruction mode change point. That is to say, the grouting slurry and asphalt mixture formed a double network structure when the compressive strength was less than 73 MPa. The rigidity of SFP material increased as the grouting slurry compressive strength increased. It causes the post-peak slope to increase, and then the FI decreases.

Moreover, when the compressive strength of the grouting slurry exceeded 78 MPa, it was adequate for the cracking load. Currently, as the fracture energy increased, theFIincreased. Therefore, the grouting slurry should be selected depending on the destruction situation.

3.4 Fracture energy

The fracture energy is defined as the area under the load-displacement curve of SCB. In this paper, the fracture energy was calculated, including the pre-peak fracture energy (Ef1), whole segment fracture energy(Ef0), andEf1/Ef0.

Fig.9 Grout compressive strength and fracture energy: (a)Whole segment fracture energy (Ef0); (b)Pre-peak fracture energy(Ef1); (c)Ef1/Ef0 index

Fig.9 shows the relationship between grouting slurry’ compressive strength and fracture energy. There was a quadratic function relationship between the grouting slurry compressive strength and the fracture energy. The quadratic function equations and the correlation coefficientR2values are shown in the Figure.The trend of the curve was influenced by the temperature. At 25 ℃, with the grout compressive strength increasing, the fracture energy decreased first and then increased. At 60 ℃, with the grout compressive strength increasing, the fracture energy increased first and then decreased. Fig.9 (a) and (b) show that the curve trends ofEf1andEf0were consistent. At 60 ℃, the fracture energy was not related to the grouting slurry compressive strength. It is shown that the failure process was largely irrelevant to the grouting slurry compressive strength at high temperature. Under these conditions, the effect of asphalt may be more obvious. More attention should be paid to the cracking property of SFP material at high temperature.

Fig.10 Fitting equation coefficient: (a) of x; (b) of x2

In order to characterize the percentage of energy before fracture in the entire failure process, theEf1/Ef0index was proposed. Fig.9 (c) shows a good linear relationship between theEf1/Ef0and the grouting slurry’ compressive strength, which only existed at 25℃ with 50 mm/min. However, the other three loading conditions of theEf1/Ef0data had a poor linear correlation with a lowR2value. It is obvious that all theEf1/Ef0curves had a common similarity. With the grouting slurry compressive strength increasing, theEf1/Ef0increased. That is to say, the SFP materials required more energy to reach the peak load with a higher grout compressive strength.

Table 4 and Fig.10 show the relevant parameters of the fitting curves. It can be seen that temperature had a very significant effect on the slopes of the fitting curves. It was often the same value of the fitting equation coefficients ofxat the same temperature. For thecurves ofEf1andEf0, thex2coefficient was positive at 25 ℃ and negative at 60 ℃. It is because the temperature affected the property of the maternal asphalt mixture. TheA60/A25index shows the property of the asphalt mixture. It can be seen that theA60/A25values of the load and theEf1were 0.1. In other words, the cracking property of the asphalt mixture at 60 ℃ was just 1/10 the size of the one at 25 ℃. It shows that the SFP material’s cracking property is influenced by temperature. Consequently, the property of the maternal asphalt mixture should also be paid attention to, especially in areas with a large temperature range.

Table 4 Coefficients in fitting curves

3.5 CRI

TheEvalues of specimens were similar at the same temperature. There were multiple peak loads of the SCB test. TheCRIvalue combined theEvalue with peak load. If mixtures have similarEvalues, the brittle mixture will have a lowerCRIvalue. In other words,the flexible mixture has a higherCRIvalue, which indicates a better resistance to cracking. TheCRIof SCB specimens at different temperatures and loading rates were presented in Fig.11.

As shown in Fig.11, theCRIvalues indicated a more regular trend than that for theEvalues and theFIvalues. It is potentially because theCRIvalues considered the energy and load. The SFP materials were most susceptible to cracking at 60 ℃ and 50 mm/min of loading rate. That means grouting slurry compressive strength had obvious effects onCRIat a high temperature and high loading rate. However, in other situations,theCRIvalues were similar. The highest grouting slurry compressive strengths had similarCRIvalues at different temperatures and loading rates. The higher grouting slurry compressive strengths had lowerCRIvalues.

Fig.11 Compressive strength-CRI curves

3.6 Destruction form

The destruction paths of the SFP material specimens were difference with different loading rates and temperatures.

Fig.12 The characterization for different loading rate: (a)-(f) 60 ℃, 1 mm/min; (g)-(l) 25 ℃, 50 mm/min

Fig.12 shows the characterization of the cracking path at different loading rates and temperatures. When the loading rate was 1 mm/min at 60 ℃, the cracking route presented the diffusion damage. It means the damage through the asphalt interface, and there was no cracking through the aggregates or grouting slurry,as shown in Fig.12 (a)-(f). Indeed, the poor cohesion between the binders created a preferential cracking path, weakening the specimen. When the loading rate was 50 mm/min at 25 ℃, the failure paths show a clean damage. The destruction path was through the grouting slurry and aggregates. The cracking paths were straight,as shown in Fig.12 (g)-(l). There was a tropism of the diffusion damage cracking path, whereas the clean damage path showed non-orientation. According to the results, cracking path was one of the methods to determine the destruction form of the SFP material.

4 Conclusions

The cracking performances of SFP material with different compressive strength grouting slurry were evaluated by the SCB test. The effectors were investigated to be the peak load,E,FI, andCRI. The cracking path of semi-flexible mixtures on different temperatures and loading rates were also evaluated:

a) The cracking performance of SFP was influenced by the compressive strength of the grouting slurry. The higher compressive strength of grouting slurry had the bigger peak load of SFP. TheFIof the SFP decreased initially and then increased slightly at 25 ℃and 50 mm/min. The rigidity of the SFP material increased when the grouting slurry compressive strength increased based on CRI values.

b) Temperature index has a significant effect on the slopes of the load-displacement fitting curves. It is one of the main factors of the cracking properties of SFP, considering theEvalues.

c) The cracking path was one of the ways to determine the cracking mode of SFP. At medium temperature and high loading rate, the cracking failure paths were through the grouting slurry and aggregates,presenting a clean damage route. On the other hand,the diffusion damage path was through the asphalt interface, while there was no cracking through the aggregates or grouting slurry at the low loading rate and high temperature.

d) Grouting slurry compressive strength, temperature, and loading rate are effective when to evaluating the cracking performance of SFP materials.

Further investigation of the effects of the cracking performance of SFP, such as interface bonding strength and aggregate grade, is highly needed.