Bonding Performance and Microstructural Mechanism between Rapid Repair Materials and Old Concrete Pavement

Abstract

In China, airports predominantly utilize airport cement concrete pavement, which inevitably undergoes deterioration in service. To uphold pavement durability and functionality, and ensure aircraft operational safety, prompt repairs of affected areas are imperative. Therefore, ordinary Portland cement mortars were used as the control group to compare and analyze the bonding performances of two common airport pavement repair materials: modified Portland cement mortars and phosphate cement mortars. Meanwhile, through microscopic experiments, the microscopic characteristics of the interfacial transition zone (ITZ) were studied, and the interface bonding mechanism was analyzed. The research results indicate that the interface bonding strength between phosphate cement mortar and old concrete pavement is the highest. This was because the elements in phosphate cement penetrated the old concrete pavement through hydration reactions, forming van der Waals forces and chemical bonding forces. In addition, the research results indicated that the presence of old concrete pavement made the three repair materials produce similar sidewall effects with the old concrete pavement, leading to a low hydration degree of the repair materials. However, the chemical bonding and penetrating structure of phosphate cement compensated for the weakening effect of the ITZ in the repair materials.

1. Introduction

Although asphalt pavement is widely used on highways due to its excellent flexibility, smoothness, and ductility, cement concrete pavement remains important in airport engineering because of its high strength and good durability [1,2,3]. However, with the increase in service life, cement concrete pavements inevitably experience damage such as edge spalling, joint deterioration, and surface functional impairments, including cracking, peeling, and aggregate exposure. To ensure the structural durability and normal functional performance of the cement concrete pavement, as well as the aircraft operation safety, it is necessary to promptly repair the damaged areas of the pavement. Generally speaking, considering the special requirements for airport pavement repair, two aspects should be emphasized. On one hand, the repair time for airport pavement is usually limited, especially at airports with high operational pressure, where repairs can only be carried out at night after flight operations have ceased, resulting in a very short repair time. Therefore, to avoid affecting daytime flight operations, it is required that the repair material can quickly reach the pavement strength requirements. On the other hand, repair materials should have good bonding performance with the old concrete pavement. If the bonding performance is poor, secondary damage will occur shortly after repair, not only increasing the maintenance workload and leading to a waste of funds but also potentially causing man-made foreign object debris (FOD) due to spalling repair blocks, thus affecting aircraft operational safety [4,5,6]. Therefore, rapid repair materials with good interface bonding performance are the preferred materials for airport pavement repair.

Currently, materials used for repairing cement concrete pavement can be divided into inorganic and organic types. The organic type mainly includes materials with organic polymers such as epoxy resin or polyurethane. Although this kind of material has high toughness and adhesiveness at air temperature, it is only suitable for small areas and emergencies under special circumstances due to its easy aging, poor coordination with the original surface, and high production and construction costs [7,8]. The inorganic type mainly consists of cement-based repair materials, including ordinary Portland cement, modified Portland cement, sulfoaluminate cement, phosphate cement, and so on. This kind of material has similar properties to the old pavement. After repair, the interface of the old and new pavement has high bonding strength, and the deformation coordination between them is good under external load. At the same time, it has good wear resistance and salt-freeze resistance, which can ensure long-term use after repair, so it is commonly used in pavement repair [9,10].

Hence, research results on rapid repair materials for cement concrete pavements have sprung up and have obtained applications in engineering [11,12]. Overall, modified Portland cement and sulfoaluminate cement have relatively high early strength and bonding performance, but their hydration products are unstable, which can lead to a decrease in later strength and the occurrence of reduction phenomenon. Phosphate cement has good long-term stability, but it undergoes intense early hydration, making it challenging to control the hydration rate, which is not favorable for construction. Gholami [13] studied a cement-based material containing traditional high cement content and a calcium chloride accelerator and found that this type of material has high early shrinkage but good compatibility and bonding performance. Xiao [14] found that the occurrence of strength reduction is due to the continuous growth of ettringite crystals with age, which generates significant expansion stress inside the hardened slurry, leading to the continuous appearance of microcracks inside the material. Tao’s [15] review summarized how the hydration rate and degree of sulfoaluminate cement depend on the quantity and reactivity of calcium sulfate and other small phases. Meanwhile, sulfoaluminate cement faces the problem of low carbonization resistance and high chloride ion diffusion ability, so its durability needs to be improved in application. Yang [16,17] and Wang [18] conducted relatively systematic studies on the setting time, mechanical properties, volume stability, durability, and interface bonding performance of phosphate magnesium cement. Hall [19] compared the retarding effects of borax, boric acid, and sodium tripolyphosphate on phosphate cement, and the results showed that the effects of borax and boric acid were better than sodium tripolyphosphate. In conclusion, after years of research, the reduction phenomenon in modified Portland cement and sulfoaluminate cement has been mitigated. Moreover, the development of retarders has effectively controlled the hydration rate of phosphate cement. As a result, inorganic regenerated repair materials such as modified Portland cement and phosphate cement have been extensively applied in airport pavement repairs.

In various performance studies, interfacial bonding strength is a key focus of repair materials. Whether the repair system can achieve a dense interface and strong adhesive strength is one of the critical issues faced by repair materials. Through continuous experiments and improvements, researchers have developed a variety of test methods, such as the tensile strength test, flexural strength test, shear strength test, compression-shear test, and tensile-shear test, to characterize the interface bonding performance between new and old pavements after repair. Additionally, scanning electron microscope (SEM), energy dispersive spectrometer (EDS), and nanoindentation techniques were used to analyze the microstructure at the interface, so as to explain the mechanism of the macro properties at the interface [20,21,22]. Formosa [23] characterized the interface between phosphate cement mortar and ordinary Portland cement mortar using SEM and EDS, and the results showed that the phosphate slurry penetrated the ordinary Portland cement mortar side. Qin [24] analyzed the materials in the interfacial region of the specimen after the failure of the interface bond strength by X-ray diffraction (XRD) and found that potassium magnesium phosphate cement formed a chemical bond with the ordinary Portland cement clinker or hydration products at the interface. Lyu [25] determined the porosity and thickness of the ITZ through Back Scattered Electron (BSE) analysis and studied the effect of aggregate particle size on the overlap phenomenon in the ITZ. While these studies have predominantly examined the bonding performance between rapid repair materials and old concrete pavement, there remains insufficient research on the specific interface bonding strength requirements for airport cement pavement repair. In airport engineering, rapid repair materials must not only exhibit strong bonding properties with existing pavement but also possess adequate interfacial bonding strength to withstand external loads effectively. Furthermore, a systematic analysis of the evolution of interfacial bonding performance over time and the formation mechanism of inorganic rapid repair materials is lacking.

Therefore, the main objective of this paper is to study the bonding performance and microstructural mechanism between rapid repair materials and old concrete pavements. Modified silicate cementitious materials and phosphate cementitious materials, two of the most popular inorganic forms of quick-repair materials utilized in airport cement surfaces, were chosen for comparison with regular silicate cements. A pull-out test and an interfacial flexural test were performed to assess the bonding performance and development law. At the same time, the micromorphology of the ITZ was analyzed through SEM and EDS, and the microstructure of ITZ was quantitatively characterized using BSE and image-processing methods. Then, the bonding mechanism between different repair materials and old concrete pavement was studied. By combining interface bonding strength, specimen failure modes, and strength correlation analysis, the main factors affecting interface bonding strength were identified. The results of this study can enhance the level of cement pavement repair and provide references and bases for the formulation of cement concrete pavement repair technical standards in airports.

2. Materials and Methods

2.1. Materials

The ordinary Portland cement used in this study (P.II 52.5R) included 3.7% limestone fly ash, 4.9% gypsum, and 1% CaCl₂ as an early-strength agent to accelerate hydration. Additionally, 0.3% polycarboxylic superplasticizer was added to lower the water–cement ratio and enhance strength.

Table 1. Physical and mechanical properties of ordinary Portland cement.

Fineness (0.08/%)	Specific Surface Area (m2/kg)	Setting Time (min)		Compressive Strength (MPa)		Flexural Strength (MPa)
		Initial	Final	3 d	28 d	3 d	28 d
0.2	349.4	126	166	32.6	60.6	6.3	8.8

shows the mechanical properties of ordinary Portland cement. The modified Portland cement is produced by increasing the fineness of the C₃S and the content of the C₃S and gypsum, as well as incorporating calcium carbonate. Enhanced fineness improves the reactivity and strength of the cement paste, while higher C₃S content contributes to increased early strength and durability of the concrete. The addition of gypsum serves to regulate the cement setting time and prevent flash setting. The XRD results depicting crystalline phases are illustrated in Figure 1. Phosphate cement is generally made of magnesium oxide, ammonium dihydrogen phosphate, borax retarder, and mineral admixture in a certain proportion. It is usually divided into A and B components, whose XRD results are shown in Figure 2. The fine aggregate used for preparing mortar specimens is river sand with a fineness modulus of 2.67.

2.2. Test Methods

Bond strength is an important index for evaluating the interface bonding performance. Tests for interface bond strength after repairing the new and old airport pavement surface include test methods placed roughly into the following categories: (1) The pull-out test method, in the old concrete substrate covered with a layer of repair material, through the pulling head connected to the pulling instrument, tests the new and old material interface bond at the tensile capacity (Figure 2a). (2) After curing, the formed benchmark specimen is cut in half and placed with one side in the flexural strength test specimen mold, with repair material poured on the other side. After the specified curing period, the bond flexural strength at the interface is tested (Figure 2b). (3) The shear resistance method is used to test the shear resistance at the interface. This method produces stress concentrations at the edges of the bond plane and does not provide a good representation of how shear forces are transferred between the old and new materials (Figure 2c). (4) In the compression-shear method, in the process of load loading, the interface surface generates compressive and shear stresses at the same time (Figure 2d). (5) The pull-shear method tests the effect of shrinkage and expansion deformation of the repair material on the bonding performance of the interface, as well as the permeability and corrosion resistance at the interface (Figure 2e). Among the more widely used methods are the pull-out test and interfacial flexural methods.

2.2.1. Pull-Out Test

The interface bonding performance was characterized by a pull-out test as shown in Figure 3. In the test, the size of the base plate was 40 cm × 40 cm × 4 cm. To achieve a rough surface similar to the in situ repair surface, the base plate was formed by layering pouring. First, a 3.5 cm base layer was poured, followed by the remaining 0.5 cm surface layer. The surface layer concrete mix included a 0.1% sodium citrate retarder based on the standard mix ratio, so the setting time of the surface layer concrete lagged several hours behind the base layer. About 10 h after the surface layer was poured, the base layer concrete was sufficiently hardened, and the surface layer concrete could be removed using a steel brush and water to simulate the uneven condition of the old concrete pavement in actual construction. Additionally, the surface texture depth of the base plate was controlled at approximately 0.6 mm using the sand patch method. When preparing the specimens for the pull-out test, a flexible silicone plate with 16 pre-formed holes of 40 mm × 40 mm × 5 mm was placed on the base plate. Then, the prepared rapid repair material was filled into each hole and compacted, and the metal pull-out rod was placed on the repair material and pressed firmly. After curing to the specified age, the testing was conducted. The pull-out strength of the specimen was calculated by Equation (1) [26].

$$R_b=\frac{F}{S}$$

(1)

where R_b is the pull-out strength of the specimen, MPa; F is pull-out force, N; and S is the bonding area between the repair material and the base plate, 1600 mm².

2.2.2. Interfacial Flexural Test

The interfacial bonding performance was also characterized by the interfacial flexural test as shown in Figure 4. In the test, a reference block was pre-formed to simulate the old cement pavement. The reference blocks were formed using a 40 mm × 40 mm × 160 mm three-gang mold. After forming for 1 d, the mold was removed, and the reference blocks were placed into water for curing for 28 d. Then, the reference blocks were cut in half along the short side and air-dried for 4 d. When preparing specimens for the interfacial flexural test, the processed reference block was first placed into the mold (40 mm × 40 mm × 160 mm), and then the prepared rapid repair material was filled into the other side of the mold and vibrationally compacted. After curing to the specified age, the testing was conducted. The interfacial flexural strength of the specimen was calculated by Equation (2) [26].

$$R_f=\frac{1.5Fl}{b{h}^2}$$

(2)

where R_f is the interfacial flexural strength, MPa; F is the load pressure, N; l is the support spacing, mm; b is the specimen cross-section width, mm; and h is the height of the specimen interface, mm test surface area.

2.2.3. SEM and EDS

To reveal the interface bonding mechanism between different rapid repair materials and old concrete pavement, SEM was used to observe the microstructure of the ITZs between rapid repair materials and old concrete pavement and obtain the BSE images. EDS was used to analyze the chemical composition of the ITZs.

Samples for SEM and EDS were obtained from the interfacial flexural strength specimens. The cube blocks with a side length of about 10 mm were cut from the flexural strength specimens by the concrete cutter equipped with a water-spray system. Then, the cube blocks were soaked in isopropanol for 2 d and then dried under vacuum for 2 d. To ensure the accuracy of the test results, the surfaces of the cube blocks needed to be polished. First, the dried cube blocks were placed in cylindrical rubber molds with a diameter of 30 mm, and the prepared epoxy resin was poured in. Then, the blocks were vacuum embedded to increase the penetration depth of the epoxy resin and remove bubbles from the resin. After that, the blocks were cured in an environment at 20 ± 2 °C for 24 h. Finally, the blocks were demolded and polished. The sample production schematic is shown in Figure 5.

In the BSE image analysis, the image size was calibrated, the interface between the new and old pavements was determined, the interface was labeled, and the old pavement was removed sequentially, as shown in Figure 6. To avoid the influence of noise generated during the experiment on the research results, a 3 × 3 median filter was used to filter the image after removing the old concrete pavement. To quantitatively characterize the properties of the rapid repair materials at the interface between the new and old pavements, the Gaussian mixture model (GMM) was used to determine the phase segmentation threshold. However, to prevent the GMM from iterating to a local extremum and making the calculated results seriously different from the actual results, the results were corrected by the frequency-gray histogram method and cumulative frequency-gray histogram method.

3. Results and Discussion

3.1. Interfacial Bonding Strength

3.1.1. Failure Modes

Regarding interfacial flexural strength specimens of various cement varieties, Figure 7 depicts the two types of interfacial damage forms. The interfacial surfaces of ordinary Portland cement mortar and modified Portland early-strength cementitious materials with old concrete are smoother, as shown in Figure 7a,c; in addition, phosphate cement mortar, shown in Figure 7b, can pull out cement stones or aggregates at the interface with old concrete. It is evident from the bonded pull-out strength specimen damage form, shown in Figure 8, that phosphate cementitious materials have high interfacial bonding capabilities. In the pull-out test, the concrete at the interface pulls out because the simulated old cement pavement concrete is not as strong as the phosphonate cement mortar. The area of pull-out grows as the specimen’s drying duration lengthens.

The form of interfacial strength damage suggests that the interfacial bond between ordinary Portland cementitious materials and modified Portland early-strengthening cement concrete primarily originates from the role of physically embedded locking, whereas the interfacial bond between phosphate cementitious materials and old concrete originates from the combined effect of physically embedded locking and chemical bonding. The weak point of the entire combined specimen is the interface between the old and new materials, and the substantial variations in bond mechanisms between different repair materials and old concrete account for the observed discrepancies in interfacial bond strength.

3.1.2. Bonding Strength

The test results of interfacial flexural strength and bond pull-out strength of rapid repair cement mortar and old concrete are shown in Figure 9. The interfacial flexural strength and bond pull-out strength of the repair mortar follow essentially the same growth patterns as they age, growing more quickly in the early stages and stabilizing gradually in the latter stages. There are significant differences in the interfacial bonding between various repair mortars and old concrete. Phosphate cement mortar and old concrete have the strongest interfacial bonding. At 28 days, the interfacial flexural strength and bond pull-out strength can reach 10.1 MPa and 3.4 MPa, respectively. Between Portland cement mortar and old pavement, the interfacial flexural strength and bond pull-out strength can reach 5.3 MPa and 1.6 MPa at 28 days, respectively. Modified Portland early-strength cement mortar exhibits the lowest interfacial bond with old concrete; its interfacial flexural strength and bond pull-out strength are essentially maintained at around 1.8 MPa and 0.8 MPa after 1 d, respectively, significantly lower than those of the other two materials.

The ultimate tensile qualities at the old and new interfaces are largely reflected by both the interfacial flexural strength and the bond pull-out strength. The two were fitted using regression analysis, as shown in Figure 10, and it was discovered that they had a strong linear association, with a correlation coefficient R² over 0.8. The slopes of the fitted equations for various repair materials can be seen, from the fitted equations, to be close to one another, suggesting that there is an intrinsic relationship between the bond pull-out strength and the interface flexural strength that is less influenced by the type of material.

3.2. Microstructural Analysis

3.2.1. Microtopography

A typical SEM picture of the microscopic morphology of the area where the old and new pavement surfaces meet after restoration is shown in Figure 11. With penetration cracks and severe separation, a distinct border between the two Portland cement mortars and the previous pavement can be seen at 1000× magnification. At 5000× magnification, the maximum crack widths measured were 3.0 μm and 2.4 μm, respectively. Both the phosphate-based cement mortar and the old pavement exhibit very good adhesion, and the interface at 1000× magnification demonstrates a high degree of Integrity and densification. Even at 5000 times magnification, it is difficult to distinguish the interface segmentation line with accuracy; no gap or microcracks are visible, indicating that the microstructure between the two surfaces is well developed. At the interface, there may be other embedded interactions that synergistically enhance the bonding ability of the phosphate cement material. The interfacial bond strength of phosphate cement material is much higher than that of the other two materials, and the failure fracture of the interfacial bond strength specimen occurs at the interface, which explains the aforementioned events.

3.2.2. Element Distribution

The distribution law of each element at the interface can be tested using an energy dispersive spectrometer (EDS), and the findings are displayed in Figure 12. The distribution of aggregates and cement stones on both sides of the interface primarily affects the ratio of each element. For ordinary Portland cement mortar, the element types on both sides of the interface are the same and primarily consist of Si, Ca, Al, O, S, and other elements constituting Portland cement clinker and hydration products. When a Portland cement mortar is poured, Ca(OH)₂ at the original airport pavement surface’s interface may react with the reactive components in the mortar to produce products like C-S-H gels, which create a chemical connection there. The mechanical occlusion effect is the main source of the interfacial bond strength, as shown in Figure 11, and the previously noted gap between the original airport pavement surface and the cement mortar of the two Portland cement systems, which suggests that the chemical bonding is not as strong.

Following the repair of the phosphate cement mortar, there is a more noticeable difference in the distribution of the characteristic elements between the two sides of the interface. Ca and Si are mainly distributed in the right original pavement surface, with a small amount of Si on the left mainly derived from fly ash added in the phosphate cement component, and Ca elements come from calcium silicate impurities in magnesium oxide particles [24]. The distribution of P components is more even on both sides, whereas Mg is mostly found on the left side of the phosphate cement mortar, making it easy to distinguish between the old and new pavement. For the elemental species and percentage investigation, one point per side at the interface boundary line was chosen, as shown in Figure 13. The composition of 1# point was primarily O, Si, Ca, P, and other elements, suggesting that P elements entered the original airport pavement surface through pores and microcracks. According to the relevant literature, the secondary phosphoric acid phase was formed by the reaction of soluble acid phosphate with Ca(OH)₂ or other alkaline fractions. The substance that made up the 2# point included elements such as Ca, P, O, Mg, and Si, indicating that chemical reactions between the new and old materials happen at the interface [27]. Phosphate cement mortar creates a strong bond at the interface with the original airport pavement surface through chemical bonding effects and embedded locking when combined with experimental analysis and research from the literature. To create an embedded locking effect, phosphate cement mortar can, on the one hand, fill and penetrate the pores and cracks at the interface with the original airport pavement surface after pouring; on the other hand, because phosphate dissolves quickly, phosphate cement slurry contains a lot of H+ ions. In addition, under acidic conditions, hydration products and unhydrated cement particles at the interface of the original airport pavement surface dissolve and react with the phosphate cement slurry to form a chemical bond [28]. Furthermore, through microcracks and pores in the old specimens, liquids with a lot of ions (H+, H₂PO⁴⁻, HPO₄²⁻, etc.) seep in and strengthen the alkaline components’ chemical compositions.

3.2.3. Phase Distribution

According to the physical phase segmentation threshold determined by the Gaussian mixture model algorithm, the gray-scale ranges of pores, hydration products, and unreacted particles were determined, and the percentage of each physical phase in the image was calculated. Figure 14 shows the distribution of each physical phase of the quick-repair cement material at the interface, from which it can be visualized that the modified Portland cement has more unhydrated particles, and the phosphate cement has more pores at the interface. Figure 15 shows the percentage of each physical phase at the interface for different quick-repair cement materials, where the percentage of each physical phase is the average of multiple images. Figure 15 illustrates the increased degree of hydration at the interface of typical Portland cement mortar, with hydration products accounting for 79.31%, and unreacted particles and pores contributing around 10% each. A significant amount of cement clinker builds up at the interface without taking part in hydration, as evidenced by the proportion of unreacted cement particles at the modified Portland cement mortar’s interface reaching 26.55%. This implies that the transition zone of the interface has a low degree of hydration in the modified Portland cement. Phosphate cement mortar has a relatively large percentage of both unreacted cement particles and porosity at the interface, where the larger percentage of unhydrated cement particles is mainly due to the accumulation of a lot of spherical fly ash not involved in the reaction at the interface, which can be clearly seen from the BSE image. The large porosity at the interface can be explained by the “sidewall effect” between cement paste and aggregate in cement concrete [29,30]. This effect arises due to the presence of the old airport pavement surface, which hinders the random distribution of cement particles. Consequently, smaller-sized particles tend to predominate in the cement at the interface, creating a transition zone where larger particles cannot effectively accumulate. This phenomenon results in the formation of a loose and porous structure at the interface.

The same occurs with ordinary Portland system cement, in which modified Portland cement has more pores and more unreacted particles compared to ordinary Portland cement, which can explain the lower interfacial bond strength of modified Portland cement mortar. Phosphate cement mortar also has more pores and unreacted particles, but the interfacial bond strength is high, mainly due to the high chemical bond or van der Waals forces between the phosphate cement and the original channel surface, which can be seen based on the interfacial bond strength specimen damage section and phosphorus elemental distribution diagram. When using ordinary Portland cement materials for pavement repair, it is recommended that an interfacial agent that chemically binds the old and new materials be applied to the surface of the old pavement.

To further study the properties of the repair material at the interface and analyze the scope of influence of the presence of the old airport pavement surface on the microstructure of the nearby repair material, several strips were delineated by the method of boundary line translation to calculate the ratio of each physical phase of the repair material at different locations from the interface of the airport pavement surface. Figure 16 shows the changes in the ratio of each physical phase of the repair material at different locations from the old pavement interface. As the distance from the interface edge line increases, the proportion of hydration products in both ordinary Portland cement mortar and phosphate cement mortar increases, whereas the proportion of pores and unreacted particles decreases, as shown in Figure 15. Among them, when the distance exceeds 15–20 μm, the change of the proportion of each phase in ordinary Portland cement tends to level off; when the distance exceeds 20–25 μm, the change of the proportion of each phase in phosphate cement mortar tends to level off. The above phenomenon shows that the “sidewall effect” of the airport pavement surface will make the repair material form a low degree of hydration and loose porous structure at the interface.

However, the changes in the proportion of the physical phase of the modified Portland cement mortar as the distance to the interface edge line increases are quite different from those of the other two materials, which are mainly manifested in the fact that the proportion of hydration products gradually decreases in the range of 0–15 μm from the interface, while the proportion of unreacted particles gradually increases. This phenomenon further indicates that the hydration degree of modified Portland cement in the transition zone of the interface is low, and a lot of cement clinker accumulates at the interface without participating in hydration, which is the same phenomenon as the phenomenon of patches of unhydrated cement particles visualized in Figure 14b. The reason for this phenomenon may be that the old airport pavement surface has large water absorption at the interface, resulting in insufficient cement hydration in a certain range of the Interface transition zone.

3.2.4. Thickness of the ITZ

When calculating the proportion of each object phase on the strip at different distances from the interface edge line, the mean values of multiple images at the same distance from the interface edge line were used. To analyze the lateral distribution of each object phase at the same distance from the interface edge line, and to understand the statistical significance of the mean values, the lateral coefficients of variation of the object phases on each strip were calculated. The coefficients were calculated using Equation (3).

$$C_i=\frac{{\mu }_i}{{\sigma }_i^2}\times 100\%$$

(3)

where $C_i$ is the transverse coefficient of variation of the individual phases on strip i, and ${\mu }_i,{\sigma }_i^2$ are the mean and variance of the percentage of individual object phases on the ith strip of different images, calculated using Equation (4).

$${\mu }_i={{\displaystyle \sum }}_{n=1}^N\frac{p_{ni}}{N} , {\sigma }_i^2={{\displaystyle \sum }}_{n=1}^N\frac{{\left(p_{ni}-{\mu }_i\right)}^2}{N}$$

(4)

where p_ni is the percentage of individual object phases of the nth image on the i strip, and N is the number of images.

The transverse coefficients of variation of pore space, hydration products, and unreacted particles across the strips are shown in Figure 17. The percentage of pore space and unreacted particles varied more with increasing distance to the interface margins for the various materials, falling within the range of 15% to 55%. This is mainly due to the fact that the occupancy ratio of both is relatively small and the sensitivity of the coefficient of variation to changes in the value of the occupancy ratio is high. In addition, the pore space has the largest variation due to its more complex composition, including capillaries, bubbles, cracks, etc., and its mean value is superimposed on a complex irregular pattern. In contrast, the variation of the transverse coefficient of variation of the percentage of hydration products basically stays between 5% and 20%, indicating that the percentage of hydration products at different transverse locations at the same distance from the interface edge line has a low dispersion and a low degree of variability. To more accurately describe the changes in the microstructure of the repair material at the interface with the increase in the distance to the interface edge line, to determine the weak performance area of the repair material at the interface, and to avoid the influence of the incidental factors of the image selection process on the analysis results, this paper chooses the average value of the percentage of hydration products of different images to carry out the next step of analysis.

To determine the range of weak performance of the repair material at the interface, the difference in the percentage of hydration products occupying adjacent strips was investigated and evaluated by the coefficient of variation [31]. The coefficient of variation was calculated using Equation (5).

$$D_i=\frac{\left|p_{hi}-p_{h1}\right|}{p_{h1}}\times 100\%$$

(5)

where $D_i$ is difference coefficient of the hydration product’s proportion between the ith strip and the first strip; $p_{hi}$ is the hydration product’s proportion of the ith strip; and $p_{h1}$ is the hydration product’s proportion of the first strip.

The percentage of hydration products of the repair materials and their coefficient of variation vary as the distance from the interface margin increases, as shown in Figure 18. When the distance to the interfacial boundary increases from 0 μm to 20 μm, the proportion of hydration products in silicate cement mortar increases from 0% to 30%, with a fluctuation of around 30%. A similar phenomenon exists for modified Portland cement mortar and phosphate cement mortar. This phenomenon indicates that there is a difference in the microstructure between the interface region near the old airport pavement surface and the interface region far away from the old airport pavement surface. The region with a different microstructure of the repair slurry is called the interface transition zone, and the thickness of the interface transition zone is the distance to the interface edge line when the difference coefficient of the percentage of the hydration product starts to stabilize. In the study of hydration product proportion difference coefficient in different strips of the law of change, the horizontal coordinate is used in each strip based on the interface of the median distance from the borderline. In this paper, when determining the thickness of the transition zone at the interface, a conservative treatment was carried out; that is, the maximum value of the distance from the strip where the boundary line of the transition zone is located to the interface edge line was taken as its thickness.

Table 2. Thickness of interfacial transition zone of rapid repair cement mortar.

Repair Material Type	Ordinary Portland Cement Mortar	Modified Portland Cement Mortar	Portland Cement Mortar
ITZ thickness	20 μm	25 μm	25 μm

shows the thickness of the interfacial transition zone for quick-repair cement mortars. The interfacial transition zone thicknesses of the three materials are all between 20 μm and 25 μm, with little difference, indicating that the presence of the original airport pavement surface has approximately the same range of influence on the three materials.

3.3. Interfacial Bonding Mechanisms

Combined with the previous interface strength test and interface microstructure analysis, the interface bonding mechanism is studied according to the following four aspects:

(1)

The process of forming a rapid repair cement material involves pouring the cement particles into the pores and cracks on the original roadway section, followed by a hydration reaction where the hydration products progressively become more hydrated. The original airport pavement surfaces are then riveted to each other, creating a mechanical occlusion effect, as shown in Figure 19. Interface bond strength and material body strength correlation analysis shows that this role is influenced by the repair material hydration product strength, which increases macro-performance as material body strength develops. The primary cause of early interfacial bond strength at the repair interface is mechanical occlusion, which has varying effects depending on the material’s early-strength development rate, the hydration product characteristics, and degree of interfacial defect filling in the original course surface;

(2)

Some of the dissolved ions penetrate into the original pavement through the microcracks under the effect of adsorption and have a series of chemical reactions with unreacted cement clinker or hydration products to generate new products, and through the hydration products in the microcracks, the repair material and the original pavement are bonded together, forming a structure that penetrates through the original pavement of cement stones and aggregates, as shown in Figure 20. This sort of structure considerably improves the interfacial bonding qualities, and pulling out of the original pavement components happens during strength testing. However, its production is tied to the qualities of the repair material, and in combination with the microstructure of the interface and the type of damage to the bonded specimens, this structural form is obvious at the interface between the phosphate cement mortar and the original pavement;

(3)

The unreacted particles and hydration products of the original airport pavement surface will be partially dissolved in the repair material’s solution at the interface between the old and new pavement surfaces. The ions of the old and new materials will be drawn to one another, creating intermolecular forces, or van der Waals forces, while some of the ions will simultaneously react to form chemical bonds, as shown in Figure 21. Both of these processes make a substantial contribution to the interfacial bonding qualities, with van der Waals forces present at most repaired cement interfaces, while the creation of chemical bonding is dependent on the chemical composition of the repair material;

(4)

Due to the sidewall effect, the repair material’s side interface area has a low degree of cement hydration, creating a loose and porous performance-weakening area as shown in Figure 22. Based on prior experiments and data analysis, it was discovered that the performance of various repair cement materials in the weakened region had similar thicknesses. This suggests that the influence of the original airport pavement surface on the repair material’s performance is not very different. However, the fraction of microscopic phases in this area is varied, and its influence on the total interfacial bonding performance needs to be evaluated in connection with other aspects.

3.4. Discussion

The different repair materials exhibit substantial differences in composition and bonding form with the existing pavement. The interfacial bonding performance is contingent upon the collective influence of these factors. Phosphate cement compositions display good interfacial bonding through van der Waals forces, chemical bonding, and the creation of a through structure on the original pavement side. Although there is a weakened interfacial transition zone on the side of the repair material, the performance of the material in this region is still better than that of the original pavement, and the weakest part of the repair material–original pavement combination component is the original pavement in the region near the interface, which is no longer the interfacial bond between the old and new pavements. The aggregate and cement stone were removed from the pavement during the bond strength test since the original material was not strong enough. The interfacial bond strength of ordinary Portland cement material is relatively low, and the interfacial bond mostly derives from van der Waals forces and mechanical occlusion, and the chemical bond at the interface is weak. Among them, the hydration degree of modified Portland cement material at the interface is low, there exists more content of unreacted particles, and the strength of the slurry embedded in the interfacial pores of the original channel surface is insufficient, which leads to the reduction of mechanical occlusion, so the interfacial bond strength is relatively small.

Comparing the interfacial bonding mechanism of phosphate cement material and ordinary Portland cement material, it can be found that, compared with the adsorption effect of van der Waals forces and mechanical occlusion, the chemical bonding effect and the formation of a penetrating structure in the original airport pavement surface side have a greater enhancement of interfacial bonding performance and, to some extent, can make up for the weakening of the performance of the repair material. It is advised to treat the existing pavement surface with an interfacial agent that can chemically link the old and new materials when employing ordinary Portland cement materials for pavement restoration.

4. Conclusions

In this study, ordinary Portland cement mortar was used as the control group to compare and analyze two commonly used rapid repair materials for pavement, namely modified Portland cement material and phosphate cement-based material. The bonding performances of three types of mortar were studied through pull-out tests and interface flexural tests. The microtopography, element distribution, phase distribution, and thickness of the ITZs were analyzed through microscopic experiments such as SEM and EDS. Meanwhile, the interfacial bonding mechanisms of the rapid repair materials were analyzed based on the experimental data. The following conclusions have been drawn:

(1)

The interface bonding strength between phosphate cement mortar and old concrete pavement is higher than that of Portland cement mortar and modified Portland cement mortar;

(2)

The elements in phosphate cement permeate into the old concrete pavement through hydration reactions, forming van der Waals forces and chemical bonding forces. The interfacial adhesion of Portland cement mortar and modified Portland cement mortar mainly comes from van der Waals forces and mechanical interlocking, with weak chemical bonding at the interface;

(3)

The presence of the old concrete pavement will generate a sidewall effect, resulting in a lower hydration degree of the repair material and the formation of a porous structure at the interface. Compared to ordinary Portland cement mortar, modified Portland cement mortar had the lowest degree of interfacial hydration with 26.55% of unhydrated cement particles. The hydration degree of phosphate cement at the interface is relatively low, but its interface bonding strength is the highest, making the overall interface bonding performance optimal;

(4)

Compared to the adsorption of van der Waals forces and mechanical bite effects, chemical bonding and the formation of a penetrating structure on the old concrete pavement have a greater improvement effect on interface bonding performance, which can compensate for the weakening effect of the repair material in the ITZ to some extent.