1. Introduction
Concrete materials are commonly utilized in civil engineering, roads, bridges, and other engineering domains. However, it is important to acknowledge the presence of defects in concrete that can hinder its application. For instance, the tensile strength of steel reinforcement combined with concrete is significantly lower (only 1/8–1/12 of the compressive strength), leading to early corrosion of the longitudinal reinforcement. Additionally, the inherent brittleness of concrete further exacerbates these defects, impeding the progress of concrete construction materials. Currently, engineered cementitious composites (ECC) replace traditional ordinary concrete and are applied in engineering structures, one of the main ways to solve the above problems [
1,
2]. The reason lies in the thickness of the ECC protective layer area, which is made of ECC material instead of the original concrete. By harnessing the nonlinear deformation, energy absorption, and crack control properties of the ECC and optimizing the interface between ECC and concrete to enhance its bond performance, it transforms into a functional composite material [
3], which results in enhanced load-carrying capacity and ductility of the structural element, as well as improved control over crack width, thereby extending the service life of the element [
4,
5].
On the other hand, ECC, which stands for strain-hardening cementitious composites and pseudo-strain cementitious composites, refers to a distinct kind of concrete that has both high tensile strength and ductility. The material has a tensile strain capacity above 3% while retaining a fiber volume fraction of no more than 2% [
6]. It is well known that determining the axial tensile strength of plain concrete by a direct axial tensile test is necessary and also difficult. The reason is that there are inevitable problems, such as skewing and eccentricity during the installation of plain concrete specimens, and their geometric and physical centers do not coincide with each other. Thus, the test results determined by the direct axial tensile test fluctuate greatly. Furthermore, researchers have conducted a range of empirical and theoretical investigations on the tensile characteristics and ontological connections of ECC [
7,
8,
9]; however, a cohesive consensus has not yet been established. Given this, in order to obtain the axial tensile strength of ECC, the split tensile strength test was considered to reflect the axial tensile strength of ECC indirectly. The test results obtained from this splitting tensile strength test are less discrete and simple to implement.
Research on the splitting tensile strength of functionally graded concrete has shown that several parameters influence its splitting tensile strength. There are usually factors such as the strength class of concrete [
8], interfacial reinforcement process [
9], the thickness of ECC [
10], polyvinyl alcohol (PVA) fiber admixture [
11], and age [
12]. It was found that the splitting tensile strength of functionally graded concrete increased as the strength grade of the concrete increased [
13]. Tian et al. [
10] concluded that the roughness of old concrete, the strength of the old concrete matrix, the type of interfacial agent and binder, and the age have different degrees of influence on the bond tensile strength through the experimental study of bonding tensile properties of ultra-high toughness cementitious composite with concrete.
The damage of functional gradient concrete mostly occurs at the interface bond, and the interface becomes a key factor affecting the splitting and tensile properties of functional gradient concrete. Ibrahim et al. [
14] pointed out that the main influencing factors of the bonding properties of new and old concrete are the interface treatment method and state, the type of interfacial agent, the bond strength of reinforced concrete, and the difference in deformation of new and old concrete. Yin and Liew [
15] concluded that interfacial properties significantly affect both mechanical properties and damage modes of composites, and interface design has always been an important part of fiber-reinforced composites microstructure design research. Qian et al. [
16] proposed that there exists a transition layer between the old and new concrete interfaces consisting of three thin layers: a penetration layer, a strong effector layer, and a weak effector layer, where the surface condition of the strong effector layer, the interfacial agent, and the repair material together determine its performance. Jiang et al. [
17], through the study of shear properties of steel fiber cement mortar bonded to concrete, concluded that the type of interfacial treatment, the strength of steel fiber cement mortar, and the strength of the old concrete can significantly increase the bond surface shear strength. A study by Njim et al. [
18] found that the use of an artificial notch treatment significantly affected the splitting tensile strength of functionally graded concrete. He et al. [
19] tested the mechanical properties of old and new concrete binders. The effects of interfacial roughness and interfacial binder on the bonding properties of old and new concrete were investigated. The study showed that the transition zone between old and new concrete is the key to bonding old and new concrete. The interfacial roughness affects the penetration layer, the interfacial binder improves the reaction layer, and the proper improvement of the transition zone between old and new concrete is conducive to better bonding of old and new concrete. In their experiments on the bond surface of new and old concrete, Zhang et al. [
20] and Manawadu et al. [
21] examined the impact of bond surface roughness, age, and interfacial agent on fracture toughness. The analysis revealed that the fracture toughness of the bond surface increased substantially as the roughness of the surface increased.
Additionally, the fracture toughness of the bond surface increased in a hyperbolic fashion as the age of the bond increased. The fracture toughness of both new and old concrete increased in a hyperbolic fashion as the bonding age increased. Furthermore, the inclusion of an interfacial agent had a substantial impact on enhancing the bond fracture toughness of both new and old concrete. Zhang et al. [
22] suggested that the bonding performance between old and new cement mortar mainly depends on the following factors: first, the void area of the old cement mortar stone surface; second, the number and area of the cement particles in the new cement mortar in contact with the old mortar stone surface; and third, the microstructure of the new cement mortar formed at the bonding interface. Then, the following methods are also proposed to improve the bonding strength: first, the old mortar stone surface is soaked in water to facilitate the hydration of the new cement mortar; second, the use of fine particles, grading of cement or other materials as an interfacial agent, the choice of interfacial agent requires its own good performance, dense structure, and can be generated after the hydration of the crystals that can improve the adhesive force; third, pay attention to the quality of the construction, to ensure that the new and old materials indicate that the new and old materials are in close contact.
A study by Feng et al. [
23] found that the roughness of old concrete surfaces and the strength of encapsulated steel fiber cement mortar significantly affected the interfacial bond strength of new and old materials. Steel fibers mixed in the repair material can be a certain degree of the bond strength of the new and old materials.
The existing experimental studies on the splitting and tensile properties of functional gradient concrete have achieved certain results, but some aspects still need improvement. Regarding the sample preparation methods, some studies focused on the splitting tensile properties of functional gradient concrete. Still, the current sample preparation methods may not accurately simulate the functional gradient concrete structures in real engineering [
24,
25]. Therefore, improvement in sample preparation methods is needed to reflect the real performance of functional gradient concrete.
Regarding the test setup and loading method, the current test setup and loading method may not be able to adequately consider the non-uniformity and gradient of functional gradient concrete [
26,
27]. Therefore, there is a need to design a more appropriate test setup that can simulate the stresses in real projects and consider the special characteristics of functional gradient concrete.
Regarding the selection of test parameters, the performance of functional gradient concrete is affected by various factors, including material composition, gradient distribution form, and gradient change rate [
28,
29]. The current study needs to clarify further and optimize the selection of test parameters to obtain more accurate data on splitting tensile properties.
Regarding the analytical methods, current experimental studies on the splitting and tensile properties of functionally graded concrete mainly focus on determining mechanical property data, and there are still fewer analyses of the macroscopic and microstructures [
30,
31]. Therefore, there is a need to develop more comprehensive methods for analyzing the results to gain a deeper understanding of the performance and damage mechanisms of functional gradient concrete.
Regarding sustainability and durability, the study of sustainability and durability of functional gradient concrete as a new material is also very important [
32,
33,
34,
35]. The material’s long-term performance, durability, and environmental adaptability should be considered in the experimental study of splitting and tensile properties to assess its feasibility and application prospects in practical engineering.
The specimen size in this study is 150 mm × 150 mm × 150 mm, the thickness of the ECC material is 75 mm and 45 mm, and the thickness of the matching regular concrete is 75 mm and 105 mm. The effect of three parameters on concrete’s splitting and tensile characteristics, namely concrete strength grade, interfacial reinforcement, and ECC thickness, is examined and analyzed, as well as the trend of the effect on the split tensile properties. The influence of the three parameters on the concrete’s splitting tensile characteristics was investigated and analyzed. To investigate and analyze the concrete strength grade, interface enhancement technology, the thickness of ECC, and other aspects of the split tensile strength performance using the split tensile strength test. At the same time, the splitting tensile strength under the condition of numerous influencing elements was examined in this article, which gives the experimental reference value for its application in the engineering sector.
4. Analysis of Test Results
This section focuses on the splitting strength test of functional gradient concrete. It examines the impact of three parameters—concrete strength grade, interfacial reinforcement technology, and changes in ECC thickness–on its splitting performance and the corresponding trends of influence.
4.1. Effect of Concrete Strength Class
The functional gradient concrete splitting test data in
Table 6 is used to assess the trend and degree of influence of various concrete strength grades on the specimens’ splitting tensile strength. It was discovered that the interfacial bond strength between layers of functional gradient concrete rises with an increase in concrete strength grade, irrespective of changes made to the interfacial reinforcement procedure or the ECC thickness.
A correlation between the functionally graded concrete’s splitting tensile strength and the concrete strength grade is illustrated in
Figure 7. This correlation illustrates how the splitting tensile strength of the specimens is influenced by the various concrete strength grades, as shown in
Table 6. As will be elaborated in the following section, it has been discovered that the splitting tensile strength of the specimens is influenced differently by the interfacial reinforcement processes JM0 (interface without special treatment), JM1 (interface with notch treatment), and JM2 (interface with wire mesh treatment).
Indeed, this study conducted tests on the performance of interfacial bond strength in functional gradient concrete interlayer. These tests focused on different interfacial reinforcement processes, namely JM0, JM1, and JM2. Due to space limitations, only the test results are provided below to aid in the analysis of
Figure 7. The average interlayer interfacial bond strength of the specimen corresponding to C30 is 2.965 MPa, while the specimen corresponding to C50 has an average interlayer interfacial bond strength of 4.186 MPa. The specimen JM0-45 mm is used as an illustration. The specimen corresponding to C30 has an average interlayer interfacial bond strength of 3.522 MPa, while the specimen corresponding to C50 has an average interlayer interfacial bond strength of 4.459 MPa.
Additionally, there is a 0.937 MPa increase in the interlayer interfacial bond strength between the C30 and C50 specimens.
Figure 7 demonstrates that various treatments applied at the boundary between concrete and ECC have a substantial impact on the splitting tensile strength of functionally graded concrete. Specifically, the use of JM1 has a more noticeable effect compared with the use of JM2.
The aforementioned experimental analysis demonstrates that the functional gradient concrete interlayer interfacial bond strength increases with conventional concrete strength grade under the same interfacial reinforcement procedure and ECC thickness. The concrete strength grade parameter significantly influences the interfacial bond strength between layers of functional gradient concrete, which could be primarily due to the fact that in part of the interfacial zone between conventional concrete strength and ECC, higher strength grades of concrete contribute more to the interfacial bond strength of functional gradient concrete.
Table 6 shows that when the concrete strength class is increased from 30 to C50, the splitting tensile strengths corresponding to JM0-45 mm are 1.130 MPa and 2.799 MPa; those corresponding to JM1-45 mm are 1.478 MPa and 3.185 MPa; those corresponding to JM2-45 mm are 1.324 MPa and 2.844 MPa; those corresponding to JM0-75 mm are 1.376 MPa and 1.752 MPa; those corresponding to JM1-75 mm are 1.953 MPa and 2.14 MPa; the corresponding splitting tensile strength of JM2-75 mm are 1.455 MPa and 1.781 MPa, which makes it easy to detect that the splitting tensile strength of the specimens has increased to varied degrees. Still, the magnitude of the increase has stayed consistent. The specimens’ increases in split tensile strength were, in that order, 147.7%, 115.5%, 114.8%, 27.3%, 9.58%, and 22.4%. When the interface is treated as a notch, and the thickness of the ECC is 45 mm, the increase in the split tensile strength of the specimen is 1.707 MPa; when the interface is treated as a notch, and the thickness of the ECC is 75 mm, the increase in the split tensile strength of the specimen is 0.187 MPa. The primary cause of this might be attributed to the weight of the concrete in the functional gradient concrete specimens as well as the fact that the splitting tensile properties of the concrete are more strongly affected by changes in its strength grade. Consequently, it is evident that an increase in concrete strength grade significantly impacts the splitting tensile strength of functional gradient concrete.
4.2. Effect of the Interfacial Reinforcement Process
The quality of the interfacial bonding has a direct impact on the stress transfer effect between the matrix and the reinforcing body, which also has a bigger effect on the macroscopic mechanical properties of the composite materials. If the interfacial bonding is too weak, the composite material is subject to interfacial debonding damage under stress, and the fiber cannot properly express the reinforcing action. By appropriately changing the surface of the reinforcing material, the composite material’s interlaminar shear strength, tensile strength, and modulus can all be increased. As a result, appropriate interfacial reinforcement technology is employed in this study to reinforce the interfacial bonding zone between functional layers of functional gradient concrete in order to improve its macroscopic mechanical properties.
As seen in
Figure 8, the test in this research study uses three different approaches to treat the interface between ECC and regular concrete layers. The specimens underwent a maximum of 30 min of treatment. The 150 mm × 150 mm interlayer surface served as the bonding surface, and in order to meet the strength requirements at the interface bond, the concrete surface and the ECC surface had separate treatments.JM0 denotes that the interface is not given any special treatment; JM1 denotes that artificial grooves measuring 25 mm in diameter and 10 mm in depth are formed at the interface of the first material poured; and JM2 denotes the installation of a fine steel wire mesh with a 20 mm grid spacing at the interface of the first goods poured.
Figure 9 shows the scanning electron microscopy (SEM) images of the interfacial bonding zone under various concrete strength grades, interfacial treatments, and ECC thicknesses, displaying the microscopic morphology of the hydration products at the interfacial bonding zone of the samples. According to
Figure 8, after 28 days, both the concrete and the ECC reinforcement, acting as the matrix and reinforcement, had sufficiently hydrated the interface, resulting in the appearance of many Ca(OH)2 crystals, hydrated calcium silicate (C-S-H) gel, and calcium sulfoaluminate in the interfacial bonding zone and the absence of an obvious interfacial transition zone, as shown in
Figure 10. The hydration products have also achieved the “embedded solid” state and are well-bonded; altogether, the bonding effect between the two is good, particularly in the case of JM0, where the JM1 treatment is particularly evident.
Referring back to
Figure 8, in the instance of JM2 treatment, the steel wire mesh treatment at the functional layer’s interface causes, to some extent, the emergence of fine, short cracks that are visible to the unaided eye at the interface of the ECC and the concrete between the layers and across their surfaces, thereby disrupting the continuous and uniform distribution of the hydration products. Compared with C30-JM2-30 mm, the length and width of the cracks in C30-JM2-45 mm are more noticeable in smaller regions. This suggests that the interface treatment and ECC thickness have little influence on the microscopic composition of hydration products at the interface but have a considerable effect on the microscopic morphology.
The test demonstrated that different interfacial treatment effects resulted in different values of splitting tensile strength of functionally graded concrete when comparing the average splitting tensile strength of specimens with the same parameters, such as concrete strength grade and ECC thickness, but with different interfacial enhancement processes. When the interface enhancement process is JM0 (no special treatment), JM1 (grooving treatment), and JM2 (fine steel wire mesh treatment), the corresponding four groups of specimens splitting tensile strength values are 1.13 MPa, 1.478 MPa, 1.324 MPa; 1.376 MPa, 1.953 MPa, 1.455 MPa; 2.799 MPa, 3.185 MPa, 2.844 MPa; 1.752 MPa, 2.14 MPa, and 1.781 MPa. When the comparison in
Figure 6 is taken into consideration, it is discovered that the specimen cleavage tensile strength of the chosen interfaces increases by only 0.029–0.194 MPa when fine steel wire mesh treatment (JM2) is applied instead of the unspecialized treatment (JM0), which is not a statistically significant increase, is not readily apparent. This might be because, despite the fine wire mesh being applied to the specimen’s interface, the fine wire mesh and the load on the specimen were essentially kept parallel, meaning that the fine wire mesh’s function was ineffectively fulfilled. The specimen’s split tensile strength increased noticeably, reaching 0.348–0.577 MPa, after the interface was changed from the unspecialized treatment (JM0) to the grooving treatment (JM1). The cause is primarily due to the fact that the concrete and ECC to achieve a better “embedded solid”, concrete and ECC contact area increases, and consequently, the interface between the two mechanical occlusion forces is greater, reflected in the macro-expression is the function of gradient concrete splitting tensile strength of the ensuing increase. It is discovered that increasing the specimen’s interface roughness will improve its splitting tensile strength regardless of how the interface process is improved. Moreover, there is an obvious correlation between the interface enhancement process and the splitting performance of functional gradient concrete. As such, careful consideration must be given to the way the specimen’s interface is handled.
4.3. Effect of ECC Thickness
This section investigates the effect of ECC thickness on the specimens’ splitting tensile characteristics. When the concrete strength grade and interfacial reinforcement process parameters remain constant with the increase in ECC thickness from 45 mm to 75 mm, the corresponding specimen splitting tensile strengths are 1.13 MPa, 1.376 MPa; 1.478 MPa, 1.953 MPa; 1.324 MPa, 1.455 MPa in order, and the specimen splitting tensile strength increased by 0.246 MPa, 0.478 MPa, 0.131 MPa; 2.799 MPa, 1.752 MPa; 3.185 MPa, 2.14 MPa; 2.844 MPa, 1.781 MPa, respectively, and the reductions of specimens was 37.4%, 32.81%, and 37.38%, respectively. As can be shown in
Figure 11a, the change in ECC thickness has a significant impact on the splitting tensile strength of functional gradient concrete.
The test findings indicate that the variation in ECC thickness is a significant determinant of the splitting tensile strength of functional gradient concrete. However, the splitting and tensile characteristics of the specimens do not exhibit a rise as the thickness of ECC increases. In fact, they show contrasting patterns.
Figure 11a demonstrates that the splitting tensile strength of the specimens exhibited a rising pattern when the concrete strength grade of C30 was used. Conversely,
Figure 11b illustrates a sequential drop in the splitting tensile strength of the specimens when the concrete strength grade of C50 was utilized.
The phenomenon may be attributed to the simultaneous increase in the concrete strength grade and the thickness of ECC. As the concrete strength increases, the bonding between the cement paste, sand, and gravel in the concrete mix weakens. This lack of cohesion in the mix reduces the fluidity, resulting in poor bonding between the concrete and ECC at the interface region. Consequently, there may be some weakening at the interface, leading to a decrease in the splitting and tensile properties of the specimens.
4.4. Elaboration/Comparison with Past Literature
ECC stands out among gelling materials due to its exceptional ability to withstand high levels of tensile strain. Prior research on the tensile qualities of the material has been conducted by Kanda et al. [
37], Li [
38], Lee et al. [
39], and other researchers. In a recent research study conducted by Chen et al. [
40], the impact of various dosages (ranging from 0.25 to 1.0 vol%) of recycled tire polymer fiber (RTPF) on the splitting tensile characteristics of ECC was examined. The study also studied the influence of strain rates on these properties. RTPF was used as a substitute for polyvinyl alcohol fiber (PVAF) in the ECC samples. In their study, Qasim et al. [
41] examined how different dosages (ranging from 0.25 to 1.0 vol%) of recycled tire polymer fiber (RTPF) affected the splitting tensile properties of ECC. They replaced a portion of the PVAF -ECC and steel-polyvinyl alcohol hybrid fiber-reinforced ECC (SPH-ECC) with RTPF. The researchers conducted experiments to investigate the interfacial bond strength and compared the effects of hybrid fiber ECC with single fiber ECC. The goal of their study was to identify promising and effective retrofit materials for reinforced concrete structures. Ouyang et al. [
42] and Gao et al. [
43] examined the impact of surface ECC residual bond area damage on the split tensile strength of the repair system. The findings indicated a decline in the interfacial binding strength as the temperature increased. In their study, Tawfek et al. [
44] examined how the orientation of fibers impacts the mechanical characteristics of ECC composites. They used two distinct casting techniques for their investigation.
Refs. [
40,
41,
42,
43,
44] are derived from split tests, in which literature [
40,
41] examined the impact of various fiber dopings on the split tensile characteristics of ECC. These dopings are directly relevant to the dopings discussed in this study. The impact of ECC interfacial bond strength on the split tensile characteristics of ECC was examined in previous studies [
42,
43], which is directly related to
Section 4.2 of the current study. This study described in the literature [
44] used SEM and digital image correlation (DIC) to examine the damage process in ECC specimens subjected to compression and tensile testing, which is relevant to
Section 4.3 of the current article. Ultimately, the research conducted in this publication may serve as a valuable point of reference for future investigations, particularly when comparing them to well-established studies.
It should be noted that when studying materials with multiple variables, such as specimen design, engineering background, and influence parameters, the comparative analysis can only be conducted through research methodology and design theory. The research theory serves as a mere indication.