A Review of Research on the Interfacial Shear Performance of Ultra-High-Performance Concrete and Normal Concrete Composite Structures

Abstract

The interfacial shear performance between ultra-high-performance concrete (UHPC) and normal concrete (NC) is a critical factor in determining the overall performance of composite structures. This paper systematically reviews the research progress on the interfacial shear performance of UHPC-NC, revealing the core mechanisms of bond strength (dominated by mechanical interlocking with chemical bonding as a supplementary factor). It compares the advantages and disadvantages of single-shear, Z-shaped shear, double-shear, and inclined shear tests, clarifying the influence patterns of key parameters such as interface roughness, matrix wetness, curing conditions, and fiber content. This study found that interface treatment is the most significant factor in improving shear strength. Roughening or grooving treatments can increase the strength by more than 40%~80%, while the combination of rebar planting and grooving can further enhance ductility. The matrix wetness (saturated and moist) and UHPC age (within 7 days) need to be strictly controlled to avoid differences in shrinkage stress. Prediction models based on mechanics, finite element analysis, and experimental data each have their advantages and disadvantages and should be selected based on actual working conditions. To address common issues in practical engineering, such as insufficient interface roughness, shrinkage cracking, and fatigue degradation under cyclic loading, it is recommended to adopt composite interface treatment techniques (such as roughening + rebar planting), prestressing design, and optimized fiber distribution (with a steel fiber content of 1.5%~2.5%). This paper provides the theoretical basis and practical guidance for the design optimization and construction control of UHPC reinforcement projects and composite structures.

1. Introduction

Ultra-high-performance concrete (UHPC), an innovative and superior cement-based composite material, was first conceived in 1993. It was pioneered by France’s BOUYGUES company, building upon the foundations of macro-defect-free (MDF) cement developed in the late 1970s [1], the Densified System Containing Homogeneously Arranged Ultra-fine Pearticles (DSPs) cement-based composite introduced in the early 1980s [2], and advancements in steel fiber-reinforced concrete. The development process involved eliminating coarse aggregates to achieve greater uniformity, refining particle size to enhance compaction, and incorporating steel fibers to improve ductility. Initially referred to as Reactive Powder Concrete (RPC) [3], this groundbreaking material was later redefined by Larrard and Sedran in 1994, who proposed the term UHPC to better encapsulate its remarkable performance [4]. While MDF faced limitations due to its inadequate water resistance and DSPs struggled with issues such as cracking, UHPC successfully overcame these deficiencies. Through extensive and rigorous research, UHPC has evolved to exhibit a compressive strength typically exceeding 150 MPa, with peak values soaring to 810 MPa [5], and boasts unparalleled durability.

Utilizing this technology, we can construct innovative structures that transcend the limitations of conventional design. The Hangrui Dongting Bridge [6], for instance, employs a steel–UHPC composite deck, achieving a 40-fold increase in deck stiffness compared to traditional designs. The pedestrian bridge spanning the Pearl River at Guangzhou Tower features a 15-centimeter-thick deck paved with UHPC, satisfying stringent crack-resistance requirements while ensuring remarkable lightweightness. Notably, the Yunnan Honghe Bridge pioneered UHPC composite deck paving in China by using natural curing conditions, effectively addressing temperature fluctuations at high altitudes. Despite UHPC’s exceptional performance, its commercial scalability remains hindered by high costs, limited availability, incomplete design standards, and complex manufacturing/curing processes—making it impractical to fully replace traditional concrete [7]. Consequently, the integration of UHPC with NC has emerged as a viable trend. A prime example is Malaysia’s Kampung Linsum Bridge [8], constructed in 2010. This composite beam bridge uniquely utilizes UHPC in the compression zone and NC for the deck, optimizing material strengths to enhance structural efficiency and reduce self-weight by up to 60%–80% UHPC usage compared to full UHPC beams, while sacrificing only 5%–6% in ultimate load capacity [9]. Its cracking load is 67% higher than that of conventional NC composite beams. Switzerland further validated UHPC’s potential by pioneering its use in reinforcing the Chillon Viaduct’s deck [10], demonstrating its feasibility for structural strengthening. Today, UHPC is increasingly deployed as a high-performance reinforcing material in hybrid structural systems, combining with other materials to balance cost-efficiency and superior performance. This approach not only leverages UHPC’s strengths but also significantly reduces overall production costs.

Since the 20th century, with China’s rapid economic development, a large number of concrete structures have sustained varying degrees of damage over prolonged use due to factors such as natural environmental impacts, overloading, and unreasonable design. These damages compromise the functionality, durability, and safety of the structures, rendering the repair and reinforcement of damaged concrete structures a critical focus in contemporary engineering research [11]. In newly constructed composite structures, the interface between UHPC and NC frequently emerges as a structural weakness. Interface failure often precedes both reinforcement failure and global structural collapse [12]. The interfacial shear performance is pivotal for structural cooperation [13], as shear behavior directly influences structural integrity, crack resistance, and load-bearing efficiency. Macroscopically, the shear capacity of the reinforced concrete interface commonly lags behind that of the individual concretes. Shrinkage in newly cast concrete induces tensile and shear stresses at the interface, debilitating interfacial bonding and jeopardizing the composite structure’s durability. At the microscopic scale, the base concrete exhibits water absorption. When reinforced with new concrete, bleeding on the surface of the base concrete becomes pronounced, elevating the water/cement ratio in the interfacial transition zone (ITZ) between old and new concretes [14]. The transition zone primarily consists of ettringite and calcium hydroxide. The calcium hydroxide concentration in the ITZ can degrade bond strength, rendering the interface mechanically vulnerable [15]. Hence, investigating the interfacial shear behavior between UHPC and NC is of paramount importance.

2. Sources and Mechanisms of Interfacial Bonding Force

The main sources of bond strength at the interface of UHPC include physical bonding and chemical bonding. Physical bonding primarily relies on friction and mechanical interlocking between the interfaces [16]. When two interfaces are in contact, factors such as surface roughness, texture, and shape directly influence the magnitude of friction. Meanwhile, minute asperities and pores between the interfaces can provide mechanical interlocking points, thereby enhancing bond strength. Additionally, the bridging effect of fibers plays a significant role in physical bonding, particularly at the UHPC-UHPC interface, where fibers can effectively transmit stresses, enhancing structural integrity and ductility [17].

Chemical bonding primarily relies on the chemical bonds formed through chemical reactions at the interface [16]. When UHPC bonds with other materials (such as NC, asphalt, etc.), some chemical reactions may occur at the interface, such as cement hydration and pozzolanic reactions. These reactions generate new chemical bonds, thereby enhancing the bond strength between the interfaces. Additionally, appropriate interfacial agents can also improve the interfacial chemical interaction by enhancing wettability and chemical compatibility between the interfaces, further strengthening the bond strength.

In terms of mechanisms, Bijen J, Salet T, and others [18] first proposed in the 1990s that the interfacial forces originate from physical forces such as mechanical interlocking, van der Waals forces, and surface tension. This theory was introduced in the research on the bonding between new and old concretes by Chinese scholars in 2011. Fiebrich MH et al. [19] believed that the interfacial bonding between new and old concretes is a thermodynamic behavior. In subsequent research, based on numerous macro-tests and micro-studies, Xie Huicai et al. [20] concluded that, in general, mechanical interlocking plays a dominant role, and the interfacial forces are also significantly influenced by the type of interfacial agent. In specific situations, for example, when using polymer interfacial agents, van der Waals forces become the dominant force, whereas when using fly ash mortar interfacial agents, the existence of chemical forces can be clearly confirmed. Both physical and chemical interactions at the interface jointly affect the generation and development of bond strength, and the mechanism for enhancing interfacial properties is also crucial, including the control of interfacial roughness, the bridging effect of fibers, and the selection of interfacial agents. These factors will directly impact the stability and reliability of the interfacial bond strength.

The significant difference in elastic modulus between UHPC and NC (with UHPC’s elastic modulus reaching 40–50 GPa and NC’s approximately 20–30 GPa [21]) will have a certain impact on their interfacial shear behavior. Due to UHPC’s high stiffness, its deformation under load is much smaller than that of NC, resulting in a deformation difference between the two materials and disrupting the deformation compatibility at the interface [22]. When subjected to external loads, NC, with its lower elastic modulus, tends to undergo greater shear deformation first, while rigid UHPC tends to maintain its original shape. This deformation mismatch leads to local stress concentration near the interface, often manifesting as interface failure predominantly on the NC side, with crack propagation occurring at a certain angle to the loading axis [23]. Furthermore, the difference in the elastic modulus also impairs the cooperative load-bearing capacity between the two materials: under cyclic or dynamic loading, repeated deformation differences between UHPC and NC may cause interfacial bond fatigue degradation. M. An et al. [24] conducted freeze–thaw cycling tests on UHPC and C50 concrete using seawater as the medium and the quick-freeze method. The results indicated that after 200 freeze–thaw cycles, the dynamic elastic modulus of flawless UHPC specimens remained stable; however, even after 300 cycles, the dynamic elastic modulus of UHPC specimens with initial defects was still over 99%. In contrast, the dynamic elastic modulus of C50 specimens decreased significantly [25], and failure ensued. Therefore, in the design of composite structures, interface treatment is required to mitigate the adverse effects caused by the mismatch in elastic modulus.

Furthermore, environmental factors such as temperature and humidity can also affect the interfacial bond strength of UHPC. For instance, changes in temperature may alter the rate and extent of chemical reactions at the interface, thereby influencing the formation and development of bond strength. Dong et al. [26] found that polypropylene fibers inhibited the spalling behavior of 3D-printed ultra-high-performance concrete (3DP-UHPC) after high-temperature exposure by mitigating moisture loss and enhancing thermal stability, but they may also lead to interface weakening, resulting in decreased interfacial bond strength. Chen et al. [27] demonstrated that low-temperature environments can improve the bonding strength of UHPC-NC, but the low-temperature performance of the interfacial agent somewhat limits the enhancement of interfacial bonding strength. Changes in humidity may affect the wettability and moisture content between interfaces, thereby influencing the stability and durability of the bond strength. UHPC-NC specimens with wet interfaces exhibit better interfacial bond shear strength compared to those with dry interfaces [28].

3. Research on Test Methods for Shear Performance of UHPC-NC Interface

During shear performance testing, the vertical force is primarily used for loading, and this force must be precisely applied to the predetermined loading area of the specimen. The accuracy of assessing the shear strength of UHPC interfaces highly depends on the testing method employed, as variations in methods can result in significant differences in the measured values of UHPC interfacial bonding strength. Currently, the main testing methods for determining the shear performance of UHPC interfaces are categorized into two categories: direct shear tests and inclined shear tests [14], as illustrated in Figure 1.

There are three types of direct shear test methods, namely the single-shear test [29], the Z-shaped direct shear test, and the double-shear test [30]. In order to better understand the testing methods and their applications for measuring the shear properties of concrete interfaces,

Table 1. Testing methods for shear properties of concrete interfaces and their applications.

Testing Methods	Testing Standards (Not Limited to the Following)	Applicable Fields	Brief Description	Application Examples
(a) Single-shear test	AASHTO LRFD [31]	Evaluation of shear performance at interfaces between new and repaired materials (NC interfaces); Shear resistance of the contact surface between concrete slabs and bedding materials	In the early 20th century, Morsch et al. [32] proposed the rectangular short-beam direct shear test method, which is one of the earliest experimental methods for determining the shear strength of concrete.	Evaluation of CFRP-Concrete Interfacial Debonding [33]
(b) Z-shaped direct shear test	ACI 318-14 [34]	Analysis of shear behavior at composite interfaces (such as fiber-reinforced concrete, UHPC-NC composite structures)	Proposed by Hofbeck et al. [35] in 1969 and widely used ever since, it can effectively reduce the influence of bending stress. The provisions on shear design in the specifications and the requirements for material testing provide a theoretical basis for the experiment.	The shear strength of the interface between new and old concrete can be verified, and the results are in good agreement with the calculated values provided by specifications such as ACI 318-14 [36].
(c) Double-shear test	JGJ/T 221-2010 [37]	Measurement of the shear strength of structural connectors (bolts, anchors) or prefabricated components between concrete	It was improved and proposed by Momayez et al. [38] based on the single-side shear test. In the relevant specifications issued in China in 2010, the application of the double-side shear test for measuring the shear strength of fiber-reinforced concrete was clearly stated, promoting the popularization of this method in engineering practices in China.	Evaluation of Shear Connection Behavior in Composite Beams with Stiffened Cold-Formed U-Shaped Steel and Concrete [39].
(d) Oblique shear test	ASTM C882 [40]	Research on Multi-Angle Shear Behavior of Inclined Joints, Sloped Structures, or Adhesive Interfaces	This standard describes a method for oblique shear testing, which combines compression and shear actions to avoid issues such as local compression failure and crack bending [41].	Investigation of the Shear Resistance Performance of Joints in Prefabricated UHPC Segmental Bridges [42].

provides a detailed overview of four major testing methods, including their corresponding testing standards, applicable fields, brief descriptions, and application examples. It is worth noting that the current specifications for UHPC are still inadequate.

3.1. Single-Shear Test

The single-shear test measures the interfacial shear strength directly under shear loading by bonding the UHPC-NC specimen on one side, as shown in Figure 1a. This method is widely used due to its simplicity. The formula for calculating the interfacial shear stress using this method is

$$\tau ={\displaystyle \frac{P}{A}}$$

(1)

In Equation (1), P represents the ultimate load, and A represents the effective shear area.

Feng S et al. [43] utilized UHPC and NC as repair materials, incorporating them with the base concrete to form cubic bonded specimens for single-shear testing. The research results indicated that the microscopic structural characteristics of the overlay transition zone (OTZ) of the UHPC repair material exceed those of NC, and its combination with concrete substrates with rough surfaces is an effective choice for repair materials.

The advantages of this test method lie in its simple specimen preparation and intuitive loading method. However, the disadvantage is that the process of applying the shear load in the test is difficult to control. Guan et al. [44] found that specimens in single-shear tests often fail due to interface slippage or substrate destruction, making it impossible to accurately characterize the pure shear strength. Additionally, specimens are prone to stress concentration at the loading ends and are affected by additional bending moments, which can increase the dispersion of test results [45]. Future research is needed to develop new equipment or technological methods to reduce the impact of end effects.

3.2. Z-Shaped Direct Shear Test

The Z-shaped direct shear test is an interfacial shear test method documented in various internationally recognized standards and codes, such as ACI 318-14 [34] and PCI [46]. This method is designed to reduce the influence of additional bending moments caused by eccentric loading, as shown in Figure 1b. By clamping the Z-shaped specimen at both ends and applying shear loads, the interfacial failure mode is made closer to pure shear failure, effectively overcoming the defects of single-shear tests. The geometric design of the Z-shaped specimen ensures relatively uniform stress distribution across its shear plane. From a broader perspective concerning Z-shaped specimens, the multi-directional shear performance of Z-shaped columns subjected to bidirectional shear can be studied by adjusting the loading directions (such as 0°, 45°, and 90°) [47].

Compared to single-shear tests, this test method offers advantages in more uniform stress distribution, standardized specimen shapes and loading methods, and better data reproducibility. The geometric design of the Z-shaped specimen effectively simulates the actual shear stress state of the UHPC-NC interface in composite structures, particularly suitable for studying the effects of interface treatment methods such as dowel bars and roughening in high-precision scenarios. For example, during the test, parameters such as dowel bar ratio and roughening depth can be varied to directly observe the mechanical behavior of the interface. Jang H et al. [48] evaluated the shear performance of plain concrete construction joints through Z-shaped shear tests involving two types of concrete combinations and multiple interface treatments, highlighting the significant impact of steel fibers, keyway technology, and coarse aggregate interlocking mechanisms. The research showed that steel fibers and groove technology significantly affected the shear performance of UHPC combinations, while the coarse aggregate interlocking mechanism was crucial for UHPC-NC combinations. Cao C et al. [49] analyzed the effects of surface roughness, shear reinforcement ratio, and inclination angle of shear reinforcement on shear stress behavior through Z-shaped direct shear tests. The research indicated that the ultimate shear strength of the interface was mainly provided by the interfacial bond strength, while the residual shear strength depended on the shear reinforcement and surface roughness. Zhang Y et al. [50] explored the shear performance of the interface of UHPC-NC specimens through Z-shaped direct shear tests, studying interface treatment methods such as smooth surfaces, roughened surfaces, grooves, dowel bars, and combinations of grooves and dowel bars. The results showed that interface roughness had the greatest impact on the cracking load, and dowel bars significantly improved shear resistance and ductility, with a synergistic effect when combined with grooves. Yang J et al. [51] investigated the shear behavior of UHPC-NC interfaces with different types of mechanical interlocks (smooth, with aggregates, shear keys) and shear reinforcement ratios, using Digital Image Correlation (DIC) technology to observe crack propagation. The study found that the combined action of mechanical interlocks and dowel bars effectively restricted crack propagation in the interface transition zone and improved shear strength. Dowel bars significantly enhanced the shear strength of toothed key interfaces but had a limited impact on rough interfaces.

The test can simultaneously record parameters such as load-slip curves, shear strength, and elastic stiffness, providing a basis for validating theoretical models. Yang Jun et al. [52] proposed a formula for calculating shear capacity based on the fracture surface method through Z-shaped direct shear tests on UHPC-NC interfaces. Jiang H et al. [53] conducted direct shear tests using 10 Z-shaped specimens to study factors such as the number of keys, confining stress, and steel fiber type. They found that confining stress significantly enhanced the crack resistance and shear capacity of the specimens, especially at triple-key joints. Additionally, they proposed a new prediction formula that considers concrete tensile strength, enabling more accurate predictions of the shear strength of dry reactive powder concrete (RPC) joints in prefabricated RPC segmented bridge connectors. Zhang W et al. [54] analyzed the effects of factors such as roughness density on the interfacial shear performance of Z-shaped specimens through direct shear tests. The results showed that increasing roughness could improve structural strain compatibility and also validated the reliability of the tests, leading to the development of a groove model without locating bars. Yu K et al. [55] considered different interface types and restraint conditions and studied the direct shear performance of UHPC adhesive joints through 24 Z-shaped shear tests. The study found that passive restraint could enhance interface strength and ductility, with the passive restraint force tending to stabilize as the load increased. The epoxy resin coating demonstrated high reliability. Under passive restraint, the shear capacity of UHPC-keyed joints was mainly provided by key shear resistance, interfacial friction, and UHPC surface bonding force. Based on the tests, the friction coefficient was determined, a fitting formula was established for the shear strength of the UHPC surface and passive restraint force, and a formula for direct shear strength was derived according to Mohr’s stress circle theory. Although some achievements have been made in theoretical models for single factors at this stage, the long-term shear performance of UHPC under different environmental conditions has not been fully studied, and the universality of the models still needs to be developed. Understanding the durability and stability of UHPC under various environmental conditions is crucial for its widespread application.

There is still a risk of stress concentration at the corners of specimens, which may lead to local fracture or substrate failure, necessitating the correction of test results through numerical simulation. Xian X et al. [56] combined direct shear tests with finite element analysis to study the impact of dowel reinforcement on the shear strength of UHPC-NC specimens. They found that specimens without dowel reinforcement exhibited brittle failure, while dowel reinforcement caused interface crack propagation, enhancing shear strength and increasing slip, presenting ductile failure characteristics. Most current tests employ static loading, making it difficult to simulate the interface fatigue performance under cyclic or impact loading in actual structures. Additionally, the specimen sizes are relatively small, making it challenging to simulate actual large-scale structures. These limitations need to be addressed through research combining numerical simulation, dynamic testing, and other methods.

3.3. Double-Shear Test

To investigate the influence of different testing methods on the shear performance of concrete interfaces, the double-shear test is a newly designed shear test that increases the shear plane of the specimen [57], as shown in Figure 1c. This test involves synchronous loading from both sides, forming double shear planes between the central UHPC layer and the NC layers on both sides. Compared to single-shear tests, it further optimizes stress distribution, significantly reduces additional bending moment effects caused by eccentric loading, and enhances the accuracy of test results. Wang et al. [58] found through comparison that the interface shear strength measured by double-shear tests is 20%–30% higher than that of single-shear tests, and the failure mode is mainly interface peeling, which is more consistent with actual engineering scenarios. However, double-shear tests require high-precision equipment and strict control of loading synchronization, as any deviation may introduce torque errors [59].

The double-shear test demonstrates good adaptability to the UHPC-NC interface after interface treatment (such as roughening, grooving, dowel reinforcement, etc.) and can effectively evaluate the mechanical interlocking effect. Zhang Yang et al. [60] conducted seven sets of double-shear push-out tests to evaluate the influence of different NC surface treatments (such as smooth, roughened, exposed reinforcement, grooved, drilled, and dowel-reinforced) on the shear performance and failure modes of the UHPC-NC interface. The results indicated that NC surface roughness is the primary influencing factor, with roughened or grooved interfaces providing the best shear capacity and dowel reinforcement and grooved interfaces exhibiting better ductility. Xie Z et al. [61] investigated the influence of the number of shear stirrups, interface roughness, NC strength grade, and steel fiber morphology on the interfacial bond strength through ten sets of double-shear push-out tests. The research showed that an appropriate number of shear stirrups, roughened interface treatment, and increased NC strength can significantly improve the shear capacity, while steel fiber morphology has a minimal impact. Zhang Y et al. [62] explored the shear performance of the UHPC-NC interface, including shear capacity and failure modes, through double-shear direct shear tests and analyzed multiple influencing factors: NC strength, UHPC age, curing state, NC surface treatment, wettability, addition of UHPC expanding agent, and other stresses on the interface. The addition of an expanding agent slows the early development of shear capacity and slightly reduces long-term capacity, while a roughened NC surface significantly enhances interfacial shear capacity. Additionally, the interfacial shear capacity is also affected by the number and area of UHPC dovetail joints, groove joints, or post-installed reinforcement. Furthermore, Chen L et al. [63] proposed a refined finite element model that combines cohesion, dowel action, and friction to explore the shear performance of the UHPC-NC interface. This model was validated through double-shear tests and showed that interfacial reinforcement is crucial for enhancing the shear strength of UHPC-NC, with NC being a key consideration in designing interfacial reinforcement.

The double-shear test is also applicable for verifying the shear capacity of interfaces in composite structures made of prefabricated components or other novel high-tech concretes, such as Engineered Cementitious Composites (ECC). Wen Xiaodong et al. [64] conducted 12 sets of UHPC-NC double-shear tests to investigate the effect of the density and distribution of shear studs on the UHPC surface. The research results indicated that the density and distribution of shear studs significantly affect the shear strength of the interface, particularly the density, which has a parabolic relationship with shear strength. At the same density, increasing the spacing between shear studs can enhance shear strength. Wang D et al. [65] validated the shear contribution of groove density in post-cast NC and prefabricated UHPC specimens through double-shear tests. When the NC strength is the same, a higher groove density results in higher interfacial shear strength. Wei XY et al. [66] conducted double-direct-shear tests to evaluate the shear performance of ECC-NC interfaces with dot grooves and rectangular grooves. The results indicated that the bonding performance of the interface with dot grooves was superior to that with rectangular grooves, with a 73% increase in shear strength.

Due to the high accuracy of the double-shear test, many scholars have used it to provide experimental data support for interface bond-slip models, such as deriving shear strength formulas based on Coulomb’s friction theory. Duan M et al. [67] introduced a new type of perfobond connector to enhance the performance of the UHPC-NC interface. By testing 20 push-out specimens with four different parameters, they found that the shear capacity is mainly provided by adhesion, while dowels have a limited impact on capacity but can significantly increase initial stiffness. Based on the test results, they established predictive equations for the concrete dowel and interfacial bond capacity. Zhang Y et al. [68] conducted eight sets of double-shear tests to investigate the effects of various interface treatment methods, including shallow roughening, deep roughening, interface adhesives, exposed reinforcement, grooving, drilling, and post-installed steel threaded studs, on shear performance. The study found that increasing concrete strength and surface roughness can enhance interfacial shear strength. Considering factors such as interfacial cohesion, mechanical interlocking, grooves, and dowel action, they proposed a comprehensive formula to predict the shear strength of the UHPC-NC interface.

Furthermore, the embedment length of the reinforcing bars has a significant impact on the interfacial shear strength. As the embedment length of the reinforcing bars increases, the interfacial shear strength gradually improves. Zhao Y C et al. [69] conducted UHPC-NC double-shear tests and found that when the embedment length of the reinforcing bars increased from 1.5 Ø to 2 Ø, the interfacial shear strength increased by 58.5%; when the embedment length was further increased to 2.5 Ø, the interfacial shear strength increased by an additional 78.3%. This indicates that increasing the embedment length of the reinforcing bars can significantly enhance the shear strength of the interface.

The limitations of the double-shear test are mainly reflected in its inadequate simulation of complex stress states. It can only simulate single shear or simple compression-shear conditions and cannot reflect the multiaxial stress coupling effects in actual structures. Therefore, it is necessary to combine oblique shear tests or numerical simulations to supplement the analysis of interface performance under complex stress conditions [15]. Additionally, due to its sensitivity to size effects, specimen dimensions (such as groove density and interface area) have a significant impact on the results. Specimens must be designed strictly in accordance with specifications. However, there is currently a lack of unified standards for double-shear tests targeting the UHPC-NC interface, and adaptive adjustments are required for UHPC [70].

3.4. Oblique Shear Test

The oblique shear test is used to measure the interfacial bond strength of specimens under a combined stress state consisting of shear and compressive stresses [71]. The specimen is bonded by UHPC and NC through an oblique interface, as shown in Figure 1d. This method ensures a uniform stress distribution across the interface but requires strict control of the oblique angle, typically ranging from 30° to 60° [72].

The oblique shear test is one of the direct methods for establishing the τ-σ constitutive relationship. Zhang B et al. [72] systematically studied the shear mechanical behavior of the NC-UHPC interface. Through oblique shear tests and microstructure analysis, they revealed the mechanical mechanisms within the interface transition zone and proposed a constitutive model based on the relationship between shear strength, interface roughness, and substrate strength.

Existing research commonly adopts interface roughness quantification and reinforcement configuration as variables, combined with Digital Image Correlation (DIC) technology to monitor interface slip and strain distribution. Feng S et al. [73] prepared 135 composite specimens and evaluated the interfacial bond strength of UHPC-NC through oblique shear tests. They conducted an in-depth exploration of the influence of interface parameters and testing methods on the adhesive force between UHPC and NC. The results showed that bond failure occurred in the NC substrate when the surface roughness of the substrate (average sand-filling depth ≥ 0.63 mm) was high. Zhang Y et al. [74] investigated the bonding characteristics between NC and UHPC layers through oblique shear, splitting, and direct tensile tests, observing bond strength and failure modes. They also examined the effects and enhancement mechanisms of seven factors on bond strength, including substrate surface roughness, UHPC age, substrate moisture, UHPC curing conditions, NC strength, adhesive, and expanding agent. The research indicated that the UHPC overlay exhibited excellent interfacial bonding performance under appropriate surface roughness and substrate moisture conditions. Based on the results of direct tensile and oblique shear tests, the friction coefficient of the interfacial bond strength was inversely calculated.

The advantage of the oblique shear test lies in its ability to simulate complex loading conditions. By generating shear components through axial loading, it more closely approximates the actual stress conditions of engineering joints. Additionally, it allows for the separation of normal and tangential stresses by adjusting the angle. Austin et al. [75] further proposed the theory of critical angles, pointing out that the optimal test angle is determined jointly by interface roughness and the internal friction angle. The critical angle for rough interfaces is 19°, while for smooth interfaces, it is 27°.

4. The Main Factors Influencing the Interfacial Shear Performance of UHPC Composite Members

The interfacial shear performance of UHPC composite components is influenced by a multitude of factors, with significant differences in the mechanisms and degrees of influence among these factors. Based on existing research, this section systematically reviews the specific impacts of key factors on shear strength and outlines corresponding enhancement strategies. In summary, the main factors affecting the interfacial shear performance of UHPC can be broadly classified into eight categories: interface treatment methods, strength and moisture content of the base concrete, curing conditions, age of UHPC, load type and stress state, fiber content and distribution, environmental factors, and pouring sequence and technique.

4.1. Interface Treatment Method

Interface treatment is one of the most significant factors affecting shear strength. Interface roughening involves increasing the micro-roughness of the interface through physical or chemical methods to enhance the mechanical interlocking force of the interface. Several common interface treatment methods are shown in Figure 2.

The surface roughness of the NC substrate has a significant impact on the bond strength at the UHPC-NC interface [76]. Studies have shown that a rough interface can increase shear strength by 30% to 80%, while a smooth interface is prone to brittle failure [54,68]. Zhang D et al. [77] investigated the bond characteristics of the UHPC-NC interface after bush-hammering treatment through oblique shear tests and found that bush-hammering could effectively enhance the interfacial shear performance. With bush-hammering treatment, the interfacial shear strength was significantly increased by 40% to 70%. Jiang et al. [78] found that when the number of keyways gradually increased from 1 to 3, the oblique shear resistance of the C40 substrates increased by 94% and 178%, respectively. Yang Jun et al. [52] conducted direct shear tests and discovered that modifying the grooved angle could increase the interfacial shear strength by approximately 0.2 times, with an approximate 117% increase in shear strength for additional 5 mm of grooved depth. Aaleti S et al. [79] conducted oblique shear tests to investigate the influence of interface roughness on shear strength. The research indicated that when the roughness is relatively low (<3 mm), the shear capacity is higher, but it decreases significantly as craAcks propagate. Conversely, when the roughness is higher (>3 mm), the shear capacity tends to stabilize, approximately ranging between 15 and 20 MPa. Additionally, the exposed area of coarse aggregates on the NC surface exhibits a linear relationship with its interfacial shear strength, providing more mechanical anchor points for the interface [80].

The use of groove or keyway designs can further enhance the shear strength by more than 80%. Zhang Y et al. [50] found that the interfacial shear strength of a groove combined with dowel bars can reach 8.7 MPa, representing a 315% increase compared to a smooth interface (2.1 MPa). When the groove spacing is reduced to 50 mm, the shear strength further increases to 9.5 MPa [62]. Compared to rough surfaces, interfaces treated with grooves exhibit a distinct cracking phase [81]. However, further research is needed to determine the most suitable interface treatment technology for different application scenarios and to balance the relationship between construction costs and performance improvements. Additionally, through oblique shear tests and double-sided direct shear tests, it was found that as the groove density increases, the ultimate strength of the specimens gradually increases [82]. Specifically, for shear specimens with a 30° inclination angle, the ultimate strength of UHPC remains relatively constant with increasing groove density, but the ultimate strength of NC gradually increases. For shear specimens with a 60° inclination angle, the ultimate strength of both UHPC and NC gradually increases with increasing groove density. Therefore, the enhancement effect on shear strength is more pronounced at a 60° inclination angle. Different groove shapes also have a significant impact on shear strength. Wang Y et al. [83] conducted oblique shear tests and found that larger inclination angles can more effectively improve the interfacial shear performance. Yang J et al. [84] found that changing the angle of trapezoidal grooves can improve the interlocking effect, leading to higher shear strength. As shown in Figure 3a, when the short side of the trapezoidal groove opens inward (11°), shear cracks develop towards the interface, exacerbating damage. However, when the short side of the trapezoidal groove opens outward (−11°), shear cracks propagate towards the interior of NC, inhibiting cracking and increasing shear strength by 1.25 times compared to the former.

Research on chemical mechanisms primarily focuses on the impact of gel materials and interfacial agents on interfacial shear performance. The chemical reactants of these two can fill interfacial capillaries and microcracks [85], positively contributing to enhancing interfacial shear strength. Portland cement, serving as the gelling agent in NC, reacts with water through its core mineral phases (C3S, C2S) to produce calcium silicate hydrates (C-S-H) and calcium hydroxide (CH). Among them, C-S-H forms the skeletal structure of NC, while CH accumulates in the ITZ. The silica fume (SiO₂) incorporated into UHPC undergoes a pozzolanic reaction with CH at the NC interface, generating a dense secondary C-S-H gel [86] that fills interfacial pores. Analysis by Feng et al. [87] indicates that C-S-H can effectively reduce the thickness of the interface’s strong effect layer, optimize its pore distribution, and enhance interfacial bond strength. Various interfacial agents exhibit significantly different bonding effects on the UHPC-NC interface, primarily categorized into polymer materials and cement-based composites [88]. Ganesh et al. [89] found that epoxy resin plays a crucial role in improving interfacial bond strength. Xie Huicai et al. [20] pointed out that using epoxy resin as an interfacial agent can increase shear strength by 25%, while fly ash mortar interfacial agents enhance strength by approximately 15% through chemical bonding.

In summary, for conventional engineering projects aiming to enhance the shear strength of the UHPC-NC interface, mechanical treatment is preferred. Methods such as roughening, sandblasting, and grooving increase the bonding force by expanding the mechanical bite area. Several common interfacial treatment methods during construction are shown in

Table 2. Several common interface treatment methods.

	Interface Treatment Methods	Comparison Parameters	Impact Effects
UHPC-NC (A: Single-sided Shear; B: Double-sided Shear; C: oblique shear)	roughening (exposing coarse aggregate)	Smooth interface	C: Compared to the smoother interface (6.88 MPa), there is a 69% increase [77].
	quantity of grooves	Single groove interface	C: The shear strength of interfaces with two grooves and three grooves increased by 94% and 178%, respectively [78].
	Grooved angle	Right-angle groove	A: There is a 0.2-fold increase; approximately a 117% improvement for additional 5 mm of grooved depth [52].
	Combination of grooving and rebar embedding.	Smooth interface (2.1 MPa)	A: The strength reaches 8.7 MPa, representing a 315% increase. When the grooving spacing is reduced to 50 mm, the shear strength further increases to 9.5 MPa [50].
	45° and 60° trapezoidal grooves	Rectangular groove	C: The average interfacial bond strength of 60° trapezoidal grooves is 80% higher than that of rectangular grooves, and 45° trapezoidal grooves are also 28% higher than rectangular grooves [83].
	rough surface	Smooth, drilled holes and grooves.	B: It is a 54% improvement compared to the smooth interface, a 35% improvement compared to the drilled hole interface, and a 10.4% improvement compared to the grooved interface [62].
	Same grooving density	The strength of post-cast NC is upgraded from C30 to C50.	B: The interfacial strength can be increased by 16.05% to 58.59% [65].
	rib	When comparing specimens with rib widths of 15 mm and 20 mm, and with rib spacings increased from 40 to 100.	A: The interfacial strength of specimens with a rib width of 15 mm decreased by 29%, while those with a rib width of 20 mm increased by 40%. Increasing the rib width helps to improve the strength [91].

, with roughening or grooving being recommended. When the roughness depth of the NC substrate is controlled at an average of 4 mm, it can ensure sufficient shear strength at the interface with UHPC, meeting the standards in ACI 546.3R-14 [90]. Optimization of the depth and spacing of keyways can increase shear strength by 25% to 40% [52]. However, further research is needed on the impact of the depth-to-spacing ratio (ratio of depth to spacing) of grooves on the shear strength of the interface under different UHPC interfaces and conditions. Additionally, embedding steel bars or shear studs can significantly improve interfacial ductility and residual strength. Xian Xuelei et al. [56] found that the shear strength of specimens with embedded steel bars is more than 50% higher than that of specimens without. However, when the steel bars in both UHPC and NC are fully anchored, part of the NC may first be crushed under shear forces, as shown in Figure 3b. When designing specimens or calculating their shear capacity, the strength of NC needs to be considered [69], and it generally should not be less than C30. Overall, mechanical treatment is low in cost and effective but has low construction efficiency; chemical treatment is convenient but has poor durability; and embedding steel bars offers the best overall performance but at a higher cost.

4.2. Strength and Wetness of the Substrate Concrete

Adding a rapid-hardening agent can significantly increase the early strength of concrete [92], which aids in accelerating the enhancement of interfacial shear strength. In general, for every 10 MPa increase in the strength of NC, the average interfacial shear strength can increase by 8% to 15% [62]. This value can vary depending on various parameter conditions. Experiments have shown that when the strength of the matrix concrete is increased from C30 to C50, the interfacial shear strength increases from 15.89 MPa to 24.64 MPa (a 55% increase) [74]; and when it is increased from C40 to C50 and C55, the shear strength increases by 57.67% and 77.47%, respectively [93]. The failure mode of the C30 concrete interface is primarily dominated by interface failure (partial bond failure of the rough surface + stripping of the NC surface layer), while for C40 and C50 concrete, the proportion of shear failure within the NC bulk increases to 69%~100%, indicating that the interface strength has surpassed the shear capacity of the NC itself [62]. Based on this, it is recommended that the strength of the base concrete should not be less than C30, and high-strength aggregates should be preferentially used to enhance mechanical interlocking.

Bentz [94] demonstrated that pre-wetting the surface of NC has a significant impact on the interfacial bonding performance between new and old materials. Proper wetting of the NC substrate can effectively enhance the shear strength of the UHPC-NC interface. Wetting the substrate can increase the interfacial shear strength by 15%–30%. Within a certain range, the higher the strength or wetness of the normal concrete, the greater the shear strength at the UHPC-NC interface [74]. This is because when water molecules are present at the NC interface, they can significantly slow down the migration of water molecules within the UHPC reinforcement layer. Furthermore, they promote the formation of hydration products (C-S-H gel) in the ITZ and help reduce the number of capillary pores in this area, thereby making the ITZ denser [95,96]. As shown in Figure 4, Pan J [28]’s experiments indicated that, under the same roughness, the interfacial shear strength of the wetted substrate was increased by an average of 4%–12% compared to the dry substrate. Before construction, the substrate should undergo saturated wetting treatment (soaking in water for 48 h). When constructing in high-temperature environments, the exposure time of the substrate during construction should be shortened, and spray misting for moisture retention should be used to delay water evaporation [62]. Additionally, silica fume or nanomaterials can be appropriately added to the ITZ to improve the microstructure of the substrate and reduce the enrichment of calcium hydroxide [15].

4.3. Curing Conditions

The curing temperature and humidity directly affect the hydration reaction of UHPC and the density of the ITZ. High-temperature steam curing can enhance the early strength of UHPC by 30% to 50%, but improper curing conditions (such as excessively low humidity) can lead to the propagation of microcracks in the ITZ, resulting in a 10% to 15% reduction in shear strength [62,74]. Zhang et al. [62] found that high-temperature curing (at 60 °C) can increase the 3-day interfacial strength to 1.5 times that of standard curing at room temperature, but the long-term strength (at 28 days) increases by only 5%. Studies have shown that specimens cured at room temperature for 28 days exhibit the highest interfacial oblique shear strength. Specimens subjected to steam curing at 60 °C for 72 h have an interfacial oblique shear strength comparable to those cured at room temperature. However, specimens cured under steam at 90 °C for 48 h show a significant decrease in interfacial oblique shear strength compared to room-temperature-cured specimens, with an average reduction of 17.3% [60], as shown in Figure 5. It should be noted that temperature gradients can lead to stress concentration at the interface [26,83]. Zhu Y et al. [97] compared the free shrinkage of UHPC with its shrinkage when constrained by an NC matrix under both room temperature and steam curing conditions. Their research found that different temperature curing conditions did not affect the final shrinkage value, but NC constraint significantly reduced the shrinkage of UHPC, and the trend of shrinkage under constraint was minimally influenced by the curing conditions. Pre-wetting the surface of NC can also effectively enhance the bond strength at the interface [98].

4.4. Age of UHPC

The early strength development of UHPC is crucial for interfacial performance. Regarding early strength, experiments by Zhang Yang et al. [60] showed that when the age of UHPC increases from 3 days to 7 days, the interfacial shear strength increases from 16.57 MPa to 18 MPa, representing a 8% increase, and then stabilizes at 18.58 MPa after 28 days. The experimental results of Wang et al. [99] indicated that compared to the interfacial shear strength of UHPC-NC at 2 days of age, the shear strengths at 7 days, 14 days, and 28 days of age increased by 23.6%, 35.9%, and 43.0%, respectively. In terms of long-term performance, Zhang Y et al. [62] found that higher NC strength and wettability lead to stronger interfacial shear capacity. The shear capacity of UHPC significantly increases when its age increases from 3 days to 7 days. However, the interfacial strength of 180-day-old UHPC increases by only 5% compared to that at 28 days, indicating limited strength growth in the later stages, as shown in Figure 6. During construction, it is essential to ensure that the age difference between UHPC and the substrate concrete is less than 7 days to avoid differences in shrinkage stress [60].

4.5. Load Type and Stress State

The shear strength is significantly affected by the stress state at the interface (tension-shear or compression-shear). As the load increases, the shear strength at the interface between UHPC and NC initially increases and then rapidly decreases [91]. The combined tension-shear action accelerates interface debonding, resulting in a 40%–50% reduction in strength [55,100]. Zhang Y et al. [74] discovered that the shear strength of the UHPC-NC interface increases significantly with normal stress. Specifically, for low-roughness interfaces, when the normal stress was 10.80 MPa and 12.32 MPa, the shear strengths reached 18.71 MPa and 21.34 MPa, respectively. Furthermore, the splitting tensile strength (3.85 MPa) exceeded the direct tensile strength (2.92 MPa) but was lower than the oblique shear strength. These findings confirm that variations in stress states significantly influence interfacial strength. However, it is worth noting that compressive stress greater than 5 MPa may pose a risk of crushing the matrix.

Cyclic loading can lead to the accumulation of micro-cracks at the interface and a decrease in shear strength. Xia J et al. [101] divided the fatigue shear slip evolution of specimens into two stages: crack formation and slow development. Through fatigue push-off tests, they found that using a reinforcement ratio of 0.78% and high-pressure water treatment at the interface can enhance the fatigue damage resistance of the UHPC-NC interface. During design, it is recommended to avoid load combinations dominated by tension-shear at the interface and to improve the stress state by applying prestressed compression [102].

4.6. Fiber Content and Distribution

Fibers in UHPC serve multiple purposes, enhancing both the toughness of UHPC and the interfacial performance between UHPC and NC. The shrinkage of post-cast UHPC is constrained by the substrate concrete, leading to tensile stresses [103]. This can easily result in micro-cracks at the interface, thereby affecting the overall structural durability. According to the research by Wu Y et al. [104], fibers have two main effects: in thinner reinforcement layers, fibers can reduce the shrinkage of UHPC, significantly decreasing the risk of cracking and delamination caused by material shrinkage; fibers can also effectively inhibit the propagation of micro-cracks, redistribute the stress around the main cracks, increase the residual stress after cracking, and thus alter the force transmission mode. Current research on the effects of fibers on UHPC performance mostly focuses on the flexural strength of UHPC-NC [105], the tensile properties of UHPC [106], and the pull-out performance of fiber-reinforced UHPC [107]. However, there is relatively little direct research on the shear performance of the UHPC-NC interface.

Steel fibers inhibit crack propagation through bridging action, as illustrated in Figure 7, resulting in an increase in interfacial residual strength. Within a reasonable range of fiber content (2% to 3%), there is a significant enhancement in bond strength and viscoelastic properties [92]. However, excessive fiber content leads to a decrease in bond strength instead [16,48,108]. As the steel fiber content increases from 0% to 1%, 2%, and 3%, the shear strength of the UHPC-NC interface increases by 23.20%, 68.10%, and 156.32%, respectively [93]. However, an excessive amount of fibers (>3%) can lead to stress concentration at the interface [16]. Furthermore, as the corrosion level of steel fibers increases, the performance of UHPC gradually declines, but the bond strength at the UHPC-NC interface increases due to an increase in surface roughness [109]. In high-temperature environments, the use of polypropylene fibers can improve the high-temperature performance, but the interface bond strength may decrease by 10% to 15% [26]. Chen et al. [110] found that the incorporation of 3% polypropylene fibers can enhance the ductility of the interface, with the residual strength reaching 49% of the peak strength.

During construction, the volume fraction of steel fibers can be set at 1.5% to 2.5%. Mixing processes can be optimized to improve fiber distribution and avoid local agglomeration. Alternatively, directional fiber distribution techniques can be employed, using magnetic or electric fields to control the orientation of steel fibers, optimizing stress transfer paths and enhancing mechanical properties [111]. Additionally, Chiseling treatment can be used to increase the exposed area of fibers, but the improvement in strength is limited (1.6%), in contrast, grooving treatment can maximize the synergistic effect of fibers, resulting in a significant improvement (12 times) [17].

4.7. Environmental Factors

Studies have shown that the bond strength of the UHPC-NC interface generally increases at −60 °C compared to 20 °C due to a smaller coefficient of variation and less dispersion at lower temperatures [27]. Appropriate high-temperature exposure can promote the secondary hydration reaction in the concrete at the UHPC-NC interface, resulting in a moderate increase in bond strength [112]. However, excessively high temperatures (>60 °C) or prolonged exposure times may lead to interfacial cracking due to thermal expansion differences [26,27]. Therefore, polymer-based interfacial agents are preferred in high-humidity environments, while low-temperature-resistant interfacial agents are used in low-temperature environments [27]. When exposed to chloride salt environments for an extended period, the interfacial shear strength decreases by about 5% to 8% annually. To enhance durability, anticorrosive coatings or the incorporation of mineral admixtures can be used [113]. Studies have shown that as the strength of the base concrete, the curing age of UHPC, and the interface roughness increase, the chloride ion concentration at the UHPC-NC interface gradually decreases [114], effectively improving its durability.

Yu et al. [115] found that after 10 freeze–thaw cycles, a large number of pores appear at the UHPC-NC interface. The shear strength of interfaces treated with freeze–thaw water, composite salts, and chloride salts decreased by 57.3%, 67.8%, and 53.5%, respectively, compared to those under normal conditions. Additionally, the shear strength of smooth interfaces begins to decline after 40 salt-freeze–thaw cycles, while for grooved interfaces, the decline starts after more than 60 cycles [116], as shown in Figure 8.

During construction, weather-resistant materials such as polypropylene fibers or expanding agents can be added to alleviate temperature stresses. Meanwhile, the UHPC mix proportions and interface treatment techniques should be selected based on the specific engineering environment to ensure a good match.

4.8. Pouring Sequence and Process

The pouring sequence affects the interfacial microstructure. When casting UHPC over a prefabricated NC substrate in situ, the interfacial strength is 15% to 50% higher than when pouring in the reverse order, as the freshly cast UHPC can better fill the pores in the substrate [117,118]. Analysis of existing databases has found that pouring the substrate concrete first and then the UHPC results in an 47.6% increase in interfacial strength compared to the reverse sequence [118]. Liu et al. [119] studied the effect of delayed pouring of NC on its interfacial strength, with delays of 0, 25 min, 1 h, 3 h, 7 h, 10 h, and 24 h. The results showed that under standard curing conditions, appropriate delayed pouring can both ensure interfacial strength and reduce water loss. However, pouring intervals exceeding 2 h between layers can lead to cold joints Which will result in a decrease in shear strength, necessitating the use of interfacial agents or secondary vibration techniques [120]. During construction, priority should be given to pouring the substrate concrete first and leaving a rough interface. Layered pouring should be adopted to reduce material shrinkage differences at the interface.

5. Prediction Model for Shear Strength of UHPC-NC Interface

After decades of research, related computer algorithms and mathematical theories have been further developed, and a wealth of valuable shear test data has been accumulated. Based on this, scholars have continuously developed, studied, and refined predictive models for the shear strength of UHPC composite members. Predictive models for interfacial shear in concrete structures can be roughly divided into three categories: models based on mechanical principles, models based on finite element simulations, and models based on experimental data. As shown in

Table 3. Comparison of the advantages, limitations, and applicable scenarios of different predictive models for interfacial shear strength.

Model Type	Advantages	Limitations	Applicable Scenarios
mechanical model	The physical meaning is clear, and the computational efficiency is high.	Poor adaptability to complex boundary conditions.	Simple Force Member Preliminary Design
finite element model	It can simulate complex loads and interface characteristics.	High computational cost and reliance on experimental calibration for parameters.	Refined Structural Analysis and Optimization
experimental data model	Has a strong ability to capture multi-parameter nonlinear relationships.	Relies on large amounts of high-quality data and has limited extrapolation capabilities.	Rapid evaluation of engineering cases with multi-factor coupling

, the three categories of models exhibit distinct differences in their core strengths, limitations, and applicable scenarios, providing a systematic reference framework for model selection in engineering practice.

5.1. Model Based on Mechanical Principles

This type of model is primarily based on mechanical principles and mathematical derivations and is established on the foundation of force decomposition and stress–strain relationships. It assumes that the shear behavior at the UHPC-NC interface can be described by fundamental mechanical equations. By establishing calculation formulas or mathematical models for interfacial shear strength, it predicts the interfacial shear strength. The advantage of this approach lies in its clear physical meaning and high accuracy. However, its derivation process may be relatively complex, and it requires numerous assumptions and preconditions.

Ji He [121] compared the shear test data (ultimate load) of UHPC-NC composite beams with the prediction results of the truss-arch model. Approximately half of the data had errors controlled within 5%, and 90% of the data had errors below 10%. This indicates that the model can accurately reflect the shear contributions of the stirrups and the UHPC layer.

Zhang X et al. [102] proposed a new model to quantify the interfacial friction force in order to investigate its effect on the shear performance of steel-UHPC stud connections. This friction force was then integrated into the prediction of shear strength based on the compression dispersion model and the elastic foundation beam theory. Forty-seven push-out tests were selected from published literature, and the average ratio and standard deviation of the predicted shear strength to the experimental results were found to be 0.97 and 0.09, respectively, which verified the high prediction accuracy of the proposed model.

5.2. Model Based on Finite Element Simulation

These predictive models utilize numerical techniques such as finite element analysis to construct three-dimensional models that recreate the loading conditions and material property evolution during testing. The model parameters are finely tuned to match the results of the numerical analysis with experimentally observed data, thereby establishing a predictive model for interfacial shear strength. The advantage of this model lies in its ability to effectively simulate various complex loading scenarios and interfacial characteristics while being relatively cost-effective. However, its prediction accuracy is limited by the numerical analysis method employed and the degree of refinement of the computational model [100].

Farzad M et al. [122] developed a numerical method that more closely resembles reality compared to the tie-rod interaction model, aiming to predict the load-bearing capacity of structures where the base layer and overlay concrete are connected or repaired. They focused on material combinations with UHPC as the overlay and normal-strength concrete as the base layer, conducting three-point bending, direct shear, and oblique shear tests. Based on the shear friction theory, the compressive strength of the contact layer was taken as that of the weaker concrete side; the uniaxial tensile response and friction coefficient were calibrated and calculated based on experimental data. Compared to the experimental results, the maximum error of the contact layer model was 18%, while the tied model showed discrepancies exceeding 150%. The contact layer model provided more accurate estimates of failure loads than the tied model.

Farouk A I B et al. [123] innovatively proposed a shear strength prediction model for long-span UHPC-NC composite beams with grooved interfaces. They constructed a corresponding finite element model and conducted in-depth analysis and verification of the interfacial shear behavior between prefabricated NC and cast-in-place UHPC with different interface designs based on data obtained from push-out tests. Compared to the experimental results, the load–displacement relationship calculated by the finite element analysis model can accurately reflect the shear behavior of all interfaces. Meanwhile, the coefficients of variation for all specimens in both experimental and numerical studies are below 5%, which confirms a high degree of consistency between the numerical results and the experimental results.

Huang S et al. [124] established a nonlinear finite element model of UHPC-reinforced T-beams using ABAQUS and simulated the actual bond behavior between UHPC and the original concrete by employing the surface-to-surface cohesive behavior in the software. The load–displacement curves obtained from the model analysis were consistent with those from the experiments in terms of trend, and the deviations in ultimate load for all specimens were less than 5%, demonstrating the model’s effectiveness in simulating crack development.

5.3. Model Based on Experimental Data

These models primarily rely on experimental research on the shear performance of UHPC composite member interfaces. By designing different experimental parameters, accurate measurements of shear strength, deformation, and other performance indicators of the specimens are obtained, thereby establishing a mathematical relationship between interfacial shear strength and these parameters. The advantage of this type of model is that it can directly reflect the material properties and interfacial characteristics in actual engineering. However, its disadvantage lies in the high experimental costs and limitations imposed by experimental conditions and sample size [125].

Yuan S et al. [117] integrated data from 563 splitting tests and 338 slant shear tests to construct an artificial neural network (ANN) model and derive an explicit formula. Validated with 38 improved slant shear tests, their ANN model achieved an R² value of 0.828. The modified shear friction formula, which incorporated physical modeling of individual components (cohesive force and frictional force), demonstrated higher accuracy compared to various code-based formulas. Du et al. [118] further proposed an improved ANN model (R² = 0.828) and a shear friction formula, introducing a casting sequence influence coefficient (0.9–1.0). When compared to classic models such as AASHTO and Mattock, their approach showed a reduction in prediction error of over 35%.

Liu K et al. [126] employed four models—artificial neural network (ANN), random forest (RF), adaptive boosting (ADAB), and categorical gradient boosting (CATB)—based on data from 95 slant shear tests. The input variables included joint angle, material compressive strength, surface treatment, interfacial moisture, curing age, and method (normal temperature curing and 90 °C steam curing). The validation results indicated that the CATB model performed the best (with an R² of 0.948 and an RMSE of 1.408 on the test set). They also quantified feature importance using SHAP (Shapley additive explanations) and partial dependence plots (PDPs), demonstrating that surface treatment, joint angle, and NC compressive strength were key factors. The CATB model significantly outperformed the empirical formulas in the AASHTO and European codes.

6. Conclusions and Outlook

6.1. Conclusions

(1) A comprehensive review of the sources and mechanisms of the bond strength at the interface between UHPC and NC is presented. The bond strength at the UHPC-NC interface originates from physical bonding and chemical bonding. Physical bonding relies on friction, mechanical interlocking, and fiber bridging effects, while chemical bonding depends on the chemical bonds formed through interfacial chemical reactions, and the use of appropriate interfacial agents can enhance the chemical interaction. The test methods for interfacial shear performance include single-sided shear, Z-shaped shear, double-sided shear, and inclined shear tests. The single-sided shear test is easy to operate, but difficult to control the loading, which may lead to stress concentration and additional bending moments. The Z-shaped shear test provides uniform stress distribution, can simulate actual loading conditions, and allows for the study of multi-directional shear properties, but there is a risk of stress concentration at the corners of the specimen. The double-sided shear test optimizes stress distribution by increasing the shear area, resulting in accurate test results and applicability to various composite structures, but it requires high equipment precision and has limited ability to simulate complex stress states. The inclined shear test can simulate combined loading conditions and closely resemble the actual stress state of engineering joints, but it requires strict control of the inclination angle.

(2) The factors that primarily influence interfacial shear performance include interface treatment methods, the strength and moisture content of the base concrete, curing conditions, the age of UHPC, load types and stress states, fiber content and distribution, environmental factors, pouring sequence, and construction techniques. In practical engineering, interface failure problems often stem from insufficient roughness, concentration of shrinkage stress, fatigue under cyclic loading, and poor environmental durability. Roughening (roughness ≥ 0.5 mm) can enhance mechanical interlocking, while rebar planting (embedded length ≥ 2 Ø) can improve ductility. Using a combined interface treatment technique of both during construction can increase the shear strength of the structure by 40%~315%. Controlling the age difference between UHPC and NC (<7 days), adopting layered pouring or secondary vibration techniques, and adding expansion agents can reduce interfacial shrinkage stress. Treating the interface with high-pressure water and configuring a reinforcement ratio of 0.78% can improve the fatigue resistance of composite structures. In chloride environments, using anti-corrosion coatings or mineral admixtures can refine the pores in the ITZ. In freeze–thaw zones, air-entraining agents should be added (reducing shear strength loss to within 10%). During construction, the following parameters need to be strictly controlled: the strength and moisture content of the base concrete are positively correlated with interfacial shear strength, so the base concrete should be saturated and moistened during construction, and its strength should not be less than C30. Steam curing can increase the early strength of concrete and has a positive effect on early interfacial shear performance, but relative humidity must be strictly controlled above 90% to avoid shrinkage cracks. Regarding fiber content and distribution, an appropriate amount of steel fibers (1.5%–3%) can suppress crack propagation, improve residual strength, and optimize fiber distribution, or using hybrid fibers can enhance performance. The pouring sequence and technique can affect the interfacial microstructure. During construction, priority should be given to the combination of prefabricated NC and cast-in-place UHPC, with layered pouring and intervals not exceeding 2 h.

(3) There are mainly three types of prediction models for interfacial shear strength: those based on mechanical principles, finite element simulations, and experimental data. Mechanical models (with an error of ≤10%) are suitable for simple designs, finite element models are applicable for complex analyses (with a deviation of <5%), while experimental-based models rely more on data support. It is recommended to combine multiple models for cross-validation in engineering to improve reliability.

6.2. Outlook

(1) Currently, research on the interfacial shear performance of UHPC-NC predominantly focuses on the influence of single factors, with insufficient investigation into its long-term performance under complex environments. In the future, multi-field coupling experiments should be conducted to develop degradation models for the durability of shear interfaces and to quantify the cumulative effects of environmental factors on shear strength. These efforts will provide theoretical guidance for engineering applications in marine, alpine, and other similar environments.

(2) With the rise in intelligence, future efforts should focus on the application of intelligent construction technologies. By combining 3D printing with directed fiber distribution technology, the UHPC pouring process can be optimized to reduce shrinkage cracks. Additionally, sensors can be introduced to monitor the evolution of interfacial stress, enabling real-time feedback and adjustment of curing parameters.

(3) Currently, UHPC-NC composite structures have been successfully implemented in bridge reinforcement and lightweight bridge deck applications. In the future, further promotion of composite interface treatment and prestressed design is needed to address issues such as shrinkage cracking and fatigue degradation, thereby supporting the development of new green infrastructure.