Research Status and Prospects of Grouted Sleeve Connections in Prefabricated Structures

Abstract

The application and promotion of grouted sleeve connectors in prefabricated structures are closely related to their high efficiency and intensive advantages. Numerous scholars have conducted experimental studies on the performance of sleeves, but there has been no systematic consolidation of these efforts. In this study, the latest developments in grouted sleeve connection technology are systematically reviewed and analysed, focusing on its applications and characteristics, performance testing, influencing factors, load-transfer mechanisms, and performance evaluation. First, the differences in sleeve code formulation across various countries are compared, the advantages and disadvantages of different sleeve types and grouting techniques are reviewed, and the application scenarios of sleeves are summarized. Second, an overview of the performance of grouted sleeves in tensile, fatigue, and seismic tests is provided, highlighting key factors affecting structural performance and experimental results. Furthermore, the effects of various factors (the anchorage length, diameter and strength of reinforcing bars; types and defects of grout materials; sleeve tube design; and temperature) on the performance of sleeves are investigated, and some beneficial conclusions are drawn. The load-transfer mechanisms of different sleeve types are subsequently compared, and the common features of the sleeves that meet the performance evaluation criteria are analysed. Finally, potential future research directions and innovations in sleeve technology are suggested to provide researchers and scholars with innovative ideas and research perspectives for developing new sleeves and advancing the application of grouted sleeve connectors.

1. Introduction

In the context of the “dual carbon” goals, prefabricated construction methods exhibit significant emission reduction effects and efficient energy utilization, becoming an essential means to guide the construction industry towards low-carbon and environmentally friendly economic development [1]. As shown in Figure 1, prefabricated building structures are assembled via various connection methods (grouted splice connections, welded connections, socket connections, dry mechanical connections, steel box joints, and wet grouted sleeve connections), forming a composite structure that is inherently discontinuous. The performance of the connectors is a critical factor influencing the overall performance of the structure [2].

Grouted sleeve connectors are widely used as alternatives for solving issues such as poor ductility, high assembly precision requirements, weak seismic resistance, long anchorage lengths, easy blockage, and complex construction associated with rebar welding, socket connections, and grouted splice connections [3,4,5]. Grouting sleeves have gradually developed from their initial application in civil buildings to municipal bridges and underground structures. Since then, these connectors have formed a wide variety of types with different structures. According to ownership characteristics, grouted sleeves can be categorized into two major types: proprietary commercial sleeves and standard sleeves. Common proprietary commercial grouted sleeves, such as the Lenton Interlok, NMB Splice Sleeve, and TTK Tops-joint, are cast in moulds in factories [6]. However, for commercial and copyright reasons, research on the performance of proprietary grouted sleeves has been limited, with only a few studies [7,8] evaluating the improved performance of NMB proprietary grouted sleeve connections. Furthermore, due to the monopolization of technical standards by proprietary sleeve manufacturers, grout-filled sleeve connectors currently lack unified international standards. This absence results in discrepancies in performance comparisons across regions, which hinders cross-border engineering collaboration and technical exchanges, thereby constraining the global standardized development of this technology. Additionally, different types of prefabricated building structures have their own unique requirements for connectors. For example, the sleeves used in building frame structures have higher requirements for durability and seismic resistance. In municipal bridge structures, the heavy self-weight of piers and bearing components and frequent dynamic loads from vehicles require sleeves with high strength and good fatigue resistance. In underground structures, sleeves must possess not only good seismic resistance but also certain waterproofing and corrosion resistance [9,10,11,12]. Therefore, relying solely on a limited variety of proprietary sleeves, which have a narrow range of applications and are expensive, severely constrains the development of prefabricated construction. In contrast, extensive performance tests have been conducted on standard sleeves worldwide, providing real and intuitive insights into the mechanical behaviour of sleeve joints under loading. On the basis of these tests, theoretical research has been carried out and gradually promoted for engineering applications, thereby promoting the development, advancement, and maturity of grouted sleeve connection technology.

However, a systematic review of the widely used sleeve grouting connection technology has not been conducted. On this basis, in Section 1 of this paper, the use and characteristics of grouting sleeves from different perspectives, including specification requirements, structural forms, technical requirements, and application scenarios, are comprehensively summarized. Section 2 provides detailed descriptions of three types of mechanical property tests, including the tensile, fatigue, and seismic performance of the sleeves, and identifies the factors affecting performance. In Section 3, the influence mechanisms of different factors are discussed on the basis of the conclusions drawn from the mechanical property tests in Section 2. Section 4 and Section 5 summarize the different load-transfer mechanisms of ordinary sleeves and the key factors of qualified sleeves identified in feasibility evaluations. Finally, suggestions and prospects for potential future research directions and innovation points are proposed, such as discussing the degree of the contribution of sleeve toughness and its optimization methods, studying sleeve life prediction methods, establishing reinforced grout slip constitutive models, analysing the coupling effects of multiple factors, developing grouting admixtures, building sleeve corrosion degradation models, and developing high-strength aluminium alloy sleeves. The summary provided in this paper is expected to deepen the understanding of sleeve research and provide innovative ideas and research perspectives for the future development of grouted sleeve technology.

Figure 1. Connection method for assembled structure [9,10,11,12,13,14].

Figure 1. Connection method for assembled structure [9,10,11,12,13,14].

2. Use and Characteristics of Sleeves

As early as the 1960s, the 38-story Ala Moana Hotel in Honolulu first used the NMB grouted sleeve, invented by Dr. Alfred A. Yee, to connect its frame columns. The NMB grouted sleeve demonstrated superior seismic performance, leading to its rapid adoption in Europe, North America, and Japan [15,16]. Figure 2 illustrates the working principle of the sleeve. With the rise of the prefabricated construction industry in China, further advancements in grouted sleeve technology have been made, yielding rich research outcomes. In this section, the current status of grouted sleeve research both domestically and internationally is reviewed on the basis of code requirements, structural forms, technical requirements, and application scenarios.

2.1. Standard Regulation

Grouted sleeve connection technology has undergone more than six decades of development and application. During this period, extensive experiments and engineering practices have been carried out in numerous countries, advancing in-depth research on sleeves and refining the related knowledge and technical systems. Complete construction processes and standardized industry standards have also been established. For example, in the United States, the American Concrete Institute (ACI) lists grouted sleeve connections as one of the primary technologies for reinforcing bar connections in their reports [17]. Following the introduction of sleeve connection technology to Japan, the Japan Society of Civil Engineers (JSCE) recognized this connection system and established relevant standards [18]. In China, a series of standards related to sleeves have been established [19,20,21,22], indicating the mature application of grout-filled sleeve systems. However, there are significant differences in standards across countries. These differences include the following: China classifies sleeves based on casting and machining processes, while the United States does not specifically categorize sleeves and instead focuses on material versatility. Japan, by contrast, primarily relies on standards monopolized by enterprises. Additionally, China prefers a balanced application of frame structures and shear walls, whereas the United States adopts multi-storey shear wall and frame structures, and Japan mainly uses frame structures without a tradition of shear walls. In terms of similarities, most countries currently widely use ductile iron for sleeves, which requires high costs and stringent manufacturing processes.

The “Technical Specification for Mechanical Splicing of Steel Reinforcing Bars” JGJ 107-2019 states that grouted sleeve connections are a type of mechanical connection and categorizes joint performance into three levels, namely, I, II, and III, with Level I having the highest performance requirements [23]. In JGJ 107-2019, the performance requirements for Level I standard mechanical joints are lower than those specified in JGJ 355-2015. The Japan Society of Civil Engineers (JSCE) classifies mechanical connections for reinforcing bars into four grades on the basis of performance, from highest to lowest: SA > A > B > C.

Table 1. Mechanical connection joint performance index.

Compare Content		“Technical Specification for Mechanical Splicing of Steel Reinforcing Bars” JGJ 107-2019 [23]		“Technical Specification for Grout Sleeve Splicing of Rebars” JGJ 355-2015 [22]	Recommendation for Design, Fabrication and Evaluation of Anchorages and Joints in Reinforcing Bars [18]
Project	Target	Grade I joint	Grade II joint	Grouting sleeve joint	SA-Grade	A-Grade
Unidirectional tensile test	Intensity	fu > fstk bar break, fu > 1.10fstk connector failure	fu > fstk	fu > fstk bar break, fu > 1.15fstk connector failure	fj > 1.35fyn or fun
	Residual deformation (mm)	u0 ≤ 0.10 (d ≤ 32) u0 ≤ 0.14 (d > 32)	u0 ≤ 0.14 (d ≤ 32) u0 ≤ 0.16 (d > 32)		δs ≤ 0.3 mm
	Ultimate elongation	Asgt ≥ 6.0			εu ≥ 20εy and εu≥ 0.04	εu ≥ 10εy and εu≥ 0.02
High stress repeated tension	Intensity	fu > fstk bar break, fu > 1.10fstk connector failure	fu > fstk		fj > 1.35fyn or fun
	Residual deformation (mm)	u(20) ≤ 0.3			δs(20c) ≤ 0.3 mm
Large deformation and repeated pressure	Intensity	fu > fstk bar break, fu > 1.10fstk connector failure	fu > fstk		fj > 1.35fyn or fun
	Residual deformation (mm)	u(4) ≤ 0.3; u(8) ≤ 0.6			/

lists a comparison of performance indicators for the mechanical joints of reinforcing bars according to various specifications. The main difference between JGJ 355-2015 and JGJ 107-2019 is that when failure occurs outside the reinforcing bar, JGJ 355-2015 specifies that the tensile strength of the joint should reach 1.15 times the standard tensile strength of the connected reinforcing bar. This requirement is consistent with American standards [17]. Additionally, JGJ 355-2015 introduces offset unidirectional tensile strength testing, which is not listed in Table 1 [22]. In the Japanese standard, the strength, elongation, and other properties of SA-grade connections are similar to those of the base reinforcing bars, making them suitable for applications with higher requirements for reinforcing bar joints, such as the plastic hinge zones of important structures. A-grade connections have strengths and stiffnesses similar to those of base reinforcing bars, but other properties are somewhat weaker. In Japan, the performance of grouted sleeve joints must meet A-grade requirements.

2.2. Structural Form

As shown in Figure 3, a grouted sleeve joint involves inserting deformed reinforcing bars into a prefabricated tube and injecting grout through the grout port. Once the grout solidifies, it forms a structural assembly. The load transfer and continuity are ensured through the confinement effect of the sleeve on the grout and the bond between the grout and the reinforcing bars. Grouted sleeve structures can be classified into two types: fully grouted sleeves and half-grouted sleeves [20]. The distinction between traditional mechanical couplers and grout-filled sleeves lies in their force transmission mechanisms and connection continuity. Mechanical couplers rely on mechanical locking mechanisms, such as threads or keyways, to transmit loads. While they allow on-site adjustment and are suitable for large-diameter rebars, they carry risks of microscopic interface slippage, which may lead to cumulative deformation under long-term dynamic loads. In this study, unless otherwise specified, fully grouted sleeves are used for the connections.

At present, the proprietary sleeves commonly used in engineering include Nisso Master Builders, Tokyo Steel BOLTOPS and TOPS-JOINT, Fuji sleeves, Beijing JM sleeves, and Taiwan Ruentex sleeves. Jansson [24] and Ameli [25] conducted a series of tests using NMB fully grouted sleeves, providing detailed and objective evaluations of their tensile, fatigue, and seismic performance. Wang et al. [26] performed load-bearing capacity tests using Tokyo Steel BOLTOPS and TOPS-JOINT sleeves, whereas Wang [27] conducted further research on optimizing sleeve performance via Beijing JM series grouted sleeves. Research on ordinary sleeves is even more diverse. As early as 1995, Einea et al. [28] began studying ordinary sleeves. Since then, the structural forms of ordinary sleeves have shown a trend towards diversification and combination. In this study, ordinary sleeve structures are categorized into circular and noncircular types, as shown in

Table 2. Summary of research on shape of full-grouted sleeves.

Classification	Shape	Name	Reference	Technology	Recommended Anchorage Length (d)	Sleeve Schematic Diagram	Feature
Specialist sleeves	Roundness	NMB	Yee [15]	Foundry technique	4.9~7.1		The conical end is the prefabricated end, while the other end is the on-site assembly end. The outer surface of the conical segment is equipped with longitudinal ribs.
		Boltops	Zheng [32]		6.8~7.4		The inner and outer diameters of sleeves used for vertical component connections are both larger than those of sleeves used for horizontal component connections. Sleeves used for horizontal component connections are equipped with bolts for fixing the reinforcing bars.
		Tops joint
		Taiwan Ruentex	Wu et al. [33]		6.4		The outer diameter of the sleeve is uniform along its entire length, and the internal structure features symmetrically arranged annular ribs.
		JM sleeve	Wang [27]	Machining	5.4~7.1		The machine cuts continuous, bamboo-joint-like grooves on the inner side of the sleeve wall to serve as shear keys.
	Special-shaped	Fuji sleeves	Gao [34]	Extrusion forming	7.1~7.5		The unique waveform structure is formed by squeezing the annular grooves distributed along the sleeve wall at intervals.
Ordinary sleeve	Roundness	Type 1	Einea et al. [28]	Bar splicing	7		The inner part of the sleeve is lapped with a steel bar.
		Type 2		Welded bar			Four steel bars are welded inside the sleeve.
		Type 3		Welded steel ring			Two steel rings are welded inside the sleeve.
		Type 4		Welded steel plate			Two steel plates are welded to the end of the steel pipe.
		Integral full-grouted sleeve	JG/T398-2019 [20]	Foundry technique	8		The connection method is simple, and it offers high stability and safety.
		Segmented full- grouted sleeve		Foundry technique + machining			The structure is simple, and the cost is lower compared to an integral full-grouted sleeve. However, the connection method is relatively complex and requires higher installation precision.
		HSS sleeve	Henin and Morcous [35]	Machining			HSS features threads machined on the inner wall of the sleeve to increase the bond strength between the grout and the sleeve. Type P, on the other hand, includes additional steel plates at the ends, which significantly enhance the tensile strength compared to HSS.
		Type P
		Pattern sleeve B	Kim [36]	Machining	5~7.5		The sleeve ends are designed with a tapered shape.
		GWS	Lu et al. [37]	Welded connection	6~6.4		A small-diameter steel pipe wedge is welded inside the sleeve.
		GWST		Welded connection +machining			A small-diameter steel pipe wedge is welded inside the sleeve, and the inner surface is machined with threads.
	Special-shaped	Square	Dai et al. [38]	extrusion process	7.5		The inner cavity is formed by cold pressing, creating threads or protrusions.
		Large-diameter threaded sleeve	Huang et al. [14]	Machining	6.2		Internal threads and bolts provide mechanical resistance.
		GFRP sleeve	Sayadi et al. [39]	GFRP	75~125 (mm)		Symmetrical conical ribs are set at both ends of the sleeve.
		GDPS sleeve	Zheng et al. [40]	Rolling technique	7		The low-alloy seamless steel pipe is cold-processed using a three-axis roller compression technique.
		Pattern sleeve A	Kim [36]	Machining + heat treatment	5~7.5		The sleeve surface features continuous annular grooves.
		THS	Ling et al. [41]	Foundry technique	4.8~7.8		The cylindrical sleeve tapers at both ends, presenting an overall conical shape.

, which lists the names, recommended anchorage lengths, schematic diagrams, and features. In addition to the categories listed in Table 2, Ling et al. and Rahman et al. [29,30] designed and developed split cylindrical sleeves that can be assembled on site and sleeves that transform the connecting reinforcing bars into conical rods [31]. The exploration of diverse sleeve structural forms has injected significant vitality and innovation into the construction process of prefabricated buildings. Currently, prefabricated buildings are receiving increasing attention and promotion, and sleeves, as key connection components, offer a wide range of choices and diverse types. This diversity enhances the compatibility and stability of joints and connections.

2.3. Technical Requirements

This section focuses on the technical requirements for sleeve materials and grouting processes. As shown in

Table 3. Sleeve material requirements [20].

Material	Nodular Cast Iron			Rolled Steel
	QT500	QT550	QT600	45#Round Steel	45#Round Tube	Q390	Q345	Q235	40Cr
Tensile strength/MPa	≥500	≥550	≥600	≥600	≥590	≥490	≥470	≥375	≥980
Elongation after breaking/%	≥7	≥5	≥3	≥16	≥14	≥18	≥20	≥25	≥9
Note	The spheroidizing rate is ≥85%.			When the yield phenomenon is not clearly defined, the specified plastic extension strength R0.2 is used as a substitute.

, the JG/T 398-2019 [20] standard indicates that when manufacturing grouted sleeve tubes by casting processes, ductile iron is preferred as the raw material. When mechanical processing methods are used, various types of steel that meet tensile strength requirements can be selected [42]. Grout, as the medium for load transfer in grouted sleeves, must possess good flowability, high compressive strength, and a high expansion rate. Despite significant differences in the types and performance of commercially available grouts, they must all meet the technical indicators listed in

Table 4. Technical performance of sleeve grouting material at room temperature/low temperature [21].

Test Item		Performance Index
Fluidity/	Initial	≥300
	30 min	≥260
Compressive strength/	1 d/−1 d	≥35
	3 d/−3 d	≥60
	28 d/−7 + 21 d a	≥85
Vertical expansion rate/%	3 h	≥0.02
The difference between 24 h and 3 h		0.02~0.05
Chloride ion content b/%		≤0.03
Bleeding rate 1%		0

^a In the table, −1 d stands for maintenance at negative temperature 1 d, −3 d stands for maintenance at negative temperature 3 d, and −7 d + 21 d stands for maintenance at negative temperature 7 d. ^b The chloride ion content is based on the total amount of grout material.

. In addition, different grouting processes have a significant effect on the grouting quality and tensile performance of sleeves. On the basis of the application scenarios and technical requirements of grouted sleeves, commonly used grouting processes include single-end grouting, double-end grouting, and vacuum-assisted grouting. The characteristics of these grouting processes are summarized in

Table 5. Grouting process [43].

Process	Principle	Advantages	Disadvantages	Applicability
Single-end grouting method	Using slurry fluidity and filling force, the sleeve is injected by pressure.	The operation is simple, the cost is low, and the construction speed is fast.	When the sleeve is longer, the slurry filling may be uneven.	Suitable for small, prefabricated components, such as prefabricated walls, prefabricated balconies.
Two-end grouting method	The slurry is injected simultaneously or alternately from both ends of the sleeve to ensure that the gap between the steel bar and the sleeve is evenly filled.	The slurry distribution is uniform, with high density and strength.	The operation is complex and requires more equipment and coordination.	Suitable for longer sleeve connections, such as prefabricated piers, prefabricated bridge panels, etc.
Vacuum-assisted grouting	Evacuate one or both ends of the sleeve, and then inject the stock from the other end to ensure that the sleeve is evenly filled.	The slurry filling effect is good, with high density and fewer bubbles.	The cost of vacuum suction equipment investment and maintenance is increased, and the operation of equipment increases the difficulty of construction.	Suitable for situations where high-density connections are needed, e.g., integrated pipe corridor, prefabricated tunnel segments, etc.
Connected cavity grouting method	The connected cavity is designed so that a plurality of sleeves form a continuous cavity.	This reduces the number of separate grouting applications and simplifies the construction process.	The structure design of prefabricated components is complicated, and the difficulty of implementation increases.	Suitable for longer sleeve connections or multipoint connections and prefabricated connections between columns and beams.
Micropressure filling method	In the connected cavity mode, the grouting device is added to continuously replenish the grout material to ensure that the grout material is dense.	Coordinated grouting prevents overflow and allows repeatable use.	It is difficult to guarantee the density of grouting.	Suitable for connection scenarios that require fine control and uniform filling.
Gravity grouting method	Adjust the amount of mortar to the appropriate level, tamp the air out, and reduce the prefabricated components.	The precision is easy to control, and the construction is convenient.	The volume of grout material needs to be calculated precisely, and it is difficult to lift the prefabricated component.	Suitable for prefabricated tunnel components and prefabricated pier abutments with a large volume.

. Each grouting process has advantages and disadvantages, and in actual engineering practice, the choice of grouting process is typically based on construction conditions and the structural form of the components, considering both grouting quality and efficiency [43].

This study believes that for emerging prefabricated frame tunnels, particularly when grouted sleeves are used to connect prefabricated top arch and sidewall components, existing grouting processes are relatively traditional and require further research and improvement. Combining the principles of multiple grouting processes may help eliminate some adverse factors. For example, integrating the principles of chamber grouting with micropressure grouting and designing multichannel grouting devices can effectively combine the advantages of both processes, thereby improving the grouting quality and construction efficiency.

2.4. Application Scenario

Sleeves have been widely applied in prefabricated structures, including buildings, utility tunnels, municipal bridges, and subway stations, promoting the development of prefabricated building structures, as shown in Figure 4. Early building framework systems connected using grouted sleeves have successfully withstood earthquakes of up to magnitude 8.1 and demonstrated excellent seismic performance during a 7.2 magnitude earthquake in Manila, Philippines, and a 6.9 magnitude earthquake in Kobe, Japan [16]. Moreover, when sleeves are applied to the lower nodes of utility tunnels, controlling the length of the embedded bars and ensuring the quality of grouting allows the joints to exhibit stable and reliable bonding performance. This results in the average ductility coefficient of the prefabricated tunnel specimens being approximately 5.9% lower than that of the cast-in-place specimens [44]. Compared to lap splice connections with grout, sleeves can effectively transmit the stress of reinforcing bars until component failure and ensure the seismic performance of the structure [45]. With advancements in modern material science and construction techniques, the use of high-strength grout and large-diameter reinforcing bars in grouted sleeves has addressed issues such as time-consuming construction and cracking in traditional joint connections in large, prefabricated bridge structures, particularly in pier sections. The mechanical properties of prefabricated bridges are comparable to those of cast-in-place piers, indicating that grouted sleeves are suitable for rapid bridge construction [4].

In recent years, the application of grouted sleeves has extended to subway station structures. For example, the Jinan Bridge Station node on the western extension line of Beijing Subway Line 6 uses grouted sleeve connections, achieving the complete integration of prefabricated reinforced concrete structures. Research on the seismic performance of prefabricated assembled nodes, which is based on the Beijing Subway Line 6, further demonstrated that grouted sleeves have excellent deformation capacity, load-bearing capacity, initial stiffness, and energy dissipation capability [46]. Because of their unique technical advantages and broad applicability, as well as stable and reliable structural performance, grouted sleeves play a significant role in various structural forms in the construction field.

Figure 4. Application field [4,14,45,47,48,49,50].

Figure 4. Application field [4,14,45,47,48,49,50].

3. Performance Test

As a crucial connection component in prefabricated buildings, the tensile performance, fatigue resistance, and seismic behaviour of grouted sleeves directly affect the overall stability and safety of structures. This section reviews performance tests conducted on individual grouted sleeve components and their structural applications, categorizing and summarizing the research progress on the basis of the domestic and international literature.

3.1. Tensile Properties

In the field of prefabricated buildings, grouted sleeves can resist tensile loads to achieve secure connections between components. Uniaxial tensile tests can be used to evaluate the mechanical properties of new types of grouted sleeves, as well as promote their application.

By reviewing different nonproprietary sleeve designs proposed by various scholars, we can explore effective structural forms to increase the tensile strength of sleeves, providing references for the subsequent development and design of new sleeves. For instance, Einea [28] conducted axial tensile tests to compare the tensile performance of ordinary circular sleeves under four different structural forms and reported that welding steel plates at the sleeve ends effectively increased the bond strength, thus enhancing the tensile performance of the sleeves. Improving on this work, Kim [51] designed and fabricated full-grouted sleeves with tapered ends and proposed the hoop effect of steel pipes. Lu et al. [37] increased the bond strength by welding wedges at the ends, reducing the anchorage length from 8 d to 6.4 d while meeting the required tensile strength standards. In addition to modifying the ends, Wang et al. [13] increased resistance by drilling holes in the sleeve body and inserting bolts. This revealed that the closer the bolts are to the sleeve opening, the greater the improvement in bond strength. Huang et al. [14] combined the addition of bolts with internal mechanical threading to further increase the tensile strength of sleeves. Other methods to increase resistance to grout slippage within the sleeve include adding shear keys or forming annular ribs inside the sleeve via traditional welding techniques, although these methods result in higher mould production costs than ordinary mechanical processing.

Additionally, altering the shape of the sleeve body is another research focus. For example, Kim [36] conducted tensile tests on sleeves with continuous annular grooves machined on the outer surface. Sayadi et al. [39] designed symmetrical conical ribs. Similarly, Zheng et al. [6] used a three-axis roller compression technique to cold-process multiple inverted trapezoidal grooves. As shown in Figure 5, the primary failure modes of these different sleeve structures are categorized into three main types. According to relevant standards [17,22,23], the ideal failure criteria for sleeves require that the joint strength exceeds that of the parent rebar material, such that rebar fracture occurs while grout shear failure, grout delamination, or the circumferential cracking of the sleeve is observed. Specifically, rebar pullout, grout slippage, and cracking are deemed non-conforming failure modes. These studies indicate that altering the smoothness of the sleeve body surface can effectively increase the tensile strength. Furthermore, Ling et al. [41] manufactured conical sleeves by casting techniques, achieving a 25–35% increase in bond strength compared to that of cylindrical sleeves. However, the tapered ends limit the tensile tests to rebar diameters of 16 mm or less. Additionally, square sleeves [38] and other noncircular sleeves have limited practical applications in engineering. As shown in Figure 6, the load–displacement curves from the above tensile tests show similar trends for different types of grouted sleeves, including the elastic stage, yield stage, strengthening stage, and slip failure before reaching the yield point of the rebar. Moreover, an increase in rebar diameter generally leads to greater elongation, but the length of the reserved clamping section also affects elongation [13].

In conclusion, this study demonstrates that the structural configuration of sleeves significantly impacts bond strength. Specifically, these characteristics involve modifying the inner surface smoothness of the sleeve or altering the original flatness of the port, which increases structural resistance to grout flow. Therefore, future research should prioritize these influencing factors to achieve optimal performance and desired outcomes.

Figure 5. Tensile test and failure mode [6,14,37,52,53].

Figure 5. Tensile test and failure mode [6,14,37,52,53].

Figure 6. Typical load–displacement curves [14,54,55,56,57,58,59,60].

Figure 6. Typical load–displacement curves [14,54,55,56,57,58,59,60].

3.2. Fatigue Performance

Fatigue damage is a dynamic process of cumulative damage, and its mechanical behaviour differs significantly to that of static failure. As previously mentioned, the axial tensile strength and deformation characteristics of grouted sleeves only reflect their basic performance under static loading. To gain a comprehensive understanding of the fatigue performance of sleeves in practical applications, high-stress and large-strain cyclic tension–compression tests are typically conducted. The cyclic loading procedure is illustrated in Figure 7.

Over the past decade, researchers have extensively studied the fatigue damage and performance degradation of grouted sleeves under cyclic loading. Zheng and Guo [61] conducted cyclic tension–compression tests via low-cost seamless ribbed steel pipe grouted sleeves and reported that the ultimate bond strength of the reinforcing bars was approximately 10% lower than that of uniaxially tensioned specimens. Wang et al. [62] noted that increased temperature leads to a greater compressive deformation of the sleeves under cyclic loading, potentially causing plastic hinges at the connection ends, leading to structural yielding and a reduced load-bearing capacity. Zhang et al. [63] investigated the effects of reinforcing bar diameter and grout type on load-bearing capacity and revealed that the degree of structural damage and stiffness degradation is significantly influenced by the type of grout and that adding fibres can effectively mitigate fatigue damage under cyclic loading. After repeated large-deformation tension–compression cycles, the yield plateau of the reinforcing bars disappears. As shown in Figure 8, different fibres increase the load-bearing capacity of the specimens by 5.85%, 13.8%, and 5.4%.

Additionally, Fan et al. [64] studied ultra-high-performance concrete (UHPC) grouted sleeves and reported that cyclic loading reduces the bond strength of the sleeves and that shorter anchorage lengths are more significantly affected when there are grout defects. Li et al. [65] further investigated the impact of supplementary grout on structural load-bearing performance and revealed that when the length of grout defects is less than 20% of the specified anchorage length, the structural performance remains acceptable. Lu et al. [66] compared sleeve body materials and reported that ductile iron joints exhibit better energy dissipation and plastic deformation capabilities than seamless steel pipe joints do. The damage caused by cyclic loading is also noteworthy; as shown in Figure 9, the depth of damage accounts for approximately 8% of the total sleeve length, and the yield displacement of the joint is significantly smaller than that of the same-type reinforcing bars. Moreover, Xu et al. [67] designed cyclic tension–compression tests for sleeve joints with reinforcing bars of different diameters and observed severe damage to the grout at the sleeve ends. Under repeated tension–compression cycles, the initial stiffness of the specimens with modified sleeve inner walls decreased by an average of approximately 38.5%, and the load–displacement curve exhibited a slight reverse S shape [68]. Li et al. [69] modified the inner walls of sleeves and conducted fatigue tests, finding that the size of the sleeve threads and the angle of the wedges significantly affect the stress distribution and damage state of the grout, and proposed an inner wall modification scheme suitable for reinforcing bars with a diameter of 12 mm.

In summary, research on the fatigue damage performance of grouted sleeves has yielded substantial results in terms of structural form, reinforcing bar diameter, grout type, and grout defects. This research has deepened our understanding of the behaviour of grouted sleeves under cyclic loading and provided a solid theoretical foundation and guiding principles for their practical application in engineering.

Figure 7. Cyclic load loading program [64].

Figure 7. Cyclic load loading program [64].

Figure 8. Load–deflection curves of fully grouted sleeve connections with different anchorage lengths under cyclic loading at high stress (CH) [63].

Figure 8. Load–deflection curves of fully grouted sleeve connections with different anchorage lengths under cyclic loading at high stress (CH) [63].

Figure 9. Failure modes of specimens after cyclic tensile loads [62,66].

Figure 9. Failure modes of specimens after cyclic tensile loads [62,66].

3.3. Seismic Performance

Research on fatigue performance focuses on the individual connection level of grouted sleeves, aiming to thoroughly investigate the structural performance of a single grouted sleeve. However, whether due to natural earthquakes, artificial earthquakes, or vibrations caused by machinery operation and traffic loads, these events can damage structural buildings. For prefabricated buildings connected via grouted sleeves, the seismic performance of the joints is crucial for structural safety. Therefore, many researchers have explored the seismic behaviour of grouted sleeve connections under various conditions. As shown in Figure 10, complex physical models and experimental schemes are constructed to replicate various vibration scenarios encountered in actual engineering environments to comprehensively evaluate and verify the stability and disaster resistance of the structures.

Zhao and coworkers [70,71] focused on the seismic performance of grouted sleeve connections in prefabricated columns with HRB500 high-strength reinforcing bars. These scholars reported that even if the reinforcing bars did not reach the yield state, damage still occurred at the component interfaces, with the reinforcing bars pulling out of the sleeves, as shown in Figure 11. This finding indicates that simply increasing the strength of the reinforcing bars is insufficient to increase the bond strength between the grouted sleeves and the reinforcing bars. Li et al. [72] explored the seismic performance of grouted sleeve connections in prefabricated columns after fire exposure and revealed that different durations of fire exposure led to variations in failure modes. In addition, the hysteresis loops showed more pronounced pinching effects, as shown in Figure 12A,B. Similar conclusions were drawn in Liu’s [73] study, but the research study did not delve deeply into the impact of fire on the sleeve structure and subsequent damage mechanisms. Lei et al. [71] used UHPC as the grout material to study the seismic performance of prefabricated columns and proposed a method to determine the critical height of grouted sleeve connections in short columns. This study determined that when the sleeve is located above the critical height, its position has a significantly reduced impact on seismic performance. If there are grouting defects in the prefabricated column connections, then an increase in defects leads to noticeable pinching effects and asymmetry in the load–displacement hysteresis loops, with the ductility coefficient potentially decreasing to 58% of that of intact specimens, along with significant reductions in the stiffness and energy dissipation capacity. As shown in Figure 13, Li et al. [74] built refined models on the basis of the aforementioned experiments, revealing the failure mechanisms of prefabricated column splices and the lateral forces and displacements at the column tops. As shown in Figure 12C, Zheng et al. [63] further confirmed that grouting defects lead to characteristic sliding failures of reinforcing bars and pinching phenomena in hysteresis loops, highlighting the importance of grouting quality for seismic performance. Xin et al. [75] conducted comparative tests on prestressed and nonprestressed grouted sleeve connections in prefabricated concrete bridge piers and proposed a method for estimating the load-bearing capacity of components with and without prestress, as illustrated in Figure 14.

Additionally, studies on the application of grouted sleeve technology in the sidewalls of utility tunnels and prefabricated station sidewalls have shown that this technology can effectively transmit the stress of reinforcing bars until the failure of prefabricated concrete composite sidewall specimens, demonstrating excellent ductility and durability. In summary, the application of grouted sleeve technology in different structures and research on its seismic performance provide a solid theoretical foundation and technical support for the development of prefabricated buildings.

4. Factors Influencing Sleeve Connection Performance

The above review summarizes the performance research and testing methods for grouted sleeves under different structural forms and load types. In addition, performance tests have been conducted to investigate the factors affecting the connection performance of grouted sleeves. According to the findings from these performance tests, the material properties and temperature are the primary factors influencing the mechanical performance of grouted sleeves. On this basis, the research progress is categorized and reviewed according to relevant worldwide studies.

4.1. Bar

Reinforcing bars serve as the primary medium for force transmission between components in grouted sleeve connections, significantly impacting the strength and ductility of the structure. Therefore, numerous scholars have conducted extensive research on how changes in reinforcing bar parameters affect structural performance. This subsection reviews and analyses the influence of different anchor lengths, diameters, and strengths of reinforcing bars on the mechanical performance of grouted sleeves.

4.1.1. Anchorage Length of Steel Bar

According to the code [20], when the tensile strength f_u ≥ 1.15f_stk for grouted sleeves, the minimum anchorage length should be eight times the diameter of the reinforcing bar, which is calculated via Equation (1). In practice, although an anchorage length of 8 d provides additional safety margins, it can lead to practical issues such as the congestion of reinforcing bars at the lap joints, increased structural self-weight, higher costs, and reduced space utilization [40]. Therefore, many scholars have investigated methods to shorten the anchorage length.

In addition to modifying the structural form of the sleeves to decrease the anchorage length, Shao [76] utilized steel fibre bridging in UHPC grout and gravity-fed grouting connections in sleeves. On the basis of Equations (2)–(4), the researcher proposed reducing the anchorage length of sleeves to 5.5 d under critical conditions. Under the same grouting method, the anchorage length for ordinary commercial UHPC can be reduced to 7 d. Additionally, when UHPC grout was used, Huang [14] increased the mechanical resistance to reduce the anchorage length to approximately 6.2 d, but the specimens were prone to failure when the length was below the critical value. The type of sleeve material is also a significant factor affecting the anchorage length. For instance, Sayadi and coworkers [39] conducted uniaxial tensile tests on glass fibre-reinforced polymer (GFRP) joint sleeves and reported that as the number of GFRP layers increased, the tensile strength of the sleeves improved at the same anchorage length. In addition, these investigators proposed equations for calculating the elastic and inelastic segments of the anchorage length of grouted sleeves (Equations (5) and (6)).

Studies comparing different materials, such as seamless steel pipes and ductile iron sleeves [66], through tensile tests have shown that the higher the stiffness of the sleeve, the shorter the required anchorage length of the reinforcing bars under the same load. Therefore, effective methods to reduce the anchorage length include modifying the structural form of the sleeves and enhancing the strength of the grout.

$$L_{\mathrm{m}\mathrm{g}}=1.06{\displaystyle \frac{d{f}_{stk}}{f_{cu}}}$$

(1)

In the formula, d is the diameter of the steel bar; $L_{\mathrm{mg}}$ is the anchorage length; $f_{\mathrm{cu}}$ is the tensile strength of the grouting material, N/mm²; and $f_{\mathrm{stk}}$ is the standard tensile strength of the steel bar, N/mm².

$${\tau }_{\mathrm{u}}={\displaystyle \frac{F_d}{\pi d{L}_{m\mathrm{g}}}}={\displaystyle \frac{F_d}{n\pi d^2}}$$

(2)

where F_d is the ultimate tensile load and τ_u is the bonding stress between the reinforcement and grout.

$${\tau }_{\mathrm{u}}\pi d{L}_c=A{f}_{su}$$

(3)

$$L_c={\displaystyle \frac{f_{su}}{4{\tau }_{\mathrm{u}}}}$$

(4)

Here, L_c is the critical anchoring length, and $f_{su}$ (N/mm²) is the tensile strength test value of the specimen.

$$L_e={\displaystyle \frac{d{F}_d}{4u_{\mathrm{e}}}}$$

(5)

$$L_{in}={\displaystyle \frac{d(F_d-F_y)}{4u_{\mathrm{iu}}}}$$

(6)

where L_e is the elastic segment length, L_in is the inelastic segment length, u_e is the bond stress in the elastic region, and u_in is the bond stress in the inelastic region.

4.1.2. Diameter

The durability of the structure and the bond strength generated by the concrete vary with changing the diameter of the reinforcing bars. It is crucial to select the optimal reinforcing bar diameter that ensures that the minimum inner diameter of the grouted sleeve meets the specified nominal diameter of the connected reinforcing bar. Liu [77] conducted 27 groups of bond performance tests on grouted sleeves and finite element analysis and reported that the smaller the reinforcing bar diameter, the greater the bond strength at the interface, which affects the failure mode of the sleeve. Studies [30,51,76] have shown that as the reinforcing bar diameter increases, the relative bond area decreases, leading to a reduction in the bond strength between the reinforcing bar and the concrete. The main reason for this is that larger-diameter reinforcing bars exhibit more pronounced necking during failure, with a significant reduction in diameter during tension, which greatly weakens frictional resistance and mechanical interlocking.

As shown in Figure 15, the length of the sleeve increases linearly with increasing the reinforcing bar diameter, but the rate of increase begins to slow when the reinforcing bar diameter exceeds approximately 20 mm [78]. Sayadi [39] revealed that, with unchanged parameters for grouted joints, increasing the reinforcing bar diameter from 14 mm to 16 mm changed the failure mode of the sleeve from reinforcing bar fracture to sleeve fracture. Wang [13] conducted bolted sleeve tests and reported that when the sleeve provides high resistance, increasing the reinforcing bar diameter tends to cause grout debonding failure in the sleeve. Some researchers [14,29,41] have conducted tensile tests on sleeves with various reinforcing bar diameters. The results showed that increasing the reinforcing bar diameter can effectively enhance the tensile strength and ductility. However, increasing the reinforcing bar diameter reduces the thickness of the grout, making it more susceptible to splitting cracks that can extend inside the sleeve. The sensitivity of the sleeve to grout splitting deformation increases, improving the confinement effect and reducing reinforcing bar slip, thus increasing the stiffness of the specimen before yielding [79]. Therefore, when the reinforcing bar diameter is selected, the impacts of the sleeve diameter and anchorage length must be considered.

4.1.3. Intensity

The strength of reinforcing bars connected by grouted sleeves directly affects the overall strength and ductility of a structure. For a grouted sleeve to be considered qualified, its tensile strength must reach 1.15 times the tensile strength of the reinforcing bar. Therefore, studying the impact of different ordinary threaded reinforcing bars and high-strength reinforcing bars on the performance of components is critical for selecting appropriate reinforcing bar strengths in practical engineering applications.

Research has shown that when the sleeve and grout provides sufficient bond strength, increasing the reinforcing bar strength from HRB400 to HRB500 slightly improves the load-bearing capacity of the specimen, but the deformation ability decreases owing to the significant weakening of the bond behaviour between the reinforcing bar and the grout [13]. According to the literature [33], the connection load-bearing capacity of HRB500 reinforcing bars is approximately 5.4% to 23.7% greater than that of HRB400 reinforcing bars. Via a theoretical analysis combined with numerical simulation, Gao [68,80] from Tongji University investigated the relationship between the reinforcing bar strength and grout bond strength. The scholar concluded that increasing the reinforcing bar strength from HRB400 to HRB500 enhances the load-bearing capacity, but the anchorage length can remain unchanged. However, when the strength is increased to HRB600, the load-bearing capacity increases significantly, and the anchorage length must be increased to 8 d to meet the tensile requirements.

Additionally, increasing the reinforcing bar strength alters the failure mode of the component and the maximum displacement at the ultimate strength [59,71]. Overall, when higher-strength reinforcing bars are used, the difference in their properties results in a reduced deformation rate under stress, affecting the bond strength between the grout and the reinforcing bar. This leads to differences in the performance of sleeves under different reinforcing bar strengths.

4.2. Grout Material

4.2.1. Grouting Material Type

Grouted sleeve cementitious grout differs from ordinary concrete slurry in that it is a high-strength cement-based binder without coarse aggregates. As shown in Figure 16, the compressive strength of the grout is a key indicator of its performance, directly affecting the bond strength between the grout and the reinforcing bar [78]. As shown in Table 6, the grout types currently used in sleeve testing in this study are categorized into three classes, namely, specialized grout, modified grout, and UHPC grout. Specialized grout is a high-strength micro-expansive grout developed specifically for sleeve structures. Modified grout involves the addition of specific materials to ordinary grout to enhance certain properties. UHPC is a relatively mature industrial material characterized by the absence of coarse aggregates and high compressive strength, making it highly compatible with grouted sleeves. Consequently, UHPC grout is widely used in joint connections today.

In addition to the three categories of grout mentioned above, some researchers have developed grouts specifically for use in low-temperature conditions. Xie et al. [81] developed a low-temperature grout suitable for environments ranging from −5 °C to 5 °C, whereas Li et al. [82] conducted research on subzero-temperature grout suitable for environments ranging from −10 °C to 0 °C. Unfortunately, these low-temperature grouts have not yet been tested in actual grouting tensile tests.

According to the specifications, Table 6 lists key parameters such as the 28 d compressive strength and 30 min flowability of the grout, as well as the ultimate tensile strength of the reinforcing bars and the ultimate tensile strength of the sleeves and the ratio of the latter to the former. Comparative analysis reveals that when the compressive strength of the grout is low, specimens are prone to failure because of reinforcing bar slippage. When higher-strength UHPC is used as the grout, the tensile strength of the sleeve increases, altering the failure mode of the sleeve. Additionally, adding fibres significantly reduces the flowability of the grout, increasing the likelihood of producing grouting defects. Therefore, increasing the compressive strength of the grout to increase the tensile strength of the sleeve is an effective measure. However, regardless of the type of grout used, the ultimate tensile strength f_stk of the grouted sleeve has not met the specification requirement of 1.15 times the ultimate tensile strength f_u of the reinforcing bar.

Thus, it is recommended that in actual engineering design and construction, appropriate and cost-effective grout should be selected rather than the grout with the highest compressive strength.

Table 6. Grouting material classification.

Sort	Reference	Type	28 d Compressive Strength (MPa)	30 min Fluidity	Bar Diameter (mm)	Anchorage Length (mm)	Ultimate Tensile Strength of Rebar fu (MPa)	Ultimate Tensile Load of Sleeve (kN)	Ultimate Tensile Strength of Sleeve fk (MPa)	Failure Mode	fk/fu Ratio
Special grout	Huang et al. [14]	Ordinary nonshrinkage material	60.8	290	28	8 d	596	369.6	600.24	Slip of steel bar	1.01
	Shao et al. [76]	Special grout material	103.2	280	32	7 d	612.06	499.8	621.45		1.02
	Lu et al. [37]	Ordinary grout material	79.2	/	20	8 d	591	182	579.32	Fracture of steel bar	0.98
	Zheng et al. [40]		70.2	290	25	7 d	625	300.8	612.79		0.98
	Kim [51]	Mortar grout material	98	/	25	7.5 d	689	322.72	658.00		0.96
	Lin and Wu [52]	Ordinary grout material	84	270	32	6 d	586	463	575.69	Slip of steel bar	0.98
Improved grout	Zhang et al. [83]	Polypropylene fibre	87	365	18	8 d	649	164.7	647.23	Fracture of steel bar	1.00
		Polyvinyl alcohol fibre	86.1	380	18	8 d	649	164.8	647.60		1.00
		Basalt fibre	90	380	18	8 d	649	165.1	648.8		1.00
	Wu et al. [84]	High strength concrete	80.79	/	20	7.15 d	632.5	472.81	637.65		1.01
	Lei et al. [71]	Custom fibre grout	143	290	18	8 d	652.3	168.84	663.50		1.02
UHPC grout	Wang et al. [13]	Unfibre	105.2	289	28	5 d	624	391.6	636.00		1.02
		Reinforced fibre	126.0	258	28	5 d	624	325.7	528.90		0.85
	Huang et al. [14]	Unfibre	89.9	289	28	8 d	596	373.4	606.41		1.02
		Add basalt fibre	97.3	253	28	8 d	596	374.1	607.55		1.02
	Shao et al. [76]	Reinforced fibre	120.8	268	28	7 d	602.19	377	612.26		1.02
			120.8	268	32	8 d	612.06	501	622.94		1.02
	Fu et al. [85]	Unfibre	119.6	280	32	8 d	836.7	674.14	838.65		1.00
		Reinforced fibre	128.4	262	32	8 d	836.7	674.75	838.41		1.00
	Lei et al. [71]	Commercial UHPC	97.2	310	16	8 d	654.7	128.48	639.01		0.98

Table 6. Grouting material classification.

Sort	Reference	Type	28 d Compressive Strength (MPa)	30 min Fluidity	Bar Diameter (mm)	Anchorage Length (mm)	Ultimate Tensile Strength of Rebar fu (MPa)	Ultimate Tensile Load of Sleeve (kN)	Ultimate Tensile Strength of Sleeve fk (MPa)	Failure Mode	fk/fu Ratio
Special grout	Huang et al. [14]	Ordinary nonshrinkage material	60.8	290	28	8 d	596	369.6	600.24	Slip of steel bar	1.01
	Shao et al. [76]	Special grout material	103.2	280	32	7 d	612.06	499.8	621.45		1.02
	Lu et al. [37]	Ordinary grout material	79.2	/	20	8 d	591	182	579.32	Fracture of steel bar	0.98
	Zheng et al. [40]		70.2	290	25	7 d	625	300.8	612.79		0.98
	Kim [51]	Mortar grout material	98	/	25	7.5 d	689	322.72	658.00		0.96
	Lin and Wu [52]	Ordinary grout material	84	270	32	6 d	586	463	575.69	Slip of steel bar	0.98
Improved grout	Zhang et al. [83]	Polypropylene fibre	87	365	18	8 d	649	164.7	647.23	Fracture of steel bar	1.00
		Polyvinyl alcohol fibre	86.1	380	18	8 d	649	164.8	647.60		1.00
		Basalt fibre	90	380	18	8 d	649	165.1	648.8		1.00
	Wu et al. [84]	High strength concrete	80.79	/	20	7.15 d	632.5	472.81	637.65		1.01
	Lei et al. [71]	Custom fibre grout	143	290	18	8 d	652.3	168.84	663.50		1.02
UHPC grout	Wang et al. [13]	Unfibre	105.2	289	28	5 d	624	391.6	636.00		1.02
		Reinforced fibre	126.0	258	28	5 d	624	325.7	528.90		0.85
	Huang et al. [14]	Unfibre	89.9	289	28	8 d	596	373.4	606.41		1.02
		Add basalt fibre	97.3	253	28	8 d	596	374.1	607.55		1.02
	Shao et al. [76]	Reinforced fibre	120.8	268	28	7 d	602.19	377	612.26		1.02
			120.8	268	32	8 d	612.06	501	622.94		1.02
	Fu et al. [85]	Unfibre	119.6	280	32	8 d	836.7	674.14	838.65		1.00
		Reinforced fibre	128.4	262	32	8 d	836.7	674.75	838.41		1.00
	Lei et al. [71]	Commercial UHPC	97.2	310	16	8 d	654.7	128.48	639.01		0.98

4.2.2. Grouting Material Defects

As shown in Figure 17, human factors, environmental conditions, technology, processes, and equipment are the primary causes of defects in grout, leading to issues such as grout leakage, interface delamination, internal voids, reinforcing bar eccentricity, and poor grouting [86]. Among these defects, grout leakage and reinforcing bar eccentricity are the most common. Grout leakage can result in different types of defects depending on the arrangement of the sleeves. These defects can include horizontal upper voids, vertical top voids, and lower voids [87]. Studies have shown that when grout leakage defects exist in the structure, the tensile strength of the grouted sleeves decreases with changes in the defect rate, altering the failure mode of the sleeves. For example, Zheng et al. [56] designed 24 groups of specimens with different defect rates and types of voids using 16 mm diameter HRB400 reinforcing bars and conducted uniaxial and cyclic loading tests. The results showed that under uniaxial tensile loading, when the anchorage length at the defective end was less than 4 d, the failure mode shifts from reinforcing bar fracture to reinforcing bar slip. Under cyclic loading, the critical anchorage length for the change in failure mode is 5 d. Under the same loading conditions, Li et al. [65] reported that when the length of grout leakage defects at the sleeve joint end exceeds 20% of the reinforcing bar anchorage length of 8 d, the mechanical behaviour of the structure is significantly altered. Similarly, related studies [88,89] indicate that if defects exist in the sleeves, then the load-bearing capacity, stiffness, ductility, and energy dissipation of prefabricated connections (such as prefabricated frames, walls, and columns) are weakened.

Research has revealed that secondary grouting to fill defects in grouted sleeves, as shown in Figure 18, can result in structural performance similar to that of sleeves fully filled in one operation. Therefore, by using effective defect detection techniques to identify the defect rate, location, and type, the timely repair of sleeve defects can be performed. Additionally, reinforcing bar eccentricity refers to the misalignment of the centre axis of the reinforcing bar with the centre axis of the sleeve after the bar is inserted [76,86]. This misalignment affects the interaction among the reinforcing bar, the grout, and the inner wall of the sleeve. As the eccentricity increases, the constraint provided by the grout is weakened, reducing the bond strength and potentially increasing reinforcing bar slip. However, when the anchorage length is sufficiently long and the compressive strength of the grout is sufficiently high, the impact of reinforcing bar eccentricity on performance is minimal and can be neglected.

4.3. Sleeve Pipe Sleeve

As shown in Figure 19 and Figure 20, in sleeve performance tests, tensile loads are transmitted from the reinforcing bars and grout to the sleeve tube, causing strain in the tube; notably, severe cases can lead to fracture [39]. This finding indicates that the material properties and processing methods of the sleeve tube play crucial roles in the load-bearing capacity of the sleeve. As shown in Figure 21, materials such as mechanically processable low-carbon steel, corrugated aluminium tubes, carbon structural steel, and cast iron and ductile iron produced by casting processes are currently selected for sleeve tubes [40,41,52,61]. Studies typically use malleable and cost-effective structural steel instead of commercially available cast iron and ductile iron sleeves, which are more complex and expensive to process. However, the smooth inner surface of structural steel can cause the grout to easily slip out. To increase the bond strength between the sleeve and the grout, measures such as setting up annular ribs, welding wedge inserts, welding protruding ribs, welding reinforcing bars, and machining threads are employed. However, the processing of a sleeve tube can weaken the mechanical properties of the material. For example, setting up annular ribs can cause severe stress concentration at the first protruding rib, forming a weak point. Deep thread machining can reduce the elastic modulus of the sleeve, weakening its tensile strength. Therefore, to ensure the safety of the sleeve’s tensile performance, these parameters need to be kept within a reasonable range. Relevant studies have shown that the wall thickness of the sleeve should not be less than 9 mm when setting up annular ribs, the depth of thread machining should not exceed 1/8 of the sleeve wall thickness, and the sleeve wall thickness should not be less than 1/13 of the diameter of the connected reinforcing bar [29,35,37,40,41]. Changes in the structural form of the sleeve tube, as mentioned in the performance test in Section 2.1, are not repeated here. Notably, the ratio of the sleeve’s outer diameter to its wall thickness (OD/WT) significantly influences the load-bearing capacity of the structure. Some researchers believe that a higher OD/WT ratio results in a greater joint load-bearing capacity and smaller residual deformation. Reasonably increasing the wall thickness of the sleeve can enhance the constraint effect on the grout [13].

In addition to the use of structural steel for sleeve tubes, some researchers have used composite manufacturing processes to create glass fibre-reinforced polymer (GFRP) sleeve tubes [39]. Similar to the results for structural steel, corrugated GFRP sleeves with uneven surfaces provide better constraint effects. When the thickness of the GFRP sleeve is reduced to 4 mm, the sleeve fails, and the tensile strength decreases by approximately 7%. Therefore, the thickness of the GFRP sleeve should not be less than 5 mm (equivalent to five layers of GFRP). Overall, the material and processing methods of the sleeve tube play crucial roles in the structural performance of the sleeve. Within reasonable limits, the wall thickness of the sleeve is positively correlated with the constraint effect it provides.

4.4. Temperature

Under fire disasters, the internal temperature of prefabricated structures significantly increases, with the temperature effects being particularly noticeable at joint and seam locations. Since grouted sleeves are composed of different materials, their thermal responses at high temperatures vary, leading to the potential failure of the specimens. Therefore, investigating the critical temperature at which the mechanical properties of grouted sleeve connections degrade is an important topic. Many scholars have conducted studies on the performance changes in grouted sleeves at different temperatures, with temperatures ranging from room temperature (20 °C) to high temperatures (up to 1000 °C). As early as 2006, Zhao et al. [91] pioneered research on the performance of grouted sleeves with different anchorage lengths at high temperatures ranging from 250 °C to 500 °C. Other scholars subsequently extended the temperature range to higher levels. The results revealed that the critical temperature at which the failure mode of the sleeves changes is approximately 400 °C. When the temperature reaches 600 °C, the load-bearing capacity decreases by less than 10%, and the failure mode changes from pull-out after reinforcement yielding to pull-out before reinforcement yielding. Xiao et al. [92] conducted high-temperature performance tests on grouted sleeves and reached the same conclusion, further validating that 400 °C is a critical point for the transition in sleeve performance. Additionally, the presence of defects in the grout and variations in the thickness of the protective layer can significantly alter the critical temperature for performance transition. When defects exist in the grout, the critical temperature for the transition in the failure mode decreases to 300 °C. Increasing the thickness of the protective layer can increase the critical temperature to 600 °C [77].

As shown in Figure 22, Wang et al. [93] reported that as the temperature increases from 200 °C to 800 °C, the difference between the Δu values after heating and during heating begins to increase, and the trend of load reduction becomes more significant, with a clear difference in the ultimate load. The scanning electron microscopy (SEM) of the grout at the same temperature conditions revealed that the degradation in sleeve performance is due to the decomposition of hydration products, increased porosity, and changes in crystal phases, leading to a reduction in bond stress, which ultimately makes the grouted sleeve more prone to failure under loading. Therefore, when temperatures are excessively high, new anchorage lengths should be reconsidered. Furthermore, high temperatures have a relatively minor impact on the strain development of the sleeves. The sleeve tube remains elastic at high temperatures. Studies have shown that the elastic modulus of reinforcing bars slightly increases from 100.2% to 100.3% below 400 °C and then decreases almost linearly from 400 °C to 600 °C, with the reduction in elastic modulus being permanent [94].

5. Force Transfer Mechanism

As shown in Figure 23, the load-transfer mechanism of grouted sleeves involves the interaction among reinforcing bars, grout, and sleeve. When reinforcing bars are inserted into sleeves filled with high-strength micro-expansive cement-based grout, the bond strength formed between them is crucial for maintaining continuous load transfer. The specific load transfer path is as follows: external forces act on the reinforcing bars, which are then transferred to the grout through the bond strength between the reinforcing bars and the grout. The grout subsequently transfers the load to the sleeve through the bond strength between the grout and the sleeve, and finally, the sleeve transmits the load to the external structure or component. Conversely, the compressive resistance of sleeves is significantly influenced by factors such as grout compressive strength, sleeve rigidity, and grout compaction. However, the current literature on the compressive failure mechanism remains limited, and future studies are encouraged to further explore this mechanism. In contrast, this study focuses on investigating the tensile failure mechanism of sleeves.

First, consider the load-transfer mechanism between the reinforcing bars and the grout. As shown in Figure 24, the presence of ribs on deformed reinforcing bars exerts pressure and frictional resistance on the grout. These forces can be decomposed into components parallel and perpendicular to the axis of the reinforcing bar. The parallel component provides the primary part of the interfacial resistance between the reinforcing bar and the grout, whereas the remaining part is composed of chemical bonding forces, which collectively resist tensile loads. The perpendicular component acts as the interfacial pressure between the reinforcing bar and the grout, with the opposite reaction being the constraint force provided by the sleeve to the grout. These combined forces enable the reinforcing bar to function in load transfer within the sleeve [95].

Next, the load-transfer mechanism between the grout and the sleeve is considered, as shown in Figure 25. When the inner surface of the sleeve is smooth and angle-free, the frictional and chemical bonding forces form a horizontal force to resist tensile loads. When the inner surface of the sleeve is smooth but angled, the pressure component exerted by the grout on the sleeve, along with the frictional and chemical bonding forces, forms a horizontal force. This results in a higher tensile load-bearing capacity for conical sleeves. Since the sleeve opening provides the least constraint, wedge blocks with different angles can be added at the sleeve opening to enhance the tensile load-bearing capacity, with the load-transfer mechanism being similar to that of conical sleeves [37,41]. When the inner surface of the sleeve has ribs, the radial component of the contact pressure provided by the concentric ribs against the grout can resist the load transferred by the grout, as well as the splitting expansion caused by the “wedging” effect between the reinforcing bar and the grout [6]. When the inner surface of the sleeve has threads, the bond resistance varies depending on the thread profile. For example, with rectangular and triangular threads, the mechanical interlock between the inner rectangular threads of the sleeve and the grout, along with the axial slip prevention provided by the threads, significantly reduces the axial slip of the grout. Additionally, the passive constraint of the sleeve effectively restrains the splitting expansion deformation of the grout, enhancing the bond strength between the sleeve and the grout [14]. When the inner surface of the sleeve has triangular threads, the structure not only provides mechanical resistance but also offers resistance from the grout perpendicular to the thread slope. The radial component of this resistance, together with other resistances, resists the external load transferred by the grout.

Figure 23. Sleeve force diagram [96].

Figure 23. Sleeve force diagram [96].

Figure 24. Reinforcement stress [95].

Figure 24. Reinforcement stress [95].

Figure 25. Sleeve stress diagram [14,37,40,41,69,97].

Figure 25. Sleeve stress diagram [14,37,40,41,69,97].

6. Performance Evaluation

Through the evaluation indicators of yield ratio R_y, strength ratio R_s, ductility ratio R_d, and capacity ratio R_c, the structural performance of sleeves under loading can be quantitatively assessed [22,41,98,99]. According to standards [22], the tensile strength of test specimens must be at least 125% of the rebar yield strength, corresponding to a strength ratio (R_s > 1.25). The yield ratio (R_y) is used to determine whether deformed rebars yield during testing, with performance-qualified grout-filled sleeves requiring R_y ≥ 1.0. Additionally, joints must exhibit sufficient ductility; for regions with moderate-to-low seismic activity, the ductility ratio (R_d ≥ 4.0) is mandated. The capacity ratio (R_c) indicates the utilization efficiency of the sleeve’s capacity, where values closer to 1 signify higher efficiency in sleeve usage. As shown in

Table 7. Sleeve evaluation.

Serial Number	Specimen Name	Anchorage Length/mm	Bar Diameter	Ultimate Tensile Strength	Yield Load Fy/kN	Ry ≥ 1.0	Rs ≥ 1.25	Rd ≥ 4.0	Rc	fust/fstk	Failure Mode
1	WBS-5 [100]	125	16	128	116.8	1.16	1.27	7.64	0.33	/	Slip of steel bar
2	WBS-7 [100]	175	16	133.5	113.8	1.13	1.33	13.13	0.45	/	Fracture of steel bar
3	THS-5 [100]	125	16	135.4	115.9	1.15	1.35	12.7	0.46	/
4	THS-7 [100]	175	16	137.6	116.5	1.16	1.37	11.56	0.65	/
5	GW-D20-2 [37]	140	20	182.7	140.25	1.12	1.45	5.21	/	1.08
6	GT-D20-2 [37]	140	20	181.9	139.31	1.11	1.45	5.1	/	1.07
7	S57-400-28-G1-B1-5d [13]	140	28	391.6	254	1.03	1.59	34.69	0.96	1.02
8	S57-500-28-G1-B1-5d [13]	140	28	394	309.9	1.01	1.28	37.19	0.97	0.92
9	S48-t8-28-8d-G1-C2/2 [14]	224	28	373.1	/	1.04	1.45	4.17	0.76	1.02
10	L350-18B-1 [101]	170	12	78.9	74.6	1.2	1.34	2.65	1.05	/	Grout slip
11	3512-2R6 [39]	170	12	78.94	/	1.28	1.44	2.2	/	/

, parameters such as the ultimate tensile load, yield load, four evaluation indicators, and failure modes are listed. By reviewing the assessment results of various types of sleeves from the literature [13,14,37,39,100,101,102], it can be concluded that the structural performance evaluation of sleeves is not influenced by the failure mode or the diameter of the reinforcing bars. The performance evaluation reflects the numerical representation of the sleeve’s energy dissipation capability before reaching the ultimate state, which is affected mainly by changes in the structural form.

According to the literature [100], when the sleeve wall thickness is reduced to 4.5 mm, the corresponding evaluation parameters transition from compliant to non-compliant. Similarly, studies in [13,39] show that moderate adjustments to sleeve wall thickness within a reasonable range significantly alter the constraint effect of the sleeve. Thus, sleeve wall thickness is a critical factor influencing the performance of grout-filled sleeves, and modifications to this critical parameter directly affect the evaluation results of sleeve performance. In addition to wall thickness, the specimens listed in row 7 of Table 7, which use UHPC grout and incorporate bolts, tend to have R_y, R_s, and R_d values greater than 1.0, 1.25, and 4.0, respectively, with Rc values generally ranging from 0.80 to 1.00. Furthermore, the increase in the angle of the wedge blocks at the sleeve ends has the most significant impact on the performance evaluation, serving as a key factor for WBS-type sleeves. This aspect needs to be carefully considered in sleeve design.

7. Conclusions

The rise in grouted sleeve technology has played a crucial role in the development of prefabricated structures. In this paper, grouted sleeve connection technology is comprehensively reviewed, focusing on its application characteristics, performance tests, influencing factors, and a detailed analysis of the load-transfer mechanisms and performance evaluation criteria for different types of sleeves. The main conclusions from the literature review are summarized as follows:

(1)

Japanese standards are more detailed in the classification of sleeve performance indicators, whereas Chinese standards are more specific in the definition of performance test parameters. Noncircular sleeves perform better than circular sleeves do in conventional applications. Currently, grouting techniques for sleeves are relatively single-purpose across different structural forms, and the integration of different techniques needs to be considered. With continuous improvements in the material and processing properties of grout, reinforcing bars, and sleeves, the application scope of sleeves has expanded to various building structural forms.

(2)

Modifying the end of the sleeve (e.g., welding steel plates, wedge blocks, or reducing the diameter) significantly enhances the tensile performance of the sleeve. Altering the smooth inner surface of the sleeve to add shear keys (e.g., threads, welded ring ribs, annular grooves) further improves tensile performance. Fatigue performance tests have shown that factors such as grout defects, increased fibre content, and elevated loading temperatures negatively affect sleeve performance. Damage to the grout at the sleeve end can reach approximately 8% of the total sleeve length, and the initial stiffness can decrease by an average of approximately 38.5%. In seismic tests, reinforcing bar slip and yielding often occur, preventing the grouted sleeve from fully exploiting its structural performance.

(3)

The anchorage length of reinforcing bars should not be less than the critical anchorage length, and the anchorage segment includes both elastic and inelastic sections. Theoretical calculations of the anchorage length can serve as a basis for structural design. The diameter of reinforcing bars is most influenced by the inner diameter of the sleeve; larger diameters are not always better, as they result in greater displacement. Increasing the strength of reinforcing bars can increase the load-bearing capacity by approximately 5.4% to 23.7%, but the anchorage length must be correspondingly increased to meet the tensile requirements.

(4)

Grout can be classified into three types: specialized grout, modified grout, and UHPC grout. The tensile strength of the sleeve is positively correlated with the compressive strength of the grout, but fibres can reduce the flowability of the grout and increase the likelihood of defects. The most common defects in sleeves are grout leakage and reinforcing bar eccentricity. Grout leakage can significantly affect the load-bearing capacity and durability of the sleeve. When the reinforcing bar is offset from the centre line of the sleeve by less than 6 mm, the impact is relatively limited. Defect filling and monitoring technologies are now quite mature, allowing for the combination of different techniques to address various defects.

(5)

Sleeve casings can be made of cast iron, alloy steel, or GFRP. Processing the casing can weaken the mechanical properties of the material, so the depth of the threads should not exceed one-eighth of the casing wall thickness. The ratio of the outer diameter to the wall thickness (OD/WT) significantly affects sleeve performance. The wall thickness of the sleeve should be greater than 1/13 of the diameter of the anchored reinforcing bar but should not be excessively thick. The impact of temperature on the tensile performance of the sleeves is most significant at 400 °C.

(6)

The load-transfer mechanism involves the interaction among reinforcing bars, grout, and sleeve. The ribs on the reinforcing bars provide pressure and frictional resistance to the grout, along with chemical bonding forces. The casing enhances tensile performance by increasing surface friction and providing shear keys, combined with the passive constraint effect of the casing. Performance evaluation theories have confirmed that adjusting key factors in conventional sleeves can significantly influence performance.

8. Prospects

(1)

Grouted sleeve connection technology is well understood within the traditional framework of strength and stiffness assessment; however, its contributions to enhancing structural resilience have not been systematically investigated. Therefore, conducting theoretical analyses to quantify how changes in sleeve parameters affect overall structural resilience and proposing specific metrics for resilience contribution hold promising prospects for future research.

(2)

The long-term performance and reliability of sleeve connections are highly important. Currently, accurate and high-precision life prediction models are needed to ensure and prevent safety issues arising from structural damage or ageing. Further research is needed in this area.

(3)

The constitutive model for the slip between reinforcing bars and grout in grouted sleeves needs further clarification. The coupled effects of high temperatures, different loading conditions, and the presence of defects require more attention. Research on and the development of admixtures for grout, degradation models for sleeve performance after corrosion, and the selection of high-strength aluminium alloys and other materials for casings need to be explored further.