Investigating the Application of Smart Materials for Enhanced Maintenance of Rubber Expansion Joints in Bridge Expansion Joint Systems

Featured Application

Superelastic shape memory alloy expansion joints are demonstrated as an alternative to traditional rubber expansion joints, with the potential for widespread application in bridge maintenance.

Abstract

In South Korea, the public infrastructure encompasses 172,111 facilities, with bridges accounting for a significant segment (totaling 34,199). These bridges undergo expansion due to traffic, vehicular loads, and temperature fluctuations. Expansion joint devices are installed to maintain vehicle stability and driving performance across expansion gaps. While these devices effectively ensure vehicular stability and performance, they do not address issues such as leakage and debris fall; therefore, rubber expansion joints should be installed. However, these rubber joints are prone to damage from various factors, resulting in secondary issues such as girder corrosion and accidents under bridges. Because of these inherent vulnerabilities, these joints require frequent replacements, making continuous bridge maintenance challenging. Therefore, this study explores the development of novel expansion joints using superelastic shape memory alloys to overcome the limitations of traditional rubber expansion joints. A comparative finite element analysis was conducted on the developed superelastic shape memory alloy and traditional rubber expansion joints. This study also assessed the long-term usability of these novel joints, particularly their ability to revert to their original shape post load removal. This research presents a promising alternative to conventional expansion joints and holds potential implications for enhancing the durability and safety of bridge infrastructure.

1. Introduction

Since the 1980s, South Korea has experienced significant construction development, resulting in the establishment of a wide array of infrastructure facilities. The National Land Safety Management Corporation of South Korea oversees these structures through the Facility Management System [1]. Notably, bridges constitute the largest category among non-building infrastructure facilities, as detailed in

Table 1. Status of infrastructure facilities in South Korea (as of 2 January 2024).

Facility	Type			Total
	1	2	3
Bridge	5214	7741	21,761	34,716
Tunnel	2010	2345	849	5204
Port	173	440	-	613
Dam	120	567	-	687
Building	3624	77,670	30,029	111,323
River	649	6466	20	7135
Water and Sewage	580	1791	1	2372
Retaining wall	-	3835	721	4556
Cut Slope	-	4769	1	4770
Utility-Pipe Conduit	-	50	-	50
Others	-	-	685	685
Sum	12,370	105,674	54,067	172,111

. These bridges are classified into types one to three, depending on their length and type. The classification of type one to type three bridges is listed in

Table 2. Criteria for the classification of bridge facilities.

Division		Standard of Division
Type 1	Road	Bridges whose super structure type is a suspension bridge, cable-stayed bridge, arch bridge, or truss bridge.
		Bridges with a maximum span length of 50 m or more (excluding single-span bridges).
		Bridges with a length of 500 m or more.
		A covering structure with a width of 12 m or more and an extension of 500 m or more.
	Railway	Bridge for a high-speed train.
		Urban railway bridges and overpass bridges.
		Bridges whose super structure is a truss bridge or an arch bridge.
		Bridges with a length of 500 m or more.
Type 2	Road	One-span bridge with a span length of 50 m or more.
		Bridges that do not fall under type 1 and have a length of 100 m or more.
		Covering structures that do not fall under type 1 and have a width of 6 m or more and a length of 100 m or more.
	Railway	Bridges that do not fall under type 1 and have a length of 100 m or more.
Type 3	Bridge	Road bridges with a length of 20 m or more and less than 100 m, as per the ⌈Road Act⌋.
		Bridges with a span length of 20 m or more as per the ⌈Act on the maintenance and improvement of road networks in agricultural and fishing villages⌋.
		Bridges with a length of 20 m or more.
		Railway bridges with a length of less than 100 m.
	Pedestrian Overpass	Pedestrian bridge over 10 years old.

. As observed in Table 1 and Table 2, most of the bridges in Korea are small- and medium-sized.

Bridges serve as essential national infrastructure assets, playing vital roles in transportation and logistics. These structures are engineered to adapt flexibly to various forces, including traffic and vehicular loads, creep, drying shrinkage, and temperature-induced expansion. This is achieved by designing them with adequate expansion allowances to accommodate movements along and perpendicular to the bridge axis. However, such expansions can lead to the formation of gaps between the upper girders, which may cause vehicle damage and degrade driving performance. To mitigate these issues, bridges are typically equipped with expansion joint devices between the upper girders. These devices are essential for preventing gap formation, thereby ensuring vehicle safety and driving performance.

Expansion joint devices are primarily classified into steel and rubber types based on their material properties. Steel expansion joint devices, such as finger and rail joints, are characterized by their durability and adjustable expansion capacity, making them suitable for long-span and medium-sized bridges. In contrast, monocell joints, featuring a rubber driving surface, offer superior shock absorption and enhance vehicle driving performance. However, their application is limited to smaller bridges, due to their restricted expansion capacity. Additionally, the rubber material of monocell joints is inherently susceptible to wear.

Expansion joint devices in bridges are designed with a specific gap to flexibly accommodate the displacement between the upper girders. However, this gap can lead to issues such as the corrosion of the upper girders (concrete and steel box girders), due to rainwater penetration and debris accumulation from vehicle traffic. To mitigate these problems, rubber expansion joints, often termed rubber seals, are installed within these gaps, functioning similarly to water trays. These joints are effective in diverting rainwater away from the bridge structure and preventing debris from falling underneath. Despite their utility, rubber expansion joints, as consumable components, are susceptible to damage, due to continuous load-bearing and external impacts.

Figure 1 illustrates the primary forms of damage to expansion joint devices, with rubber expansion joints constituting 18.4% of the total damage. Figure 2 shows the actual cases of damage to the rubber expansion joints, with the leading causes being traffic and vehicular loads (54.9%), debris accumulation (22.8%), and material degradation (22.3%), as summarized in

Table 3. Detailed analysis of the primary damage causes in expansion joints.

Type	Cause	Ratio (%)
Sediment	Defective Cleaning	100.0
Rubber Seal	Vehicle load Sediment Deterioration	54.9 22.8 22.3
Leak	Functional Decline Defective Construction Rubber Seal not Installed	61.4 36.2 2.4
Corrosion	Rainwater, etc.	72.1 27.9
Distance Lack	Defective Construction Displacement Defective Repair	82.4 16.0 1.6
Damage/Transform	Vehicle load	100.0

[2]. Despite this damage, rubber waterproofing materials are treated as consumable parts, and these parts are replaced after the damage occurs. Therefore, research on the maintenance of rubber index materials is not widely conducted in Korea.

This study proposes an innovative approach to address the limitations of rubber expansion joints in existing bridge designs. This research explores the use of superelastic shape memory alloys as a substitute for conventional rubber materials in expansion joints. These alloys can uniquely return to their original shape after plastic deformation, offering adaptability to bridge expansions. Additionally, these alloys can be tailored to various bridge types and possess properties akin to general steel, improving maintenance efficiency. Previous preliminary research has confirmed the efficacy of superelastic shape memory alloys for use in smaller bridges as an alternative to rubber expansion joints. Displacement control was performed on the superelastic shape memory alloy, wherein it reverted to its original shape without residual displacement [3]. However, in the preliminary research, the approximate shape of a small-sized water-stop material applied to small-scale bridges was created and reviewed. Based on this foundation, the current study extends the application to larger bridges, modeling both rubber and superelastic shape memory alloy expansion joints similar to those used in real-world scenarios. Furthermore, theoretical verification was conducted through finite element analysis to evaluate the practical applicability, without performing separate field application.

2. Material Characteristics

2.1. Rubber Expansion Joints

Table 4. Material properties of rubber expansion joints.

Items	Standard	Test Methods
Tensile Strength (MPa)	Over 15 MPa	KS M 6518
Strain (%)	300	KS M 6518

presents the material properties of the rubber expansion joints used in expansion joint devices [4]. As discussed in the introduction, rubber expansion joints primarily serve as water trays and are not subject to specific regulations based on bridge type. Consequently, they conform to basic standards, such as a maximum tensile strength of 15 MPa and a strain capacity of 300%, in accordance with KS (Korean Standards & Certification). In particular, the material characteristics used for this finite element analysis are graphically represented in Figure 3.

2.2. Superelastic Shape Memory Alloys

Recent developments in the construction industry have shown a growing interest in smart construction materials capable of actively responding to various loads and autonomously restoring their original form through intelligent capabilities. Superelastic shape memory alloys, among these smart materials, are increasingly recognized for their exceptional behavior characteristics and are the subject of extensive research in engineering. These alloys, proposed as alternative materials for rubber expansion joints in expansion joint devices, demonstrate flag-shaped behavior characteristics, as depicted in Figure 4. Unlike conventional steel, superelastic shape memory alloys can revert to their original form after undergoing plastic deformation beyond the elastic limit [5,6,7]. A critical feature of these alloys is the presence of austenite, which exhibits a superior performance under stress and temperature conditions, while martensite, in comparison, shows a relatively poorer performance. At sub-room temperatures, the martensite in superelastic shape memory alloys is primarily in the form of twinned martensite, which transforms into detwinned martensite under external loads. Consequently, these alloys experience lower stress at temperatures below room temperature and tend to exhibit significant residual displacement. Addressing this issue involves the heat treatment of the superelastic shape memory alloys, transforming martensite into the austenite phase, thus facilitating the restoration of the original shape. Furthermore, in the austenite phase, superelastic behavior is exhibited, allowing the material to return to its original shape without additional heat treatment, even after plastic deformation, at temperatures above room temperature.

Rubber expansion joints in public bridge infrastructure are engineered to withstand continuous, repeated loads, reflecting the behavioral characteristics of the bridge girders. Consequently, not only durability, but also the shape memory effect, are critical aspects for these rubber expansion joints. Various types of shape memory alloys can serve as replacements for rubber expansion joints, with compositions including alloys mixed with copper or iron and nitinol-based shape memory alloys consisting of nickel (Ni) and titanium (Ti). In this study, nitinol-based shape memory alloys were selected, due to their superior shape memory effects and deformation range compared to copper or iron-mixed alloys, as detailed in

Table 5. Characteristics of shape memory alloys.

Process Factors	NiTi	Cu-Based	Fe-Based
Maximum Recoverable Strain	8%	5%	Less than 5%
Cost	High	Low	Low
Shape Memory Effect	High	Moderate	Low
Workability	Moderate	Low	Good
Fabrication	Low	Good	Moderate
Processing	Demanding	Easy	Easy

. The material properties of these nitinol-based shape memory alloys, such as their yield strength and tensile strength, are superior to those of general structural steel, as shown in

Table 6. Comparative characteristics of superelastic shape memory alloys and structural steel.

Type		Nitinol		Structural Steel
		Austenite Phase	Martensite Phase
Physical Properties	Melting point	1240–1310 °C		1500 °C
	Density	6.45 $\mathrm{g}/{\mathrm{c}\mathrm{m}}^3$		7.849 $\mathrm{g}/{\mathrm{c}\mathrm{m}}^3$
	Thermal conductivity	0.28 W/cm°C	0.14 W/cm°C	0.65 W/cm°C
	Coeff. of thermal expansion	11.3 × 10−6/°C	6.6 × 10−6/°C	11.7 × 10−6/°C
Mechanical Properties	Recoverable elongation	up to 8%		0.2%
	Modulus of elasticity	30–83 GPa	21–41 GPa	200 GPa
	Yield strength	195–690 MPa	70–140 MPa	248–517 MPa
	Ultimate tensile strength	895–1900 MPa		448–827 MPa
	Elongation at failure	5–50% (typically ~25%)		~20%
	Poisson ratio	0.33		0.27–0.30
	Hot workability	Quite Good		Good
	Cold workability	Difficult due to rapid work hardening		Good
	Machinability	Difficult, abrasive techniques preferred		Good
	Hardness	30–60 RC		Varies
	Weldability	Quite Good		Very good
Electrical Properties	Resistivity	100 μΩ∙cm	80 μΩ∙cm	13–125 μΩ∙cm
Chemical Properties	Corrosion performance	Excellent (similar to stainless steel)		Fair

, and their elongation rate exhibits a significant difference, being approximately 40-fold higher [8,9,10].

3. Research Method

3.1. Finite Element Analysis

The shape memory alloy used in this study was assumed to be a nitinol-based shape memory alloy comprising 50.7% Ni and 49.3% Ti. To simulate the hyperelastic behavior of the shape memory alloy, the properties were generated using the UMAT function within the Abaqus program [11]. Figure 5 shows the representative parameters required for modeling the properties of the superelastic shape memory alloy with UMAT. The critical material parameters include the initial elastic modulus (E_A), tangential elastic modulus (E_T), forward and reverse phase transition stresses (σ_Fs, σ_Ff, σ_Rs, σ_Rf), and maximum strain (ε_T) [12]. The austenitic Poisson’s ratio and martensitic Poisson’s ratio were fixed at 0.33. For the finite element analysis of the expansion joint devices, the specifications of the devices currently in use on public bridges were examined. Figure 6 demonstrates the determination of the gap distance between girders based on the amount of expansion and the selection process for the height and length of the block out, which ultimately influences the size of the expansion joint device. The representative specifications for these devices are summarized in

Table 7. Standard specifications for the expansion joint devices.

Product	Amount of Expansion (mm)	Spacing Distance (mm)			Bolt
		Min	Mid	Max
1	120	60	120	180	M20
2	160	80	160	240	M20
3	200	100	200	300	M20
4	250	125	250	375	M22
5	300	150	300	450	M22
6	350	175	350	525	M22
7	400	200	400	600	M24

. It should be noted that these standards are adaptable to specific on-site conditions. In this study, the design concept for the rubber expansion joints in the expansion joint devices was developed based on the standard specifications provided in Table 7. A detailed concept of this design is presented in Figure 7.

In the finite element analysis of the superelastic shape memory alloy expansion joints, as illustrated in Figure 7, a uniform thickness of 5 mm was maintained for all joints, irrespective of the expansion amount. To evaluate the behavior characteristics across different bridge gap distances, four distinct models were selected, as detailed in

Table 8. Detailed specifications for the expansion joint devices.

Type	Maximum Allowed (mm)	Distance (mm)
		Minimum	Standard	Maximum
Case-1	120	60	120	180
Case-2	200	100	200	300
Case-3	300	150	300	450
Case-4	400	200	400	600

. The table presents the normal gap distance as the standard measurement, with the minimum and maximum gap distances corresponding to the least and greatest extents of expansion, respectively. This approach enables the determination of the maximum permissible expansion amount for the girders, labeled as “Maximum Allowed”.

Based on the established design concept, modeling was performed using the finite element analysis software Abaqus [13,14,15,16,17]. The expansion joint modeled through Abaqus is shown in Figure 8. During the meshing stage, all water-stop material parts were meshed in a hexagonal form based on the “structured technique”. The hexagonal mesh enhances the speed and stability of the finite element analysis in this study, owing to its stable rectangular parallelepiped shape. The modeled expansion joints were designed to maintain the specified gap distance and to accommodate movements both along and perpendicular to the bridge axis. Expansion joint devices are typically fabricated and installed either as complete units or in segments of 1 m, depending on specific requirements. Consequently, the models employed in this analysis represent expansion joint devices with a length of 1 m. The displacement control in the models was executed based on the aforementioned minimum and maximum gap distances, with each expansion joint device model undergoing displacements of ±60 mm for Case-1, ±100 mm for Case-2, ±150 mm for Case-3, and ±200 mm for Case-4. In case of economic conditions, as shown in Figure 9, bolts were used to restrain it. Moreover, as damage from bolts may occur during actual water-stop material installation, general steel plates were reinforced on both sides of the contact area. At this time, because the water-stop material does not directly, but rather indirectly, affect the deformation of the expansion joint, the general steel plate part on one side was restrained in the y and z directions, and a load was applied in the x direction. Additionally, the general steel plate section to which no load was applied was restrained in all x, y, and z directions.

3.2. Displacement History Curve

Displacement history curves for the finite element analysis of both rubber and superelastic shape memory alloy expansion joints were derived from Internet of Things (IoT) data gathered from actual bridges where these joints are in use. Figure 10 provides an overview of a bridge equipped with IoT sensors. This specific bridge is an RC slab bridge completed in 2000, which has been operational ever since. It spans a total length of 50.0 m and consists of four spans.

In this study, IoT sensors were installed at the abutment sections of the bridge, as direct attachment to the expansion joint devices was not feasible. Figure 11 illustrates the detailed placement of the lateral displacement IoT sensors, with four sensors installed on each side of the bridge abutments. To capture comprehensive displacement data, including the effect of seasonal temperature variations, lateral displacement data were collected over the course of a year. This dataset comprised the average daily displacement values spanning from January to December 2022.

The measured displacements along the axial direction and those perpendicular to the bridge axis are presented in Figure 12 and Figure 13. The analysis of these data indicates that displacements in both directions were less than 4 mm. This limited range of expansion attributed to the bridge’s specific location, which experiences lower public loads, necessitated a tailored approach for the finite element analysis. To facilitate an accurate analysis of potential replacement materials for the existing rubber expansion joints, the maximum gap distance displacement in the axial direction was utilized. However, displacements perpendicular to the bridge axis were not considered in the analysis, in line with findings from previous studies, due to their relatively minor extent [3].

Consequently, the displacement history curves employed in each finite element analysis model were idealized, as depicted in Figure 14. Recognizing that actual bridge axial displacements occur over an extended duration, the displacement history curve for short-term repeated load behavior verification was designed with a maximum displacement duration of 1 s, within a total cycle of 4 s.

4. Results

4.1. Rubber Expansion Joints

The finite element analysis was performed on four models, employing displacement history curves based on IoT sensor data for the repetitive load control of existing rubber expansion joints. The outcomes of this analysis are showcased in Figure 15 and Figure 16. This approach allowed for a thorough evaluation of the behavior of both existing and potential new materials under simulated conditions reflecting actual bridge dynamics.

The analysis of the rubber expansion joints shows that the maximum load borne by the joints decreased as the displacement load increased. Specifically, the maximum loads were approximately 1.57 kN for Case-1, 0.6 kN for Case-2 and Case-3, and 0.44 kN for Case-4, as shown in

Table 9. Member force results of rubber expansion joints.

Type	Maximum Force (kN)	Minimum Force (kN)
Case-1	1.57	−0.14
Case-2	0.57	−0.05
Case-3	0.62	−0.05
Case-4	0.44	−0.03

. These results indicate the joints’ increasing maximum displacement in each case, demonstrating similar load-bearing capabilities regardless of the rubber expansion joint’s tolerance at specific displacement levels. Notably, all models of the rubber expansion joints remained within their elastic range, showcasing their ability to return to their original shape after deformation.

4.2. Superelastic Shape Memory Alloy Expansion Joints

The axial direction analysis of the superelastic shape memory alloy expansion joints, as depicted in Figure 17 and Figure 18, exhibited a pattern akin to that of the rubber expansion joints, with a gradual reduction in maximum load corresponding to increased displacement load. The observed maximum loads were approximately 3052.62 kN for Case-1, 1999.60 kN for Case-2, 1348.90 kN for Case-3, and 967.36 kN for Case-4, as shown in

Table 10. Member force results of shape memory alloys.

Type	Maximum Force (kN)	Minimum Force (kN)
Case-1	3052.62	−172.35
Case-2	1999.60	−182.22
Case-3	1348.90	−106.01
Case-4	967.36	−75.38

. This behavior is attributed to the superior tolerance of superelastic shape memory alloy expansion joints to increased gap distances, despite the increase in the maximum displacement for each case. Moreover, when subjected to minimal displacement, the load values were smaller, as the joints did not bear direct loads. All of the models equipped with superelastic shape memory alloys experienced plastic displacement but demonstrated a flag-shaped recovery to their original form upon load removal. Significantly, none of the four models exhibited residual displacement, returning to their original shape post displacement-load removal. This characteristic highlights the potential effectiveness of superelastic shape memory alloys in bridge expansion joint applications, offering resilience and durability under varying load conditions.

5. Discussion

Rubber seals for expansion joints are recognized worldwide as consumables; therefore, they are replaced when damaged or when damage occurs. Therefore, there are almost no research cases on rubber seal materials for expansion joints. This study explored the utilization of superelastic shape memory alloys as alternative materials to enhance the maintenance efficiency of rubber expansion joints in bridge expansion joint devices. The NiTi-based shape memory alloy used in this study is more difficult to manufacture and process compared to the typical Cu-based and Fe-based shape memory alloys, but it has an excellent shape memory effect and a higher maximum strain capacity. In addition, it can be restored to its original form through separate heat treatments or stress relief at room temperature, allowing continuous use. These findings suggest that, as the displacement tolerance increased, the maximum load in superelastic shape memory alloy expansion joints decreased, primarily due to the widening gap distance in the expansion joint devices. Remarkably, these alloys could revert to their original shape after load removal, even when subjected to plastic displacement beyond their elastic limits, without experiencing any residual displacement. This behavior closely resembled that of rubber expansion joints under similar analytical conditions. The analysis of the maximum load indicated that superelastic shape memory alloy expansion joints can endure significantly higher loads compared to their rubber counterparts, likely attributable to their material properties closely aligning with those of conventional steel types. Despite bearing higher loads, the primary role of expansion joints in bridges is not to directly resist external forces, but rather to ensure material restoration to the original shape and uphold durability. In future studies, based on the research results, in order to confirm the usability of the shape memory alloy, we plan to conduct verification through experimental methods.

6. Conclusions

Current rubber expansion joints are subject to periodic replacement due to natural aging, vehicle loads, and sedimentation, leading to ongoing maintenance costs. Conversely, superelastic shape memory alloys exhibit the same durability as regular steel; therefore, they can be used semi-permanently, without the need for separate maintenance. The analysis of the IoT sensor data shows that bridge girders exhibit repetitive expansion behavior over extended periods, suggesting that superelastic shape memory alloy expansion joints could offer a more flexible performance. Furthermore, considering that most bridges in South Korea are small-to-medium in size, with relatively modest expansion amounts, as evidenced in this study, they are less likely to be subjected to extreme loads. The manufacturing of these superelastic shape memory alloys is more expensive than that of rubber. However, as the service life of the bridge increases, if the rubber stopper material sustains damage, not only does the cost of material replacement rise, but also the expense of labor. These costs can be continuously reduced over time, leading to potential economic benefits in the future. Therefore, optimizing the cross-section for enhanced efficiency could render these superelastic shape memory alloy expansion joints more effective. In conclusion, superelastic shape memory alloy expansion joints are an innovative alternative to traditional rubber expansion joints, with the potential for widespread application in bridge maintenance.