Expansion Joint For Road Bridge

To meet the requirements of bridge deck deformation, expansion joints are usually installed between two beam ends, between the beam end and the abutment, or at the hinge position of the bridge. The expansion joint is required to be able to expand and contract freely in two directions parallel and perpendicular to the bridge axis, be firm and reliable. When vehicles drive over it, the driving should be smooth without sudden jumps or noises. It should also prevent rainwater, garbage, and dirt from infiltrating and blocking. Installation, inspection, maintenance, and removal of dirt should be simple and convenient. At the location where the expansion joint is installed, the railing and the bridge deck pavement should be disconnected.

Expansion Joints

1.1. Function of Bridge Expansion Joints: The function of bridge expansion joints is to adjust the displacement and connection between the upper structures caused by vehicle loads and bridge construction materials. Once the expansion device of an oblique bridge is damaged, it will seriously affect the driving speed, comfort, and safety, and may even cause traffic accidents.

• Ensuring the free expansion and contraction of the beam body;
• Enable vehicles to drive smoothly;
• Having good water tightness and drainage performance;
• Facilitating the removal of dirt in the groove.
• Harms of Missing or Damaged Expansion Joint Functions:

When the expansion is blocked, the pier cap may be damaged, or the internal force of the beam body may increase; vehicles will experience jumping and unstable driving; the impact force will increase, causing damage to the bridge, especially the end part; and water seepage will endanger the durability of the bridge.

2.1, Bridge Expansion Joint Models: The models of bridge expansion joints include GQF - C type, GQF - Z type, GQF - E type, GQF - F type, and GQF - MZL type.

All of them are bridge expansion joint products designed with hot - rolled integrally formed special - shaped steel.

The GQF - C type, GQF - Z type, GQF - E type, and GQF - F type bridge expansion devices are suitable for bridges with an expansion amount of less than 80mm.

The GQF - MZL type bridge expansion device is a modular bridge expansion device composed of side beams, middle beams, cross beams, and a linkage mechanism, suitable for medium - and large - span bridges with an expansion amount of 80mm - 1200mm.

2.2, Code Representation Method:

The code representation method is consistent with the representation method of the communication industry standard of the People's Republic of China. Taking GQF - C60, GQF - F80, GQF - MZL480, GQF - C60(NR), and GQF - F80(CR) as examples,

GQF is the code for expansion joint devices specified in the communication industry standard.

The type code: - MZL represents the modular, straight - beam connecting - rod chain type;

C, Z, F, L represent the shapes of special - shaped steel;

the number represents the displacement amount of the expansion device: 0 - 1200mm;

NR and CR represent the types of rubber: NR represents natural rubber, and CR represents neoprene rubber.

2.3, Product Display:

• 3.1, Seamless Type: Hidden - joint type (continuous bridge deck, TST)
• 3.2, Butt - Joint Type: Filled butt - joint type, embedded butt - joint type (MM - like type)
• 3.3, Steel - Supported Type (Comb - Plate):
• 3.4, Combined Type: Rubber - Plate Type
• 3.5, Modular Type: MM Type

3.1, Seamless Type

The seamless expansion device is a structure where when the joint structure does not protrude from the bridge deck, an elastic material is filled in the expansion gap at the end of the bridge, and a waterproof material is laid. Then, a viscoelastic composite material is paved on the bridge deck pavement layer, making the bridge deck pavement at the expansion joint form a continuous body with the road surface. The deformation of materials such as asphalt concrete and elastomer at the joint is used to absorb the expansion and contraction of the beam body, while providing support for the wheels. Common forms mainly include continuous bridge deck and TST gravel elastomer expansion joints.

1), Main Characteristics of This Type of Expansion Device:

• It can adapt to the expansion and contraction deformation and a small amount of rotational deformation of the upper structure of the bridge;
• It makes the bridge deck pavement form a continuous body, and there is no impact or vibration during driving, providing good driving comfort;
• The expansion device itself forms a multi - layer waterproof structure, with good waterproof performance;
• In cold regions, it is easy to carry out mechanized snow - removal maintenance without damaging the joint;
• The construction is simple and easy, and convenient for maintenance and replacement.
• This type of expansion device is generally a structure formed by cutting the road surface with a cutting machine after the road (bridge deck) construction is completed and injecting a joint - filling material into the groove. It is only suitable for parts with a small expansion amount (generally < 40mm).
• The seamless expansion joint adhesive installed in strict accordance with the process requirements has a service life approximately twice that of the general modified asphalt pavement.

Continuous expansion joints of the bridge deck

TST Gravel Elastomer Expansion Joint:

The TST bonding material usually does not become brittle on the bridge deck at - 40°C, and does not flow at a high temperature of 80°C in summer. It can be used normally throughout the country. Due to the high - temperature adhesion characteristics of TST, it can be firmly bonded to the existing road surface quickly during construction. It is not sticky at room temperature and will not be carried away after cooling. TST is a special high - viscosity elastoplastic material, which is in an elastoplastic state at room temperature. After melting at high temperature, it can be hot - poured into gravel, and after forming, it is like asphalt concrete. It can withstand vehicle loads and has elasticity, and can replace the function of small expansion joints. The construction is convenient and fast. After the pavement cools, traffic can be opened. When the expansion joint needs to be replaced, it can be constructed on one side, without interrupting traffic on busy traffic sections.

TST Seamless Expansion Joint Construction Process:

• Grooving: Setting out the groove width according to the design requirements, cutting the joint, remove the cut - off road surface material, and clean the groove.
• Embedding Rebar: Drive an expansion bolt every 25cm in the transverse direction of the bridge at a distance of 5 cm from the edge of the groove. The height is 1/2 of the groove depth, and weld a φ12 steel bar along the seam direction on the inner side nut of the bolt.
• Filling Sponge Body: Clean the groove with high - pressure water, then heat and dry the surface of the groove with a flame. Fill the gap between adjacent beam ends with a sponge rubber strip, trying to fill it as much as possible without leaving gaps.
• Uniformly brush the special TST adhesive on the exposed surface of the groove, wait for 15 minutes, then pour in the melted TST, and evenly smear it on the bottom and side of the groove with a scraper, with a thickness of 1 - 2mm. Then place the cross - joint steel plate, fix it with positioning nails, and pay attention to centering.
• Start from one end of the groove, put in the pre - heated (130 - 150°C) large stones, with a thickness that can show the bottom TST. Then pour in TST to submerge the stones. Lay layer by layer in this way.
• Spread the pre - heated small stones, 10mm higher than the bridge deck, compact them with a flat - plate vibrator, and then scrape them flat with a scraper. Generally, to prevent settlement, it is 1 - 2mm higher than the bridge deck. At this time, it can be trimmed arbitrarily and patted flat with a shovel.
• Pour in enough TST to submerge the stones. At this time, to prevent TST from flowing onto the two sides of the bridge deck, use wooden boards to block the two sides of the groove to keep the edges neat.
• Trim the edges, remove the baffle plates on both sides, cool for 1 - 2 hours, and open to traffic.

(TST Gravel Elastomer Expansion Joint)

3.2, Butt - Joint Type

1), Filled Butt - Joint Expansion Device:
The filled butt - joint expansion device is an expansion device that uses the elasticity of the expansion body to bear the wheel load. The materials used for the expansion body include sand and gravel, crushed stone, and various shaped rubber products. Foam plastic boards or synthetic resin materials can also be used. The expansion body is always in a compressed state. Common types include U - shaped galvanized iron sheet type, wooden board filling type, asphalt filling type, rectangular rubber strip type, and tubular rubber strip type. The U - shaped galvanized iron sheet expansion device was a widely used filled butt - joint expansion device in the 1970s and 1980s.

Main Characteristics:
Low cost;
the required materials are easy to process;
the construction is simple and easy.

This type is generally suitable for bridges with an expansion amount of less than 40mm. Due to its poor durability and waterproof performance and short service life, it is rarely used at present.

2), Embedded Butt - Joint Expansion Device:
The embedded butt - joint expansion device, also known as the special - shaped steel type or MM - like type expansion device, has a structural principle of embedding rubber products of different shapes with steel components of different shapes, and then anchoring them to the beam body or the abutment back wall at the joint as a whole through the anchoring system. The special - shaped steel provides support for the wheels, and the expansion and contraction of the beam end are absorbed by the tension and compression of the rubber strip or rubber belt. The expansion body can be in a compressed or tensile state. This is a widely used expansion device in domestic highway bridge construction at present. Common types include W type, SW type, M type, and PG type.
Suitability: It is suitable for bridge structures with an expansion amount of less than 80mm, that is, the joint width is 20mm - 80mm.

1), Main Characteristics:

•  Simple structure, clear force - bearing, and low cost;
• The main components of the expansion device are processed and completed by the manufacturer, and installed on - site.
• The connection with the beam end is generally welded through steel bars, with a reliable structure and easy - to - guarantee construction quality;
• Good durability;
• Good waterproof and drainage performance;
• Good driving comfort.

2), Two Design Principles of MM Expansion Joint: "Rigid Anchoring" and "Sealed Waterproofing".
     Rigid anchoring:
The quality of the expansion joint anchoring directly affects the service life of the expansion joint. The anchoring metal plate mainly plays the role of force transfer. The anchoring device that has passed the fatigue test is directly welded to the side beam. At the same time, the side beam is rigidly connected to the upper structure of the bridge to ensure that the expansion joint can bear the maximum traffic load. In the case of long - term bearing of dynamic traffic loads, the method of connecting other expansion joints to the upper structure of the bridge with screws or bolts is not feasible. The MM expansion joint has taken a leading - edge design in this regard, separating the two functions of bearing and waterproofing and dealing with them separately, which is more conducive to strengthening and improving the two functions.
       Thorough waterproofing:
One of the characteristics of the MM expansion joint is that the neoprene rubber sealing strip is effectively embedded in the groove of the side beam, which can ensure thorough waterproofing. At the same time, it can be replaced on the bridge deck with simple tools or repaired by vulcanization. Protected by the side beam, the sealing strip is not directly rolled by the wheels, and its "V" - shaped structure can play a role in self - cleaning of sediment. The sealing strip can resist tensile force and can also perform lateral and vertical displacements. In contrast, the water leakage of the expansion joint will cause certain damage to the bridge structure.

3.3, Steel - Supported Type
The steel - supported expansion device is assembled with steel and can directly bear the wheel load. This type of expansion device was mostly used in steel bridges before and is now also used in concrete bridges. There are various types, current situations, and sizes of steel - supported expansion devices. The widely used one is mainly the steel comb - type. The structure of the steel comb - type bridge expansion device is composed of comb - shaped plates, connecting parts, and an anchoring system. In some steel comb - type bridge expansion devices, synthetic rubber is filled between the comb teeth to play a waterproof role, and special drainage grooves are also used to solve the drainage problem. The steel comb - type bridge expansion device is also a steel plate finger - shaped joint. According to the support conditions of the comb teeth, it can be divided into supported type and cantilever type.

Main Characteristics:

• All components are processed and assembled with steel, with high structural strength;
• It can provide continuous support for the wheels, with good driving comfort;
• The connection with the beam body uses pre - embedded steel components, with reliable connection;
• Strong impact and vibration resistance, good durability;
• It can adapt to large - scale horizontal displacement and can be used for large - scale bridges.

Suitability:

It is suitable for bridges with an expansion amount greater than 40mm. However, due to its high cost, its application range is not very wide.

3.4, Rubber - Plate Type
The rubber - plate expansion device makes full use of the low shear modulus characteristic of rubber materials. A load - bearing steel plate and an anchoring steel plate are set in the rubber body, and bolt holes are set. It is connected to the beam end as a whole through bolts. This structure relies on the shear deformation of the rubber body between the upper and lower grooves to absorb the expansion and contraction displacement of the beam. The steel plate embedded in the rubber body spans the beam - end gap and bears the wheel load. This device was applied earlier in China. There are many manufacturers in the country, and it has different names. It was mainly applied in the 1980s and 1990s. The rubber - plate expansion device has the advantages of simple structure, convenient installation, and economic applicability. It is mainly suitable for secondary and below - grade highway bridges with an expansion amount of 30mm - 60mm and is widely used in China.

Performance Characteristics:
It relies on the shear deformation of the rubber body between the upper and lower steel plates to meet the deformation requirements of the structure. After the device is deformed, there is a certain amount of deformation energy stored in the rubber body, which will have a certain restraining force on the structure; the load - bearing cross - joint steel plate is embedded in the rubber body. Compared with the steel - structure expansion device, it has a certain buffering effect on the impact force of the wheels, effectively protecting the expansion device and the beam body and improving the driving conditions; the angle steel of the expansion device effectively strengthens the strength of the beam - end. The horizontal deformation internal force of the expansion body of the rubber - plate expansion device is relatively large, generally about 30 - 35N/m. The greater the deformation, the greater the horizontal force, and the greater the possibility of overall damage to the device.

Selection Consideration:
Therefore, when selecting a rubber - plate expansion device, it is necessary to consider factors such as installation errors and temperature errors. The selected deformation allowance should not be less than 30mm to ensure the normal use of this type of device.

3.5, Modular Type
The modular bridge expansion device is an expansion device composed of longitudinal beams (special - shaped steel), cross beams, displacement control boxes, rubber sealing strips, and other components. A rubber sealing strip (belt) with a V - shaped cross - section or other cross - sectional shapes is embedded in the side beams and middle beams of the special - shaped steel to form a telescopic seal. The special - shaped steel directly bears the wheel load and transmits the load to the cross beam, and then the cross beam transmits it to the beam body and the abutment. The displacement control box ensures that the gap between the special - shaped steels remains uniform when the expansion device absorbs the beam - end deformation. The rubber sealing strip prevents debris from entering and provides waterproofing. The modular expansion device can increase the number of middle - beam steels and seals according to the actual expansion amount requirements, and can form an expansion device that meets large - displacement requirements. It is generally used for bridges with an expansion amount greater than 80mm. From a single - joint of 80mm to a multi - joint of 1200mm, there are 15 levels in total.
1) Main Characteristics:
The entire expansion device is composed of various components such as special - shaped steel longitudinal beams, steel cross beams, control transmission mechanisms, displacement boxes, and sealing rubber strips, with a relatively complex structure;
Good sealing performance, good waterproof and drainage performance;
It can be suitable for bridges with large expansion amount requirements;
High overall structural stiffness, good durability;
Good driving comfort.
2), Limitations:
However, due to its complex structure, maintenance and replacement require professional technicians from the manufacturer. Coupled with its high cost, it is generally only used for high - grade large - scale bridges.

(MM modular bridge expansion device)

The expansion amount of the beam body is the primary basis for selecting expansion joints.

Influential Factors on the Expansion Amount of Expansion Devices
4.1 Temperature Changes
Temperature changes are the main factor affecting the expansion amount of bridges. They can be divided into linear temperature changes and non - linear temperature changes, with linear temperature changes playing a dominant role in influencing the bridge expansion amount. In a specific external temperature environment, the temperature distribution inside the bridge structure is uneven. The end of the beam body undergoes angular displacement due to changes in the thermal properties of the material. For small - span bridges (L\leq8m), the linear expansion coefficient is very small and can be ignored. For large - span bridges, sufficient attention must be paid during the design process. Generally, the linear expansion coefficient can be referred to in the following table.

Bridge Type Linear Expansion Coefficient Temperature Change Range

(Temperature variation range and linear expansion coefficient)

4.2 Shrinkage and Creep of Concrete
The shrinkage and creep of concrete are inherent properties of concrete components and are also random phenomena. The mix proportion, water - cement ratio, slump, cement type, temperature, relative humidity, loading age of concrete, load - bearing time, and strength of concrete have a significant impact on its shrinkage and creep. Both reinforced concrete bridges and prestressed concrete bridges need to consider shrinkage and creep. The creep amount is calculated by multiplying the elastic deformation of the beam under prestress by the creep coefficient ф=2. The shrinkage amount is converted based on a temperature drop of 20℃. When installing the expansion joint, the shrinkage and creep have already developed to a certain extent. During the calculation, the installation time should be used as the reference, and the shrinkage and creep amounts of concrete should be reduced. The reduction coefficient β can be selected with reference to the following table.

(Shrinkage and creep reduction coefficients)

4.3 Longitudinal Slope of the Bridge
In bridges with a longitudinal slope, movable bearings are usually made horizontal. When the bearing displaces, the expansion joint not only undergoes horizontal displacement but also vertical misalignment (Δd), and its value is equal to the horizontal displacement value multiplied by the tangent of the longitudinal slope tgθ.

4.4 Displacements of Skew Bridges and Curved Bridges
When skew bridges and curved bridges undergo displacement (ΔL) in the direction of bearing displacement, displacements also occur along and perpendicular to the bridge end line, that is:
Δd=ΔL·SINα;
ΔS=ΔL·COSα;
where α is the inclination angle and ΔL is the expansion amount.

4.5 Deflections of Bridges Caused by Various Loads
Under the action of live loads and dead loads, the ends of the bridge undergo angular displacement, causing vertical, horizontal, and angular displacements of the expansion device. If the beam body is relatively high, vibration will also occur.

4.6 Earthquakes
The impact of earthquakes on the displacement of expansion devices is relatively complex and difficult to grasp at present. Generally, it is not considered in the design. However, if reliable data are available and the settlement, rotation, horizontal movement, and inclination of bridge piers and abutments caused by earthquakes can be calculated, it should be considered in the design.

Calculation of Beam Expansion and Contraction
▲Expansion and contraction caused by temperature changes
Calculation formula:
∆Lt=Tmax-Tmin)×α×L
∆Lt+=Tmax-Tset)×α×L
∆Lt-=Tset-Tmin)×α×L

Where: ∆Lt --- Expansion and contraction due to temperature changes;
∆Lt+ --- Elongation due to temperature changes;
∆Lt- --- Shortening due to temperature changes;
Tmax --- Design maximum temperature;
Tmin --- Design minimum temperature;
Tset --- Installation temperature;
α --- Coefficient of linear expansion;
L --- Length of the expansion beam.

▲ ▲Expansion and contraction caused by concrete creep and shrinkage
Creep-induced expansion and contraction formula:
∆Lc = ( α_p/Ec)×Φ×β×L;
Shrinkage-induced expansion and contraction formula:
∆Ls = 20×〖10〗^(-5)×β×L;
Where: ∆ Lc --- Expansion and contraction of concrete creep;
∆Ls --- Expansion and contraction caused by concrete shrinkage;
α_p--- Average axial stress of prestressed concrete;
Ec --- Elastic modulus of concrete;
Φ--- Creep coefficient of concrete;
β --- Reduction coefficient of concrete shrinkage and creep;
L --- Length of the expansion beam.

▲▲▲Example:
A prestressed concrete beam bridge has a beam length of 40m.
The temperature change range is -4°C to 42°C.
The coefficient of linear expansionα = 〖10〗^(-6) ; shrinkage strain ε = 20×〖10〗^(-5).
The creep coefficient φ= 2.0; the shrinkage and creep reduction coefficient β = 0.6.
The average axial stress of prestressed concrete α_p = 80kg/cm^2.
The elastic modulus of concrete Ec = 3.4×105kg/cm^2;
The installation temperature is 20°C.
▼(1). Temperature Changes
∆Lt=Tmax-Tmin)×α×L
=46×(10×〖10〗^(-6))×40000
=18.4mm

∆Lt+=Tmax-Tset)×α×L
=22×(10×〖10〗^(-6))×40000
=8.8mm
∆Lt-=Tset-Tmin)×α×L
=24×(10×〖10〗^(-6))×40000
=9.6mm
▼(2), Creep
∆Lc = ( α_p/Ec)×Φ×β×L;
=(80⁄340000)×2×0.6×40000
=11.3mm
▼(3). Shrinkage
∆Ls = 20×〖10〗^(-5)×β×L
=2×〖10〗^(-6-5)×40000×0.6
=4.8mm
Therefore, the expansion and contraction ∆L = 18.4 + 11.3 + 4.8 = 34.5mm.
The beam elongation is 8.8mm.
The beam shortening =9.6 + 4.8 + 11.3 = 25.7mm, which can be regarded as an initial compression of 25.7mm.
It should be noted that when selecting an expansion device, a certain safety margin (about 30%) is generally required to be considered for the expansion and contraction to ensure the service effect and durability of the expansion device. In this example, the initial compression can be taken as 34mm.

5.1 Failure Modes and Cause Analysis
Under non - overloading conditions, the recommended fatigue life of expansion devices is 10 - 15 years.

• For the filled - butt - type expansion device, when the angle steel falls off, the concrete on both sides is broken, the concrete on the abutment side is completely broken, the rubber strip is broken, or there are deep potholes, its service life can be judged to have ended.
• For the seamless - type expansion device, when there is obvious vehicle jumping, partial cracking of the concrete on both sides, serious fragmentation, or wrinkling, its service life can be judged to have ended.
• For the embedded - butt - type expansion device, when there is obvious vehicle jumping and serious damage to the bridge deck pavement, its service life can be judged to have ended.
• For the plate - type rubber expansion device, when the anchor bolts fall off, the rubber ages and deforms, and the concrete cracks, its service life can be judged to have ended.

5.1.1 Seamless - type
The main failure modes of seamless expansion joints include: obvious ruts and cracks on the surface of the elastomer, wavy or local detachment of the elastomer surface, local or large - scale peeling of aggregates; or cracks at the joint with the bridge deck pavement, which gradually break and fall off; or damage to the bridge deck pavement within the range of the expansion device.
Cause Analysis of Damage: Problems with the material properties of the elastomer filler itself, such as insufficient ability of the elastomer material to absorb the deformation of the beam end, insufficient material strength, and the quality of the binder not meeting the actual use requirements, as well as non - compliance with the manufacturer's construction requirements during construction; displacement and rotation of the bridge caused by external factors such as temperature and load, resulting in cracking and damage of the elastomer; and the structure of the expansion device itself, such as insufficient strength of the cross - joint plate.
5.1.2 Butt - type
The main failure modes include: the rubber strip bulges in hot weather, falls off in cold weather, and has local perforations and water leakage; cracks and fragmentation in the anchorage area concrete; and fragmentation and peeling of the bridge deck pavement.
Cause Analysis of Damage: It is difficult to install the rubber strip in an ideal state; the connection between the main anchor parts and the embedded parts of the beam body is weak. In addition, the paving concrete is thin, and the post - cast concrete surface layer lacks vibration during construction, resulting in problems with density and strength, making the concrete on both sides prone to damage; the connection strength between the anchorage area concrete and the bridge deck pavement is insufficient, and small cracks develop into local fragmentation and peeling.
5.1.3 Steel - supported Type
The main failure modes of this type of expansion device include: welding seams opening, with some welds difficult to weld firmly due to technological problems, resulting in the entire steel plate falling off, and weak anchor parts causing looseness; and fatigue fracture of individual steel toothed plates.
Cause Analysis of Damage: This type of expansion device is prone to deformation during processing and use, making it difficult to ensure the fit between the toothed plate and the cushion plate. Once a gap is generated, it is unfavorable for the stress of the connection part, causing noise and vehicle jumping. In addition, due to round - the - clock operation, the toothed plate undergoes repeated loading, resulting in premature fatigue, loosening of the fastening bolts, and the rotation and lift up of the comb - shaped plate, making it exposed.
5.1.4 Rubber - plate Type
The main failure modes of this type of expansion device include: the rubber plate peeling off, the embedded steel plate being exposed, falling off, or breaking, the anchor bolts being sheared off and flying out of the holes, the concrete on both sides cracking and fragmenting, and the appearance of potholes and other damage phenomena.
Cause Analysis of Damage: First, it is due to the structure itself (design reasons). The principle of this type of expansion device is to use the shear deformation of the rubber between the upper and lower grooves to meet the expansion and contraction of the beam body. There are steel plates embedded in the expansion body, which span the beam end gap and bear the load. There are anchor steel plates on both sides, which are connected to the beam end through bolts, and are installed in sections per meter, resulting in poor integrity. Also, due to the large horizontal friction force of this type of expansion device, the requirements for the anchoring system are extremely high. Second, the product quality is poor. For example, the performance of the rubber material, the material and reasonable layout of the stiffening steel plates, the bonding strength between the steel plates and the rubber, and the control of temperature and humidity during production all have very strict requirements. Slight quality problems often lead to phenomena such as the entire plate breaking, rubber peeling, rubber layer wear, steel plate exposure, and the anchor bolts shearing off and the rubber plate flying out, which are directly related to the quality of the rubber expansion device itself, its large transverse width, and large stiffness difference.
5.1.5 Modular Type
The main failure modes of this type of expansion device include: welding seams of the main middle beam components opening, resulting in shaking and noise; poor expansion and contraction uniformity; aging, falling off, or jumping out of the sealing rubber strip, with serious water leakage; cracks and potholes in the concrete on both sides of the device, local fragmentation of the bridge deck pavement, and unsatisfactory anchoring systems, resulting in local or overall damage.
Cause Analysis of Damage: First, the side beams and middle beams of this type of expansion device used in China are mostly composite structures formed by welding steel plates or sections into special - shaped parts. The welding quality is difficult to guarantee. In addition, the method of using pressure strips (or clips) and screws to fasten the sealing rubber strip makes the fasteners prone to rust and breakage, resulting in poor structural integrity, a large amount of welding work, and difficult - to - guarantee welding quality due to unqualified welding processes, leading to welding seam opening or the rubber strip falling off or even jumping out. Second, in the pre - reserved slot for installing this type of expansion device, there are both anchor boxes and a large number of anchor reinforcement bars, including the main reinforcement bars in the beam body and the pre - embedded anchor reinforcement bars, which makes it difficult to pour concrete, and problems such as cavities, difficult - to - guarantee compactness, and insufficient strength are likely to occur. During use, phenomena such as bite - marks, cracks, and local potholes will appear. If not dealt with in a timely manner, serious problems of overall damage to the anchorage part will occur.
5.2 Main Diseases
5.2.1 Analysis of the Disease of Narrow Expansion Joints
The width of the expansion joint during construction and installation is not appropriate, resulting in insufficient reserved compression amount, the expansion joint being squeezed shut, increased internal stress, damage to the concrete of the expansion joint body, and the appearance of potholes and other pavement damages.

The width of the expansion joint has an abnormal change compared with the normal joint width reserved during design.
5.2.2 Analysis of the Disease of Expansion Joint Height Difference
Due to reasons such as abutment settlement, installation errors, and fragmentation of bearing pad stones, one side of the bridge is lower than the road surface side, resulting in vehicle jumping at the bridge head. Inspection shows that the abutment settlement has not caused serious damage to the lower structure. At the same time, the two diseases of vehicle jumping at the bridge head and damage to the expansion joint are interrelated. The large impact load caused by vehicle jumping at the bridge head directly acts near the expansion joint, causing damage to the expansion joint.

5.2.3 Analysis of the Disease of Expansion Joint Blockage
Due to the accumulation of sand, stones, and other debris, the expansion joint is prone to losing its free expansion and contraction ability. When the temperature rises in summer, if the main beam cannot expand freely, thrust is likely to be generated between adjacent main beams or between the main beam and the abutment. In severe cases, the main beam may even be jacked up or the abutment back wall may crack.

5.2.4 Analysis of the Disease of Damaged Expansion Joint Rubber Strips
In addition to aging, the above three diseases of expansion joints are extremely likely to cause cracking, damage, and warping of the rubber strips in the expansion joint.

5.2.5 Analysis of the Disease of Damaged Anchorage Areas
During construction, the strength of the post - cast strip concrete in the anchorage area is insufficient, or the maintenance is not in place. Or there is a height difference with the bridge deck, resulting in vehicle jumping. Coupled with the frequent action of overloaded vehicles, damage occurs, which is likely to cause damage to the steel structure part of the expansion joint.

5.2.6, Water Seepage in Expansion Joints
This is a secondary disease caused by damaged rubber strips or broken anchorage areas. The harm caused by water erosion due to seepage is extremely great.
★Direct Hazards:
Seepage acts on the following parts, causing corresponding hazards.

• The rubber of piers (abutments) and bearings ages and cracks, and the steel plates rust.
• The concrete of piers (abutments) and solid slab girders is eroded, resulting in pitted surfaces, and the steel bars expand due to rust.
• Water accumulates in the cavity of hollow slab girders.
• The ends of steel - structure girders rust.

★Indirect Hazards:
Water erosion can spread to the bridge decks, girders, and hinge joints, damaging the upper load - bearing components. If the bridge deck pavement is permeable, the following diseases will be aggravated.

• Water leaks from hinge joints, and in severe cases, the hinge joints fall off.
• Cracks appear in the webs of hollow slabs.
• The bridge exhibits single - slab loading (this disease is more serious in small and medium - sized hollow slab beam bridges).

★Examples:
At around 7 am on June 10, 2004, the Tianzhuangtai Bridge in Panjin City, Liaoning Province, suddenly collapsed. The bridge broke in the middle by 27 meters, with three cars falling into the water. The two drivers and passengers of the agricultural vehicle managed to escape, and fortunately, there were no deaths. The cause of the accident was overloading.

Long - term water seepage at the expansion joint of the cantilever beam end of the bridge deck led to a decline in the durability of the corbels. When heavy vehicles passed, the corbels suddenly broke, causing the hanging beams to fall off.

Water Seepage in Expansion Joints
Water seepage in the expansion joint, water marks on the hollow beam, drilling holes for water drainage

Waterproofing of expansion joints of steel - structure bridges is particularly important.

5.2.7, Other Types of Diseases

(Severe concrete fragmentation Failure of the continuous expansion joint of the bridge deck.

Loose nuts of the rubber expansion joint

5.3, ● Maintenance of Expansion Joints ●

(Cross - section after the collapse of the Mingyangtan Bridge in Harbin)

Completely out of control

The expansion joints are completely replaced by asphalt concrete

• Clean the expansion joint.
• Repair or replace the rubber strip.
• Repair the anchorage area.
• Replace the entire expansion joint.

Cleaning the expansion joint is the most important part of daily maintenance, but it is often verlooked.

Note: This figure describes the consequences of a single phenomenon of expansion joint blockage. The arrows represent trends and possibilities, and are not the only or necessary conditions for causing diseases.)

The expansion joint should generally be cleaned once a month. For road sections that are easily contaminated, the cleaning frequency needs to be increased.

When cleaning, sharp tools should not be used to prevent damage to the rubber strip. Equipment such as high - pressure water guns and high - strength blowers can be used.

Once the rubber strip is damaged, it must be repaired or replaced.

For small local cracks and damages, epoxy resin can be used for bonding. If the damage is severe or the rubber strip is seriously aged, it needs to be replaced. When replacing, use a crowbar similar to that for changing tires to pull out the old rubber strip, and then install the new rubber strip in the same way.

If there are cracks or damages in the anchorage area, they must be repaired immediately.
For cracks in the anchorage area, epoxy resin can be used for grouting. For wider cracks, epoxy mortar can be used for repair. If the damage is severe, chisel away the damaged part to expose the steel bars and steel components, remove rust, and then repair by pouring steel - fiber concrete or rapid - setting concrete.

6.1, Process Flow

Measurement → Marking → Cutting the Joint → Removing Concrete and Debris → Installing the Foam Board between Beam Joints → Lifting and Positioning the Expansion Joint → Adjusting the Plane Position of the Expansion Joint → Adjusting the Elevation of the Expansion Joint → Anchoring → Releasing the Lock → Providing Protection → Pouring Concrete → Finishing and Curing → Opening to Traffic

Examples:
MM expansion joint as an example:

1), Cutting and Grooving:
The installation of the expansion joint device should preferably be carried out after the road surface is paved, and the groove size should meet the requirements for installing the expansion joint device.
2), Cleaning the Groove:
All dirt, dust, and other unnecessary substances must be completely removed.
3), Checking the Gap:
Check whether the gap between each beam of the expansion joint device meets the requirements of the installation temperature. If not, adjustments must be made under the guidance of the engineering and technical personnel of the manufacturing factory to ensure that the gap between each beam of the expansion joint device meets the design requirements. After adjustment, install the fixture in preparation for installation.
4), Positioning and Leveling:
Taking the asphalt pavements on both sides as the elevation reference, place the expansion joint device in the groove, and adjust the expansion joint device so that its top surface is at the same elevation as the road surface, and its longitudinal and transverse slopes are consistent with those of the bridge road surface.
5), Checking the Position:
Check the position of the expansion joint device to ensure that its position in the direction perpendicular to the joint and along the joint meets the design requirements. If individual pre - embedded steel bars at this time interfere with the correct installation of the expansion joint device, they can be cut off by gas cutting.
6) Welding the Anchoring Steel Bars:
First, connect and weld the anchoring steel bars on one side of the expansion joint device to the pre - embedded steel bars in the reserved groove. When welding, weld one by one at intervals, and then weld the anchoring steel bars on the other side in the same way. After confirming that the expansion joint device is firmly fixed, the fixture can be removed, and then weld all the remaining un - welded anchoring steel bars to the pre - embedded steel bars to ensure reliable anchoring of the expansion joint device.

7) Welding of Segmented Expansion Joints:
If the expansion joint device is installed in sections, the joints must be welded. The welded joints of the steel sections are pre - fabricated in the manufacturing factory. When two adjacent joints are aligned, installation can be carried out. After all the beams are welded, proceed with the anchoring steps as described above.
8) Installing the Formwork:
Install the formwork at the beam end. The formwork is fabricated according to the external dimensions of the expansion joint device and the notch of the reserved groove. The formwork should be made very tight to prevent mortar from flowing into the displacement control box or into the beam - end gap.
9) Pouring Concrete:
After checking that the installed formwork is tightly sealed, clean the reserved groove, and then pour concrete (using steel - fiber concrete) and vibrate it compactly. The concrete should have at least the same strength as the structural concrete at that location. When pouring concrete, keep the top surface of the expansion joint device clean.

6.2, Allowable Deviations for Expansion Joint Installation

Precautions:

• Construction temperature and the pre - adjusted gap width at the factory.
• Protect the structural joint by filling it with foam board. The bottom of the V - shaped rubber strip should also be blocked with polyethylene foam board to prevent mortar leakage.
• Control the top surface flatness.

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