Retrofitting of Existing Bridge Using Externally Bonded FRP Composite–Applications and Challenges

The increase in deterioration of bridge infrastructure is a large-scale national problem. Maintaining the existing bridge infrastructure network and adapting it to new capacity requirements has become one of the most challenging tasks for today’s engineers. Bridges designed and built only a few years ago are now subjected to traffic loads well above the design ones. At the same time, it has become evident that the durability of bridges is not always guaranteed, even for relatively recent constructions. For bridges in urban areas, the space concerns are predominant and building a new bridge alongside an existing one is not a viable option. It is therefore necessary to intervene on the structure with most feasible, economical and efficient methods to upgrade, repair or strengthen the existing bridges while preserving, at least partially, its traffic bearing capacity. These challenges are greater than the ones required to design and build a new bridge. Fiber Reinforced Polymer (FRP) composites represent a new and promising solution to the shortcomings of several traditional materials and upgrading techniques and has a great potential to integrate into the bridge infrastructure. In the recent years several, researchers have investigated the performance of external bonded FRP composites and found to be a successful effective technique for upgrading structural element. This paper highlights the applications of external bonded FRP composites overlay for bridge infrastructure, the challenges involved and path to implementation.

Introduction and Background

The bridge infrastructure has been deteriorating for many years, a result of sometimes like harsh environmental conditions, heavy loads, insufficient maintenance, and, frequently, unintentionally damaging maintenance practices and it has be come a large-scale national problem. It has been widely reported that approximately 40% of India’s bridges are either structurally deficient or functionally obsolete. Structural “deficiency” does not imply that a bridge is unsafe or likely to collapse. With proper weight restrictions and enforcement, most deficient bridges can continue serving traffic safely when limited to posted maximum loads. The main reasons for classifying a bridge as structurally deficient are low load ratings (or weight restrictions), deteriorated decks or deteriorated substructures. Maintaining the existing infrastructure network and adapting it to new capacity requirements has become one of the most challenging tasks for today’s structural engineers. Bridges designed and built only a few tens of years ago are now have to carry traffic loads well above the design ones. At the same time, it has become evident that the durability of bridges is not always guaranteed, even for relatively recent constructions. In this context, the engineer is often asked to design refurbishments, enlargements and repairs on structures that are under heavy usage. This presents challenges that are even greater than the ones required to design and build a new bridge. For bridges in urban areas the space concern are predominant and building a new bridge alongside an existing one is not a viable option. It is therefore necessary to intervene on the structure while preserving, at least partially, its traffic bearing capacity. Due to budget constraints, many authorities are forced not to proceed with strengthening but to post load restrictions on their bridges as a temporary measure. A significant number of bridges all over the world need rehabilitation and strengthening.

The economic impact of bridge replacement is represented by not only the direct costs associated with demolition and construction of a new bridge, but also by the indirect costs associated with the loss of roadway use and traffic disruption. The latter are often difficult to quantify and foresee. In addition, high traffic volumes, tight construction budgets, and challenging roadway construction areas have put a strain on the ability of conventional materials to meet the public need for rapid construction, long-lasting structural components, lightweight and easy constructed facilities. This problem has created an urgent need for effective means of structural retrofit, repair and rehabilitation of bridge infrastructure as an alternative to bridge replacement. Casting additional elements, increasing cross-section size, and bonding steel plates are techniques that have been used in the past when widening or strengthening an existing bridge. These solutions can be expensive and difficult to implement, especially for working constraints (low-vertical clearance, head room,) scheduling constraints, economical constraints (minimum labor cost, minimum shut-down costs, material costs, etc) and long-term performance with strength and durability.

In the recent past, steel plate bonding was considered as an effective method for upgrading structural members. It began in South Africa and France, where steel plates bonded with epoxy resins were used for strengthening of concrete members [5]. This method originates from the strengthening of steel beams by means of adding steel plates. But, bridges that had been strengthened using steel plates were showing signs of corrosion after few years in service that resulted in the deterioration of the original structure. In Addition to the above, steel plate bonding included high installation costs, maximum labor, low on-site flexibility of use, aesthetically poor due to large changes in member size after repair, increased dead load and traffic interruption and this called for engineers to investigate for the alternatives. Considering all the above factors Fiber Reinforced Polymer (FRP) composites represent a new and promising solution to the shortcomings of several traditional materials and this shows a great potential for integration into the bridge infrastructure. In the past years, experiments have been conducted to investigate the applicability of FRP composite in bridge structures, including the applications of FRP composite to bridge deck, column and beam strengthening, etc. [1-4]. The performance of external bonded FRP composite have found to be a successful technique and an effective method for upgrading structural element. This paper highlights the applications of external bonded FRP composites overlay in bridge infrastructure, the challenges involved and path to implementation.

Fiber Reinforced Polymer (FRP)

FRP composite materials, which have been used for some time in the aerospace and military communities, is at present as a potential solution to some of the highway community’s infrastructure. During the past decades, a significant amount of exploratory and basic research has been conducted all over the world on the use of FRP composite materials for civil infrastructure applications.

Typically, FRP materials has superior properties with respect to strength, weight, durability, creep, fatigue, light in weight and easy to construct, FRP provide excellent strength-to weight characteristics; can be fabricated for “made-to-order” strength, stiffness and geometry. The bridge can be strengthened without reduction of vertical clearance, and FRP can be applied in severe exposure environments that may have resulted in the deterioration of the original structure, corrosion resistant, low axial coefficient of thermal expansion and relatively low cost of maintenance. FRP composite may be used to extend the life of bridge infrastructure because their low dead weight allows for an increase in live-load carrying capacity.

FRP composites are anisotropic and characterized by excellent tensile strength in the direction of the fibers. They do not exhibit yielding, but instead are elastic up to failure. Figure 1 demonstrates some typical response of uni-axially loaded fiber materials including High Modulus (HM) steel (modulus of elasticity) and High Strength (HS) steel. Fibers have a linear elastic behavior until failure which is brittle. Table 1 compares the material properties of carbon fiber, concrete and steel. FRP composites exhibit several properties that make them suitable for use with several traditional material structures.

The current commercially available FRP reinforcements are made of continuous fibers of Aramid (AFRP), Carbon (CFRP), or Glass (GFRP) impregnated in a resin matrix each of which has its own advantages and disadvantages. FRP composites can be produced by different manufacturing methods in many shapes and forms (Figure 2).

The Key features and benefits of FRP composite are tabulated in Table 2 and the qualitative comparison of the performance of carbon, glass, and aramid composites is presented in Table 3 [17]. Commonly used FRPs for concrete reinforcement are bars, prestressing tendons, pre-cured laminates/shells and fiber sheets. Bars/rods have various types of deformation systems, including externally wound fibers, sand coatings, and separately formed deformations, these types are commonly used for internal concrete reinforcement. FRP procured as laminates/shells and sheets are commonly used for external concrete reinforcement. Based on the results of the literature search, the various bridge-related applications of FRP can be divided into five Major FRP Composite Application categories:

1. Repair and retrofitting (laminate applications),
2. FRP composite reinforcement (rebar and tendons),
3. Seismic retrofitting,
4. FRP composite bridge decks and superstructures, and
5. Unique applications.

Among these, the most predominant application for retrofit involves the externally bonded FRP composite (laminates/shells and sheets) that have been used to replace traditional method such as bonded steel plates, jacketing, etc for providing additional tensile strength and stiffness for existing structural element.

Retrofitting of existing Structures by externally bonded FRP overlay

Externally bonded FRPs have been used to retrofit structural members such as columns, slabs, beams, and girders in structures such as bridges, parking decks, smoke stacks, and buildings [30] and have proved to be an effective way of improving strength and stiffness of existing structural elements. A large number of projects, both public and private, have used this technology and escalating deployment is expected, especially in seismically active regions [10]. The application of FRP as external reinforcement to concrete infrastructure has been studied by many groups. This technology is fairly mature; extensive research on FRP exists on bond performance, creep effects, ductility of the repairs, fatigue performance, force transfer, peel stresses, resistance to fire, and ultimate strength. Today, there are numerous manufacturers of FRP composite systems for repair and retrofitting. Guidelines for the design and application of these materials for flexural retrofit of concrete elements are available from the manufacturers. The American Concrete Institute (ACI) has incorporated the Design and Construction of Externally Bonded FRP Systems for Strengthening Concrete Structures in its 2002 Code. The new Canadian Highway Bridge Design Code (2001) also provides some detail on FRP composite. These manual can serve as a model for FRP composite specifications and a comprehensive guide to address various design issues. Figure 3 illustrates fully and partially immersed five varieties of configurations of flexural strengthening with an externally bonded; round and square bars, and strips (Laminate or/and Sheet). The implementation of these techniques with various available FRP composites such as laminates/shells and sheets for strengthening existing reinforced concrete infrastructure have been demonstrated around the world in the recent years.

Three methods are used for application of external FRP reinforcement-adhesive bonding, hand lay-up or wet lay-up, and resin infusion. Summary of these methods are shown in Table 4. The important functions of matrix material in FRP composite include: i) bind the fibers together and transfer the load to the fibers by adhesion and/or friction; ii) provide rigidity and shape to the structural member; iii) isolate the fibers so that they can act separately, resulting in slow or no crack propagation; iv) provide protection to the fibers against chemical and mechanical damages; v) influence performance characteristics such as ductility, impact strength; and vi) provide finish color and surface finish for connections. To perform these desirable functions, the fibers in FRP composite must have: i) high modulus of elasticity for use as reinforcement; ii) high ultimate strength; iii) low variation of strength among fibers; iv) high stability of their strength during handling; and v) high uniformity of diameter and surface dimension among fibers.

Another area that has received considerable attention in the recent years is that of seismic retrofitting of concrete bridges using FRP composites. Axial strength and ductility increase of concrete columns, piers is needed whenever repair and strengthening are involved. Repair may be required when columns are damaged under excessive external loads or due to erosion in exposed environments. Lateral confinement has been known to add both strength and ductility in the axial direction for concrete columns and this idea was originally developed back in the 1920s’ [25]. Lateral confinements for concrete columns can be in various forms. They appear chronologically as (i) spiral and circular reinforcements; (ii) concrete jacketing; (iii) steel jacketing; and (iv) FRP composite jacketing, wrapping. Steel has been a conventional and widely used construction material. However, corrosion is one of the largest drawbacks of such material. Weight can be another problem because the construction costs can surge when the installation is labor intensive. Concrete jacketing, though has a lower cost, but simply adds weight and cross sectional area to the original structure and may be undesirable. On the contrary, FRP composite jacketing systems have emerged as an alternative to traditional construction, strengthening, and repair of reinforced concrete columns and bridge piers [30]. The primary FRP application-column FRP wrapping is commonly used in seismic retrofits even under worst site conditions such as for repairing corrosion damage in chloride contaminated concrete in high seismic zone. These procedures, which can be used in place of steel jackets, provide additional confinement for the column. This leads to additional column ductility and can also enable rebar splices with insufficient laps to more fully develop. It should be noted that column wrapping could also lead to increased axial capacity. Although this is not the objective in a seismic wrapping application, it can be used as a retrofit technique for column strengthening. Extensive laboratory investigations have been conducted, and several manufacturers have products that are being marketed for this application. Several countries have conducted field projects involving column jacketing using FRP composites. Some of the field applications in California were applied prior to the Northridge earthquake. Thus far, FRP composite jacket applications have performed well.

Challenges in implementing FRP composite materials in the bridge infrastructure

From the above discussion it can be observed that the fiber reinforced composites are very attractive proposition for repair and upgradation of damaged concrete structures. However, the success of the method depends on several factors:

Understanding FRP composite materials’ mechanics–The mechanics of FRC is fundamentally different from other construction materials. Therefore, one needs to understand the material before its use. Application of FRP composite at current state requires knowledge in material behavior and manufacturing process far more than that required for the conventional materials.

One example is the prediction of failure mode of FRP composite, which requires knowledge of fiber orientation and fiber-matrix interaction.

Understanding Structural Performance: In general, ductile materials are used in bridge engineering to ensure ductile failure, obvious deformation and warning to users. For example, steel is used for members and reinforcement because of its inherent linear elastic/perfectly plastic behavior. However, FRP composite, exhibits almost elastic linear behavior to failure. FRP composite is also very sensitive to strength concentration. When stress concentration occurs at joint, stress is not distributed over this area due to linear elastic property of FRP composites.

Therefore, if this is not taken into account, a structure built from FRP composite can fail in brittle manner. Another aspect of structural performance, about which bridge engineers are still skeptical, is the dependence of strength of FRP composites on fiber orientation and placement.

Since matrix provide little resistance to load, alternative load paths are almost nonexistent in FRP composites. This situation is critical in bridge engineering because load paths are very difficult to predict, even for a simple structure built from steel or concrete. So it is considered safer to use materials that can provide load resistance from unpredictable directions. In addition to this, the design of civil structures is generally governed by stiffness performance, such as maximum deflection criterion. Since FRP composites have low stiffness, their use can result in civil structures being significantly over-designed for strength. Although this issue can be overcome by using FRP with high stiffness, such as carbon fibers, they tend to be very expensive.

Durability: The proponent of FRP composites claims that environmental, chemical, and mechanical durability is one of the advantages over traditional materials. A number of constituent materials have potential resistance against moisture effect, ultraviolet radiation, chemical attack, freeze thaw cycles, dynamic loading, and material aging effects. Several experiments conducted in the past show this potential, such as fatigue test. However, this is not obvious in current civil engineering application of FRP composites because all of them have been in service for a short period of time. Therefore, long-term durability data are required to strengthen such claim.

These results in difficulty for a bridge engineer to design bridge structures using FRP composites.

A large body of research work exists however; an efficient dissemination of knowledge is of utmost importance.

ii. Availability of design methods- The design manuals is already developed. The principle obstacle to greater diffusion of FRP is lack of codes and standards. Unlike conventional material industries, FRP composite producers do not currently have any representative organization to interact directly with the bridge engineer community, i.e. the national Subcommittee on Bridges and Structures. Bridge engineers are trained to utilize appropriate material in appropriate manner.

They do not need expertise in material science to design, construct, and maintain bridges of conventional material like concrete or steel. However, application of FRP composite at current state requires knowledge in material behavior and manufacturing process far more than that required for the conventional materials. One example is the prediction of failure mode of FRP composite, which requires knowledge of fiber orientation and fiber-matrix interaction. This results in difficulty for a bridge engineer to design bridge structures using FRP composite. Prescriptive nature of standard specification also makes FRP composite not preferred material because bridge engineers do not want to be liable for any structural failure associated with using this new material. Applications and considerations are made easier if official codes and guidelines applicable to specific structures or members are made available. The availability of such codes and guidelines will enormously assist FRP dissemination. However, they need wider publicity and possible inclusion in Indian code of practice.

iii. Availability of materials-GFRP is available locally in adequate quantities. CFRP needs to be imported. New production facilities will be extremely important as the use of FRP Composite is gaining popularity.

iv. High cost of material-In the current marketplace, FRP composite materials are relatively expensive compared with steel and concrete. Although the costs vary depending on specific components and manufacturing type, a bridge deck made from FRP could cost two to four (or more) times (on a per-square-foot basis) what a bridge deck made from traditional materials would cost. In the near term, it is difficult to anticipate a change in the relative costs. Without government or industry subsidy, many FRP composites will not find their way into service. The factors affecting cost are not entirely understood, but they are likely to include economies of scale, perceived profit opportunity, and labor costs associated with a highly skilled workforce traditionally working on defense and aeronautics applications. Because of their cost, a compelling case will have to be made for specifying FRP composite materials for bridge construction instead of steel and concrete. It may be that the offsetting economy of speedier construction and/or reduced weight, the lower life-cycle costs of FRPs than those of traditional materials can justify the use of FRP. Costs incurred in a construction project using FRP composites are categorized as shortterm and long-term costs. Short term cost includes material cost, fabrication cost, and construction cost. However, light-weighted and modular components made from FRP can help decrease construction cost. This includes easy erection or installation, transport, and no need for mobilization of heavy equipment. More saving, though difficult to quantify, can also be achieved from less construction time, less traffic disruption, or other factors commonly affected by construction project. These advantages have to be considered on case-by-case basis.Long term cost of FRP composites is more complicated to evaluate because it involves various unpredictable costs, such as maintenance, deconstruction, and disposal costs. Some costing techniques have been developed. One of them is the “Whole of Life (WOL)” technique, derived from life-cycle costing, includes initial cost, maintenance cost, operating cost, replacement and refurbishing costs, retirement and disposal costs, etc, throughout the expected life-span of the project. Using this technique, FRP composite and traditional materials can be compared by calculating economic advantages for structures designed for same performance criteria. As environmental awareness increases, long-term cost of project becomes more important. Along with performance characteristics, such as stiffness and strength, sustainability has become one of the criteria in selecting construction material. FRP composites are considered a potential candidate because they allow virtually any combination of material properties. Unlike other industries, in which FRP composites have been successfully introduced, construction industry is very cost-sensitive. It is really difficult to justify the use of FRP composite over other cheaper construction materials, when a project does not require a specific advantage of FRP composites.

The claim of lower life-cycle cost is also difficult to justify because limited number of relevant projects have been built using FRP composites.

v. Availability of technology-The repair technique needs knowledge about several new construction materials. Successful application of these materials would require fundamental understanding of the behavior of these materials. Although, the research base is already available, awareness in India about these materials is scant. There is a need to include these materials in basic curriculum.

The major challenges mentioned above should not be viewed as barriers but progress to make good on what the materials have promised to deliver. They will serve as opportunities to improve the materials to ensure that the product will be durable and reliable.

Steps involved for implementation and widespread application of FRP Composite in the Bridge Infrastructure includes:

1. Prioritizing the bridges which are structurally deficient
2. Identification of the needs of transportation agencies
3. Determining the condition of each structural member and identifying the members to be strengthened
4. Determining the required extra capacity to be achieved
5. Evaluating the feasibility of using the appropriate FRP composites for strengthening schemes.
6. Establishment of the important engineering properties and structural behavior
7. Identification of appropriate tests for determining these properties
8. Development of design and implementation specifications for the use of these materials
9. Development of guidelines and procedures for inspection and maintenance of FRP composite structures and
10. Development of the documentation and necessary training for widespread understanding of this technology by the engineering, fabrication, and construction industries. A cohesive strategic plan is needed to effectively accomplish this implementation.

Conclusion

The repair and rehabilitation of aging and deteriorating of concrete bridges and infrastructure poses an urgent challenge for the civil engineering community. FRPs can play key roles in meeting these challenges. FRP composite materials show great potential for integration into the bridge infrastructure. Despite these beneficial superior properties over the other traditional materials, widespread application of FRP composites to the bridge infrastructure has been slow and uneven. With FRP composites, the Western and neighboring countries are already changing the way they build and maintain their bridges. Although a large research base is already available about these materials, only a small portion has resulted in actual applications in bridge infrastructure systems in India. However, there are several significant, but not insurmountable, challenges to overcome before widespread implementation occurs. These challenges include a lack of familiarity with the material among practicing bridge engineers, the cost of the material, and the lack of a unified effort (especially from a widely accepted coordinating agency) to move implementation efforts forward. Practicing civil engineers and even most newly graduated civil engineers typically have very little knowledge of FRP composite materials. If successful widespread application is to occur, these engineers are the ones who will apply FRP composite materials to the infrastructure. With the removal of certain obstacles to implementation, FRP composite materials have a place in the bridge infrastructure. Quality control is crucial to the successful application of FRP systems. Most FRP strengthening systems are simple to install. However, improper installation (e.g., not properly mixing epoxy components or saturating the fibers, misaligning the fibers, etc.) could be avoided with careful attention. Even though FRP component costs are higher than traditional materials on a square-foot basis, they may be competitive in terms of life-cycle costs. FRP composite materials may be the most cost-effective solution for repair, rehabilitation, and construction of portions of the bridge infrastructure if used intelligently.

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