Introduction to Strengthening with FRP
It is well known that concrete is a building material with a high compressive strength and a poor tensile strength. A beam without any form of reinforcement will crack and fail when subjected to a relatively small load. The failure occurs suddenly in most cases and in a brittle manner. The most common way to reinforce a concrete structure is to use steel reinforcing bars that are placed in the structure before the concrete is cast. Since a concrete structure usually has a very long life, it is quite common that the demands on the structure changes with time. The structures may have to carry larger loads at a later date or fulfil new standards. In extreme cases, a structure may need to be repaired due to an accident. Another reason can be that errors have been made during the design or construction phase so that the structure needs to be strengthened before it can be used. If any of these situations should arise it needs to be determined whether the structure should be strengthened or replaced. Over the past decade, the issue of deteriorating infrastructure has become a topic of critical importance in Europe, and to an equal extent in the United States and Japan.
The deterioration of decks, superstructure elements and columns can be traced to reasons ranging from ageing and environmentally induced degradation to poor initial construction and lack of maintenance. Added to the problems of deterioration, are the issues related to the needs for higher load ratings and the increased number of lanes to accommodate the ever-increasing traffic flow on the major arteries. As an overall result, a significant portion of our infrastructure is currently either structurally or functionally deficient. Beyond the costs and visible consequences associated with continuous retrofit and repair of such structural components, are the real consequences related to losses in production and overall economies related to time and resources caused by delays and detours. As we move into the twenty-first century, the renewal of our lifelines becomes a critical issue.
However, to keep a structure at its original performance level, or sometimes only at acceptable performance level, it needs to be maintained at predestined time intervals. If lack of maintenance has lowered the performance level of the structure, the need for repair can be required. In cases when higher performance levels are needed, upgrading can be necessary. Performance level here means load carrying capacity, durability, function or aesthetic appearance. Upgrading refers to refined calculation models, strengthening, increased durability, and change of function or improved aesthetic appearance. In this book, mainly strengthening is discussed. Restoration, reparation and reinforcement of old concrete structures are becoming increasingly common. If one considers the capital that has been invested in the existing infrastructures, then, it is not always economically viable to replace an existing structure with a new one. The challenge must be taken to develop relatively simple measures such as rebuilding, restoration, reparation and reinforcement that can be used to prolong the life of structures. This challenge places a great demand on both consultants and contractors. For example, there are difficulties in assessing the most suitable method for an actual subject; as for example, two identical columns within the same structure can have totally different lifespan depending on their individual microclimate. Also strengthening structures to carry higher load or change a structures use can be complicated, especially when the load in the service limit state is high. It is therefore important to analyses the problem thoroughly to be able to select the correct measure. The choice of an unsuitable reparation method can even deteriorate the structure’s performance. In the cases where upgrading is appropriate, the intention should be to increase durability or load-bearing capacity.
There are many different methods to strengthen an existing concrete structure such as; change of cross section, external pre-stressing, change of static system or design that is even more accurate where real material data and loads are considered. Another alternative strengthening method is FRP (Fiber Reinforced Polymers) plate bonding. A more frequently used method of improving a structure’s load-bearing capacity is to attach sheets of fabric or fiber composite to the structure. The fiber in the composite can either consist of glass, aramid or carbon. The latter has been proven again and again to be favorable on building structures of concrete. The adhesive that is used to bond the fabric or the laminate to the concrete surface is a hardy two-component epoxy adhesive, which together with the fiber then becomes a polymer composite on the surface of the structure. The old structure and the new bonded material create a structural relationship that has a greater strength than the original structure.
The most common way to strengthen structures with advanced composites has been for bending but strengthening for shear, torsion and axial loads is also often needed. Strengthening a structure for shear or torsion can be justified by the same reason as for bending, but strengthening a structure for bending can also lead to the structure needing to be strengthened for shear since the failure mode can be changed Strengthening a structure for shear is often more theoretically complicated then for bending since the shear behavior of concrete is not as well understood as the bending behavior. In cases strengthening are needed for columns, considerably confinement effects can be obtain by wrapping the column with FRP sheets. However, when unidirectional FRP materials are used consideration must also be given to the anisotropy of the materials. One must remember that these materials have first and foremost been developed for the space, aviation, boat and car industries. The building industry has totally different demands when looking at the use of advanced polymer composite materials. For example a bridge will have a designed lifespan exceeding 50 years, sometimes 120 years and building structures are loaded during their lifetime with large static loads whereas in the aviation industry the loads are dynamic and carried over a relatively short time.
FRP materials are a group of advanced composites consisting of high strength and high modulus fibers embedded in a matrix with distinct interface characteristics. Both fibers and matrix retain their physical and chemical identities, yet they produce distinct properties that cannot be achieved by either constituent acting alone. The matrix is the binder material of the composite and plays a significant role in transmitting the applied load to the fibers. The most common type of matrix is polymeric material, including epoxies and polyesters. Several types of fibers such as glass and carbon have been used in structural applications. In recent years, the increased availability and reduced cost of FRP materials has stimulated increased research into reinforced timber. Some researchers developed a computer program to predict the behavior of wood beams reinforced with FRP sheets bonded to the tension surface. Some other, conducted similar work using pre stressed FRP sheets as reinforcement. The programs showed that small amounts of FRP reinforcement could produced significant gains in strength and stiffness. Both studies confirmed the computer predictions with experimental tests, but the work was restricted to small samples of clear wood. Many researchers have investigated the use of FRP for reinforcing timber beams. FRP sheets, CFRP strips, and GFRP strips have all been used as external reinforcement for timber. Each of these studies reported an increase in the strength and stiffness of the beams. Some other researcher reported two events that occurred simultaneously at failure. A tensile failure in one of the bottom laminations was accompanied by partial or total delamination of the FRP strip.
Researchers at the University of Maine are performing detailed investigations into the behavior of FRP-reinforced glulam beams in order to develop design specifications. Researchers present a case study of the first highway bridge constructed with FRP-reinforced glulam. Al1 of the above studies used glulam or laminated veneer lumber (LVL). Glulam and LVL are engineered timber products constructed by bonding together multiple laminations of smaller timber beams. The laminations are selected to reduce the presence and effects of knots, producing less variability in strength than for sawn lumber.
The three most common fibers used in structural applications are glass, aramid, and carbon. Glass fibers have high tensile strength combined with good mechanical propones, high chronical resistance, and excellent insulating properties. Some disadvantages of glass are low tensile modulus, sensitivity to abrasion, and reduced tensile strength in the presence of water and sustained loads due to creep rupture characteristics. The types of glass fiber cornrnonly found in structural applications are E (electrical) and S (high strength). Aramid fibers have the lowest specific gravity and the highest tensile strength-to-weight ratio of the main fiber types. In bending, aramid fibers exhibit a high degree of yielding on the compression side that is not observed in other fibers. This non-catastrophic failure mode gives aramid fiber composites superior damage tolerance to dynamic and impact loading. Some disadvantages of aramid are low compressive strength, difficulty in machining, and sensitivity to ultra-violet radiation. Carbon fibers have very high tend strength, high elastic modulus, and high fatigue strength. The main advantages of carbon fibers are their high strength-to-weight ratios, excellent durability, and tow relaxation under sustained toad. Some disadvantages include low impact resistance and high cost. Due to their high cost, carbon fibers have been used for very specific applications such as pre stressing of structural members and flexural reinforcement of concrete structures.
Necessity of strengthening with FRP
Masonry structures are ancient and were built on times when no appropriate theory and good knowledge were available. People usually built their houses according to the available knowledge and experience. So many buildings which still exist do not satisfy the present guidelines. Also the recent worldwide earthquakes make people more conscious about the safety of life and property. Some of the famous building which becomes valuable in terms of culture and history demand longer service life. It is also a common issue that the place which was residential area some years ago now becomes industrial area, so people will usually want to change to use of their previous building. Sometimes there may be mistake while construction. So lot of reasons may be claimed for strengthening existing buildings. It is summarized as follows:
- To eliminate structural problems or distress which results from unusual loading or exposure conditions, inadequate design, or poor construction practices. Distress maybe caused by overloads, fire, flood, foundation settlement, deterioration resulting from abrasion, fatigue effects, chemical attack, weathering, inadequate maintenance, etc.
- To be conform to current codes and standards.
- To allow the feasibility of changing the use of a structure to accommodate a different use from the present one.
- Durability problems due to poor or inappropriate construction materials.
- Design or construction errors.
- Aggressive environments not properly understood during the design stages.
- Increased life-span demands made on ageing infrastructure.
- Exceptional or accidental loading.
- Varying life span of different structural or non-structural components.
Also, Repairing/strengthening means to increase one or more than one of the following parameters:
- Tensile capacity
- Shear capacity
- Flexural capacity
- Compressive capacity
- Member stability
- Strength or stiffness or both
Applications of FRP
In the past, the high cost of FRP materials had restricted their applications to areas where weight reduction was more important than cost, such as the aerospace and sporting goods industries. More recently, the need to upgrade and repair infrastructure has led to increased research and use of FRP materials in structural engineering applications. For the repair of existing structures, FRP laminates (approximately 1 to 2-mm thickness) have ken bonded to the tension surface of beams and slabs. FRP sheets and fabrics have been wrapped around circular columns to improve ductility and strength and have been wrapped around the webs of beams to enhance the shear strength. The non-corrosive characteristics and high strength of FRP has made it an attractive replacement for steel reinforcement in concrete structures. FRP in the form of bars and cables have been used as both regular and pre stressed reinforcement and as stirrups in new concrete structures.
The first question, which should be posed about fiber composites in the building industry, is whether they offer any advantages in comparison to the materials that are used today? The answer to this is without a doubt ‘yes’. Durability of concrete structures are often related to corrosion in the steel reinforcement; simply put – no steel, no corrosion. Furthermore, these materials offer high strength, low weight and flexibility, e.g. new types of structures can be built that a few years ago were not dreamed possible. From a shorter perspective, possibly the most interesting applications are within the areas of maintenance, repair and strengthening of already existing structures. The existing buildings in the western world are becoming older and older and even though a large proportion are functionally competent, there are also a large amount that are in need of repair or reinforcement. Infrastructure such as bridges is influenced by society’s demands for increased loads due to increased permissible axel pressure and total weight per vehicle. This leads in the turn to the need for upgrading with respect to the load carrying capacity. Current norms can differ from when a structure was built or errors could have been made during the construction or production stages. Changing demands on structures occurs in Europe, Japan and the USA. If one looks to Eastern Europe there is a huge need for strengthening of structures, mostly due to neglected maintenance. In these cases a measure, which can recover the full function of the structure may be the adhesion of composite to the surface of the structure. Other applications for composites can be traditional reinforced or pre-stressed concrete structures in general. Interesting applications can also be completely new structures of fiber composite material. Most likely, these materials will be used in combination with traditional building materials such as steel and concrete. The use of composite material within the infrastructure sector understandably poses different demands than in other applications. Compared to the aviation industry, the following differences are obvious:
- Reduced demands on very high dimensional stability;
- Increased need for material that under a long time (> 50 years) can resist different environmental and even static loads. Usual outdoor environments include temperature fluctuations, UV-radiation, moisture cycles etc. In normal indoor environments there are requirements concerning fire and low emissions from the material that must be taken into consideration. In the vicinity of bridges and roads there is often the presence of de-icing salts;
- Irregular maintenance;
- Requirements for low initial costs and low maintenance costs;
Here are some FRP usages:
Bridge strengthening: FRP have been embraced as a cure for steel–RC bridges and structures that are crumbling from the corrosive effects of de-icing and marine salts, environmental pollutants, and from the long-term effects of traffic loads that exceed design limits. Glass and carbon-reinforced polymers provide an attractive and economical solution for strengthening existing concrete bridges and structures because of their high strength and low weight. The material can be used where longer, unsupported, spans are desirable, or where a reduced overall weight combined with increased strength could mean greater seismic resistance. A lightweight FRP-reinforced structure can reduce the cost of columns and foundations and can accommodate the increasing demands of heavier truck loads. One of the great benefits of using FRP to strengthen an existing concrete bridge is the speed of application. However, this speed is potentially compromised if the traffic is halted on the bridge during the works. Therefore, it is clearly beneficial for the bridge to be able to withstand dynamic live loading during cure of the adhesive. FRP composites have also been used to rehabilitate pre-stressed concrete (PC) bridge members. PC members are susceptible to steel strand fatigue and may require strengthening to prevent further loss of pre stress, some researches showed that PC girders could be strengthened with externally bonded CFRP composite plates to increase their ultimate flexural capacity. Also both flexural and shear capacities of a 30-year-old damaged PC girders could be substantially increased with externally bonded CFRP composite sheets. They used CFRP U-wraps as shear reinforcement along the length of the girder to delay de-bonding failure.
Flexural strengthening: The bending capacity of concrete elements can be increased through the use of externally bonded FRP plates, strips or fabrics. Alternatively, near-surface mounted strips (NSM) or rods with the fiber direction parallel to the member axis can be utilized. The use of externally bonded plates and NSM CFRP systems to strengthen RC beams in flexure has been well researched. There is a state-of-the-art paper, review papers and books on FRP composites on the civil infrastructure and on the rehabilitation of existing civil structures. Composites that are fabricated either through wet processes on-site or prefabricated in plates and then adhesively bonded to the concrete surface provide an efficient means of strengthening, which can be carried out with no or little disruption in use. The efficacy of the method depends mainly on the appropriate selection of the composite material and on the efficiency and integrity of the bond between the composite and the concrete surface. That is to say that flexural strengthening can be done by bonding FRP strips to the soffit of the beam.
Shear strengthening: When an RC beam is deficient in shear, or when its shear capacity is less than the flexural capacity after flexural strengthening, the shear strengthening of the respective beam has to be considered. It has been realized that the FRP bonded to the soffit of an RC beam does not modify significantly the shear behavior from that of the un-strengthened beams. Therefore, the influence of FRP strips bonded to the soffit for flexural strengthening may be ignored in predicting the shear strength of the beam. Various bonding schemes of FRP strips have been utilized to improve the shear capacity of RC beams. The shear effect of FRP external reinforcement is maximized when the fiber direction coincides with that of maximum principal tensile stress which generally is at approximately 45 degrees to the member axis. However, sometimes it is more practical to attach the external FRP reinforcement with the principal fiber direction, perpendicular to the member axis direction. That is to say that the shear capacity of concrete members can be enhanced by providing externally bonded FRP with the fibers oriented in the transverse direction to the member axis direction, in the case of columns and beams, or in the direction of both the column and the beam direction in the case of beam–column joints.
Flexural strengthening of RC plates: FRP composites were used for the flexural strengthening of one-way slabs and two-way slabs using FRP sheets. These sheets were bonded along the middle of the slab or distributed along the slab width. Pre-stressed sheets were used for two-way slabs. When the RC plates are simple supported, the one-way plates were strengthened by bonding FRP strips to the soffit along the required direction. For two-way plates, strengthening must be applied for both directions by bonding FRP strips in both directions. There are a limited number of theoretical studies on FRP-strengthened RC slabs when compared to those on beams, but the finite element analysis has been one of the most effective numerical methods for modelling their behavior.
Strengthening against poor detailing defects: The flexural cracking may be followed by cover concrete spalling, failure of transverse steel reinforcement, and buckling of longitudinal steel reinforcement or compressive crushing of concrete due to the poor detailing in the regions of flexural plastic hinges. This mode of failure is usually accompanied by large inelastic flexural deformation. By adding confinement in the form of FRP jackets with fibers placed along the column perimeter, the spalling of cover concrete is prevented and the buckling of the longitudinal steel bars is restrained. In this way, more ductile responses can be developed and larger inelastic deformations can be sustained.
Strengthening of RC columns: The strengthening of existing RC columns using steel or FRP jacketing is based on a well-established fact that lateral confinement of concrete can substantially enhance its axial compressive strength and ductility. The most common form of FRP column strengthening involves the external wrapping of the FRP straps. The use of FRP composites provides a means for confinement without the increase in stiffness (when only hoop reinforcing fibers are utilized), enables rapid fabrication of cost effective and durable jackets, with little or no traffic disruption in most cases. In FRP-confined concrete subjected to axial compression, the FRP jackets are loaded mainly in hoop tension while the concrete is subjected to triaxle compression, so that both the materials are used to their best advantages. As a result of the confinement, both the strength and the ultimate strain of concrete can be enhanced, while the tensile strength of FRP can be effectively utilized. Instead of the brittle behavior exhibited by both materials, the FRP-confined concrete possesses an enhanced ductility. For FRP wrapped axially loaded columns, the design philosophy relies on the wrap to carry tensile forces around the perimeter of the column as a result of lateral expansion of the underlying column when loaded axially in compression. Constraining the lateral expansion of the column confines the concrete and, consequently increases its axial compressive capacity. Columns can be strengthened axially by providing confinement by wrapping them in the hoop direction. For circular columns, the original guidelines treated strengthening in compression and flexure separately. When circular concrete columns are wrapped using FRP material, there are many potential beneficial effects on strengthening.