RC Structures Strengthened with FRP for Earthquake Resistance

Design of RC beams with longitudinal reinforcement made of steel bars has adopted the philosophy of Under-Reinforced Design. But, Concrete Beams with FRP longitudinal bars need a reconsideration of this philosophy. The large strength of FRP bars and low strength of concrete obviates Over-Reinforced Design. By suitably increasing the amount of FRP longitudinal bars, it is possible to enhance the deformability of the FRP-reinforced beams. While Limit State Method of design will be applicable still, the partial safety factors on loads and on materials need to be adopted after due experimental studies.

Fiber reinforced polymer (FRP) materials are being used for the last two decades as reinforcement in new concrete structures and prestressing tendons in pretressed concrete structures. In addition, they are being used at large scale in external strengthening and/or retrofitting of deficient structures to restore the strength and stiffness of the structure to the design values or to meet the current code provisions. But, to make such structures capable of resisting the effects of strong earthquake shaking, at least a minimum stiffness, sufficient strength and adequate ductility are to be ensured. FRP laminates and FRP fabric sheets are used in external strengthening to provide the needed stiffness, strength, and ductility by suitably orientating the fibers and creating confinement. These FRP materials possess high tensile strength with moderate stiffness and are non-corrodible; they can be used efficiently for improving performance of structural elements, without increasing the earthquake forces. In this chapter, a unified design approach has been presented for earthquake retrofitting of RC columns; a design example is provided to demonstrate the effectiveness of FRP for seismic strength and ductility enhancements.

Traditional methods for repairing impaired structures such as concreting, steel jackets, or timber splicing are impractical because of the inherent constraints associated with these materials. They would be susceptible to the same deterioration as the existing structure, leading to an ongoing cycle of repairs. Fibre-reinforced polymer (FRP) composite jackets offer a wide range of advantages including superior corrosion resistance, lightweight properties, and long-lasting durability. These characteristics make FRP composite jackets highly advantageous compared to conventional repair systems. Additionally, they can be effectively utilized for repairing various types of structures, including those made of timber, steel, and concrete. FRP composite jackets can be implemented through several techniques. However, in the experimental investigation presented in this chapter, automated fibre placement (AFP) was used to overwrap and reinforce two sets of thin-walled square hollow sections (SHS), columns and beams, with thermoplastic carbon fibre reinforced polymer (CFRP). The results obtained were then compared with the control samples without CFRP reinforcement. For the control columns, a good agreement was observed between the predicted and experimental ultimate compressive loads. The ultimate loads of CFRP reinforced columns exceeded the ultimate loads of the control columns. Inward and outward buckling was observed in each column. De-bonding, tearing, and snapping of the CFRP plies was observed in column specimens with thermoplastic CFRP reinforcement. For the control beams, there was a comparable agreement between the predicted ultimate load and the experimental ultimate load. It was found that the ultimate loads for some strengthened beams were higher than that of the control beams. For all beams, there was inward deformation on the upper surface of each beam, and outward deformations were observed on the two side walls of the SHS beams. This experimental investigation showed that the current strengthening processes using AFP is not comparable to traditional CFRP strengthening methods which use epoxy and FRP plies and that further research is required in this space. Due to the failure modes observed, future research is planned to improve the reinforcement method using AFP. The planned improvements are surface preparation, AFP processing conditions, number of CFRP layers and orientation of the CFRP.

The present study aims to the evaluation of the effectiveness of hybrid building systems to resist extreme earthquake forces. The study is conducted by considering a five-storey building frame with different variants consisting of different types of primary structural members: steel, reinforced cement concrete, concrete-filled steel tube (CFST), and truss system. The seismic performance of five variants of building frames is evaluated under the effect of near-field earthquakes also referred to as extreme earthquakes by performing nonlinear time history analysis (NTHA). The results obtained are compared for different cases considering various seismic parameters of the building model. The performance of hybrid frames was evaluated in terms of inter-storey drift, top-storey displacement, top storey acceleration and base shear. The study concludes that hybrid building systems consisting of CFST columns and steel beams perform outbound as compared to the other variants with the decreased storey drift, and displacement and the hybrid structure consisting of RCC beam produced lower accelerations.

The conventional practices of strengthening RC elements like Reinforced Concrete (RC) jacketing, Ferro cement jacketing, and Steel jacketing lead to an increased cross-sectional size of the member, which interrupts the functionality and proves to be nondurable when exposed to an aggressive environment. The use of Fiber Reinforced Polymer (FRP) composite technique in strengthening work addresses these issues over the other conventional practices. The simpler execution practice, high strength-to-weight ratio and faster setting time make FRP an efficient material in RC strengthening works. Many varieties of FRP, such as Glass, Carbon, Basalt, Aramid, etc., are available in construction practice that have unique tensile properties. The properties of tensile strength, Young’s modulus and rupture strain of FRP play a major role in the strength and post-yield deflection of the associated RC members. The property of adhesive used as a bonding source between FRP and concrete plays a significant role in transferring the forces and determining the failure mode. Strengthening techniques primarily depend on the demand/deficiency of structural elements. This chapter presents an overview of the strengthening techniques and experimental behaviour of FRP-strengthened RC elements.

Retrofitting the seismically damaged structures and strengthening the existing structures due to the upgradation of seismic zones continuously taking place. In fact, learning from occurring earthquakes and study of structural damages has become a very important aspect of earthquake engineering. Most of the developing countries have large numbers of habitats made of masonry, while the focus on the safety of these poor men’s habitats is relatively very less. Developing countries can also afford concrete structures for dwellings, while steel structures are mostly used for industrial purposes. On the other hand, the developed countries have large numbers of steel structures as well as shear wall based structures as they can afford the cost of the same. To have a comprehensive idea about retrofitting and strengthening measures for all categorial structures, the current chapter is an attempt to bring together the existing techniques for retrofitting and strengthening all the structures under separate subheadings and also underlines the promising new techniques which are coming up for this purpose. At the end, the chapter summarizes a few important points on this aspect. The chapter may be helpful for having an overall picture of the retrofitting and strengthening of structures.

Aging and deteriorating infrastructure, and more strict seismic design standards, call for the need of seismic retrofitting. Amongst various seismic retrofit strategies, use of fibre reinforced polymer (FRP) is both effective and advantageous over many other techniques, chiefly due to its high strength-to-weight ratio and good fatigue strength. Further, the speed of installation of FRP being high, it results in reduced downtime, that is extremely beneficial in case of seismic retrofitting of crucial infrastructure. The retrofit techniques with FRP are designed to add ductility, confinement, moment and shear capacity to existing structural members. Thus, local strengthening of members is achieved and alterations to overall structural dynamic properties are minimal. Amongst the commonly used fibres, glass fibres are suitable in low-cost seismic retrofit applications in comparison to carbon. However, caution needs to be exercised over the performance of FRP under elevated temperatures, as fire is a common hazard associate with earthquakes. This chapter discusses the various aspects of FRP retrofitting of reinforced concrete structural elements, as well as of masonry infill walls, for enhanced seismic performance.

Fibre-reinforced polymer (FRP) has gained significant applications in concrete composites. A FRP consists of tendons or fibres encased in a polymer matrix or bonding agent [1]. Different FRPs can be produced by selecting fibres and polymers of suitable materials.

Fiber Reinforced Polymer (FRP) based strengthening and retrofitting methods have gained importance even in developing countries such as India, with the advent of professionally managed and technically sound contracting companies. Numerous advantages of FRP, such as fast and clean application, exceptional tensile strength, free of corrosion, negligible deadload addition, etc. are being capitalized while staying within the limits of safety by understanding the limitations of the same. The author has completed over 2,500 retrofitting projects on reinforced cement concrete (RCC) structures in professional capacity in India and abroad, from industrial and commercial structures to bridges, dams and heritage buildings. Most of these projects required major utilization of FRP, in the form of FRP rods, laminates, wraps, anchors, NSM applications, crack-stitch and fiber reinforced cementitious matrix (FRCM) hot-wrap. In this chapter we shall discuss some of these projects as case studies and get an introduction into some of its design concepts and practical aspects.

In the present study, Unplastisized Poly Vinyl Chloride (UPVC) tubes filled with concrete are axially loaded to investigate their load carrying capacity and associated ductility. Total twelve specimens of UPVC tubes of diameters 140 and 200 mm with effective length of 900 mm were cast. M20 grade of concrete was filled inside the tubes for the casting of UPVC concrete filled tube (CFUT) column specimens. All these specimens were prepared to investigate the effect of outer diameter to thickness (D/t) ratio on the strength, confinement and ductility of columns specimens. The column specimens were tested for axial loading in the INSTRON machine of capacity 250 tonnes. Their load-deformation curves and modes of deformation were recorded. The test results indicate that as D/t ratio decreases, the confinement and ductility increases. The magnitude of the displacement corresponding to peak load increases as diameter of specimen increases. It can also be concluded from the experiments that concrete filled UPVC tubes can be used as columns.

The development of Fibre Reinforced Polymer (FRP) can be traced to the expanded use of composites after the World War II in the early 1940s, though the use of FRP was considered seriously for use as reinforced concrete until 1960s. Fibre Reinforced Polymers are well recognized as an effective seismic retrofit/fire resistant material for existing concrete buildings. This strengthening domain in civil engineering, a critical part of overall lifecycle aspect of any infrastructure, is more than two decades old and several successful projects have been installed using FRP as reported in several literatures. Many of these retrofitted buildings have experienced significant earthquakes and performed as designed, validating the effectiveness of the FRP and technology. Extensive laboratory testing and actual earthquakes have led to the growth of dependable design methodologies and guidelines for FRP to be used by the engineering fraternity. FRP materials have a high strength-to-weight ratio, which make them a perfect material for seismic retrofit. Although they do not add significant mass to a structure, they certainly enhance the capacity of various structural components. This also avoids the mandate of performing the analysis again without appreciable weight change and further consequential effect on foundation after due strengthening. FRP possesses innate characteristics to deal with fire and heated environment. This article is an attempt to highlight some of the features of FRP Reinforced High-Strength Concrete Structures from both Seismic and Fire viewpoints.

Due to the ageing of infrastructures, there is a need for repair and rehabilitation of the structures at later stages. The usage of carbon fiber-reinforced polymer for strengthening is one such practice that has become common in the Indian scenario. This paper specifically discusses the comparison of different application methods of carbon fiber-reinforced polymers for enhancing the shear capacity of the reinforced concrete beams. Near surface mounted carbon fiber-reinforced polymer strips, externally bonded carbon fiber-reinforced polymer strips and externally bonded carbon fiber-reinforced polymer strips with U–wrap are the methods adopted for strengthening reinforced concrete beams for shear. Three-point loading flexure tests were conducted on the strengthened beams, and it was found that the near surface mounted method exhibited a 28.5% increase in the shear capacity of the beam. In contrast, the externally bonded method exhibited an 18.5% increase, and externally bonded with U–wrap exhibited a 142% increase in the shear capacity of the beam compared to the control beam.

Most of the RC structures exposed to corrosion environment are getting distressed and there is a loss of capacity of structural elements and components. In addition, existing RC structures are generally weak at joint levels and causing failures due to external events like earthquakes. Present chapter is made explaining the detailed calculations to improve the concrete properties by FRP confinement. A detailed procedure is provided to evaluate the improved strength of RC elements strengthened with FRP plate. Also procedure explained to evaluate improved strength of RC structural component (joint). All these procedures are validated with published related experimental data.