S. K. Singh, Senior Principal Scientist, CSIR-Central Building Research Institute, Roorkee, India
Fibre-reinforced polymer (FRP) composite materials with their high strength-to-weight ratio, is considered an innovative way for strengthening of concrete structures, while adding minimal additional weight to structures. Masonry structures are also strengthened with FRP, which is an integrated system based on fibers such as carbon, glass, aramid etc. and epoxy resins. This is an ideal technique as it combines the advantage of using non-corrosive and lightweight advanced composite materials in the form of FRP. In addition, by pre-stressing the Carbon Fiber reinforced polymer (CFRP), laminates are also used more efficiently in strengthening the structures by utilising its tensile capacity.
Gopal L Rai, Managing Director, Dhirendra Group of Companies (DGC), Mumbai, India
Repair and rehabilitation of rapidly deteriorating structures is a matter of concern for most countries in the world. Deterioration is observed in the form of cracking and corrosion (Riveros et al., 2018). It is very difficult to assess in totality the causes of damage, mapping the extent of damage, proper corrective prescription, prediction of residual life with and without repair measures, instrumentation, monitoring and maintaining the health, and, most importantly, to strike a cost benefit ratio of the repair. This makes the industry large and multifaceted and there is substantial amount of knowledge available on the issues.
Fibre reinforced polymer (FRP) composites such as glass fiber reinforced polymer (GFRP) and carbon fiber reinforced polymer (CFRP) are being widely used for enhancing the load carrying capacity of reinforced concrete structural members (Jayasuriya et al., 2018 and Karatas et al., 2018). These composites consist of high strength fibers bonded in a resin matrix with the fibers as the main load carrying elements, whereas the resin or polymer matrix acts as a load transfer medium and protects the fiber from environmental damage.
These FRP composites are heterogeneous, anisotropic and do not exhibit plastic deformation (Karatas et al., 2018). The provision of FRP around reinforced structures improves the bond strength between steel reinforcement and concrete by providing axial stress. It possesses good tolerance under various types of loadings, such as monotonic or cyclic (Boroujeni et al., 2018). The FRP wrapping prevents the entry of aggressive ions such as chloride ions, which delay the corrosion rates of reinforcement in concrete and slows down the degradation of structures. The confinement of FRP slows the degradation due to cracking, inhibits seepage of water through these cracks and helps in repair of cracks that are formed within the structures (Liang et al., 2019).
The effectiveness of FRP composites for rehabilitation work mainly depends on structural assessment, suitable design, its manufacturing and ion-site application. The performance of the FRP fabrics depends on the bond characteristics between the fabrics and the concrete structures. The bond governs the load transfer mechanism between the FRP fabrics and the structural matrix. Hence, the concrete crack spacing, crack width and reinforcement development length is controlled by FRP application (Meng et al., 2018).
The increasing application of these materials is owing to the advantages it offers with respect to the other conventional methods of strengthening. They are effective as strengthening materials due to their lightweight, non-corrosive, high tensile strength, high fatigue resistance, low density, high stiffness and durability. In addition, these materials can be made into any size and geometry and require less efforts in installation in comparison with other conventional materials of strengthening. The structures can be strengthened using these materials in relatively less amount of time without causing any hindrance to the normal functionality or affecting the aesthetics.
This method of strengthening is extensively studied, and various codes and guidelines are for the ease of application in actual structures (ACI 440 and CEB-FIP Report). FRP can be used for strengthening of concrete, masonry, timber and steel structures. Extensive research has been carried out on their successful use in strengthening of RCC slabs and beams to increase their flexural and shear capacity (Irwin et al., 2001) and increase of confinement in axially loaded columns. In addition, it has been found that application in masonry structures has reduced cracking and has increased the load carrying capacity and ductility.
Figure 1 : Causes of distresses in concrete buildings
Repair & Rehabilitation Industry In India
Any concrete structure exhibits signs of distress and deterioration at an early age of construction due to various reasons, leading to repair works as shown in Fig. 1 (Suryawanshi, 2012).
As repair is inevitable in any structure, the repair industry is present in every nook and corner of India. However, the industry is not well organized. Thus, the exact figure of annual cost to stakeholders of the industry is not available. However, cost involved is found to be greater than that of the developed western countries.
Approach Towards Repair and Retrofitting of Structures
The engineering analysis, design and constructability are necessary steps for finalization of repair and retrofitting strategies, keeping in mind the following aspects (Chander, 2014), which define the basic approach towards developing a repair system.
Functionality Aspect: The basic function / operation of the structure should not be hampered.
Structural Safety Aspect: The susceptibility of the structure to an earthquake event has to be within acceptable standards.
Importance Level Aspect: Historic buildings / structures with immense archeological importance are sometimes beyond the cost factor for retrofitting. Such structures have to be rehabilitated without changing its aesthetics and architectural features.
Construction Methodology Aspect: The retrofitting has to be performed using latest construction techniques that have a minimal impact on the normal functioning of the buildings.
Economy Aspect: The entire cost of construction has to be practical and logical towards extended life of the structure.
Skilled Labour Availability: The repair & retrofitting practices need unusual construction method and is a highly technical job that and calls for utmost care while implementing. A very skilled workmanship must be provided to implement the suggested measures.
Based on the above aspects, the repair method adopted may be surface repair, protection, waterproofing, rehabilitation, strengthening etc. To implement this successfully, skilled expertise and specialized training is necessary. In India, the same contractor works on all kinds of works, which includes execution of new construction work, repair work and protection works. In addition, unskilled and semi-skilled labourers are involved in repair works due to non-availability of skilled labour. This leads to repair works of sub-standard quality. In addition, it is important to use state of the art equipment and technologies for different types of repair jobs.
Various Principles Adopted in Repair and Retrofitting
Passive and Active Strengthening System
Strengthening of structures can be done using a passive or an active strengthening system. In case of active system, external forces are introduced in the member, which counteracts the effects of internal forces. In case of passive system, strengthening will come into picture when additional loads were applied. Concrete jacketing, bonding steel plates and FRP wrappings on structural members are examples of passive strengthening system. This system is relatively easy in application but does not utilize the materials full properties.
|Table 1: Various principles adopted in repair of concrete|
|1||Protection against ingress||
bandaging of cracks
Filling of cracks
Transferring of cracks into joints
Erecting external panels
Hand applied mortar
Adding/replacing embedded/external reinforcing bars
Resistance to chemicals
Preserving or restoring passivity
Increasing cover with additional mortar or concrete
|9||Cathodic control||Limiting oxygen content (at the cathode) by saturation or surface coating|
|Applying an electrical potential|
Control of anodic areas
Active coating or barrier of the reinforcementApplying corrosion inhibitors in or to the concrete
The Active system, even though not relatively easy in application, utilizes the materials properties completely. Such a system generally improves the performance of the structure and increases its serviceability by reducing cracks and deflections. Examples of such a system includes external post tensioning system, bonding prestressed FRP laminates on the structural member, etc. The nature of strengthening system to be applied depends on the magnitude of deficiency in the member. In addition, a combination of both the methods can be used for improvement in the structural performance (Mandara et al., 2002).
Prestressed FRP System
FRP composites and non-prestressed laminates are externally wrapped / bonded on the structural members to enhance load carrying capacity. This application is advantageous in several ways: It helps in enhancement of tensile strength in the direction of fibers and lowers the density of FRP composites. It develops high resistance against corrosion and harsh environmental conditions. Its application is user friendly as well. It requires the least number of equipment and is applied in a short period. However, only a partial strength of the FRP laminates is utilized by the structure. Only 30 - 35% of the tensile strength is utilized during the serviceability stage of structures. The cause for such an issue is the premature debonding failure of FRP laminates from concrete surfaces that limits the development of tensile stresses (Peng et al., 2016 and Serega et al., 2018).
Figure 6: Typical load-deflection curves by prestressed & non-prestressed laminates
(El-Hacha et al., 2001)
Prestressed FRP laminates on the tensile face of the flexural member improves its serviceability. It helps in increasing the fatigue life and durability of RC structures on which it is applied. It also helps to close the existing cracks, delay the formation of new cracks and reduce structural deformation within the structures (Guo et al., 2018; Rojob et al., 2018 and Hosseini et al., 2014). Prestressing is hence a useful technique as it easily overcomes the defects of passive reinforcement of FRP laminates on structures. Fig. 6 shows the typical load-deflection curves for RCC beams strengthened with non-prestressed and prestressed FRP composite laminate system.
Various techniques were developed to induce stressing in the CFRP composites for flexural strengthening (Fig.7), shear strengthening, confinement and axial load strengthening. This pre-stressing can be induced either on the surface of the structural members or on the outside. The amount of prestressing forces to be applied is carefully designed as it affects the strengthening behaviour. In case of high prestressing forces, the failure in members is possible. Hence, it is required to apply proper amount of force at the ends of the laminates.
Figure 7: Application of non prestressed FRP laminates in slabs & wrap on girder
Prestressing forces should not exceed 50% of the ultimate strength of the laminates (CEB-FIP Report). Minimum level of prestressing should be 25% of the ultimate strength to improve the strength properties (El-Hacha, 2001). Mechanical anchors in the form of metallic plate can be used to provide sufficient anchorage to the laminates. Gradient anchors can also be used for the same. Prestressing of CFRP laminates can be done indirectly by cambered beam system or by inducing prestress against external steel frame, followed by bonding on the structural member to be strengthened (El-Hacha, 2001). But tensioning of CFRP laminates against the beam to be strengthened is one of the most widely used methods of prestressing. In this method, the FRP laminates are bonded on the member to be strengthened followed by application of prestress in the beams with the help of prestressing jacks at the ends of laminates. It is kept in the stretched position till the curing of the adhesive is complete. After this, the hydraulic jacks are removed and the laminate transfers the stresses to the concrete. Proper anchorages need to be placed at the ends to avoid debonding failures. This method of strengthening is highly effective.
Strengthening of Slab Using Prestressed FRP System
Figure 8: Excessive deflected slab (Courtesy: M/s DGC, Mumbai)
After carrying out an analysis of the RCC structure, it was decided to provide prestressed CFRP laminate in alternate manner at the negative moment regions at the tension face with a pre-stressing force of 50 kN. The properties of the materials used in strengthening are as per Table 2. The details of prestressing are shown in Fig. 8 to Fig. 12. After carrying out the strengthening of the slab it was subjected to load testing using water load. The slab was subjected to loading as per specifications given in IS 456: 2000. The loading was applied in stages so as to avoid any untoward mishap due to distress in structure. After full loading was applied, the readings of deflections and strain were measured, and after keeping the load on the slab for 48 hours the readings were further monitored. To check the rebound of the slab, the dewatering was done in stages and readings was recorded at consecutive intervals of time. The recovery observed in strengthened flat slab panel even after 48 hours loading post strengthening is 80.97% thereby indicating that the proposed method of strengthening has worked effectively.
Strengthening of Brick Masonry Building With FRP System
The FRP systems are also being used to confine and strengthen masonry structures. A residential building, about 45 years old, was distressed. After preliminary visual inspection, the requirement for strengthening of structural member with repair of building’s other non-structural member were finalized. Fig. 13 shows ongoing retrofitting work of the building, step wise.
Performance of Repair Works
Repair/retrofitting is a complex process due to various factors involved. It is important that the engineer who addresses the issues understands the nature of the structure and its importance. The root cause of damage before suggesting a repair methodology; and understanding that life and performance of the repair work not only depend on the materials or workmanship, but also on the compatibility between the existing structure and the new materials used for repair. Due to the lack of specialists and specifications, many a times repair work does not meet the performance criteria and in turn leads to repair of repairs. Thus, the field of repair and rehabilitation of structures has not gained the confidence of people and is always looked down upon. A survey also shows that of the total repair works carried out, 50% of the works performed unsatisfactorily.
Repair and retrofitting of structures have never been performed on a large-scale basis in India. Therefore, there are no established guidelines available for this purpose. Existing building codes do not address this problem adequately. It is the need of the day to have a vision to improve the status and performance of the repair and rehabilitation industry in India. The main objective shall be to create a repair/rehabilitation code to establish assessment, design, materials and construction practices to raise the level of repair/protection performance, establish clear responsibilities and provide building officials with means to issue permits for such work.
|Table 2: Material properties of FRP composite system|
|Type of laminates||Thickness of laminate (mm||Width of laminate (mm)||Ultimate Tensile strength (MPa)||Ultimate strain (%)|
|Alternate Prestressed CFRP laminates||1.4||100||3200||1.5|
There are challenges in the existing structures due to non-visible damage and unknown structural conditions, which affect the assessment of causes of distress. The lack of specific requirements for variations in repair practice and materials affects the choice of materials and hence the failure in compatibility of the old to new material, which further affects the performance and life of the repair work. Lack of safety and reliability checks or performance standards leaves the building officials with no directions, post repair.
Taking into consideration all the practical difficulties and shortcomings in repair and rehabilitation of structures, it may be proposed to develop standard guidelines for the process from diagnostics to implementation stage, along with performance standards for its acceptance.
Figure 13: Retrofitting of brick masonry building (Courtesy: M/s DGC, Mumbai)
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