Fiber-reinforced Polymer Wrapping Research Articles

Failure patterns and mechanical behavior of the cement mortar column partially confined by the FRP jacket

Fiber-reinforced polymer (FRP) composite has been recently adopted to maintain the integrity of infrastructure made of cement mortar, attributed to its high strength-to-weight ratio and easy-to-construct. To obtain an in-depth understanding about the compressive behavior of cement mortar samples with variable FRP center wrapping heights, both the acoustic emission (AE) response and deformation characteristics of which were comprehensively investigated in the present research. The relationship between the confinement provided the exterior FRP jacket and the mechanical characteristic was also explored, followed by the economic analysis on this reinforced technique. Test results indicated that both the deformation ability and the load-bearing capacity gradually increased with the increased center wrapping height of the FRP jacket. When the larger the AE energy generated at the initial compression stage, the more dispersed AE localization. Meanwhile, these main cracks occurred on the surface of reinforced cement mortar generally transformed from the FRP wrapping strip towards other extremities. Correspondingly, the number of tensile cracks of FRP partially wrapped cement mortar decreased with the increased axial deformation. Note that the residual integrity of these reinforced cement mortar with larger FRP-center wrapping height is much superior than its counterparts. The force state of partially reinforced cement mortar with FRP confinement was also discussed in the present research and the results of which were compared with existing theoretical models. Compared to other theoretical models, Teng's model shows the most accuracy in predicting the mechanical behavior of FRP partially reinforced cement mortar.

Study on damage characteristics and application of gypsum pillar strengthened with FRP

The instability of mine pillars has posed a serious threat to the safety of mines in China. In order to reduce the risk of pillar instability, this study used fiber-reinforced polymer (FRP) to reinforce gypsum specimens and conduct mechanical loading experiments. The study investigates the reinforcement effect of gypsum specimens with different height-diameter ratios and different FRP wrapping thicknesses, and analyzed the uneven distribution of constraint force along the height direction of FRP-reinforced gypsum specimens. An optimized wrapping method was proposed, and a stress–strain constitutive model for gypsum materials under optimized wrapping was established. By utilizing a statistical method based on strain energy density for heterogeneous rock damage identification, a simulation study was conducted on FRP-reinforced gypsum-like specimens. A comparative analysis was carried out with experimental results to verify the feasibility of the constitutive model. The research findings indicate that the use of FRP reinforcement significantly improves the uniaxial compressive strength and strain values of gypsum-like specimens. Moreover, the smaller the aspect ratio, the more pronounced the effect of FRP reinforcement on gypsum-like specimens. The stress–strain curve of FRP-reinforced gypsum-like specimens shows a distinct combination of curves and straight lines, which can be divided into two distinct stages based on different elastic moduli. The peak value of acoustic emission energy of FRP-reinforced gypsum specimens appears at the junction of the first and second stages of the stress–strain curve, and then the acoustic emission energy curve becomes relatively flat, with an increase in peak energy before failure. The constraint force acting on FRP-reinforced gypsum specimens exhibits an obvious uneven distribution along the height direction, based on which an optimized reinforcement scheme is proposed. Based on the established constitutive model, numerical simulation was conducted using real data from a specific mine. The research findings demonstrate that when the optimized reinforcement method is employed, the load-bearing capacity of the mine pillars was significantly enhanced. The maximum displacement was reduced by 33%, the maximum vertical stress was reduced by 3.2%, and the volume of the plastic zone was reduced by 39.87%.

Compressive behavior of FRP-confined 3D printed ultra-high performance concrete cylinders

Three-dimensional (3D) concrete printing technology has attracted increasing applications due to its merits such as labor-saving. Due to difficulties in implementing reinforcements in 3D printed concrete (3DPC), 3DPC structures are commonly designed to predominantly resist compressive loadings. This paper proposes to further enhance the compressive performance of 3D printed ultra-high performance concrete (3DPU) elements by fiber-reinforced polymer (FRP) wrapping. Axial compression tests on FRP-confined 3D printed UHPC (FC3DPU) and unconfined 3D printed UHPC (UC3DPU) cylinders were conducted. The key variables include the loading directions (i.e., X, Y, Z directions) of the 3DPU cylinders and the FRP confinement thickness (i.e., one and two layers). Test results show that FRP wrapping can substantially enhance the strength and deformation capacity of 3DPU. Furthermore, the compressive strengths of the UC3DPU and FC3DPU in the X-direction are the highest, while they are the lowest in the Z-direction. The actual confinement ratio threshold for sufficient confinement of FC3DPU is 0.1. Two existing models of FRP-confined concrete were assessed, and results show that models of Liao et al. and Teng et al. have a comparable accuracy in predicting the ultimate axial stresses and strains of FC3DPU. Microscopic analysis reveals that 3DPU has more large defects (i.e., equivalent diameter (Eq) > 2 mm) in interlayers but less small defects (Eq < 2 mm) than the cast counterparts.

Hybrid strengthening scheme for reinforced concrete beams: Flexural behaviour and large-scale testing

Revision of design standards, new regulations, and planned/unplanned increases in loadings potentially make buildings vulnerable to changes in capacity demands. This may result in some structural components of an existing structure requiring strength or stiffness improvement. Strengthening of capacity deficit members is widely recognised as the preferred and sustainable choice in terms of conserving resources and minimising the carbon footprint. Strengthening techniques such as Reinforced Concrete (RC) jacketing, steel jacketing and Fibre Reinforced Polymer (FRP) wrapping have been explored and widely used. However, when adopted solely, each scheme has its drawbacks. RC jacketing results in a considerable rise in the dead load, influencing the loading on the foundation and restricting the functional space available post-jacketing. Steel jacketing often lacks flexibility as a significant effort is required for installation. FRP improves the flexural strength response but produces low stiffness enhancement and lesser ductility. A hybrid strengthening option, i.e. a combination of the above strengthening methods, presents a viable option to address the limitations cost-effectively. This paper presents a pioneering attempt focused on improving the flexural performance of RC beams using a hybrid strengthening scheme comprising of RC jacketing and FRP. Four large-scale RC beams were prepared, and three of them were strengthened. The first was strengthened by RC jacketing, the second by a combination of RC jacketing and flexural FRP, and the last by the combination of RC jacketing, flexural FRP and FRP ‘U wrap’ in the shear zones. The specimens were tested using the four-point bending test. The flexural behaviour was assessed in terms of the peak moment capacity, flexural stiffness, load-displacement response, crack pattern and failure modes. The test results confirmed that hybrid strengthening leads to strength gain and stiffness gains of 105% and 90% respectively, as compared with the unstrengthened beam. The efficacy of the theoretically predicted strengths against the test strengths is assessed.

Axial compressive behavior and design-oriented model for large-rupture-strain (LRS) FRP-confined concrete in rectangular columns

Polyethylene terephthalate (PET) fiber-reinforced polymer (FRP) composites, which is a kind of low-carbon production and have a large rupture strain, may have prosperous application outcomes. PET FRP wrap has been explored as confining reinforcement for concrete, and the majority of investigations are limited to PET FRP-confined concrete in circular and square columns, whereas PET FRP-confined rectangular columns have barely been touched. To fill this research gap and gain a fundamental understanding of the confinement mechanism, comprehensive compressive tests on 36 columns were conducted in this study, in which the confinement thickness, cross-section aspect ratio, and specimen size were the focal points. All specimens exhibited a three-segment behavior (i.e., strain hardening-softening-hardening response). In addition, it is demonstrated that specimen size had an insignificant effect on the axial stress-strain response of small and medium columns with a height less than 400 mm, provided that their confinement levels were the same (or similar). Furthermore, the minimum effective confinement stiffness for PET FRP-confined rectangular columns to achieve sufficient confinement was 0.004, which was extracted from all available test results. A design-oriented model with improved performance was proposed and the simulated full axial stress-strain showed close agreements with the test results.

Seismic behavior of RC square columns strengthened with LRS FRP under high axial load ratio

A new type of fiber-reinforced polymer (FRP) with a tensile rupture strain of more than 5% was used for the seismic retrofitting of reinforced concrete (RC) square columns in this paper. A control column and 9 FRP-wrapped RC square columns were prepared with different number of FRP layers and FRP types. The seismic performance was comprehensively discussed and analyzed in terms of the failure modes, hysteretic behaviors, cumulative energy dissipation and FRP strain evolution. Experimental results showed that the ductility of specimens had a remarkable improvement as the confinement stiffness increased. Moreover, the buckling of longitudinal bars was restricted and postponed. Numerical analysis was conducted in OpenSees to further research the seismic performance of specimens based on an LRS FRP-confined concrete model and a reinforcing bar buckling model considering FRP confining effect. The simulated results were in close agreement with the experimental results. Parametric analysis was also conducted to investigate the influence of the FRP wrapping thickness, axial compression ratio and longitudinal reinforcement ratio on the seismic performance.

Evaluation of Hybrid Fiber Multiscale Polymer Composites for Structural Confinement under Cyclic Axial Compressive Loading

Fiber reinforced polymer (FRP) confinement is recognized as the most promising technique for the strengthening and retrofitting of concrete structures. In order to enhance the performance of conventional epoxy-based FRP composites, nano filler modification of the epoxy matrix was implemented in the current study. In particular, the cyclic loading response of standard concrete specimens externally confined by epoxy-based natural and hybrid fiber reinforced polymer systems was investigated. The confinements were realized with sisal fiber reinforced polymer (SFRP) and hybrid sisal basalt fiber reinforced polymer (HSBFRP). Moreover, the effects of multiwalled carbon nanotubes (MWCNT) were also investigated. Three different specimen sets were considered for study: (i) unconfined specimens, (ii) epoxy-based FRP confined specimens and (iii) MWCNT incorporated epoxy-based FRP confined specimens. The specimens were tested in repeated compressive mode in loading-unloading cycles at increasing displacement levels. The test results revealed that FRP wrapping could enhance the mechanical behavior of unconfined columns in terms of strength and ductility. Moreover, it was evident that the mechanical properties of the epoxy matrix were enhanced by MWCNT incorporation. The developed epoxy-based FRP confinement containing MWCNT ensures improvement in axial strength by 71% when compared with unconfined specimens. The epoxy-based FRP confinement, with and without MWCNT, exhibited a high strain redistribution behavior around the concrete core. In comparison to the unconfined specimens, the confinement could increase the sustained axial strain from 0.6 to 1.4% using epoxy-based FRP confinement and to 1.6% with MWCNT incorporated epoxy-based FRP confinement. Further, an empirical model was developed to predict the ultimate axial stress of concrete columns confined externally with FRP jackets. The ultimate compressive strength obtained from the experimental study was compared with the proposed model, and the observed deviation was lower than 1%.

Numerical and experimental study of the mechanical behaviour for FRP-wrapped cement mortar-coal composite disc

The instability of the interface could induce the failure of the composite structure, and fibre-reinforced polymer (FRP) can be applied to solve this problem. The effect of FRP wrapping on the tensile strength and failure pattern of the cement mortar-coal composite (CCC) disc at different interface angles were investigated by the Brazilian splitting experiment. The effect of FRP wrapping on microcracks and contact force evolution was further revealed through PFC3D-FLAC3D coupling numerical simulation, and the mechanism of FRP wrapping was demonstrated. The results indicate that FRP wrapping significantly improved the nominal tensile strength of the specimens. In addition, FRP wrapping reduced the anisotropy of the specimens. Three types of failure were observed in the composite specimens: fractures in the coal and cement mortar elements were tensile cracks, while fractures at the interface were shear or tensile. FRP wrapping increased the degree of damage to the coal and cement mortar elements when the specimen was unstable, but reduced the extent of damage at the interface. The degree and extent of the stress concentration were increased with FRP wrapping, and the anisotropy of the contact force distribution was reduced.

Symbolic Regression Model for Predicting Compression Strength of Prismatic Masonry Columns Confined by FRP

The use of Fiber Reinforced Polymer (FRP) materials for the external confinement of existing concrete or masonry members is now an established technical solution. Several studies in the scientific literature show how FRP wrapping can improve the mechanical properties of members. Though there are numerous methods for determining the compressive strength of FRP confined concrete, no generalized formulae are available because of the greater complexity and heterogeneity of FRP-confined masonry. There are two main objectives in this analytical study: (a) proposing an entirely new mathematical expression to estimate the compressive strength of FRP confined masonry columns using symbolic regression model approach which can outperform traditional regression models, and (b) evaluating existing formulas. Over 198 tests of FRP wrapped masonry were compiled in a database and used to train the model. Several formulations from the published literature and international guidelines have been compared against experimental data. It is observed that the proposed symbolic regression model shows excellent performance compared to the existing models. The model is easier, has no restriction and thereby it can be feasibly employed to foresee the behavior of FRP confined masonry elements. The coefficient of determination for the proposed symbolic regression model is determined as 0.91.

Strengthening RC square columns with UHP-ECC section curvilinearization and FRP confinement: Concept and axial compression tests

In this paper, a novel column strengthening technique incorporating section curvilinearization (SC) using ultra-high performance engineered cementitious composites (UHP-ECC) and fiber-reinforced polymer (FRP) confinement is proposed and validated. Eleven large-scaled FRP-confined square reinforced concrete (RC) columns with SC using UHP-ECC as the additive material were prepared and tested. The main parameters investigated in this paper included the FRP thickness, the rise-to-span ratio, and the strength of the additive concrete. Test results show that the SC technique with FRP jacketing and UHP-ECC as the additive material enhances the load-bearing capacity of the RC columns by 52% with a 17% section increment. By assessing the existing design-oriented stress–strain models of confined concrete in curvilinearized square columns (CSCs) using the test results obtained in this study, it reveals that the current models could underestimate the ultimate strength of the confined concrete in CSCs with a two-layer FRP wrap, and therefore further research on the improvement on the stress–strain model of confined concrete in CSCs becomes necessary.

Effects of metakaolin on the shear capacity of EBFRP RC beams: An experimental and numerical investigation

The main aim of this study is to find the shear resistance capacity of the fiber-reinforced polymer beam with metakaolin. The RC beams used in this work were prepared with the replacement of cement by 35 % metakaolin. The major objective of this study is to understand the overall behavior of metakaolin-based reinforced concrete beams strengthened using fiber-reinforced polymer (FRP) composites. First, several trials were prepared with different dosages of metakaolin, biochar, and eggshell powder for achieving a target cubic compressive strength of 30 MPa. From the developed optimized mix, 12 RC beams were cast and strengthened with different configurations of carbon FRP wrapping. Test results revealed that the RC beam developed with metakaolin and strengthened with full wrapping (CFRP-FW) showed better overall performance in terms of peak strength and ductility. In addition to the experimental program, a detailed non-linear finite element (FE) analysis was performed using the commercial software ABAQUS. The predictions from the FE analyses matched well with the experimental results in terms of peak shear strength and failure patterns. The validated FE model was used for performing a detailed parametric analysis aiming to determine optimum design outputs for external bonding strengthening of metakaolin-based RC beams under shear.

Experimental study of reinforced concrete piles wrapped with fibre reinforced polymer under vertical load

This research mainly focuses on the nature of Fiber-Reinforced Polymers (FRP) wrapped piles for the axial compression load. Normally, the degradation of Reinforced Cement Concrete (RCC) structures occurred due to corrosion, inadequate quality materials, aging, live load increments, fatigue, damages and deterioration, and so forth. RCC pile foundations are commonly affected by the above reasons. So, these structures can be strengthened with suitable methods such as jacketing, stitching, overlaying, sealing, grouting, coating, Near Surface Mounted (NSM) systems, FRP wrapping, repairing, and blanketing. FRP wrapping is well suited for strengthening pile structures because of its easy application, and this method is done without using heavy tools and skilled labor. Due to its being non-metallic, FRP reinforcing has desirable properties such as high resistance to chemicals as well as heat, flexibility, large tensile strength, permeability, and also no oxidation. Thus, FRP wrapping was selected as the chosen approach for this ongoing research. In this study, we apply an axial compression load as well as a skin friction condition to a pile member that has been modified to assess its performance and behavior.

Experimental studies on confined compressive strength of Stainless Steel Wire Mesh (SSWM) wrapped concrete columns

The findings of an experimental evaluation of the axial compressive strength of an M25 grade Plain Cement Concrete (PCC) column confined with locally available Stainless Steel Wire Mesh (SSWM) are presented in this paper. Various types of Fiber Reinforced Polymer (FRP) are usually employed to strengthen the load-bearing components of concrete structures. However, these FRP materials exhibits brittle failure and debonding of FRP strip or wrap. Stainless Steel Wire Mesh (SSWM), on the other hand, is a cost-effective substitute for FRP materials when it comes to strengthening of structural elements. In this study, effectiveness of SSWM for confinement of concrete columns is demonstrated through experimental study. For the research, column specimens with a diameter of 150 mm and a height of 300 mm are used. Test specimens are strengthened with one-layer circumferential wrap of SSWM. Three variants of SSWM i.e. SSWM 30 × 32, SSWM 40 × 32 and SSWM 50 × 34 are considered for the study. After 28 days of moist curing, SSWM is applied to the column surface using epoxy SIKADUR 30LP. Axial compressive load-carrying capacities of strengthened specimen are compared with that of control specimens. From the comparative study it is found that SSWM 40 × 32 performs better compared to other variants of SSWM considered for this study. Compared to the control specimen, the load-carrying capacity of the column strengthened with SSWM 40 × 32 is increased by 29.35%. For all the specimen, failure patterns are observed in addition to the load–displacement and stress–strain behaviour. Results of present experimental investigation establishes the suitability of SSWM for confinement or strengthening of concrete elements subjected to compression.

An Example-Guide for Rapid Seismic Assessment and FRP Strengthening of Substandard RC Buildings

This paper presents a rapid seismic assessment and Fibre Reinforced Polymer (FRP) retrofit design methodology which relies on the European design guidelines recently published in Chapter 8 of fib Bulletin 90 on the use of externally applied FRP reinforcement in the seismic retrofitting of reinforced concrete (r.c.) structures. For this purpose, an example-guide is developed with step-by-step hand calculations aiming to facilitate engineers of practice and researchers working in the field to easily understand the proposed methodology. A three-storey, pilotis-type residential r.c. building is selected typical of the Mediterranean construction practice in the 1970s. The methodology followed only aims to provide preliminary results on seismic assessment and retrofitting before the implementation of more sophisticated analysis if need be (e.g., in case of irregular buildings). The assessment procedure identified that the columns of the ground storey, being the most critical structural elements for the stability of the structure, are vulnerable to brittle failure modes. To remove all the brittle failure modes attributed to inherent deficiencies and enhance the overall deformation capacity of the building, the strengthening schemes applied in the ground storey (pilotis) is a combination of local strengthening measures, such as FRP wrapping, and global interventions. The latter may refer to the addition of r.c. jacketing to the central column to remove slenderness and of metal X-braces to modify the lateral deflection shape of the building and thus moderate the interstorey displacement demand.

Parametric investigation on behaviour of carbon fiber reinforced polymers wrapped reinforced concrete columns under eccentric loads

AbstractThe use of carbon fiber reinforced polymer (CFRP) has proved to be significantly efficient in improving the overall behaviour of reinforced concrete columns. The use of high‐strength concrete in columns not only reduces the size but also gives adequate durability to the structure. The behaviour of reinforced concrete columns depends on numerous parameters, including concrete compressive strength, percentage of steel, cross‐section dimensions, ties spacing and the slenderness ratio. In this work the following parameters are investigated: Concrete compressive strength, percentage of steel, and the slenderness ratio. The focus of experimental investigation was to study the influence of carbon fiber reinforced polymer wrapping on the behaviour of plain concrete columns. Following the experimental investigation, a detailed numerical investigation was conducted to study the influence of the above parameters on the behaviour of eccentrically loaded plain and reinforced concrete columns. The influence of carbon fiber reinforced polymer wrapping on the overall performance was also studied using the available models for predicting the load‐moment interaction of columns to confirm the findings of the numerical investigations. Based on the detailed experimental and numerical investigation it was found that the concrete compressive strength significantly influences the overall behaviour of the reinforced concrete columns subjected to eccentric loadings.

Vehicular Impact Resistance of FRP-Strengthened RC Bridge Pier

As-built bridge piers are likely to be subjected to vehicular collisions, causing severe damage to the bridge substructure and even resulting in the collapse of the entire bridge. This study aimed to evaluate the effectiveness of fiber-reinforced polymer (FRP) strengthening in enhancing the vehicular impact resistance of as-built bridge piers. A typical local, refined finite-element (FE) model of a simply supported two-span, double-column, reinforced-concrete (RC) bridge was established. The adopted material constitutive models and the FE analytical approaches were then validated through reduced vehicle model lateral impact tests on bare and carbon fiber–reinforced polymer (CFRP)-strengthened RC columns. The effective schemes for FRP strengthening of bridge piers against vehicle impact are discussed from the aspects of fiber orientation, number of layers, and strengthened height as well as by FRP type. Based on the FRP strengthening scheme, 48 vehicle–bridge collision scenarios were designed to evaluate the collapse of bridges with/without FRP-strengthened piers. It was found that (1) external FRP wrapping can effectively enhance vehicular impact-resistance of as-built bridge piers (e.g., reducing the degree of damage and deformation of the impacted pier); (2) for typical bridge piers, the effective strengthening scheme comprised a fiber orientation of 0° in reference to the circumferential direction, a 3-m strengthened height, and four-layer CFRP wrapping; (3) compared with bare bridge piers, FRP-strengthened piers could improve the vehicular impact-resistant safety redundancy of as-built bridges and, to some extent, avoid bridge collapses under truck collisions at high speed. Furthermore, through introducing the dynamic increase factors of concrete strength, reinforcement rebars, and FRP, and considering the nonuniform distributions of the strain rate of FRP-strengthened bridge piers caused by vehicular collision, the dynamic shear capacity of strengthened bridge pier was formulated.

Axial performance of grouted C-Shaped concrete block masonry columns jacketed by carbon and glass FRP

To overcome the relatively low ductility and capacity of masonry elements, the confinement technique may be an effective alternative for improving their performance, especially the columns. Fiber Reinforced Polymers (FRP) jacketing is a potential approach has benefits in terms of strength enhancement and gaining ductile behavior. Therefore, in this study, two types of FRP were used to improve column axial capacity and ductility: carbon and glass FRP. The effect of FRP wrapping on the compressive behavior of concrete masonry columns was investigated experimentally. Fully grouted scaled masonry columns constructed using C-shaped concrete blocks were wrapped with FRP jackets and tested under concentric compressive load until failure. For this purpose, 18 half-scale masonry prisms under variations in FRP jacketing and FRP thickness were developed and tested in six sets. The effect of FRP confinement on the unconfined masonry columns was evaluated. The CFRP and GFRP jacketing increased the peak strength and ultimate axial strain. Confinement improves post-peak performance by softening the descending stress–strain relationships. Increasing FRP jacket thickness improved the axial strain, strength, and ductility of masonry columns. The CFRP wrap gives 10 ∼ 16% more strength than GFRP. Ultimate strain enhanced by 76 % and 57 % for CFRP and GFRP respectively.

Cyclic behaviour of as-built and strengthened existing reinforced concrete columns previously damaged by fire

A structure, during its life, may be subjected to multiple hazards. These hazards are sometimes combined over a short period of time, or in some cases occur many years apart, with the first hazard influencing the structural response under a second hazard. A reinforced concrete (RC) structure previously damaged by fire and then exposed to seismic loading is one such example. To assess such structures, the effects of fire on the cyclic performance of RC elements needs to be better understood. Moreover, it is also important to develop and validate strengthening methods that can reinstate or improve the seismic performance of fire-damaged RC elements. This paper presents the results of a novel experimental campaign where six full-scale RC columns with detailing representing existing Mediterranean buildings designed to old seismic codes are subjected to fire and then cyclic loading. Four RC columns were damaged after exposure to 30 or 90 min of the ISO 834 standard fire curve in a furnace and then tested under uniaxial cyclic lateral loading up to failure. Two of these columns were repaired and strengthened post-fire with Carbon Fibre Reinforced Polymer (CFRP) wrapping. The strengthening method aimed to increase the concrete strength through confinement, and to increase the displacement ductility and energy dissipation capacity under seismic loading. Two additional control columns, one as-built and another strengthened, were cyclically tested for comparison with the fire-damaged columns. It was found that the 30 min fire exposure resulted in few concrete cracks, whilst cover spalling and general cracking was observed in the 90 min fire exposure. A significant decrease in the displacement ductility and dissipated energy of the columns was observed following fire exposure, even for the 30 min fire. The columns that had post-fire repair and CFRP strengthening, showed better cyclic performance than the control column without fire exposure. It was also found that post-fire strengthened columns may reach similar seismic performance than similarly strengthened columns without previous fire damage.

Crack propagation law at the interface of FRP wrapped coal-backfilling composite structure

The coal-backfilling (CB) composite structure jointly bears the excessive stress. The interface between the coal pillar and backfilling is the weakest area. The instability of the interface can induce the splitting failure of the composite structure, and fibre-reinforced polymer (FRP) could be applied to solve this problem. The Brazilian splitting laboratory tests were carried out on the CB composite specimens and FRP wrapped CB composite specimens (WCB) with different interface angles (α = 0°, 15°, 30°, 45°, 60°, 75°, 90°). The failure modes of the above specimens were monitored by using the digital image correlation (DIC) technology. The stress intensity factors (SIFs) and the energy release rates (ERRs) at the interface centre of the composite specimens were calculated. The result shows that three types of failures, namely interfacial crack, tensile crack, and concentrated failure zone, appeared on the composite specimen. The interface angle was an important factor affecting the failure modes of the composite specimens. Interfacial stability of composite specimens could be enhanced by FRP wrapping. The SIFs KI and KII increased, and the expansion of the interface cracks was hindered by FRP wrapping. The critical load in the stable stage and the ERR of specimens were improved by FRP wrapping.

A fragility-oriented approach for seismic retrofit design

This study proposes a practical fragility-oriented approach for the seismic retrofit design of case-study structures. This approach relies on mapping the increase of the global displacement-based ratio of capacity to life-safety demand ( CDRLS) to the building-level fragility reduction. Specifically, the increase of CDRLS due to retrofitting is correlated with the corresponding shift in the fragility median values of multiple structure-specific damage states, observing that a pseudo-linear trend is appropriate under certain conditions. Accordingly, a practical approach is proposed to fit such a (structure-specific) linear trend and then use it by first specifying the desired fragility median and subsequently finding the corresponding target value of CDRLS that must be achieved through retrofit design. The validity of the proposed approach is illustrated for an archetype reinforced concrete (RC) structure not conforming to modern seismic design requirements, which has been retrofitted using various techniques, namely, fiber-reinforced polymers wrapping of columns and joints, RC jacketing, and steel jacketing.