Strengthening System Research Articles

Pull-out test of carbon-fiber composite cable (CFCC) tendon for an internal anchorage

An internal strengthening system involving the embedding of a post-tensioned prestressing tendon in a wedge-shaped anchorage of an existing concrete member was developed previously. The previous study confirmed the adequate load capacity of the prestressing tendon for the internal strengthening technique. The tendons used in the previous investigation were steel-rigid bar and steel-wire cables. These steel tendons are prone to corrosion under severe environmental conditions, such as bridge deck slabs sprayed with deicers. Carbon fiber composite cables (CFCC) are occasionally employed as tendons in prestressed concrete bridges. This study focused on the non-metallic material and examined the load capacity of CFCC tendon in internal strengthening systems to achieve the feasibility of the technique under corrosive environment. The main concern of the novel strengthening system was the anchoring system of the CFCC in the internal wedge. In general, both ends of the CFCC tendon can be gripped with external fasteners and be provided a tensile force using a hydraulic jack. In an internal prestressing system, a CFCC end must be anchored without any external fastener. It is preferable to simply fix the CFCC in a small wedge anchor made of existing concrete. This study focused on developing a CFCC anchoring method in a wedge-shaped hole and examined the tensile load capacity of the CFCC tendon fixed in the anchorage. The end of the CFCC tendon was reversely twisted to improve the pullout resistance by increasing the diameter of the cable. To examine the load-bearing capacity of the developed CFCC tendon, a mock-up reinforced concrete (RC) block with an internal wedge-shaped anchorage was used. A CFCC tendon equipped with a reverse-twisted tip was inserted into a wedge-shaped hole filled with high-strength grout. A pull-out test of the CFCC tendon was conducted to examine its load-bearing capacity. The test results revealed that the CFCC tendon with a reverse-twisted tip embedded in the anchorage exhibited adequate load-bearing capacity. This study confirmed that the post-tensioned prestressing system using the CFCC tendon is appropriate for strengthening RC members having the thickness of at least 200 mm, such as bridge deck slabs.

Efficiency of CFRP torsional strengthening technique for L-shaped spandrel reinforced concrete beams

Abstract The present study aims to get experimentally a deeper understanding of the efficiency of carbon fiber-reinforced polymer (CFRP) sheets applied to improve the torsional behavior of L-shaped reinforced concrete spandrel beams in which their ledges were loaded in two stages under monotonic loading. An experimental program was conducted on spandrel beams considering different key parameters including the cross-sectional aspect ratio (i.e., web height/web thickness), and the availability of the CFRP strengthening system. The ledge of the spandrel beams was exposed during testing to a very high eccentric load, which was transferred to the web of the spandrel beam causing high shear, torsion, and bending moments. Consequently, the applied load resulted in in-plane and out-of-plane deformations of the web accompanied by flexural and shear cracks. This article demonstrates the advantage of using CFRP sheets to strengthen the mentioned members. The applied CFRP sheets increased the failure torsional load by about 37% compared to the identical L-spandrels without strengthening. The outcomes indicate that using CFRP sheets show improvement in restricting the deflections and rotation of L-spandrels due to increasing spandrel stiffness. The reduction in the degree of rotation attained more than 33% in comparison to the spandrel beams without strengthening. The experimental program confirmed the applicability of the proposed strengthening technique for compacted and slender L-shaped spandrel-reinforced concrete beams.

On Mechanical Behavior of Metal Anchors in Historical Brick Masonry: Testing and Analytical Validation

The repair and strengthening of historical masonry buildings is a fundamental aspect in the conservation of the built cultural heritage. Temporary shoring or strengthening are often used and, usually, involve the introduction of new metallic elements. The connection between the original substrate and the new elements must be analyzed carefully to prevent further damage to the building. This paper presents a study on the mechanical behavior of metal anchors applied to brick masonry walls. An experimental campaign is developed, and a series of pull-out tests are carried out on masonry walls built in a laboratory with natural hydraulic lime mortar and low mechanical strength bricks. Two groups of tests are conducted, namely, with the actuator in the direction of the anchor axis and with the actuator inclined with respect to the fastener axis. Moreover, two types of anchoring systems are used, namely, adhesive (chemical and cementitious grout) and mechanical anchors. The experimental results are compared to predictive analytical formulas available in the literature for estimation of the ultimate load capacity, according to the type of failure. From the comparison between experimental and analytical values, it is proven that the analytical formulation originally developed for concrete substrates cannot be directly extrapolated to brick masonry cases, and specific predictive formulas should be developed. The presented research can be used to select the most efficient anchoring system for strengthening and retrofitting of historical brick masonry structures.

Modelling of CFRP-Strengthened RC Shear Walls with a Focus on End-Anchor Effects

This study first provides an overview of the development of a novel tube anchor system for the seismic strengthening or repair of reinforced concrete (RC) shear walls with carbon fibre-reinforced polymer (CFRP) sheets. The new anchor system can significantly improve the load transfer mechanism between the CFRP and supporting RC structural elements, resulting in ductile behaviour of the strengthened shear walls with increases of lateral load capacity and ductility by up to 2.6 and 8.3 times, respectively. The study then presents a new finite element modelling technique capable of capturing the complete cyclic response, i.e., from the elastic behaviour to the ultimate collapse of CFRP-strengthened RC shear walls with the newly developed tube anchor system. Two different modelling approaches are proposed to consider the effects of the tube anchor system. Additionally, other important CFRP- and RC-related mechanisms, including CFRP debonding effects, confinement enhancement, tension stiffening, compression softening, and strength and stiffness degradation under cyclic loads, are also considered in the model. By comparing the analytical and experimental results, it is demonstrated that the proposed modelling approach can accurately replicate the complex behaviour of CFRP-strengthened shear walls with a wide range of aspect ratios, from the ductile flexural behaviour of slender walls to the brittle shear failure of squat walls, without requiring detailed modelling of the anchor system.

Strengthening System Effects on the Out-Of-Plane Mechanisms Activation of Masonry Walls under Tsunami Loads

The vulnerability of masonry structures against tsunami loads is a highly debated topic in the research community due to the impact in the risk evaluation. The main aim of this paper is to examine the structural response of masonry walls against tsunami loads in terms of out-of-plane local mechanism activation. Furthermore, a critical discussion is proposed about the influence of strengthening system parameters on the out-of-plane response of the masonry wall. Results of parametric analyses are shown in dimensionless form to analyse the effects of main parameters, both for masonry walls and tsunami waves, on the structural response. The analyses results are the bases to design strengthening systems with fiber-reinforced composite materials in order to reduce the vulnerability of masonry structures under tsunami loads.

A novel electropulsing treatment to improve the surface strength and repair the pore of additively manufactured Ti-6Al-4V alloy

The surface strength and pore content in electron beam melted (EBM) Ti-6Al-4V alloy play a key role in its tribological performance and fatigue life. In this study, a novel surface electropulsing treatment was applied to generate local phase transformation strengthening on the surface of EBM Ti-6Al-4V while simultaneously repairing the pores in the hardened zone. The surface microhardness was enhanced by 36 % from ∼260 HV0.2 to ∼355 HV0.2 when electropulsing treated for 120 ms, and keep the hardness with the prolongation of time to 240 ms. The thickness of the hardened surface increased from 1.3 mm to 1.7 mm with the increase in processing time. In addition, the hardened zone showed much fewer pores than the matrix. Mathematical simulation indicated that the high cooling rate in the treated zone was sufficient to result in a martensite transformation, and the compressive stress induced by thermal stress and electrode loading could cause deformation and furtherly improve the strength. More importantly, the combination of the large-scale compressive stress with the current crowding effect induced compression in the hardened zone is a benefit for repairing the pore. Predictably, this novel electropulsing treatment can be extended to other AM α + β Ti alloy systems and AM martensite steel systems for strengthening and pore repair.

Improving the shear behavior of RC dapped-end beams using precast strain-hardening cementitious composite (P-SHCC) plates

Dapped-end beams (DEBs) are frequently used in precast concrete structures in order to facilitate the construction procedures. However, premature failure is most likely to happen at their re-entrant corner owing to the immense shearing stresses in this region. To mitigate such a problem, a new strengthening system making use of Strain-Hardening Cementitious Composites (SHCC) material was utilized in this paper. For this purpose, an experimental study on ten DEBs specimens, including one defected un-strengthened specimen (BC), was prepared and tested under a monotonic 4-point loading protocol to assess the efficiency of the adopted strengthening system. The traditional reinforcement at the re-entrant corner was substituted by precast SHCC (P-SHCC) plates to upgrade the shear strength and serviceability of the tested DEBs. The P-SHCC plates were pre-cured for about 4 weeks before being used to mitigate any volumetric change issues related to SHCC. A slight amount of reinforcement was provided inside the P-SHCC plates to enhance their cracking behavior and avoid strain localization. The studied parameters were the scheme of P-SHCC plates at the re-entrant corner (continuous or intermittent/discontinuous), the orientation of P-SHCC plates (vertical or 45°-inclined), as well as their position with regard to the beam's cross-section (external or internal). The thickness of each external P-SHCC plate was kept constant at 20 mm so as to be easily incorporated into the concrete cover, whereas the internal P-SHCC plates had varied thicknesses (40, 55, or 70 mm). The test results revealed that providing DEBs with P-SHCC plates could significantly improve both shear capacity and the developed ductility. The gain in the ultimate load reached up to 91 % relative to the un-strengthened specimen (BC). Furthermore, a great reduction in the developed major shear crack width for all strengthened DEBs was attained. At the service load level, all strengthened DEBs showed a decreased crack width, with an average value of about 77 % compared to the specimen BC. Finally, two equations considering the contribution of P-SHCC plates were proposed to predict both the shear strength and the average crack width of the strengthened DEBs. The proposed equations showed good agreement with the experimental results.

Global inequity creates local insufficiency: A qualitative study of COVID-19 vaccine implementation challenges in low-and-middle-income countries.

The COVID-19 pandemic has amplified pre-existing challenges to health promotion and care across the world, and particularly in low- and middle-income countries (LMICs). This qualitative study draws on data from a panel of immunisation experts and uses a novel framework of vaccine delivery domains to explore perspectives from those who live and work in these settings on the challenges to implementing COVID-19 vaccine programs in LMICs. We conducted a thematic content analysis of 96 participant free text replies to questions from Round I of a three-round Delphi consensus study amongst global experts on COVID-19 vaccine implementation. Participant responses highlighted challenges to vaccine program implementation including issues related to equity; governance, decision-making, and financing; regulatory structures, planning, and coordination; prioritisation, demand generation, and communication; vaccine, cold chain, logistics, and infrastructure; service delivery, human resources, and supplies; and surveillance, monitoring, and evaluation. We reflect on our findings in light of global efforts to address vaccine inequity and emphasise three key areas salient to improving vaccination efforts during novel infectious disease outbreaks: 1) Ensuring safe and sustainable service delivery in communities and at points of care; 2) Strengthening systems for end-to-end delivery of vaccines, therapeutics, diagnostics, and essential supplies; 3) Transforming structural paradigms towards vaccine equity.

Modeling and prediction of short/long term mechanical behavior of FRP-strengthened slabs using innovative composite finite elements

The analysis of plates or slabs strengthened with fiber reinforced polymer (FRP), especially when it comes to geometric irregularity and the time-varying characteristics of adhesive, presents multi-faceted simulation, modeling and computational challenges. In this paper, two new multilayer composite plate elements are proposed to model and analyze FRP-strengthened slabs, aiming at meeting these challenges with efficiency. The novelty relies on that the innovative elements realize the modeling of substrate, adhesive layer and FRP sheet in one “macroscopic” element rather than modeling each component by separate elements. One of them is capable of emulating geometric irregular structures while the other has better accuracy, and the flexible combination of them can be used to achieve accurate simulation and calculation of various strengthening systems. Another novelty is the extension of the study from elastic to viscoelastic by introducing an appropriate rheological interface and correspondence principle. The consistency with literature verifies the correctness of the proposed elements, subsequently, they were used to investigate the influence of the FRP tapering ends and strengthening forms on the reinforcement effectiveness. It is found that the proposed method is convenient to predict the short/long term mechanical behavior of FRP strengthened slabs with accuracy and computational efficiency, thus contributing to the design and optimization of the strengthening system.

Effect of the strengthening patterns on the flexural performance of RC continuous beams using FRP reinforcements

Statically indeterminate RC elements such as continuous beams are the most widely used structural form. Their flexural behavior and failure modes are considerably different from those of simply supported ones. The use of FRPs for flexural strengthening and retrofitting continuous RC beams has hardly been studied. Therefore, this paper compares the impacts of NSM-FRP and EB-FRP systems in two-span beams. The overall experimental program of this study consisted of fourteen large-scale bending tests. The performance of each strengthening system is presented and discussed in terms of failure mode, yielding and load-carrying capacities, ductility, and energy absorption capacities, FRP debonding strain, and serviceability state. The experimental results showed that the failure mode is mainly dependent on the type, length, and position of the FRPs, however, a general improvement in the flexural performance of strengthened beams was observed. The NSM technique allows the FRP tensile strength to be better exploited than the EB technique.

STRENGTHENING OF ANCHOR CHANNELS ON THE CONCRETE SURFACE

AbstractFor the installation of curtain wall systems, the positions of anchor channels gradually move closer to the edge of the concrete slabs. But close to the edge of the slab, conventional supplementary reinforcements have problems to redistribute the loads to the reinforcement due to the short bond length that is available after concrete edge failure occurred. Therefore, tests have been conducted to investigate the behaviour of anchors channels, close to the edge. To achieve this, a strengthening system on the surface of the concrete with post installed fasteners was developed. First results indicate a high potential for strengthening the channels close to the edge under shear load. With these test results numerical simulations with Atena Science were performed and, after its calibration, the model was used for further investigation.

Evaluation of flexural strengthening methods for beams with simulated deterioration using spike-anchored FRP externally bonded sheets and near-surface-mounted strips

• 20 beams were tested to evaluate FRP flexural strengthening techniques. • Specimens were detailed to mimic the common deterioration of bridge beams. • The effect of FRP spike anchor arrangements were evaluated. • Results show that more FRP spike anchors may not translate to superior performance. • Unsymmetrical steel and NSM reinforcement layouts were considered. In consideration of past research on both externally bonded and near-surface-mounted (NSM) fiber-reinforced polymer (FRP) reinforcement for the strengthening of deteriorated reinforced concrete elements, additional work is needed to investigate the application of FRP for flexural strengthening in specific scenarios and for specific member types. Furthermore, the need exists for guidance regarding reliable anchorage of externally bonded sheets for flexural strengthening. An experimental investigation was conducted on 20 reinforced concrete beam specimens with details and simulated deterioration patterns representative of the beams of in-service adjacent box beam bridges. Special focus was placed on the effective arrangement of FRP spike anchors for externally bonded FRP, unsymmetrical deterioration of steel reinforcement, and the eccentric placement of NSM strips. The results show that spike anchors located only at the ends of the externally bonded FRP sheets provided superior performance compared to anchors placed at multiple locations along the sheets. Moreover, the eccentricity of reinforcement did not play a significant role in the effectiveness of the strengthening systems. The test results also highlight the importance of bond on the performance of members strengthened with NSM reinforcement. Overall, the findings extend beyond adjacent box beam bridges and have a wide applicability to other flexural members.

Flexural strengthening of full-scale RC columns with Adhesively-bonded longitudinal CFRP plates: An experimental investigation

Conventional method of flexural strengthening of RC columns using carbon fiber reinforced polymer (CFRP) composites is transverse wrapping. However, for rectangular columns, this method is not effective or requires modification of column geometry. In this paper, a new configuration of rectangular RC column flexural strengthening system is investigated. The new system includes prefabricated CFRP plates vertically attached on column surfaces, and a short concrete jacket to prevent debonding of the CFRP composites. Five full-scale rectangular RC columns strengthened by either concrete jacket or vertical CFRP plates were tested under cyclic loading. Cyclic performance of the strengthened columns, and strain distribution of CFRP composites on the tension side were investigated. Test results showed that failure modes of rectangular RC columns were changed by application of CFRP composites. Vertical CFRP plates are effective in improving ultimate capacity and stiffness of rectangular RC columns under cyclic loading.

Shear strengthening of RC beams using FRCM system made with industrial wastes

In the present study, industrial wastes were used as a partial replacement for cement to form cement-based repair mortars, and the feasibility of using these mortars as the matrix in fiber reinforced cementitious matrix (FRCM) system for shear strengthening of reinforced concrete (RC) beam specimens was investigated. Accordingly, an experimental program comprising two parts was carried out. Firstly, the properties of the mortars were examined through compressive strength, tensile strength, sulfuric acid resistivity, and electrical resistivity tests. Finally, 6 shear-critical RC beam specimens were manufactured, two of which remained un-strengthened acting as control specimens. Two specimens were strengthened in shear by carbon fiber reinforced polymer (CFRP) strips and two specimens were strengthened by fiber reinforced cementitious matrix (FRCM) layers made with prepared mortars. The shear capacity of the specimens was measured through 3-point bending test. The parameters investigated in the beam specimens included the effect of shear strengthening system (FRP/FRCM) and shear span/depth ratio. It was shown that the specimens strengthened by the FRCM system made with waste materials had similar or slightly better performance compared to the FRP strengthened specimens.

Prediction of compressive strength and evaluation of different theoretical standards and proposed models of brick columns confined with FRP, FRCM, or SRG system

The strength capacity of confined masonry column is one of the topics that need to be studied. In this study, the efficiency of using different types of advanced composite (non-corrosive materials) such as fiber reinforced polymer (FRP), fiber reinforced cementitious matrix (FRCM), or steel reinforced grout (SRG) in confining masonry columns is investigated. A wide range of experimental database of masonry column specimens has been collected from the results that available in scientific literatures. Different theoretical standards and proposed models that used to predict the capacity of masonry columns confined with FRP and FRCM are evaluated based on collected experimental database. Since there is no standard code or specific proposed model for SRG system, the confined capacity of this system is predicted and evaluated using the FRCM proposed models. The justification of using these models is that both FRCM and SRG systems have the same concept of using inorganic material as a paste material. An index named “equivalent fiber reinforcement index (EFRI)” is proposed to capture the key factors that control the behavior of the confined masonry columns with different advanced composite. This index is used as reference parameter for the purpose of the comparison between different strengthening systems. As a result, all types of advanced composite presented a significant increase in ultimate capacity. Also, the behavior of the masonry columns was significantly dependent on the type of fabric used. Different modes of failure were reported, including crushing of masonry block, as well as a deboning of FRP from the masonry substrate and deboning or slippage of fabric within inorganic paste matrix. Compared with other models and standards, the American Concrete Institute Committee 440 (ACI 440) and American Concrete Institute Committee 549 (ACI 549) shows very good predictions for the confined capacity of masonry columns strengthened by FRP and FRCM or SRG respectively.

Experimental and analytical study of RC beams strengthened with prestressed near-surface mounted 7075 aluminum alloy bars

Recently developed high-strength aluminum alloys (AA) have great potentials for strengthening and repairing RC structures because of their merits in high strength-to-weight ratio, high ductility, and inherently corrosion-resistant behaviors. However, few studies have been conducted on taking advantage of AA bars for rehabilitation. This study was therefore carried out to reveal the feasibility of 7075 AA bars on near-surface mounted (NSM) applications for flexural strengthening of reinforced concrete (RC) beams. Experiments of four-point flexural tests were presented to assess the performance of NSM AA bar strengthened beams through eight concrete beams, including a control specimen without strengthening, a specimen strengthened by conventional basalt fiber reinforced polymer (BFRP) bars served as reference, and other six ones using NSM AA strengthening technique with various prestress level and bar arrangement. Results indicate that the application of the strengthening system was able to prevent premature debonding failure and increase the load-bearing capacity by up to 102.1%, but reduced the deformability. In addition, an analytical model for capturing the load–deflection behavior of various strengthening schemes was developed. Good agreements between the results from observations and predictions validated the proposed model, suggesting its potential for guiding the design of the AA strengthening schemes.

Flexural behavior of strengthened concrete beams with multiple retrofitting systems

Due to material deterioration, structure age, changes in building function, fault design, and decreased structural reliability brought on by natural catastrophes, it may require to strengthening the reinforced concrete (RC) beams. The entire carbon footprint is increased by the usage of building materials like cement. Therefore, restoring/improving their overall functionality may be accomplished more sustainably by strengthening and repairing damaged members. In this study, a series of RC beams were tested in three-point bending to determine the ability of various strengthening techniques to improve the beams' flexural capacity. Three strengthening techniques were proposed: External bonding with glass fiber-reinforced polymer (GFRP) laminates, near-surface mounted (NSM), and U-jacket. The experimental study compared the various strengthening methods in terms of ultimate loads, crack loads, maximum deflections, gains in ductility, and energy absorption. The finite element program (ABAQUS) was used for the numerical computations because it is capable of properly simulating experimental research on the flexural behaviour of reinforced concrete beams. Compared to all techniques, the strengthening using GFRP in the different techniques (GFRP laminate, NSM, and U-jacket) propagation of cracks before failure occurred in the RC beam. The sample B3-St. plate-NSM, which used the NSM technique, had the highest peak load of 77.2 MPa compared to all samples and a 36.64% increase over the control sample B0-Control. Moreover, the sample B3-St. Plate-NSM has improved stiffness which contributed to reducing the deflection at peak load by 21.12% than the control sample. The strengthening system using steel plate as NSM showed the lowest cost ($/m`) among the other strengthening systems. The specimen B1-GFRP, which used the GFRP technique, had the highest toughness of 1099.2 kN.mm by an increase of 40.25% over the control sample. Finally, the results obtained by using numerical FE models show good agreement with the experimental results.

Structural strengthening of masonry elements by reinforced repointing combined with FRCM and CRM

Within the framework of the seismic vulnerability of existing masonry constructions, a current issue is represented by the need of performing structural strengthening interventions to increase or restore the capacity of these buildings to resist seismic actions. Retrofitting solutions are therefore often proposed using innovative materials and techniques, such as Fiber Reinforced Cementitious Matrix (FRCM). It should be highlighted that the application of FRCM usually consists of a continuous reinforcing system covering the entire surfaces of the masonry panels. These strengthening solutions are very effective in improving the structural behavior of existing masonry elements, but they are not applicable when the surface of the masonry should remain visible, i.e., in historical buildings. Therefore, other techniques should be adopted, such as the reinforced repointing technique. In this paper, a series of diagonal compression tests are performed on clay brick masonry panels strengthened using different techniques, involving the use of reinforced repointing combined with the application of Fiber Reinforced Cementitious Matrix (FRCM) and Composite Reinforced Mortar (CRM). The tested masonry panels are strengthened either symmetrically, by using FRCM on both sides, or unsymmetrically, by applying the reinforced repointing technique on one side and FRCM or CRM on the other side of the panels. The results of the experimental campaign shows that the strengthening systems can improve the structural behavior of the masonry panels, both in terms of bearing capacity and ultimate deformations.

Preliminary considerations on coupled pin-supported walls as a strengthening solution for existing buildings

In the last years, attention to the seismic strengthening of existing buildings has increasingly grown, especially since a huge number of national incentives has been allocated for a deep renovation of the existing building stock. Various research has pushed toward new solutions suitable to account for the entire life cycle of the retrofit solution and that would overcome the main barriers to the renovation, which are, for example, the need to relocate the inhabitants. In this work, a strengthening system that involves the use of coupled pin-supported walls is proposed. The solution could be carried out from outside of the building thus not requiring the inhabitants’ relocation. In particular, two pin-supported walls are coupled through horizontal beams connected to the walls by unbounded post-tensioning cables. The effectiveness of the proposed solution has been evaluated through the application to a reference case.

Intrados CFRCM strengthening of masonry arches

This paper describes the preliminary experimental results obtained from destructive cyclic tests on masonry arches strengthened with FRCM (Fiber Reinforced Cementitious Matrix) strengthening system applied onto the arch intrados.The scaled models of the arches used in the tests, reproduce the geometry of the high span ratio bridges typically found in the Italian railway lines. They were tested with a non-symmetrical vertical load that involves a collapse mechanism with 4-hinges similar to the typical seismic collapse mechanism of masonry arches. The experimental campaign made it possible to assess the increase in the arch resistance with the FRCM strengthening system, to simulate the load-displacements curve and to analyze the collapse mechanism of the unstrengthened and strengthened specimens.