Proposed Strengthening System Research Articles

Strengthening of reinforced concrete slabs using carbon fiber reinforced polymers rods and concrete jacket with a mechanical anchorage system

This study presented the development of a new method for strengthening of Reinforced Concrete (RC) slabs using Carbon Fiber Reinforced Polymer rods (CFRP) and an Ultra-High Performance Fiber Reinforcement Concrete (UHPFRC) external jacket with a Mechanical Anchorage System (MAS). The mechanical anchorage system includes two components: high-carbon steel plates and the Mechanical Expansion Anchorage Bolt System (MEABS), and it’s employed to prevent any premature de-bonding between the existing concrete surface and the externally strengthening layers (CFRP rods and UHPFRC jacket). The efficiency of the proposed strengthening method was evaluated through conducting an experimental test on a few fortified slabs by applying cyclic loads using a dynamic actuator. For this purpose, three distinct concrete slabs were tested to assess various design parameters, including a control slab, a strengthened slab with the UHPFRC jacket only, and a strengthened slab with CFRP bars at the bottom of the slab with an external UHPFRC jacket. The results of experimental tests reveal that the proposed strengthening system significantly enhanced the load capacity of the slab and prevented premature debonding failures between the old concrete of the slab and the new UHPFRC layer until the stage of slab failure. Accordingly, the new proposed strengthening system played a crucial role in enhancing the tension zone of the slabs and delaying the occurrence of diagonal crack loads. On the other hand, embedding CFRP bars within the UHPFRC jacket in the strengthened slab led to a notable enhancement of 82 % in the ultimate load capacity of the slab. The FE and analytical models were developed to predict the behavior of the specimens. The models' outcomes were in good agreement with the experimental data. Consequently, the new proposed strengthening system emerges as a reliable technique for enhancing the performance of reinforced concrete slabs, eliminating the risk of debonding between old and new parts.

Strengthening of two-way slabs with fiber-reinforced polymer composites: A new system using grooving technique and fans

The flexural capacity of a reinforced concrete (RC) two-way slab can be efficiently enhanced using fiber-reinforced polymer (FRP) composites if FRP debonding is postponed. On the other hand, with the increase in flexural capacity, the vulnerability of the slab to punching shear failure increases. In this context, the current study examines FRP strengthening of lightly reinforced slabs using the externally bonded reinforcement on grooves (EBROG) technique, which has already been proven successful in postponing FRP debonding. Four slab specimens are tested in two groups under concentric loading. The first group consists of two specimens reinforced with headed shear studs: one reference specimen and one specimen strengthened in flexure with FRP. The second group includes two specimens without internal punching shear reinforcement: one reference specimen and one specimen strengthened with a new simultaneous flexural and punching shear strengthening system, taking advantage of FRP fans. According to the results, flexural strengthening of the slab in Group 1 increased its ultimate load capacity by 77%, and the proposed strengthening system in Group 2 led to a 94% enhancement of the ultimate load capacity. A calculation method based on the yield-line theory is presented to predict the load capacity of the test specimens. According to this method, graphs simplifying the estimation of the flexural capacity of FRP-strengthened slabs are proposed.

Bed joint reinforcement of shear walls using threaded titanium rods

Shear walls in historic constructions often require retrofit. The lateral load capacity is frequently insufficient to withstand seismic actions. Threaded titanium rods embedded in mortar bed joints have emerged as a viable solution to reinforce brickwork shear walls. The use of a high tensile strength and ductile material (titanium alloys), with a high degree of reversibility of the retrofit intervention, is promising as it represents a new opportunity in masonry conservation. Aiming at studying the effectiveness of the titanium rod repair, this research had a twofold goal: 1. laboratory tests were performed to investigate the structural response of reinforced walls at macro-scale level; 2. a numerical modelling strategy was implemented to fill the knowledge gap regarding the material properties of brickwork masonry (English and Flemish bond). Quasi-static cycling loading tests performed on full scale (1200x1200 mm, two wythes thickness) specimens demonstrated how bed joint reinforcement using threaded titanium rods produced a significant increase in the lateral load and deformation capacities. To provide an effective tool for predicting all material parameters, as input data, and for designing the proposed strengthening system, a step-by-step procedure - based on the use of digital image processing (DIP) techniques, homogenization theories and micro-modelling strategies - was therefore proposed.

Experimental assessment of the thermo-mechanical bond behavior of NSM CFRP with cement-based adhesives

Cement-based materials, which are incombustible and show low thermal diffusivity, show great potential as a valid alternative to polymeric matrices for bonding near surface mounted (NSM) carbon fiber reinforced polymer (CFRP) systems for the strengthening of reinforced concrete (RC) structures. The performance of these materials in different loading conditions, including high temperature exposure, still needs to be better explored. This paper presents a study on the characterization of the bond behavior using cement-based adhesives and CFRP strips with innovative surface treatment. The CFRP strips used in this study were manufactured specifically with sand-treated surfaces to improve the bonding performance between CFRP and cement-based adhesive by taking advantage of the better grip of increased roughness surfaces. The results of the pull-out tests in ambient conditions, as well as exposed to elevated temperatures, are presented and discussed. The effect of using sand particles of different sizes, adhered with two different resins on the CFRP strips surface embedded in different cement-based matrices, was compared. The obtained results showed that by applying a sand treatment on the surface of CFRP strips, the maximum pull-out force was up to 12 times higher than when using smooth surfaced strips. Pull-out tests showed that, by using the NSM sand treated CFRP strips and CFRP bars with cement-based adhesive, an average bond strength up to about 9 MPa and 11 MPa, respectively, could be achieved. Thermo-mechanical pull-out tests showed the proposed strengthening system is able to withstand up to 30 min of heat exposure before bond failure.

Design of filling self-stressing concrete to use in strengthening of columns by prefabricated jacketing

This study proposes the design and characterization of an expansive self-compacting concrete (SCC) with self-stressing properties for its use as a filling in reinforcement systems by external prefabricated jacketing. The self-compacting characteristic ensures the correct filling of the inner space and the characteristic of expansiveness (self-stressing) guarantees a correct transmission of tensions from the column to the prefabricated reinforcement jacket. In the design, a K-type expansive agent based on calcium sulfoaluminate is used in the doses of 10% and 15% and the influence of its addition on the fresh properties and the compressive strength on the reference mix of SCC is analysed. The restrained expansion is evaluated in prisms by the standard method ASTM C878 and by means of an alternative method based on a double ring device that simulates the configuration of reinforcement systems using carbon fiber reinforced polymers (CFRP) or steel external jacketing. Finally, the efficiency of the proposed strengthening system is compared to that usually used for CFRP jacketing directly glued on the element like concrete specimens and reinforced concrete columns. The results show that the double-ring based test device presents a high degree of correlation with the prism-based standard method and that the proposed strengthening based on filling self-stressing SCC and external prefabricated jacketings is an efficient solution that allows to move from a passive reinforcement system (CFRP jacketing glued directly to the column) to an active one that allows the reinforcement to contribute from the initial moments of the cast-in-place of the self-stressing SCC.

Flexural strengthening of RC slabs with nonprestressed and prestressed CFRP strips using EBROG method

This paper proposes a fully nonmetallic system for strengthening RC one-way slabs with prestressed CFRP composites. The flexural behaviors of RC slabs strengthened with nonprestressed and prestressed CFRP strips were compared through four-point bending tests. The externally bonded reinforcement on grooves (EBROG) method was utilized to delay premature debonding of the CFRP composites and was compared with conventional externally bonded reinforcement (EBR). The experimental results indicated a significant enhancement in slabs flexural performance of slabs that were strengthened via the EBROG method and the novel anchorage system. The nonprestressed and prestressed CFRP plates attached using the EBROG method increased the bending capacity by 34% and 77%, respectively, compared with the nonprestressed EBR CFRP plate. Furthermore, the maximum CFRP strain reached 8.9‰ for the nonprestressed EBROG CFRP strip, whereas that for the nonprestressed EBR CFRP strip was only 5.9‰. Thus, the EBROG method delayed CFRP debonding and increased the strain in the reinforcement by 50% compared with the EBR method. In the prestressed CFRP strip, the strain reached 12.4‰ prior to the debonding, confirming the high efficiency of the proposed strengthening system.

Flexural behavior of RC beams strengthened with BFRP bars-reinforced ECC matrix

This paper has proposed a new strengthening system made of basalt fiber-reinforced polymer (BFRP) bars-reinforced engineered cementitious composite (ECC) matrix (hereinafter referred to as “BFREM” for brevity) for repairing of RC beams. Six specimens including a control beam and five beams strengthened with the BFREM layers were prepared and tested. The test variables included the reinforcement ratio of BFRP bars (0.94% or 1.41%) and the installation method (prefabricated or cast-in-place) used for the BFREM layer as well as the presence of the end anchorage system (with steel bolts). The test results showed that two failure modes of the strengthened beams were the tensile rupture of BFRP bars and the concrete cover separation with the strengthening layer attached. The cast-in-place method incorporating the end anchorage system was capable of inhibiting the debonding failure of the BFREM layer, thereby making better utilization of the strengthening materials. Moreover, the proposed strengthening system was efficient in enhancing the ultimate loads of the strengthened beams, which were 34%–48% higher than that of the control beam. A theoretical analysis was then conducted to predict the flexural strengths of the tested beams, and its reliability was verified by comparing the test results and the theoretical predictions.

The effectiveness of FRCM-U-wrap joist: An alternative solution in flexural strengthening of reinforced concrete beams

This paper presents both experimental and analytical research results for predicting the enhancement of the flexural capacity of reinforced concrete (r.c.) beams strengthened in flexure with an innovative system based on the use of fabric reinforced cementitious matrix (FRCM). The patented system, called FRCM-U-Wrap Joist Tech provides a new methodology for beam strengthening capable of generating a higher internal couple of the cross section with no incisive change of the element weight. Experimental evidence gives back a 100% of increase in flexural strength than the non-reinforced beam. Analytical prediction shows a good accordance with the experimental data. The proposed strengthening system seems to be more effectiveness than the beam strengthened by means of traditional technology consisting in externally bonding the fibers directed to the beam intrados.

Design and testing of an adhesively bonded CFRP strengthening system for steel structures

In the framework of sustainable development policies, it is essential that infrastructure owners can rely on effective repair or strengthening solutions, designed and tested in relevance to actual service conditions. In the case of steel structures, fatigue damage is a major concern that can significantly affect the lifespan of the structure, and so far, there are very few operational methods capable of preventing fatigue cracks in the field. Adhesively bonded carbon fiber reinforced polymer (CFRP) composites are being successfully applied to the rehabilitation of concrete structures for more than two decades, and they are currently receiving much interest for the strengthening of steel elements, but mainly for curative purpose after severe damage has occurred.In the present study, which is part of a European project called FASSTbridge, a specific CFRP system has been developed as a preventive method against fatigue damage of steel structures. The proposed system consists of a commercially available ultra-high modulus (UHM) CFRP composite plate compatible with the stiffness of the host steel structures, which is bonded to the steel support using a novel hybrid epoxy/polyurethane adhesive. A first part of the paper presents the main specifications that should be adopted in the design of CFRP strengthening systems applied to steel structures, and that were identified from an extensive literature survey. These specifications have guided the development of the polymer adhesive and the choice of a peculiar commercial CFRP plate in the preliminary phase of the project. Experimental characterizations were then conducted (i) on the formulated hybrid polymer adhesive to optimize its curing schedule and check the previous specifications are fulfilled, and (ii) on CFRP reinforced steel specimens in order to verify the effectiveness of the proposed strengthening system. This experimental program involved both short term and durability tests that were performed by different laboratories. Such an inter-laboratory study made it possible to verify the performances of the developed strengthening system and to assess the influence of installation parameters and environmental conditions.

CFRP shear strengthening system for steel bridge girders

This paper presents an investigation undertaken to study the effectiveness of using small-diameter CFRP strands for shear strengthening of steel bridge girders. The study includes a comprehensive experimental program to study effects of the CFRP reinforcement ratio and orientation of the strands. An analytical model, calibrated by the experimental tests, was used to provide design recommendation. Results of the study showed that the proposed strengthening system is effective in increasing the shear capacity of steel bridge girders and there was no sign of CFRP debonding or rupture failure commonly observed by CFRP laminates up to approximately 80% of the steel yield stress.

Behaviour of partly stiffened cold-formed steel built-up beams: Experimental investigation and numerical validation

To address the various instability problems in cold-formed steel members, many researchers have mainly focused on developing innovative sectional profiles wherein geometry of the section plays a vital role in enhancing the inherent resistance of such sections against premature buckling. However, the process of forming such innovative shapes is not only complex and time-consuming but sometimes such sections fail to mobilize their complete reserve strength. Hence, a stiffening arrangement of weaker zones for mobilizing the untapped reserve strength is suggested. The contribution of this simple, effective and partly stiffening arrangements, aimed at eliminating/delaying the premature local buckling, is studied both experimentally and numerically and also compared with existing codes. Experimental study was carried out on different simply supported cold-formed steel beams with judiciously proposed stiffening arrangements under four-point loading. An equivalent hot-rolled steel beam was also tested to compare the efficiency of the cold-formed steel beams. The cold-formed steel beams investigated had different width-to-thickness ratio, different geometries and different stiffening arrangements. The test strengths, failure modes, deformed shapes, load versus mid-span displacements and geometric imperfections were measured and reported. The test strengths of the beam models are also compared with the design strength predicted by North American Standards and Eurocode for cold-formed steel structures. To validate the test results further, a numerical study was carried out on such stiffened cold-formed steel beams using finite element software ABAQUS. All these results show that the proposed strengthening system is efficient and economical and allow cold-formed steel beams to reach greater load carrying capacity.

Shear strengthening of steel plates using small-diameter CFRP strands

This paper presents the results of a comprehensive research program, including experimental and analytical studies, to examine the use of small-diameter CFRP strands for shear strengthening of steel structures and bridges. The experimental program examined the effectiveness of the proposed strengthening system to increase the shear capacity of steel plates subjected to pure shear stresses using a unique test set up. A nonlinear finite element analysis (FEA), calibrated the experimental results, was used to study parameters which were not included in the experiments. Research findings indicated that the proposed system is effective for shear strengthening of steel structures and eliminated the typical debonding failure commonly observed by CFRP laminates.

Flexural Strengthening of RC Slabs Using a Hybrid FRP-UHPC System Including Shear Connector

A polymeric hybrid composite system made of UHPC and CFRP was proposed as a retrofit system to enhance flexural strength and ductility of RC slabs. While the effectiveness of the proposed system was confirmed previously through testing three full-scale one-way slabs having two continuous spans, the slabs retrofitted with the hybrid system failed in shear. This sudden shear failure would stem from the excessive enhancement of the flexural strength over the shear strength. In this study, shear connectors were installed between the hybrid system and a RC slab. Using simple beam, only positive moment section was examined. Two full-scale RC slabs were cast and tested to failure: the first as a control and the second using this new strengthening technique. The proposed strengthening system increased the ultimate load carrying capacity of the slab by 70%, the stiffness by 60%, and toughness by 128%. The efficiency of shear connectors on ductile behavior of the retrofitted slab was also confirmed. After the UHPC top is separated from the slab, the shear connector transfer shear load and the slab system were in force equilibrium by compression in UHPC and tension in CFRP.

Fibre reinforced mortar application for out-of-plane strengthening of schist walls

The aim of the present work is to assess the effectiveness of an innovative strengthening technique for the rehabilitation of masonry buildings deficiently prepared to resist to loading conditions typical of seismic events. This technique is based on the application of outer layers of fibre reinforced mortar (FRM) by spray technology and it is used for increasing the load carrying capacity and deformation ability of masonry elements. For this purpose three almost real scale schist walls prototypes were strengthened and tested. The experimental program is described and the relevant results are presented and discussed. For estimating the properties of the schist walls and FRM taking into account the application conditions, the tested prototypes were simulated with a FEM-based computer program that has constitutive models for the simulation of the nonlinear behaviour of these materials. By using the derived properties, a parametric study was conducted to identify the influence of the FRM properties on the performance of the proposed strengthening system.

Structural behaviour of masonry panels strengthened with an innovative hemp fibre composite grid

Sustainability goals are essential driving principles for the development of innovative materials in the construction industry. Natural fibres represent an attractive alternative as reinforcing material due to both good mechanical properties and sustainability prerequisites. The present work investigates the shear behaviour of masonry panels strengthened with a mortar-based system reinforced with an innovative hemp fibre composite grid. The objective of the study is to assess the feasibility of using the proposed strengthening system for external retrofit of existing masonry walls and to compare its performances with typical retrofitting solutions.

Behavior of thin masonry arches repaired using composite materials

The innovative technique described in this article is the result of the evolution of a traditional building technique (tabicada technique or tile vaulting method) belonging to the constructive Spanish tradition. This technique consists in the realization of thin masonry vaults by placing several layers of thin bricks alternated with coatings of hydraulic mortar. Since the use of a modern technology may improve the mechanical performance of the traditional materials, the proposed strengthening system is based on the combination of the typical features of tabicada technique with the high mechanical properties of the composite materials. In order to provide extra tensile strength to the vaulted structure, composite grids were inserted between the brick layers into the mortar coating as well as on the outer surface of the vaults. Seventeen arch specimens were tested under a monotonic vertical compression load applied at the keystone. The influence of the type of strengthening arrangements, arch thickness and properties of hydraulic mortars were investigated. Test results are presented and discussed considering the mechanical behavior of the specimens and the axial stress-axial strain relationships.

Performance of steel beams strengthened with prestressed CFRP laminate

Prestressed Carbon Fibre Reinforced Polymer (CFRP) system is generally used for strengthening reinforced concrete beams where CFRP laminate can improve both the strength and serviceability behaviour of reinforced concrete beams via increasing the overall member stiffness. However, the applicability of this technique to strengthening steel structures is still lagging behind its application to concrete structures. In this study, the flexural behaviour of steel I-beams strengthened with prestressed CFRP laminate using a mechanical anchorage system is experimentally investigated. A total of nine steel beams subjected to flexural loading are tested in various conditions to evaluate the effectiveness of the proposed strengthening system. The experimental investigation confirmed that CFRP prestressing increases the ultimate load of the strengthened steel beams and moderately delays the premature debonding failure of the CFRP laminate. Even with low level of CFRP prestressing, significant enhancement in the ultimate load of the strengthened beam was recorded. Beams strengthened using non-prestressed CFRP laminate mainly failed due to premature debonding of the laminate with a slight increase in the failure load. Mechanical end anchorages maintain the CFRP laminate prestress after releasing the jacking force without encountering any debonding of the CFRP laminate till final failure of the strengthened steel beam. The results of the experimental programme and its outcomes are presented and discussed.

Strengthening of Thin Masonry Arches

The innovative technique here illustrated is the result of historical evolution of an ancient system of reinforcing tiled vaults belonging to the constructive Spanish tradition. Such a traditional technique consists in the lamination of flat rectangular tiles or thin bricks into thin vaults known as boveda tabicada. Since the use of modern technologies may improve the mechanical performance of the traditional materials, the core of the proposed strengthening system is based on the idea of combining the peculiar features of tabicada technique with the good tensile properties of composite materials. More in detail, it is possible to obtain reinforced masonry vaults or arches by overlapping different layers of tiles or thin bricks and laminates, embedded within an hydraulic mortar, so that the entire assembly may act as a single structural unit. Eighteen prototypes of tiled arches were tested under a monotonic vertical load applied at the keystone. The influence of the types of reinforcement, number of layers and properties of hydraulic mortar has been investigated. Laboratory outcomes are presented and discussed in the paper considering mechanical behavior of specimens and axial stress-axial strain relationships.

Tests on reinforced concrete slabs with cut-out openings strengthened with fibre-reinforced polymers

This paper presents the results of experimental investigations on reinforced concrete slabs strengthened using fibre-reinforced polymers (FRP). Eight tests were carried out on four two-way slabs, with and without cut-out openings. Investigations on slabs with cut-outs revealed that the FRP can be placed only around the edges of the cut-out when retrofitting the slabs whereas, in the situation of inserting cut-outs combined with increased demands of capacity, it is necessary to apply FRP components on most of the soffit of the slab. The proposed strengthening system enabled the load and deflection capacities of the FRP-strengthened slabs, in relation to their un-strengthened reference slabs, to be enhanced by up to 121% and 57% for slabs with and without cut-outs respectively.

Behavior of Beams Strengthened with Novel Self-Anchored Near-Surface-Mounted CFRP Bars

A commonly observed failure mode in laboratory tests involving surface bonded fiber-reinforced polymer (FRP) laminates or near-surface-mounted (NSM) bars is premature delamination, that is, the separation of the FRP from the substrate well before the FRP reaches its ultimate strain capacity. To delay the onset of delamination and to ensure that the NSM FRP reinforcement continues to contribute to member strength after partial delamination, a new self-anchored carbon fiber-reinforced polymer (CFRP) bar was developed and tested for this investigation. This bar is made with a series of monolithic spikes that can be anchored deep inside the concrete. In addition to cutting grooves into the concrete cover for the placement of the primary reinforcing bar, holes are drilled deep into the concrete to insert the spikes. To test the performance of this bar, six large, simply supported, reinforced, concrete beams were retrofitted with NSM bars and tested in four-point bending. Two beams were strengthened with NSM bars without anchors or spikes but were otherwise similar to the self-anchored bar and served as control specimens (Series B1). Two beams were strengthened in flexure with the new self-anchored NSM bars (Series B2), and the remaining two beams (Series B3) were strengthened in flexure and shear by using the self-anchored NSM bars as partial shear reinforcement. The effect of the proposed strengthening system on the beams’ strength, failure mode, deformability, and ductility are discussed on the basis of the experimental results. The anchors delayed delamination and enabled the NSM bar to experience at least a 77% higher strain at failure than the companion bar without anchors. The anchors also increased beam displacement ductility and energy ductility at a 20% strength degradation by at least 34% and 42%, respectively.