Bond Capacity Research Articles

Evaluation on the Bond Capacity of the Fire-Protected FRP Bonded to Concrete Under High Temperature

To figure out the change in the reinforcing effect of FRP system used for the retrofit of RC beam when it is exposed to high temperature, it is required to evaluate not only the behavior of the entire beam, but also the bond performance at anchorage zone through a bond test according to the increase of external temperature. Moreover, the study to find various fire-protection methods is necessary to prevent the epoxy from reaching the critical temperature during an exposure to high temperature. In this manner, the fire-resistance performances of externally bonded (EB) FRP and near-surface-mounted (NSM) FRP to concrete block were evaluated by high-temperature exposure tests after performing a fire-protection on the surface in this paper. Board-type insulation with mortar was considered for the fire-protection of FRP system. After the fire-protection of the FRPs bonded to concrete blocks, an increasing exposure temperature was applied to the specimens with keeping a constant shear bond stress between concrete and the FRP. Based on the result, the temperature when the bond strength of the FRP disappears was evaluated. In addition, a finite element analysis was performed to find a proper method for predicting the temperature variation of the epoxy which is fire-protected with board-type insulation during the increase of external temperature. As a result of the test, despite the same fire-protection, NSM specimens were able to resist 1.54–2.08 times higher temperature than EB specimens. In the design of fire-protection of FRP system with the board-type insulation, it is necessary to consider the transfer from sides as well as the face with FRP. If there is no insulation of FP boards on the sides, the epoxy easily reaches its critical temperature by the heat penetrated to the sides, and increasing the thickness of the FP board alone for the face with FRP does not increase the fire-resistance capacity. As a result of the FE analysis, the temperature variation at epoxy can be predicted using the analytical approach with the proper thermal properties of FP mortar and board.

Study of the Bond Capacity of FRCM- and SRG-Masonry Joints

Fiber-reinforced cementitious matrix (FRCM) and steel-reinforced grout (SRG) have been increasingly applied as externally bonded reinforcement to masonry members in the last few years. Unlike fiber-reinforced polymer (FRP), FRCM and SRG have good performance when exposed to (relatively) high temperature and good compatibility with inorganic substrates, and they can be applied to wet surfaces and at (reasonably) low temperatures. Although numerous studies investigated the mechanical properties and bond performance of various FRCM and SRG, new composites have been developed recently, and their performance still needs to be assessed. In this study, the bond behavior of three FRCM composites and one SRG composite applied to a masonry substrate is investigated. Sixteen single-lap direct shear tests (four tests for each composite) are performed. The FRCM studied comprised one layer of carbon, PBO (polyparaphenylene benzobisoxazole), or alkali-resistant (AR)-glass bidirectional textile embedded within two cement-based matrices. The SRG composite comprised one layer of a unidirectional stainless-steel cord textile embedded within a lime-based matrix. The results show a peculiar bond behavior and failure mode for each composite. Based on these results, the behavior of the carbon and PBO FRCM is modeled solving the bond differential equation with a trilinear cohesive material law (CML).

Prediction models for bond strength of steel reinforcement with consideration of corrosion

Corrosion phenomena is one of the main deterioration causes, which remarkably affects the behavior of structural reinforced concrete (RC) members in seismic regions. Researches on reducing rehabilitation cost, performance assessment, and accurate modelling of corrosion-affected RC structures are progressively becoming popular in recent years. Corrosion diminishes bond capacity between reinforcement and surrounding concrete, which induces reduction in strength and ductility of members. The aim of this investigation is to provide a prediction approach based on a large number of results from published researches related to corroded reinforcement in concrete members using artificial neural networks (ANN). The minimizing mean square error criterion and increasing regression value of predicted results are considered for evaluation of training performance of ANN models. The validity of proposed model is checked using collected experimental database. Results show that estimated model has acceptable agreement with experimented data.

Scattering and phase-contrast X-ray methods reveal damage to glass fibers in endodontic posts following dental bur trimming

ObjectivesThere is concern that the integrity of fiberglass dental posts may be affected by chairside trimming during treatment. We hypothesize that hard X-ray methods of phase contrast-enhanced micro-CT (PCE-CT) and synchrotron based X-ray refraction (SXRR) can reliably identify and help characterize the extent of damage. MethodsFiberglass posts were imaged both as manufactured and following trimming with a diamond bur. Each of the posts was imaged by SXRR and by PCE-CT. Datasets from PCE-CT were used to visualize and quantify 2D and 3D morphological characteristics of intact and of damage-affected regions caused by trimming. ResultsThe SXRR images revealed fiber inhomogeneities from manufacturing with a significant increase in internal surfaces in sample regions corresponding to damage from trimming. PCE-CT volumes unveiled the micromorphology of single fibers in the posts and some damage in the trimmed area (e.g. fractures, splinters and cracks). Area, perimeter, circularity, roundness, volume and thickness of the glass fibers in the trimmed area were statistically different from the control (p < 0.01). SignificanceThe integrity of single fibers in the post is critical for bending resistance and for long-term adhesion to the cement in the root canals. Damage to the fibers causes substantial structural weakening across the post diameter. Glass fragments produced due to contact with the dental bur may separate from the post and may significantly reduce bond capacity. The above mentioned synchrotron-based imaging techniques can further facilitate assessment of the structural integrity and the appearance of defects in posts (e.g. after mechanical load).

Assessment of Existing Bond Models for Externally Bonded SRP Composites

This manuscript assessed the applicability of the existing ultimate bond formulas originally developed for externally bonded carbon fiber-based composites in the analysis of steel-reinforced polymers (SRPs). In the first part, the methods of predicting the bond capacity are reviewed, the differences are indicated, and the factors determining the bond are discussed; then, using the bond test results of over 400 samples available in the literature, the bond prediction methods are assessed by graphical comparison. The evaluation mainly concerned concrete elements and epoxy adhesives; however, to a lesser extent, a similar analysis was performed for the masonry substrate and grout matrices. The results showed the relatively good applicability of the majority of bond prediction models for the analysis of SRP composites. In most cases, the ultimate bond force was slightly underestimated, which was beneficial in the design of this type of strengthening. Larger discrepancies concerned weaker grout matrices.

Interfacial bond performance between self-expansion polymer and concrete

Polymer grouting technology is a new trenchless method to restore leakage and subsidence of underground concrete structures, and the bond between polymer and concrete is critical to estimate the deformation and ultimate conditions of the repaired structures. In this paper, scanning electron microscopy (SEM) and a series of direct shear tests were used to evaluate the interfacial bond properties between self-expansion polymer and concrete. Based on the test results, the effect of polymer density, the strength of concrete, the moisture content of grouting interface before and after grouting on interfacial microstructure, failure mode, shear stress-slip curves, and bond capacity were evaluated and discussed. The results demonstrated that the polymer could infiltrate into the pores of the concrete grouting surface, the infiltrate value and the polymer strength increased with the amount of grouting and resulted in large bond strength. Pre-moistened the grouting surface reduce the polymer migration into the irregularities of the concrete substrate and affect chemical reactions, show un-full filled pores of the interface and increased the diameter of the polymer cells around the interface, which is thought to exerts a negative influence on the bond behavior.

Bond investigations of prestressed, near-surface-mounted, ribbed memory-steel bars with full bond length

The effectiveness of iron-based shape memory alloy (Fe-SMA, ‘memory-steel’) reinforcements has been proven in several studies and site applications. Due to the novelty of ribbed memory-steel reinforcement bars, limited research has been performed on their application in structural engineering, however. In this study, the bond behaviour of memory-steel bars in a near-surface-mounted configuration designed for prestressed strengthening of concrete structures was investigated. Memory-steel bars were installed on concrete blocks and activated by resistive heating before being subjected to pull-out load. The effects of heating and hence prestressing, bar ductility, bond length and rigidity of the fixation were studied based on obtained prestress, crack development, slip and bar strain. During activation of the bars, a longitudinal splitting crack with insignificant crack width occured. Two failure modes were observed during subsequent external loading, including tensile rupture of the bar when less ductile material was used, or bond failure when material with higher ductility was under investigation. Initial prestressing did not result in a reduced bond capacity. Based on the differential equation of bond behaviour, an analytical calculation procedure was developed that enables prediction of bond shear stress, slip, bar strain, pull-out load, as well as the use of a non-linear material behaviour.

General Analytical Model for the Bond Capacity of NSM FRP-Concrete Joints

Abstract Fiber-reinforced polymer (FRP) near-surface mounted (NSM) reinforcement represents an effective solution for strengthening and retrofitting existing concrete structures. As it is embedded ...

Novel hybrid lipid nanocapsules loaded with a therapeutic monoclonal antibody – Bevacizumab – and Triamcinolone acetonide for combined therapy in neovascular ocular pathologies

The aim of this study was to design and develop a novel hybrid formulation based on lipid nanocapsules containing bevacizumab (BVZ), an effective therapeutic antibody, on the surface and triamcinolone acetonide (TA) in the inner core (BVZ-TA-LNC) intended to improve ocular therapy. Hence, a phase inversion-insertion one step method was developed to drug loading and surface modification of lipid nanocapsules by post-insertion of a bifunctional polymer, followed by antibody coupling using “click” chemistry. The covalent bond and antibody capacity binding to its specific antigen were confirmed by thermal analysis and immunoassay, respectively. BVZ-TA-LNC presented nanometric size (102 nm), negative surface potential (−19 mV) and exhibiting 56% of TA in the lipid core. BVZ-TA-LNC tended to prevent the endothelial cell migration and significantly prevented the capillary formation induced by the vascular endothelium growth factor (VEGF). The novel hybrid system allowed the co-loading of two different therapeutic molecules and may be promising to improve the therapy of eye disorders that occur with inflammation and/or neovascularization.

Flexural Behavior of Reinforced Concrete Beams Strengthened by Textile Reinforced Magnesium Potassium Phosphate Cement Mortar

Magnesium potassium phosphate cement (MKPC), made of sintered magnesium oxide and potassium phosphate, is a new material for construction and structural repair. It possesses high early strength and bond capacity, low drying shrinkage, excellent fire resistance and compatibility with concrete. These characteristics render it an ideal material for strengthening reinforced concrete (RC) structures. This study developed a MKPC-based textile reinforced cementitious composite (TRCC) to strengthen RC beams at soffit. Three-point flexural tests were conducted on two control specimens without strengthening and two strengthened specimens. Experimental results showed that the TRCC system exhibits good bonding with substrate concrete. The strengthened specimens failed due to rupture of textile meshes in TRCC. TRCC could improve load capacity by 4%-25%, postpone propagation of cracks, reduce tensile strain of main reinforcement and reduce deflection of beams at the same load level. High relative textile ratios contributed to the load resistance while impaired the ductility of strengthened beams. Design formulae are proposed to predict the flexural capacity of beams strengthened by MKPC-based TRCC, and reasonable prediction close to experimental results can be obtained from the formulae.

Anchorage behaviour of headed bars as connection system for precast reinforced concrete structural components

Precast reinforced concrete (RC) structural joint is a critical region which demands appropriate connection system for adequate shear transfer. Prevalent dowel bars connection for precast components is prone to damage during the seismic motion. This paper proposes an innovative modified dowel bar connection, termed as headed bars, wherein the reinforcement bars are attached to the steel head anchor through welding and embedded into the concrete. To investigate the anchorage behaviour of headed bars, pull-out tests were conducted on a total of 144 specimens, which were differentiated on the parameters of head profile i.e., plain, grooved, ribbed and without head (representing dowel bar); varying length of the head and varying embedment depth into the concrete. The anchorage behaviour of headed bars was studied from the standpoint of failure mode, bond capacity and slip of bar. Dowel bars demonstrated pull-out failure with low bond capacity, whereas headed bars exhibited slip of bar and concrete failure along with higher bond capacity. Based on the experimental results, geometrical specifications for headed bars are recommended. A numerical analysis was carried out to evaluate the efficacy of headed bars as a connection system, where precast RC wall and columns connected using headed bars of proposed geometry were subjected to displacement controlled lateral loading and compared with the structural performance of connection using conventional dowel bars. Numerical analysis exhibited superior performance of precast RC wall-columns connected through headed bars in comparison to the dowel bar connection.

Polarizable charges in a generalized Born reaction potential.

The generalized Born (GB) model is a fast implicit solvent model that is used as an approximation to the Poisson equation for solutes described by point charges. Due to the simple analytical form, GB models are widely used in molecular dynamics simulations to account for (implicit) solvation effects. In this work, we extend the application of the GB model to polarizable charges by coupling it to the bond capacity (BC) model. The resulting BC-GB model is a non-variational polarization model where the reaction potential is calculated from a GB expression and included in the polarization equation to account for solvation effects. Being non-variational, the BC-GB makes use of a Lagrange formulation for an efficient evaluation of energy gradients. The stability of the algorithm in molecular dynamics simulations is tested in the microcanonical ensemble, and the results show energy conservation as well as small fluctuations. The inclusion of implicit solvation increases the computational cost by only 15% compared to vacuum. Combined with a significant reduction in system size by describing the solvent as a continuum makes the BC-GB model an interesting model for applications requiring the combination of solute polarization and extensive conformational space sampling.

Experimental study on local bond failure of tension reinforcing bars embedded in concrete

AbstractIn this study, to evaluate the strength on the local bond failure, pull‐out tests of deformed bars straightly embedded in concrete were conducted. As a result, the test results were classified into entire bond failure and local bond failure. The effects of the beam width and inner transverse ties on the entire bond failure were evaluated by the previous bond capacity formula. In this study, split modes in which more than one bar split the concrete for the upper or lateral side were assumed, and a method to evaluate the strengths on the local bond failure and a way to expect the split mode was proposed.

Effect of liquid rubber modification on the bond behavior of externally bonded FRP laminate-concrete interface under dynamic loading

Interfacial debonding between fiber-reinforced polymer (FRP) laminate and concrete substrate is one of the critical issues in the externally bonded FRP strengthened concrete members. Existing studies have shown that the addition of liquid rubber into the bonding adhesives can increase debonding resistance of FRP laminate-concrete interface and consequently improve the comprehensive performance of FRP strengthened concrete members under quasi-static loading. The purpose of this study is to clarify the effect of liquid rubber modification on the bond behavior of FRP laminate-concrete interface under dynamic loading. Six groups of double-lap shear specimens with neat or rubber-modified epoxy were tested under static and two dynamic loading rates. The parameters of peak load, FRP strain distribution and interfacial bond stress-slip relationship are analyzed. Test results indicate that the liquid rubber modification can significantly improve the bond capacity of FRP laminate-concrete interface under dynamic loading. Further, the influence mechanism of liquid rubber modification on the dynamic bond behavior of FRP laminate-concrete interface is discussed. Finally, an empirical dynamic bond stress-slip model considering the effect of liquid rubber modification is proposed and validated.

A cohesive model to predict the loading bond capacity of concrete structures repaired/reinforced with HPFRC/UHPFRC and stressed to mixed mode

The risk of cracking/debonding of a cement overlay used to repair or strengthen an existing structure is still a key issue. Current bond test methods are not designed to measure the combined effect of peeling (mode I) and shear (modeII) on the interface. A few existing models propose theoretical approaches to predict that, but they were fitted on specific cases and lack in generality. In addition, controversial opinions about the influence of both the moisture level of the substrate surface prior to the application of the overlay and properties of the latter on the loading bond capacity call for further investigations. In this work, a cohesive model is developed to predict the loading bond capacity of an existing concrete structure overlaid by a layer of HPFRC/UHPFRC. Different bond tests were specifically designed for calibrating the cohesive parameters employed into the model, which also takes into account the type of the overlay used and the moisture conditioning level. An experimental campaign confirmed the reliability of the predictions of the proposed theoretical model.

Repair of historic stone pagoda: Flexural behavior of granite reinforced with stainless steel bars

In the stone-made architectural heritage, one of the most widely used stones is granite. Given its superior material properties and abundant reserves, most Korean stone pagodas were built using granite. For the repair and restoration of the ancient stone pagoda made of granite, understanding of the structural behavior of reinforced granite is crucial. In this study, flexure tests for seven simply supported full-scale granite reinforced with stainless steel bars were conducted to investigate the failure mode and load–displacement relationship. Through the experiments, the feasibility of the suggested method for reinforcement of the damaged granite was also explored. Test specimens were manufactured after injecting epoxy resin by inserting stainless steel bars into the drilled holes, and then each granite substrate was bonded with adhesive epoxy resin. The test results showed that the load–displacement relationship of reinforced granite was highly dependent on the presence of the reinforcement, positioning of the reinforcing bars, and bond capacity of the reinforcing bars. Also, reinforcing bars significantly affected the deformation of the reinforced granite. Based on the test results, four distinct flexural responses of reinforced granite are presented.

Anchorage of naturally corroded, plain reinforcement bars in flexural members

Reinforced concrete structures are often damaged by corrosion, which affects the interaction between reinforcement bars and concrete. Earlier studies mostly applied artificial corrosion to test the bond between deformed bars and concrete. However, there is a lack of knowledge on the effects of natural corrosion on plain bars. In this paper, 20 beams with naturally corroded plain bars and varying amount of damage were taken from an 80-year-old bridge and tested in three-point bending. All but three of the specimens anchored the yield force of the bars after the opening of one or two major bending cracks. At large deflections, the load-carrying mechanism changed from beam to arch action. Eventually, end-slip of the reinforcement bars was observed. The bars were extracted, cleaned, three-dimensionally scanned, and tested in tension. The average bond strength in the unyielded zone was found to be equal to 7.39 MPa, with a standard deviation of 3.33 MPa. The casting position was identified as an important factor: when uncorroded, bottom-cast bars had a higher bond strength than that of top-cast bars. However, they were more prone to splitting cracks and, consequently, loss of bond strength for small corrosion levels. Top-cast bars had increasing bond strength with increasing corrosion levels, owing to the absence of external cracks. These differences were likely related to a denser concrete surrounding the bottom-cast bars. The remaining bond capacity in the yielded zones was evaluated to be approximately 1.0 MPa.

Bond behaviour of ribbed near-surface-mounted iron-based shape memory alloy bars with short bond lengths

Many existing concrete buildings worldwide require improvement in structural integrity. Previous research has proven the effectiveness of iron-based shape memory alloy (Fe-SMA or memory-steel) reinforcements for the prestressed strengthening of structures. The unique function of the material, i.e., the so-called shape memory effect, enables cost-effectiveness and presents simple ways to prestress defective building components. Ribbed memory-steel bars have only been available recently. This study aimed to investigate their performance in a novel strengthening technique, named near-surface mounted (NSM) method. Bond experiments with short bond lengths were performed to investigate the feasibility of this configuration and to identify the effects of several design parameters including: groove dimensions, bar diameter, bar material, cover depth, mortar strength, and concrete strength. The use of a digital image correlation system enabled detailed measurements of slips, crack width, and surface strain.An analytical procedure based on the differential equation of bond behaviour was developed, which enabled the calculation of slip, strain, bond shear stress, and load capacity. The results show that ribbed memory-steel bars can be used in an NSM configuration due to high bond capacity. The cover depth, substrate strength and elastic modulus had a significant effect on the failure mode and bond capacity.

The facilitative interpersonal skills method: Difficult psychotherapy moments and appropriate therapist responsiveness

AbstractThe Facilitative Interpersonal Skills (FIS) task is a performance test of therapists’ use of common relational skills (e.g. empathy, building expectations). The FIS method includes (a) materials that simulate difficult client moments on video, which are used to collect therapists’ responses to these situations; and (b) independent ratings of these responses. Many of the FIS items are informed by psychotherapy processes that have been linked to outcome and facilitative conditions that have been reframed as individual therapist skills (e.g. alliance bond capacity). Overall, the FIS has predicted psychotherapy outcome. A single study is described in which FIS predicted the therapist effect using multilevel modelling of a large sample of clients who were nested within therapists. We also summarise two additional outcome studies that used experimental designs. One future direction is to better understand how therapists form responses to these difficult moments. We conclude that forming an optimal therapeutic response during challenging, emergent in‐session situations involves responsiveness (Stiles et al., 1998), or finding a response that fits the clients’ needs within any moment.

The impact of alliance-focused and facilitative interpersonal relationship training on therapist skills: An RCT of brief training

Research on standard methods of therapist training has found mixed evidence to as to whether standard training methods are effective. This study investigated the impact of a novel, research-informed training protocol that integrated elements of alliance-focused training (AFT) and facilitative interpersonal skills (FIS). Beyond traditional training techniques of didactics and lecture, the AFT/FIS intervention incorporated empirically supported video simulations of therapy, which were reinforced by role plays and deliberate practice on key therapeutic interpersonal skills. Fifty-eight graduate-level therapy trainees and professional therapists from various helping fields were randomized to one of two brief trainings in a multi-site RCT: (i) the AFT/FIS workshop or (ii) a more traditional demonstration training (DT) workshop. Participants were assessed on critical, relational therapeutic skills before and after the training. After controlling for relevant covariates, participants in the AFT/FIS training saw a marginally higher post-intervention level of overall therapeutic skills. Subsequent exploratory analyses revealed AFT/FIS participants also had significantly higher levels of specifically targeted post-training therapist skills (i.e., empathy, alliance bond capacity, and alliance rupture-repair responsiveness) compared to participants in DT. Implications for future empirical investigations and training initiatives are discussed.