Bond Capacity Research Articles

Investigations on the effects of rebar diameter on the post-fire bond capacity of RC flexural members and development of a novel post-fire bond model

This study is part of a comprehensive research effort focused on examining the impact of various parameters on the post-fire bond behaviour of RC flexural members. Precisely, the impact of the rebar diameter is thoroughly discussed. Beam-end specimens, which realistically simulate the boundary conditions of a full-scale beam while maintaining a relatively compact size, were subjected to ISO 834 fire curve. To investigate the behaviour of lap-splice in a slab and beam, two different fire exposure scenarios, where one side (slab) and three sides (beam) of the specimens were exposed to fire, were considered. It is evident that although the initial and residual load-carrying capacities increase with larger rebar diameters in absolute terms, the differences in residual bond capacities remain relatively insignificant across nearly all examined fire-exposure durations. Based on the evaluation of the test results, a model for local bond stress-slip curve of bond affected by fire is proposed.

Experimental Study on Shear Strengthening of Reinforced Concrete Beams by Fabric-Reinforced Cementitious Matrix.

In this study, an experiment was conducted to investigate the shear performance of reinforced concrete (RC) beams strengthened using fabric-reinforced cementitious matrices (FRCM). Four reinforced concrete beams, including a control specimen, were fabricated, and the shear strengthening effect of the FRCM was investigated on eight shear specimens, with the strengthening type and shear reinforcement as key variables. In particular, the digital image correlation (DIC) technique was applied to closely analyze the deformation of reinforced concrete beams subjected to shear forces. The average shear strain-shear stress curve of each specimen was derived, and the contributions of shear and bending to the vertical deflection and the change in the principal strain angle with increasing shear force were analyzed. The experiment results showed that all specimens failed with diagonal cracks within the shear span. In the specimens without shear reinforcement, the shear strength increased by up to 65% according to the FRCM strengthening, while in the specimens with shear reinforcement, only the sided bond strengthened specimen showed a strength increase of 16% compared to the control specimen. Based on displacement data of the DIC, it was confirmed that FRCM strengthening can control the deformation of the RC beam. To evaluate the shear strength of the FRCM-strengthened RC beams, a shear strength model was proposed by considering the contributions of the concrete section, shear reinforcement, and FRCM. The proposed model was capable of reasonably evaluating the shear strength of RC beams strengthened with FRCM, considering the shear contribution of FRCM and bond capacity between FRCM and concrete substrate, in which the shear strength of specimens was underestimated by 28% to 35%.

Experimental and numerical investigations on the residual bond capacity of RC flexural members exposed to fire considering twin rebars

Experimental and numerical investigations on the residual bond capacity of RC flexural members exposed to fire considering twin rebars

Innovative Application of Self-Healing Technology to Masonry: A Proof of Concept

ABSTRACT Cracks represent a prevalent form of damage in masonry structures, posing not only aesthetic concerns but also compromising structural durability; therefore, they are undesirable and need to be repaired. The repointing technique is traditionally implemented in this context, especially in historical masonry. However, this method fails to provide a long-term solution, leaving structures vulnerable to future damage. The paper investigates the applicability of a bio-based self-healing mortar to enable autonomous repair of masonry. This innovative mortar, developed to repair concrete structures, was implemented to explore the capacity of couplets to recover their original bond capacity and aesthetic aspect after multiple damaging events. Specimens built with calcium-silicate and clay bricks were subjected to subsequent cracking cycles using a crack-mouth-opening-displacement controlled bond-wrench test. Experimental results showed that self-repair, in terms of bond restoration and aesthetic filling of cracks, occurs even after multiple cracking cycles when the bio-based mortar is used with both types of bricks, optimizing the autogenous healing (intrinsic) of cement-based mortars. The effectiveness varied also according to the types of brick and healing environment used, e.g. under humid conditions (RH ~ 95%), 50% vs 80% of the original capacity was regained in fully separated couplets made respectively with clay and calcium-silicate bricks.

Corrosion Protection of Embedded Steel Reinforcement Using Canarium Schweinfurthiia Exudate for Coastal Concrete Structures

This experimental study evaluated the effects of corrosion on the bond behavior and structural integrity of reinforced concrete. A total of 36 concrete cube specimens containing reinforcing steel bars were divided into control, uncoated, and Canarium schweinfurthiia exudate/resin coated groups and immersed in 5% NaCl solution for 360 days. Periodic tests were conducted to analyze corrosion levels, bond strength, steel bar properties, and failure loads. Results showed the control specimens exhibited little corrosion while uncoated bars experienced significant corrosion after exposure. In contrast, exudate/resin coated bars demonstrated a substantial reduction in corrosion, indicating the coating's inhibitory effects. Pull-out bond strength and maximum slip tests consistently revealed lower strengths and higher slips in corroded samples compared to controls. However, coated specimens maintained higher bond capacities and lower displacements, validating the coating's protective performance. Measurement of bar diameters, cross-sectional areas, and weights before and after corrosion highlighted the proportional losses associated with the corrosion process. Dimensions and masses of corroded reinforcements decreased by 2-7% on average from initial values. In comparison, controls showed minimal variations. Failure load analyses found controls withstood the highest loads, while corroded members exhibited reduced capacities. Overall, the study demonstrated corrosion negatively impacts the critical steel-concrete bond and undermines composite action essential for structural integrity. Exposure to NaCl solutions markedly increased corrosion and decomposition of unprotected bars. However, natural Canarium schweinfurthiia exudate/resin coatings were highly effective at mitigating corrosion effects by preserving bond strength, slip resistance, steel geometries and load capacities close to uncorroded levels. The findings validate protective coatings as a viable solution for durable reinforced concrete design in marine environments. Proper corrosion prevention through coatings or inhibitors can help ensure structures withstand service loads over their design life.

Room temperature phosphorescence from intermolecular hydrogen-bonded quinolone quinoline carboxylic acid/polyacrylamide material and the application for anti-counterfeiting

Polymer-based organic room temperature phosphorescence (RTP) materials are of critical importance in organic light-emitting diodes, displays, and anti-counterfeiting applications. Nevertheless, the development of heavy-atom-free materials that exhibit long RTP lifetimes and high quantum yields via a straightforward, cost-effective, and environmentally friendly preparation remains a significant challenge. In this study, we design and construct an environmentally sustainable and highly efficient hydrogen-bonding network (QA/PAM) employing commercially available quinoline carboxylic acid (QA) derivatives and the water-soluble polymer polyacrylamide (PAM) with high hydrogen bond capacity. By increasing the conjugation degree and the number of carboxyl groups in QA, we can effectively enhance the π-π* and n-π* transitions, thereby reducing the energy gap between the S1 and T1 states, resulting in high quantum yields and offering the ability to adjust colors from blue to green and subsequently to yellow-green. Particularly, BQDA/PAM-0.3 wt% system shows an average lifetime of 392.46 ms and QY of 26.93 % under 300 nm excitation at 297 K. Anti-counterfeiting patterns and 2D code fabricated by the BQDA/PAM solution by environmentally friendly screen printing can be facilely deciphered by mobile phone, offering potential applications in optical and digital information security for commercial production.

Effect of high-strength and stainless steel reinforcement on the flexural behavior of UHPC beams

Ultra-high performance concrete (UHPC) is an advanced cement-based composite which shows remarkable properties including very high compressive strength, increased tensile resistance, impressive toughness and excellent durability. One promising application of UHPC is in heavily-loaded beams and slabs where its use can allow for increased design efficiency and improved durability. Conversely, the high bond capacity of UHPC can lead to early bar fracture failures, especially when using low amounts of ordinary steel reinforcement. This paper examines the influence of reinforcement grade on the flexural ductility of UHPC beams. As part of the study, three beams built with either Grade 400 MPa ordinary steel reinforcement, Grade 690 MPa high-strength reinforcement or Grade 520 MPa stainless steel reinforcement, are tested under flexural loading. The key test parameters include the influence of concrete type (UHPC vs. conventional high-strength concrete) and steel type (high-strength or stainless steel vs. ordinary steel). The results show that the flexural strength and ductility of the UHPC beams is influenced by the steel grade/type. The beam with ordinary steel shows a well-defined yield point but fails by bar rupture. Increased resistance by 47% is observed when using high-strength bars, however bar rupture occurs owing to the low strain-capacity of this steel type. Combining UHPC with higher ductility stainless steel does not increase capacity, but leads to remarkable ductility by 75%. As part of the numerical study the behaviour of the beams is predicted using FE modelling and the effects of tension steel ratio, UHPC tensile strength and high-strength or stainless steel type are investigated.

The methylome of motile cilia.

Cilia are highly complex motile, sensory, and secretory organelles that contain perhaps 1000 or more distinct protein components, many of which are subject to various posttranslational modifications such as phosphorylation, N-terminal acetylation, and proteolytic processing. Another common modification is the addition of one or more methyl groups to the side chains of arginine and lysine residues. These tunable additions delocalize the side-chain charge, decrease hydrogen bond capacity, and increase both bulk and hydrophobicity. Methylation is usually mediated by S-adenosylmethionine (SAM)-dependent methyltransferases and reversed by demethylases. Previous studies have identified several ciliary proteins that are subject to methylation including axonemal dynein heavy chains that are modified by a cytosolic methyltransferase. Here, we have performed an extensive proteomic analysis of multiple independently derived cilia samples to assess the potential for SAM metabolism and the extent of methylation in these organelles. We find that cilia contain all the enzymes needed for generation of the SAM methyl donor and recycling of the S-adenosylhomocysteine and tetrahydrofolate byproducts. In addition, we find that at least 155 distinct ciliary proteins are methylated, in some cases at multiple sites. These data provide a comprehensive resource for studying the consequences of methyl marks on ciliary biology.

Bond behaviour at high temperatures between concrete and CFRP or steel strengthening bars applied according to the embedded through-section (ETS) technique

The embedded through-section (ETS) technique is an efficient shear-strengthening method for reinforced concrete (RC) structures, in which fibre reinforced polymer (FRP) or steel bars are bonded in holes predrilled in RC members. Compared to conventional FRP strengthening methods, ETS systems are expected to present better performance when exposed to high temperatures/fire, due to the insulation provided by concrete to both adhesive and FRP/steel components. Yet, the mechanical/bond properties of the strengthening materials are highly affected when the glass transition temperatures (Tg) of their components is attained/exceeded; however, there are no studies available about the influence of elevated temperatures on the performance of ETS systems. This paper presents an experimental study that addresses the effects of elevated temperatures on the bond behaviour of ETS systems – pull-out tests were conducted on steel rebars or sand-coated carbon-FRP (CFRP) rods bonded with epoxy adhesive on concrete cylinders at temperatures up to 90 °C. The results obtained confirm that the strength and stiffness of the bonded interfaces decrease when the temperature in the adhesive approaches/exceeds its Tg; moreover, with sand-coated CFRP rods, failure was also influenced by the bond between the sand coating and the CFRP rods core, and thus the average bond capacity was significantly reduced for temperatures below the Tg of the epoxy adhesive. Bond-slip relations were calibrated for different elevated temperatures for the steel/CFRP-concrete interfaces using data obtained from the pull-out tests; these bond laws can be used to simulate the mechanical behaviour of ETS-strengthened-RC members subjected to elevated temperatures.

Enhancing fire resistance of masonry structures: The potential of ultra high performance concrete (UHPC)

The potential of ultra high performance concrete (UHPC) for strengthening masonry structures is evaluated in this work, both under normal service conditions and after exposure to high temperatures, as a significant gap has been identified on this topic at the present time. The mechanical properties of UHPC exposed up to 800 °C are analyzed by means of destructive and non-destructive tests, and the UHPC-to-masonry bond capacity is tested, depending on temperature and substrate preparation. The strengthening of masonry columns by confinement is studied, proving that UHPC jackets can double the compressive strength and provide ductility, even after exposure to high temperatures. Strengthening of masonry walls to in-plane loads is also evaluated, finding that UHPC layers can increase the shear strength and prevent brittle failures. Experimental results are compared with the predictions of theoretical models from some design guides, slightly adapted to the context of this research, obtaining a good degree of accuracy.

Experimental Performance Evaluation of RC Beams Strengthened by TRM with Improved Bond Capacity

Experimental Performance Evaluation of RC Beams Strengthened by TRM with Improved Bond Capacity

Effect of local bond behavior degradation on tension stiffness in reinforced concrete with pre-existing longitudinal cracks

Reinforced concrete structures often have cracks when exposed to corrosion of rebars. This induced crack can deteriorate the bond capacity between concrete and rebar, affecting the structural performance of the member. This study conducted pull-out and uniaxial tension tests on cracked specimens to investigate the effect of bond behavior degradation on the tension stiffening effect. It uses an innovative approach to simulate concrete cracking by inserting an aluminum pipe into the concrete and filling it with an expansion agent to replicate the volumetric expansion caused by rebar corrosion. The local bond-slip behavior degradation is modeled using Popovics formulation based on pull-out cracked bond specimens. Furthermore, uniaxial tension tests reveal that induced cracks decrease transverse crack numbers and reduce the tension stiffening effect, primarily due to bond loss. However, this degradation becomes insignificant after a total deformation of 1.5 mm, corresponding to about 0.1% in average strain. Finally, analytical results confirm that tension stiffness does not necessarily decrease with increased induced crack width. Instead, the tension stiffness may increase in case of high bond loss due to induced crack.

Three-dimensional finite element modeling of debonding failure of skew FRP-bonded concrete joints

Despite the extensive investigations on the shear strengthening of reinforced concrete (RC) beams using externally bonded fiber-reinforced polymer (EB-FRP) composites, the critical problem of how the skew angle (angle between FRP and crack opening direction) influences the bond capacity of FRP has drawn little concern and remains unsolved. This paper presents the first three-dimensional (3D) finite element (FE) model for studying the failure of skew FRP-bonded concrete joints. The concrete was simulated using the concrete damaged plasticity model, and the FRP was modeled as an orthotropic material equipped with the LARC05 model as the damage criterion. The FRP-to-concrete interface was described using a linear elastic cohesive law. The 3D FE model, which provided a relatively large convergent mesh size of 2.5 mm, was calibrated and validated using the bond tests of aligned and skew FRPs. Both the tests and FE results revealed a notable decrease in the bond capacity of FRP at the skew angle of 45°. The parametric analysis demonstrated that the adverse skew effect was reduced when the FRP width increased or FRP thickness decreased, by delaying the onset of the stable peeling-off of concrete along the FRP fiber axis. Compared with FRP, steel plate suffered less from the skew effect, due to its isotropic nature. This study can provide valuable implications for more effective shear-strengthening methods of RC beams and facilitate novel insights for the development of more robust design models. The 3D FE framework can also serve as a versatile tool for analyzing the complicated failure modes of various FRP-strengthened concrete systems.

Interfacial bond capacity prediction of concrete-filled steel tubes utilizing artificial neural network

Interfacial bond capacity prediction of concrete-filled steel tubes utilizing artificial neural network

Exploiting Deep Eutectic Solvent-like Mixtures for Fractionation Biomass, and the Mechanism Removal of Lignin: A Review

Green solvents, which include deep eutectic solvent-like mixtures (DES-like mixtures), are categorized as ecological and economical solvents for the pretreatment and fractionation of different types of biomasses. DES-like mixtures represent a group of the most promising green solvents for lignocellulosic pretreatment and are currently used effectively in the biomass pretreatment process. The present work describes the latest applications of DES-like mixtures in biomass delignification processes and, at the same time, summarizes the mechanism of action and influence of DES-like mixture systems on the removal of lignin from different types of biomasses. The results of this review indicate that the physicochemical properties (acidity, hydrogen bond capacity, polarity, viscosity, and water content) of DES-like mixtures have a significant effect on the biomass fractionation process. In addition to the nature of components forming DES-like mixtures, the reaction conditions (temperature, time) influence the efficiency of delignification. Active protons obtained from the hydrogen bond donor facilitate proton-catalyzed bond cleavage during fractionation, where the most significant step is the destruction of the ether and ester bonds between polysaccharides and lignin. DES-like mixtures can depolymerize lignin with subsequent breakdown of the β−O−4 bonds.

Theoretical effects of corrosion on lateral stress along reinforcement bars in concrete

Literature review of latest research findings on the bond behavior of corroded reinforcing steel bars suggests that the bond mechanism is minimally affected by corrosion. However, discrepancies exist among research concerning the influence of corrosion on the critical parameters that govern bond behavior of bars.Former studies have primarily focused on the diminishing bond strength caused by corrosion. Several key factors influencing bond strength have been identified, including corrosion of the main bar, concrete cover thickness, concrete type, stirrups, stirrup corrosion, and corrosion rate. However, existing bond strength models are largely based on specific test results and lack generalizability.This work is a part of a wider study aiming to evaluate main concrete and steel parameters effect on corroded reinforced concrete structures. The scope of present paper is to evaluate the effect of several parameters on bond behavior. Some concrete exposure classes are considered, and corrosion process is simulated using a non-linear analytical model. Three bond theories are considered to have comparative results. Corrosion simulation has been done using different concrete and steel parameters to evaluate their effects on RC corroded elements’ bond capacity.

A Graphical Solution to Bond Capacity

This study proposes an analytical model, referred to as the "Wine Glass model", offering an elegant graphical solution to the bond capacity. It is built on the basis of two key assumptions: (i) the bond length is sufficiently long (longer than an effective bond length) and (ii) the stress-strain behavior of the adherent monotonically increases, which is typically met by the majority of engineering materials. The tensile stress-strain (σ - ϵ) curve of the adherent can be visualized as a wine glass when plotted against the vertical axis (σ-axis). In this analogy, the fracture energy of the adhesive bond divided by the thickness of the adherent strip (Gf/t) represents the wine poured into the glass. The height of the wine within the glass corresponds to the level of adherent tensile stress, with respect to the bond capacity (Fb). This Wine Glass model, which suits lap-shear joints with both linear and nonlinear adherents, is validated on experimentally measured bond capacities of various types of lap-shear joints, including carbon fiber reinforced polymer (CFRP)-to-steel joints and iron-based shape memory alloy (Fe-SMA)-to-steel joints. It unveils the mechanism of bond capacity.

Shear stress transfer at the SRG-concrete interface: influence of the number of layers

Externally bonded SRG (steel reinforced grout) is a promising technique for strengthening existing RC (reinforced concrete) and masonry structures. SRG is a composite material made of high-strength steel fibers embedded in a geo-polymeric (inorganic) matrix. The failure of structural members strengthened with SRG composites typically occurs due to debonding when an interfacial crack develops at the fiber-matrix interface. This paper presents the results of single-lap shear tests performed in order to study the debonding phenomenon of SRG composites bonded to a concrete substrate. Specimens with one and two textile layers were tested to assess the dependency of the debonding mechanism and bond capacity on the number of layers. The results of the single-lap-shear tests were then used to derive a local CML (cohesive material law), which describes the interaction between the concrete substrate and the SRG strips.

Repair and strengthening of existing structures using FRP composites externally-bonded with toughened epoxy adhesives

This literature review investigates the efficacy of toughened epoxy adhesives for structural repair and strengthening of existing structures with FRP composites. Highlighting the limitations of traditional epoxy adhesives, such as susceptibility to fatigue loading and impact actions, the study explores the potential of toughened formulations to mitigate fracture propagation issues. Through the incorporation of additional phases like rubber particles or nanomaterials, toughened adhesives offer increased fracture toughness and improved structural performance. However, this could also lead to trade-offs, including potential reductions in mechanical properties, glass transition temperature reduction and possible health and environmental concerns. This study presents an overview of the main toughening mechanisms occurring in modified epoxy resins. Two toughening techniques are described and discussed, namely the rubber-toughening and nanomaterial-toughening. The study further investigates the effects of the use of toughened adhesives in externally-bonded FRP strengthening/repair applications, involving both steel and concrete substrates. Eventually, the difference in bond-slip behavior of toughened and traditional adhesives is described, with reference to currently available literature studies on the topic. This review paper shed light on the promising advantages of using toughened epoxy adhesives in FRP applications, in terms of higher bond and strain capacity and enhanced bond transfer mechanisms between composite and substrate, if compared to traditional adhesives.

Analytical study of FRP debonding in a real-scale concrete strengthened beam

The exploitation of the mechanical properties of fiber-reinforced polymers (FRP) externally bonded to concrete members is limited by the occurrence of debonding. Single- and double-lap shear tests were performed to study the debonding process. However, the results of these tests fail to describe the debonding process when multiple substrate cracks occur, as in the case of reinforced concrete beams strengthened in flexure. In this paper, an analytical model is proposed to describe the load response of a concrete beam strengthened in flexure with an externally bonded FRP composite. The profiles of the FRP-concrete slip, FRP axial force, and cross-section rotation, the evolution of the cracking process, and the failure mode may be obtained by this model. The results of the model shed light on the debonding process in FRP-strengthened beams and may be used to investigate the relationship between the bond capacity of FRP-concrete interfaces with a single crack and that observed in real-scale strengthened beams.