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.

Bond responses and anchorage length of GFRP bar in precast recycled aggregate concrete

Precast recycled aggregate concrete (PRAC) employs precast reject-derived recycled aggregates (PRAs) and exhibits mechanical properties on par with or superior to those of natural aggregate concrete (NAC). Nevertheless, the durability of PRAC is hampered by aged mortar. To address this issue, integrating high durability glass fiber reinforced polymer (GFRP) bars into PRAC emerges as a potential solution for constructing enduring structures. Building upon this context, the study focuses on the bond responses between PRAC and GFRP bars, an area with limited existing research. Three main tasks were conducted: Firstly, a comprehensive test program involving 108 pullout samples was conducted to investigate the key variables' effects on bond failure mode and stress-slip relationship. Subsequently, an equation was developed to determine the peak bond strength of recycled aggregate concrete (RAC) based on available test data. Additionally, reliability analyses were performed to discuss the anchorage length of GFRP bar embedded in RAC. The findings are as follows: (a) The PRAC's bond capacity initially increased then decreased with rising PRA content. At full replacement, PRAC's bond strength was 28.3% lower than NAC. However, a two stage mixing approach restored PRAC's bond capacity to NAC levels. (b) Helical wrapping of the GFRP bar consistently exhibited the highest bond capacity, with an average bond strength 51% greater than a sand coated GFRP bar and 21% higher than a GFRP bar treated with both helical wrapping and sand coating. (c) Current codes specifying the critical anchorage length for FRP bar in NAC were often unsafe for RAC. Hence, a new expression, factoring in RA content, type and RAC mixing method, was proposed to determine the suitable anchorage length.

Experimental Study on Bond Behavior between CFRP and Concrete with a Convex-Circular Arc Interface

The bond performance of CFRP to concrete plays a vital role in CFRP strengthening on concrete structures. In this paper, an experimental study was carried out to investigate the bond performance of CFRP to concrete with a convex-circular arc interface. The main factors were the curvature of the concrete surface and the bond length and the layers of the CFRP laminate. Based on the experimental results, the failure mode of the bond specimens, the variation of the bond capacity, the CFRP strain, and the bond–slip constitutive model are analyzed. The results showed that most of the specimens failed to peel off the interface concrete, and the bond capacity tended to increase with the increase in bond length when the bond length was within an effective value. When the interface curvature increased to 1/0.8 m, the bond capacity tended to increase due to the CFRP exerting a certain pressure on the concrete surface. The prediction formula of the bond capacity between the CFRP and concrete is proposed considering the influence of the interface curvature. The bond–slip curves are given out based on the finite differential analysis of the strain distribution of CFRP laminates. The accuracy and applicability of the proposed model are verified with a comparison to the test results and other existing models.

Influence of activation temperature and prestress on behavior of Fe-SMA bonded joints

The prestressed strengthening of structures via use of bonded iron-based shape memory alloys (Fe-SMAs) has proven promising, albeit with concerns regarding the temperature dependency of the adhesive properties. In this study, the effect of activation temperature and generated prestress are investigated experimentally. Six Fe-SMA-to-steel adhesively bonded joints, comprising different Fe-SMA strips (non-prestrained and prestrained) and activation strategies (full activation and partial activation), were prepared, activated via electrical resistance heating, and tested under quasi-static loading. It is found that the bond–slip behavior of a joint with activation can be modeled by that of an equivalent non-activated joint. The generated prestress influences the full-range behavior by raising the base tensile stress level of the Fe-SMA strip, with negligible effects on further aspects of the full-range behavior. With the increasing activation temperature, the fracture energy is initially increased and eventually reduced, while the bond capacity and effective bond length are retained almost constant.

Bonded Behavior of Hybrid-Bonded CFRP to Heat-Damaged Concrete Interface

The reliable bond of CFRP to heat-damaged concrete is fundamental to ensuring cooperative working when the CFRP is applied for strengthening fire-damaged concrete structures. In this paper, the bond performance of hybrid-bonded (HB) CFRP to a heat-damaged concrete surface was investigated by using single shear tests. The concrete blocks were initially heat-damaged to temperatures of 100 °C, 200 °C, 300 °C, and 400 °C. The heat-damaged blocks were subsequently bonded with CFRP using the HB technique. The primary experiment parameters were the exposure temperature, the number of mechanical fasteners, and the bonded layers of CFRP. The test results show that the external-bonded (EB) and HB-CFRP joints with concrete exposed to a temperature of 400 °C were prone to fail with concrete shear-tension due to the decreased shear strength of concrete at high temperatures. The EB- and HB-CFRP joints with heat-damaged concrete anchored with no more than three fasteners present a higher bond than the reference joints with unheated concrete, while the HB-CFRP joints anchored with three fasteners provide a decreased bond capacity with the increase in exposure temperature. The utilization rate of single-layer CFRP joints with unheated concrete increased by 57.9%, 139.5%, and 136.8% with the mechanical fasteners in numbers of one, two, and three compared with the reference specimen. Accordingly, the bond capacity increased by 111.8%, 128.4%, and 186.7%. Finally, a model was proposed to estimate the bond strength of HB-CFRP joints with heat-damaged concrete.

Are Universal Adhesives Effective for Bonding to Zirconia in the Long Term?

The bond capacity of universal adhesives should be comparable to a specific primer for zirconia. Thus, this study evaluated the bond strength to zirconia of four universal adhesives and a zirconia primer over long-term storage. The surfaces of 75 samples of zirconia were sandblasted with 50 µm aluminum oxide particles and then divided into groups (n = 15): G1 - Single Bond Universal (SBU); G2 - All Bond Universal; G3 - Peak Universal Bond; G4 - Ambar Universal (AU), and G5 - Z-Prime Plus (ZP). A cone of resin composite was constructed on the applied materials. The samples were submitted to a tensile bond strength test after 24 h using a universal testing machine. Then, the remaining materials were removed from the sample surfaces, and the surfaces were polished and sandblasted again as previously described to obtain the same groups. These new samples were stored in distilled water at 37°C for 12 months and then submitted to a tensile bond strength test. The data were analyzed by two-way analysis of variance and Tukey's test (α =0.05). The material factor (P = 0.001) and the storage factor (P = 0.001) were significant, and the interaction was not significant (P = 0.117). According to Tukey's test, bond strength mean values (in MPa) followed by distinct letters were significantly different. After 24 h, G5 = 21.12 A, G1 = 20.55 A, G4 = 19.19 AB, G2 = 14.22 B, and G3 = 8.44 C. After 12 months, G1 = 7.37 A, G5 = 5.61 AB, G4 = 4.97 B, G2 = 3.32 C, and G3 = 1.93 D. After 12 months of storage, all groups' bond strength significantly decreased. SBU and AU had bond strengths comparable to ZP after 24 h. No material resisted water degradation.