Relative Slip Research Articles

Bond performance and damage assessment of self-sensing steel–fiber composite bar with geopolymer concrete

This study employed a novel steel–fiber composite bar (SFCB) enhanced with Distributed Optical Fiber Sensors (DOFS) technology to reveal the local bonding and damage mechanisms between composite reinforcement and seawater sea sand geopolymer concrete. By embedding DOFS within the steel core of the SFCB reinforcement, the local bond stress between the reinforcement and geopolymer concrete was monitored. The continuous measurement provided by DOFS revealed a non-uniform strain distribution along the length of the SFCB reinforcement and an uneven distribution of bond stress within the bonded section. Based on test results, a modified bond–slip model was utilized to predict the bond–slip relationship between the SFCB reinforcement and the geopolymer concrete, taking into account the influence of surface treatment on the descending branch of the bond–slip curve. Additionally, a damage model was employed to evaluate the extent of damage at the bond interface between the SFCB reinforcement and the geopolymer concrete under different load levels. Finally, the modified bond–slip model was also used to propose the anchorage length for SFCB reinforcements, which was compared with ACI, CSA, and JSCE codes.

MoS2/Ti Co-deposited Coatings and Their Fretting Wear Properties at Elevated Temperatures

Abstract Fretting wear refers to the damage phenomenon experienced by the mechanical components undergoing micro-amplitude relative slip at their contact region due to vibration. Titanium alloys find their extensive application in aerospace industry components such as splines and dovetail joints, where they experience fretting wear phenomenon. This research work investigates the effect of MoS2/Ti co-deposition coatings with varying Ti content, deposited on TC4 titanium alloy substrate using magnetron sputtering. Fretting wear tests were conducted at room temperature, 100 °C, and 200 °C, using a specially designed fretting test fixture with a ball-on-flat contact configuration, mounted on a servo-hydraulic fatigue testing machine. The results indicated that the coating becomes denser with an increase in the Ti content. The coating exhibited the highest hardness and better anti-fretting wear performance at room temperature. However, the effect of Ti content on the fretting wear behaviors changed at elevated temperatures. At the highest Ti content coating, excessive oxide particles were found at the worn surface at elevated temperatures, inducing an abrasive effect and localized cracks. However, coatings with moderate Ti content (9.62 at. %) effectively protected the substrate from significant wear at room temperature and maintained a low coefficient of friction at high temperatures without failure.

Study on bending performance of prefabricated glulam-cross laminated timber composite floor

Abstract The glulam-cross laminated timber (CLT) composite floor is a type of prefabricated composite floor that integrates glulam beams and CLT slab into a unified structure using shear connectors. To investigate the bending performance of the glulam-CLT composite floor, the bending test was conducted on a full-scale composite floor under static load. The study comprehensively analyzed the failure mechanism, load–deflection behavior, interface slip and strain distribution of the glulam-CLT composite floor. The test results of the composite floor indicated that the failure mode was tensile fracture of the wood beam at the bottom. As the load increased, the deflection deformation of the mid-span beam exceeded that of the edge beam. When the load reached its ultimate limit, the deflection deformation of the mid-span beam increased by 14.4% compared to the edge beam. In the early loading phase, the strain distribution of the composite section satisfied the assumption of a plane section. However, the strain distribution deviated from this assumption with the increased load due to the relative slips between the glulam beam and CLT flange. To calculate the bending performance of the composite floor, the M-shaped section of the glulam-CLT composite floor was simplified as T-section composite beams. The linear-elastic method for determining the flexural rigidity and ultimate bearing capacity of the glulam-CLT composite floors was proved to be accurate and reliable. The findings provided valuable insights into the bending behavior of the CLT flange under load and emphasized the non-uniform stress distribution caused by shear lag effects.

Experimental and analytical investigations on bond–slip behavior of steel reinforcement in UHPFRC considering yielding

AbstractThe bond behavior between ultra‐high‐performance fibre‐reinforced concrete (UHPFRC) and steel rebars plays an important role in the load‐bearing capacity and serviceability of structural members. A suitable bond–slip model is essential for predicting the load resistance and deformation capacity of members, after cracking of the UHPFRC. This paper studies the bond strength between ribbed steel bars and the surrounding UHPFRC before and after yielding of the steel reinforcement. To determine the corresponding bond–slip relationship, pull‐out tests, with varying embedment lengths of the reinforcement, namely 2 times and 10 times the rebar diameter, and diameter of reinforcing bars ranging from 10 mm to 16 mm were conducted. The bond stress, strain and slip along the embedment length of the steel rebars were measured by strain gauges and displacement transducers. Based on the test data, a bond stress–slip model was derived before (elastic stage) and after (inelastic stage) yielding of the reinforcement. The slip at the yield point is suggested through theoretical equations. The post‐yield bond–slip model is based on yield bond stress and frictional bond stress. The two‐part model was validated through iterative procedures and is able to predict the applied force, bond stress and strain distribution along the reinforcing bar. A comparison between the test data and analytical results demonstrates a good predictive capacity on the bond behavior of reinforcement in UHPFRC both before and after yielding of the reinforcement.

Bond behavior between the deformed steel bar and different types of cementitious grout under reversed cyclic loading

To systematically investigate the bond behavior of deformed steel bars in cementitious grout, direct pullout tests were conducted on 15 sets of specimens by considering five types of cementitious matrixes and three different loading schemes. The failure modes, bond stress-slip curves, as well as the bond degradation and cumulative energy dissipation of specimens under reversed cyclic loading, were compared and analyzed. Results indicate that splitting pull-out or pull-out failure modes were observed, and pull-out failure was the most common failure mode of the specimen (accounting for 91 %) due to the installation of stirrups and the sufficiently short anchorage length. Meanwhile, regardless of the loading scheme, the general relationship between the magnitude of maximum bond stress of specimens composed of different cementitious materials was shown as C60＞CGM-270＞CGM-300＞CGM-340＞CGM-380, while there was no consistent pattern in relative slippage. Moreover, the relationship between the degradation of the maximum bond stress and stiffness of each specimen under reversed cyclic loading was shown as CGM-380 >CGM-340 ≈CGM-300 >CGM-270 >C60. The degradation mainly occurs in the first two cycles, and then gradually stabilizes. Furthermore, the cumulative energy dissipation relationship of specimens with different materials was shown as C60 >CGM-270 >CGM-300 >CGM-340 >CGM-380 under the reversed cyclic loading. The above investigations could provide theoretical support for the evaluation of ductility and energy dissipation of concrete members strengthened with cementitious grout under earthquake actions.

Bond behavior between geopolymer recycled brick aggregate concrete and steel bars after exposure to high temperatures

The use of recycled brick aggregate (RBA) as the replacement for natural coarse aggregate in geopolymer concrete is an effective method for waste recycling. When the formed geopolymer recycled brick aggregate concrete (GRBC) is employed in structures, it may encounter the risk of fire. Whether the steel bars and GRBC can work together after a fire is the basis for structural safety and analysis. However, previous research on the bond behavior between GRBC and steel bars after exposure to high temperatures is still lacking. In this paper, pull-out tests on 50 GRBC and 50 ordinary recycled brick aggregate concrete (ORBC) were conducted to investigate the effects of temperature (20 °C, 200 °C, 400 °C, 600 °C, and 800 °C), and RBA replacement ratio (0 %, 25 %, 50 %, 75 %, and 100 %). The test results show that the failure pattern of GRBC specimens changes from splitting-pull-out failure to pull-out failure as the heating temperature increases. The bond strength decreases with the increase of temperature or RBA replacement ratio. When the temperature rises from 200 °C to 800 °C, the bond strength of GRBC pull-out specimens decreases by 11.54 %–72.33 % compared with the specimens at room temperature, and the decrease rate is lower than that of ORBC. The bond strength of GRBC specimens with a 100 % replacement ratio decreases by 46.34 %–53.74 % compared with those without replacement. The peak slip of GRBC specimens generally increases with increasing temperature or decreasing RBA replacement ratio. Based on the post-temperature test results and the room temperature model, suitable models for the bond strength and peak slip of GRBC after high temperatures were established. The predicted results of bond stress−relative slip curves were in good agreement with the test results. The findings of this study can provide a foundation for the application of GRBC, and provide a basis for its structural analysis after high temperatures.

Bridging Behavior of Palm Fiber in Cementitious Composite

This study addresses the growing need for sustainable construction materials by investigating the mechanical properties and behavior of palm fiber-reinforced cementitious composite (FRCC), a potential eco-friendly alternative to synthetic fiber reinforcements. Despite the promise of natural fibers in enhancing the mechanical performance of composites, challenges remain in optimizing fiber distribution, fiber–composite bonding mechanism, and its balance to matrix strength. To address these challenges, this study conducted extensive experimental programs using palm fiber as reinforcement, focusing on understanding the fiber–matrix interaction, determining the pullout load–slip relationship, and modeling fiber bridging behavior. The experimental program included density calculations and scanning electron microscope (SEM) analysis to examine the surface morphology and diameter of the fibers. Single fiber pullout tests were performed under varying conditions to assess the pullout load, slip behavior, and failure modes of the palm fiber, and a relationship between the pullout load and slip with the embedded length of the palm fiber was constructed. A trilinear model was developed to describe the pullout load–slip behavior of single fibers, and a corresponding palm-FRCC bridging model was constructed using the results from these tests. Section analysis was conducted to assess the adaptability of the modeled bridging law calculations, and the analysis result of the bending moment–curvature relationship shows a good agreement with the experimental results obtained from the four-point bending test of palm-FRCC. These findings demonstrate the potential of palm fibers in improving the mechanical performance of FRCC and contribute to the broader understanding of natural fiber reinforcement in cementitious composites.

Explainable AI based slip prediction of steel-UHPC interface connected by shear studs

Recently, ultrahigh performance concrete (UHPC)-steel composite structures with shear studs have garnered interest for their potential high ductility and resilience in infrastructure. Ensuring their ductility is crucial for failure prediction and resilient infrastructure development. However, pull-out tests show that studs in UHPC often fail to meet a minimum relative slip of 6 mm, with no established direct methods for predicting this slip. This study proposes a novel approach to evaluate the shear slip at the UHPC-steel interface connected by shear studs, utilizing explainable artificial intelligence. We integrate a Light Gradient Boosting Machine (LightGBM) algorithm with SHapley Additive exPlanations (SHAP) to predict and interpret shear slip. The model development utilizes a dataset of 184 instances from UHPC-steel pullout tests, encompassing 16 features related to UHPC slab characteristics, shear pockets, and stud shear connectors. Engineered features and shear strength are strategically incorporated as predictors under four scenarios to achieve the most optimal model. The best performing model achieves an RMSE of 0.814 mm, an MAE of 0.598 mm, and an R2 of 0.838. The model’s superiority is reaffirmed through comparative analysis against five alternative tree-based ensemble models. SHAP analysis demonstrates that features related to the UHPC slab and stud shear connectors contribute 65 % and 35 % respectively to slip prediction. The top influential features are dst, sl and fc from the stud, and fu from the slab. The engineered features and ultimate load per stud indicate limited efficacy in improving model precision. Higher values of dst and fu positively contribute to shear slip prediction, whereas higher values of sl negatively impact shear slip prediction. Based on several SHAP analyses, a set of parameters for the potential ductile design of stud shear connectors in UHPC slabs is proposed. Overall, the integration of LightGBM and SHAP maintains high generalizability and enhances model transparency, instilling confidence for practical deployment and promising tangible advancements in real-world structural applications.

Seismic Performance Analysis of the Internal Joint in the New Demountable Fabricated Concrete Frame with Prestressed Mortise–Tenon Connections

This paper proposed a novel demountable fabricated joint in frame, which is connected by the prestressed and mortise–tenon connection. The prefabricated components of the demountable structures are designed to be reused, and the joint presented in this paper will promote the sustainable application of prefabricated components in future. The damage process and damage pattern of the internal joints under the horizontal load were analyzed using the refined numerical analysis model based on ABAQUS 6.14. Parametric analyses were conducted simultaneously for five parameters: axial compression ratio, the area and effective initial stress of unbonded prestressed strands (UPSs), the local reinforcement ratio in the core zone of the demountable joint, and the friction coefficient between the interface of concrete. The results showed that the demountable joint exhibits excellent energy dissipation potential under horizontal loads, but the damage was concentrated in the core zone. The deformation of the joint mainly consisted of the self-deformation of the prefabricated components, including bending, bearing and shear, as well as the relative slip deformation between the prefabricated components. The axial compression ratio has a more significant effect on the hysteresis performance compared to the areas of the UPSs and the reinforcement ratio. The initial effective stress of the UPS and the friction coefficient have relatively minor influence on the hysteretic performance of the joint. Finally, this paper recommends the design parameter values (axial compression ratio should not exceed 0.4, area of unbonded prestressed reinforcement should be not lower than Asn=0.02 and not higher than Asn=0.1, the initial stress of the UPS takes the value of 0.75fpu) and outlines optimization measures.

Probing fretting wear behavior of gauge-changeable spline axle under rotational bending loads

The latest generation of gauge-changeable trains in China feature designs with their wheels and axles connected by spline couples and cylindrical fits. In comparison to the interference fitted wheelset, the spline axle with clearance fit is more prone to fretting damage. In this study, the stress distribution of spline axle was analyzed using finite element method. Then, a rotatory bending test on scaled wheel/axle samples was conducted to reveal the fretting wear behavior of the gauge-changeable axle. During traction, the contact stress of the spline peaks at 311 MPa on the tooth’s root side, with a corresponding relative slip of 14 μm. Fretting wear is observed in the spline axle, the damage zone of the wheel seat can be categorized into five bands (A–E), and severely damaged areas are covered with red-brown debris. Moreover, the main damage mechanism of untreated spline axle and surface-modified spline axle is delamination accompanied with oxidation. The improved properties of plasma nitriding and grease lubricant are attributed to the high surface hardness and residual compressive stress, as well as low frictional stress.

Experimental and analytical investigation of bracket moment connection for precast concrete beam-to-composite column joints

In large industrial buildings, precast prestressed concrete beams and composite columns are appealing to achieve longer span and higher story height, with temporary supporting and scaffolding minimized. To facilitate field erection of moment connection between precast beams and composite columns, a bracket connection transferring beam loads to the column by connecting the beam to a composite bracket attached to the column has been developed. Since precast beams are lap-spliced and integrated with the bracket by concrete engagement at the interfaces, the connection performance depends on beam lap length and load transfer details (i.e., concrete shear keys). In this study, the seismic performance of precast prestressed concrete beam-to-composite column joints with the bracket connection was experimentally and analytically investigated. Cyclic loading tests were conducted on four full-scale specimens with different beam lap lengths and load transfer details. For the specimens with a longer beam lap length, the bracket connection successfully transferred the beam flexural strength to the column with the connection barely damaged. The governing failure mode was beam flexural yield occurring at the bracket end. However, for the specimen with a shorter beam lap length, relative slips at the interfaces of the beam and bracket occurred and thus the strength was governed by the bracket connection. Excellent ductility satisfying the acceptance limit of special moment frame in AISC 341 was achieved in all specimens. The load transfer at the bracket connection through concrete shear keys and other details were further investigated through finite element analysis. Design models to predict the connection strength were proposed and the validity of the models was verified through comparisons with the tests and analyses.

Shear behavior of short stud in I-beam-ultra-high performance concrete (UHPC) composite structure

As the connecting component of I-beam and UHPC, the stud is the basis of the cooperative work of each component, and is crucial to the stability and safety of the structure. To further study the shear behavior of studs in I-beam-UHPC composite structures, the push-out test was carried out by numerical simulation. The numerical simulation is in line with the load-relative slip curve and failure mode obtained by the experiment, verifying the accuracy of the numerical model. On the basis of verifying the model, the influences of the diameter, height and ultimate tensile strength of the stud on the relative slip curve, shear strength, yield strength and shear stiffness of the stud are studied. The findings indicate that a stud with a diameter of 22 mm exhibits a shear strength 156% greater than that of a stud with a diameter of 10 mm. Additionally, the yield strength increases by 135%, and the shear stiffness by 160 kN, highlighting the significant impact of stud diameter on shear resistance. When the ratio of length to diameter is less than 2.7, the shear strength of the stud increases with the height of the stud. The shear strength of a stud with a length-to-diameter ratio of 2.7 is 35% higher than that of a stud with a ratio of 1.5. The ultimate tensile strength of the stud ranges from 360 to 510 MPa, with only an 11 kN increase in yield strength, indicating that the ultimate tensile strength has a limited influence on shear properties. As the height of the stud decreases, its ultimate tensile strength gradually increases, bringing the numerical model closer to the predicted values. For shorter studs with higher ultimate tensile strength, the finite element simulation results are closer to the predicted values.

Investigation of Dual Network Construction for Toughening in Bio-Based Polyamide Composites.

This study investigated the role of constructing a dual network in toughening bio-based long-chain polyamide 610 (PA610) composites. Rheological studies were conducted to reveal the effects of toughening agent type and content on the material properties. According to the variation trend of mechanical properties and the appearance of a rheological low-frequency plateau of the materials, the percolation network concentration ϕc of the toughening agent in the PA610 matrix was determined to be 13.5 vol.%. The interfacial interaction of the composite was evaluated through the percolation theory, and the scaling value v = 1.36 for both indicated the good affinity between PA610 and the toughening agent. Rheology results found that the combination of ethylene terpolymer (PTW) and maleic anhydride-g-styrene-b-(ethylene-butylene)-b-styrene (MAH-SEBS) could achieve an optimal balance between the mechanical properties and fluidity of the composites. Furthermore, the addition of ultra-high-molecular-weight polytetrafluoroethylene (PTFE), in conjunction with the toughening agent, facilitated the construction of a dual semi-interpenetrating network. The strengthened intermolecular interactions restricted the relative slippage and mobility of the polymer chains and therefore enhanced the strength and toughness of the material. This study provides new possibilities and approaches for optimizing the comprehensive properties of bio-based polyamide materials.

Study on interfacial mechanical properties of bonded pipe joint under dynamic load

This study analyses the interfacial mechanical behaviour of adhesive-bonded pipe joints under dynamic loading conditions. First, a mechanical model of the bonding interface of a pipe joint was established. Next, computational formula for the interfacial slip and shear stress in the pipe joints were obtained using variable separation, eigenfunction expansion, and Laplace transform methodologies. Further, the effects of various parameters, including the loading duration, bond length, adhesive layer thickness, stress rate, adhesive shear modulus, elastic moduli of the main pipe and coupler pipe, and adhesive-layer thickness, on the mechanical response of the pipe joints were assessed. The goal is to study the impact of these parameters on the maximum shear stress, interfacial slip, and normal stress in the main and coupler pipes, their effects on the pipe joint strength can be determined, thereby providing a theoretical basis for the design of practical pipe–joint configurations. The innovation of this study is as follows: it considers high-stress-rate dynamic loads for formulating complete mathematical equations. Further, it employs theoretical methods to calculate the relative slip and shear stress in pipe joints, through which theoretical formulas for engineering applications of adhesive-bonded pipe joints were derived. Additionally, the study analyses the impact of various parameters on the dynamic mechanical behaviour of the adhesive layer at the pipe joint interface, leading to a deeper understanding of the interface mechanical behaviour characteristics and key parameters to be considered while receiving dynamic loads in pipe joints.

A Novel Geothermal Wellbore Model Based on the Drift-Flux Approach

Geothermal energy, being a clean energy source, has immense potential, and accurate wellbore modeling is crucial for optimizing the drilling process and ensuring safety. This paper presents a novel geothermal wellbore model based on the drift-flux approach, tested under three different temperature and pressure well conditions. The proposed model integrates the conservation equations of mass, momentum, and energy, incorporating the gas–liquid two-phase flow drift-flux model and heat transfer model. The key features include handling the heat transfer between the formation and the wellbore, addressing the slip relationship between the gas and liquid phases, and accounting for wellbore friction. The nonlinear equations are discretized using the finite difference method, and the highly nonlinear system is solved using the Newton–Raphson method. The numerical simulation, validation, and comparison with existing models demonstrate the enhanced accuracy of this model. In our tests, the model achieved a high accuracy in calculating the bottom-hole pressure and temperature, with mean relative errors (MREs) significantly lower than those of other models. For example, the MREs for the bottom-hole pressure and temperature of the Rongxi area well in Xiongan, calculated by this model, are 1.491% and 1.323%, respectively. These results offer valuable insights for optimizing drilling parameters and ensuring drilling safety. Comparisons indicate that this approach significantly outperforms others in capturing the complex dynamics of geothermal wellbores, making it a superior tool for geothermal energy development.

Mechanochromism of π-conjugated borate derivatives and the effect of intermolecular non-covalent forces on molecular packing and mechanochromic property

We prepared three π-conjugated pinacol borate derivatives anthryl dioxaborolane (ANB), pyrenyl dioxaborolane (PYB) and perylenyl dioxaborolane (PEB) based on anthracene, pyrene and perylene moieties, and studied their optical properties in solid and solution. Interestingly, the three molecules were structurally similar, yet exhibited different optical properties and mechanochromic behavior. Crystal structure studies showed that the mechanical fluorescent response of ANB, PYB and PEB were closely related to the molecular packing modes. Although there were multiple π-π and σ-π interactions in all three molecules, the conjugation degree of ANB was relatively low, resulting in a weak π-π interaction. Therefore, the effective partial parallel packing between adjacent ANB could not form in the initial state, and external mechanical stimulation could not induce the change of optical properties. In PYB and PEB molecules with a high conjugation degree, the π-π interaction was stronger. A partial parallel packing state was formed between the molecules under the synergistic action of π-π and σ-π interaction. Under external disturbance, the relative slippage between molecules was easy to occur. Thus, the characteristic static excimer fluorescence was displayed due to the enhancement of parallel packing. Since the intermolecular interactions of PYB and PEB were a kind of weak non-covalent forces, the packing state changed by external force could return to the initial state at room temperature, showing reversible mechanochromic characteristic. This kind of room temperature reversible force sensitive material constructed by weak non-covalent interactions was expected to be applied and popularized in the fields of encrypted communication and anti-counterfeiting display.

Outstanding interlaminar strength of carbon fiber reinforced epoxy resin via graphene oxide chemical bridge bonding

Carbon fiber reinforced polymers (CFRPs) are susceptible to interlaminar failure under high shear stress, which reduces their service life. Herein, we report a strategy to address the interlaminar damage problem of CFRPs by utilizing graphene oxide (GO) as a bridge to chemically bond carbon fibers and epoxy resin. Amino groups were grafted on the surface of carbon fibers to react with GO, then active amino functional groups were introduced onto GO, enabling crosslink with the epoxy resin. The chemical bonding force and mechanical interlocking effect of GO tightly integrated the carbon fibers with the epoxy resin, greatly reducing relative slip under high shear stress. This flexible and robust connection resulted in a significant improvement in the interfacial adhesion strength of CFRPs. The GO content and type of amino reactants were optimized to fabricate CFRPs, and the interlaminar shear strength and transverse fiber bundle tensile strength were increased to 76.36 MPa and 34.2 MPa, which were 46.2 % and 77.2 % higher than those of pre-modification materials, respectively. This research demonstrates that the chemical bonding provided by grafting graphene oxide significantly enhances interlaminar bond strength, thereby extending the service life of CFRPs and offering a promising path for manufacturing high-strength composites.

Experimental and numerical investigation on flexural behavior of steel-UHPC composite slabs with PBL shear connectors

A full-scale flexural test was conducted to investigate the flexural and cracking behavior of Steel-UHPC Composite Slabs (SUCS) with PBL (Perfobond rib) shear connectors, with a focus on analyzing the impact of the tensile reinforcement ratio on flexural performance. During the test, the flexural performance of SUCS was assessed through measurements of strain, mid-span deflection, relative slip, crack width, and crack morphology. A section analysis was performed to predict the flexural and cracking behaviors of SUCS, which was further verified by experimental results. The results show that the PBL shear key effectively ensures the overall force of steel plate and UHPC plate, resulting in good ductility of SUCS under negative bending moment. As tensile reinforcement ratio increased from 0.89 % to 1.69 %, the cracking load remained stable while the yield load and ultimate load demonstrated significant improvements of 54 % and 57 %, respectively. The cross-section analysis indicated that with the increment in tensile reinforcement ratio, the contribution of tensile reinforcement to flexural capacity increased, whereas the contribution of UHPC to tensile capacity notably decreased. The equivalent uniform stress reduction coefficient of the UHPC's tensile zone, determined through simplified negative flexural bearing capacity, was calculated to be 0.87. The failure mode of negative flexural resistance differs significantly from positive flexural resistance, as it involves the breaking of the tensile steel bar.

Experimental and analytical investigations into flexural behavior of composite beams with UHPC T-section and HRS H-section

This study investigated the flexural behavior of composite beams with ultra-high performance concrete (UHPC) T-section and hot rolled steel (HRS) H-section. First, a four-point bending test on six specimens was conducted. The effects of stud spacing and tensile reinforcement ratio on flexural performance were investigated in terms of crack patterns, failure modes, ultimate load, load-strain response, relative interfacial slip, and ductility. The experimental results indicated that the stud spacing and the presence of tensile reinforcement had a visible effect on the crack patterns of UHPC web. The distribution of group cracks was observed at UHPC web of specimens with unreinforced UHPC bulb. During the failure stage, the whole HRS H-section in the constant moment region yielded. In particular, the partial cross section of HRS entered the strain hardening stage. In addition, an analytical approach verified by six tested specimens was developed to study the moment-curvature response of HRS-UHPC composite beams (HUCBs). The comparison results indicated the good accuracy of the analytical approach. Then, the proposed sectional analysis model was utilized to investigate the effects of the strength of HRS and UHPC, the total depth of cross section, and the depth of HRS H-section on yielding moment, including ultimate moment and ductility. The analytical results revealed that the strength of HRS and the total depth of cross section had a significant impact on the moment-curvature response of HUCBs. The sectional analysis also illustrated that the yielding point was mainly determined by the yielding of HRS H-beam. Compared with UC120, the ultimate moment and ductility coefficient of the composite beam using UC200 increased by 20.6 % and 87.7 %, respectively. However, the tensile strength of UHPC had a slight influence on the yielding and ultimate moment as well as ductility.

Estimation of the bar stress based on crack width measurements in reinforced concrete structures

AbstractEstimating the stress of reinforcing bars and its variations in service conditions can be useful to determine the reserve capacity of structures or to assess the risk of fatigue in the reinforcement. This paper investigates the use crack width measurements to estimate the stress in the bars. In existing structures, crack width formulations can be used to estimate the stress in the reinforcement from crack width measurements, profiting from additional information that can be measured in‐situ, such as the crack spacing. Recent experimental results show that the values of the mean bond stress typically considered in code formulations overestimate the actual bond stresses activated in cracked concrete specimens. This paper presents the results of an experimental program consisting of reinforced concrete ties and beams instrumented with Digital Image Correlation and fiber optical measurements. The results confirm the differences with typically assumed bond stresses. A formulation to estimate the bond stresses in service conditions is derived from the results of the numerical integration of a previously developed local bond–slip relationship. Their pertinence for the estimation of the stress in the reinforcement from the measured crack width is evaluated with satisfactory results for monotonic loading and for the maximum force in cyclic tests.