Bond-slip Relationship Research Articles

Numerical Investigation on the Seismic Behavior of Novel Precast Beam–Column Joints with Mechanical Connections

Traditional cast-in-place beam–column joints have the defects of high complexity and high construction difficulty, which seriously affect the efficiency and safety of the building construction line, and precast beam–column joints (PBCJs) can greatly improve the construction efficiency and quality. At present, the investigations on the seismic behavior of precast reinforced concrete structures are still mainly focused on experiments, while the numerical simulations for their own characteristics are still relatively lacking. In the present study, the seismic behavior of novel precast beam–column joints with mechanical connections (PBCJs-MCs) is investigated numerically. Based on the available experimental data, fiber models for four PBCJs-MCs are developed. Then, the simulated and experimental seismic behaviors of the prefabricated BCJs are compared and discussed. Finally, the factors influencing the seismic behavior of the PBCJs-MCs are further investigated numerically. The numerical results indicate that the fiber models can consider the effect of the bond–slip relationship of concrete and reinforcement under reciprocating loads. The relative errors of the simulated seismic behavior indexes are about 15%. The bearing capacity and displacement ductility coefficients of the PBCJs-MCs decrease rapidly as the shear-to-span ratio (λ) increases. It is recommended that the optimum λ for PBCJs-MCs is 2.0–2.5. The effect of the axial load ratio on the seismic behavior of PBCJs-MCs can be negligible in the case of the PBCJs-MCs with a moderate value of λ.

Experimental investigation of dynamic bond behaviors between LRS-FRP and concrete

The dynamic bonding behaviour at the interface between a large-rupture-strain fibre-reinforced polymer (LRS-FRP) and concrete was investigated via drop weight impact tests of notched beam specimens. The main experimental variables included the concrete strength grade, number of LRS-FRP layers, and loading rate. With increasing loading rate, the location of the peeling damage at the interface between the LRS-FRP and concrete shifted from the interior of the concrete layer to the concrete–binder layer interface, binder layer, and even binder layer–FRP interface. The ultimate debonding load of the LRS-FRP strips increased with increasing strain rate, reaching more than 1.7 times the static value at a strain rate of approximately 4/s, while the effective bond length did not show a clear increasing or decreasing tendency with increasing strain rate. For the local bond-slip relationship, both the peak interfacial bond stress and interfacial fracture energy increased with increasing strain rate, and the influences of the concrete strength and number of FRP layers on the strain rate sensitivity were insignificant. Based on the experimental results, improved models to predict the ultimate debonding load and effective bond length at the LRS-FRP–concrete interface under both static and dynamic loading conditions were proposed. Furthermore, dynamic increase factor (DIF) formulas for the peak interfacial bond stress and interfacial fracture energy for bond-slip modelling of the interface between a polyethylene naphthalate (PEN)-FRP and concrete were established.

Bond behavior of the FRP grid-concrete interface with geopolymer mortar as an adhesive

This study conducts an investigation into the effect of the geopolymer mortar (GPM) thickness on the bond behavior of the FRP grid-concrete interface. Eighteen specimens were tested under the single-lap shear loading to evaluate the effect of the mortar thickness (8/10/15/20/25 mm) and macro fiber reinforcement from failure mode, load response, strain distribution, and bond stress distribution. The study further assessed the applicability of existing calculation models for the FRP grid-concrete interface. The results indicate that the ultimate load initially increased and then decreased as the mortar thickness increased, and the macro fiber reinforced mortar enhanced the ultimate load. However, mortar thickness variation and macro fiber reinforcement had little effect on the failure mode, strain distribution, and bond stress distribution, while lager thickness and macro fibers significantly reduced crack occurrences. Additionally, existing models for the FRP sheet-concrete interface tended to underestimate the effective bond length and interface fracture energy of the FRP grid-concrete interface. Regression analysis, based on existing models, enabled the development of effective bond length and bond-slip relationships, showcasing satisfactory accuracy.

Experimental investigation of flexural bond behavior of sand-coated GFRP rebar embedded in concrete

Steel rebars used in Reinforced Concrete (RC) elements may be subjected to corrosion due to aggressive environmental conditions. Rebar corrosion reduces the durability and the structural capacity of RC elements. On the other hand, since the steel rebars affect the magnetic fields, it is not preferred to be used in RC structures which is used to carry the equipment that propagate magnetic waves such as toll plazas on highways. For these reasons, the use of Fiber Reinforced Polymer (FRP) rebars is increasing, especially in RC infrastructure constructions. The objective of the present study is to investigate the bond behavior of Glass Fiber Reinforced Polymer (GFRP) rebar with a sand-coated outer surface in concrete using the arched beam test method. The concrete strength and the embedment length were chosen as variable parameters and 34 beam test specimens were tested in the laboratory. A comparative analysis of the crack patterns, the diagonal crack angles and the failure modes of the specimens is made. In addition, the experimentally determined bond-slip relationships were compared with different analytical models in the literature. Consequently, a bond-slip model, which takes into account CMR (Cosenza-Manfredi-Realfonzo) for the increasing part and mBPE (modified Bertero-Popov-Eligehausen) for the decreasing part, is proposed to be used in the numerical models of RC beams with sand-coated GFRP rebars. A prediction model for the bond strength is proposed, along with a recommendation for an upper bound value for the bond stress, which indirectly offers a minimum embedment length for a bond without slippage.

Evaluation of residual bond behaviour between rebar and recycled aggregate concrete after high-temperature exposure followed by water spray cooling

This paper presents an evaluation of the residual bond behaviour between rebar and recycled aggregate concrete (RAC) using granite natural coarse aggregates and different recycled coarse aggregate (RCA) replacement levels (δ = 0%, 30%, 50%, and 100%) for both pull-out and cubic specimens. After high-temperature exposure (T = 200, 400, and 600 ℃) followed by water spray cooling, the results revealed a consistent decline in bond strength and compression strength with increasing temperature. Compared to ambient conditions, bond strength and compression strength decreased by 65.2% ∼ 77.8% and 53.2% ∼ 58.4%, respectively, after exposure to 600 ℃. The reduction rate of residual bond strength changed significantly at about 400 ℃. There was a linear relationship between RAC compression strength and rebar–RAC bond strength after high temperature exposure followed by water spray cooling. With increasing RCA replacement levels, the relative bond strength gradually improved at the same temperature. Based on the regression analysis, empirical formula for the rebar–RAC bond strength and bond–slip relationship models were proposed, all taking into account RCA replacement levels and temperature.

Experimental investigation on withdrawal resistance performance of nails in southern pine

The objective of this study is to evaluate the structural performance and withdrawal capacity of three types of nails embedded in southern pine (SP) framing members. Pull-out tests on 430 specimens were conducted until complete withdrawal occurred. Representative variables derived from practical wall-to-wall or wall-to-floor connections were considered, including nail type, nail diameter, embedment length, and nail direction. Results reveal that the failure modes of specimens depend on nail shank roughness. Smooth shank nails (SSNs) and concrete nails (CNs) mainly experience nail-wood interface sliding and wood fiber tearing failures, while zinc-coated helically threaded nails (ZHTNs) exhibit wood fiber shearing and tearing failures. As expected, the ultimate withdrawal capacity increased with greater embedment length, with the highest capacity of nails in the perpendicular-to-grain direction, followed by parallel-to-grain. The withdrawal load-slip relationships of nails in SP consist of three stages: linear-elastic rise, plunge, and failure stage for SSNs and CNs, while ZHTNs exhibit elastic-plastic behavior during the second phase. The modified Rafi model suggested in this study can be used to translate the withdrawal bond-slip relationships of nails in SP. Finally, a withdrawal resistance calculation equation for SP was put forward and validated, showing an average difference of 10.33% between theoretical and experimental values.

Experimental study on bonding properties of Fe-SMA-to-steel bonded interface

The interfacial bonding performance between Fe-based shape memory alloys (Fe-SMA) and steel is of utmost importance for the reinforcement of steel structures using Fe-SMA. In order to better reveal the bonding performance and ensure the reliability of Fe-SMA-to-steel bonded interfaces, the single-lap-shear tests of 27 adhesive joints were conducted. The experimental results indicate that the bearing capacity increases with the bonding length until reaching the effective bonding length (Le), with SA, Sika, and LC adhesive joints possessing Le of around 90 mm, 90 mm, and 100 mm, respectively. Better bonding performance can be achieved with 1–1.5 mm thickness for the 3 types of adhesive joints, while the bearing capacity of LC adhesive joints is less affected by the adhesive thickness. The SA joints experienced interface debonding failure, the Sika joints experienced the mixed failure mode of interface debonding and cohesive failure, and the LC joints exhibited cohesive failure. For the bonded interfaces, cohesive failure is the preferred mode, evidenced by Fe-SMA plates in LC joints reaching 60% of their tensile strength at failure. Furthermore, utilizing the established constitutive model of Fe-SMA and the strain propagation from experiments, the shear stress evolution and interfacial bond-slip relationship of Fe-SMA-to-steel bonded interfaces were analyzed, demonstrating that the nonlinear behavior and high ductility of Fe-SMA significantly affect the load-bearing performance and failure evolution. This paper verifies the feasibility of reinforcing steel structures with Fe-SMA through adhesive bonding connection, providing an experimental basis for the theoretical analysis and practical application of Fe-SMA-to-steel bonded interfaces.

Enhancing bond performance of CFRP-steel epoxy-bonded interface by electrospun nanofiber veils

The epoxy-bonded interfaces between carbon fiber reinforced polymer (CFRP) and steel usually have insufficient strength and toughness, and the toughening of bonded interface is a key problem for the usage of CFRP in steel structures. In this study, electrospun nanofiber veils were first proposed to enhance the bond performance of CFRP-steel epoxy-bonded interfaces. Firstly, shear tests were conducted on neat epoxy and nano-modified single-lap aluminum-aluminum joints to determine the optimal areal density and number of layers of nanofiber veils, as well as the optimal curing processes. Then, a series of neat epoxy and nano-modified CFRP-steel double-lap joints with different bond lengths were tested to investigate the size effect of the bond behavior. The displacement and strain field evolution of the joints were captured by the digital image correlation (DIC) technique, allowing for visualization of the detailed failure process. The failure modes, load-displacement curves, CFRP strain distributions, and bond-slip relationships of the CFRP-steel joints were obtained. Both the tests on aluminum-aluminum and CFRP-steel joints show that the optimal modification strategy is incorporating 3 layers of nanofiber wels with an areal density of 4.5 g/m2, with 5 h room-temperature and 2 h 80 °C high-temperature curing. The primary failure mode of CFRP-steel joints is CFRP delamination accompanied by CFRP-adhesive interface or steel-adhesive interface debonding. The bond strengths of the modified joints with 3 layers of 1.5 g/m2 and 4.5 g/m2 nanofiber veils are increased by 7 % and 25 % compared to those of un-modified joints, respectively. The 4.5 g/m2 nanofiber veils modified bonded interface has an effective bond length of about 152 mm, with a corresponding ultimate bearing capacity of 117 kN. Different from the triangular (brittle) shape of most neat epoxy interfaces, the nano-modified interfaces have trapezoidal (ductile) bond-slip relationships, providing superior cracking resistance. Moreover, a comparison with the bond strength of SiO2 nano-particles and carbon nanotubes (CNTs) modified joints revealed that nanofiber veil modification comes to higher bond strength in most cases. The proposed electrospun nanofiber veil modification technique provides a great insight into the interfacial toughening of CFRP-steel composite structures.

Bond-slip behavior of deformed rebar in grouted duct connection: Experiment, theoretical analysis and cohesive-zone element model

In precast bridge piers, the grouted corrugated duct (GCD) connection can offer advantages of accommodating large construction tolerances and being cost-effective. However, this connection is relatively new, and its bond strength and effective bond length are a concern in structural design. To fill the research gap of the bond behaviors of the GCD connection, experimental tests of 36 pull-out specimens with/without GCD were conducted in this paper. The influencing factors of bond strength were investigated, including duct, rebar diameter and anchorage length. The results revealed that the GCD connection required a shorter anchorage length and exhibited a plumper bond-slip curve and higher bond strength compared to the specimen without GCD connection. Then, an analytical model based on thick-walled cylinder theory was proposed to construct the bond-slip curve of GCD connection, and to identify the criteria for mortar strength. To simulate the bond-slip relationship in finite element model, a cohesive-zone element model realized by the zero-thickness user defined element was developed at the interface between rebar and dummy rebar. Finally, a comparison was made among the solution of proposed bond-slip model, finite element analysis, and the tested data, demonstrating good consistency between the three.

Analytic solution of pull-out failure on bond-slip relationship between deformed rebar and ultra-high-performance concrete

Numerous studies have explored the bond performance between deformed rebar and ultra-high-performance concrete (UHPC), yet an analytical solution to the bond-slip relationship remains elusive. This study presents an analytical solution of pull-out failure for the staged bond-slip constitutive relationship between rebar and UHPC, taking into account the meso-scale bond properties of fibers. The critical point coordinates for different stages are determined by the analytical solution. Utilizing the elastic mechanic solution of thick-walled cylinder under uniform internal pressure, the fictitious crack model, and the fiber-matrix discrete model, analytical solutions and simplified formulas for bond stress and slip at the critical points are provided respectively. The simplified formulas for predicting bond characteristics are proposed. The bond-slip test data are composed of 107 groups from 15 literatures, and the predictions of different formulas are compared. The average ratios of the theoretical ultimate bond stress and peak slip to the test results are 0.995 and 1.033, with a standard deviation of 0.179 and 0.231, and the error is generally within 20%. Compared with the bond-slip relationship, it is more reasonable that the ascending and descending sections are exponential. Moreover, the parameters in the bond-slip relationship can be calculated by the theoretical predictions of bond characteristics at each critical point.

Experimental and Analytical Investigation of Moment-Carrying Capacities of Reinforced Concrete Frame Systems Corroded by Accelerated Corrosion Method

Technical investigations carried out after the devastating earthquakes in Turkey in the last 25 years show that one of the main causes of damage to reinforced concrete structures is reinforcement bar corrosion. The deterioration caused by reinforcement corrosion in reinforced concrete elements reduces the parameters that define the performance level of the structure, such as ductility, stiffness, bond-slip relationship, load carrying capacity, and energy absorption capacity. Therefore, determining the specified performance levels of reinforced concrete structures exposed to corrosion before any seismic activity occurs is of great importance in preventing loss of life and property. Cross-section behavior in reinforced concrete structures represents the load-bearing element behavior In order to accurately determine the section behavior, the Moment-Curvature relationship must be defined. In this study, the structural behavior of reinforced concrete frames exposed to corrosion was examined experimentally and analytically. For this purpose, 4 of the 5 reinforced concrete frame specimens constructed were corroded at different ratios using the accelerated corrosion method. After the corrosion process was completed, in the experimental part of the study, all specimens were tested under the effect of a 20% constant axial load and reversal-cyclic loading. In the analytical study, as a result of the cross-sectional analyzes carried out by taking into account the material-mechanical properties changing with the effect of corrosion, it was determined that the calculated moment values were in high predicted with the experimentally measured moment values.

Experimental study on the mixed mode debonding by using a modified CFRP-to-steel double strap joint

The bond response of CFRP-to-steel joints in mixed mode-I/ΙΙ is a big concern since the loads consistent with mode-I may decrease the shear load capacity of the joints. For this motive, the current work introduces a modified CFRP-to-steel double strap bonded joint in which different initial peeling angles can be created. The specimens were monotonically loaded with a tensile machine until failure that subjected the CFRP-to-steel joint to a pull-pull condition. Although the specimens were geometrically symmetric throughout their length, the results showed that the initial debonding only occurred on one side of the two bonded CFRP strips. The side that initially debonded showed an interfacial failure between the CFRP strip and the adhesive, while the other side showed a CFRP delamination. The increase of the peeling angle significantly affected the initial shear load capacity, which decreased from the baseline 0° by 50% and 70% when 2° and 3° were adopted, respectively. Moreover, a peeling angle of 4º was found to be the critical peeling angle for the adopted CFRP-to-steel joints, i.e. the angle beyond which the specimens debond when subjected to pure mode-I loading even before any loads consistent with pure mode-II were applied to the bonded joints. The initial debonding induced a quick increase in the global slip and lower stiffness in the load-slip relationship. For the bonded region near the CFRP-loaded end, the maximum bond shear stress decreased from 20 to 7.5 MPa when the initial peeling angle varied from 0º-3º. To represent these effects on the local bond behavior, a new nonlinear bond-slip relationship is proposed and adjusted with the Integrate Absolute Errors (IAE) to the experimental results, which was achieved with IAE values lower than 4%. The results show, therefore, a high precision and versatility of the model to locally represent the bond behavior of the present CFRP-to-steel joints with different initial peeling angles.

Shear performance and size effect of CFRP strengthened RC beams

In this study, a 3D meso-scale numerical simulation method is established for analyzing the shear failure of carbon fiber reinforced polymer (CFRP) strengthened RC beams without stirrups. The inhomogeneity of concrete, the bond-slip relationship between steel bars and concrete, and the interaction between CFRP and concrete are considered. Based on this method, effects of shear-span ratio and CFRP fiber ratio on the shear performance and nominal shear strength size effect of CFRP strengthened RC beams are investigated. Results indicate that the size effect of CFRP strengthened RC beams is obvious, however, the shear-span ratio has little impact on the size effect of nominal shear strength of RC beams. The CFRP fiber ratio enhances the shear strength of RC beams on the one hand, while weakens the shear size effect of RC beams on the other hand. However, its influence on the shear performance of RC beams remains basically unchanged with varying shear-span ratios, indicating that there is no coupling effect between the shear-span ratio and CFRP fiber ratio. The shear-span ratio significantly influences both the shear bearing capacity of RC beams and the shear contribution of CFRP sheets, with the increase in the shear-span ratio, the shear bearing capacity of RC beams decreases to varying degrees, and the shear contribution of CFRP sheets increases initially and then decreases. Finally, the shear strength size effect law (SEL) of CFRP strengthened RC beams is proposed, which can quantitatively describe the effects of the shear-span ratio and the CFRP fiber ratio.

Mechanical behaviors of steel-CFRP plate bonded joints after freezing-thawing cycles

The effective bonding between the steel element and carbon fiber reinforced polymer (CFRP) is critical for applying the CFRP to strengthen steel elements. The severe service environment always affects the interfacial bond strength, resulting in a reduction in strengthening efficiency. To research the interfacial bond performance between the steel element and CFRP plate after freezing-thawing environment, this paper conducted studies on the mechanical behaviors of steel-CFRP plate joints. Experiments indicated that the freezing-thawing environment did not influence the failure mode, but decreased the interfacial ultimate capacity of double-shear specimens. The ultimate load is reduced by 14.5% after exposure to 300 freezing-thawing cycles. An approximate bilinear bond-slip relationship is employed for expressing the interfacial bond-slip behavior of all specimens, and the freezing-thawing environment has a negative influence on the bond-slip behavior. By determining the degeneration laws of bond-slip parameters after experiencing freezing-thawing cycles, the bilinear bond-slip model considering freezing-thawing effect was developed. Moreover, the three-dimensional FE modelling was implemented based on the mixed-mode cohesive zone model. The comparisons of the test and FE results show that the FE results are consistent with the test results, and the interfacial damage and debonding process can also be precisely predicted. The numerical modelling also reveals that the asymmetric debonding of the two interfaces can affect the ultimate load, which leads to a higher scatter of test results for the double-shear joint.

Study on Bond-slip Behavior between Non-uniformly Corroded Reinforcement and Concrete

In order to improve the stress concentration in concrete at steel nodes caused by zero thickness springs, the improved Spring2 nonlinear spring model from ABAQUS is used to simulate the bond-slip behavior between non-uniformly corroded reinforcement and concrete. Based on the collected test results with different stirrup spacing, fitting the influence coefficients of bond stress under stirrup corrosion. With the combination of a non-uniformly corroded rebar distribution model, the refined numerical simulation of the drawing process of reinforced concrete specimens was carried out using ABAQUS. The study shows that the improved nonlinear spring model is better than the zero thickness spring model in terms of computational accuracy and efficiency. The stirrup correction coefficient is approximately linear with the stirrup spacing; the bond-slip relationship considering the effect of stirrup spacing, has a better consistency with the relevant tests in the simulation of the pullout test. Weibull distribution for the fitting of non-uniform corrosion of reinforcement is more reasonable than Gumbel distribution.

Crack monitoring of tension members with distributed fiber optic sensor considering substrate strain redistribution and coating/fiber interfacial slip

An analytical model is proposed to describe the strain transfer from crack opening displacement (COD) of a tension member substrate to the distributed fiber optic sensor (DFOS), considering the DFOS/fiber coating interfacial slip and strain redistribution of the cracked structural substrate. The DFOS/fiber coating interfacial bond-slip relation is simplified by using a three-phase linear model, and the analytical expression of the substrate strain distribution of a tensile plate with different depth of crack is derived. The effect of the COD and crack depth on the strain distribution of the DFOS is obtained analytically, and tensile tests were also conducted on aluminum plates with different depth of precut crack to validate the developed model. Analytical results showed that the strain distribution of the DFOS is dependent on the COD and the crack depth, and the effect of the COD on the strain distribution of the DFOS is more significant than the crack depth. The analytical results of the developed model are in good agreement with those of the tensile tests.

Mechanical behavior of longitudinally continuous slab tracks reinforced by adhesive anchors under nonlinear temperature load

Adhesive anchors have been widely used as a post-reinforcement measure to strengthen longitudinally continuous slab tracks. However, the mechanical behavior of the longitudinally continuous slab tracks reinforced by adhesive anchors under nonlinear temperature load has not been thoroughly investigated. Therefore, this paper was motivated to conduct experimental and numerical investigation on the temperature-induced deformation and damage of the reinforced track. Firstly, experimental tests were carried out to investigate the influence of the borehole conditions on the bond properties of post-installed adhesive anchors. Next, the bond-slip relationships between anchors and concrete were established for anchors in various borehole conditions. Then, a novel finite element model of the reinforced track incorporating a nonlinear cohesive zone model of structural interfaces was developed and validated by filed monitoring data. Finally, the effects of borehole conditions and number of invalid anchors on the deformation and damage behavior of the reinforced track under nonlinear temperature load were systematically studied. Results show: (1) The cubic compressive strength grade of concrete has a significant effect on the bond-slip curves between concrete and anchors of the longitudinally continuous slab track. (2) Wet boreholes and uncleaned boreholes would reduce elastic stiffness and ultimate bond strength between anchors and concrete by 11–37% respectively, compared with dry and cleaned boreholes. (3) The maximum vertical displacement of slabs could increase by 7% and 4% if the boreholes are wet and uncleaned respectively. (4) The interface at the center of slab end near damaged joint will not fail for the track with 1 or 2 invalid anchors when the temperature reaches its highest. However, interface debonding will occur with a temperature rise of 34.6 ℃ if there are 3 invalid anchors.

Bond Behavior of Recycled Tire Steel-Fiber-Reinforced Concrete and Basalt-Fiber-Reinforced Polymer Rebar after Prolonged Seawater Exposure

The integration of basalt-fiber-reinforced polymer (BFRP) rebars into concrete design standards still remains unrealized due to limited knowledge on the performance of the rebars in concrete, particularly in terms of bond durability in harsh conditions. In this work, we investigated the bond durability characteristics of BFRP rebars in fiber-reinforced self-compacting concrete (FRSCC) structures. To this aim, a number of 24 FRSCC pullout specimens reinforced with either BFRP rebar or glass-fiber-reinforced polymer, GFRP, rebar, which is a commonly used type of FRP, were fabricated. Half of these specimens were submerged in simulated seawater for a two-year span, while the other 12 similar specimens were maintained in standard laboratory conditions for comparative purposes. Subsequently, all 24 specimens underwent monotonic and fatigue pull-out tests. The exploration in this study focused on investigating the influence of the environmental condition, reinforcement type, and loading type on the bond stress versus slip relationship, maximum bond stress, and failure mode of the specimens. Based on the results obtained and by adopting the durability approach of industry standards for prediction of the bond retention of FRP-reinforced concrete, the bond strength retention between BFRP/GFRP and FRSCC after 50 years of exposure to seawater was estimated. The outcomes of the study are expected to enhance engineers’ confidence in the use of FRP, especially BFRP, for constructing durable and sustainable reinforced concrete structures in aggressive environments.

Effects of fiber content and concrete cover on the local bond behavior of helically ribbed GFRP bar and UHPC

The bond performance of GFRP bar to UHPC is of great concern in GFRP reinforced UHPC elements and structures. In this study, pullout tests were conducted on 45 specimens to investigate the local bond behavior of helically ribbed GFRP bar to UHPC which was characterized by the failure mode, the bond strength and the bond stress-slip curve. The test parameters included the fiber content in UHPC Vf (0 %–3 %) and the concrete cover C (12–66 mm) and their effects on the local bond behavior were accessed. Specimens with C ≥ 36 mm and Vf ≥ 2 % trended to exhibit the direct pullout failure while those specimens with C < 36 mm and Vf < 2 % generally behaved in the splitting pullout failure. GFRP bars with diameter of 18 mm embedded in UHPC had the bond strength of 18.76–54.11 MPa. The bond strength could be significantly enhanced by increasing the fiber content in UHPC or the cover thickness within a certain range. As the fiber content increased from 0 % to 2 %, the bond strength of GFRP bar to UHPC grew by 74.81 %–92.22 %. However, the bond strength showed almost negligible growth when the fiber content further increased from 2 % to 3 %. Similar phenomenon was observed in the effect of cover thickness on the bond strength. The critical cover thickness was found to be approximate 3 times of rebar diameter D in the case of embedment length Le = D. In addition, some interactions were observed in the effects of these two factors on the bond performance. Based on the test results, an updated equation including the influence of fiber content was proposed to predict the bond strength of ribbed GFRP bars embedded in UHPC. Further, a fitted model was developed to describe bond-slip relationship between GFRP bars and UHPC.

Mesoscale numerical investigation of dynamic spalling fracture in toughness concrete

This study presents a two-phase mesoscale numerical model to investigate the dynamic spalling fracture behaviour of hybrid fibre ultra-high toughness cementitious composites (UHTCC). The model simulated the presence of random steel fibres and ductile UHTCC independently, and accurately incorporated the fibre-matrix bond-slip relationship, with carefully considering the influence of the inclination angle of the steel fibres on the pullout behaviour. Validation of the numerical model involved a meticulous comparison between numerical outcomes and results from spallation tests, encompassing strain-time and velocity-time histories, failure modes, and locations of cracking or fracture. Subsequently, the developed model was employed to analyse the impact of fibre inclination, stress wave amplitude, and waveform (based on the impulse equivalence principle) on the dynamic spalling fracture behaviour of hybrid fibre UHTCC. Evaluation of spalling fracture performance was based on both maximum velocity and pullback velocity of resulting fragments or free surfaces. Findings revealed that fibres parallel to the stress wave propagation direction exhibited optimum performance. It was also observed that stress waveforms with rectangular and sinusoidal shapes presented the highest risk of spalling fracture, while exponentially attenuated waves were comparatively safer. Additionally, stress waveforms with either rise time or duration at maximum pressure were deemed hazardous. Further, the hybrid fibre UHTCC demonstrated multiple spalling cracking/fracture behaviours under all stress waves except rectangular ones. This comprehensive exploration provided insights for the utilization of hybrid fibre UHTCC in impact engineering.