Bond Stress Research Articles

Bond–slip constitutive relation of mortar anchor under different loading rates

Purpose This paper aims to focus on establishing the bond-slip constitutive relation of mortar anchor under high loading rates by the dynamic pull-out test. Design/methodology/approach Self-made specimens were made for the dynamic pull-out test to explore the bond performance of mortar anchor, and the bond-slip constitutive relation of mortar anchor under high loading rates was established according to the analysis of test data. Findings During the loading process, the position of the peak bond stress was observed to translate to the free end. The bearing capacity of the mortar anchor was enhanced to some extent due to the increase of the loading rate. Originality/value The bond-slip constitutive relation of mortar anchor under high loading rates was established with the introduction of the position function and dynamic-load expanded coefficient.

Research on the Bond Behavior of Preplaced Aggregate Concrete-Filled Steel Tube Columns.

In order to investigate the bond behavior of preplaced aggregate concrete-filled steel tube (CFT-PAC) columns and the difference of bond behavior between CFT-PAC columns and normal concrete-filled steel tube (CFT-NC) columns, a total of 11 columns were prepared and the push-out tests were conducted. The experimental parameters included the type of concrete (preplaced aggregate concrete and normal concrete), concrete strength (C40, C50 and C60), cross-section dimension (D = 219 mm, 299 mm and 351 mm) and the thickness of steel tube (t = 6 mm and 8 mm). The results indicated that the CTF-PAC columns had a similar load-slip curves with CFT-NC columns. The bond stresses of the CFT-PAC columns were higher than that of the PAC-NC columns at the same concrete strength. Increasing compressive strength of PAC increased the critical bond strength and bond strength of CFT-PAC columns. With an increase of the L/D ratio, both of the slip corresponding to peak load and bond strength of CFT-PAC columns exhibited an increasing trend. A rise in the D/t ratio led to a decrease in the bond stress of CFT-PAC columns and an increase in slip corresponding to the peak load of CFT-PAC columns. The proposed bond stress–slip relationship model considerably matched the bond stress–slip relationship of CFT-PAC columns.

Improvement of bond performance between concrete and CFRP bars with optimized additional aluminum ribs anchorage

The slippage of fiber-reinforced polymer (FRP) bars in concrete occurs frequently because of the insufficient anchorage capacity of FRP bars in concrete, and it considerably affects the reliable application of FRP bars as reinforcing steel bars in civil engineering structures, especially in prestressed structures. In this paper, an optimized additional rib (AR) anchorage system was applied to the carbon-FRP (CFRP) bars to improve this weak anchorage characteristic of CFRP bars. The extrusion technology of the additional ribs was optimized, and only a slight deterioration of approximately 1.85% in the tensile strength of the CFRP bars was noted, compared to that corresponding to our previous extrusion technology (a loss of approximately 7.84%). Subsequently, pull-out tests were performed to investigate the influence of the number of additional ribs and bond length on the bond performance between the CFRP bars and concrete. The experimental results demonstrated that the existence of additional ribs, which produced an end pressure, transformed the bond stress transferring mechanism and reduced the radial force exerted on the surrounding concrete, thereby delaying the occurrence of the concrete splitting failure. In addition, the CFRP bars anchored with the additional ribs exhibited a remarkable enhancement in the pull-out strength, whereas this improvement was influenced by the embedment length of the CFRP bars and the ratio of the additional rib length to the bond length (lar/L). Finally, an empirical expression to calculate the development length of the CFRP bars with and without an AR anchorage was proposed and compared with the expressions provided in the existing design standards. It was concluded that the applied AR anchorage system could effectively reduce the development length of the CFRP bars. Compared to that of the control specimens, there was approximately 16.7% decrease in the development length for the specimens anchored with one additional rib.

Experimental study on CFRP-concrete dynamic debonding behaviour

Carbon fibre reinforced polymer (CFRP) is widely used in strengthening structures against dynamic loading. However, debonding is one of the primary failure modes in CFRP strengthened reinforced concrete (RC) structures. This phenomenon is controlled by the interfacial shear bond-slip behaviour between the two materials, viz. CFRP and concrete. Although the quasi-static bond-slip response has already been extensively investigated, studies on their dynamic behaviour are rather limited, especially those under high loading rate regime. Limited knowledge of the interface behaviour will affect high-fidelity simulations of FRP strengthened RC structures under dynamic loading regime. To shed light on the FRP bond-slip behaviour under high loading rates (above 800 mm/s), the authors proposed a novel experimental method for high loading-rate impact tests using a modified Split Hopkinson Pressure Bar (SHPB) set-up. Based on this set-up, slip rate could be derived through single-lap shear tests under impact loading regime. Dynamic enhancing effect on ultimate load, shear bond stress and fracture energy could be quantified for a wide range of loading rates varying from 0.02.mm/s to 2150 mm/s. It was found that the bond-slip properties showed remarkable dynamic enhancing effect under high loading rate (above 800 mm/s). However, at the loading rate around 2000 mm/s, dynamic effect could be limited by CFRP load capacity in which the CFRP sheet might directly fractured rather than debonded from the concrete surface. The effect of parameters on the bond behaviour was better understood from a detailed experimental parametric study. Constitutive equations were proposed to model the dynamic bond-slip behaviour of the CFRP-concrete interface.

Bond Stress between Steel‐Reinforced Bars and Fly Ash‐Based Geopolymer Concrete

Geopolymer concrete has been regarded as one of the most important green construction materials, which has been restrained in engineering applications partially due to a lack of bond studies. The structural performance of the reinforced concrete components primarily relies on the sufficient bond between the concrete and the reinforcing bars. Before being utilized in any concrete structure, GPC must demonstrate that it possesses understandable bond behaviour with commercial steel reinforcements. This work presents an experimental investigation on the bond stress of steel bars in reinforced geopolymer concrete (GPC) structures. Standard beam‐end pull‐out tests were conducted on GPC specimens reinforced with 16 mm plain and ribbed bars that were equipped with electrical resistance strain gauges. The longitudinal variation in the bond stress in the GPC beams during the pull‐out tests was calculated and plotted, as well as the stress in steel bars. The cracks on the bond area of the GPC were compared with those of the corresponding ordinary Portland cement concrete (OPC), as well as the steel stress and bond stress. The results showed that the relative slip between plain bar and geopolymer concrete varies from 30–450 microns from the loaded end to the free end when the bond stress decreased by 83%. The relative slip between ribbed bar and geopolymer concrete varies from 280–3,000 microns from the loaded end to the free end when the bond stress decreased by 57%. Generally, GPC is different from OPC in terms of bond stress distribution.

Bond properties of steel and sand-coated GFRP bars in Alkali activated cement concrete

The bond performance of glass fibre reinforced polymer (GFRP) bars and that of steel bars embedded in Alkali Activated Cement (AAC) concrete are analysed and compared using pull-out specimens. The bond failure modes, the average bond strength and the free end bond stress-slip curves are used for comparison. Tepfer’s concrete ring model is used to further analyse the splitting failure in ribbed steel bar and GFRP bar specimens. The angle the bond forces make with the bar axis was calculated and used for comparing bond behaviour of ribbed steel bar and GFRP bars in AAC concrete. The results showed that bond failure mode plays a significant role in the comparison of the average bond stress of the specimens at failure. In case of pull-out failure mode, specimens with ribbed steel bars showed a higher bond strength while specimens with GFRP bars showed a higher bond stress in case of splitting failure mode. Comparison of the bond stress-slip curves of ribbed steel bars and GFRP bars depicted that the constant bond stress region at the peak is much smaller in case of GFRP bars than ribbed steel bars indicating a basic bond mechanism difference in GFRP and ribbed steel bars.

Bond stress and behavior of interface between untreated aluminum alloy surface and concrete

This paper explores the viability of using aluminum alloy (AA) plates as an externally bonded reinforcement (EBR) material that could overcome some of the shortcomings of the prevailing steel and FRP materials and provides new advantages. A total of 32 prisms with different concrete strengths have been prepared as single shear test specimens with AA plates bonded to the prisms at different lengths using epoxy adhesive. Single shear tests were conducted on the prism specimens until failure. Load-extension and load-strain values were recorded. Bond stress and bond slip values were calculated. Test results have shown that plain untreated AA surface is susceptible to premature debonding; however treating the AA surface, by roughening it, increases its bond strength and ultimate load capacity considerably. Generally, the ultimate load and maximum bond stress increased with the increase of the bonded length and concrete strength, on the average, for the AA roughened surface compared to the AA plain untreated surface, by up to 457%, 327%, respectively. It is also observed that the modes of failure were influenced by the changes in the bonded length, concrete strength and AA surface treatment.

Temperature effect on behavior of structural light weight concrete filled steel tube (LWCFST) column

Expanded clay lightweight aggregates (Sintagg) are more light weight aggregate made by heating clay to around 1200 °C (2190 °F) in a rotary kiln. The structural light weight concrete of M40 grade was developed by using Sintagg aggregates, which used to fill in steel tubes columns (circular, square, and rectangular). This paper investigated the effect of temperature variations (600–1000 °C) on structural behavior of LWCFST column under axial loading and research findings about load deflection behavior, failure patterns, comparison of experimental and numerical fire resistance load (EN 1994-1-2:2005–Annex G) and temperature effect on bond stress of LWCFST. The results shows that LWCFST has a good fire resistant capacity and bond stress value after exposure to temperature variations, with 25.30% reduction in dead weight of column than normal concrete filled column.

Assessing provisions in Eurocode2 for anchoring reinforcing bars with bends

In Eurocode2, the design provisions allow for the anchorage length of reinforcing bars with bends to be calculated as the center line of the bar from the critical section of the member to the end of the straight portion of the bar beyond the bend, with no limit on the maximum value of the length beyond the bend (extension length). This paper thus presents statistical analysis for test results of reinforcing bars anchored with bends, collected from the literature, as a means to assess the performance of the provisions of Eurocode2. Strength of bent bars at an anchorage length depends on many parameters, including concrete compressive strength, embodied as a constant bond stress along the anchorage length; diameter of the bar; concrete side cover; spacing between bars, and confining reinforcement. Test results incorporated in this analysis include 113 simulated beam-column joint specimens with 16, 19, 22, 25, and 36 mm beam bars with stresses at anchorage failure ranging from 213 to 943 MPa, and concrete compressive strengths ranging from 18 to 114 MPa. Each specimen contained 2, 3, or 4 bent beam bars with clear spacing between them ranging from 2db to 11.4db. All specimens contained no confining reinforcement within the joint region and a constant concrete side cover of either 65 mm or 90 mm. The analysis showed that, for a given bar diameter, the predicted bar stress at anchorage failure with limitations as originally integrated on bond stress, concrete cover, and spacing between bars is reasonably safe. However, the ratio of test-to-calculated bar stress decreases as the ratio of extension to embedment length increases, resulting in an unconservative design when the extension length exceeds half of the embedment length. For rational designs, an upper limit on the extension length of half the embedment length is thus suggested.

Behavior of Prefabricated Full-Depth Precast Concrete Bridge Deck Panel System: Optimum Prestress Level

This paper reports on results and findings obtained from a nonlinear finite element analysis (NLFEA) of a prototype prefabricated full-depth precast concrete bridge deck panel system under different level of prestressing force. The NLFEA were validated with experimental results obtained from full-scale testing of the prototype bridge system. The benefits of the NLFEA can be highly appreciated when visualizing the substantial time and cost savings, the ability to change any parameter of interest, and the capability of demonstrating any interesting response at any load value and at any location in the system. The most attractive results were: (1) the system is capable of withstanding and maintaining its integrity under eight times the simulated AASHTO truck service load without considerable reduction in its ultimate strength capacity and stiffness, (2) The NLFEA showed that the live load-induced bond stresses for 4 lines prestress level was almost 0.74 times the live load-induced bond stresses for 6 lines prestress level.

Steel stress patterns between two primary cracks in concrete

The national and international codes and standards and the literature consider the stress patterns either in case of a single crack or of equidistant primary cracks which developed at the same load level. Moreover, most FE models consider smeared cracks which do not allow for any insight in the real inner behavior of structural concrete. This paper considers the stochastic character of the concrete tensile strength’s distribution along the reinforced concrete element and a realistic local bond stress vs. slip relationship with its hysteretic character when the sign of local slip changes due to the occurrence of a new primary crack.

Bond Behaviour of Steel and Concrete with Pull-Out Test

This experimental investigation presents the influence of rebar’s which has protecting coating, rested rebar and fresh rebar and there bond strength development between the steel and concrete. Pull-out experiment was conducted Universal Testing Machine (UTM) which has a capacity of 400 KN as per IS code procedure. The tested rebar includes rusty rebar, acid preserved rebar and cement chemical compound anticorrosive coated rebar. The Concrete mix design for M25 grade of concrete were used and therefore 18 concrete cube specimens with external projection of steel rod were tested. The various load slip behaviour was studied at the free end finish and loaded end victimization dial gauges. The last word bond stress just like the lowest load worth of 0.025 mm metal slip and 0.25 mm slip was thought of as a result of the usable bond strength of steel rebar’s and concrete. The check results blatant correlation exists between Load at 0.025 mm free finish slip and 0.25 mm loaded finish slip. It had been found that presence of rust and cement compound anticorrosive coating among the steel concrete interface appreciably can increase the bond strength of the order of 20 % and 27 % severally for 16mm diameter bars as compared to rust free rebar. For 20mm dia. Bars has totally different bond strength for rusty rebar’s and therefore the increase in bond strength for coated bars are compared with 2 differing kinds of uncoated bars and rusted rebar’s were determined. It’s over that presence of rust influences in reduction / increase in bond strength hoping on the character of rust at the interface among the initial ages. Application of cement compound coating has been improves the bond strength of the order of 31% – 37 % to satisfies the necessities of Burse Indian Standards code (IS)

Experimental Study on Mix Proportion Parameter Optimization of Cement Anchoring Material.

To prevent major deformation of surrounding rock and improve the stability of unstable rock mass, this study optimized the ratio of cement anchoring material based on the orthogonal design method and rock mechanics tests. Using the six influencing factors of coal ash content, sodium silicate content, ettringite content, naphthalene sulfonate content, water–cement ratio, and sand–cement ratio, a total of 18 sets of material matching schemes was designed. Compressive strength and pull-out force tests were carried out. The results are as follows: the compressive strength of the test piece between 43 and 55 MPa; the bond stress between the bolt and the anchoring agent range is from 103 to 136 kN; and the adhesion stress between the anchoring agent and the rock masses range is from 76 to 112 kN. The main order and degree of influence of various factors affecting the bond stress and the adhesion stress of the anchoring material were determined by range analysis and analysis of variance. The results showed that the amount of coal ash had a clear controlling effect on the bond stress, and the water–cement ratio, sand–cement ratio, and sodium silicate dosage had a significant influence on the bond stress. Moreover, the amount of sodium silicate had the greatest influence on the adhesion stress, and the water–cement ratio had the second largest influence on the adhesion stress. The amount of naphthalene sulfonate had the least influence on the adhesion stress. The above experimental data and results were subjected to multiple linear regression analyses, and the empirical equations of the mechanical indexes of the test pieces and the proportion parameters of the anchoring materials were obtained to guide the engineering support design. The engineering application showed that the optimized anchorage material could be applied to further improve the safety of the surrounding rock anchoring system.

Longitudinal shear behaviour of composite slabs with profiled steel sheeting and ECC

This paper studies the longitudinal shear behaviour of composite slabs composed of profiled steel sheeting and engineered cementitious composite (ECC), i.e., referred to as “ECC composite slab” in the following. A total of eleven full-scale ECC composite slab specimens were designed and tested under four-point flexural loading. The main testing parameters included the shear span ratio, the thickness of the steel sheeting, the depth of the composite slab, and the arrangement of the shear studs. The influence of these parameters on the failure modes and the longitudinal shear resistance of the specimens were evaluated based on the test results. The behaviour of the ECC composite slab was compared with that of the composite slab with traditional concrete topping. The formulae for calculating the longitudinal shear resistance and the ultimate shear bond stress of ECC composite slabs were proposed according to the m-k method and the partial shear connection (PSC) method, and the predictions agreed well with the test results. Finally, an advanced finite element model (FEM) was established and validated against the test results, which further proved that adopting shear studs as end anchorage could significantly improve the longitudinal shear resistance of an ECC composite slab.

Bond strength of steel reinforcement under different loading rates

This paper addresses the bond strength of steel reinforcement subject to different loading rates. In the experimental programme, a total of 15 pull-out tests were conducted on steel reinforcing bars with various embedment lengths. The loading rate varied from 0.1 mm/s to 100 mm/s. The applied load, slip at the loaded end and strains along the embedment length of steel reinforcement were measured during testing. Bond stresses and strain rates along the embedment length of reinforcement were calculated from the measured strains. Test results suggested that a higher loading rate could increase the bond stress between steel reinforcement and concrete. The increase in bond stress was mainly induced by the rapid slip and the relatively high dynamic yield strength of reinforcement, depending on the stress state of the reinforcement. Finally, conclusions were drawn from the experimental results to facilitate the analysis of the bond-slip behaviour of steel reinforcement under high loading rates.

Study on the interfacial bond slip constitutive relation of I-section steel and fully recycled aggregate concrete

17 standard push-out specimens which are designed to study the interfacial bond-slip behavior of I-section steel and reinforced recycled concrete with 100% replacement ratio of coarse aggregate (SRRC). The parameters of test are strength of recycled aggregate concrete, thickness of I-section steel protective layer, embedded length of I-section steel and ratio of stirrups. The test results show that the strain and stress of the I-section steel at the interface of SRRC distribute exponentially, and the four test parameters have a certain influence on the interfacial bond strength of SRRC. Based on the analysis of the test data, the calculation formulas of the interfacial bond strength of SRRC are established. The constitutive relations of the bond-slip at the loading end are established, and the multistage linear results are more satisfied with the test results than power-type. Finally, the position function of the bond-slip F(x) and the bond stress G(x) are introduced, and the bond-slip constitutive relation with changes of position are established, which provides a reference for related specification design and engineering construction.

Anchorage length of reinforcing bars in tension: an assessment of the Australian Standard AS 3600:2018

When designing a reinforced concrete member for strength, ductility and robustness, it is essential that a sufficient length of reinforcement is developed on both sides of a critical section in a zone of tension. The specifications of the existing codes of practice for calculating the development length of reinforcement are statistically based on test data mostly from small-scale pull-out and beam specimens, and their applicability outside the range of the test data is uncertain. This paper describes an experimental research program to assess the effects of different structural parameters on the development length requirements for modern high strength steel reinforcing bars, including the case of lapped splices in large-scale reinforced concrete members in bending. The factors of safety obtained in the tests are assessed according to the Australian Standard (AS3600:2018) provisions for the anchorage length of reinforcing bars in tension and also in terms of attainable average bond stress in different anchorage conditions. The study has revealed that modification(s) to two of the coefficients included in the provisions of AS3600:2018 would provide a more consistent and reliable factor of safety for the anchorage of reinforcement in large-scale concrete members, including at lapped splice locations.

Bending model for composite UHPFRC-RC elements including tension stiffening and crack width

The application of a thin ultra-high performance fiber-reinforced concrete (UHPFRC) layer at the tensile side of reinforced concrete (RC) elements has lead to an efficient structural concept with promising capacities. Composite UHPFRC-RC elements have an increased bending capacity with respect to the RC member. In addition, the serviceability and durability are also improved as the composite element has higher stiffness and smaller crack widths. In order to widen the application of composite UHPFRC-RC elements, practitioners and designers need analytical and numerical tools with a sound mechanical background, if possible conceptually analogous to well-established models for conventional concrete structures. The paper starts with a discussion of the main drawbacks of existing models for UHPFRC-RC elements in bending. Secondly, new experimental basis is provided with own tests where the digital image correlation (DIC) technique is exploited to understand the crack pattern development and interfacial behaviour of UHPFRC-RC. Finally, the main goal of the paper is the proposal of a new model for UHPFRC-RC elements in bending which includes tension stiffening and the interaction at the steel-concrete and concrete-UHPFRC interfaces by means of a new composite tension chord model. The tension chord model is based on the stress transfer mechanisms between adjacent cracks, which include the explicit consideration of bond stresses between the constituent materials (i.e. steel and concrete, and UHPFRC and concrete). The compatibility condition between the UHPFRC and the RC layers is established in terms of the extension of each layer between adjacent cracks, as the classic compatibility of strains based on perfect bond assumption cannot be used upon macrocrack formation. Rather, the extension of the UHPFRC layer includes the contribution of cross-sections within the elastic, hardening or softening stages of the UHPFRC. The proposed model allows estimating the whole bending response, including calculation of crack widths and average curvatures between cracks.

A Computational Model for Prestressed Concrete Hollow-Core Slab Under Natural Fire

Performance-based approach, introducing a new two-phase computational model for determining the response of prestressed hollow-core concrete slab exposed to natural fire including heating and cooling phase, is presented. Firstly, the two-dimensional coupled hygro-thermo-chemical model is used to determine time dependent temperature and moisture field in the characteristic cross-section of the concrete hollow-core slab during fire. In addition, the influence of opening on the temperature distribution over prestressed hollow-core concrete slab is accounted for. Secondly, stress–strain state of prestressed concrete hollow-core slab is determined with a newly developed one-dimensional geometrical and material non-linear model, which includes a slip between concrete and tendon. Temperature dependent mechanical properties of concrete, tendon and bond stiffness are accounted for in the model. Model validation showed that the presented two-phase computational model is suitable for the analysis of prestressed hollow-core concrete slab exposed to natural fire. Furthermore, parametric studies revealed that heat exchange between the concrete section and the opening has a significant influence on the development of temperatures in the slab, particularly in the cooling phase, and consequently also on the development of slab displacements. In addition, it was identified that accounting for the slip between concrete and tendon enables the determination of the bond stress distribution and evaluation of the load bearing capacity of the contact.

Promotion of the mechanical properties and thermal conductivity of epoxy by low Si3N4 whisker content and its mechanisms

ABSTRACTSi3N4 whisker (Si3N4w) is selected as epoxy filler. The influence of filler content on the bulk density, porosity, bending strength, Young's modulus, critical stress intensity factor, work of failure, morphologies of fracture surface, and thermal conductivity of Si3N4w/epoxy is investigated. The bending strength is 82.63 MPa at a Si3N4w content of 5 vol% and increases to 25.29% more than that of neat epoxy. Compared with that of neat epoxy, the work of failure and thermal conductivity increase by 455% and 34.78% to 18 248.92 J·m−2 and 0.31 W·m−1·K−1, respectively, at a Si3N4w content of 7 vol%. However, Si3N4w/epoxy becomes sensitive to precrack due to a weak CN bond and residual tensile stress at the interface, thereby resulting in the decline of critical stress intensity factor. The coexistence of various energy dissipation mechanisms, namely, steps, craters or depressions, stress whitening, plastic flow, pull out of Si3N4w, and rough fracture surface, is observed in Si3N4w/epoxy. © 2019 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2020, 137, 48721.