Average Bond Stress Research Articles

Bond behavior between reinforcing bar and reactive powder concrete

AbstractTo study the bond–slip behavior between reinforcing bars and reactive powder concrete (RPC), eccentric pull‐out tests of 20 groups of bonding specimens were conducted, and the variables in the test specimens included RPC strength, cover thickness, bond length, and reinforcing bar diameter. The effects of various parameters on the bond stress and peak slip were analyzed. A formula for calculating the average bond stress and between the reinforcing bar and RPC were established, and the bond–slip relationship between reinforcing bar and RPC was determined. The results indicated that the bond length had the most significant effect on the bond performance, followed by the strength of concrete, cover thickness, and reinforcing bar diameter. When the bond length increased from 3d to 4d, the average bond stress decreases by 13.2% to 23.7%. When reinforcing bar diameter increased from 16 to 20 and 25 mm, the average bond stress increased by 6.58% and 15.9%, respectively. The average bond stress increased with increase of the cover thickness. When the RPC strength increased from 105.2 to 124.8 MPa, the average bond stress increased by 7.0% to 13.2%.

Local bond strength prediction of deformed reinforcing bars in concrete considering heterogeneity at interface regions

An analytical model is proposed to predict local bond strength (τf) by incorporating heterogeneity at interface regions for deformed reinforcing bars centrally anchored in concrete. The rib width on the bar surface is introduced as an interfacial characteristic parameter G in the proposed model; this accounts for the heterogeneity. Both τf and the local interfacial fracture energy (GIIf) of each specimen were found to be linked to G and can be determined analytically from the maximum pull-out loads (Fmax) from tests. It was found that the predicted τf was larger than the maximum average bond stress (τavg-max); the discrepancy between the two values reduced with an increase in L/G. Moreover, with an increase in L/G, the predicted τf showed a certain decrease, with the reduction decreasing with stronger interfacial homogeneity. The predicted GIIf was found to be significantly increased because of the weaker boundary effect. The validity of the proposed model was verified using comparisons of predicted Fmax (using the determined values of τf and GIIf) and the experimental Fmax, with the only failure mode being bar pull-out. Moreover, the model can be applied to steel or fibre-reinforced polymer bars and the concrete refers to all types of cementitious materials.

Transfer stage parameters for concrete beams prestressed with BFRP bars: Experimental and finite element studies

This work is a vital step in enhancing the potential use of a newly developed basalt fibre reinforced polymer (BFRP) bars for prestressing applications. An experimental investigation of four concrete beams has been conducted to determine the transfer length, end-slip, average bond-stress and prestress losses associated with prestressing of concrete beams with BFRP bars. Strain gauges, linear potentiometers, and load cells have been used to measure the strains, slip and loads, respectively, during the pretensioning of the BFRP bars. An initial prestress load of 19 ± 0.5 kN corresponding to an initial prestress of 380 ± 15 MPa has been applied to the BFRP bars, which corresponds to approximately 50% of the ultimate strength of BFRP bars utilized in the present study. The transfer length, end-slip and average bond-stress for 8 mm diameter BFRP bars in concrete with 28-day compressive strength of 40 MPa were 250–500 mm, 1.6–2.7 mm and 1.8–2.6 MPa, respectively. A non-linear FE model has been developed, and the finite element results for the transfer-stage parameters for prestressed concrete beams prestressed using BFRP bars have been estimated and compared with the experimental data.

Experimental study on the bond-slip behavior and stress transfer mechanism between shaped steel and high-performance fiber-reinforced concrete

In light of the weak bond strength between the two materials in steel reinforced concrete (SRC) and the brittleness of high-strength concrete, the use of high-performance fiber-reinforced concrete (HPFRC) was proposed with the addition of cellulose fiber and rice husk ash (RHA) in high-strength concrete. The purpose of this study is to investigate the bond behavior of shaped steel embedded in HPFRCs. Thirteen push-out test specimens of steel high-performance fiber-reinforced concrete (SHPFRC) were cast. The test parameters considered in this study were the HPFRC strength, cover thickness, embedment length, and stirrup ratio. The experimental process and failure modes were analyzed, and the influence of various test parameters on the average bond strength was studied. The average effective bond stress was introduced to study the whole process of bond failure and the stress transfer mechanism. Then, a method for calculating the bond stress was established. The results showed that the failure modes of the specimens were classified into bond splitting failure, bond anchorage failure, and mixing failure. The parameters have various effects on the average bond strength. The introduction of the average effective bond stress reflected the transfer mechanism of bond stress at the interface. The largest component of bond stress was chemical adhesion, and mechanical interaction was the smallest.

Application of Superabsorbent Polymer as Self-Healing Agent in Self-Consolidating Concrete for Mitigating Precracking Phenomenon at the Rebar–Concrete Interface

Improved autogenous healing capacity of concrete using superabsorbent polymers (SAPs) was used as an efficient approach for mitigating damage between steel rebar and self-consolidating concrete (SCC). The results for normal concrete (NC) and those for SCC mixtures were compared. Two SAPs with different particle sizes and chemical compositions were used in the experimental program. Results showed that despite the greater reduction effect of SAP with a smaller particle size on compressive strength, SCC containing this type of SAP had the highest bond strength in uncracked specimens, compared with SAP with larger particle sizes, for SAP-modified NC and SCC mixtures. Moreover, results showed that SCC and NC containing SAP had considerably greater healing improvement factors for large crack widths (w≥0.30 mm) compared with mixtures without polymers; almost 46%, 30%, and 24% healing improvement factors were obtained for average bond stress, bond strength, and residual bond stress of SAP-containing concrete mixtures, respectively. Furthermore, complete strength recovery (100% healing improvement factor) was obtained for SCC mixture with w=0.10 mm after a 28-day healing period.

Local bond strength prediction of deformed reinforcing bars in concrete by considering heterogeneity at interface regions

An analytical model is proposed in this paper to predict the local bond strength τf by incorporating the heterogeneity at interface regions for deformed reinforcing bars centrally anchored in concrete. The rib width on bar surface is introduced as an interfacial characteristic parameter G in the proposed model accounting for the heterogeneity. Both the τf and local interfacial fracture energy GIIf from each specimen are linked to the G and determined analytically if the maximum pull-out loads Fmax are given from the test. It is found that the predicted τf is larger than the maximum average bond stress τavg-max. The discrepancy between the two values is reduced with the increasing of L/G. Moreover, as the L/G increases, the predicted τf shows a certain decrease and the reduction becomes smaller with stronger interfacial homogeneity. The predicted GIIf is significantly increased because of the weaker boundary effect. The validity of the proposed model is verified upon the comparisons between the predicted Fmax using the determined τf and GIIf and the experimental ones with the only failure mode of bar pull-out. Moreover, the bars here can be steel or FRP (fiber-reinforced polymer) bars, and the concrete refers to all types of cementitious materials.

Study on Dynamic Bond-Slip Constitutive Relation of Corroded Mortar Anchor

In order to study the bonding performance of corroded mortar anchor under dynamic load and reveal the mechanism of dynamic bond-slip, dynamic pull-out tests for mortar anchors with different corrosion degrees were carried out. On the basis of the average bond stress and relative slip, the basic bond-slip constitutive relation of corroded mortar anchor was established according to the measured data. The bond stress under different loading frequency were fitted, and the dynamic influence coefficient affecting the bond performance of mortar anchor was obtained. Based on the bond stress-slip relationship of each point in the anchorage section under different corrosion degrees, the position function in anchorage section was deduced. According to the above basic bond-slip relation, dynamic influence coefficient and anchorage position function, the dynamic bond-slip constitutive relation of corroded mortar anchor regarding to anchorage position was established, which could provide reference for the study on the durability of mortar anchor and its dynamic anchorage performance.

Basic Mechanical Properties of Duplex Stainless Steel Bars and Experimental Study of Bonding between Duplex Stainless Steel Bars and Concrete.

In recent years, as a result of the large-scale use of stainless steel bars in production and life, people’s demand for stainless steel bars has increased. However, existing research information on stainless steel bars is scant, especially the lack of research on the mechanical properties of duplex stainless steel bars and the bonding properties of duplex stainless steel bars to concrete. Therefore, this paper selects 177 duplex stainless steel bars with different diameters for room temperature tensile test, and then uses mathematical methods to provide suggestions for the values of their mechanical properties. The test results show that the duplex stainless steel bar has a relatively high tensile strength of 739 MPa, no significant yield phase, and a relatively low modulus of elasticity of 1.43 × 105 MPa. In addition, 33 specimens were designed to study the bonding properties of duplex stainless steel bars to concrete. In this paper, the effects of concrete strength, duplex stainless steel reinforcement diameter, the ratio of concrete cover to reinforcing steel diameter, and relative anchorage length on the bond stress were investigated, and a regression model was established based on the experimental results. The results show that, with the concrete strength concrete strength from C25 to C40, the compressive strength of concrete increased by 56.1%, the bond stress increased by 27%; the relative anchorage length has been increased from 3 to 6, the relative anchorage length has doubled, and the bond stress has increased by 13%; and, the ratio of concrete cover to reinforcing steel diameter increased to a certain range on the bond stress has no significant effect and duplex stainless steel reinforcement diameter has little effect on the bond stress. The ratio of concrete cover to reinforcing steel diameter from 3.3 to 4.5 and the bond stress increased by 24.7%. A ratio of concrete cover to reinforcing steel diameter greater than 4.5 has no significant effect on the bond stress, with the average bond stress value of 20.1 MPa. The duplex stainless steel bar diameter has little effect on the bond stress for the diameters of 12 mm, 16 mm, 25 mm duplex stainless steel bar, and their average bond stress is 19.9 MPa.

Bond stress distribution analysis between non-prestressed steel strand and concrete in novel beam-to-column connection

The Precast Prestressed Assembled Structural (PPAS) system frame is a novel prefabricated concrete structure, which mainly use 15.2-mm high-strength steel strands as the bottom longitudinal reinforcement. In addition, a certain length of non-prestressed steel strands with 90-degree hooked anchorage type are reserved at the beam end and anchored into concrete column to form a novel beam-to-column connection. The good bond and anchorage performance of steel strand in concrete column is the prerequisite for this connection. However, the bond behaviors such as the bond stress distribution, the bond stress transfer mechanism of the steel strand-to-concrete interface etc. has not been fully understood. For this reason, pull-out tests on six specimens, including thirty-six steel strands with various anchorage type, embedment length and anchorage position, were performed in this research. The results indicated that, compared with straight-line anchorage type, the steel strand with 90-degree hooked anchorage type could increase the ultimate average bond stress and bond stiffness. In addition, the embedment length and anchorage position of the steel strands also had different degrees of influence on the average bond stress. The evolution of strain-pullout load behavior of steel strand was strongly related to the bond position. The anchorage type of steel strand greatly affected the distribution pattern of bond stress between steel strand and concrete. However, the bond stress distribution was always non-uniform along the embedment length, regardless of the anchorage type or embedment length.

Use of unstressed strands for connections of precast concrete members

Precast concrete members need to be connected effectively to form an integral structural system. Use of unstressed strands provides a cost-effective and practical solution to reinforce the connection regions of precast concrete members, especially for precast concrete bridge girder applications. With the limited understanding of bonding characteristics for unstressed strands, a combination of experimental and analytical programs, which focused on unstressed strands as a connection between precast concrete girders and cap beams for seismic applications, was designed to investigate the fundamental load-transfer characteristics of unstressed strands anchored in grout and concrete based on pullout tests. The relationship between strand stress and loaded-end displacement was developed, and the bond stress of unstressed strands embedded in concrete and grouted duct was examined. The average bond stress of unstressed strands anchored in concrete and grouted duct is recommended to be five and seven times the square root of concrete compressive strength, respectively. The results of this research provide qualitative embedment length requirements to design connections between precast concrete members using unstressed strands.

Experimental investigation of 0.6 in. diameter strand lifting loops

Very little experimental data have been published relating to the pullout capacity of prestressing strand lifting loops. To address this gap in knowledge, 13 pullout tests were conducted on strand lifting loops with 0.6 in. (15.24 mm) diameter, 270 ksi (1860 MPa) strand. Straight and bent orientations were tested for single loops at different embedment depths. Loops were embedded in 12 in. (304.8 mm) wide and 44 in. (1117.6 mm) deep concrete blocks and subjected to monotonic, static loading until failure. Marginal bond quality of the strand (18.2 kip [81 kN]), Mohs hardness (3.6), and concrete strength (3000 psi [20.7 MPa]) resulted in an average bond stress value of 400 psi (2758 kPa) at failure. Most tests exhibited pullout failure modes and adequate ductility. Three loops tested at 32 in. (812.8 mm) embedment with 6 in. (152.4 mm), 90-degree bends experienced brittle side-face blowout failures. These failures were due to inclination of the lifting, which led to a reduced edge distance. A safe uniform bond stress of 199 psi (1372 kPa) is recommended for 0.6 in. diameter strand.

Mechanical performance of overlap connections with grout-filled anchor reinforcements in embedded metal corrugated pipe

To study the mechanical performances of a new kind of overlapping connection with grouted anchor reinforcements in embedded metal corrugated pipes, a total of 24 groups of 72 specimens were prepared and tested. The overlap connection performance of grouting anchor reinforcements in embedded metal corrugated pipes was studied by bi-directional tension test. The factors such as the diameter of anchor reinforcements, anchor length and location of reinforcements were taken into account. The results show that the bi-directional tension test method can effectively reflect the actual stress state of the overlap connection. When the connection satisfies the structural requirements, the location of the reinforcement has little influence on the anchorage performance of the connection. Under the same conditions, the failure load of the specimens will increase with the increase of the diameter of reinforcements and the anchorage length. Meanwhile, under the same conditions, the average bond stress decreases with the increase of the reinforcement diameter and the anchorage length. Experimental results and analysis show that overlap connection with grout-filled anchor reinforcements in embedded metal corrugated pipe has good anchoring performances, and also has the advantages of simple structure and convenient construction.

Surface modification of steel fibers using chemical solutions and their pullout behaviors from ultra-high-performance concrete

To improve the pullout resistance of conventional straight steel fiber from ultra-high-performance concrete (UHPC), its surface was modified using various chemical solutions. Calcium carbonate (CaCO3) particles were precipitated on the surfaces of the steel fiber in an aqueous solution. The morphology and particle size of CaCO3 were controlled by ethylenediaminetetraacetic acid (EDTA) and investigated via scanning electron microscopy; further, its chemical composition was verified through energy-dispersive X-ray spectroscopy. The test results indicate that longer duration of immersion in a solution of CaCl2 and Na2CO3 yield larger CaCO3 particles on the surface owing to Ostwald ripening. The addition of EDTA changes the morphology of the CaCO3 particle and decreases its grain size from 16.2 to 3.1 μm. In order to investigate the effects of the precipitated CaCO3 particles on the fiber pullout resistance, single fiber pullout test was carried out; the modified steel fibers were embedded in UHPC matrix. The CaCO3 particles precipitated on the fiber surface improved the pullout resistance of the steel fiber from the UHPC. The smaller CaCO3 particles were more effective for enhancing the pullout resistance. The average bond stress and pullout work were increased by up to 15% and 37%, respectively, owing to the precipitated CaCO3 particles. The EDTA solution could peel off the surface of steel fiber regularly and render it rough; thus, it greatly increased the pullout resistance (more than 80%) than the plain fiber.

Properties of Alkali-Resistant Glass Fiber Reinforced Coral Aggregate Concrete

The intention of this paper is to analyze the properties of coral aggregate concrete (CAC) that is reinforced by alkali-resistant glass fibers (ARGF) and the bond performance with BFRP (basalt fiber reinforced polymer) bars. Two types of ARGF, denoted by Type A and Type B with different manufacturing technologies and fiber lengths, are used in the test. Tests of compressive strength, splitting tensile strength, and flexural performance were performed on ARGF-CAC with four different contents for the two types of ARGF. It is found that the cubic compressive strength is slightly reduced when the fiber volume fraction exceeds 0.5%, but almost keeps invariable if the fiber content further increases. However, the tensile strength, residual strength retention and flexural toughness are improved as more ARGFs are added into CAC, and even higher with Type B ARGF addition. The optimized volume fraction is 1.5% for both the two types of ARGF based on the evaluation of the workability and mechanical performance. Moreover, central pull-out test was performed to study the bond properties of ARGF-CAC with BFRP bars. It is found that both the maximum average bond stress and residual frictional stress are generally reduced as the bond length is longer. The addition of Type B ARGFs can significantly improve the bond strength; however, the Type A ARGFs seem to have marginal effect.

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.

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.

Experimental Studies on Bond Performance of BFRP Bars Reinforced Coral Aggregate Concrete

Basalt fiber reinforced polymer (BFRP) rebars reinforced coral aggregate concrete is a new type of concrete used in ocean engineering. In order to investigate the bond performance between BFRP rebars and coral concrete, 30 pull-out tests were carried out in 10 groups with different diameters of BFRP rebars, bonding lengths and strength of the coral concrete. The results show that good bonding between BFRP rebars and coral concrete were achieved. The main failure modes can be categorized as BFRP rebars pull out destruction, splitting failure of coral concrete and BFRP rebars fracture. The bond slip (tau{text{-}}s) curves of the BFRP rebars and coral concrete were obtained during the tests. It was found to be similar to the common concrete using fiber reinforced polymer (FRP) bars. The bond-slip relation can be roughly divided into micro-slip phase, slip phase, decline phase, and the residual stress stage. The bond between BFRP rebars and coral concrete increases with the increase of the bond length and diameter of BFRP rebars, but the average bond stress will decrease. Moreover, increasing the strength of coral concrete is effective to improve the bond performance of BFRP rebars. In this paper, the continuous bond slip model (Gao et al. in J Zhengzhou Univ 23:1–5, 2002) was used to represent the tau{text{-}}s constitutive relationship of BFRP rebars and coral concrete. The analysis show that the proposed model has a high degree of accuracy in representing tau{text{-}}s curve of BFRP rebars and coral concrete.

Study on bond performance between FRP bars and seawater coral aggregate concrete

Application of FRP (fiber reinforced polymer) and SCAC (seawater coral aggregate concrete) in island construction can overcome the shortcomings in conventional steel and concrete. This paper is concerned with the bond performance between FRP bars and SCAC by pullout tests. The effect of fiber type, diameter and bond length of FRP bar and curing condition on the bond behavior is analyzed. It is found that the maximum average bond stress decreases with the increasing of the bond length and the bar diameter. The bond strength is slightly reduced for the specimens immersed in seawater compared to those cured in standard condition for FRP bars bonded to SCAC. Moreover, a four-linear model for local bond stress-slip relationship is proposed with four characteristic parameters, and an analytical approach is then presented to predict the maximum pullout load based on the model. The analytical results show good agreement with the experimental ones, and the proposed four-linear model with four characteristic parameters proves to be reasonable in the present study.

Effective bond length of CFRP sheets externally bonded to concrete beams under marine environment

Externally strengthened marine concrete structures by using carbon fiber-reinforced polymer (CFRP) are being used more extensively because of their exceptional properties, including high corrosion/environmental degradation resistance. Debonding of CFRP sheet from the concrete substrate is one of the typical failure modes observed by using this technique. Therefore, the strengthening technique efficiency strongly depends on the effectiveness of the CFRP-concrete bond. Numerous experimental studies have been conducted to investigate the bond behavior and most of the proposed bond strength models considering the influence of the effective bond length. This study was conducted to experimentally investigate the effective bond length of CFRP sheets subjected to marine environment exposure, which is identified as one the major gaps in this discipline. The concrete beam specimens exposed to marine environment were tested to determine the effective bond length. The three-point bending-type shear bond test was used to obtain the stress versus load relationship. The test variables were the type and exposure duration. The test results showed that the marine environmental exposure significantly influenced bond stress and effective bond length. Two factors, CFRP stiffness and concrete compressive strength, contributed to fix the effective bond length of the CFRP-concrete interface. The maximum bond stress, after 12 months of wet/dry cyclic exposure, was found higher than that subjected to full immersion exposure. The average maximum bond stress decreased with an increase in full-immersion exposure time to 12 months. Results indicated that the exposure condition influences the capacity of CFRP-concrete bond and resulted in a reduction in effective bond length by 16–29%. Moreover, debonding was observed as the dominating mode of failure for all tested specimens. The prediction models of effective bond length, considering the influence of marine environment, are also established. Therefore, the outcome of this study will help to resolve one of the debonding issues in strengthening marine structures.

Bond behaviour of GFRP reinforcement in alkali activated cement concrete

Bond plays a key role in the performance of reinforced concrete structures. Glass fiber reinforced polymer (GFRP) reinforcing bar and alkali activated cement (AAC) concrete are promising alternative construction materials for steel bars and Ordinary Portland Cement (OPC) concretes respectively. In this study, the bond behaviour between GFRP bars, and AAC and OPC concretes is investigated by using beam-end test specimens. Sand-coated GFRP bars with 12.7mm and 15.9mm diameters and embedment lengths of six and nine times the bar diameter were used. The free end and the loaded end bond slip relationships, the failure mode and the average bond stress were used to analyse each of the specimens. Additionally, the distribution of tensile and bond stress along the embedment length was investigated by installing strain gauges along the embedment length. The results of the study indicate that the tensile and bond stress distribution along the embedment length are nonlinear, and the nonlinearity changes with the load. Finite element analysis was also performed to further study the bond stress distribution along the embedment length of the bar. From the finite element analysis, it was found that the bond stress distribution depends on the embedment length of the specimens; approaching uniform distribution as the embedment length decreases.