Free-end Slip Research Articles

Acoustic emission-based analysis of bond behavior of corroded reinforcement in existing concrete structures

Summary This experimental study exploits acoustic emission (AE) monitoring to assess the bond behavior of corroded bars in reinforced concrete prism samples under pull-out tests. The analysis of AE parameters obtained in this study including amplitude, duration, cumulative number of hits, and signal strength was performed to identify different stages of bond degradation. The results of bond behavior and free end slip were analyzed and compared with the corresponding AE data. An intensity analysis of AE signal strength was also completed to quantify the bond damage using two additional AE parameters: historic index (H (t)) and severity (Sr). The outcomes of AE analysis enabled the detection of early stages of bond deterioration (micro-cracking) as well as identification of the macro-cracking stage prior to the occurrence of bond splitting failure. The results also showed a good correlation between AE cumulative number of hits and AE signal strength parameters with the steel-to-concrete bond degradation because of corrosion. Finally, the results of H (t) and Sr were employed to generate intensity classification charts to characterize the stages of micro- and macro-cracking and range of slip of corroded reinforcement. Copyright © 2016 John Wiley & Sons, Ltd.

Development of bond strength model for CFRP-to-concrete joints at high temperatures

Abstract Because of temperature sensitivity of the adhesives used in the interface, temperature variations may significantly affect the bond behavior of carbon fiber reinforced polymer (CFRP) to concrete joints. An analytical solution is used in this work to derive the two-parameter bond–slip relationship of CFRP to concrete bonded joints at high temperatures. The values of the key parameters at elevated temperatures can be directly calculated from the expressions for the strength and the stiffness of CFRP to concrete joints. Considering the effect of the free end slip on the bond behavior, a simplified formula of the peak load for a given joint is presented. Based on regression analyses of existing test data, it is found that the normalized value of interfacial bond property at elevated temperatures should be a function of ΔT (service temperature subtracts glass transition temperature). The proposed bond strength-temperature model here represents the test data well, despite the large scatter.

Numerical Modeling of Fully Grouted Rockbolts Reaching Free-End Slip

AbstractIn this study, numerical simulation was conducted to model the behavior of rockbolts with free-end slip in tension. A nonlinear bond–slip relationship was input to the numerical model to represent the interaction mechanism of the interface between rockbolt and grout. The results of the model were validated against laboratory tests. The numerical model matched the experimental results in terms of load–displacement relationship, rockbolt strain distribution, and interfacial shear-stress distribution. This paper indicates that (1) the rockbolt element in the numerical model is capable of describing the strain-softening behavior of the rockbolt–grout interface; and (2) the pullout tests of rockbolts with free-end slip can be used to derive the bond–slip relationship, and thus provide insights into the mechanical behavior of rockbolts with free-end slip.

Bond Strength between Steel and Concrete Experiment Analysis at Different Temperatures

It is known in the literature, at different temperatures bond strength bond between steel and concrete and relevant test data segment free end slip magnitude rarely, it is necessary to study this type of test, in order to more in-depth study of steel and variation between concrete bonding properties at different temperatures in order to verify different temperatures we derive between steel and concrete expressions are appropriate bond stress and slip. This test is primarily measured under different temperature conditions, reinforced concrete specimens under axial tension force central role, respectively, at the free end of the specimen and loading end bond segment relative slip steel and concrete at different temperatures . Because when the temperature reaches about 200 °C, the specimen began to appear on the adhesive debonding phenomenon, so this calculation is only done by experiments and theoretical consideration a temperature in the range of 20 °C to 190 °C.

Bonding between high strength rebar and reactive powder concrete

A central pullout test was conducted to investigate the bonding properties between high strength rebar and reactive powder concrete (RPC), which covered ultimate pullout load, ultimate bonding stress, free end initial slip, free end slip at peak load, and load-slip curve characteristics. The effects of varying rebar buried length, thickness of protective layer and diameter of rebars on the bonding properties were studied, and how to determine the minimum thickness of protective layer and critical anchorage length was suggested according the test results. The results prove that: 1) Ultimate pull out load and free end initial slip load increases with increase in buried length, while ultimate bonding stress and slip corresponding to the peak load reduces. When buried length is increased from 3d to 4d(d is the diameter of rebar), after peak load, the load-slip curve descending segment declines faster, but later the load rises again exceeding the first peak load. When buried length reaches 5d, rebar pull fracture occurs. 2) As thickness of protective layer increases, the ultimate pull out load, ultimate bond stress, free end initial slip load and the slip corresponding to the peak load increase, and the descending section of the curve becomes gentle. The recommended minimum thickness of protective layer for plate type members should be the greater value between d and 10 mm, and for beams or columns the greater value between d and 15 mm. 3) Increasing the diameter of HRB500 rebars leads to a gentle slope in the descending segment of the pullout curve. 4) The bonding properties between high strength steel HRB500 and RPC is very good. The suggested buried length for test determining bonding strength between high strength rebars and RPC is 4d and a formula to calculate the critical anchorage length is established. The relationships between ultimate bonding stress and thickness of protective layer or the buried length was obtained.

Analytical model for rock bolts reaching free end slip

This paper presents the difference of behaviour between fully grouted bolts with and without free end slip when loaded in tension. An analytical approach is proposed for fully encapsulated bolts when the free end of the bolt slips. This model is based on the existing bond–slip relationship of bolt–grout interface with no free end slip. The derived analytical solutions of load–slip relationship, slip distribution, the shear stress and strain distributions presented in this paper are all connected with free end slip. The analytical approach is validated by experimental results. Free end slip has a significant influence on rock bolt behaviour and should not be ignored.

Slip distribution model along the anchorage length of prestressing strands

An analytical model to predict strand slips within both transmission and anchorage lengths in pretensioned prestressed concrete members is presented. This model has been derived from an experimental research work by analysing the bond behavior and determining the transmission and anchorage lengths of seven-wire prestressing steel strands in different concrete mixes. A testing technique based on measuring the prestressing strand force in specimens with different embedment lengths has been used. The testing technique allows measurement of free end slip as well as indirect determination of the strand slip at different cross sections of a member without interfering with bond phenomena. The experimental results and the proposed model for strand slip distribution have been compared with theoretical predictions according to different equations in the literature and with experimental results obtained by other researchers.

Flexural response of reinforced concrete (RC) beams strengthened with near surface mounted (NSM) fibre reinforced polymer (FRP) bars

The behaviour of RC beams strengthened with NSM FRP bars was experimentally investigated. Eight beams were tested under four point bending. The effects of material type, epoxy properties, bar size and the number of NSM bars were studied. The tested beams were strengthened with a limited bond length in order to imitate as much as possible work-place conditions, as the grooves could only be cut up to the faces of the supporting columns with difficulty. The load capacity, deflection, mode of failure, FRP strain, concrete strain, free end slip and the transverse strain in epoxy and concrete of the tested beams were all analysed. Comparison of strengthened and control beams showed enhancement of 155.8% and 129.8% in the yielding loads, while the increase in the ultimate loads was 166.3% and 159.4% for beams strengthened with carbon fibre reinforced polymer (CFRP) and glass fibre reinforced polymer (GFRP) respectively. The beams strengthened with CFRP bars experienced higher stiffness than the corresponding beams with GFRP bars. Epoxy properties, size and number of bars had little effect on the load capacity of the strengthened beams with failures mainly occurring either in epoxy or as a result of concrete cover separation.

Bond-Slip Relationship for EB-FRP Joints by Including Free End Slip

Closed form solutions are derived in this work by including free end slip, relating free end and loaded end slips to bond resistance, as well as relating slip or strain at any position to free end slip. To identify the bond-slip relationship, indirect analytical identification from load-slip response curve is used. Using the identification procedure developed in this paper, very good agreement between the theoretical solutions and experimental results was observed, which validates the theoretical results derived in this work.

Predicting Strand Transfer Length in Pretensioned Concrete: Eurocode versus North American Practice

Prestressing strands are commonly used in pretensioned prestressed concrete bridge construction. Transfer length is an important parameter for structural design. This paper presents a comparative study on strand transfer length provisions from Eurocode 2 and North American practice, and identifies similarities and differences between both models. A database of measured transfer lengths according to several authors has been compiled and compared with predictions according to code provisions. The intervals of predictions are smaller than those corresponding to the experimental results, and they are smaller when code provisions are more simplified. The interval from Eurocode 2 is greater than that from American Concrete Institute (ACI) code 318, which, in turn, is greater than the interval from AASHTO. The number of underestimated cases is lower for Eurocode 2 because of the higher predicted values, but situations in which a short transfer length is unfavorable are neglected by all models because they are not good predictions of shorter measured transfer lengths. When a transfer length estimation criterion is based on an allowable free end slip, more cases are excluded from the ACI code 318 provisions.

An experimental study of different factors affecting the bond of NSM FRP bars in concrete

Near-surface mounted (NSM) fiber reinforced polymer (FRP) reinforcement has proven to be an efficient technique for strengthening concrete members. The bond behavior of the NSM system depends on the bond in the two existing interfaces, while the response in terms of load–slip curves and load capacity may be strongly affected by many other parameters. As a consequence of the variety of factors affecting the bond, different studies into the behavior of NSM FRP reinforcement have been carried out in recent years. In this paper, an experimental study is performed in which the effect of adhesive properties, bar type, bar size, FRP properties, groove geometry and the use of mechanical interlocking on the capacity and bond behavior of a NSM joint is investigated. The failure load, average bond stress, loaded end slip, free end slip, transverse strains (on adhesive and concrete), and the mode of failure for the tested joints are reported and discussed. The results show that the adhesive properties, FRP bar size and bar surface treatment were critical factors in the capacity and mode of failure of the specimens tested.

Performance of near surface mounted glass fiber reinforced polymer bars in concrete

Near surface mounted fiber reinforced polymer reinforcement has become very popular method in strengthening of reinforced concrete structures. The major problem that accompanied the near surface mounted technique is the bond between fiber-reinforced polymers and concrete that corresponds to the stress transfer from concrete into fiber-reinforced polymers. This article presents the test results of pullout tests of near surface mounted glass fiber reinforced polymer bars to investigate the effect of different parameters on the bond performance of this strengthening technique. The test parameters include: adhesive type, groove size, bonded length, environmental condition, and concrete strength. A total of 40 near surface mounted–fiber-reinforced polymer bars, installed in C-shaped concrete blocks, were constructed and tested to failure. Five types of cement and epoxy-based adhesives were used. Two groove sizes and three bonded lengths were also investigated. Normal and high strength concrete were also used. In addition, the effect of two harsh environmental conditions on the performance of the strengthening system was also investigated. The results are presented in terms of pullout loads, free end slip, and mode of failure.

Experiment on the Bonding Performance of BFRP Bars Reinforced Concrete

Through the pull-out test methods, the concrete strength、reinforcement diameter Basalt Fiber Reinforced Plastics Bars、the anchorage length、 stirrup rate and other factors on the bonding properties of the BFRP reinforced concrete is analyzed. The BFRP bars and reinforcing steel bars bonding properties is compared. BFRP reinforced concrete bond failure mode has two types .As the concrete strength increases, the bond strength of the BFRP reinforced concrete increased. With the increase BFRP bars diameter and shear lag relationship, the cohesive force of the BFRP reinforced concrete decrease accordingly. And the failure modes of the shape of the BFRP reinforcement concrete in BFRP bonding properties with a big impact for the specimens’ configuration stirrups on the ductility. When BFRP bars loading under the same load level, the end of the slip is greater than the free end slip.

Analytical identification of bond–slip relationship of EB-FRP joints

Bond–slip relationship of externally-bonded (EB) fiber reinforced polymer (FRP) joints can be determined by either directly or indirectly from a pull-off test. The indirect analytical method is gaining popularity and has been recommended in recent literature because of the more consistent and accurate results it yields. To date, loaded end slip vs applied load curve is used for indirect identification of the bond–slip relationship, ignoring the free end slip. This work shows that ignoring free end slip may result in significant errors in measuring the bond–slip relationship. The analytical method is used in this work to derive closed form solutions for FRP strain, interface slips, and bond strength of EB-FRP joints. The analytical solutions are validated by experimental results. Using the analytical solution, it is clearly shown in the paper that free end slip affects results and should be considered in deriving bond–slip relationship of EB-FRP joints, especially when bond length is short.

Study on push-out test and bond stress-slip relationship of circular concrete filled steel tube

According to the results of 9 circular concrete filled steel tube (CFT) push-out tests, a new theoretical model for average bond stress versus free end slip curve is proposed. The relationship between verage bond stress and free end slip is obtained considering some varying influential parameters such as slenderness ratio and diameter-to-thickness ratio. Based on measured steel tube strain and relative slip at different longitudinal positions, the distribution of bond stress and relative slip along the length of steel tube is obtained. An equation for predicting the varying bond-slip relationship along longitudinal length and a position function reflecting the variation are proposed. The presented method can be used in the application of finite element method to analyze the behavior of CFT structures.

Effect of FRP bar degradation on interface bond with high strength concrete

An experimental investigation is described on the durability performance of FRP bar-concrete interface bond, purposely focused on the surface material degradation of FRP bar by using a concrete mix with high compressive strength . A total of 48 pullout specimens with four different types of FRP bars were cast for bond strength tests . Additional specimens were also cast for concrete compressive strength test, visual inspection of FRP-concrete interface, and scanning electron microscopy analysis to evaluate the environmental effects. Specimens were exposed to the following conditions: tap water maintained either at ambient temperature or 60 °C, and air of thermal cycling from −20 to 60 °C. The environmental conditioning resulted in bond strength reductions of 4–10% for CFRP bars and 0–20% for GFRP bars. The conditioning also increased the free-end slip before the peak bond strength was reached. The degradation of bond was related mainly to the deterioration of FRP bar and less to concrete. The interface bond failure mode and degradation mechanism due to FRP bar damage are characterized.

Effect of elevated temperature on bond between steel reinforcement and fiber reinforced concrete

The bond behavior between fiber reinforced concrete and 20-mm reinforcing steel rebars was evaluated under elevated temperatures. Fifty modified pullout specimens (100×100×400 mm) were prepared using high strength concrete with basalt aggregate and different volumetric mixtures of three types of fibers, namely brass-coated steel fibers, hooked steel fibers, and high modulus polypropylene fibers, before being cured for 28 days at 40 °C. Specimens, designated for heat-treatment, were then subjected to elevated temperatures, ranging from 350 to 700 °C, whereas unheated (control) ones were left in laboratory air. The overall response of control and heat-damaged specimens, pulled out up to failure, and cracking extent and continuity were described. Standard cubes (100 mm 3) were cast, cured, and heat treated under similar conditions, then tested to evaluate compressive and splitting strengths. The results showed marked reductions in residual compressive, splitting and steel–concrete bond under high temperatures with dramatic changes in bond stress–free-end slip trend behavior. Use of fibers minimized the damage in steel–concrete bond under elevated temperatures and hence the reduction in bond strength. Specimens which incorporated hooked steel fibers attained the highest bond resistance against elevated temperatures followed, in sequence, by those prepared with the mixture of hooked and brass-coated steel, the mixture of hooked steel and polypropylene, and brass-coated steel fibers. Statistical models for bond stress versus free-end slip and bond strength versus exposure temperature were developed. These showed excellent agreement with the trend behavior of present experimental data.

Bond Durability of Glass Fiber-Reinforced Polymer Bars Embedded in Concrete Beams

An experimental and analytical investigation of bond durability of E-glass fiber-reinforced polymer reinforcement bars in concrete beams is presented. Beams were conditioned with sustained flexural loads in indoor, outdoor, 60°C alkaline solution, or freeze/thaw environments for up to 3 years , after which they were subjected to eccentric three-point flexure tests to evaluate bond. Experimental bar force and slip were used to draw direct conclusions on bond durability, and also to calibrate a proposed local bond–slip model that incorporates concrete cover splitting. Experimental bar force at the onset of free-end slip varied little after any of the conditionings, although the characteristic of bond failure was noted to be less ductile in the moister environments. The interfacial fracture energy associated with bond–slip did not change with conditioning time in any of the environments except freeze/thaw, where a monotonic reduction versus time was seen. The effective bond length of the bar under various co...

Investigation of bond between fibre-reinforced polymer bars and concrete under sustained loads

Although the use of fibre-reinforced polymer (FRP) bars to replace steel in reinforced concrete is becoming more common, uncertainty remains concerning the long-term performance of FRP, including the effect of a sustained load on the bond between the FRP bars and the concrete. An experimental study was therefore undertaken to investigate the long-term durability of the bond for various types of bars embedded in concrete: one type of glass FRP, two types of carbon FRP, and conventional steel reinforcing bars. Pullout specimens were tested both statically to failure and under sustained loads for periods of up to 1 year while free-end slip was monitored. Results revealed lower short-term bond strengths for FRP bars relative to steel and significant variability in long-term bond-slip performance among FRP bars of different types. Post-testing investigations revealed damage to bar surfaces at the macroscopic level, as well as broken longitudinal fibres and damage to the surface coatings at the microscopic level.Key words: reinforced concrete, fibre-reinforced polymer (FRP), bond, creep, pullout, sustained loads.

Effect of alkali-silica reaction and freezing and thawing action on concrete–steel bond

An experimental program was conducted to investigate the effect of stresses and cracks, caused by alkali-silica reaction (ASR) and freezing and thawing (F/T), on bond between reinforcing steel and concrete. Pullout test cylinders, reinforced with 18 mm steel bars, were used to evaluate bond behavior. Concrete prisms (50 × 50 × 300 mm) were also cast to evaluate expansion and reduction in ultrasonic velocity due to ASR and F/T cycles, respectively. Specimens were cured for 40 days before being either immersed in sodium hydroxide solution of 0.5 normality in order to accelerate ASR, or subjected to different cycles of F/T. Bond behavior, expansion, and ultrasonic pulse velocity tests were carried out as ASR progressed or under F/T cycles. The progress of ASR resulted in significant losses in critical bond stress and ultimate bond strength capacity reaching as high as 44% and 24%, respectively, accompanied by a significant increase in free-end slip at failure. The loss in bond due to ASR was higher for specimens prepared using concrete with lower concrete strength and higher percentage of reactive aggregate. F/T action caused a significant reduction in critical bond stress and ultimate bond strength that reached as high as 100% and 55%, respectively, and an increase in free-end slip at failure. Neither ASR nor F/T cycles affected trends in the behavior of bond stress versus free end slip curves.