Deformed Steel Rebars Research Articles

Bond properties of deformed steel bars coated with quartz sand modified enamel in concrete under both monotonic and reversed cyclic loading

Reinforced concrete structures located in seismic zones require significant ductility, which is affected by the bond properties between steel and concrete. The bond may be negatively impacted by anti-corrosion coatings. Quartz sand modified enamel (QSME) enhances the bond ductility for steel rebars in concrete under monotonic loading. This study investigates the bond properties of QSME-coated deformed steel rebars under monotonic and cyclic loading. For comparison, uncoated, fusion-bonded epoxy (FBE)-coated, and unmodified enamel-coated steel rebars are also studied. In addition to the coating type, the effects of steel rebar diameter, anchorage length, and concrete strength are considered. Acoustic emission (AE) technique was utilized to capture the AE energy released during the pull-out tests. Results showed that the bond stiffness and strength properties of QSME-coated steel rebars increase when concrete strength increases, while they decrease with rebar diameter under monotonic loading. The energy dissipation capacity of QSME-coated deformed rebar specimens is in average 1.43, 2.11, and 1.56 times greater than that of uncoated, FBE- and enamel-coated specimens, respectively. Among all coating types, the QSME-coated steel rebars exhibit the highest degradation index for the residual bond strength and the most AE activity, attributed to their improved chemical bonding and interfacial friction.

Bond between deformed steel rebars and concrete at elevated temperatures

This paper presents an experimental study on the bond behavior between deformed steel rebars and concrete at elevated temperatures, focusing on the effect of heating rate on the flexural bond strength. Most of the earlier research on this topic involved axial pull-out tests in cooled-down conditions. None of the earlier research has studied flexural bond behavior in hot conditions. Also, there is a lot of variability in the test conditions used in these studies. To overcome these challenges, a number of beam specimens whose flexural behavior was governed by bond failure were prepared and tested at elevated temperatures. Two different rebar diameters (12 mm and 20 mm), and two different rates of heating (2 °C/min and standard fire curve) were used in these tests. The results indicate that a rapid rate of heating affected the bond strength more significantly. Also, the 20 mm-diameter rebar had lower bond strength per unit surface area in comparison to the 12 mm-diameter rebar. The bond ductility reduced more rapidly than the bond strength as the interface temperature rose.

Experimental study with DIC technique on the bond failure of structural concrete under repeated reversed loading with different amplitude

The bond between deformed steel rebar and concrete plays a critical role against seismic action, thus it is important to capture the dynamic bond loss and establish the bond-slip model with cyclic loading. To explain the macroscale performance such as bond-slip relationship and establish the bond strength model, it is meaningful to capture the mesoscale and local behaviors between ribs and concrete. In this paper, pull-out specimens with rebar embedded in concrete and small windows excavated in the cover concrete are adopted with DIC technique to monitor the strain variation and crack development along the interfaces under static and reversed loading of different amplitude. From the test, it is found that DIC is an effective approach to obtain the localized characteristics including the normal and shear deformations. Through parametric study, the effects of concrete cover depth and the existence of stirrups on bond deterioration in terms of both global and local behaviors are studied. Interestingly, the global slip and local crack-width show synchronically increase under the pulling-out action, which demonstrates that the macroscale behaviors could be clarified from mesoscale measurement with the aid of DIC technique.

Bond conditions in wall-type steel-reinforced self-compacting concrete element

This work looks into the variations in the bond conditions between steel and self-compacting concrete (SCC). The bond conditions along the height and span of wall elements are compared with column and beam elements. The study involved pull-out bond tests and compressive strength tests on module samples derived from a wall element having 2240 × 160 × 1600 mm in length, width and height, respectively. SCC mixture casting was performed from a single point located at one of the short edges of the element. The pull-out tests were conducted on samples having deformed steel rebars with a diameter of 16 mm. We examined in detail the top-bar effect occurring across the entire wall element. The bond strengths at different heights within the element were compared to that at its bottom. On average, the reduction in the bond strength was 22%, and as much as 39% at the top of the wall. The bond strength increased by an average of 13% within the end part of the element as a result of the rebound effect. The phenomenon of bond strength reduction with an increasing depth of concrete under a reinforcing bar seems universal across different types and heights of elements.

Experimental study and modeling on bond-slippage behavior of steel bar in polyvinyl alcohol-steel hybrid fiber engineered cementitious composite

In order to understand and estimate the composite action of steel bar and fiber reinforced low drying shrinkage engineered cementitious composite (LSECC), bond strength-slip relation of steel bar embedded in LSECC was studied in this paper. Series pullout tests, smooth steel bar and deformed steel rebar, with four LSECC mixtures and two embedded lengths were performed. Failure on bond between steel bar and LSECC under pullout load can be divided into debonding and pullout stages. In debonding stage, constant bond strength that principally controlled by chemical and physical bond of steel bar and cement matrix can be used to describe the pullout behavior. In pullout stage, bond strength first increases with slippage (slip-hardening stage of bond). After the peak, bond strength decreases with slippage (slip-softening stage of bond). In the slip-hardening stage, much higher bond strength is observed in slip-hardening stage of rebar than that of the smooth bar, while much smaller slippage corresponding to the peak bond strength is observed of rebar than that of the smooth bar. An analytical three stage bond strength-slippage model was proposed based on pullout test results with small embedded length (30 mm). Meanwhile, pullout model of steel reinforcement from ECC with arbitrary embedded length was developed based on debonding and pullout theory, which can well simulate initial ascending, slip-hardening and slip-softening stages under pullout load. Good agreement between model and test results in terms of load-slippage relationship of longer reinforcement embedded in ECC was observed.

Flexural bond evaluation of deformed steel rebars in recycled aggregate concrete

An accurate quantification of the bond between embedded rebar and recycled aggregate concrete (RAC) is a prerequisite for confident use of RAC. However, there is a noticeable lack of experimental data that can faithfully represent the actual bond stress state in real RAC structures. To address this limitation, a total of 37 beam specimens were tested in strict conformity with the RILEM testing protocol. The strength grade of old concrete was among several control variables studied for their impact on the bond strength. The findings reveal that the incorporation of coarse or/and fine recycled aggregate (RA) does not alter the expected mode of bond failure for a beam; however, it significantly affects the beam’s bond strength in flexure. Using RA sourced from weaker parent concrete leads to much inferior interfacial bonding, but there exists an almost linear relationship between the bond strength of RAC relative to that of natural aggregate concrete (NAC) and their relative water absorption ratio. This relationship seems to hold true regardless of the strength of RA. Based on the probabilistic analyses conducted herein, the anchorage length specified in existing codes developed for NAC is found inadequate and unsafe for RAC. A modifier is therefore proposed to remedy this unconservatism.

Bond Characteristics of Deformed Steel Rebar in Palm Kernel Shell (PKS)-Rubberised Concrete Composite

The bond mechanism between concrete and steel is an important input parameter in the design of reinforced concrete elements. The reuse of waste materials as aggregates in concrete has led to the discovery of different types of concrete with unique bond characteristics. This paper reports on the bond characteristics of concrete produced from waste automobile tire chips and palm kernel shell aggregates and deformed mild steel rebars. A total of 125 concrete cubes (150 x 150 x 150mm) with metal inserts were cast from 21 concrete mixes with varied content of PKS and waste automobile tire aggregates. Pullout test was carried out to evaluate the strength of the bond mechanism between the steel and the various concrete mixes. The results revealed that bond strength decreased with increasing PKS and tire content. Moreover, increasing the bar size and embedment length reduced the bond stress. The ratio of the bond stress and compressive strength was found to be averagely 63.88%. Regarding the bond failure mechanism, it was identified that failure of the specimen occurred through either rebar pullout or tensile splitting of the concrete. Based on the results obtained, it was concluded that PKS and tire can be used as partial replacement of granite aggregates in concrete since the resultant concrete can develop adequate bond with steel bars in structural applications.

Investigation into corrosion-induced bond degradation between concrete and steel rebar with acoustic emission and 3D laser scan techniques

Corrosion-induced bond degradation of deformed steel rebar in concrete is experimentally investigated with acoustic emission (AE) and 3D laser scan technique. Concrete specimens were fabricated and subjected to direct pullout test after being corroded to different levels. The number and width of cracks present during the corrosion tests and the pull-out tests were recorded. The energy released during the pullout tests were captured with AE probes, and the frequency characteristics was analyzed. After pullout tests, the surface morphology of corroded steel rebars was determined with a 3D laser scanner. A modified bond deterioration model was proposed and the parameters associated with the model were analyzed. Results indicated that two types of AE signals were acquired during pullout tests: concrete cracking in high frequency range of 35 ~ 41 kHz and steel-concrete friction in low frequency range of 3 ~ 15 kHz. The bond strength and the bond-slip characteristics depend upon the level of corrosion as well as the number and width of cracks. The reduction factor of the bond-slip model exponentially decreases as a function of the average cross-sectional area loss and linearly decreases with an increase of the rib area loss.

Moisture transport and steel rebar corrosion in repair composites incorporating Nano-Fibrillated Cellulose (NFC)

Repair of deteriorating concrete infrastructure remains a major challenge. The concern is that current repair materials are not able to prevent premature failure after a repair has been performed. Thus, an engineered repair mortar that can enhance the durability of repaired composites by mitigating moisture transport and its debilitating influence on deterioration mechanisms such as steel rebar corrosion would be highly coveted. In this study, the influence of reinforcing a repair mortar with 0.1% by volume of Nano-Fibrillated Cellulose (NFC) on the capillary water transport in monolithic and composite specimens was investigated. Micro Computed Tomography (CT) imaging was also utilized in evaluating the microstructure of sorptivity test specimens. Thereafter, the influence of repair mortars on the corrosion behavior of deformed steel rebar was investigated via natural and accelerated tests. In these tests, concrete substrates had an embedded rebar in it, and repair mortars were applied on the top creating a composite specimen with a repair interface. Some of the parameters measured were surface resistivity, half-cell potential, overlay delamination time, residual rebar pullout load and rebar corrosion degree.Sorptivity tests indicated that the NFC is indeed capable of refining the pore structure through internal curing and thus reducing the capillary uptake of water. Moreover, the tendency of the NFC to entrap micro voids, especially near the repair interface, thereby indirectly reducing moisture transport and saturation of composite specimens was confirmed by CT imaging. Natural and accelerated corrosion test results demonstrated that NFC addition inhibited rebar corrosion as indicated by reduced rebar corrosion degree and delayed delamination of the repair overlay. The NFC also provided superior surface resistivity and a higher rebar pullout load capacity after corrosion.

Bond performance of deformed steel rebars in HSC incorporating industrially produced recycled concrete aggregate

This paper presents an experimental study on the local bond behaviour of deformed steel rebars in high-strength concrete (HSC) incorporating high-quality recycled concrete aggregate (RCA) produced from a recycling plant in Flanders, Belgium. To examine the influence of the coarse RCA on the local bond behaviour between the rebars and concrete, a total of 5 concrete mixtures (target strength class C50/60) are prepared, in which the replacement percentage of the coarse RCA to the natural aggregate is 0, 10, 20, 50 and 100%, respectively. Deformation controlled pull-out tests with relatively short embedded length (3 times the diameter) of the steel rebars are carried out to investigate the local bond-slip behaviour between the steel rebars and HSC. The test results reveal that the incorporation of the RCA does not have a significant effect on the bond performance between the rebars and concrete. Based on the test data, a new model for predicting the maximum bond stress is proposed. The analytical relationship in fib Model Code 2010 generally agrees well with the measured bond-slip curve, irrespective of the RCA content in the concrete, provided that the measured maximum bond stress is used. A more accurate description of the bond-slip behaviour up to the relative slip of 1.0 mm can be achieved by adjusting the α-value in the equation in fib Model Code 2010.

Bond Strength Behavior for Deformed Steel Rebar Embedded in Recycled Aggregate Concrete

This study investigated the bond strength behavior of a deformed steel bar embedded in RAC through an experimental program and numerical analysis. In the experimental work, eighteen push-out specimens were tested. The compressive strength of RAC, the recycled aggregate replacement ratio, the embedded length of the reinforcing bar, the size of the rebar, the concrete cover, and the yield stress of the reinforcing steel bar were the main parameters investigated. The effect of these parameters on bond strength, bond-slip behavior, and modes of failure are discussed. Analysis of the test results indicate that the bond strength in concrete is reduced by 13% when using a specimen constructed from recycled aggregate compared with conventional concrete. The failure modes in a reinforcing bar embedded in RAC representing splitting failure and push-out failure, were similar to those in conventional concrete. The finite element analysis presented in this study was used to analyze forty-four push-out specimens. Through numerical analysis, the bond strength of RAC was related to the 0.57 power function of compressive strength. A design equation for bond strength of reinforcing bars embedded in RAC is proposed. The proposed equation was calibrated through the numerical and experimental results.Â

Finite element modelling of monotonic pull-out test of deformed steel rebar embedded in well-confined concrete

A finite element study carried out using LS DYNA and aimed to simulate the monotonic pull-out test of deformed steel rebar embedded in concrete is presented in this paper. Three models of the interface between deformed steel rebar and well-confined concrete, i.e. perfect bond model and two bond-slip models are observed and compared. Bond stress-slip response and rebar stress-slip response obtained numerically are validated with experimental data and empirical equations available from the literature. The full bond model overestimates the response, providing higher rebar stress. In the bond-slip models, good agreement is observed between numerical and experimental bond stress and rebar Stress–slip responses. The empirical equation of bond-slip proposed by Murcia-Delso and Shing (2014) is found to overestimate the peak bond stress.

Effect of Fibers on the Bond Behavior of Deformed Steel Bar Embedded in Recycled Aggregate Concrete

A large volume of concrete debris is being produced in many countries on the globe due to the demolition of old concrete structures and testing of concrete specimens in laboratories. One of the ways to reuse concrete debris is to produce Recycled Aggregates (RA) and use them in new concrete. In recent years, Recycled Aggregates Concrete (RAC) has experienced increasing demand in various non-structural and structural applications. In reinforced concrete structures, one of the sources of brittle failure is sudden loss of bond between reinforcing bars and concrete in anchorage zones. Therefore, for the structural application of any new kind of concrete such as fiber reinforced RAC, knowledge of bond characteristics of reinforcing bars embedded in concrete becomes essential for determining the overall structural response under different modes of loading. In this regard, this study experimentally investigated the effect of fibers on the bond stress-slip behavior of deformed steel re-bar embedded in RAC. Concrete mixes having 0, 50 and 100% RAs were prepared with and without the addition of fibers. Two types of fibers were investigated in mono form: hooked-ends steel and polypropylene fibers. The dosage of steel and polypropylene fibers was kept 40 and 4.4 kg/m3, respectively. Axial compression and standard pull-out tests were performed. Test specimens for pull-out test were prepared using deformed steel re-bars of 19mm (#6) diameter. The results of strength tests confirmed that the compressive strength of concrete is decreased by replacing Natural Aggregates (NA) with RAs. For bond behavior of steel re-bar, the results of this study showed that replacement of 50% NA with RAs did not affect the bond response of steel bar, however, 100% replacement of NA with RAs showed detrimental effect on bond stress slip behavior. The results further showed that the addition of both types of fibers made it possible to recover the loss in compressive strength, bond strengths and bond toughness occurred because of replacing NA with RAs. In case of RA concrete mixes containing hooked-ends steel fibers, strength values were found even greater than the strength values of Natural Aggregates Concrete (NAC). From the results of this study, it was found that it is possible to design a structural concrete mix using 100% RAs and steel fibers at relatively low dosage of 40kg/m3.

Crack width evaluation of fiber-reinforced cementitious composite considering interaction between deformed steel rebar

This study aims to propose a simple evaluation method of crack width in reinforced FRCC members. Uniaxial tension test is conducted for steel-reinforced FRCC prism specimens with slits to measure crack width clearly. The test parameters are cross-sectional size of prism and fiber volume fraction of FRCC. Through the loading test, steel strain – crack width relationship is obtained. The theoretical calculation formula to predict crack width in steel-reinforced FRCC is led by solving the force equilibrium and compatibility conditions between FRCC and reinforcing bar considering bond stress – slip relationship, fiber bridging law (bridging stress – crack width relationship) and condition of crack occurrence. The steel strain – crack width curves predicted by the proposed formula show a good adaptability with the experimental results in each parameter.

Simplified Analytical Model for Interfacial Bond Strength of Deformed Steel Rebars Embedded in Pre-cracked Concrete

AbstractAlthough extensive bond models have been developed for use in the numerical simulation of uncracked reinforced concrete, no simplified method exists providing satisfactory accuracy and effi...

Experimental and numerical investigation on bond between steel rebar and high-strength Strain-Hardening Cementitious Composite (SHCC) under direct tension

Sufficient bonding between Strain-Hardening Cementitious Composites (SHCC) and steel rebars is required to guarantee the stress transfer in structural applications. Although the bond between medium-strength SHCC and rebars has been extensively studied, the present understanding of this property for high-strength SHCC is very limited. Moreover, the previous pullout or bending test methods have some limitations. This study investigates the bond between deformed steel rebars and high-strength SHCC with a new direct tension approach, which is able to determine the fundamental interfacial stress versus displacement relation under pure tension. The effects of concrete block material and cover thickness, rebar diameter and embedment length, as well as confinement condition on the bond behavior were experimentally evaluated. The test results showed that high-strength SHCC can bond much better with deformed steel rebars than ordinary concrete and high-strength SHCC matrix (without fibers) due to the well-controlled splitting cracks, and that the geometry has certain influences on the bond strength. A simple but reasonably accurate finite element model for engineering applications was also proposed to analyze the bond behavior in structural components. In this model, all the non-linear behaviors were integrated into rebar/SHCC interfacial elements, so that the complicated steel surface geometry and/or the local splitting of concrete do not need to be explicitly modelled. Through a coupled experimental-numerical approach, the bond stress-displacement relationship was extracted from a small number of test results. The derived relationship was then employed for the prediction of additional test results, with good agreement achieved. With its general applicability demonstrated, the proposed numerical approach can facilitate the design of reinforced SHCC elements and structures.

Influence of artificial cracks and interfacial defects on the corrosion behavior of steel in concrete during corrosion initiation under a chloride environment

In this study, the effect of both artificial crack and defects of the steel-concrete interface on the initiation and propagation of steel reinforcement corrosion in concrete under a chloride environment is investigated. This work was based on two groups of reinforced concrete blocks, with or without artificial crack. The microstructures of the steel-concrete interface, the cement hydration products at the steel-concrete interface, the chloride profiles, the galvanic corrosion current, and the distribution of pitting corrosion on steel rebar were studied and compared. Experimental results show that the location of initial corrosion is related to top-casting defects but not to the location of air bubbles. Pitting corrosion mainly initiated at the rib foot of the bottom side of the deformed steel rebar with top-casting defects. Steel in concrete with an artificial crack exhibited a fast initiation period immediately followed by a propagation period. The presence of both artificial crack and top-bar effect thus led to faster deterioration than in un-cracked structures.

Bond behaviours of deformed steel rebars in engineered cementitious composites (ECC) and concrete

This paper presents the bond behaviors of deformed steel rebars in engineered cementitious composite (ECC) and concrete under direct pullout condition. A total of 12 specimens were casted and tested, it was found that the deformed steel rebar has a much higher bond strength in ECC than in concrete due to the superior tensile ductility of ECC material. A new finite element analysis (FEA) model, which considered the influences of steel ribs, was proposed and verified with the experimental results. Based on the proposed FEA model, the failure mechanisms and bond deterioration processes for both ECC and concrete specimens were analyzed and compared. It was found that the plastic area ratio for ECC specimen was as high as 32.2%, which is 3.3 times higher than that for concrete specimen. Also, the ultimate damage zone for ECC specimen is 36% lower than that for concrete specimen. It was concluded that ECC specimen has more compatible deformability with steel rebar as more ECC elements could participate in the transfer of tensile load.

Reinforced fibrous recycled aggregate concrete element subjected to uniaxial tensile loading

In this study, effect of recycled aggregates and polypropylene fibers on the response of conventionally reinforced concrete element subjected to tensile loading in terms of tension stiffening and strain development was experimentally investigated. For this purpose, concrete prisms of 100x100 mm cross section and 500 mm length having one central deformed steel re-bar were cast using fibrous and non-fibrous Recycled Aggregate Concrete (RAC) with varying percentages of recycled aggregates (0%, 25%, 50%, 75% and 100%) and tested under uniaxial tensile load. For all fibrous RAC mixes, polypropylene fibers were used at constant dosage of 3.15 kg/m3. Effect of recycled aggregates and fibers on the compressive strength of concrete was also explored in this study. Through studying tensile load versus global axial deformation of composite and strain development in concrete and steel, it was found that replacement of natural aggregates with recycled aggregates in concrete negatively affected the cracking load, tension stiffening and strain development, and this negative effect was observed to be increased with increasing contents of recycled aggregates in concrete. The results of this study showed that it was possible to minimize the negative effect of recycled aggregates in concrete by the addition of polypropylene fibers. Reinforced concrete element constructed using concrete containing 50% recycled aggregates and polypropylene fibers exhibited cracking behavior, tension stiffening and strain development response almost similar to that of concrete element constructed using natural aggregate concrete without fiber.

Temperature effects on the bond behavior between deformed steel reinforcing bars and hybrid fiber-reinforced strain-hardening cementitious composite

The authors have recently developed a hybrid fiber-reinforced strain-hardening cementitious composite (HFR-SHCC) using steel and polyvinyl alcohol (PVA) fibers. When subjected to high temperatures (up to 800 °C), this HFR-SHCC retains a higher proportion of its tensile strength compared to SHCCs containing only PVA fibers. The objective of the current study is to experimentally investigate whether the improved material properties of HFR-SHCC at high temperatures lead to better bond with conventional deformed steel reinforcing bars (rebar) compared to other SHCCs and conventional concretes. Rebar pullout specimens with a deformed rebar centrally embedded inside a SHCC/concrete cylinder were used in this study to determine the rebar-matrix bond. The bond behavior of HFR-SHCC with deformed steel rebar was compared with two concretes of different compressive strengths, a commonly used PVA-SHCC containing only PVA fibers, and a PVA-SHCC with very high content of fly ash. The rebar pullout specimens were subjected to temperatures of 100 °C, 200 °C, 400 °C, 600 °C and 800 °C and were tested after cooling down to room temperature to determine the residual bond strength. Results show that while the specimens made with the conventional concretes exhibit the largest bond strength at room temperature, the specimens made with HFR-SHCC retain the largest proportion of their room temperature bond strengths after exposure to high temperatures and show a more ductile pullout behavior instead of the abrupt splitting failure exhibited by the other materials.