Rebar Strain Research Articles

Mechanics and failure characteristics of hybrid fiber-reinforced concrete (HyFRC) composites with longitudinal steel reinforcement

While the properties of hybrid fiber-reinforced concrete (HyFRC) have been well-reported in the literature, the behavior of reinforced HyFRC (i.e., HyFRC with embedded steel rebar) is less understood. This paper investigates the mechanics and failure characteristics of reinforced HyFRC under direct tension. Samples with a low longitudinal steel reinforcement ratio were studied to evaluate the feasibility of reducing rebar congestion in structural applications through the use of fiber-reinforced concrete. Although reinforced HyFRC forms multiple cracking sites when loaded, rebar strain and HyFRC crack opening are generally concentrated at a single location under post-yield displacements. The onset of rebar plastic deformation and the exhaustion of fibers’ bridging load capacity are coincident events at a dominant crack. For a cracked reinforced HyFRC section to strengthen, the magnitude of load resistance increase from strain hardening rebar must exceed the magnitude of load resistance decrease from fiber pull-out processes. Comparisons are made with studies reported in the literature to demonstrate how longitudinal reinforcement ratio and fiber type influence cracking behavior and ultimate strain capacity. The research presented herein has far-reaching impacts on the structural design of all types of reinforced fiber-reinforced concrete materials detailed for a ductile response under large displacements.

Effect of impact loading on bar development length in CCT node

ABSTRACTCurrent design codes prescribe the development length of a reinforcing bar based on test results under static loading. In the present study, the development length of the reinforcement bar for reinforced concrete beams under impact loading was evaluated. Three-point impact loading test was performed on 16 partially bonded reinforced concrete beams to investigate the effect of loading rate on the bar development length. The test parameters were drop heights of the hammer, rebar diameter, and bar development length. The structural performance including the impact load, mid-span deflection, crack distribution, and rebar strain was evaluated. Although the bar development length was shorter than the design requirement under static loading, beam rebars were yielded under impact loading. For verification, a nonlinear numerical analysis result addressing the strain rate effect of concrete and reinforcing bars and the confinement effect of stirrups was compared with the test result. Further, considering the strain rate effect and nonuniform bond stress distribution model, a new design method was proposed to predict the bar development length under impact loading.

Validation of Finite Element Model by Smart Aggregate-Based Stress Monitoring.

Concrete compressive strength is an important parameter of material properties for assessing seismic performance of reinforced concrete (RC) structures, which has a certain level of uncertainty due to its inherent variability. In this paper, the method of concrete strength validation of finite element model using smart aggregate (SA)-based stress monitoring is proposed. The FE model was established using Open System for Earthquake Engineering Simulation (OpenSEES) platform. The concrete strengths obtained from the material test, peak stress of SA, and estimated concrete strength based on SA stress were employed in FE models. The lateral displacement monitored by Liner variable differential transformer and vertical axial load monitored by load cell in the experiment are applied in the model. By comparing the global response (i.e., lateral reaction force and hysteretic loop), local response (i.e., concrete stress, rebar strain, and cross-section moment) and corresponding root-mean-square error obtained from experiment and numerical analysis, the capabilities of validation of FE model using SA-based stress monitoring method were demonstrated.

Tension-stiffening effect in steel-reinforced UHPC composites: Constitutive model and effects of steel fibers, loading patterns, and rebar sizes

The tensile performance of steel-reinforced concrete members is closely associated with the bond interaction between concrete and the embedded rebar. Ultra-high performance concrete (UHPC) is a rapidly emerging concrete material that has an ultra-high compressive strength and bond strength. When it is reinforced with short, discontinuous fibers, it features a tensile strain-hardening behavior and a damage pattern of closely spaced narrow cracks. The present study investigated the tensile behavior of steel-reinforced UHPC members. Sixteen samples were tested, with the experimental variables including embedded rebar sizes, loading patterns, and steel fibers. The tensile responses of the steel-reinforced UHPC samples were evaluated using multiple performance measures, including the damage pattern, stiffness, load-deformation relationship, rebar strain, and the tension-stiffening behavior of UHPC. The test results showed that the enhanced bond strength due to the inclusion of steel fibers transformed the failure pattern of the steel-reinforced UHPC from multiple localized cracks into a single localized crack, which intensified the strain concentration in the embedded rebar. Although the addition of fibers substantially strengthened the tension-stiffening response of the UHPC, it also raised critical concerns about premature failure, especially when UHPC members were reinforced with small steel bars and subjected to monotonic loading. In addition to the experimental study, a tetra-linear constitutive model that was able to reasonably represent the tension-stiffening behavior of fiber-reinforced UHPC up to failure was suggested in this study.

Experimental study of the short-term and long-term behavior of perfobond connectors

Recently, perfobond connectors for steel-concrete composite structures have received extensive research attention due to their excellent bonding and fatigue performance, and the construction convenience. However, the research on the long-term behavior of perfobond connectors was seldom reported. This paper presents the results of laboratory tests on perfobond connectors, including twenty short-term specimens and six long-term specimens. Parametric study was conducted to investigate the influence of the hole diameter in the connector, the number of the holes, the thickness of the connector, the concrete strength as well as the diameter of the transverse rebar on the shear performance of perfobond connectors. For the short-term test series, the failure modes of the connector, the load-slip response, the ultimate shear bearing capacity and the load-strain curves of transverse rebar were studied. For the long-term test series, the transverse rebar strain, the relative slip between the steel beam and the concrete slab and the stress around the holes with duration of loading were discussed. The shear performance of perfobond connectors after long-term loading was also investigated. Finally, based on the test data from both this paper and open literature, a model was proposed to predict the shear bearing capacity of perfobond connectors.

Fire resistance of steel fiber reinforced concrete beams after low-velocity impact loading

Engineering structures in the city environment surrounded with combustible materials are prone to be in the danger of combined effects of blast-induced impact loading and fire. Four steel-fiber reinforced concrete (SFRC) beams were tested after pre-impact loading (impact velocity = 5.4 m/s) to explore their fire-resistance. The beams were first subjected to impact loadings and then exposed to fire with a constant load. The failure patterns of beams were observed, and the time histories of mid-span deflections, temperature field as well as rebar strains were recorded. Moreover, the fire resistance of these beams were discussed. A three-dimensional finite element numerical model considering the effects of strain rate and high temperature were established. In the simulations, a two-step analysis method was utilized. In the first-step, the impact loading process was simulated. While, in the second-step, and the failure behavior of the SFRC beams subjected to both fire and constant mechanical loading was modeled with a sequentially coupled thermal-stress analysis method, in which the simulation results obtained in the first-step were taken as the initial state. Good agreement between the simulation results and the test results illustrates the validation of the simulation method. It is found that the failure patterns of SFRC under the low-velocity impact load is altered from shear type to bending type with the increase of steel fiber dosage. When the impact energy is relative lower, though weakened by the pre-impact, the beam still work in an elastic stage and shows good fire resistance. Moreover, limited to the low-energy impact conditions, the influence of steel fiber content on the thermal and mechanical behaviors of damaged beam can be ignored.

Studies on the static behaviors of unbonded prestressed steel reinforced low-strength concrete rectangular frame beams

In this paper, four one-fifth-scale unbonded prestressed steel reinforced concrete (UPSRC) frame beam specimens were tested under vertical monotonic loading. The parameters including the amount of steel shape, ratio of rebar and the dimension of column sections were studied. The failure mode, load-deflection behavior, crack distribution, strain of rebar and steel shape were investigated. Furtherly, the tendon stress increment, ductility and post-peak load-carrying capacity of UPSRC frame beams were discussed and an analysis method was established to predict the maximum and post-peak load-carrying capacity. Test results showed that the UPSRC frame beams showed a typical beam hinge failure mode and still had good bearing capacity after the maximum load-carrying capacity. The unbonded tendons could well control the development of cracks and greatly improve the deformation restoring capacity of SRC beams. Furthermore, the value of ultimate stress increment in tendons of UPSRC members was greatly underestimated by the methods in generally design codes. By increasing the depth of web or enlarge the dimension of columns would improve the ductility of UPSRC members. Finally, the analysis method provided in the paper accurately evaluate the maximum load-carrying capacity and a little overestimate the post-peak load-carrying capacity.

Seismic performance of precast concrete frames with debonded reinforcement

This paper utilizes experimental and numerical studies to investigate the seismic behavior of precast concrete frames. The system is composed of monolithic columns and composite precast concrete beams with debonded reinforcement at the beam end, with the purpose of distributing plasticity over a larger rebar length to improve the seismic performance of traditional precast concrete frames. Two half scale precast concrete frames, with and without debonded rebar, were tested under quasi-static cyclic lateral load. The observations during the test, load–displacement curves, stiffness, energy dissipating capacity and rebar strain are discussed. The experimental findings demonstrate that rebar debonding lead to reduced strain in tensile reinforcement. The decrease in strain due to the debonded rebar was 40.2% at a drift ratio of 1%. The performance of the specimens was evaluated according to ACI 374, which demonstrates that this precast system is applicable to seismic regions. In the numerical simulation study, a macro-based finite element (FE) model was developed using fiber-section beam-column element with a modified rebar constitutive model to take into account the effect of rebar buckling. The feasibility of the FE model was verified by comparing with the experimental data.

Experimental Study on Strain Penetration Effects in Fixed‐End Rotation of RC Beam‐Column Connections with High‐Strength Reinforcement

The additional fixed‐end rotation resulting from the strain penetration of longitudinal reinforcement in a reinforced concrete beam‐column connection is a crucial factor for the plastic hinge rotation capacity. When it comes to high‐strength reinforcement, the effects of strain penetration on fixed‐end rotation become more obvious because of the increase in yield strength. In this study, 42 beam‐column connections with high‐strength hot rolled ribbed bars were designed and tested under monotonic loading at the beam end. The test results show that the rebar strains gradually decrease from the critical section towards the beam‐column connection, thereby proving the existence of strain penetration in the beam‐column connection. The slippage of the embedment reinforcement at the beam‐column interface and additional fixed‐end rotations were obtained from the test results. In addition, a parametric study involving the yield strength and diameter of reinforcement, concrete tensile strength, and embedment length in the beam‐column connection was performed to investigate the effects of various parameters on the additional fixed‐end rotation. Finally, a new simple and practical calculation model for predicting the additional fixed‐end rotation was proposed. The prediction shows good agreement with the experimental results.

Mechanical behavior of Quasi-rectangular segmental tunnel linings: First results from full-scale ring tests

The shield driven method is now a preferred choice when urban subway tunnels are constructed. In high density cities, a suitable cross-section for tunnel structures is rather important. In this study, a full-scale ring test is conducted in order to determine the mechanical behavior of Quasi-rectangular segmental tunnel linings, firstly designed and employed in Metro Line 3 in Ningbo, China. The design of experimental specimens and loading processes are introduced. Main results of tests are explored, including the failure state and measurement items, convergence deformation, strain of rebars, bending angle of joints and bolts’ strain .The ultimate bearing capacity of the structure is also reached during the test. According to the experimental results, two analysis methods are utilized to evaluate the mechanical behavior of the structure under design condition. The bearing mechanism and weakness of the lining are revealed as well. Safety factor of elastic stage and ultimate stage are also defined to evaluate safety reservation of the structure under unexpected circumstance. The test will lay the foundation for further verification and optimization of Quasi-rectangular segmental tunnel linings.

Damage monitoring of the RC frame shaking table test and comparison with FEM results

Piezoelectric smart aggregate (SA) is an internal stress sensor capable of full range seismic damage monitoring for concrete structures. In this study, the structure model is a one-bay-two-story RC frame, which is subjected to one direction horizontal excitation in a shaking table test. Stresses of four cross sections at the top and bottom of the two columns in the first floor which were symmetrical about the central axis along the excitation direction were monitored. Strain of rebar, acceleration, displacement of the structure was also monitored. The RC frame underwent Loma Prieta earthquake record with a peak value of 0.8g. After the simulated ground motion, only moderate damages were observed in the structure. Finite element analysis (FEA) using OpenSEES were conducted to simulate the shaking table test. By the comparison between the test and numerical results, the influence of the randomness of the material property identified by the SA-based stress monitoring on the failure pattern was discussed and the parameters of finite element model were checked.

Numerical simulation of mega steel reinforced concrete columns with different steel sections

SummaryIn recent years, the mega steel reinforced concrete (SRC) columns have been applied in super‐tall buildings. The previous research on SRC columns mainly focuses on the components with simple arrangement of encased steel, such as H‐shaped steel and cross‐shaped steel, with little attention paid to mega SRC columns that always have complicated encased steel and large steel ratio. The cyclic loading tests were carried out on scaled mega SRC column specimens with different cross‐section type of encased steel and steel ratio. The nonlinear three‐dimensional finite element and fiber element models were established respectively to simulate the inelastic behavior of mega SRC columns. The equivalent plastic strain of concrete, steel and rebar as well as the confinement effect on the concrete caused by the rebar and the steel plate were analyzed subsequently. By using the verified numerical model, the seismic behavior of specimens with different axial compression ratio was also studied. The test and analysis results indicate that the steel ratio and axial compression ratio have significant effects on the seismic performance of mega SRC columns, while the effect of cross‐section type of encased steel is not significant. Copyright © 2016 John Wiley & Sons, Ltd.

Characteristics of ground motion and threshold values for colluvium slope displacement induced by heavy rainfall: a case study in northern Taiwan

Abstract. The Huafan University campus is located in the Ta-lun Shan area in northern Taiwan, which is characterized by a dip slope covered by colluvium soil of various depths. For slope disaster prevention, a monitoring system was constructed that consisted of inclinometers, tiltmeters, crack gages, groundwater level observation wells, settlement and displacement observation marks, rebar strain gages, concrete strain gages, and rain gages. The monitoring data derived from hundreds of settlement and displacement observation marks were analyzed and compared with the displacement recorded by inclinometers. The analysis results revealed that the maximum settlement and displacement were concentrated on the areas around the Hui-Tsui, Zhi-An, and Wu-Ming buildings and coincided with periods of heavy rainfall. The computer program STABL was applied for slope stability analysis and modeling of slope failure. For prevention of slope instability, a drainage system and tieback anchors with additional stability measures were proposed to discharge excess groundwater following rainfall. Finally, threshold value curves of rainfall based on slope displacement were proposed. The curves can be applied for predicting slope stability when typhoons are expected to bring heavy rainfall and should be significant in slope disaster prevention.

Modeling Bar Slip in Nonductile Reinforced Concrete Columns

AbstractBar slip deformations in reinforced concrete frame structures cause additional flexibility at the boundaries of beams and columns due to penetration of rebar strain into the joints and footings. There are several available analytical models that estimate bar slip deformations, many of which have difficulties capturing the bar slip deformations that occur after flexural-axial damage. A new methodology is proposed to effectively account for bar slip deformations within the existing elements of an analytical model, without increasing the complexity of the system. This new method shows good agreement with previous experimental results and was successfully used in large-scale hybrid simulations. A series of hybrid simulations were conducted involving full-scale concrete columns instrumented to measure the bar slip deformations. The results of these tests, along with previous experimental studies, are used to develop a new model for the slip of rebar embedded in concrete. The proposed bar slip model is ...

Innovative ECC jacketing for retrofitting shear-deficient RC members

Engineered Cementitious Composite (ECC) is distinguished from conventional fiber reinforced concrete by its ductile tensile strain-hardening behavior and crack width control ability. This study investigates the performance of ECC jacketing for retrofitting shear-deficient reinforced concrete (RC) members. Six RC cantilever structural beams are prepared, and five of them are retrofitted with jackets. The experimental parameters involve the properties of the jacket, i.e., (1) ECC or mortar as the matrix, (2) presence or absence of steel meshes, and (3) welded wire or bar meshes. The performance of the various schemes of ECC jacketing is evaluated using the test results of the cantilever beams under cyclic loading. In particular, this study explores whether the appealing properties of ECC shown on the material scale can translate into better performance of the ECC jacket on the structural scale. Multiple performance measures of the beams are employed, including damage patterns, hysteretic loops, energy dissipation capacities, rebar strain profiles, shear distortions, and failure modes. The test results show that the ECC jacket without steel meshes is able to improve the cyclic behavior of the original shear-deficient beam considerably. The behavior of the retrofitted beam at the performance level of the ultimate limit state can be further enhanced by reinforcing the ECC jacket with steel meshes. In addition, the multiple performance measures suggest that the ECC jacket with a single layer of bar meshes be the best retrofitting scheme. Based on the test results, both the advantages and disadvantages of the ECC jacketing technique are reported.

The Synergistic Effects of Freeze-Thaw, Salt Attack and Bending Stress on Behavior of Reinforced Concrete

The damage experiments of reinforced concrete (RC) samples under synergistic effects of cyclic freeze-thaw, deicing-salt attack, rebar corrosion and bending stress were investigated using a comprehensive experimental method. Synergistic effects of these factors on the damage evolvement of RC were studied by measuring the change of dynamic modulus of concrete, rebar strain and concrete strain. Experimental results showed similar rule in damage evolvement but different damage rate between the normal-strength concrete (C45, the 28-day compressive strength of 52 MPa) and the high-strength concrete (C70, the 28-day compressive strength of 77 MPa). The dynamic modulus of the reinforced concrete degraded with increasing effecting factors.

Experimental Study on Serviceable Performance for Steel Reinforced Concrete Beam

Steel Reinforce Concrete Beam (SRCB) has a better flexural strength and excellent performances of ductility and seismic resistance in contrast to reinforce concrete beam.Six SRCB have been done with different compressive strength of concrete,rebar ratio,rebar cover thickness and steel cover thickness. The paper analyses the character of deflection and crack development process in following aspects: the development of maximum width and average width of crack, the law between strain of rebar and steel with the width of crack and the crack closing character. Experimental study indicates that the deflection and crack development can be devided into three stages. SRCB with reasonable rebar and H-section steel arrangement manner has excellent serviceable performance,such as deflection, resistance to crack, crack width and crack closing performance.

Experimental Study on Shear of Reinforced Concrete Beams with High Strength Rebars as Stirrups under Different Shear Span Ratios

To investigate the influence of shear span ratio for the shear behavior of reinforced concrete beam with HRBF500 high strength rebars as stirrups, an experiment was carried out, which included 8 simply supported beams with HRBF500 rebars as stirrups. Under concentrated loads, the crack, deflection, strain of rebars, bearing capacity and failure mode are observed under different shear span ratios. Some comparisons are made between test results and calculated outcome. It shows that the shear span ratio has very important influent on the shear behavior of reinforced concrete beam with HRBF500 high strength bars as stirrups. Formula in code for design of concrete structures can be used to calculate its shear capacity with enough safety.

Experimental Research on Torsional Performance of Prestressed Composite Box Beam with Partial Shear Connection

In order to research torsional performance of prestressed steel-concrete composite box beam with different shear connection, this paper designed three prestressed steel-concrete composite box beams with different degree of shear connection, and studied the stress characteristics of prestressed composite beam under static torsion process load. The torsion behaviors of prestressed composite beam such as the steel strain, concrete flange strain, rebar strain, angle of torsion, deformation and failure patterns are analyzed on the base of the experimental results. It comes to a result that shear connectors have little impact on beams cracking torque, but ultimate torque of composite beams decreases along with the reduction of shear connection degree. When the shear connection degree is 0.5, the ultimate torque is reduced by about 15%.

High strain and low cycle fatigue behaviors of rebars produced by QST and V-N microalloying technology

Based on the failure model of rebars in reinforced concrete structures (RCS) under earthquake loading, total strain controlled high strain and low cycle fatigue (HSLCF) behaviors of 400MPa grade 16mm diameter rebar produced by V-N MA (Microalloying) and QST (Quenched and Self-Tempered) were tested at room temperature. Through the fittings of Coffin–Manson and Hollomon relationship, the rates of energy-absorption under seismic loading was proposed, which was defined as cyclic toughness. The test results showed that although the static mechanical properties of two types of rebars is similar, the V-N MA rebar exhibit higher cyclic toughness, fatigue life and stress amplitude under constant strain amplitude, as well as the transition fatigue life than the QST rebar, suggesting it will have better seismic performance under seismic loading. Additionally, to understand the microstructural basis for this difference in performance, broken specimens were examined using both scanning (SEM) and transmission (TEM) electron microscopy.