Steel Strain Research Articles

The Consequence of the Involvement of Flexural, Compression, and Punching Reinforcement Upon Punching Strength

Flat slabs have an important role in concrete buildings due to their architectural flexibility and speed of construction. Punching shear is one of the most important phenomena to be considered during the design of reinforced concrete flat slabs, as this type of failure is brittle and does not predict previously raised alarms before failure. The main factors that affect punching strength in concrete are compressive strength, flexural reinforcement, and punching reinforcement in the form of stirrups, shear studs, or other shapes. This paper is part of a research program operated at the reinforced concrete laboratory of the Faculty of Engineering, Cairo University, to evaluate the contribution of horizontal flexural reinforcement, horizontal compression reinforcement, and vertical punching reinforcement on the punching strength of reinforced concrete flat slabs. In this research, fifteen half-scale specimens are cast and tested. The specimens had dimensions of 1100×1100 mm and a total thickness of 120 mm. All specimens were connected to a square column of dimensions 150×150 mm and loaded at the four corners with a supported span of 1000 mm. The main parameters considered in this research included spacing between stirrups, width of the stirrups, number of stirrup branches, ratio of the compression reinforcement, and ratio of the tension reinforcement. During testing, ultimate capacity, steel strain, cracking pattern, and deformation were recorded. The experimental results were analyzed and compared against values estimated from different international design codes. Doi: 10.28991/CEJ-2024-010-09-014 Full Text: PDF

Evolution of Confinement Stress in Axially Loaded Concrete-Filled Steel Tube Stub Columns: Study on Enhancing Urban Building Efficiency

In the context of green building and sustainable urban development, understanding the mechanical behavior of structural components like concrete-filled steel tube (CFST) columns is crucial due to their improved load-bearing capacity, energy efficiency, and optimized material usage, which enhance structural resilience and sustainability. This research addresses the complex development of confining stress and its impact on the concrete core (CC) behavior within these columns, which are essential for urban infrastructure. Through extensive numerical studies, this study proposes a model to estimate the confining stress in axially loaded CFST short columns. Study findings reveal that the confinement effectiveness is influenced by variables such as compressive strength (CS) of the concrete, cross-sectional shape, and depth-to-wall thickness percentage. Additionally, the confinement is also significantly affected by the yield strain of steel εy/εc to the peak strain of unconfined concrete εc. A three-dimensional finite element model (FEM) was built for the simulation of the columns’ nonlinear behavior and was rigorously validated against experimental data. This model aids in the design and implementation of more efficient and resilient urban structures, supporting the principles of sustainable construction. The study underscores the importance of structural integrity in sustainable urban development and provides valuable insights for improving the design of green building materials.

Test on progressive collapse of steel moment-resisting frame with upper-and-lower flange angle steel connection

Both of test and numerical simulation methods are employed to study the vertical progressive collapse and collapse assessment methods of steel frame under accidental loads. Firstly, a steel frame collapse test model with a geometric similarity ratio of 1:1.5 is designed and manufactured. The ends of column of test model is fixed. The upper and lower flange angle steel connection is used. The far end of the beam is constrained by a controllable sliding hinge support. The various stages of vertical progressive collapse of structures are simulated by a large displacement loads applied at the far end of the beam. The main focus of the testing is to examine the stress and deformation of beam and joint at various stages, particularly the development of stress and deformation in the angle steel attached to the beam-column connection. The development law of collapse load-displacement curve, joint rotation, beam strain, and angle steel strain are analyzed. The structural resistance mechanism and the transformation relationship between resistance mechanisms during collapse are studied. Subsequently, the relationship between the structural dynamic increase factor curve and the structural collapse process is analyzed using finite element method. The results show that the strain of angle steels can better reflect the destruction and deformation process of substructure during the collapse, the collapse resistance mechanism of structure, and the transition between resistance mechanisms.With the help of the characteristic points on the structural dynamic increase factor curve, the collapse limit state of structure can be reliably determined.

High-temperature low-cycle fatigue characteristics of the 12Cr10Co3MoWVNbNB turbine rotor steel

Low-cycle fatigue (LCF) experiments of the 12Cr10Co3MoWVNbNB steel at 598 °C and a strain amplitude ranging from 0.27 % to 0.55 % were executed to reveal the fatigue characteristics, fatigue fracture mechanism, and further to predict the high-temperature fatigue life of the new-style turbine rotor steel. The results indicate that the steel exhibits a high-temperature fatigue softening characteristic. During the high-temperature LCF, the cracks are generated at surface or subsurface defects of the specimens, and the fatigue striations and dimples are apparent on the fracture surfaces of the crack propagation zone and the final fracture zone, respectively. The Smith-Watson-Topper (SWT) and Manson-Coffin models are suitable to predict the high temperature fatigue life of the steel at high/low strain amplitudes, respectively. In terms of finite element analysis of the turbine set rotor, it is determined that the steady state strain of the rotor steel is approximately 0.2 %. And thus, the fatigue life of the steel rotor is estimated as 172,500 cycles at the strain amplitude, based on the Eq. (6) in this work. After 100,000 cycles at the strain amplitude, the yield and tensile strengths of the steel specimens are only reduced by 1 % and 3 %, respectively, indicating a long residual life, which is consistent with the above predicting result.

FEM simulations applied to the failure analysis of RC structure under the influence of municipal sewage pressure

The paper discusses a failure mechanism of reinforced concrete (RC) structure with steel cover that failed under the influence of municipal sewage pressure. To explain the reasons of failure, in-situ measurements, laboratory experiments and comprehensive Finite Element Method (FEM) computations were performed. Non-destructive in-situ scanning tests were carried out to determine quantity and cover thickness of embedded reinforcement bars, simultaneously, laboratory tests regarding concrete and shotcrete thickness, density and compressive strength were performed on samples prepared from core drills taken from the RC structure. FEM computations were carried out with the constitutive continuum model for concrete and steel with material parameters designated on the basis of stress–strain curves in uniaxial compression and uniaxial tension, respectively. An isotropic coupled elasto-plastic-damage formulation based on the strain equivalence hypothesis was used. In order to describe strain localization in concrete, model was enhanced in a softening regime by a characteristic length of micro-structure by means of a non-local theory. FEM analyses were carried out for different values of sewage pressure. The main attention was paid to the evolution of steel cover deformation and strain localization of the RC ceiling slab. FEM results revealed strong dependence between a bond-slip between anchors and steel cover deformation as well as between sewage pressure value and strain localization pattern of RC structure. Mechanism of the structure failure under complex loading conditions was realistically captured and its reasons were discussed in detail.

Effectiveness of Grouting and GFRP Reinforcement for Repairing Spalled Reinforced Concrete Beams

Corrosion of steel reinforcement from chloride exposure can compromise the strength of reinforced concrete structures. Rust formation expands, applying pressure on concrete, resulting in cracks and spalling. Prompt repair is crucial for severe cases of spalling. This research assessed the efficacy of repair strategies for reinforced concrete beams post-spalling, including grouting and different techniques involving Glass Fiber Reinforced Polymer (GFRP) reinforcement. The research examined four variations of reinforced concrete beams, each sized at 150 mm × 200 mm × 3300 mm. Results showed that the standard beam (BK) had an average maximum load capacity of 29.74 kN. In contrast, the grouted beam (BGR) demonstrated a reduced maximum load of 14.39 kN, along with decreased steel and concrete strain compared to BK. This suggests that the grouting repair did not fully restore the beam's flexural capacity after spalling. Incorporating GFRP strips (BGRS) led to a marginal increase in the beam's maximum load, albeit remaining below BK, with lower steel and concrete strain than BK. However, the steel and concrete approached their yield points, indicating enhanced flexural performance. The full-wrap GFRP beam (BGRSF) experienced an 8.08% increase in maximum load compared to BK, with concrete strain surpassing BK, suggesting an enhancement in flexural stiffness. Doi: 10.28991/CEJ-2024-010-07-05 Full Text: PDF

Damage parameters identification of dual-phase 800 steel based on microvoids analysis and response surface methodology

Accurately identifying the unknown parameters of the Gurson-Tvergaard-Needleman (GTN) damage model is essential to better understand the damage behavior of materials and improve the simulation accuracy. In order to solve the problem that traditional parameter identification methods are cumbersome and inefficient, this paper proposed an efficient damage parameter identification strategy with the combination of microscopic analysis, finite element simulation, and response surface analysis. The relationship between the evolution of microvoids and plastic strain in DP800 steel was quantified through the in-depth analysis of the microstructure of tensile specimens. Response surface method and Box-Behnken experimental design were adopted to efficiently identify and optimize the crucial damage parameters (fN, fC, fF) with the difference ratio of fracture strain as the response variable. Through analysis of variance, the factors that have significant influence on the damage parameters were further confirmed. Through the experimental comparison with the simulation results based on the GTN damage model, the effectiveness of the identified damage parameters was verified, and the influence of these parameters on the stress-strain curve was deeply revealed. The results show that the damage parameter identification strategy has achieved remarkable results in accuracy and efficiency, and has important engineering significance for the accurate prediction of the actual dual-phase steel sheet forming process.

Effects of carbonation modification on shear performance of recycled aggregate concrete beams: Test and mechanism analysis

The effective use of recycled aggregates (RAs) in construction significantly contributes to environmental preservation and the conservation of non-renewable natural resources. However, RAs typically exhibit inferior performance relative to natural aggregates (NAs). To improve the quality of RAs, carbonation modification has been introduced as an eco-friendly and practical method. This study explores the shear performance of concrete beams that incorporate various replacement ratios (30 %, 50 %, 70 %, and 100 %) of carbonated recycled aggregates (CRAs). The experiment employed Digital image correlation (DIC) system to evaluate the stress distribution on the concrete surface within the shear span section. The evaluation of shear performance was conducted through comprehensive analysis includes load-deflection curve, steel strain, the width of the primary diagonal crack, average principal tensile strain, orientation of principal directions, average shear strain, and shear deflection. A practical approach was adopted to separate the shear strength contributions of transverse steel (Vs) and concrete (Vc), based on transverse steel strains along the primary diagonal crack. Results indicate that carbonated recycled aggregate concrete (CRAC) beams demonstrate enhanced shear performance in comparison to recycled aggregate concrete beams. Shear strength significantly decreases with replacement rates exceeding 70 %. Furthermore, the experimental results were compared against shear strength with the design codes, and shear deflection with existing models.

Quantitative monitoring of localized pitting corrosion in steel bars through a combined use of distributed optical fiber sensors and finite element analyses

Pitting corrosion of steel bars can greatly impair mechanical properties of steel bars, so detecting pitting corrosion timely and accurately is crucial for assessing the health of structures. However, most current corrosion detection methods have some difficulties measuring pitting corrosion level of steel bars effectively. Considering that corrosion pits can cause strain redistribution of steel bar in the tensile state, this study proposes a method to quantitatively determine the pitting corrosion level through monitoring the steel strain distribution. Distributed optical fiber sensors (DOFS) based on Rayleigh scattering and capable of achieving the spatial resolution in millimeter scale were adopted to monitor the strain distributions of notched HRB400E steel bars subjected to tensile stress, and finite element (FE) method was employed to analyze the strain distributions of steel bars with different pit geometries. Experimental results showed that the bare fiber sensors (PI-FS) measured steel strain more accurately than jacketed fiber sensors. From strain distributions monitored by PI-FS, the pit location and pit length can be directly determined. Further, FE results revealed that the strain distribution along the fiber sensor position was significantly affected by the fiber sensor position relative to the pit mouth. Finally, the procedures to determine the pitting corrosion level from DOFS data and FE results were presented by mathematically comparing the similarity of strain curves given by the fiber sensor on a pitting-corroded bar and FE analyses of steel bars with different corrosion levels. In the end, the issues related to the practical applicability of the proposed method were discussed and limitations of this study were addressed. Although with limitations, this study provided a novel and promising way to quantitatively monitor the pitting corrosion level based on the strain distribution of steel bars.

Features of bond-slip relations: 3D finite element analysis based on tests of short RC ties

The interaction between concrete and reinforcement is highly complex and is of fundamental importance for predicting the behaviour of reinforced concrete (RC) structures. While the interaction of the two materials is often characterised by a bond – slip law, there are currently no such laws capable of accurately predicting serviceability characteristics, such as deflection, crack spacing, and width. This paper examines the effect of various parameters on the shape of bond – slip relation at loads before yielding of reinforcement. The study is based on a mesoscopic numerical analysis, utilizing the finite element method along with a friction cohesive zone model to explore the bond-slip phenomena in short RC ties. The numerical model is combined with an experimental campaign involving six short concrete ties reinforced with 20 mm bars. Steel strain profiles were monitored within the reinforcement using electrical strain gauges. These experimental steel strain profiles served as a robust tool for controlling and calibrating numerical models. The numerical models employed solid finite elements to simulate both reinforcement and concrete, with special zero-thickness plain elements applied to simulate the friction cohesive contact at the steel-concrete interface. Bond-slip relations were obtained from the numerically modelled reinforcement strain profiles. It was shown that concrete strains can be ignored when assessing slip. The effect of the embedded length of reinforcement under the fully confined conditions was demonstrated to have little effect on bond-slip relations. It was also shown that the maximum bond stress is strongly related to the thickness of the concrete cover. The effect of load intensity on the bond – slip behaviour was investigated. It was shown that under full confinement, the initial bond stiffness was not affected by load intensity. However, degradation of bond stiffness with increasing load due to internal cracking took place in the RC ties with a small cover/diameter ratio.

Experimental study on dynamic direct tensile performances of steel fiber-reinforced RPC at high strain rates

Reactive power concrete (RPC) exhibits excellent mechanical properties, makes it great potential for applications. RPC has gained increasing attention in academic and engineering areas and has been used in fields such as anti-explosion engineering. The insufficient tensile strength of RPC is one of the critical reasons for its structural failure. Existing studies revealed that the strain rate effect of concrete under dynamic tension is stronger than that under dynamic compression. The existing research focuses either on dynamic splitting and spalling tests, or dynamic uniaxial tensile properties with a strain rate of 0–100/s, neither of which are representative of the effect of RPC subject to explosion. In addition, the study of the dynamic uniaxial tensile constitutive model of RPC has not been reported in the existing research. To solve the existing limitations, this study conducted dynamic direct tension tests on steel fiber-reinforced RPC with a predefined strain rate higher than 100 s−1 using a Split Hopkinson Tension Bar (SHTB) apparatus. RPC dumbbell-shaped samples, including plain RPC and steel fiber-reinforced RPC (SFRPC) with 1 % and 2 % fiber volumetric fractions, were tested at high strain rates of 98–528 s−1. Based on the test results of this study and those derived from existing publications, the relationship between dynamic direct tensile properties, steel fiber fraction, and strain rate was quantitatively analyzed, and corresponding empirical formulae were proposed. To meet the demand for numerical analysis on RPC structures subjected to explosive forces, a dynamic tensile stress-strain model for steel fiber-reinforced RPC considering steel fiber, strain rate, and dynamic toughness, was established.

3D finite element performance-based study of RC interface stiffness

Although the importance of concrete-reinforcement interaction for reinforced concrete mechanics is well recognized, its intrinsic complexity needs in-depth meso-scales investigation. The contact zone between concrete and reinforcement was investigated in this study using RC ties that were short enough not to crack. The rib-scale simulation was carried out using a three-dimensional finite element technique using a simplified rib shape in the reinforcing bar. The contact between the concrete and steel was modeled using a friction cohesive zone model. An analysis was conducted to explore the impact of various contact zone parameters, suggesting numerical attributes for both normal and tangential stiffness while considering diverse rebar diameters. Concrete strain distribution and propagation of smeared splitting cracks in prisms were investigated. Additionally, a relationship between the interface's normal stiffness and rebar diameter was established through an exponential equation: Knn=2,53∙1016∙D−7,5[MN/m3]. The stiffness parameters proposed were utilized to forecast the steel strain profiles of RC ties at various load levels.

3D nonlinear finite element simulation of RC beams corroding under service loads

ABSTRACT In the present study, a 3D nonlinear finite element (FE) model has been developed to simulate reinforcement corrosion in RC beams under sustained load. The corrosion effects were modelled as bond deterioration at the rebar-concrete interface, damage in cover concrete and mechanical degradation of corroded rebars. A simplified temperature field-based methodology was proposed in the commercial program ABAQUS to simulate the evolution of corrosion degradation while a sustained load is maintained in the beam. The FE model was validated using available experimental results in terms of cracking behaviour and load-deflection response and found to be in good agreement. Parametric investigations were carried out to analyse the performance of beams under simultaneous corrosion and sustained loads. The findings demonstrated that pitting corrosion under simultaneous loading conditions can significantly compromise a beam’s structural integrity. Specifically, a scenario with 20% mass loss and 60% sustained load resulted in a 33.3% increase in concrete strain and a significant increase of 738% in the tensile steel strain leading to yielding. Furthermore, the influence of load and corrosion sequence on the behaviour of the beam was assessed by comparing its response when corroded under sustained load with a beam corroded and subsequently loaded.

Structural impact under accidental LNG release on the LNG bunkering ship: Implementation of advanced cryogenic risk analysis

The potential risk associated with the accidental release of Liquefied Natural Gas (LNG) is a notable concern during the design phase of gas-handling facilities. One of the risk is an excessive cryogenic exposure to structures or equipment, leading to hazardous situations. This study aims to introduce a novel method that implements Advanced Cryogenic Risk Analysis (ACRA) to enhance the safety of LNG bunkering ship structure in case of cryogenic flow leakage. The method involves evaluating thermal loads based on the preceding simulated LNG release scenarios using Computational Fluid Dynamics (CFD). These thermal parameters are then employed in structural analysis through the Finite Element (FE) approach.The integration of thermal data from CFD simulations into the FE model was achieved using the Random Forest algorithm. The LNG bunkering structure is represented using DH36-grade steel material. Furthermore, to accurately capture the mechanical properties of DH36 steel, cryogenic tensile tests were conducted across a temperature range from 298.15 K to 113.15 K. The resulting stress-strain data from these tests were utilized to establish a reliable material model for conducting FE analysis.This paper comprehensively presents a novel ACRA procedure by utilizing physical parameter of the gas release cosnequence such as the steel temperature profile to identify the brittle failure under the accidental gas release on the LNG bunkering ship. In addition, quantitative risk analysis (QRA) was also presented regarding the cryogenic spill hazard, to link the probability and consequence aspects of the LNG bunkering risk, particularly the results of steel temperature, stress, and strain experienced by the ship’s structure. Furthermore, the individual risk was also estimated. The implications of the findings are discussed in detail regarding the cryogenic risk, cryogenic tensile test and numerical analyses.

The axial tensile performance of DSCW-RC joints with lap-spliced bars

Joints formed between double-skin concrete shear walls (DSCWs) and reinforced concrete (RC) foundations commonly utilize lap-spliced bars to transfer forces. However, the behavior and design of these critical connections have not been thoroughly investigated. This study experimentally analyzes a steel-reinforced DSCW-to-RC foundation anchorage joint under monotonic uniaxial tension loading. Seven 1:3 scale specimens were tested to examine the effects of varying lap-spliced bar diameter, steel plate thickness, and stud bolt spacing. Crack propagation, load-displacement response, and reinforcement strain were measured. The results characterize two potential failure modes: ductile steel yielding and brittle stud shearing. Steel strains developed linearly up to yielding, demonstrating uniform transmission of tensile loads. Increased plate thickness and decreased stud spacing enhanced capacity. To mitigate sudden shear failures, the number and strength of studs must exceed the expected tensile demand. New design recommendations are proposed, including minimum stud bolt shear strength and the use of confining tie bars. This study provides an improved understanding of the behavior of DSCW-to-RC foundation lap-spliced connections, enabling resistance to axial demands while avoiding brittle failure modes. The experimental data will inform future analytical models and design provisions for these critical structural joints.

Experimental investigation of steel-reinforced precast shear wall with replaceable energy dissipators

To improve the bearing and energy dissipation of the precast concrete (PC) shear wall, this paper proposes an angle-section and C-section steel embedded PC wall with external low-yield-point steel (LYP) energy dissipators (EDs). The wall is an energy-dissipation and load-bearing bi-functional system during the seismic and a design method for this wall is also presented in this paper. Two full-size precast shear walls with and without external EDs (PWE and PW, respectively), were designed based on the proposed method and tested under an axial compression ratio of 0.2. Seismic performance in terms of failure modes, hysteretic behavior, strength envelope, stiffness degradation, energy dissipation, and steel strains was investigated and discussed. According to the crack pattern observed during the test, the PW and PWE were found to be in flexural–shear failure mode, and the PWE exhibited better seismic behavior compared with the PW, The yield and peak loads of the PWE compared with those of the PW increased by 23% and 22%, respectively. The equivalent viscous damping ratios of the two walls exceeded 0.05. When the walls were under a large displacement, the equivalent viscous damping coefficient of the PWE was higher than that of the PW. Moreover, the cumulative energy dissipation of the PWE exceeded that of the PW by 15–55%. The strain in the embedded steel in the PWE was less than that in the PW, indicating that the PW suffered more nonlinear strain and concrete damage. Test results indicate that the supplementation of replaceable external EDs can improve lateral bearing and energy dissipation capacities as well as decrease the nonlinear strain to reduce the extent of damage to wall panels. Additionally, the analytical model of PWE based on the fiber model was proposed and the rationality of the model was verified by comparison of the numerical and tested results. Furthermore, a parameter analysis was conducted to investigate the seismic behavior of PWE considering various structural properties.

Exploring key factors affecting the ultimate compression capacity of Unbonded Steel-mesh-reinforced Rubber Bearings

To address bearing-sliding damage in highway bridges during seismic events, the Unbonded Steel-mesh-reinforced Rubber Bearing (USRB) was developed. This novel bearing employs flexible, high-strength steel wire meshes as reinforcement, enhancing lateral deformation capacity and reducing bearing-sliding risk. Recent seismic specifications increased the bearings' vertical design load to 30 MPa, where existing bearings with flexible reinforcements (e.g., fiber-reinforced bearings) fail to meet this standard. Hence, for the first time, our study thoroughly investigated the ultimate compression capacity of USRBs and the key factors influencing the capacity. Through ultimate compression tests on full-scale prototypes, we demonstrated that USRBs can comply with heightened seismic requirements, with primary failure attributed to the tensile failure of the steel mesh reinforcement. These tests were complemented by 3D finite element modeling in ANSYS, employing the LINK180 elements for mesh reinforcement and defining the ultimate capacity as the point at which reinforcement stress reached tensile strength. This particular modeling approach, validated against experimental data, can accurately predict the ultimate compression capacity and the force-deformation response of USRBs. A comprehensive parametric analysis, using the validated modeling approach and an advanced statistical method LHS-PRCC, was conducted to identify the key factors influencing the compressive capacity, including bearing geometric parameters (i.e., bearing width, the second shape factor, and rubber layer thickness), steel-mesh configuration characteristics, and steel-mesh material properties. Our findings emphasize the significance of rubber layer thickness, steel mesh weight, and steel wire tensile strength and strain in influencing the USRBs' ultimate compression capacity. Based on these key factors, an empirical equation for the ultimate compression capacity of USRBs was established to guide the optimization design of USRBs. This study not only fills the gap in USRB's performance under extensive compression but also provides a foundation for future optimization of these bearings to enhance their performance in bridge engineering.

Lateral hysteretic behavior of precast concrete shear walls with unjointed vertical distribution bars and opening

Precast concrete shear walls with unjointed vertical distribution bars have desirable lateral resistance compared with cast in situ concrete shear walls and much less construction cost as no sleeve splicing joint is needed. However, the influence of opening in the precast panels on their lateral hysteretic behavior has not been fully investigated. This study presents the results of cyclic loading tests of three full size precast concrete shear walls with unjointed vertical bars and opening. The test results showed that these shear walls had similar failure modes as a cast in situ concrete shear wall, such as cracking in the corners of the opening and the concrete and steel bars crushing at the bottom corners of the shear walls. The peak load of the positive and negative loading cycles and the stiffness of the unjointed shear wall with a window opening were 14.6%, 4.3% and 1.3%, respectively, smaller and the ductility ratio was 37.6% higher, than an otherwise the same cast in situ shear wall. The lateral stiffness of the unjointed shear wall with a door opening was 35% smaller than that of the one with a window opening, due to the stiffness contribution of the wall segment underneath the window opening. The development of the longitudinal strains of concrete and steel strain along the height of the wall limbs generally met the plane section assumption. However, those next to the vertical joint interface were affected by the development length of the shear force after the bedding mortar cracked under tension. The determination of the development length needs to consider the influence of the stiffness of the link beam and the wall segment underneath the opening.

DEFLECTION AND STRAINS PERFORMANCE OF KENAF FIBRE-REINFORCED CONCRETE BEAMS

There has been a need to find a lasting means to improve beams' deflection and strain performance without necessarily increasing the beam stirrups. The limitations of steel and other synthetic fibres have reduced their application. Given its tensile strength, renewability, sustainability, and affordability compared to steel and synthetic fibres, kenaf fibre development is of great interest in Malaysia. Therefore, this research work experimentally investigated the deflection and strain performance of Kenaf Fibre Reinforced concrete beams. The volume and length of kenaf fibre used were 0.75% and 25mm, respectively. Kenaf fibre was treated with 5% NaOH to improve its performance. The alkaline-treated and untreated kenaf fibres morphology was observed under a variable-pressure scanning electron microscope. It was discovered inclusion of kenaf fibre reduced the workability of concrete. Also, plain and KFR concrete morphology was observed under a variable-pressure scanning electron microscope. Four beams were designed to study the deflection and strain performance of the KFR concrete beam; one was well reinforced but without kenaf fibre. The stirrups spacings were increased by 25%, 50%, and 75% for the other three beams. The beams were tested under centre loading. Deflection and concrete strain at the centre and along the beam span were measured using LVDTs and concrete strain gauges for the four beams. Furthermore, the strains on the shear and bottom flexural reinforcements at the centre of the beam were measured using steel strain gauges. It was discovered that kenaf fibre enhanced the deflection and strain performance of beams between 25% and 50% stirrups deficiency.

Time-dependent behavior of RC slab-column connections under high sustained loads

Reinforced concrete (RC) flat-plate slab-column connections may be exposed to high sustained stresses under gravity loads far exceeding the typical service-level stresses due to design errors, construction defects, or material deterioration. If the connections fail, then collapse of the structure could occur. Plain concrete can fail when exposed to sustained compressive stress in excess of 75% of its short-term strength. However, very little research has been conducted on the time-dependent strength and stiffness characteristics of RC slab-column connections under high sustained loads. This paper presents the experimental results of eleven aged slab-column connections under sustained concentric and eccentric gravity loading. The specimens were constructed at a 0.47-scale and had slab tensile reinforcement ratios of 0.64% and 1.0% without using shear reinforcement. Three specimens were loaded to failure in a short time, while eight specimens were tested for periods of approximately 45 days under high sustained loads (83% to 98% of their peak capacity). Test results showed that high sustained loads may lead to an eventual failure; however, the level of sustained load needs to be very close (∼95%) to the short-term capacity. Under the sustained loads, the deflection of specimens increased by 18% to 39% of the initial deflections. The deflection and steel strain creep rate approximately followed a logarithmic function, but there was a greater increase in deflection than steel strain. A sharp increase in deflection due to tertiary creep occurred only 2 to 3 min before failure, leaving little warning of the impending failure.