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Flexural behavior of concrete beams reinforced with high-strength steel bars after exposure to elevated temperatures
... Enhanced energy dissipation is essential for reducing the impact of seismic vibrations on concrete structures, contributing to greater resilience during earthquakes. In comparison, while Jun Zhao et al. (2023) successfully enhance the seismic performance of concrete walls using external MR dampers to improve energy dissipation and self-centering capacity, our study on magnetized concrete provides a more comprehensive solution. By altering the physical structure of water, our method not only enhances compressive strength and durability but also significantly reduces material usage and overall structural weight, leading to greater seismic resilience and efficiency in construction. ...
... By integrating findings on the flexural behavior of concrete beams reinforced with high-strength steel bars after thermal exposure, engineers can better design magnetized concrete structures that maintain their seismic resilience even after extreme events like fires. This ensures that the structures are not only resistant to seismic forces but also retain their integrity in post-fire scenarios, leading to safer and more reliable infrastructure in seismically active regions [4]. ...
High-Performance Concrete (HPC) offers better workability, compressive strength, and durability compared to ordinary concrete. One type of high-strength concrete is magnetic concrete. The primary objective of this empirical research is to investigate the potential of magnetized concrete in enhancing the seismic properties of concrete structures, in this technology, the physical structure of water is altered by inducing a magnetic field. The research employs empirical methods, including experimental testing and analysis, to assess the seismic performance of magnetized concrete compared to traditional concrete. As a result, the number of molecules in a molecular cluster decrease from 13 to 5 or 6, and the surface tension of the water is reduced. Using this water in concrete preparation increases the workability of the concrete mix and reduces the required water. Additionally, by facilitating cement hydration, it increases the compressive strength and durability of the concrete. This increased compressive strength enhances the stability and resistance of the structure against earthquake forces. Moreover, maintaining the compressive strength of the concrete while using magnetic water can reduce cement consumption. This reduction in cement consumption can lower the weight of the structure, which is very effective in reducing earthquake forces in tall buildings.
... Lowcycle fatigue refers to a material failure during fatigue testing owing to loading the material cyclically to strain rates large enough that cause the failure with small number of loads or strain cycles [2]. Researchers investigated the usage of HSRB in concrete beams and joints [3][4][5]. For instance, Zhao et al. investigated the bending behavior of reinforced concrete beams with HSRB after exposing the specimens to elevated temperatures and testing twelve beams [4]. ...
... Researchers investigated the usage of HSRB in concrete beams and joints [3][4][5]. For instance, Zhao et al. investigated the bending behavior of reinforced concrete beams with HSRB after exposing the specimens to elevated temperatures and testing twelve beams [4]. They examined the effects of various factors, including exposure temperatures ranging from room temperature to 1000 °C with an increment of 200 °C, exposure duration of 0 to 2 hours with an increment of 1 hour, yield strengths of (500 and 600 MPa), and the number of heated sides were changing between 2 and 3 sides. ...
This study addresses low-cycle fatigue performance of high-strength steel reinforcement bars (HSRB) when used with mechanical couplers due to the growing demand for higher-strength steel reinforcement bars in both seismic and non-seismic applications, driven by the need to reduce bar congestion, lower material quantities, and consider economic and environmental factors. Low-cycle fatigue involves material failure owing to a finite number of load or deformation cycles, generally occurring under substantial strain rates that surpasses the yielding limit. The experimental program assesses the fatigue behavior of HSRB produced using microalloying, quenching, and tempering techniques, coupled with mechanical couplers (eleven different types) from five companies in the United Stated of America. The study highlights significant differences in fatigue endurance based on the type and make of couplers and suggests potential improvements in manufacturing processes to enhance fatigue resistance. It is found that the mechanical couplers sustain a loading protocol of (-1% to 3%) when there is a clear distance of 2 times the diameter of the bar between the coupler and the gripping machine from top to bottom. The coupled bars sustained a minimum of 6 half cycles and a maximum of 38 half cycles.
... The use of high-strength and high-toughness (HSHT) steel bars has further enhanced energy absorption and crack control in reinforced concrete beams [16]. Due to the broad applicability of high-strength steel bars, not only their standard bending capacity but also their behaviour at elevated temperatures is being investigated [17]. Additionally, steel-basalt fibre composite bars (SBFCBs) have shown promise in addressing corrosion issues while maintaining ductility, indicating the potential for hybrid reinforcement [18]. ...
This paper presents a detailed analytical approach for the bending analysis of reinforced concrete beams, integrating both structural mechanics principles and Eurocode 2 provisions. The general analytical expressions derived for the curvature were applied for the transverse displacement analysis of a simply supported reinforced concrete beam under four-point loading, focusing on key limit states: the initiation of cracking, the yielding of tensile reinforcement and the compressive failure of concrete. The displacement’s results were validated through experimental testing, showing a high degree of accuracy in the elastic and crack propagation phases. Deviations in the yielding phase were attributed to the conservative material assumptions within the Eurocode 2 framework, though the analytical model remained reliable overall. To streamline the computational process for more complex structures, a simplified model utilising a non-linear rotational spring was further developed. This model effectively captures the influence of cracking with significantly reduced computational effort, making it suitable for serviceability limit state analyses in complex loading scenarios, such as seismic impacts. The results demonstrate that combining detailed analytical methods with this simplified model provides an efficient and practical solution for the analysis of reinforced concrete beams, balancing precision with computational efficiency.
... Several researchers have developed numerous methods for improving and analyzing several significant aspects of the flexural behavior of RC beams [7]. Most of these advancements focused on additives [8][9][10][11][12][13], partial or complete replacement of basic concrete materials [14][15][16][17][18], recycled materials [19][20][21], using high-strength steel bars [22][23][24][25][26], or research on the cross-section size effect on RC beams [27]. But changing the configurations of the typical shear reinforcement of RC beams, on the other hand, is a new challenge and a promising technique for improving RC beam strength [28][29][30][31][32][33][34][35][36][37][38]. ...
Many researchers have studied how modifying conventional shear reinforcement into spiral and truss systems improves the behavior of RC beams. However, there is a scarcity of studies investigating the influence of spiral reinforcement, and limited research is available on the flexural capacity of beams utilizing truss reinforcement systems. Additionally, recent designs focused only on the rectangular spiral and rectangular truss systems, underscoring the necessity of incorporating a new design of modifications in the stirrup configurations. These gaps must be addressed to identify the most effective design for achieving the desired flexural capacities. As a result, the present study conducts a simulation and experimentation on RC beams utilizing modified stirrups through the Abaqus software to describe the load-deflection relationship, determine the flexural capacity and ductility, and analyze the failure mode and crack patterns. The present study simulated seventeen finite element models, including one control beam as BN and four various designs that used rectangular spiral (BR-S), rectangular truss system (BT-R), and a new modification, namely vertical X-shaped stirrups (BV-X), and X-shaped truss system (BT-X) with four spacings of 150mm, 125mm, 100mm, and 75mm. The findings reveal that the most effective enhancement in RC beam behavior was observed within the BT-R group, particularly with BT-R 100, which demonstrated a remarkable 6.551% increase in flexural capacity compared to BN. Moreover, stirrup spacing and inclination considerably impact the beam's performance, depending on the various modifications of stirrups in RC beams. Furthermore, uniform failure modes have been observed across all models and specimens, including BN, demonstrating that modified stirrups improve RC beam performance. The present study compared and verified the finite element simulation results through an actual experiment from BN and BT-R 150 models and specimens. Doi: 10.28991/CEJ-2024-010-06-02 Full Text: PDF
... Therefore, the failure and degradation mechanisms of RC beams has been a subject of considerable interest to researchers and engineers [1][2][3][4]. Numerous studies have extensively explored the behavior of RC beams under various loading conditions [5][6][7][8][9][10][11][12][13][14]. They have studied how factors like impact velocity, reinforcement arrangement, and concrete strength influence their impact failure patterns. ...
To explore and compare the failure modes, deformation behaviors, and load-bearing capacities of single-edge notched (SEN) beams strengthened with carbon fiber-reinforced polymer (CFRP) and steel bars, static and dynamic three-point bending tests on both types of concrete beams have been carried out in this study. During the static tests, the electro-hydraulic servo machine served as a loading device to apply pressure to CFRP beams and reinforced concrete (RC) beams. During the impact experiments, different impact velocities were imparted by adjusting the drop hammer’s height. Thus, information regarding crack propagation, energy absorption, and deformation was obtained. The results from the static tests showed that the RC beams predominantly experienced shear failure. In contrast, the CFRP beams primarily exhibited bending–shear failure, attributed to the relatively weaker bond strength between the bars and the concrete. Impact tests were conducted at three different velocities in this study. As the impact velocity increased, both types of concrete beams transitioned from bending failure to bending–shear failure. At the lowest velocity, the difference in energy absorption between beams reinforced with different materials was insignificant during the bending process. However, at the highest velocity, CFRP beams absorbed less energy than RC beams. The study of structures’ impact failure modes and their mechanical characteristics offers valuable references for the anti-collision design and protection of structures.
... Scientists from Indonesia determined the shear capacity of reinforced concrete beams reinforced with steel rods or steel plates connected to the lintel [17]. Researchers from Zhengzhou University studied the flexural behavior of concrete beams reinforced with high-strength steel rods after exposure to elevated temperatures [18]. Scientists from Iran have developed a theoretical model for predicting the shear strength of reinforced concrete beams with discrete or continuous transverse reinforcement [19]. ...
The investigation pertains to the coating utilized in single-story industrial buildings. Frame constructions with spans of 24, 30, and 36 meters are examined, employing wood-based elements as rafter structures. The reinforced concrete rafter structures exhibit a pitch of 2-3 meters and are configured in the shape of an I-beam. The truss structures along their length are subdivided into seven sections, featuring variable lengths, flange widths, rib thicknesses, and cross-section heights. Deflection calculations consider the nonlinearity of concrete and reinforcement deformations, adhering to prevailing building codes. The elastic solutions method is employed in conjunction with the finite difference method. The proposed coating designs are distinguished by their ease of manufacturing, transportation, and element installation. The wood-composite rafter structure boasts a lower mass compared to reinforced concrete elements, facilitating installation with a lightweight crane and overall diminishing the coating's weight without compromising its structural integrity. Several beam characteristics for spans of 24, 30, and 36 meters include respective mid-span heights of 1.2 meters, 1.4 meters, and 1.5 meters; volumes of 8.23 cubic meters, 9.25 cubic meters, and 10.6 cubic meters; and weights of 19.8 tons, 22.2 tons, and 25.4 tons. The proposed solution allows for the integration of bending moment and stiffness diagrams for the rafter beam configuration.
... In some high-rise large-span structures, the use of low-strength steel will result in the problem of 'fat beams and columns', not only increasing the consumption of steel, but also making them difficult to tie, and more difficult to ensure the quality of concrete pouring. Vigorously promoting high-strength bars in construction projects is an important measure for energy saving and emission reduction and providing engineering structure quality [5,6], which is of great significance for promoting the structural adjustment, transformation, and upgrading of the iron and steel industry and construction. However, existing high-strength bars have poor ductility and toughness and are prone to brittle fracture, which limits their application in engineering structures. ...
Constant resistance energy (CRE) steel reinforcement has a yield strength of up to 750 MPa and an ultimate elongation of more than 20%. CRE reinforcement overcomes the contradiction between high yield strength and high uniform elongation of ordinary high-strength bars. This paper explores the flexural performance and load-carrying mechanisms of CRE-reinforced concrete beams through a series of experiments, while also presenting a theoretical analytical method for such specimens. Flexural tests on six CRE-reinforced concrete beams and two control tests on hot-rolled ribbed bar 400 (HRB400)-reinforced concrete beams were conducted in this paper. The study examines the influence of the shear–span ratio and reinforcement type on the mechanical response of the beams, including cracking load, yield load, and ultimate load, while analyzing the variation patterns of concrete strain and reinforcement strain. The experimental results demonstrate that as the shear–span ratio decreases, the crack resistance and load-carrying capacity of CRE-reinforced concrete beams improve. Under equivalent conditions, CRE-reinforced concrete beams exhibit higher load-carrying capacity compared to HRB-reinforced concrete beams, surpassing the latter by approximately 43% in terms of ultimate load. Additionally, this paper proposes a calculation method for the mechanical response of NPR-reinforced concrete beams and compares the theoretical values with the experimental values. The differences between the two are within 13%, which proves the reliability of the calculation method.
Basalt textile-reinforced concrete (BTRC) has emerged as a promising technology for strengthening fire-damaged reinforced concrete (RC) structures. This study investigates the impact of BTRC reinforcement on the flexural properties of RC beams exposed to varying temperatures (20 °C, 150 °C, 350 °C, and 550 °C) and different BTRC configurations (unreinforced, one layer, and three layers) through four-point bending tests. The results showed that elevated temperatures worsen RC beam deformation and crack initiation, while BTRC—particularly with three layers—enhances resistance to cracking. The bending damage pattern remained consistent across temperatures, but additional reinforcement layers increased susceptibility to brittle damage. BTRC improved the ultimate flexural capacity by fortifying the tensile zone, particularly at temperatures exceeding 350 °C, although with a trade-off in ductility, which is notably evident with three layers. Compared to the bearing capacity of the unreinforced test beams, the load-carrying capacity of the test beams with one layer of BTRC increased by 12.1%, 10.4%, 3.2%, and 7.7% after exposure to 20 °C, 150 °C, 350 °C, and 550 °C, respectively. In contrast, the load-carrying capacity of the test beams with three layers of BTRC increased by 18.2%, 20.2%, 26.1%, and 24.5%, respectively. These findings highlight the potential of BTRC reinforcement to enhance structural integrity after fire exposure. Based on the experimental results, it is recommended to consider incorporating BTRC reinforcement into design and repair standards for fire-damaged structures to enhance fire resistance and long-term stability, particularly under high-temperature conditions. This study provides references for the application of BTRC reinforcement in practical projects.
Hypersonic vehicles have complex structures with multiple sources of uncertainties, and the presence of uncertainties may lead to the vehicles’ structural failure. Aiming to ensure the flight safety and structural reliability of a hypersonic vehicle, this study proposes a vibration optimization control method based on non-probabilistic time-dependent reliability. Due to the complex structure of the vehicle, its forebody and aftbody are modeled as two cantilever beams with the center of mass of the vehicle as the fixed fulcrum, respectively, and a vibration control model of cantilever is derived. Considering that the uncertainties in practical engineering are not easy to be counted, the uncertain parameters are quantified by using interval variables, and the dynamic response and time-dependent reliability of the uncertain system are analyzed based on the subinterval method and the non-probabilistic time-dependent reliability analysis method (NTRAM) designed in this paper. Further, to ensure the high reliability of the vehicle, an optimal robust linear quadratic regulator (ORLQR) method with the constraint of time-dependent reliability is proposed. The analysis of the numerical examples shows that the parameter uncertainty has a certain effect on the reliability of the vehicle, and verifies that the proposed ORLQR method can effectively control the vibration and slow down the reliability decline.
Nine reinforced concrete (RC) beams were experimentally tested to assess the effect of temperatures on the flexural performance of concrete beams with hybrid bars and GFRP-steel reinforcement. The study focused on the temperature levels (25 °C, 300 °C, and 600 °C), the reinforcement ratios, and the type of reinforcement bar. The test findings revealed a significant decline in the stiffness of beams with GFRP-steel reinforcement compared to hybrid reinforcement bar (HRB) beams. The stiffness decreased by 17 % and 31 % for 300 °C and 600 °C, respectively. Moreover, all beam specimens exhibited a flexural failure mode. Non-linear finite element analysis (NLFEA) results accurately reflected the trends observed in the experimental results, and the average ratio of the experimental and NLFEA ultimate capacity was 1.02. Finally, experimental results were used to evaluate nominal flexural strength. The comparison showed that nominal flexural strength predicts flexural stress. The average experimental-nominal flexural strength ratio was 1.14.
In this study, in order to study the flexural behavior of fiber-reinforced polymer (FRP) bars with reinforced concrete beams under static loads after high-temperature exposure, seven pieces of FRP-reinforced concrete beams were subjected to static bending tests and calculation model derivations. Four-point bending tests were carried out on FRP-reinforced concrete beams after high temperature treatment. The effects of high temperature and types of FRP bars on the cracking load, crack development, deflection and ultimate capacity, and failure mode of concrete beams were investigated. The test results show that the maximum crack width, deflection, and ultimate bearing capacity of GFRP- and CFRP-reinforced concrete beams decrease obviously with a rise in high temperature. After the exposure of 400 °C for 2 h, compared with the behavior of concrete beams at room temperature, the maximum crack width of GFRP and CFRP-reinforced concrete beams increased by 42.9% and 41.7%, respectively, the deflection increases by 103.6% and 22.0%, and the ultimate bearing capacity decrease by 11.9% and 3.9%. Meanwhile, through the analysis of the existing research results and test results, the calculation models for the maximum crack width, deflection, and residual ultimate capacity of FRP-reinforced concrete beams after exposure of high temperature were proposed. For FRP-reinforced concrete beams after high-temperature exposure, the errors between the measured maximum crack width, stiffness, residual bearing capacity, and their corresponding calculated values using the model were mostly less than 10%. The calculated value using the proposed model in this research is in good agreement with the measured value. The mechanical properties of FRP-reinforced high-strength concrete structures after high-temperature exposure can be preliminarily predicted, which provides a new theoretical basis for the application of FRP-reinforced concrete structures.
An investigation has been carried out to evaluate the influence of elevated temperature on the properties of different Fiber Reinforced Concrete (FRC) of M20 grade. Concrete specimens with Aramid fiber, Basalt fiber, Carbon fiber, Glass fiber, and Polypropylene fiber were used in the investigation. FRC specimens were exposed as per ISO standard time-temperature curve. The test temperatures were 821ºC, 925ºC, and 986ºC with the exposure duration of 30 min, 60 min, and 90 min. The crack formation, morphological changes in the matrix, at the interface between the aggregate and mortar were examined by Scanning Electron Microscope (SEM). Surface characteristics and colour changes were also investigated for the heated specimens after the specimens were cooled to room temperature. SEM investigation has shown that, increase in temperature on the specimens increased the porosity. Elevated temperature on specimens induced formation of thermal cracks and further increase in temperature increased the density of cracks inside the specimen. Contribution of fibers in reducing the propagation of cracks has been confirmed by SEM investigation.
High-strength steel (HSS) and ASTM A1035 Grade 690, in particular, have been gaining popularity in the last two decades due to its considerably higher strength and corrosion resistance compared to conventional steel, i.e. ASTM A615 Grade 420. The stress-strain response of HSS is different from that of conventional Grade 420 steel because it lacks a well-defined yield point and yield plateau. Consequently, extensive research studies have been carried out to evaluate the performance of HSS reinforcement in structural concrete to examine the adequacy of current design provisions when a design yield strength of up to 690 MPa is used. In order to accommodate HSS reinforcement, the American Concrete Institute (ACI) released Design Guide for the Use of ASTM A1035/A1035M Grade 100 (690) Steel Bars for Structural Concrete (ACI 439.6R), whereas the AASHTO LRFD Bridge Design Specifications initiated the National Cooperative Highway Research Program (NCHRP) Project 12-77 to evaluate the provisions relevant to the use of high-strength reinforcing steel and other grades of reinforcing steel having no discernable yield plateau. This manuscript provides a synthesis of these two reports in a systematic way along with the other relevant research works. In addition, a comprehensive comparison between the requirements of ACI 439.6R, AASHTO, and ACI 318, relevant to the use of HSS reinforcement, is presented. Specified yield strengths for HSS, when used in different applications set by the three codes, are provided in a summary table. The provided table is intended not only for comparison purposes but also to facilitate the use of HSS reinforcement by designers because it contains all of the important and most recent relevant clauses in the three codes. Lastly, future research recommendations are proposed to revise and increase the specified yield strength of HSS in certain applications to enable designers to take full advantage of the potential benefits of HSS reinforcements.
Cement concrete is widely used as structural material in building construction where fire resistance is one of the key considerations in design. High temperature is well known for seriously damaging micro- and meso-structure of concrete, which results in a generalised mechanical decay of the concrete and even detrimental effects at the structural level, due to the concrete spalling and bar exposure to the flames, in case of fire. Concrete, at elevated temperatures undergoes significant physico-chemical changes. Fire response of the concrete structural members is dependent on the thermal, mechanical, and deformation properties of concrete. The aim of this work is to investigate the effect of elevated temperature on the mechanical properties of concrete. The mechanical properties of concrete that are of primary interest in fire resistance design are compressive strength, tensile strength, elastic modulus, and the stress-strain response in compression. To study these, cubes and cylinders of standard size have been casted and subjected to elevated temperature of different regimes from 100°C to 1000°C in an electric heating furnace and then tested. The values thus obtained have been compared with the values in Eurocode (EN 1991-1-2), ASCE (American Society of Civil Engineers) and Published literature
Many researchers have performed experimental and theoretical studies on the shear behavior of steel fiber reinforced concrete (SFRC) beams with conventional reinforcement; few studies involve the shear behavior of SFRC beams with high-strength reinforcement. In this paper, the shear test of eleven beams with high-strength reinforcement was carried out, including eight SFRC beams and three reinforced concrete (RC) beams. The load-deflection curve, concrete strain, stirrup strain, diagonal crack width, failure mode and shear bearing capacity of the beams were investigated. The test results show that steel fiber increases the stiffness, ultimate load and failure deformation of the beams, but the increase effect of steel fiber decreases with the increase of stirrup ratio. After the diagonal crack appears, steel fiber reduces the concrete strains of the diagonal section, stirrup strains and diagonal crack width. In addition, steel fiber reduces crack height and increases crack number. Finally, the experimental values of the shear capacities were compared with the values calculated by CECS38:2004 and ACI544.4R, and the equation of shear capacity in CECS38:2004 was modified to effectively predict the shear capacities of SFRC beams with high-strength reinforcement.
This paper presents the effect of temperature on the mechanical properties of high-strength alloy structural Q460 steel. The strength and stiffness properties of steel degrade with temperature and this deterioration has to be properly accounted for in the fire resistant design of steel structures. The strength and stiffness degradation is also influenced by the composition of steel. Because of a lack of data specific to high-strength Q460 steel, design standards assume the high temperature strength variation of Q460 steel to be same as that of conventional mild steel. To overcome this drawback, strength and stiffness properties of Q460 steel were measured at various temperatures in the range of 20-800 degrees C. A relative comparison of measured data indicates that high-strength steel experiences a slower loss of strength and stiffness with temperature than conventional steel. Data generated from the experiments, namely, load-displacement relationships and vibration frequency, were utilized to develop relations for yield strength, tensile strength, and elastic modulus of Q460 steel as a function of temperature. The proposed relations developed specifically for high-strength Q460 steel can lead to better fire resistance prediction of steel structure systems. DOI: 10.1061/(ASCE)MT.1943-5533.0000600. (C) 2013 American Society of Civil Engineers.
This paper discusses the behaviour of normal and high strength concrete beams exposed to elevated temperature. The paper also provides a simple, economical, and reliable technique that can be used to assess the fire resistance of concrete beams exposed to multi-action of external loads and elevated temperature simultaneously. The new technique was verified by testing (36) normal and high strength concrete beams made of concrete mixes of different w/c and s/c ratios. The tested beams comprised simply supported and restrained beams, anti-fire coated beams, and beams cooled by different methods. The results showed that the presented technique is effective and succeeded to distinguish among the different RC beams. The results also indicated that testing RC beams subjected to flexural load and then exposed to elevated temperature provided better understanding to the structural behaviour of the flexural elements in the field. The results also showed that the restrained RC beams exhibited better fire resistance when compared to the simply supported RC beams. Using of anti-fire resisting coating improves the fire resistance of RC beams made of high strength concretes. The results also emphasised on involving the influence of the cooling methods to evaluate the actual behaviour of the flexural elements. KEYWORDS NSC (Normal Strength Concrete), HSC (High Strength Concrete), Flexural Load, Silica Fume (S/C), Anti Fire coating, Temperature Development, Techniques of Cooling.
This research investigates the influence of using the recycled rice straw ash powder on performance of UHPCC exposed to axial compressive load and elevated temperature (600 oC) simultaneously. A total of 16 UHPCC of different recycled rice straw ash powder ratios was used. The tested columns included loaded, unloaded columns, and columns cooled by various methods. Using the recycled rice straw ash as concrete filler enhanced the concrete elevated temperature resistance. Results indicated that testing UHPCC at elevated temperature while exposing them to axial compression offered well understanding for the structural behavior of axially loaded columns. Rice straw powder improves the fire resistance of UHPCC. FEA using ANSYS Software version 2019-R1 was achieved to validate experimental study. FE numerical models were analysed by considering the acquired findings to the experimental testing results in a respectable agreement manner.
This experimental study investigated the compressive behavior of lightweight concrete after exposure to elevated temperatures. In total, 240 samples from 30 different mixtures were prepared and tested to evaluate the compressive strength, elastic modulus, strain at peak stress, and stress–strain relationships of lightweight aggregate concrete (LWAC) after being exposed to elevated temperature of 250, 500, and 750 °C. Test variables were composed of cement content varied between 300 and 700 kg/m³, volumetric percentage of lightweight expanded clay aggregates (Leca) substituting natural sand and gravel at 0, 25, 50, 75, and 100% (by weight), silica fume replacing cement at 5, 7.5, 10, 12.5, and 15% (by weight) and different water to cement ratios of 0.250, 0.313, 0.375, 0.438, and 0.500. The compressive strength, elastic modulus, and strain in the maximum stress were compared with predictions from the American and European standards and the proposed analytical models. The results showed that the compressive strength and elastic modulus of LWAC declined with increasing the temperature. The samples S6, S4 and S23 performed better compared to other specimens at elevated temperature as they retained about 96%, 75% and 42% of compressive strength, respectively after exposure to 250, 500 and 750 °C. A higher residual compressive strength was observed in the sample including 75% Leca at 750 °C. The experimental stress–strain relationship was verified according to the available analytical models and an analytical model was proposed to estimate the compressive behavior of LWAC at elevated temperature.
This study deals with the behavior of reinforced concrete beams exposed to elevated temperatures having different pre-existing damage levels. Three-point flexural tests were carried on 15 beam specimens, having three levels of pre-existing damages and exposed to room temperature and elevated temperatures of 200 °C, 400 °C, 600 °C, and 800 °C, to determine the sensitivity of response parameters to these variables. The variation of crack pattern and concrete microstructure, crack width, load–deflection curves were examined in the studied specimens. The results showed that both the elevated temperatures and pre-existing damages had a significant effect on the crack initiation of concrete and the maximum crack width. It was observed that the initial stiffness of beam specimens, with the maximum reduction of 58%, was reduced by increasing the exposed temperate with approximately the same reduction rate of compressive strength of concrete. The residual flexural capacity of beam specimens was also sensitive to elevated temperatures, showing up to 30% reduction after exposure to higher temperatures. The effect of the pre-existing cracks was more significant in the ductility ratio of beam specimens and with also a higher reduction rate of ductility with respect to temperature compared to other response parameters. The reduction in ductility of specimens with no and severe pre-existing damages exposed to 800 °C was 55% and 77%, respectively.
Reinforced concrete structures located in coastal areas face durability related concerns such as corrosion of reinforcing steel due to aggressive and humid environment. An accidental fire in such a scenario can lead to a severe loss of mechanical properties and load-carrying capacity. This paper examines the load-deflection response of short span RC beams on account of exposure to combinations of corrosion-induced damage and exposure to elevated temperatures. The idea pertinent to this study is to simulate a fire in an old or aged structure located in a humid environment. The study involved flexure testing on normal strength reinforced concrete beams reinforced with SD TMT (Super Ductile thermo-mechanically treated) bars. The RC beam specimens were exposed to impressed current to achieve desired corrosion levels in terms of mass loss of 4% & 8% and subsequently to elevated temperatures (400 °C and 600 °C) under steady-state. The beams were tested in flexure, and the load-deflection response was observed. The results indicate a notable reduction in load-deflection behavior due to the two non-linear phenomena’ superimposition. It can be concluded that the exposure to combinations mentioned above of corrosion-induced damage and elevated temperatures caused significant loss of bond mechanism and load-deflection response of RC beams. The results obtained through testing have been validated analytically with sectional analysis using residual stress-strain models in the literature for concrete and steel exposed to corrosion and elevated temperatures. The results were in good agreement with analytical data for various cases considered in the study.
To investigate the seismic behavior of shear walls with high-strength materials, cyclical quasi-static tests on five half-scale shear walls with an aspect ratio of 1.5 were carried out, including four conventional reinforced concrete walls and one composite wall. The steel fiber-reinforced concrete shear wall with HRB 600 MPa reinforcement located completely in the boundary element and in the wall web was taken as the reference specimen. The presence of steel fibers, the strength of wall web rebar, and the presence of high-strength reinforcement diagonal bracing (X configuration) were selected as major test parameters for the other three conventional walls. The results reveal that the high-strength concrete (HSC) shear wall with high-strength steel rebar (HSSR) showed stable hysteresis performance, and all specimens had acceptable crack widths and residual deformations before a lateral drift of 1%, except for the nonfiber-reinforced concrete specimen. The addition of steel fibers can reduce the crack width, limit the shear crack development, and increase the flexibility deformation, thus improving the deformation capacity of the specimens. Using HRB 600 MPa reinforcement instead of HRB 400 MPa reinforcement as wall web rebar can significantly reduce the residual deformation. The specimen with high-strength reinforcement diagonal bracing in the wall web and the composite shear wall with a built-in steel truss not only had desirable repairability after a large earthquake but also showed much higher energy consumption capacity under a strong earthquake. Finally, based on the experimental and theoretical analysis, a simple method for calculating the lateral strength was established, and the predicted results showed good agreement with the test data.
This paper proposes an innovative and practical strengthening method for fire-damaged RC joints using steel haunches system. A total of seven specimens consisting of beam-column sub-assemblages were designed including one control specimen without fire damage, one control fire-damaged specimen, and five fire-damaged specimens retrofitted with steel haunches system. The specimens were first exposed to the ISO834 temperature curve in a furnace chamber. Then, two sizes of steel haunches were used to strengthen the joint panel zone, and bolted side plates (BSP) or carbon fiber reinforced polymers (CFRP) were used to repair the beam. Finally, the control and retrofitted specimens were subjected to low-cycle reversed loading until failure. The effects of fire damage and strengthening method on the hysteresis behavior, bearing capacity, stiffness degradation, ductility factor, and energy dissipation capacity of the joints are reported. Results for the control specimens showed that the failure mode changed from beam-end plastic hinge failure at room temperature to shear failure in the joint zone after exposure to fire, accompanied with a reduction in load bearing capacity. Yet, the steel haunches retrofitting relocated the plastic hinge to the beam-end zone. The combination of steel haunches and BSP significantly enhanced the seismic performance in terms of bearing capacity, stiffness, ductility, and energy dissipation. Locating the BSP close to the beam bottom improved the positive load capacity of the upgraded joints. The specimen strengthened with the combination of steel haunches and CFRP also exhibited acceptable seismic performance. The proposed repair systems allow retrofitting fire-damaged existing concrete joints to meet current seismic design requirements.
The paper presents a series of tests conducted to study the post-fire (residual) performance of the reinforced concrete beams with critical (under-designed) lap splices. Furthermore, a 3D numerical modelling approach is validated against the experimental results. Two types of beams namely, Type 1 - with continuous reinforcement, and Type 2 – with critical lap splice were investigated. Three specimens of each beam type were tested: (i) in ambient conditions (reference tests), (ii) in residual state after exposure to standard ISO 834-1 fire of 60 minutes, and (iii) in residual state after exposure to standard ISO 834-1 fire of 90 minutes. Under ambient conditions, the beam with continuous reinforcement failed due to yielding of tension reinforcement and resulted in a ductile load-displacement curve. On the other hand, the beam with critical lap splice failed due to bond splitting and resulted in a brittle load-displacement behaviour. After exposure to fire, beams with continuous reinforcement showed a nominal effect of fire on its residual behaviour, while the beams with critical lap splice displayed a drastic reduction in the capacity and a poor performance after exposure to fire. It is shown that the chosen numerical approach realistically reproduces the experiments.
In recent years several studies analysed the influence of recycled aggregates obtained from concrete debris (Recycled Concrete Aggregates, RCAs) on the durability performance of structural concrete produced with them (Recycled Aggregate Concretes, RACs) and, within this, also the influence of high temperatures has been investigated. In most cases, the concrete strength has not been considered as a key control parameter for the comparison between RACs and ordinary concrete performance. In this context, starting from a fundamental characterization of the raw materials under high temperatures, this paper presents a comprehensive experimental study on normal- and high-strength RACs mixtures designed with the Compressible Packing Model, with the aim of investigating their residual mechanical and physical performance when subjected to high temperatures (up to 650 °C). This approach allows a generalized analysis by considering the overall amount of attached mortar (AM) that represents the key parameter characterizing RCAs. In fact, since one of the main issues related to the possible use of RCAs in concrete is their significant unknown heterogeneity depending on their original source, the identification of a fundamental parameter such as the AM, allows to extend the conclusion obtained herein for RAC mixtures produced with RCAs derived from different origins.
This study puts forward experimental and numerical investigations to understand the response and failure of hybrid concrete beams reinforced by Glass Fibre Reinforced Polymer (GFRP) bars and exposed to elevated temperature. Seven GFRP reinforced hybrid concrete beam specimens were designed according to ACI440.1R-15
(ACI, 2015) and exposed to temperature values between 300 °C and 700 °C and subjected to monolithically
increased static load up to failure. The effects of elevated temperature on load- displacement relationships and
failure modes were evaluated and discussed. Experimental results have indicated that shear failure is the predominate failure mode of all tested beams. Results have also shown that the maximum reduction in the ultimate
load capacity of the hybrid concrete GFRP reinforced concrete beam is about 53% when exposed to temperature
value of 700 °C compared with the ultimate load capacity of the beam at ambient temperature.
This paper also suggests and validates a numerical model to simulate the performance and failure of GFRP
reinforced hybrid concrete beams under high temperature using the finite element software ABAQUS. The numerical model was firstly verified against the experimental test results and then used to investigate the effect of several important parameters on the performance and ultimate load of GFRP-RC beams under different elevated temperatures.
Reinforced concrete (RC) structural members generally exhibit a fairly good fire resistance due to their low thermal conductivity and high thermal capacity. However, under prolonged duration of exposure, RC members experience loss of strength and stiffness. An experimental investigation is carried out to examine the influencing factors affecting the structural performance of the RC beams of different strength grades exposed to standard fire. Specimens were heated as per standard fire curve. Different grades of RC beams (i.e. M20, M30, M40 and M50) are tested under two-point loading. The effect of standard fire on the load-deflection response, first crack load, ultimate load, temperature in rebar, yield strength of rebar and moment of resistance are investigated in the RC beams. Temperature in steel at specified locations of a RC beam is also measured to determine the extent of damage. It is observed from the investigation that the mode of failure is dependent on many material parameters (w/c ratio, density, porosity) and structural factors (compressive strength of concrete, yield strength of rebar). Damage level of concrete with lower grade was higher up to 120min duration of heating and after that loss in strength of concrete with higher grade is significant. Water-to-cement ratio, compressive strength, temperature level in concrete or steel and rebar area are some of the key factors affecting the loss in strength of RC beams at elevated temperature.
To investigate the influence of longitudinal reinforcement strength grade, stirrup spacing, concrete strength and axial load ratio (ALR) on seismic behavior of high-strength concrete (HSC) columns, seven full-scale square HSC columns reinforced with high-strength steel bars (HSSB) with nominal yield strength of 600 MPa or conventional steel bars were tested under constant axial load and cyclic lateral load. Furthermore, corresponding nonlinear finite element analysis was conducted in ABAQUS. The column top deformation and lateral load at different service states, stiffness degradation, bearing capacity degradation and strain of longitudinal and transverse reinforcement were discussed. The test result shows that using steel bars with nominal yield strength of 400 MPa as stirrup is a better choice for HSC columns and ALR is an important factor which determines whether the high-strength longitudinal reinforcement can reach its compressive or tensile yield strain before maximum point. Taking HSSB as longitudinal reinforcement (equivalent volume replacement) greatly improves the seismic behavior of HSC columns. After taking the influence of strain gradient on stress-strain relationship of compressive concrete into consideration, a confined strain gradient model (CSGM) which was suitable for finite element analysis (FEA) was established based on the model established by Razvi and Saatcioglu. Specific verification of finite element model (FEM) on deformation and lateral load at different service states were presented for further parameter analysis. The trend of displacement ductility factor were investigated in detail in numerical analysis part. With proper ALR and stirrup characteristic value, HSC columns reinforced with HSSB is able to present a displacement ductility of higher than 4, which is capable to meet the ductility demand of most codes.
The results of fire tests obtained from small-sized specimens may fail to represent the real post fire behavior of structural elements. Therefore, in this study structural concrete beams were exposed to ISO fire curve and then subjected to different curing conditions to better understand the changes in concrete during the post-fire stage. Beams were tested under four-point bending to evaluate their residual mechanical performance, and scanning electron microscopy (SEM) was performed in order to monitor the changes in the morphology of concrete due to fire exposure. The results showed that high temperature exposure caused deterioration in the morphology and reduction in residual mechanical properties of concrete, while post-fire re-curing caused an improvement in residual mechanical properties and recovery in the microstructure.
Purpose
This paper aims to explain the influence of Standard Fire as per ISO 834 on the strength and microstructure properties of concrete specimens with different strength grade.
Design/methodology/approach
The strength grades of concrete considered for the experimental investigation were Fck20, Fck30, Fck40 and Fck50. The specimens were heated up to 1, 2, 3 and 4 h as per standard fire curve. Effect of elevated temperature on compressive and flexural behavior of specimens with various strength grades was examined. Effects of age of concrete, weight loss, surface characteristics and thermal crack pattern were also investigated.
Findings
Experimental investigation shows that strength grade, duration of exposure and age of concrete are the key parameters affecting the residual strength of concrete. For the beams exposed to 3 and 4 h of heating, the residual flexural strength was found to be so insignificant that the specimens were not able to even sustain their own weight. The loss in compressive and flexural strength of Fck50 concrete specimens heated up to 1 h were found to be 26.41 and 86.03 per cent of the original unheated concrete, respectively. The weight loss was found to be more for higher grade concrete specimens, and it was about 8.38 per cent for Fck50 concrete. Regression analysis was carried out to establish the empirical relation between residual strength and grade of concrete. Scanning electron microscopy and thermogravimetric analysis were carried out to examine the damage level of fire-affected concrete specimens.
Originality/value
Empirical relationship was developed to determine the residual strength of concrete exposed to elevate temperature, and this will be useful for design applications. This database may be useful for identifying member strength of reinforced beams subjected to various durations of heating so that suitable repair technique can be adopted from the available database. It will be useful to identify the proper grade of concrete with regard to fire endurance, in the case of concrete under compression or flexure.
h i g h l i g h t s Test and prediction of flexural performance for reinforced SFRELC beam was conducted. Steel fibers enhance beams' cracking resistance, stiffness, capacity and ductility. Steel fibers altered the failure pattern of beams from brittle to plastic in flexure. Flexural ductility of test beams is evaluated using the flexural ductility index. a b s t r a c t To promote the structural application of a new high-performance steel fiber-reinforced expanded-shale lightweight concrete (SFRELC), the experimental study on flexural behaviors of reinforced SFRELC beams was conducted in this paper. Compared to two reinforced beams without steel fiber, eight reinforced SFRELC beams with steel fiber in volume fraction ranged from 0.4% to 1.6% were fabricated and tested under two-point concentrated loads. Results showed that the cracking moment, the flexural stiffness, the flexural capacity and the flexural ductility of reinforced SFRELC beams were effectively promoted due to the addition of steel fibers. The crack growth was restrained with decreased space and width. Concrete strains at normal-section of mid-span fitted the assumption of plain cross-section in flexure. Based on the experimental and theoretical researches, methods for predicting cracking moment, crack width, flexural stiffness and flexural capacity of reinforced SFRELC beam were proposed.
The mechanical properties of concrete and steel degrade after exposure to fire, thus the fire-damaged reinforced concrete (RC) members may not be able to satisfy the prescribed performance. Therefore, it is of great significance to identify the degree of fire damage for RC members or even the entire RC structure. For RC beams, comprehensive studies have been conducted on the residual flexural capacity after fire, but those related to the post-fire shear properties are barely found in the literature. In this paper, the shear tests of twenty-five beams––including frame beams and cantilevers––were carried out to investigate the difference in their shear behaviors before and after fire exposure. The parameters controlling shear capacity, such as shear span ratio, the concrete slab (i.e. the flange of the beam), and the stirrup spacing, were investigated. The experimental results indicated that both the shear capacity and the stiffness of the RC beams degraded after fire. The diagonal cracks of frame beams with fewer stirrups concentrated seriously, and their crack width increased considerably. Based on the test results, a simplified evaluation process was developed to evaluate the shear capacity of RC beams after fire, and the proposed formula was proven to be able to predict the residual shear capacity of fire-damaged RC beams with sufficient accuracy.
The thermal deformation of a reinforced concrete (RC) beam in case of fire, when prevented by the surrounding structure, induce axial forces and/or bending moments in the structural element changing its resistance. A few experimental studies on this subject address the analysis of simply supported beams, and, in some cases, when thermal restraint is imposed, the level of applied stiffness is not clearly quantified. Trying to fill this gap of knowledge, this paper reports a series of fire resistance tests on axially and/or rotationally restrained RC beams under flexural bending. The main objectives of this research were to verify the failure modes and fire resistance of the RC beams comparing the behavior of restrained with those unrestrained. Other important aspects analyzed were the evolution of temperatures in the cross section, the magnitude of the restraining forces and the vertical displacements developed. The results showed that the performance of RC beams can significantly be affected by the thermal restraint, since in some cases an increase of up to 100% in the fire resistance was registered. Keywords: RC beam. Fire. Thermal restraining. Load bearing capacity.
Engineered Cementitious Composites (ECC) considers a type of ultra-ductile cementitious composites with fiber reinforcement. It is developed for applications for economic purpose in the construction industry. ECC characterizes by strain hardening and multiple cracking. This paper experimentally investigates the performance of ECC concrete beams reinforced with conventional reinforcement bars. Advanced Polyvinyl Alcohol Engineered Cementitious Composite (PVA-ECC) fibers were selected in this purpose. Twelve RC beams were poured and tested to study flexure behavior under four-point loading test. Two different longitudinal reinforcement percentages, variable volume ratios of (PVA) and polypropylene fibers (PP) were used. optimizing the usage of PVA material trails to put it in the lower layer of the section at point of maximum tension with variable thicknesses was conducted. Initial flexure cracking load, ultimate load, the ductility and the load-to-deflection relationship at various stages of loading were evaluated. Experimental outcomes revealed that the enhancement in maximum capacity is more significant in the case of using PVA rather than PP. The maximum load increases by 20% and 34% for 1.0% and 2.0% of PVA contents in total section respectively. The relative ductility factor increases by 30% and 45% for 1.0% and 2.0% of PVA content. Results also depicted that a reasonable considerable increasing in the load capacity when used limited layer thickness of PVA concrete. Nonlinear Finite Element Analysis (NLFEA) was conducted for the purpose of simulating the behavior of experimentally tested beams, regarding crack behavior and load-deflection response. Reasonable agreement was achieved between the experimental results and NLFEA results.
Lightweight self-compacting concrete (LWSCC) is an advance concrete which combines the advantages of both lightweight concrete (LWC) and self-compacting concrete (SCC). This concrete provides an excellent solution in decreasing the self-weight of the structure while easing the pouring and removing the construction challenges and complications. This study focused on studying the impact of elevated temperatures on normal-strength lightweight self-compacting concrete (NSLWSCC) and high-strength lightweight self-compacting concrete (HSLWSCC) through its residual properties vis-à-vis compressive and tensile strengths, modulus of elasticity, mass loss and spalling intensity. LWSCC were designed using lightweight aggregate (LWA) which replaces coarse and fine aggregates at certain percentages. Three types of LWA aggregate used in this study are Scoria, Perlite and Polystyrene. Mixes consist of six NSLWSCC (50% & 100% Scoria, 50% & 100% Perlite, 20% & 30% Polystyrene) and two HSLWSCC (50% Scoria). The residual properties were measured by heating the 100 × 200 mm cylinder specimens to 100, 300, 600 and 900°C. The result shows that the NSLWSCC tend to achieve maximum strength at 100°C and then gradually decreases as the temperature increases. But in the case of HSLWSCC, maximum strength was achieved at 300°C. Minor spalling with bubbles, holes and cracking was observed at only 900°C in NSLWSCC while major explosion occurred at 300°C in HSLWSCC. The overall result indicates that magnitude of loss of strength, mass loss and intensity of spalling is proportional to temperature after certain point. This study shows how the strength and thermal stability of the LWSCC made from Scoria, Perlite and Polystyrene changes after exposing it to high temperatures.
In order to study the seismic behavior of reactive powder concrete beam-column joints reinforced with high-strength bars, an experimental investigation was carried out. A total of 5 reinforced reactive powder concrete exterior joint specimens (including 2 specimens with HRB600 steel bars and 3 specimens with HRB400 steel bars) were cast and tested. The seismic behavior of 5 existing test specimens was studied, including failure mode, hysteretic behavior, deformation capacity, ductility, energy dissipation capacity, and strength and stiffness degradation. The results showed that failure modes of reactive powder concrete beam-column joints under reversed cyclic loading are mainly flexural failure of the beam adjacent to the joint core, shear failure of the joint core, or combined failure of the plastic hinge in the beam and shear in the joint core. The configuration of HRB600 bars in reactive powder concrete beam-column joints alleviates the destruction, reduces the degradation of strength and stiffness, lessens residual deformation, and enhances both deformation capacity and energy dissipation capacity. Stirrups in the joint core directly bear part of the shear forces and provide confinement to the reactive powder concrete. Therefore, the utilization of stirrups in joints can retard the development of diagonal cracks, slow the degradation of strength and stiffness, and enhance the bearing capacity and energy dissipation capacity of the joint.
In this paper, the comchors two series of tests with a total number of 84 specimens, investigated the regularity of concrete's compressive strength and tensile strength at elevated temperature(from 20 °C-1000 °C). and set up a concise mathematical expression.
This paper conducts an experimental study on the mechanical properties of the fine grain reinforced concrete beam with HRBF500 at normal temperatures and after fire. The experiments of five fine grain reinforced concrete beams under fire with different time were accomplished. The static loading test was done when the beams were cooled to room temperature naturally. Its macroscopical phenomenon and the static failure modes were investigated through the tests. Hence, the variation of the bearing capacity of the fine grain reinforced concrete beams by the fire was obtained under different conditions. The tests of two fine grain reinforced concrete beams under loads without fire were carried out. The load-deflection curves were obtained and compared with the results after fire. The results show that the concrete beams can be seriously damaged by fire. The effects of the time of fire, bar ratio and preloading on the fire resistance are revealed. Some conclusions are drawn: The longer it is exposed to fire, the weaker its residual bearing capacity is. The smaller the bar ratio is, the weaker its residual bearing capacity is; A preloading will reduce the residual bearing capacity of the fine grain reinforced concrete beam.
In experimental investigation on mechanical properties of Q235 steel after high temperature treatment, surface characteristic and influences of the important parameters including heating temperature and temperature exposure time on the mechanical properties of steel after high temperature treatment were studied. Formulas for Possion's ratio-stress ratio, tensile stress-strain, yield strength-heating temperature, tensile strength-heating temperature of steel after high temperature treatment are proposed. The result show that: with the heating temperature rising, yield strength and tensile strength tend to decrease as a whole, while elastic modulus of the steel and Possion's ratio almost remain unchanged; with increasing temperature exposure time, the mechanical properties of the steel after high temperature treatment are hardly influenced; the calculated results by above-mentioned formulas agree well with test ones.
This paper presents an approach for assessing the residual capacity of fire exposed reinforced concrete (RC) beams. The approach involves capturing response of RC beams in three stages, namely, structural response at ambient conditions, thermo-mechanical response during fire exposure, and post-fire residual response after cooling down of beams. Distinct material properties of reinforcing steel and concrete are considered during heating and cooling phase of fire exposure and residual (after cool down) phase of analysis. In addition, relevant load level, specific fire scenarios, boundary conditions, and plastic deformations that develop in a beam during fire exposure are also incorporated in evaluating residual response of fire exposed RC beams. The proposed approach is implemented using a detailed numerical model developed in the finite element computer program ABAQUS. Predictions from the numerical model show good correlation with the response parameters measured in experiments for evaluating residual capacity of fire exposed RC beams. Also, predictions of residual capacity from the finite element analysis are compared with that obtained from simplified sectional analysis based on maximum rebar temperatures consideration. This comparison indicates that the finite element analysis yields more realistic predictions of residual capacity than that predicted from simplified sectional analysis. The applicability of the proposed approach in evaluating residual capacity of fire exposed RC beams is illustrated through a case study.
Based on the static load tests of seven reinforced concrete (RC) beams after fire and three contrastive specimens at ambient temperature, the effects of fire exposure time, shear span ratio, reinforcement ratio and flange on the residual shear behavior and the residual flexural behavior of beams were analyzed. Using a finite element program, the effects of ISO 834 standard fire with rising and cooling phases on the peak temperature in the RC section were analyzed and a practical calculation method for the shear strength of RC beams after fire was suggested. The results show that the fire can decrease the bending capacity and shear capacity, and increase the deflection of RC beam on the whole. The flange has a positive influence on the improvement of residual flexural capacity but has little effect on the shear capacity of RC beam after fire. The cooling phase of fire has significant effects on the peak temperature in the section, and whether it is considered can lead to a peak temperature with the biggest difference of 283°C and 251°C in a section of 250mm width and 400 mm height after exposure to ISO 834 standard fire for 1h and 2h, respectively. The proposed formula for the residual shear capacity of RC beams after fire has certain safety margin ratio, and can be used to assess the shear performance of structural member after fire.
This paper presents experimental and numerical results of performance and fire endurance of high strength steel reinforced concrete (RC) continuous T-beams under the standard ISO 834 condition. The fire tests included six specimens using steel reinforcement of 500 MPa yield strength with different load levels and reinforcement ratios. In all cases, the failure mode was flexural failure due to the formation of a plastic hinge mechanism. However the sequence of plastic hinge appearance was different from that at ambient temperature due to the increased support bending moments in fire as a result of restrained thermal bowing. Formation of a plastic hinge mechanism indicates sufficient reinforcement ductility to enable complete redistribution of bending moments and absence of any buckling failure of the T-beam stem in the compression region at very high temperatures. The load ratio was the most critical design parameter, with the fire resistance times of the specimens being146 min, 93 min and 64 min for load ratios of 0.3, 0.5 and 0.7 respectively. For the T-section, the temperature field of the flange plate was similar to that of a one-side fire exposed RC slab, and that of the web similar to that of a three-side fire exposed RC rectangular beam. Further numerical simulation results using steel reinforcement of 690 MPa yield strength confirm that continuous, high strength steel reinforced concrete T-beams can be designed using plastic analysis.
High-strength steel reinforcement in buildings had previously been limited to specialized applications, but recently published design guidance allows expanded use. A case study, conducted to investigate productivity benefits of using 690mpa (100ksi) versus 414mpa (60ksi) reinforcement, found that there was little to no benefit in using 690mpa (100ksi) steel in slabs, post-tensioned girders, and columns, but the beam reinforcement weight was reduced by 36%. The 2010 material cost ratio of 690mpa (100ksi) to 414mpa (60ksi) reinforcement was two, outstripping the weight reduction. However, labor cost is a function of weight, bringing the overall cost of 690mpa (100ksi) reinforcement to within 35% of 414mpa (60ksi) reinforcement cost. The material cost ratio will presumably decrease over time; if it drops by 30% or more, 690mpa (100ksi) reinforcement will be more economical. Labor costs, which vary by location, strongly influence the productivity benefits of 690mpa (100ksi) reinforcement. The use of 690mpa (100ksi) reinforcement is more favorable in expensive labor markets and it appears to be currently competitive in some. The paper's primary contribution to the overall body of knowledge is the quantitative understanding of the economic factors that influence the ability of 690mpa (100ksi) steel reinforcement to have a productivity advantage over the use of conventional 414mpa (60ksi) steel reinforcement. Practitioners, designers, and researchers can use this information to understand the cost and productivity impact of high-strength reinforcing steel.
The deterioration of mechanical properties is predominant in the design of steel structures under fire condition, as it leads to associated reduction of load bearing capacity. To understand elevated temperature dependence of mechanical properties of high strength steel S690, an experimental investigation has been carried out, using both steady state and transient state methods at temperatures ranged from 20 °C to 700 °C. Comparison of results with European, American, Australian and British design standards shows no current standard can safely guide fire-resistance design of steel structures with S690. Therefore the mechanical properties of S690 at elevated temperatures obtained herein are of value.
For evaluating fire resistance of a reinforced concrete member, temperature profile in the cross section of the member is required. Current simplified approaches and design graphs do not yield reliable temperature predictions in rebar and concrete. In this paper, a simplified approach is proposed for evaluating cross-sectional temperatures in fire-exposed reinforced concrete members. The approach is derived through statistical nonlinear regression analysis, utilizing data generated from finite element analysis. The parameters that were varied in the finite element analysis include sectional geometry, concrete characteristics and fire exposure conditions. The validity of the approach for different types of concrete is established by comparing predictions from the proposed equation with data from fire tests and finite element analysis. Through these comparisons it is shown that the proposed equation gives better predictions of temperatures in reinforced concrete members. The applicability of the proposed approach in design situations is illustrated though a numerical example. The simplicity of the proposed method makes it attractive for use in design situations and for incorporation in design standards.
Idealised data have been derived for the compressive strength of a number of concretes for fire safety design. The data are derived from the author own research including test series not published before and from more than 400 other test series comprising approximately 3000 specimens known from literature and personal contact. The data cover a variety of concretes with aggregates such as siliceous materials, limestone, granite, sea gravel, pumice, and expanded clay to fire-resistant concrete based on chamotte or Danish mo-clay. Processes are briefly described that are responsible for the deterioration of the materials when they are heated and when they are cooled down after a fire. In addition, it is explained how variations of the concrete composition influences the deterioration. Necessary characteristics of design methods are derived for variable fire courses based on analyses of consequences of material deterioration. The paper serves as a basis for a number of papers presenting calculation methods developed by the author for load-bearing capacity of any structure of any concrete at any time of any fire exposure. Some of these methods and related materials data are adopted in the CEN: ENV 1992-1-2 code and national codes such as the Danish DS411. The present paper serves as a part of the documentation for the methods and it is therefore also a supporting document for these standards.
The mechanical properties of various steel bars exposed to high temperature (“residual” properties) are experimentally investigated up to 850°C, with reference to a number of steel and bar types (carbon and stainless steel; quenched and self-tempered bars; hot-rolled and cold-worked bars; smooth and deformed bars). The aim is to clarify to what extent the thermal sensitivity of the different bars affects the ultimate capacity of a typical R/C section subjected to an eccentric axial force, past a fire (“residual” capacity). As usual in the design of R/C sections under combined bending and axial loading, the ultimate behavior is represented through the “M–N envelopes”, where the materials strength decay due to high temperature is taken into account. The results show that quenched and self-tempered bars (QST), very popular in Europe, are more temperature-sensitive above 600°C than the carbon-steel bars extensively used in the States and nowadays rarely used in Europe. Furthermore, the best response is exhibited by the stainless-steel bars, provided that they are hot rolled, as it is generally the case for medium- and large-diameter bars. Similar conclusions can be drawn for the sections reinforced with the different bar types.
The load bearing capacity of reinforced concrete structures after fire can be assessed if the modified mechanical properties of the building materials, designated here as ‘residual’, are known. In this respect, the residual properties of reinforcing steels produced (a) by the Tempcore process; (b) microalloying with vanadium; and (c) work-hardening, all falling into grade FeB500S, were selected for investigation, aiming to represent a wide range of steels currently available for the construction industry in Europe. For both the microalloyed with vanadium and for the work-hardened reinforcing steels typical diameters were investigated. For the Tempcore-type reinforcing steels, due to their heterogeneous structure, the effect of size and composition have been also examined. Specimens of the above types of steel were heated at temperatures ranging from 200 °C to 800 °C for 1 h and then cooled in air to room temperature. Thereafter, their residual properties were assessed by hardness measurements, tensile and Charpy-V impact tests. Metallographic analysis was used to correlate mechanical properties to microstructural characteristics. From the above investigation it was concluded that the Tempcore steels presented the more stable behavior up to temperatures of 500 °C, while the microalloyed steel, although it presented very satisfactory tensile properties, displayed low impact toughness due to coarsening of vanadium carbides. The work-hardened steel showed a continuous drop of its properties from approximately 250 °C and suffered from brittleness in the vicinity of 200 °C due to strain aging phenomena.
The effect of fire on the flexural behaviour of rc beams is investigated. Four groups of rc beams were cast, exposed to fire at 650 °C for time durations of 0, 30, 60 and 120 min and then cooled by water. The concrete compressive strengths of the beams were determined nondestructively using a Schmidt hammer. The beams were tested in flexure by applying two transverse loads incrementally. Strains and deflections were measured at each load increment. Cracking loads, crack propagation and ultimate loads were recorded for each beam. A reduction of ultimate loads, increase in deflection, increase in both compressive and tensile strains and reduction in concrete compressive strength due to fire exposure were observed.
Based on the investigation and review of experimental studies in the past 20 years in the People's Republic of China, this paper engages in further discussion and comparative analysis of researches on the mechanical behavior of concrete both under and after high temperature exposure. The following three aspects are focused on. First, the basic mechanical behavior of concrete at high temperature, including strength, elastic modulus, peak strain and Poisson's ratio. Second, the effect of high temperature on the yield strength and elastic modulus of rebar. And last but not the least, the high temperature response to the bond behavior between concrete and rebar. This overview summaries the states-of-the-art studies on the mechanical behavior of concrete at high temperature in China.
This study investigated the behavior of corner columns under axial loading, biaxial bending and asymmetric fire loading. It is found that, under a longitudinal stress ratio of , the residual strength ratios of the columns after fire loading show: (a) the 2 and 4 h fire loadings result in residual strength ratios of 67% and 57%, respectively; a 10% reduction on residual strength results as the duration changes from 2 to 4 h; (b) reductions in reinforcing steel ratio cause lower residual strength ratios; and (c) increasing the thickness of concrete cover causes lower residual strength ratios. It was also found that the temperature distribution across the cross-section is not affected by concrete cover thickness and steel ratio. The residual strengths can be used for future evaluation, repair and strengthening.
The effect of post-fire-curing on the strength and durability recovery of fire-damaged concrete was investigated. Twenty normal- (NSC) and high-strength concrete (HSC) mixes incorporating different pozzolans were prepared and exposed to elevated temperatures till 800°C. After natural cooling, the specimens were subjected to post-fire-curing in water and in a controlled environment for a total duration of 56 days. Unstressed compressive strength, rapid chloride diffusion, and mercury intrusion porosimetry (MIP) tests were conducted to examine the changes in the macro- and microstructure of the concrete. The test results indicated that the post-fire-curing results in substantial strength and durability recovery and its extent depend upon the types of concrete, exposure temperature, method, and duration of recuring. In one case, the recovered strength was 93% of the original unfired strength. Scanning electron microscopy (SEM) investigations indicated that the recovery was due to a number of rehydration processes that regenerate the calcium-silicate-hydrate (C-S-H). The new rehydration products were smaller in size than the original hydration products and filled the internal cracks, honey combs, and capillaries created during the fire. The surface crack widths were also reduced during the recuring process, and in most cases, they were found within the maximum limits specified by the American Concrete Institute (ACI) building code.
A common application of high strength concrete (HSC) is in columns subjected to large compressive forces. However, a major problem is the insufficient ductility available in HSC columns. To determine the required lateral reinforcement to maintain sufficient ductility, a good understanding of the stress-strain behaviour of confined concrete needs to be established. This paper describes a testing program carried out to obtain experimental data of complete (ascending and descending) stress-strain relationships between axial stress, axial strain and lateral strain for HSC. Compressive strengths of concrete tested were 100 MPa and 60 MPa. The confining pressures used were 4 MPa, 8 MPa and 12 MPa. A total of 18 stress-strain curves are presented. The experimental results obtained seem to indicate that, for high confining pressures, the lateral strain at peak stress for 100 MPa concrete was 20% less than that of the 60 MPa concrete.
Concrete material in structures is likely exposed to high temperatures during fire. The relative properties of concrete after such an exposure are of great importance in terms of the serviceability of buildings. This paper presents the effects of elevated temperatures on the physical and mechanical properties of various concrete mixtures prepared by ordinary Portland cement, crushed limestone, and river gravel. Test samples were subjected to elevated temperatures ranging from 200 to 1200 °C. After exposure, weight losses were determined and then compressive strength test was conducted. Test results indicated that weight of the specimen significantly reduced with an increase in temperature. This reduction was very sharp beyond 800 °C. The effects of water/cement (w/c) ratio and type of aggregate on losses in weight were not found to be significant. The results also revealed that the relative strength of concrete decreased as the exposure temperature increased. The effect of high temperatures on the strength of concrete was more pronounced for concrete mixtures produced by river gravel aggregate. The results of the physical and mechanical tests were also combined with those obtained from differential thermal analysis, and colour image analysis.
Using the raw material of pottery’s powder to improve the behavior of high strength concrete columns exposed to elevated temperature
- El-Naby
China, in: Research of the relation between mechanics performance of concrete material after fire and temperature and time
- L Li
China, in: Flexural behavior of fiber reinforced polymer (FRP) bars reinforced concrete beams under static and fatigue loads after elevated temperature exposure
- G Li