Fire Exposure Time Research Articles

Repair of fire-damaged RC square columns with CFRP textile-reinforced ECC matrix

Fiber-reinforced polymer (FRP) textile reinforced engineered cementitious composite (ECC) matrix (TRE for short) is a newly developed composite system with great potential for the strengthening and life extension of damaged reinforced concrete (RC) columns after fire exposure. It combines the advantages of the lightweight and high strength of FRP textile and the ultra-high ductility of ECC material. However, there is a lack of research on the use of the TRE system to strengthen fire-damaged RC columns. This paper presents an experimental study on the axial compressive behavior of fire-damaged square RC columns strengthened with the TRE system. A total of 13 specimens were prepared and tested under axial compression. The test variables included the fire-exposure time (2 h or 3 h) used to produce the initial pre-damage of RC columns, the type of cementitious matrix (ECC or mortar), the number of TRE layers (2 layers or 3 layers), and the thickness of the concrete cover (20 mm or 40 mm). The failure modes, load capacity, secant stiffness, ultimate displacement, load–displacement curves of the specimens and the hoop tensile strains in the outermost layer of the CFRP textiles were investigated and discussed. The typical failure mode of the strengthened specimens was characterized by tensile rupture of the fiber rovings bridging the main crack in the strengthening jacket. The results showed that the use of the TRE system could significantly increase the load capacity, secant stiffness and ultimate displacement of fire-damaged RC columns. Depending on the design parameters, the enhancement percentages in the load capacity varied between 18% and 126%.

Prediction of the Moment Capacity of FRP-Strengthened RC Beams Exposed to Fire Using ANNs

In this study, an Artificial Neural Network (ANN) model to predict the moment capacity of Fiber Reinforced Plastic (FRP) strengthened Reinforced Concrete (RC) beams exposed to fire is developed. The software ABAQUS heat transfer analysis is verified by comparison with the fire resistance test results. Through this heat transfer analysis, the temperature distribution of the beam section is determined, and 400 datasets are obtained using the moment capacity calculation method combined with the section equilibrium method. The data consist of eight input parameters: the beam width, beam height, FRP area, rebar area, concrete compressive strength, insulation thickness, concrete cover depth, and fire exposure time. The output parameter is the moment capacity. The ANN model is developed through a sensitivity study using the algorithm type and the number of hidden-layer neurons as variables. The average error between the predicted data of the developed ANN model and the target data obtained from the moment capacity calculation method was 0.35 kN·m, and the average relative error was 0.2512%, showing high accuracy. Therefore, the ANN model developed here can determine the moment capacity without complex calculations. The effects of the input parameters on the moment capacity of the FRP-strengthened RC beams exposed to fire are investigated using the ANN model.

Report on Existing Fireproof Construction Guidelines for Dwellings against Wildfires

This work presents a state-of-the-art review of existing fireproof construction guidelines for dwellings against wildfires. The most important wildfire-proof construction guidelines and codes for dwellings are presented, and these are later associated with existing fire engineering chapters associated with building codes. It was concluded that the variability in this subject is very high, and the approach to classifying the thermal effects in construction still lags behind scientific consensus. Moreover, the constructive requirements depend severally on the country’s building code when assessing the fireproof requirements for wildfire. Moreover, the thermal actions of wildfires in dwellings are presented and compared with classical ISO-834 standard fire curves, in which the maximum equivalent fire exposure time can range from 2 to 4 h depending on the country’s code. The key contributions of this work consist of (i) thoroughly disseminating codes and guidelines to promote scientific discussion in order to advance the wildfire fireproof standards dedicated to WUI; (ii) emphasizing the void in current codes in order to promote conversation between future researchers.

Additional Fire Exposure Time for the Effect of Spalling in Reinforced Concrete Columns

Concrete exposed to fire can experience spalling when the pore pressure increases due to water evaporation resulting in damage to part of the concrete cross-section which causes the loss of the initial cross-sectional symmetry, both thermal and geometrical. This may lead to what is, in essence, a biaxial bending problem in reinforced concrete columns. This study does not deal with the prediction of the occurrence of spalling for which a fully coupled hygro-thermo-mechanical analysis would be required. This work evaluates the consequences that corner spalling and surface spalling have on the fire resistance of reinforced concrete columns and presents a methodology based on the determination of a modified fire resistance time. An advanced fiber based numerical model for evaluating the fire resistance of RC sections exposed to fire is used as a tool to analyse the effect that spalling has on the performance of the columns. The model is able to generate the interaction surfaces of the columns subjected to uniaxial or biaxial bending and 4-sided exposed to the standard fire curve ISO 834. After a parametric analysis, it is observed that the negative effect of spalling can be quantified by means of an additional fire exposure time, which ranges between 0–35 min and depends on the column cross-section configuration, dimensions and load level. Finally, a proposal for the calculation of the modified fire resistance time for the effect of spalling is presented, which may be beneficial for practitioners in design.

Bonding behavior of interface between reinforced concrete after fire and carbon fiber-reinforced polymer

This study aims to investigate the bonding behavior between the fire-damaged concrete specimens strengthened by Carbon Fiber Reinforced Polymer (CFRP) plate. Twenty-five sets of damaged concrete specimens with variable concrete covering and exposure time conformed to ASTM E119 were prepared and bonded with CFRP plates with different bond lengths. According to Modified pull-out tests, with increasing the fire exposure time to 45 and 90 min, it significantly reduced the interfacial bonding strength and ductility. The bonding performance of fired specimens is achieved by applying longer bond length. Finally, the shear stress–slip model of the fire-damaged concrete bonded with CFRP can be proposed by using the experimental relationship between strain and slip with the simple derivative of local shear–stress equilibrium. While the effect of parameters are included in this model through the predicted interfacial fracture energy.

Experimental study on fire resistance of traditional timber mortise-tenon joints with damages

This paper presents the experimental results on the fire resistance of traditional timber mortise-tenon joints with looseness, decay and cracking in the jointing area. Static loading test and fire resistance tests were conducted on six full scale joint specimens. The development of the temperature, the charring depth and rate, and the degradation of the rotational stiffness of the joints with fire exposure time was quantified. It was found that the service damage can have significant influence on the fire resistance of the traditional timber mortise-tenon joints. The charring rate calculated from the beam and column cross-sections is close to the recommended value in design code for softwood. The displacement of the damaged joints with wood decay and cracking increased by 69% and 90%, respectively, with respect to that of the undamaged joint after 30 min of dire exposure. The rotational stiffness of a damaged joint after 30 min of fire exposure can drop down to 11.4% of the initial rotational stiffness of a sound joint at ambient temperature. The influence of the stiffness degradation of joints on the moment redistribution and fire endurance of neighbouring beams and columns is worth of further investigation.

Experimental fire performance assessment of a new type of prestressed composite circular precast concrete columns

The purpose of this paper is to study the fire resistance of a new type of prestressed composite circular precast concrete column (PCCPCC) consisting of a prestressed precast concrete hollow pipe and cast-in-place core concrete filled in the hollow section through a combined numerical and experimental approach. The potential advantages of PCCPCCs lie in their low manufacturing costs and high construction efficiency. A series of fire tests were conducted on PCCPCCs with consideration of different precast concrete strengths, rebar layouts, and axial loads, where the column damage patterns, fire exposure time, and vertical displacements of all the specimens were experimentally examined. Their post-fire residual strengths were also determined via uniaxial compression tests. Finite element (FE) simulations were also conducted and the obtained results were compared to the corresponding test outcomes. The results show that the fire resistance of PCCPCCs is largely dependent on the imposed axial loading, and an effective thermal isolation effect from the prestressed precast concrete hollow pipes was identified. The further research on this new column form is necessary for the possible future wide adoption.

Failure characteristics of reinforced concrete circular slabs subsequently subjected to fire exposure and static load: An experimental study

AbstractThis study investigated the effect of fire on the ultimate load‐bearing capacity of reinforced concrete (RC) slabs. The structural response of RC circular specimens subjected to static load conditions after exposure to a hydrocarbon fire on one side of the specimen was examined. Two fire exposure times were considered (60 and 120 min) in addition to reference non‐exposed specimens. The static response was evaluated in the damaged specimens in residual conditions after natural cooling from the elevated temperatures. The temperature distribution across the thickness of the slabs and their load–displacement response was measured. The decrease in the stiffness of the slabs due to the thermal exposure was studied by means of direct ultrasonic pulse velocity (UPV) measurements made before and after the fire tests. The decrease in global stiffness was partially accounted for by UPV measurements. The experimental results showed two peaks in the load‐deflection response of the slabs. The first peak was related to an arching mechanism introduced by the specific set‐up used. The second peak, corresponding to the ultimate load, occurred due to tensile membrane action at large deflections. While the former was strongly affected by the fire exposure, with the load being halved after the 120‐min exposure, the latter was not greatly affected by either the presence of fire or the exposure time. Simplified mechanical models were used to explain the behavior of the RC slabs during the tests.

Analysis and prediction methods for the static properties of reinforced concrete beams under fire

In order to study the thermal properties and static performance of reinforced concrete (RC) beams exposed to fire and reveal the relationship between fire exposure time, concrete spalling, bond slip and mechanical properties, the numerical simulation, theoretical analysis and machine learning methods were adopted to study the effects of high temperature on thermal and static mechanical properties of RC beams and the prediction methods of residual bearing capacity were proposed in this paper. Fire tests and static load tests after natural cooling were carried out on 4 RC simply supported beams. The refined numerical analysis model of RC beam was established and its validity was verified by the fire test. The effects of fire exposure time and characteristic parameters of concrete spalling on temperature fields and interfacial bond properties were simulated and analyzed. The residual flexural capacity were calculated and compared with measured and simplified theoretical values. Finally fire exposure time, concrete spalling depth and local spalling area ratio were selected as variables to design 250 groups of sample data, and sample database was established based on the refined numerical analysis model. Several methods for predicting residual flexural capacity after fire were proposed. Their accuracy and reliability were compared and analyzed, which can provide a method reference for safety estimation and analysis of post-disaster engineering.

Structural Fire Analysis of Beams and Slabs Using ANSYS and FEAST Software Program

In recent years, building fire has become more frequent event and fire accidents have increased all over the world. Major problem with fire is it provides very less time to handle the situation and due to elevated temperature, reduction in strength of structural member with increase in fire exposure time has been observed. To evaluate this reduced strength of structural members and design for fire load, it is necessary to have the parameters such as fire load, fire exposure time and temperature variation across the structural members. In this paper, evaluation of fire load, equivalent time of exposure and temperature distribution of beams and slabs using finite element analysis software FEAST and ANSYS has been done. For temperature variation, transient heat thermal analysis, in which temperature is increasing continuously over the time, is used. For this analysis, beams and slabs are exposed to surface fire on different surfaces and the temperature distribution of cross section has been evaluated. The results are compared and found to be matching with the European Standard procedure.

Behavior of Reinforced Ultra-High Performance Concrete Slabs Under Impact Loading After Exposure to Elevated Temperatures

Steel fiber-reinforced ultra-high performance concrete (UHPC) material is prone to explosive spalling under elevated temperatures. With the addition of polypropylene (PP) fiber, thermal spalling of UHPC can be mitigated and its fire resistance can be improved. This research investigates the impact resistance of steel and PP fiber-reinforced UHPC slabs after exposure to elevated temperatures, and the structural behavior and damage were compared against normal strength concrete (NSC) slabs. Karagozian & Case concrete (KCC) model was adopted to simulate both NSC and UHPC materials. With consideration of thermal hazards, the material damage, equation of state and strain rate sensitivity were adapted. The validity of this numerical model was evaluated against available experimental results. The numerical model was used to investigate the impact resistance of the reinforced UHPC slabs after exposure to fire hazards. The effect of fire exposure time, impact velocity and impact mass on the resistance of the reinforced NSC and UHPC slabs were analyzed. The simulation results revealed that punching shear failure areas in the NSC slabs were 2.5 times, 3.4 times, 3.0 times and 1.2 times larger than the UHPC slabs after exposure to international standardization ISO-834 standard fire for 1[Formula: see text]h, 2[Formula: see text]h, 3[Formula: see text]h and 4[Formula: see text]h, respectively. After exposure to the standard fire ISO-834 for 2 h, the punching shear failure on the bottom side of NSC increased 90.9% with the increase in falling height from 1[Formula: see text]m to 7[Formula: see text]m, while for the UHPC slabs, the increment was around 67.9%. After exposure to the standard fire ISO-834 for 2[Formula: see text]h, the punching shear damage of the NSC slabs increased by 72.9% with the punch weight increased from 100[Formula: see text]kg to 700[Formula: see text]kg, whereas the damage in the UHPC slabs increased by 53.8%.

Residual flexural behaviour comparison between composite and monolithic beams after fire exposure

Fourteen T-shaped composite beams and seven T-shaped monolithic beams were tested after fire exposure. To investigate their distinct residual flexural behaviour, the beams, which had different load ratios and thicknesses of precast slabs, were exposed to specific fire exposure times (60, 90, and 120 min) in a fire furnace and loaded to failure after natural cooling. The results showed that the residual flexural capacity of the composite beams was generally lower than that of monolithic beams. The fire exposure time was found to dominantly influence the residual flexural capacity of the composite beams. Increasing the thickness of the precast slabs can slightly enhance the residual strength of the composite beams. However, the horizontal cracks at the interface between the precast and cast-in-place concrete slabs of the composite beams reduced the horizontal shear strength and further increased the horizontal slip along this interface. Accordingly, a finite element model of the composite beams was proposed. A three-dimensional spring element was introduced into the model to consider the effect of slip at the concrete–concrete interface. To predict the residual flexural capacity of the composite beams, based on the 500°C-isotherm method, three modified equations suitable for the Chinese, American, and European codes were proposed. In comparing the test values, the modified equation applicable to the European code is recommended for subsequent predictions.

Dynamic response of cross steel reinforced concrete filled steel tubular columns under impact under fire

To explore the impact response of cross steel-reinforced concrete-filled steel tubular (CSRCFST) columns under fire, a finite element model using ABAQUS software was generated. After validation against test results, parametric studies were carried out to investigate the effects of impact load as well as time of fire exposure on the impact resistance of CSRCFST columns. The numerical results show that the impact behavior of post-fire CSRCFST columns can be divided into three stages: peak stage, plateau stage, and softening stage. For CSRCFST columns, the peak and plateau stages are important, which absorbed 32.4% and 67.6% of the impact kinetic energy, respectively. After two hours of fire exposure, the stiffness and peak impact load of the column decreased by 93.7% and 71.7%, respectively. However, the peak mid-span deflection and residual deflection increased by 6.5% and 20.1%, respectively. When the drop weight tripled, the maximum deflection and residual deformation of the midspan increased by 2.8 and 3.2 times, respectively. However, the peak impact load increases only by 14.5%. When the impact energy is the similar, the maximum midspan deflection of the specimen is almost identical, whereas a larger impact momentum reduces the peak impact force but increases the impact force at the plateau stage. By increasing fire duration, the behaviors of the column deteriorate seriously, with 85% reductions in peak impact force and 39% increase in maximum midspan deflection. In addition, the effects of impact velocity are also significant regarding each stage.

Residual Shear Capacity of RC Beams without Stirrups after Fire Exposure

To investigate the shear capacity and failure mechanism of RC beams after fire exposure, fourteen full-scale beams without stirrups were tested at ambient temperature and after fire exposure. Three parameters, including the loading ratio, shear span-to-depth ratio and longitudinal reinforcement ratio, were considered in static load tests. The deterioration mechanism of the shear bearing capacity at the diagonal section of RC beams without stirrups after fire exposure was experimentally, numerically and theoretically revealed, and a calculation formula for the shear capacity of post-fire beams without stirrups was proposed. The results show that the shear capacity and stiffness of the specimens decreased after fire exposure, and the shear strength loss of the beams increased with fire exposure time. The shear capacity and stiffness of fire-damaged specimens decreased as the shear span ratio λ increased, and the shear strength loss of the beams decreased with λ. Compared with the theoretical calculation and experimental results of beams without stirrups, the average of the absolute errors was 10.48%. Therefore, this formula can better calculate the residual shear capacity of beam without stirrups after fire exposure.

Study on thermodynamic properties of RC beams considering elevated temperature

In order to study the effect of high temperatures on the thermomechanical properties of reinforced concrete (RC) beams, fire and mechanical tests were carried out on four fully dimensioned simply supported beams. A numerical analysis model based on the measured information was developed, and validated through experimental tests and theoretical calculations. The effects of fire exposure time, different physical parameters and concrete spalling parameters on the thermodynamic properties were further analysed. The results show that the measured vibration frequencies exhibit a fluctuating trend of decay with increasing fire exposure time. The simulated frequency decays as a power function of fire exposure time. The frequency decreases with increasing span-to-height ratio and increases with section width, elasticity modulus of the concrete and reinforcement ratio. Cross-sectional temperature field is most significantly affected by spalling depth, and increases rapidly with an increase in this parameter. With the increase in spalling area ratio, the increase in the temperature field gradually becomes smaller. The effect of spalling area ratio was relatively stable after more than 15%. Flexural bearing capacity shows a linear trend of decrease with increase in spalling depth. With the increase in spalling area ratio, it shows a power function of the decreasing trend.

Effects of accelerated and natural weathering on fire-retarded industrial wood composite

This study shows the impact of aging on the fire behavior of a fire-retarded polyvinyl chloride wood composite cladding intended for outdoor use. Two sets of composite wood cladding elements were examined: one aged naturally under exposed outdoors conditions for 1 year and the second, which underwent artificial accelerated aging. Optical microscopy, electron probe micro-analysis, and solid-state nuclear magnetic resonance of 27Al were used to investigate changes in the material properties after aging, distribution of chemical compounds, and aluminum environment. This study reveals the impact of different aging procedures on the physicochemical modifications of the composite material. In addition, after aging, the fire behavior of the two sets of composites as a function of fire exposure time was evaluated by cone calorimetry. Then, the natural aged composite was evaluated on a larger scale with the single burning item. The results surprisingly demonstrate that whatever the aging, the fire behavior of these wood composite was not affected.

Seismic Performance of Composite Shear Walls Filled with Demolished Concrete Lumps and Self-Compacting Concrete after Fire

In order to provide a good solution for the treatment and utilization of construction waste, especially waste concrete and to promote the development of green construction to some extent, in this paper, a new composite shear wall filled with demolished concrete lumps (DCLs) and self-compacting concrete (SCC) was proposed, and its seismic performance after fire was investigated. Based on quasi-static tests of four composite shear walls filled with DCLs and SCC, three after a standard fire and one contrastive specimen without fire, the effects of fire exposure time, fire-retardant coating on the edge constraint steel pipe column, and the width-thickness ratio on seismic performance of composite shear walls after fire were studied. The failure patterns, bearing capacity, hysteretic loops, ductility, skeleton curves, rigidity degradation curves, and energy dissipation of shear walls were analyzed and compared. Test results indicate that the bearing capacity of the shear wall after 60 min of standard fire is slightly lower than the contrastive specimen without fire, but specimens still have good seismic performance. The interlayer displacement angle of the shear wall after a fire still meets the requirement of regulations. The fire-retardant coating on the edge of concealed steel pipe column has a limited effect on the seismic performance of specimens after a fire. The horizontal bearing capacity and energy dissipation capacity of shear walls after a fire can obviously be improved by properly increasing the width-thickness ratio.

A numerical homogenization method for the determination of the thermal bowing of a masonry wall exposed to fire: Application to natural stone masonry

Thermal bowing is one of the major concerns when analysing the structural behaviour of masonry walls in fire conditions, along with the loss of resistance due to material degradation. This contribution proposes a new homogenization method aimed at determining the thermal deformed shape of a masonry wall exposed to fire. First, the homogenized thermo-elastic characteristics of a natural stone masonry wall are determined taking into account the material properties of stone and mortar as functions of temperature increase, as well as the geometrical characteristics of their assembly. As a result, membrane and bending stiffness coefficients, as well as thermal-induced efforts, of the wall modelled as a homogenized plate are obtained. Such homogenized thermo-mechanical characteristics make it possible to determine the deformed shape of the wall after a certain time of fire exposure, through the formulation and the resolution of a finite element plate problem, making use of the previous homogenized stiffness characteristics. Numerical simulations show that the presence of joints has little influence on the deformed shape of the masonry wall compared to a homogeneous stone-only wall. They appear to be in rather good agreement with previous full-scale experimental tests carried out on limestone masonry walls, therefore providing a first validation of the proposed method, even though further improvements could be envisaged as regards a more reliable determination of the temperature gradient across the wall thickness.

Numerical Analysis on Post-Fire Resistance of High-Strength Circular CFST Stub Column in Axial Compression

Concrete-filled steel tubular (CFST) structures using high-strength materials have been increasingly used in civil engineering due to their exceptional mechanical performance. A comprehensive numerical analysis was performed in this study, where a finite element model was established for a CFST stub column using high-strength materials with consideration of fire and load combinations. The influence of critical parameters to the resistance in axial compression were analyzed based on the verified model, including the fire exposure time, the axial load level, the confinement factor, etc. The results showed that the residual resistance of a high-strength circular CFST (HCFST) column in axial compression decreased with the increase of axial load level and fire exposure time, while it increased with the increase of material strength.

Assessment of Indirect Charring in Doweled Wood Connections and External Steel Plates Submitted to Fire

AbstractThe present work aims to evaluate the wood connections, with dowels and external steel plates, in double shear, subjected to fire. The presence of external steel plates, in a fire situation, leads to a non‐uniform distribution of charring in the internal wooden elements. Based on the results, it is observed that the charring rate in wood does not remain constant and increases with the time of fire exposure of the steel elements. However, based on the influence of other variables, the charring rate decreases with increasing dowel diameter and with increasing wood density.